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'[PIC]: Building a Servo'
2001\08\08@112053 by Herbert Graf

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My brother has decided to build a gas powered remote controlled car for his
"let's not get bored" summer project and of course I have been drafted for
the electronics portion. The car (which BTW isn't your typical radio
controlled car, it will be about 4 feet long!) will use a Futaba transmitter
and receiver designed for model aircraft. As some of you may know this sort
of radio system usually uses servos and therefore the output of the receiver
is a form of PWM, in this case the receiver outputs a pulse, whose width
varies from 1ms to 2ms depending on the position of the corresponding stick,
and this process repeats every 20ms. I have decided to use a PIC as my
interface beween the motor drives and this receiver. I was wondering if
anybody has code for this sort of thing, it sounds pretty simple but if
someone has already done something like this there isn't much point in me
starting from scratch. I have searched the web and the PIC LIST archive for
pointers but all I could find is code for DRIVING a servo, obviously this
isn't what I need! :) Thanks in advance for any pointers. TTYL

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2001\08\08@120547 by Alan B. Pearce

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There have been threads in the past about detecting the pulse width from R/C
type receivers using the CCP portion of the PIC hardware. These should turn
up in the archive, but it may depend what words you use to search on. I
would start with "CCP"

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2001\08\08@124130 by Herbert Graf

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Hello, I thought about that but I will be using the 16F84 so no CCP hardware
for me. Thanks, TTYL

> {Original Message removed}

2001\08\08@124845 by Alan B. Pearce

face picon face
>Hello, I thought about that but I will be using the 16F84 so no CCP
hardware
>for me. Thanks, TTYL

You would be better to use the 16F627/8 which is cheaper, pin compatible to
the 16F84 and has the CCP hardware in it.

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2001\08\08@130524 by Herbert Graf

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Unfortunately for various reasons (partially since I have 16F84s lying
around and I want to get rid of them) I am restricted to using the 16F84.
Thanks, TTYL

> {Original Message removed}

2001\08\08@135537 by John Ferrell

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A most interesting project. BTW, it is against the law in the US to use the
airplane frequencies for surface RC. Also, unless you are already skilled at
using the airplane controller, those designed for RC cars are easier to
learn.

One of my unlikely to pursue projects is to do the same thing with my full
size back-hoe. The servos will be a critical component. The high end Futaba
PCM systems have a "fail safe" provision that can be programmed (in case of
control system failure) for shut down.

I think ordinary servo feedback applied to automotive window crank motors
will work for my proposed project.
So far I have not seen a need for a PIC.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2001\08\08@173355 by Christian Dorner

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Hello!



> varies from 1ms to 2ms depending on the position of the corresponding
stick,
> and this process repeats every 20ms. I have decided to use a PIC as my
> interface beween the motor drives and this receiver. I was wondering if
> anybody has code for this sort of thing, it sounds pretty simple but if
> someone has already done something like this there isn't much point in me
> starting from scratch.

I didn't done this befor but has a similar problem with a 433MHz Project.

My idea on this is just to count the length of the received pulse.

I.e.: Use PortB for inputting the reveiver signal and trigger a PortB change
interrupt with it.
You can use PortB0 and trigger on raising or falling edge (however your
receiver works) or  PortB4-PB7 and check the rest by your software.

Use a simple counter loop to countup a "signal-length-timer" until the
signal changes its state again. Now you have a value representing the signal
length and the position of the stick.

Hope this helps or just show you an direction how to solve this.

And, sorry about my bad english :-(

cu,
Christian

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2001\08\09@091035 by Olin Lathrop

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> Hello, I thought about that but I will be using the 16F84 so no CCP
hardware
> for me.

So don't do that.  There are lots of other 16F parts that do have CCP
modules.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\08\09@115557 by Herbert Graf

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Quite frankly I have settled on the 16F84 and that is that. Thank you for
the help.

{Quote hidden}

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2001\08\09@121618 by Jeff DeMaagd

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----- Original Message -----
From: Herbert Graf <RemoveMEmailinglistspamTakeThisOuTFARCITE.NET>


> Quite frankly I have settled on the 16F84 and that is that.

How about the PIC16F628?  It's cheaper, can run faster, has more feature,
the same number of pins and same physical size AND has a CCP module.  I wish
I knew about them months ago:

www.microchip.com/10/lit/pline/picmicro/families/16c62x/devices/16c62
8/index.htm

Based on this, I simply don't understand the F84 fixation.

Jeff

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2001\08\09@122852 by Alan B. Pearce

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>Based on this, I simply don't understand the F84 fixation.

because from an earlier reply to one I suggested the 16F627/8 he already has
a number of them.

The way to do it without having the CCP is to use the INTR input to start
the timer on the rising edge of the waveform, change the edge sensitivity to
falling edge, on the next interrupt save the timer count and set the edge
sensitivity back to rising edge. Initialise the timer ready to start
counting again on the next interrupt. The value you have saved is the pulse
width.

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2001\08\09@123309 by Douglas Butler

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I believe he said a whil ago that he has a whole bunch of 16F84s on
hand, and since this is a hobby project he doesn't want to buy another
chip.
On the other hand if he is only doing a single channel R/C servo
controller most any PIC should do fine.  I am not familliar with the
16F84 but I am sure I could do it easily with the 16C54's I use.  If I
needed a doual channel controller I would use two 16C54s!

Sherpa Doug

> {Original Message removed}

2001\08\09@124724 by Scott Dattalo

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On Thu, 9 Aug 2001, Alan B. Pearce wrote:

> >Based on this, I simply don't understand the F84 fixation.
>
> because from an earlier reply to one I suggested the 16F627/8 he already has
> a number of them.
>
> The way to do it without having the CCP is to use the INTR input to start
> the timer on the rising edge of the waveform, change the edge sensitivity to
> falling edge, on the next interrupt save the timer count and set the edge
> sensitivity back to rising edge. Initialise the timer ready to start
> counting again on the next interrupt. The value you have saved is the pulse
> width.

If you want a software PWM routine that has single instruction cycle
resolution then check out:

http://www.dattalo.com/technical/software/pic/pwm256.txt

It can be configured for pulses 256 or 4096 cycles wide. It works.
I've used and others have used it too.

I suppose one could convert it into a servo controller. But special
care must be taken to ensure the code is isochronous.

Scott

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2001\08\09@132317 by Herbert Graf

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OK, this is getting annoying now. I have seen this happen before on this
list and I feel I should mention it. Why is it not enough to say that one
has decided on using a certain part? Why does it matter that I have decided
to use a part that might not be optimal but should work anyways? True I'm
making the job a little harder by not using the ideal part but when you are
working on a project for a hobby purpose there are several factors that
don't come up when designing something for a market.

One reason for my choosing the F84 is that I have them, simple as that.
There is no reason for me to spend money and wait for other parts to arrive
when I have 16F84s in my bin. Is this enough???

Sorry if I sound a bit ticked but this often happens on this list, someone
wants to use something and you get dozens of messages asking why not this
part and why not that part. Maybe I'm the only one annoyed by this, so be
it. TTYL

> {Original Message removed}

2001\08\09@132647 by Herbert Graf

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{Quote hidden}

       This is quite a good idea, and is slightly different from the approach I
was planning to take (I was going to use the change on RB interrupt since I
wasn't aware you could change the edge sensitivity on the INT pin). Thanks,
TTYL

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2001\08\09@132650 by Herbert Graf

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That's the plan, I'm actually going to do three channels and for simplicity
I plan to use a seperate PIC for each channel with the same basic code (the
differences in code will be due to the complexity of the control needed (ie.
some will have position feedback, others won't) and the motor being driven
(using a stepper on one channel and DC motors on the other two)). TTYL

> {Original Message removed}

2001\08\09@134512 by Roman Black

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Herbert Graf wrote:
>
> That's the plan, I'm actually going to do three channels and for simplicity
> I plan to use a seperate PIC for each channel with the same basic code (the
> differences in code will be due to the complexity of the control needed (ie.
> some will have position feedback, others won't) and the motor being driven
> (using a stepper on one channel and DC motors on the other two)). TTYL

Hi Herbert, first, i'm with you on the 16F84 issue,
I have hundreds of them and use them for anything
I want to whip up quick, whether "too simple" for
a 16F84 or "too complex" for a 16F84. The fact that
I have the chips, the boards, and many code projects
with register setups completed makes it easier to use
the chips you always use. You don't have to explain
WHY you're using a 16F84. :o)

Second, I can't help you directly with the gasoline
powered model car, apart from a basic suggestion.
My friend had a small-scale RC car using a gas motor,
it was about 18" long and did 40mph (real mph) and
was rather scary. My suggestion would be to take
care designing the throttle and brake systems to
be fail safe, so in the event of ANY failure the
throttle closes and the brakes are ON. Having seen my
friends car get out of RC range and crash into things
fast enough to break a human leg and destroy the
car, this is pretty important...

Maybe you could send a "good" code constantly to the
throttle and brakes, and the moment this code fails
the onboard PIC (watchdog?) etc goes into "safe"
mode. It's surprising how often an RC vehicle gets
bad info or loss of info from the controller. :o)
-Roman

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2001\08\09@135948 by Barry Gershenfeld

picon face
I also wonder why people don't pay attention to what you are
saying.  It seemed clear enough.

Better yet, your favorite PIC is programmable and you have
brains so you can make it work.  That's the fun of PIC
programming anyway.   Anyone remember the concept
of "challenge"?

I bet if you posted a marvelous way to measure pulse widths
without using CCP hardware, people would then post "aw heck
I did that years ago...in my sleep...uphill both ways..." :)

Barry



{Quote hidden}

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2001\08\09@140607 by Bob Ammerman

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Herbert,

You are certainly welcome to use the F84 for your project (and it should
work just fine). However, remember there are 2000+ others reading the
PICLIST and it would be a disservice to them not to let them know the
'better' alternatives.

The answer you receive today will be going into the archives where hopefully
several others will find it and get their answer without having to ask the
same question to the list (sometimes for the n'th time).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: "Herbert Graf" <spamBeGonemailinglistSTOPspamspamEraseMEFARCITE.NET>
To: <KILLspamPICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Thursday, August 09, 2001 1:24 PM
Subject: Re: [PIC]: Building a Servo


> OK, this is getting annoying now. I have seen this happen before on this
> list and I feel I should mention it. Why is it not enough to say that one
> has decided on using a certain part? Why does it matter that I have
decided
> to use a part that might not be optimal but should work anyways? True I'm
> making the job a little harder by not using the ideal part but when you
are
> working on a project for a hobby purpose there are several factors that
> don't come up when designing something for a market.
>
> One reason for my choosing the F84 is that I have them, simple as that.
> There is no reason for me to spend money and wait for other parts to
arrive
> when I have 16F84s in my bin. Is this enough???
>
> Sorry if I sound a bit ticked but this often happens on this list, someone
> wants to use something and you get dozens of messages asking why not this
> part and why not that part. Maybe I'm the only one annoyed by this, so be
> it. TTYL
>
> > {Original Message removed}

2001\08\09@143838 by Jeff DeMaagd

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----- Original Message -----
From: Herbert Graf <EraseMEmailinglistspamEraseMEFARCITE.NET>


> OK, this is getting annoying now. I have seen this happen before on this
> list and I feel I should mention it. Why is it not enough to say that one
> has decided on using a certain part? Why does it matter that I have
decided

Sorry about that, I didn't read all the posts in the thread.  I had archived
all my piclist mail a few days ago.

> One reason for my choosing the F84 is that I have them, simple as that.
> There is no reason for me to spend money and wait for other parts to
arrive
> when I have 16F84s in my bin. Is this enough???

That depends.  Will the workaround take longer to code than the wait for
fresh parts?  That's another consideration.  It's totally up to you to
decide the path, I am curious if there has been any determination that a
software routine will do just as well under the circumstances or not.  It
just seems like more work than it's worth, but if you are willing to do it,
good luck.

Jeff

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2001\08\09@145800 by Byron A Jeff

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On Fri, Aug 10, 2001 at 03:44:21AM +1000, Roman Black wrote:
> Herbert Graf wrote:
> >
> > That's the plan, I'm actually going to do three channels and for simplicity
> > I plan to use a seperate PIC for each channel with the same basic code (the
> > differences in code will be due to the complexity of the control needed (ie.
> > some will have position feedback, others won't) and the motor being driven
> > (using a stepper on one channel and DC motors on the other two)). TTYL
>
> Hi Herbert, first, i'm with you on the 16F84 issue,
> I have hundreds of them and use them for anything
> I want to whip up quick, whether "too simple" for
> a 16F84 or "too complex" for a 16F84. The fact that
> I have the chips, the boards, and many code projects
> with register setups completed makes it easier to use
> the chips you always use. You don't have to explain
> WHY you're using a 16F84. :o)

As one of the advocates of the new parts, I'd like to explain that the
suggestion really isn't directed to the old hats on the list. I'll usually
throw in my two cents when I see a post that starts with "I'm starting
out with PIC and I have some questions about the 16[CF]84[A]."

Most of the time those folks simple do not have enough exposure to realize
that there are newer/faster/bigger/cheaper/more fully featured parts out there.
They think 16F84 because the vast majority of pages and projects out there
specify the part.

If you're starting out there's simply no comparison. Simply from price alone
the part is the winner. Just looked up on Digikey for 25 part quantity:

16F628: $55
16F872: $70
16F84A: $92

All for the 20 Mhz part.

And since the 16F628 has an internal 4MHz oscillator it gets even cheaper
for timing insensitive applications. BTW I noticed that the 4 MHz parts were
only pennies cheaper (14 to be exact at the 25 part price), that little is
lost buying on 20 Mhz parts.

Everyone that's taken a spin around the block knows all the strengths and
limitations of each of the part subfamilies. And of course if you'd got em
and they can do the app, use them.

But I think we'd be remiss as good PICLIST citizens not to inform newcomers
about the advantages of the latest technology.

BAJ

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2001\08\09@154022 by Herbert Graf

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> Second, I can't help you directly with the gasoline
> powered model car, apart from a basic suggestion.
> My friend had a small-scale RC car using a gas motor,
> it was about 18" long and did 40mph (real mph) and
> was rather scary. My suggestion would be to take
> care designing the throttle and brake systems to
> be fail safe, so in the event of ANY failure the
> throttle closes and the brakes are ON. Having seen my
> friends car get out of RC range and crash into things
> fast enough to break a human leg and destroy the
> car, this is pretty important...
>
> Maybe you could send a "good" code constantly to the
> throttle and brakes, and the moment this code fails
> the onboard PIC (watchdog?) etc goes into "safe"
> mode. It's surprising how often an RC vehicle gets
> bad info or loss of info from the controller. :o)

       Considering the size and weight of the car safety has been one of my number
one concerns. I actually have a few layers of safety. First off the throttle
and brake are handled completely seperately, and both will default to a safe
status if a pulse from the receiver isn't received in the last 60ms (safe
for the brake being completely on and safe for the throttle being completely
off). Aside from this the steering system will default to fully left (or
right, haven't decided) in case of loss of contact with the transmitter, so
if everything else goes wrong at least the thing will go in tight circles
instead of a straight line.

       I might also add a redundant kill switch with a second transmitter and
receiver, the only problem, and this is something I was planning to ask, is
how do you stop a gas engine remotely? I thought about disconnecting the
spark plug but how would I do that considering the voltages involved?
Cutting the fuel flow is a second idea but my experience with two stroke
engines is that they keep running, without any fuel flow for at least 30
seconds, long enough to cause damage. Anybody have a better idea? Thanks,
TTYL

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2001\08\09@155337 by John Ferrell

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Speaking as one who is on the leading edge of the learning curve,  something
with training wheels would be good. No doubt about it, there is more
information and help available for the 16F84 than anything else. It is the
best place to start. I bought these and some 16F874's to start with and I
doubt I will need any thing else until some of the clouds clear.

I looked at the datasheet on the 16F628 and decided it best to wait a while
for that one!

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2001\08\09@160358 by Douglas Butler

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>         I might also add a redundant kill switch with a
> second transmitter and
> receiver, the only problem, and this is something I was
> planning to ask, is
> how do you stop a gas engine remotely? I thought about
> disconnecting the
> spark plug but how would I do that considering the voltages involved?
> Cutting the fuel flow is a second idea but my experience with
> two stroke
> engines is that they keep running, without any fuel flow for
> at least 30
> seconds, long enough to cause damage. Anybody have a better
> idea? Thanks,

How about some relay contacts across the breaker points for the ignition
coil, assuming you have mechanical points vs. electronic ignition?  The
voltages there are reasonable and without spark the engine should stop
quickly.

Sherpa Doug

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2001\08\09@160940 by Herbert Graf

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Hehe, it's funny you mention that, I had almost the exactly opposite
reaction lately. I've been working with the 16F877 on a different project
for a while and when I picked up the 16F84 datasheet I couldn't BELIEVE how
much was missing!! The register description section is so short, all the
diagrams feel so short and small! Then I looked at the 16F628 datasheet and
I said to myself "now THIS is more like it!" :) I'm kinda surprised it took
Mchip this long to come out with such a nice chip (heck flash parts are
relatively new aside from the 16x84).

Btu on the other side the 16F84 has alot fewer "gotchas", especially when
setting up the chip (I remember with the 16F877 looking through every pins
description to make sure I hadn't missed something, like disabling the ADC
for that pin, etc.). Simplicity does have it's virtues. TTYL

> {Original Message removed}

2001\08\09@161359 by Herbert Graf

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{Quote hidden}

       I was thinking about something like that but it may be hard to find a relay
that can do it. The problem is that if the relay contacts aren't far enough
apart a spark will simply develop across the relay contacts AND the plug!
:( The motor I am using (and no I can't change the kind of motor being used!
:) )uses a "coil flying by a magnet really quickly" approach to develop the
spark.

Why I don't really like this idea either is that assuming I find a relay
with contacts far enough apart it is still possible that on say a humid day
the spark WOULD jump across the contacts. I guess I could heavily modify a
relay so the contacts are say 1cm apart, I believe that would be enough, but
being a safety device I'd rather go with something a little more "sure". :)
TTYL

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2001\08\09@162221 by Mike Kendall

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Personally, I'd remove the ground path for the ignition coil to shut off the
engine.  This will work for gasoline engines.  If you have a turbocharged
runaway diesel, best way is to remove your shirt and shove it into the
intake, or have a tube of silicone sealant handy to fix the leaking gasket
causing the runaway.  It is always safer to have the ground disrupted.
Mostly, on vehicles they control  the ground so that there is not any
voltages running through the firewall.  If the wire "grounds out" the
associated brake light or whatever will just be  on all the time.
Regards,
Mike
{Original Message removed}

2001\08\09@162437 by hard Prosser

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I think the suggestion was to put a relay across the points/sensor on the
low tension side of the magneto.

Richard P



                   Herbert Graf
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> {Original Message removed}

2001\08\09@162443 by Douglas Butler

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I think you misunderstand how a spark coil works.

The spark coil has three terminals.  One is ground, one is for the spark
plug, and the third goes to the breaker points.  When the points close
the coil builds up energy in its magnetic field.  When the points
suddenly open the energy from the magnetic field causes a pulse of high
voltage at the spark plug terminal.

The terminal for the points has low voltage, maybe 100V max.  That lead
you can run to a relay.  If you have a megneto you can simply short the
points to kill the engine.  If you have an automotive type system
powered from a battery I would put the relay in series with the points
in which case opening the relay kills the engine.

Sherpa Doug

> {Original Message removed}

2001\08\09@163054 by Mike Kendall

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> :( The motor I am using (and no I can't change the kind of motor being
used!
> :) )uses a "coil flying by a magnet really quickly" approach to develop
the
> spark.

   Isn't that a "magneto" ignition that you are describing, similiar to the
old 2 cycle  dirt  bikes?
Regards,
Mike

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2001\08\09@165617 by Dale Botkin

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On Thu, 9 Aug 2001, Mike Kendall wrote:

> > :( The motor I am using (and no I can't change the kind of motor being
> used!
> > :) )uses a "coil flying by a magnet really quickly" approach to develop
> the
> > spark.
>
>     Isn't that a "magneto" ignition that you are describing, similiar to the
> old 2 cycle  dirt  bikes?
> Regards,

And lawnmowers, and chainsaws, snow blowers, Formula 500 race cars, and
most small airplanes.

Dale
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On my desk I have a workstation...

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2001\08\09@170818 by hard Prosser

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Does the motor have mechanical points - or is the ignition Electronic? -
Some small 2 strokes have electronic modules that work on the dV/dt of the
low tension coil output and use a transistor to take the place of points.

If this is the case, you might have to figure out which is the low tension
coil connection and tap into it - if that's the way you want to go.

Another "Quick stop" possibility might be to have a spring loaded lever
poised over the (bare) spark plug terminal. This is released by a soleniod
(Or held in place by a solenoid)  and shorts out the spark to stop the
motor.  Doing it this way would not require changes to the ignition system.

Richard P




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On Thu, 9 Aug 2001, Mike Kendall wrote:

> > :( The motor I am using (and no I can't change the kind of motor being
> used!
> > :) )uses a "coil flying by a magnet really quickly" approach to develop
> the
> > spark.
>
>     Isn't that a "magneto" ignition that you are describing, similiar to
the
> old 2 cycle  dirt  bikes?
> Regards,

And lawnmowers, and chainsaws, snow blowers, Formula 500 race cars, and
most small airplanes.

Dale
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2001\08\09@171156 by Max Toole

picon face
In a message dated 8/9/2001 12:17:10 PM Eastern Daylight Time,
@spam@jeffspam_OUTspam.....DEMAAGD.COM writes:

<< How about the PIC16F628?  It's cheaper, can run faster, has more feature,
the same number of pins and same physical size AND has a CCP module.  I wish
I knew about them months ago:

www.microchip.com/10/lit/pline/picmicro/families/16c62x/devices/16c62
8/index.htm

Based on this, I simply don't understand the F84 fixation.

Jeff >>
They say that the only dumb question is the question not asked so here goes:
What does CCP mean?

Thanks,
Max

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2001\08\09@174543 by Bourdon, Bruce

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Herbert:

As far as the spark jumping the relay gap (if for some reason you cannot
gain access to a low voltage section of the circuit) you could use more than
one contact/relay: wiring the contacts in series will quickly reach a "safe"
distance...

Other thoughts;
 * is there an air intake (or exhaust) port that could be blocked? doing so
could do the job.
 * how about interrupting the mechanical coupling between the wheels and
the engine?
 * for really fast stopping of the engine you could cause the
piston/cylinder to try to act on something less compressible - such as water
by using a car windshield washer fluid pump with a small reservoir and hose
directed into the intake... though this might be too drastic.
 * you might want to reconsider changing the steering wheel direction as a
fail-safe - would be messy if it intermittently activated while moving at
speed in close quarters or near people... I'd lock the wheels as-is until a
valid steering signal is received...

Bruce.

>        I was thinking about something like that but it may be hard to find
a relay
> that can do it. The problem is that if the relay contacts aren't far
enough
> apart a spark will simply develop across the relay contacts AND the plug!
> :( The motor I am using (and no I can't change the kind of motor being
used!
> :) )uses a "coil flying by a magnet really quickly" approach to develop
the
> spark.
>
> Why I don't really like this idea either is that assuming I find a relay
> with contacts far enough apart it is still possible that on say a humid
day
> the spark WOULD jump across the contacts. I guess I could heavily modify a
> relay so the contacts are say 1cm apart, I believe that would be enough,
but
> being a safety device I'd rather go with something a little more "sure".
:)
> TTYL

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2001\08\09@175015 by Walter Banks

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>

Cut off air to carb.

>
> Cutting the fuel flow is a second idea but my experience with two stroke
> engines is that they keep running, without any fuel flow for at least 30
> seconds, long enough to cause damage. Anybody have a better idea?

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2001\08\09@180733 by Olin Lathrop

face picon face
> OK, this is getting annoying now. ...
> Why is it not enough to say that one has decided on using a certain part?
> ...
> Sorry if I sound a bit ticked but this often happens on this list, someone
> wants to use something and you get dozens of messages asking why not this
> part and why not that part.

You have every right to use a screwdriver to drive a nail instead of a
hammer.  But, you should be prepared for the inevitable comments if you ask
2000 people to take time out of their lives to do you a favor to explain how
to drive a nail with a screwdriver when the right answer is to use a hammer.


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(978) 742-9014, .....olin@spam@spamEraseMEembedinc.com, http://www.embedinc.com

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2001\08\09@183910 by Dave Mumert

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Hi

There is a web page by Chuck McManis that describes hooking a Futaba radio
to a variable speed reversible motor.  It uses a 16C54 but the code should
be easy to migrate to a 16F84.

Check at
http://www.mcmanis.com/chuck/robotics/projects/servo.html

It gives the code and a schematic for the H driver.  The code appears to be
well commented.

Dave Mumert

{Original Message removed}

2001\08\09@185434 by John Ferrell

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face
OK! I give up!
I will order a few the next time I order from someone who has them.

I have a few 8088's around but I won't start any new project with them...

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



----- Original Message -----
From: "Herbert Graf" <.....mailinglistSTOPspamspam@spam@FARCITE.NET>
To: <PICLISTEraseMEspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, August 09, 2001 4:04 PM
Subject: Re: [PIC]: Building a Servo


> Hehe, it's funny you mention that, I had almost the exactly opposite
> reaction lately. I've been working with the 16F877 on a different project
> for a while and when I picked up the 16F84 datasheet I couldn't BELIEVE
how
> much was missing!! The register description section is so short, all the
> diagrams feel so short and small! Then I looked at the 16F628 datasheet
and
> I said to myself "now THIS is more like it!" :) I'm kinda surprised it
took
> Mchip this long to come out with such a nice chip (heck flash parts are
> relatively new aside from the 16x84).
>
> Btu on the other side the 16F84 has alot fewer "gotchas", especially when
> setting up the chip (I remember with the 16F877 looking through every pins
> description to make sure I hadn't missed something, like disabling the ADC
> for that pin, etc.). Simplicity does have it's virtues. TTYL
>
> > {Original Message removed}

2001\08\09@193511 by Herbert Graf

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face
But AFAIK there IS no "low tension side of the magneto". The spark is
created when the magnet on the crankshaft passes by the stionary coil, which
results in the spark. Please correct me if I'm wrong, remember, this is a
small 2 stroke engine. Thanks, TTYL

> {Original Message removed}

2001\08\09@193523 by Herbert Graf

flavicon
face
I believe you have misunderstood the engine in question, it has no breaker
points, no distributer, it's a small 2 stroke single cylinder engine. Please
correct me if I'm wrong. TTYL

> {Original Message removed}

2001\08\09@193527 by Herbert Graf

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face
I'm not sure of the name, which is why I never stated the name of the
iginition system! :) Is my understanding of how it works correct? I only
took a quick look at how it works. Thanks, TTYL

> {Original Message removed}

2001\08\09@194347 by Herbert Graf

flavicon
face
> > OK, this is getting annoying now. ...
> > Why is it not enough to say that one has decided on using a
> certain part?
> > ...
> > Sorry if I sound a bit ticked but this often happens on this
> list, someone
> > wants to use something and you get dozens of messages asking
> why not this
> > part and why not that part.
>
> You have every right to use a screwdriver to drive a nail instead of a
> hammer.  But, you should be prepared for the inevitable comments
> if you ask
> 2000 people to take time out of their lives to do you a favor to
> explain how
> to drive a nail with a screwdriver when the right answer is to
> use a hammer.

       If you read my original post I was NOT asking how to do it, instead I was
asking if anybody could point me to code of it being done. I know how to do
it, it's just easier to use proven code, if available. The point is moot
now, I've already started coding it. Please read my post before responding.
TTYL

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2001\08\09@194351 by Herbert Graf

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Perfect! Thanks alot, TTYL

{Quote hidden}

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2001\08\09@201907 by Barry Gershenfeld

picon face
>:( The motor I am using (and no I can't change the kind of motor being used!
>:) )uses a "coil flying by a magnet really quickly" approach to develop the
>spark.
>

That's a "magneto", though I rather like "coil flying by
a magnet really quickly"  (CFBMRQ?).  And I used to believe
that it was the "really quickly" part that caused the spark.
But one day I pulled the flywheel off the lawnmower and
lo and behold underneath was a set of breaker points.
So I guess the magnet's not quick enough.  V must be
equal to di/dt or something :)   Maybe it's not that way
on all magnetos, but find the coil and see if it doesn't
have another connection.  That one should be a low
voltage control.

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2001\08\09@203641 by Byron A Jeff

face picon face
On Thu, Aug 09, 2001 at 03:38:26PM -0400, John Ferrell wrote:
> Speaking as one who is on the leading edge of the learning curve,  something
> with training wheels would be good. No doubt about it, there is more
> information and help available for the 16F84 than anything else. It is the
> best place to start. I bought these and some 16F874's to start with and I
> doubt I will need any thing else until some of the clouds clear.
>
> I looked at the datasheet on the 16F628 and decided it best to wait a while
> for that one!

I do see your point. But the interesting thing about the part is that you can
treat it just like a 16F84 until you need something. So you can start out on
a project. Then a bit into it you realize that another timer would be useful.
With the 16F84 you'd be stuck. But with the F628, there it is. Same with
the hardware UART, extra programming, data, and eeprom memory. Same with
comparators. Instead of thinking about it as a 16F628, you can think of it
as a 16F84A plus. Adding features as you need them.

And of course if you're buying there's the pure cost issue. The new part is
2/3's the price of the old one.

BAJ

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2001\08\09@204507 by Mike Kendall

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You shouldn't have to look far for the lawnmower engine cutoff as all modern
lawnmowers have a "deadmans" bar that needs to be held in before even
starting the lawn mower.  I'm still wondering, what is the make and model of
the engine??
Mike
{Original Message removed}

2001\08\09@205118 by hard Prosser

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Herbert
A magneto works much the same way as a coil - except it is self excited.
There is a low tension side which is "normally" short circuited by some
sort of switch, and a high tension overwind similar to the secondary
winding of the coil or transformer. The magnet moving past the windings
induces a current in the primary which, at the appropriate moment, is
interrupted by the switch opening. The current in the primary suddenly has
"nowhere to go" and the voltage across the primary rises dramatically.
Since the secondary winding is on the same magnetic circuit, it's voltage
rises even more dramatically as it has more turns and is sufficient to
break down the spark gap & fire the mixture.
To my knowledge there are no systems that generate the secondary voltage
directly from the magnet/coil arrangement - and spark timing would be a
major problem with this system anyway.

As I noted earlier,  2 stroke engines can have points (generally well
buried under the flywheel) or more commonly these days, a transistorised
module that detects the current in the primary winding and opens the
transistor switch as soon as this current starts to decay (i.e. as the
magnet moves away from the gap in the coil) - or something like that (I
haven't looked in detail but have had to replace a module or 2).

Hope this helps. I realise you may not want to get too involved with
rewiring the magneto so suggested the spark plug shorting method. Another
advantage that comes to mind is that  it may be possible to arrange it as a
kill switch if the car flips etc. although vibration could be a problem.

Richard P




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But AFAIK there IS no "low tension side of the magneto". The spark is
created when the magnet on the crankshaft passes by the stionary coil,
which
results in the spark. Please correct me if I'm wrong, remember, this is a
small 2 stroke engine. Thanks, TTYL

> {Original Message removed}

2001\08\09@210757 by Olin Lathrop

face picon face
> Why I don't really like this idea either is that assuming I find a relay
> with contacts far enough apart it is still possible that on say a humid
day
> the spark WOULD jump across the contacts. I guess I could heavily modify a
> relay so the contacts are say 1cm apart, I believe that would be enough,
but
> being a safety device I'd rather go with something a little more "sure".
:)

You could use a SPDT relay that routes the spark line to either the spark
plug or some dummy load.


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2001\08\09@210816 by Olin Lathrop

face picon face
> The spark coil has three terminals.  One is ground, one is for the spark
> plug, and the third goes to the breaker points.

But he's got one of those ignitions where mechanical action slowly builds up
then quickly breaks the magnetic circuit.  These don't have points or a
primary circuit.  The electrical part is only the high voltage coil.


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2001\08\09@210822 by Olin Lathrop

face picon face
> What does CCP mean?

"Capture, Compare, Pulsewidth modulation".  Microchip calls their standard
module that can do these things the "CCP module".  For details see the data
sheet of any PIC that has a CCP module, like the 16F876 and many others.


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2001\08\09@215219 by Dan Michaels

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Herbert Graf wrote:
..........
 Aside from this the steering system will default to fully left (or
>right, haven't decided) in case of loss of contact with the transmitter, so
>if everything else goes wrong at least the thing will go in tight circles
>instead of a straight line.
...........

As long as you are going to do this, why put a centrifugal force
detector fail-safe on it which will kill the engine after the thing
spends a couple of seconds in circle joy mode?

From what you are describing, I might consider using a 5 pound
weight on the end of a 6" boom, which is normally hanging straight
down, but which will start hanging out horizontally once you go
into the spin. Have a limit switch mounted near the horizontal
"hang" position. Might use a dashpot too, so normal insane
cornering won't trip the kill switch.

[sometimes I wonder why more piclisters aren't already dead]
;-)

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2001\08\09@223017 by Bob Ammerman

picon face
Since it is for a kill switch:

Imagine a grounded spring steel strip held away from the top of the spark
plug by an electromagnet.

Signal loss occurs (or explicit kill command received), then:

1: Electromagnet loses current

2: Spring steel snaps over to top of spark plug

3: Spark shorted to ground.

4: Engine stops pretty quickly.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


----- Original Message -----
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To: <RemoveMEPICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, August 09, 2001 4:16 PM
Subject: Re: [PIC]: Building a Servo


> > {Original Message removed}

2001\08\09@225831 by Herbert Graf

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{Quote hidden}

       That's true, but do remember it's NOT an 84 and has many of the "gotchas"
that the 16F877 and other later flash parts have, ie. LVP enabled, analog
instead of digital inputs enabled by default, etc. TTYL

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2001\08\09@225846 by Herbert Graf

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{Quote hidden}

       Thank you very much for the description, I've never taken one apart so I
simply assumed it was a coil and a magnet, obviously I was wrong! :(

> Hope this helps. I realise you may not want to get too involved with
> rewiring the magneto so suggested the spark plug shorting method. Another
> advantage that comes to mind is that  it may be possible to
> arrange it as a
> kill switch if the car flips etc. although vibration could be a problem.

       Hmm, the car flipping, there's something I don't even WANT to think about!
:) TTYL

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2001\08\09@225848 by Herbert Graf

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{Quote hidden}

       That's an idea, however this event SHOULD never occur, since I plan to have
a kill switch on a seperate transmitter receiver. Plus at this point I
simply want it to go, then I can add some more cool features! :) TTYL

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2001\08\09@232356 by Dan Michaels

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>> Herbert Graf wrote:
>> ..........
>>   Aside from this the steering system will default to fully left (or
>> >right, haven't decided) in case of loss of contact with the
>> transmitter, so
>> >if everything else goes wrong at least the thing will go in tight circles
>> >instead of a straight line.
>> ...........
>>
>> As long as you are going to do this, why put a centrifugal force
>> detector fail-safe on it which will kill the engine after the thing
>> spends a couple of seconds in circle joy mode?
>
>        That's an idea, however this event SHOULD never occur, since I plan
to have
>a kill switch on a seperate transmitter receiver. Plus at this point I
>simply want it to go, then I can add some more cool features! :) TTYL
>


Herbert, I disagree whole-heartedly :-0.

"SHOULD" - guess you never heard of Murphy, say what.

Crazy komakazi devices bristling sharp objects had best have
fail-safes "built-in", rather than at the behest of some guy
punching a button on a "2nd" xmtr who's battery just ran down,
or who's xmit range has just been exceeded. Better safe than
heading off to engage a barrister ;-).

- dan
================

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2001\08\10@040227 by Alan B. Pearce

face picon face
>> The way to do it without having the CCP is to use the INTR input to start
>> the timer on the rising edge of the waveform, change the edge
>> sensitivity to
>> falling edge, on the next interrupt save the timer count and set the edge
>> sensitivity back to rising edge. Initialise the timer ready to start
>> counting again on the next interrupt. The value you have saved is
>> the pulse
>> width.

>        This is quite a good idea, and is slightly different from the
approach I
>was planning to take (I was going to use the change on RB interrupt since I
>wasn't aware you could change the edge sensitivity on the INT pin). Thanks,
>TTYL

It would be possible to do the same thing using the RB interrupt, but then
you are potentially tying up the whole RB port to have an interrupt on one
pin (check the archive for interrupt on RB change problems if you are not
already aware of them), and also you have no control over which edge of the
waveform is causing the interrupt. This latter item may not be a problem,
but you may wish to average the collected timer value over several pulses to
ensure that a noise glitch does not give you problems.

Another trick you may like to implement is to have a dip switch on some pins
that enables you to set up a centre or end value, or sensitivity. If using
two dip switches you could go into a programming mode to write calibration
values into EEPROM giving minimum timer value, maximum timer value, some
sort of sensitivity per timer increment, or any other bits you may desire.
These are just some ideas I floating without knowing what you expect to do
as outputs (and also without reading the string of messages on this thread
that were here on my arrival this morning).

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2001\08\10@043205 by Alan B. Pearce

face picon face
>That's an idea, however this event SHOULD never occur, since I plan to have
>a kill switch on a separate transmitter receiver. Plus at this point I
>simply want it to go, then I can add some more cool features! :) TTYL

Apply Murphy's Law here real rapidly - anything that should not happen most
definitely will. Also I would be worried about having a kill switch on a
separate transmitter - if your receiver cannot hear the first transmitter
then there is an extremely high chance that the kill switch transmitter will
not be heard either - for the same reason the first one cannot be heard, out
of range, interference or whatever. Any kill switch must rapidly work on
being unable to hear its control source unless you are operating this thing
in a robot wars arena.

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2001\08\10@050035 by Wolfgang Maier

picon face
Herbert,

I would like to thank you personally for starting a thread, the answers to
which, have surprised even me.

OK. Now that 2000+ PIClisters have agreed to let you use a 16F84, would you
consider changing the vehicle type ? Perhaps a boat, rocket,
lighter-than-air vehicle, magnetic levitation vehicle ? Just cause you
already got an engine and other stuff, don't let that narrow your field of
view.

Regards,

Wolf

" De idle mind is da devil's playstation "

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2001\08\10@051528 by Peter L. Peres

picon face
Herbert, look in the PIC archives, there was someone who built a
multi-channel decoder. You can write the code to just count while testing
for the edge, after finding the rising edge of the servo pulse. You can
also do it using a timer and interrupt on pin change.

The tricky part is the analog feedback from the mechanics which requires
an A/D. You can try the digital A/D approach but maybe better try to use a
16C711 which already has an A/D inside. An A/D is harder to make than a
CCP ;-).

Peter

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2001\08\10@051542 by Peter L. Peres

picon face
I have never heard of the idea of a 'magnet flying past a coil really
quickly' to generate a spark. Imho there is a hall sensor in there or a
transistor circuit that does something or other to shape the pulse when
the field is maximum. If you can find this, then you will be able to tap
into it at low voltage.

Another way is to add a small insulated winding of your own (1-10 turns)
and short it out with a relay or a mosfet. The shorted winding will shield
the magnet's field and there will not be enough energy to spark (I hope).
This used to work with small motors whose points were impossible to
access safely.

Peter

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2001\08\10@051547 by Peter L. Peres

picon face
>   * for really fast stopping of the engine you could cause the
> piston/cylinder to try to act on something less compressible - such as
> water by using a car windshield washer fluid pump with a small reservoir
> and hose directed into the intake... though this might be too drastic.

Actually you can soak the engine by opening the choke in some cases with
the carburator air at 50% or so. The engine will suck so much fuel it will
go out or at least slow down to idle puff-puff. This needs to be tried
for each engine condition.

Much better is to vent a small canister of nitrogen or halon into the
air intake. This will almost certainly stop the engine immediately.

Doing what you suggest (allowing the engine to suck liquid) is very
dangerous. In an aircraft it will almost certainly loosen the propeller
nut and throw it and the propeller off at high speed, or break the
propeller if it is large enough. The shock will probably remove the engine
from its mount. In a car it may break every gear tooth in the transmission
and it may bend the crank or jam the piston. If the engine is big enough
it may actually explode by throwing the cylinder head in addition to all
the above.

Peter

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2001\08\10@051603 by Peter L. Peres

picon face
> But AFAIK there IS no "low tension side of the magneto". The spark is

The magnet passing the coil is not enough to make a spark. A normal
magneto has a pair of points to break the circuit when the magnet passes
the coil and generates maximum field (and thus current).

There are very small engines that do not have a separate HV coil. The
ignition assembly looks like a tiny wedge of cheese with a HV wire to the
spark plug. This is screwed with two (or more) machine screws onto the
engine block facing a magnet on the crankshaft. The Holes are oval for
advance adjustment. Here they use a Hall effect sensor or a dB/dt coil and
an electronic switch buried in the main coil to do this. I suspect you
have this kind of engine.

In this case, try to find the ground lug of the assembly and put a relay
between that and chassis. Adding a set of windings around the wedge and
shorting them to stop will also work.

Peter

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2001\08\10@070551 by Alan Gorham

flavicon
face
Hi

I appreciate the need to kill the motor if you lose the signal, but....

Why do you want the kill switch on a second transmitter and receiver?

Surely it would be simple to arrange the motor kill as a failsafe operation
in the same manner
as you already described for the steering?

Might save you a bit of work!

Regards

Alan

Embedded Systems Engineer
Microtima Ltd
Ouseburn Mews
3-7 Stepney Bank
Newcastle upon Tyne
NE1 2PW

Tel: 0191 2304411
Fax: 0191 2304422

{Original Message removed}

2001\08\10@070753 by Max Toole

picon face
In a message dated 8/9/01 9:09:11 PM Eastern Daylight Time,
RemoveMEolin_piclistspamBeGonespamRemoveMEEMBEDINC.COM writes:

{Quote hidden}

[172.18.149.36])
> by air-yc02.mail.aol.com (v79.27) with ESMTP id MAILINYC210-0809210911;
Thu,
> 09 Aug 2001 21:09:11 -0400
>  Received: from  cherry.ease.lsoft.com (cherry.ease.lsoft.com
[209.119.0.109])
>  by rly-yc04.mx.aol.com (v79.20) wi
Thanks Olin, I had figured that out by going to their web site and printing
off the data sheet.  Sounds like a great device.
Max

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2001\08\10@073119 by Byron A Jeff

face picon face
On Thu, Aug 09, 2001 at 10:59:21PM -0400, Herbert Graf wrote:
{Quote hidden}

Of course. In fact the placement of the LVP pin is the only thing that really
annoys me. I even understand why they did it: It had to be somewhere on
portA or portB that didn't have a function attached. Every end pin on both
parts have a secondary function. So you had to pick a pin in the middle of
portB. But the placement still sucks.

Another caveat is that for the F628 the LVP pin is on port B4. not B3 like
the 16F87X series.

As for the analog inputs, it takes one register write. And IMHO it's a
minor cost for the advantages that analog inputs give. I'm currently testing
a LM34 temp sensor. the 16F877 A/D converter measures the temp to within
1 degree. Note the F628 doesn't have A/D but I was just illustrating the
point.

BAJ

> TTYL

I had to go look this acronym up. I'll Talk To You Later too.

BAJ

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2001\08\10@080358 by Herbert Graf

flavicon
face
> >That's an idea, however this event SHOULD never occur, since I
> plan to have
> >a kill switch on a separate transmitter receiver. Plus at this point I
> >simply want it to go, then I can add some more cool features! :) TTYL
>
> Apply Murphy's Law here real rapidly - anything that should not
> happen most
> definitely will. Also I would be worried about having a kill switch on a
> separate transmitter - if your receiver cannot hear the first transmitter
> then there is an extremely high chance that the kill switch
> transmitter will
> not be heard either - for the same reason the first one cannot be
> heard, out
> of range, interference or whatever. Any kill switch must rapidly work on
> being unable to hear its control source unless you are operating
> this thing
> in a robot wars arena.

       Actually that's perfect, since it will not be a kill switch but an "alive
switch" the motor will be kept alive as long as it receives a valid signal
from the second transmitter, that way if anything goes wrong with the first
receiver/transmitter pair all I have to do is shut off the second
transmitter. TTYL

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2001\08\10@080615 by Herbert Graf

flavicon
face
> Herbert, look in the PIC archives, there was someone who built a
> multi-channel decoder. You can write the code to just count while testing
> for the edge, after finding the rising edge of the servo pulse. You can
> also do it using a timer and interrupt on pin change.
>
> The tricky part is the analog feedback from the mechanics which requires
> an A/D. You can try the digital A/D approach but maybe better try to use a
> 16C711 which already has an A/D inside. An A/D is harder to make than a
> CCP ;-).

       True, for the one with full position feedback (the throttle BTW) I was
thinking of using the "resister and capacitor hanging off a port and timing
the time constant" method, I don't need high accuracy and I believe this
should be "good enough" for that purpose. Aside from that I have no qualms
over adding a chip (got a TON of SPI ADCs lying around in my bins) so my
might just use a real ADC. TTYL

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2001\08\10@080629 by Herbert Graf

flavicon
face
> I appreciate the need to kill the motor if you lose the signal, but....
>
> Why do you want the kill switch on a second transmitter and receiver?
>
> Surely it would be simple to arrange the motor kill as a failsafe
> operation
> in the same manner
> as you already described for the steering?
>
> Might save you a bit of work!

       Yup, mainly though I want complete reduncancy for this thing, in case
ANYTHING insane happens with the main transmitter and power supply I want to
have a backup option. I don't think one can be too paranoid when dealing
with this sort of thing! :) TTYL

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2001\08\10@081020 by Roman Black

flavicon
face
Herbert Graf wrote:

>
>         I might also add a redundant kill switch with a second transmitter and
> receiver, the only problem, and this is something I was planning to ask, is
> how do you stop a gas engine remotely?


Ha ha! I've been reading all the replies here and
chuckling quietly to myself. Reminder; never ask
an electrical engineer to fix your gasoline engine...
:o)

It's very simple killing a magneto driven gasoline
engine, just CLOSE THE THROTTLE. Engines need to have
the throttle slightly open for "idle" and use an
idle screw or similar mechanical setpoint. Many race
engines do away with the idle setting, giving closed
throttle operation, which gives increased engine
braking off-throttle, and motor cutout if the throttle
is ever left closed (safety!). But requires more
skilled driving of the vehicle...

If your engine uses a safe (sprung) throttle that
relies on the PIC to hold it open to the correct amount,
it can be designed so any failures give the throttle
springing back to CLOSED and the motor brakes and stalls.
So how do you kill your lawnmower's engine?? ;o)
-Roman

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2001\08\10@082645 by Roman Black

flavicon
face
Herbert Graf wrote:
>
>         True, for the one with full position feedback (the throttle BTW) I was
> thinking of using the "resister and capacitor hanging off a port and timing
> the time constant" method, I don't need high accuracy and I believe this
> should be "good enough" for that purpose. Aside from that I have no qualms
> over adding a chip (got a TON of SPI ADCs lying around in my bins) so my
> might just use a real ADC. TTYL



Herbert, thinking again of safety, if the PIC fails
and locks up, you would expect the output pin to stay
LOW or HIGH. I would try and design the driver hardware
so that it needs the PWM in a specific range to keep
the throttle open, and any sustained high or low at
the pin would result in a closed throttle.

Then you only have to worry about the very rare fault
that may leave the PIC oscillating with the output
pin oscillating, but i'm sure you could add extra
safeguards around this.
-Roman

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2001\08\10@085016 by Herbert Graf

flavicon
face
> I would like to thank you personally for starting a thread, the answers to
> which, have surprised even me.
>
> OK. Now that 2000+ PIClisters have agreed to let you use a 16F84,
> would you
> consider changing the vehicle type ? Perhaps a boat, rocket,
> lighter-than-air vehicle, magnetic levitation vehicle ? Just cause you
> already got an engine and other stuff, don't let that narrow your field of
> view.

       ROTFLMAO! Actually the TYPE of vehicle is completely out of my control,
I've been drafted only for the electronic portions of the project, it's not
even MY project, I'm just hanging along for the ride! :) I guess building
this sort of thing has some instinctual draw for most people as evidendenced
by the great response on this list! TTYL

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2001\08\10@090709 by Olin Lathrop

face picon face
> To my knowledge there are no systems that generate the secondary voltage
> directly from the magnet/coil arrangement - and spark timing would be a
> major problem with this system anyway.

Hmm.  I'll have to examine my lawnmower more closely.  I had so far thought
it had only a magnet on the flywheel going past a stationary coil, but there
could be a primary and contacts buried somewhere I haven't gone yet.

As far as the timing though, I don't see why that would be a problem.  The
magnet has to be mounted at the right angle on the shaft, but that is no
different from the care needed to mount whatever opens the contacts.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spamBeGoneolinspamembedinc.com, http://www.embedinc.com

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2001\08\10@112008 by Mike Mansheim

flavicon
face
>   * for really fast stopping of the engine you could cause the
> piston/cylinder to try to act on something less compressible - such as
> water by using a car windshield washer fluid pump with a small
> reservoir and hose directed into the intake... though this might be too
> drastic.

fast stopping ONCE!  Auto mechanics have a name for this - "hydrolocking"
(not sure about the spelling).  It locks up the engine and destroys it.
Saw the results once - a friend tried to drive a car out of a flooding
parking lot, and it stalled.  Asked me to help him get it started the
next day - engine would not turn over - found a hole in the pan with
pieces hanging out of it - engine was junk.  The air intakes on this car
are apparently quite low to the ground.

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2001\08\10@134213 by John Ferrell

flavicon
face
Everything you want is available off the shelf except what you have in the
subject line. Actuators & feedback are the hard part.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2001\08\10@142005 by Douglas Butler

flavicon
face
All of the lawnmower and chainsaw engines I have ever seen have a
magneto with points.  Though I can believe new ones may have some
electronic circuit instead of points.  But you still need something to
suddenly change the current path in the coil.  Doing this machanically
may be possible, by hinging the core laminations or such, I doubt it
would be practical.

Sherpa Doug

P.S. Actually I do remember a Kohler engine that used a Piezo stack to
generate the spark, no coils or magnets involved!  I think you shut it
off by shorting the spark plug with a spring steel strap.

> {Original Message removed}

2001\08\10@150618 by Douglas Butler

flavicon
face
I have heard of this effect causing steam locomotives to jump the
tracks!
It is generally viewed as a bad thing to do.

Sherpa Doug

> {Original Message removed}

2001\08\10@191427 by Larry Williams

flavicon
face
Hmm.  I'll have to examine my lawnmower more closely.  I had so far thought
> it had only a magnet on the flywheel going past a stationary coil, but there
Look closely at the coil, the coil is actually the secondary of a step up transformer with the primary being the metal core. The magnetic field couples into the core and thus into the secondary field. Or at least that is the way it was explained to me.

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2001\08\10@194609 by Barry Gershenfeld

picon face
>>Hmm.  I'll have to examine my lawnmower more closely.  I had so far thought
>> it had only a magnet on the flywheel going past a stationary coil, but there

>Look closely at the coil, the coil is actually the secondary of a step up
transformer with the primary being the metal core. The magnetic field
couples into the core and thus into the secondary field. Or at least that is
the way it was explained to me.

Oh, and if you only see two wires, the third may be completed through the
ground or frame.

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2001\08\11@230041 by Graeme Zimmer

flavicon
face
Hi Doug,

> But you still need something to
> suddenly change the current path in the coil.  Doing this machanically
> may be possible, by hinging the core laminations or such, I doubt it
> would be practical.

About a million years ago I saw an ancient "magneto" which did just that....

It has a magnet buried as the center limb of a transformer, and the end pole
piece was hinged.

The magnetic field normally held the pole tightly in place, but when a lever
was moved
away (by the rotation of the cam) a spring pulled the pole piece open.

Due to the action of the spring (v/s the magnetic attraction) the pole piece
opened with an abrupt snap action,
causing a fat spark in the secondary winding.

I assumed that some manufacturer was trying to avoid a patent infringement
or some such ........

........................... Zim

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2001\08\14@081002 by Olin Lathrop

face picon face
>         Actually that's perfect, since it will not be a kill switch but an
"alive
> switch" the motor will be kept alive as long as it receives a valid signal
> from the second transmitter, that way if anything goes wrong with the
first
> receiver/transmitter pair all I have to do is shut off the second
> transmitter.

You can do this better with a single transmitter.  That way if the control
info isn't getting there, the engine is killed.  It also sounds easier to
deal with in a panic since you probably already have the controls for the
first transmitter in your hand.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spam_OUTolinSTOPspamspamembedinc.com, http://www.embedinc.com

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2001\08\14@081008 by Olin Lathrop

face picon face
> So how do you kill your lawnmower's engine?? ;o)

My lawnmower has a bar that I have to keep up against the push bar.  It is
designed so that you pretty much have to be grabbing the push bar to hold
the safety bar in the ON position although a pair of vice grips defeat this
nicely when working on the engine.  This safety bar actuates three separate
mechanisms if let go.  First, it is adjusts the tension on the governor
feedback to effectively close the throttle.  Second, it releases a
mechanical break so that the engine won't coast long once shut off by other
means.  Third, it shorts out the spark.  Seems to work.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, TakeThisOuTolinspamspamRemoveMEembedinc.com, http://www.embedinc.com

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2001\08\14@112738 by Herbert Graf

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{Quote hidden}

       Actually I WILL be doing this with the first transmitter, the second will
be a redundant kill switch. TTYL

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2001\08\15@202713 by Brandon Fosdick

flavicon
face
Herbert Graf wrote:
>      Considering the size and weight of the car safety has been one of my number
> one concerns. I actually have a few layers of safety. First off the throttle
> and brake are handled completely seperately, and both will default to a safe
> status if a pulse from the receiver isn't received in the last 60ms (safe
> for the brake being completely on and safe for the throttle being completely
> off). Aside from this the steering system will default to fully left (or
> right, haven't decided) in case of loss of contact with the transmitter, so
> if everything else goes wrong at least the thing will go in tight circles
> instead of a straight line.
>
>         I might also add a redundant kill switch with a second transmitter and
> receiver, the only problem, and this is something I was planning to ask, is
> how do you stop a gas engine remotely? I thought about disconnecting the
> spark plug but how would I do that considering the voltages involved?
> Cutting the fuel flow is a second idea but my experience with two stroke
> engines is that they keep running, without any fuel flow for at least 30
> seconds, long enough to cause damage. Anybody have a better idea? Thanks,

I missed the first part of this thread, but you keep mentioning
transmitter and servos, so I'm assuming you mean regular hobby stuff. In
that case, have you looked at PCM systems? They have a failsafe mode
that the receiver will enter when it looses the signal from the
transmitter. The failsafe simply sets all of the servos to predefined
positions. The actual positions used are user-configurable through the
transmitter. You could easily set the failsafe to shut off the engine.

Generally for R/C stuff the servo throws are configured such that the
minimum stick throw idles the engine, but if you also run the engine
trim all the way down the engine will shut off. That effectively creates
the idle-screw type behavior that someone else mentioned, while still
allowing you full throttle control for safety purposes. Not sure how it
works for magneto engines though.

At the field I fly at, non-PCM systems are forbidden for safety reasons.
If you're creating a new vehicle I strongly suggest that you use a PCM
system, not just for the failsafe but for its greater resistance to
interference as well.

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