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PICList Thread
'Truck tachometer'
2000\06\01@023816 by Jacky Joulin

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Hello ,
I need realize an application to read truck speed motor ( a tachometer ).
The origin speed information is the "W" output alternator.
Some of the readers as technical information on the "W" output !.
I research also stage input electronique diagram of a truck tachometer for
PIC adaptation .

Thank for your help.

Jacky

spam_OUTWeslayTakeThisOuTspamaol.com

2000\06\01@032723 by Jacky Joulin

picon face
Hello ,
I need realize an application to read truck speed motor ( a tachometer ).
The origin speed information is the "W" output alternator.
Some of the readers as technical information on the "W" output !.
I research also stage input electronique SCHEMA (diagram) of a truck
tachometer for PIC adaptation .

Thank for your help.

Jacky

.....WeslayKILLspamspam@spam@aol.com

2000\06\01@094659 by Chris Eddy

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Jacky;

For tachometer measurement, I suggest using the interrupt on port B0 feature.
The software can increment a count every time a pulse arrives, and at the same
time timer 0 or timer 1 is running a crystal time.  At some predetermined time
increment, the counts and time are computed to arrive at RPM's.  RPM's are
really RPS, so you divide counts by time period, scale it to arrive at the
correct units, and you have your RPM's.  I would go one step further and fold
each new reading into a mild low pass filter so that you get a smooth result.

Just a side note, if the alternator is belt driven, you may not get a precise
result.  The belt might have 'slippage' under various RPM's and alternator
load.  Picking up the ignition pulse is more exact.

Chris Eddy

Jacky Joulin wrote:

> Hello ,
> I need realize an application to read truck speed motor ( a tachometer ).

2000\06\01@095505 by Alan B Pearce

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>Picking up the ignition pulse is more exact.

Most trucks are diesel and don't have one of these. Perhaps this is why he is
looking at the alternator?

My idea would be to have some form of sensor mounted to sense the teeth on the
starter ring gear. This would probably give a more consistent pulse pickup then
alternator frequency.

2000\06\01@111714 by Midgley John

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Chris

Interesting. I have a tacho application in mind, and was wondering about
whether to time the interval between adjacent pulses, or to count a
number of pulses over a fixed time. You've gone for the latter - are
there pros and cons for the two options?

Regards

John Midgley

>{Original Message removed}

2000\06\01@115129 by M. Adam Davis

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Pulse width measurement:
 Con:
   More math code (or large lookup table and comparisons)
   Need special hardware (either flip flop, or the portB:0 interrupt)
   May need averaging to slow updates (flipping from 5 to 4 to 5 very
   quickly can be annoying)
 Pro:
   Very fast measurements (update multiple times per second)
 Comments:
   The pulse width should NOT be measured.  What you actually want to measure
   is the period of one complete cycle.  You can either convert a full cycle
   to a single pulse and measure that (flip-flop) or use the PORTB:0
   interrupt to measure the time between the same type of transistion ( time
   between the two low to high or the two high to low)

Count the pulses:
 Con:
   Slow updates (use a moving average to have fast updates)
 Pro:
   Easy to implement
 Comments:
   To get high resolution, you need to wait a long period of time to get
   enough pulses.  To get quick updates you need to measure pulses only a
   short period of time:
   Engine going at 2357 RPM
   Update once/second gives you 3 readings of 39 then one reading of 40.
   After multiplying by 60, you get 2340, but every 4th reading is 2400.
   This is quick update, but low resolution.
   At one reading every 3 seconds, you flip from 2340 to 2360, staying on
   2360 4 out of every 5 readings.

   To cover this, many (if not most) systems use a moving average of sorts.
   You measure the number of pulses in 1/2 second for 6 total seconds.  Store
   each measurement in a seperate register and average them.  Now you have
   six seconds of measurements (giving you 2360 more often than 2350) and you
   can update the display every half second.  The new 1/2 second readings
   overwrite the oldest reading before the new average.  The only drawback to
   this is speed changes are up to 6 seconds behind the actual speed.  For
   small or slow speed changes this is fine, but if the engine suddenly
   siezes at 2357 the display will update as follows in the next 6 seconds:
   2350, 2160, 1960, 1760, 1570, 1370, 1170, 980, 780, 580, 390, 190, 0

-Adam

Midgley John wrote:
{Quote hidden}

> >{Original Message removed}

2000\06\01@115932 by Chris Eddy

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Both methods are possible, as the ignition timing actually comes in no more than
hundreds of hertz, which is comparatively slow if you want to time between
pulses.  The down side to measuring this time is usually the resolution of the
reading at the higher rpm's.. if you need say 2% accuracy at high end, and you run
the RTC at 1MCPS, then you might not get a super accurate value.  But most folks
just want an indicator, and if you grab a handful of pulses, you will bury the
resolution error in a number of readings.  What was 10% error for one pulse is 1%
error for 10 pulses, etcettera.  There are lots of ways to skin this cat.

Chris

Midgley John wrote:

> Chris
>
> Interesting. I have a tacho application in mind, and was wondering about
> whether to time the interval between adjacent pulses, or to count a
> number of pulses over a fixed time. You've gone for the latter - are
> there pros and cons for the two options?

2000\06\01@171957 by steve

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> >Picking up the ignition pulse is more exact.
>
> Most trucks are diesel and don't have one of these. Perhaps this is why he is
> looking at the alternator?
>
> My idea would be to have some form of sensor mounted to sense the teeth on the
> starter ring gear. This would probably give a more consistent pulse pickup then
> alternator frequency.

I have an alternator based tacho. The only real disadvantage is that
you have to calibrate it to your setup using an absolute method of
some sort. The W output is a sine wave output of moderately constant
amplitude and frequency proportional to belt speed.
For normal driving use, it is perfectly adequate. You probably only
read it to the nearest few hundred rpm anyway. If it's on a toothed
belt there isn't any slippage.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebspamKILLspamtla.co.nz      fax +64 9 820-1929
======================================================

2000\06\02@040707 by Jacky Joulin

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Thank for yours many reply.

I have test this circuit for read the "W" output alternator:



   input  >------ R 2,2K-------------*-------------- R 10K ------    >  to
pin RB6 ( input  pic 16F84 )
                                            I
                                        I
                                        I
                                 Zener  5,1v
                                        I
                                        I
                                        I
                                    0v

Results:
Truck at   600T/mn  pulse = 2,2 ms  F= 227 Hz
Truck at   700T/mn  pulse = 2,0 ms  F= 250 Hz
Truck at 2550T/mn   pulse = 0,7 ms  F= 715 Hz


My problem :
Some parasites on the "W" output alternator degrade the signal end the result
is wrong.

How it's possible to perfect the electronique circuit ?

Thank for your help

Jacky

.....WeslayKILLspamspam.....aol.com

2000\06\02@101020 by Chris Eddy

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Jacky;

If I were you (but my French is not that good) I would do this:

Use a comparator powered from the 12V side, with a pullup on the output to
generate a 5V signal.  Zeners are such a crappy way to shape input.  Set a
threshold on one of the comparator input pins to half of the signal, and put the
signal from the alternator into the other.  BUT, run thealternator signal through
an RC circuit, say 10K and 100nF, to act as a filter.

That ought to do the trick, but if not, there are ways to AC couple the signal to
compensate for poor average value, as well as more sophisticated ways of
filtering.

I suggest the comparator because the way you have it with a zener is susceptable
to noise, but also, you do not have good control over the threshold of where the
signal is converted.  You should not prototype with a microprocessor, where you
measure the exact voltages of binary threshold.  The manufacturer does not
guarantee a precise threshold, only a range.  With a comparator, you will have a
precise threshold, and since the input is high impedance, you also get the
ability to filter.

Chris Eddy

Jacky Joulin wrote:

{Quote hidden}

2000\06\02@163742 by Clyde Smith-Stubbs
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On Thu, Jun 01, 2000 at 12:45:07AM -0400, Chris Eddy wrote:

> load.  Picking up the ignition pulse is more exact.

Not on a Diesel engine. Where did you anticipate getting the ignition
pulse from?

--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: clydespamspam_OUThtsoft.com          |          Phone            Fax
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PGP:   finger @spam@clydeKILLspamspamhtsoft.com   | AUS: +61 7 3355 8333 +61 7 3355 8334
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HI-TECH C: compiling the real world.

2000\06\02@183510 by Chris Eddy

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There is a brief pulse of RF from the ignition event, even in a diesel engine,
and you amplify, peak detect, and threshold the event.  You get a clean train
of pulses, even cleaner than the same method on a gasoline engine.  I have done
it.  I know, a ton of you are going to scream that there is no electrical event
on a diesel piston fire.  The circuit designed for a gasoline engine RF pickup
worked very well on a diesel, to our amazement.  Don't ask me where the RF
comes from on diesel.  We all know where it comes from on gasoline.

So there.  That is three people that beat me over my dumbness.  That ought to
keep the hecklers at bay.

Tongue in cheek
Chris Eddy

Clyde Smith-Stubbs wrote:

> On Thu, Jun 01, 2000 at 12:45:07AM -0400, Chris Eddy wrote:
>
> > load.  Picking up the ignition pulse is more exact.
>
> Not on a Diesel engine. Where did you anticipate getting the ignition
> pulse from?

2000\06\03@150836 by Peter L. Peres

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Hi,

I will ask about the RF pulse in the Diesel engine ;-) Where did you put the
pickup coil and was it inductive or capacitive coupling ?

thanks,

       Peter

PS: By where I mean, just poked it in the engine compartment, wound a coil
on an injection pipe <grin> or in some other place like that.

2000\06\04@012250 by Chris Eddy

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Weelll, I suppose it doesn't matter.  The pickup was a metal contact with any
point on the vehicle.  The tailpipe was our own specific pickup point.  The car
is isolated by the tires, and the RF is like a big electrostatic flash on the
body of the vehicle.  When I was working on this stuff, I saw some neat stuff.
You can see the start of the dwell time, when the coil just begins ot charge
with current.  You can see a lot of nonsense RF on a Mercedes.  You have a hard
time picking up anything on some Hondas.  I can't say more on just how the
pickup/amp worked.

The existing European solution uses a figure eight antenna which lays on the
hood.  It works very poorly, and is the reason that folks are looking hard for
alternatives.

In secrecy,
Chris Eddy

"Peter L. Peres" wrote:

{Quote hidden}

2000\06\05@020552 by Jacky Joulin

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Hello,


I research always the schema to interface and convert the "W" output alternator ----> TTL signal.

This is very important for me.

Thanks

jacky

2000\06\05@115919 by Jacky Joulin

flavicon
face
Hello,


I research always the schema to interface and convert the "W" output alternator ----> TTL signal.

This is very important for me.

Thanks

jacky

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