Searching \ for '[EE]: PIC Robot Update' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/devices.htm?key=pic
Search entire site for: 'PIC Robot Update'.

Exact match. Not showing close matches.
PICList Thread
'[EE]: PIC Robot Update'
2001\01\29@101758 by Tom Handley

picon face
  This is an update on my effort to design a PIC-based Robot. For those of
you following the earlier dialog with Dan, Steve, Roman, and others on this
subject, I finally have a good (re: great) mechanical platform. After
tearing apart VCRs, tape recorders, and little green men with no shoes and
no hair, I finally decided to just buy a decent gear box from Tamiya. Dan
and Steve already bought the dual motor Twin Gear box which offers ratios of
58:1 or 203:1. It became apparent that it would not have the torque to meet
my needs so I bought the "Worm Gear Box H.E" (High Efficiency), P/N: 72004,
which I'll describe in detail. Note, most vendors charge around $20 for this
but at Tower Hobbies ( http://www.towerhobbies.com ) it's $10.99.

  I had been `hacking' a Radio Shack R/C car that was made for speed, not
robotics. While I learned a great deal during the process, I wanted to start
from scratch with a dual motor, front-wheel-drive system with a single
castor rear wheel. Tank-style treads were also an option but it became
apparent that I would end up with a rather crude vacuum cleaner `clinching'
cat hairs off the rugs... My goal was simply to navigate around a single
floor and to provide a test-bed for various sensors.

  I had an old HeathKit Ultrasonic Intrusion Alarm with a `Book Cover' so
it could hide in your library (remember that kit? ;-). Anyway, it was a good
platform to test Ultrasonics as related to robotic applications. As a bonus,
it has two light gage aluminum (alyouminyum for our friends in the UK ;-)
covers measuring 9" x 6" with about a 1/4" L-bend around the edges. The
`gawds' must have been smiling on me as this was absolutely perfect for my
needs. I found a cheap castor wheel with ball bearings at the local hardware
store for around $3.

  After weeks of searching for a reasonably-priced motor/gear system to
suit my needs, the Tamiya Worm Gear box was ideal for my application. First,
I'm amazed neither Tamiya, nor Hobby stores really promote these motor/gear
boxes as they are ideal for robotics. There are several packages to choose
from. Back to the the Worm Gear box, it comes in a colorful box with a great
deal of detail (can I say that? ;-) on the box and simple instructions in
English and Japanese included with the kit. Yes, this is a kit but it's
trivial to assemble. It provides gear ratios of 216:1 and 336:1 which is
what I'm using. The motor is a Mabuchi F160-style but there was no part
number so I don't have the exact specs. It also supports Mabuchi F140-style
motors and includes an adapter to mount the smaller motor to the grear box.
I'm impressed with the attention to detail when it comes to the included
accessories. In addition the the housing, the motor, and gears, you get a
`star/crossbar' attachment, two levers, and a wheel with 6 holes for an
optical sensor. They also include a spacer tool molded into the plastic jig
that holds the parts. This is used to set the worm gear on the motor shaft.
To top it off, they include a lubricant for the gears. I wish they included
a little more of that but it was not a problem even after tearing the
assembly apart and modifying it for an IR sensor (see below). The main shaft
is 80mm with 6mm threaded on each end and a groove for included E-rings as
well as holes with spring-pins to mate with the external attachments. In
most cases, they included extra hardware. They even provide an extra shaft
and a `crude' plastic assembly to hold it. The extra shaft is identical with
the exception of lacking the holes for the spring-pins. Another nice,
although trivial touch, is the included Allen wrench to set the drive shaft
hex fitting that mates with the shaft gear. Again the motor is a Mabuchi
F160-style and operates from 3V to 4.5V. You can easily add your own motor
that fit's the F140/160 form factor and shaft diameter.

  After assembly and mounting the two gear boxes, my next priority was
some form of tachometer. I first looked at modifying the gear box to get an
optical pickoff from the higher speed gears but there was no room without
some precision mechanical work. I have a Dremel tool. The included wheel
with six holes mounts nicely on the unused end of the shaft but it turns too
slow for a tach. I tried `reverse gearing' to take the other ratio gear and
drive another gear for a higher speed but I was unable to fabricate a
housing with what I had on hand. It's still an option. I ended up going back
to the gear housing and moving back a gear, drilling 4 holes in the gear and
two holes in the housing with a matched IR emitter/detector pair. This feeds
into a LMC662 dual Op Amp with a rail-rail output. It's configured as a
simple comparator with a little hysteresis. At around 3V, I'm getting 9-10
PPS, too slow for a tach for a robot in a house but still useful for
distance measurements and long-term speed feedback. To turn with two motors,
you either shut off one and drive the other or drive one forward and the
other in reverse (pivot around a point). I prefer the latter and I've tested
both. If you decide to use a dual motor drive with a rear castor, spend a
few $ and get a good castor with ball bearings. This thing runs in reverse
in a straight line and I'm impressed. Actually, the differences between the
two motor's speed is so small that I really don't need a tach to keep it
going straight.

  As far as motor control, I had built several H-Bridges based on various
MOSFET configurations. Bipolar designs were not an option since I was
running off of batteries and I wanted efficiency. I won't join the MOSFET vs
BIPOLAR debate other to say that MOSFETs are more efficient and just as
reliable despite apparent failures in VCRs ;-). My favorite was two
P-channel and two N-channel HEXFETs but it was mainly designed for 6-24V
motors. Since these motors run off 3-4.5V, I went back to an all N-channel
design with a 12V switching regulator to provide gate bias and source
another 5V switching supply for 5V logic. I used TI devices with a 2V Vgs
for the low side of the bridge and since I had some HEXFETs with a 4V Vgs, I
used them for the high side of the bridge. All of the devices had built-in
diodes. The TI devices combined dual MOSFETs in one package with common
Drain and common Source versions which are ideal for this application but
for some `Lord only knows' reason, TI canceled the product and there is no
second source... I was fortunate to still have some engineering samples.
Though the design is an overkill for these Mabuchi motors, I can run up to
6A without a sink due to the low Rds. The board is 4x2" and in addition to
the H-Bridge, it supports the 12V and 5V supplies, the dual tach signal
conditioners, and a 3.3V/3A low-dropout linear regulator to provide a
constant voltage to the motors. This get's around the problem of low battery
conditions so I don't have to worry about that affecting the motor
performance. Note, these supplies are very efficient with extremely low
quiescent current. The motor supply is 4 AA batteries and is seperate from
the main Logic supply for the CPU and sensors which uses 6 AAs. Finally,
there is a 16F84 controlling the motors and monitoring the tach. Again, I
wanted to keep this modular. Right now the host CPU uses 4 lines to control
the direction and to turn the motors On and Off. This will end up as a
single serial line for motor control with another serial line for tach
feedback so the host CPU can use the data for distance measurements. With
the current tires, I get 16 pulses per revoloution and each revoloution is
9.8".

  The current status of sensors are simple bumper switches made from stiff
steel wire and microswitches with levers. I've finished testing an IR sensor
with Left, Middle, and Right outputs. Basically, it uses the 40KHz Sharp
sensor with a 12C508 modulating Left and Right IR LEDs. Again, this is just
another module for the host CPU. I looked at several designs and one problem
that always comes up is that these sensors can be too sensitive. Some
designs fix or vary the IR LED resistor. I've found it easier to just
`de-tune' the modulator frequency. My design varies from 32KHz to 40KHz.

  The next step is adding the Ultrasonic sensors. They will be mounted on
an R/C servo. The one I have will do 180 degrees and is rather unusual in
that the pulse varies from around 0.8 to 2ms. It acts like a normal servo
within the typical 1 to 2ms range. This will be another PIC-based module.

  The host CPU is currently a 16F876. I have several `test beds' built on
MicroEngineering Labs' proto boards. I've drilled holes for boards
supporting both the 28-pin and 40-pin devices. My 40-pin proto board
supports a 16F877, a custom Lattice CPLD and 512KByte SRAM. Both boards
support a keypad and an LCD interface as well as an SPI port and auxillary
chip-selects.

  The current state of software is rather crude. Right now the bot can
roam around, even climb the thick braided rugs on the hardwood floors,
bump into things, backup, and more often than not, get around objects.

  BTW, I named it Bozoterous or Bozo from an old Firesign Theater Album.
If you have not heard of that album, you are young, if you have, you are
an old fart like me ;-)

  - Tom

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\01\29@212810 by Roman Black

flavicon
face
Tom Handley wrote:
>
>    This is an update on my effort to design a PIC-based Robot. For those of
> you following the earlier dialog with Dan, Steve, Roman, and others on this
> subject, I finally have a good (re: great) mechanical platform. <snip>

Hi Tom! I enjoyed reading your robot story. :o)
So where's the pictures??

Good to hear the bot tracks straight without any
compensation, that's one of the worst problems
with DC motor driven differential drive bots.
I have used a "trim" series resistor at times
(very low ohms) to make two motors run at the same
speed, assuming relatively constant load. The
other solution of course is to go to proper
encoders with closed loop speed control.

Does he/she/it have a purpose yet? Or just
as a development platform for now? :o)
-Roman

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2001\01\30@062127 by Tom Handley

picon face
  Roman, all I have is a Kodak webcam that does 640x480 right now but I
definitely want to put pics and other info on my web site.

  I was concerned about speed control and I spent a lot of time looking
into a tach. There just isn't enough room in the gear box around the motor
shaft and the first gear. It's a worm gear on the motor shaft driving a
second gear, driving a third which drives the output shaft gear. I was able
to drill four holes in the third gear and fit a matched IR pair in the
housing but it's still not really useful for a tach at short distances. I
can still add gears to the other end of the output shaft to gear-up and get
a decent pulse rate though it's very inefficient. Right now I get one pulse
for around every 5/8" traveled so that's good enough for distance
measurements. I eventually plan on adding a Vector 2x electronic compass.
Given the identical Mabuchi motors, the quality of the gearboxes, 336:1 gear
ratio, regulated H-Bridge supply, and matched MOS/HEXFETs, it's really not
surprising that it tracks so well.

  I thought the rear castor would give me `fits' when backing up but it
works great. It's a small castor with ball bearings available for around $3
from any hardware store. One big factor is the front wheels which came from
the R/C car I was hacking. They are around 3" in diameter with 1/8" knobs
and 1.5" wide.

  As far as purpose, it's mainly an R&D platform but I'd also like to
freak-out the cat. The problem is that I would have to detonate some C4 near
the cat to get it's attention ;-)

  - Tom

At 01:25 PM 1/30/01 +1100, Roman Black wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email spam_OUTlistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body


2001\01\30@071159 by Roman Black

flavicon
face
Tom Handley wrote:
>
>    Roman, all I have is a Kodak webcam that does 640x480 right now but I
> definitely want to put pics and other info on my web site.

What's wrong with that? :o)


{Quote hidden}

As a suggestion, I normally use reflective IR sensor
pairs (pigsnouts) which are about 4mm square. If you
can get one close to a cog (closer than 5mm) and
put some black/white paint stripes etc on the cog face
it will work fine. This is what most of the newer
VCRs do, it's smaller and cheaper and more convenient
than an opto-interrupter.

The Sharp RHPX 0181 GEZZ is about $3 US in singles
and is tiny! Any TV repairer should have them in
stock or could order them if you want some. I could
get you a couple if you like.

>I eventually plan on adding a Vector 2x electronic compass.
> Given the identical Mabuchi motors, the quality of the gearboxes, 336:1 gear
> ratio, regulated H-Bridge supply, and matched MOS/HEXFETs, it's really not
> surprising that it tracks so well.

Obviously your motor drivers are good but it
mainly comes down to the quality control of the
motors, which is obviously good! :o)


> I thought the rear castor would give me `fits' when backing up but it
> works great. It's a small castor with ball bearings available for around $3
> from any hardware store. One big factor is the front wheels which came from
> the R/C car I was hacking. They are around 3" in diameter with 1/8" knobs
> and 1.5" wide.
>
>    As far as purpose, it's mainly an R&D platform but I'd also like to
> freak-out the cat. The problem is that I would have to detonate some C4 near
> the cat to get it's attention ;-)

Cool! You could get it to chase the cat and make
electric shocks?? ;o)

-Roman

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body


2001\01\30@072651 by dre Domingos F. Souza

flavicon
face
>Cool! You could get it to chase the cat and make
>electric shocks?? ;o)

       Bad man...The Lord of the Cats will take you to visit a really hot place... :oD


--------------8<-------Corte aqui-------8<--------------

       All the best!!!
       Alexandre Souza
       xandinhospamKILLspaminterlink.com.br
       Linux User #85093

--------------8<-------Corte aqui-------8<--------------

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam.....mitvma.mit.edu with SET PICList DIGEST in the body


2001\01\30@142707 by Peter L. Peres

picon face
>I have used a "trim" series resistor at times
>(very low ohms) to make two motors run at the same
>speed, assuming relatively constant load. The
>other solution of course is to go to proper
>encoders with closed loop speed control.

No, the other solution is to connect the motors together in parallel using
a relay or by putting the resistor in series with the supply of the two
H-bridges. Or by using crappy batteries which will be like the resistor,
only better ;-).

In general, if the motor voltages are never low (>>1.2V) then the H bridge
reverse diodes will couple the motors together fairly well if they run out
of sync. MOSFET (again! ;-) H-bridges will couple them even if they run on
lower voltage.

Peter

PS: To all diy homemade dc-motor driven robot users: You really should use
a small resistor in series with your batteries, going to the motors. The
starting current is several times the run current...

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email EraseMElistservspam_OUTspamTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body


2001\01\30@210355 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> >I have used a "trim" series resistor at times
> >(very low ohms) to make two motors run at the same
> >speed, assuming relatively constant load. The
> >other solution of course is to go to proper
> >encoders with closed loop speed control.
>
> No, the other solution is to connect the motors together in parallel using
> a relay or by putting the resistor in series with the supply of the two
> H-bridges. Or by using crappy batteries which will be like the resistor,
> only better ;-).


How is this going to fix it? I have bought numerous
small DC motors and the big problem with differential
drive is that for an identical voltage the two motors
run at fractionally different speeds. :o)
-Roman

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listservspamspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body


2001\01\31@060644 by Tom Handley

picon face
  Roman, I have a couple of tiny reflective IR sensors from a VCR. I used
one in the R/C car hack. In this case, the high speed gear is too small and
there is no room. Also, gear grease would get in the way. Again, it would be
easier to add gears to the other end of the shaft but right now, I don't
think I need it. Eventually, the direction will be controlled by the compass
which would be more accurate and get around problems such as a slipping
wheel.

  - Tom

At 11:08 PM 1/30/01 +1100, Roman Black wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com hint: To leave the PICList
@spam@piclist-unsubscribe-requestKILLspamspammitvma.mit.edu


2001\01\31@060657 by Tom Handley

picon face
  Peter, good point. In my case the devices can more than handle the
startup current and I use a 3.3V regulator for the bridge voltage which
provides reverse battery protection, thermal shutdown, low dropout
voltage, and low quiescent current.

  - Tom

At 09:20 PM 1/30/01 +0200, Peter L. Peres wrote:
>PS: To all diy homemade dc-motor driven robot users: You really should use
>a small resistor in series with your batteries, going to the motors. The
>starting current is several times the run current...


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com hint: To leave the PICList
KILLspampiclist-unsubscribe-requestKILLspamspammitvma.mit.edu



'[EE]: PIC Robot Update'
2001\02\01@161257 by Peter L. Peres
picon face
Roman, permanent magnet DC motors are also generators and unless they are
very wildly different they have similar characteristics ;-). This will
make them run at nearly the same speed if they are connected to the same
voltage with sensibly equal loads. Now, if they are connected directly in
parallel and the source is connected through a resistor that is as high as
possible (a few ohms or less -- the internal resistance of older batteries
f.ex.) then the motors have a fair chance to act one upon the other and
try to synchronize, to within 5% or so. If they happen to run in N-1
phases (one collector wire cut on rotor in each, or one rotor magnet wound
with more or less turns than the others in each) then they will sync real
good and try to lock in phase too. Not that I recommend this as a syncing
method for serious purposes. Some el cheapo dc motors are wound sloppily
anyway and have this asymmetry built in. Perhaps they do this on purpose
;-). The syncing effect is better, the higher the speed of the motors.

Peter

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\01@182435 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> Roman, permanent magnet DC motors are also generators and unless they are
> very wildly different they have similar characteristics ;-). This will
> make them run at nearly the same speed if they are connected to the same
> voltage with sensibly equal loads. Now, if they are connected directly in
> parallel and the source is connected through a resistor that is as high as
> possible (a few ohms or less -- the internal resistance of older batteries
> f.ex.) then the motors have a fair chance to act one upon the other and
> try to synchronize, to within 5% or so. If they happen to run in N-1
> phases (one collector wire cut on rotor in each, or one rotor magnet wound
> with more or less turns than the others in each) then they will sync real
> good and try to lock in phase too. Not that I recommend this as a syncing
> method for serious purposes. Some el cheapo dc motors are wound sloppily
> anyway and have this asymmetry built in. Perhaps they do this on purpose
> ;-). The syncing effect is better, the higher the speed of the motors.


I understand what you're saying, but having purchased
many small DC motors for hobby robot projects it annoys
me that they almost never match speeds for the same voltage.

Even decent quality Mitsubishi (etc) VCR motors can be
off by 5% or worse if both driven from the same regulated
voltage. OK, so it's not a big difference, but when your
robot travels in a curve instead of a line it is an extreme
pain in the behind! :o)

Sometimes a simple trim (series) resistor fixes this
pretty well, if you have one main forward speed. The
other option is code the bot's behaviour so it moves in
short spurts with lots of turning and looking around...
This looks ok to an observer but is still a pain.

Going to encoders and fixing the speeds is the nice way
of doing it. I just use steppers lately, haven't bothered
with small DC motors for a while. Steppers make very
impressive little bots, eveyone loves the whine and
the "industrial" looking motion. :o)
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\01@185429 by Barry Gershenfeld

picon face
(snip fore and aft...)

>I understand what you're saying, but having purchased
>many small DC motors for hobby robot projects it annoys
>me that they almost never match speeds for the same voltage.

I keep wondering...and no one's mentioned it...what would
you get if you made sure the *current* to each motor
was the same?  I've heard of things called "current
mirrors" and this sounds like a good use for one.

Barry

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\01@192156 by Roman Black

flavicon
face
Barry Gershenfeld wrote:
>
> (snip fore and aft...)
>
> >I understand what you're saying, but having purchased
> >many small DC motors for hobby robot projects it annoys
> >me that they almost never match speeds for the same voltage.
>
> I keep wondering...and no one's mentioned it...what would
> you get if you made sure the *current* to each motor
> was the same?  I've heard of things called "current
> mirrors" and this sounds like a good use for one.
>
> Barry


Hi Barry, I am not sure this would fix this particular
problem?? Since the motors were running at different
speeds with the same applied voltage, we have to guess
at why...

* different winding resistance?
* different brush resistance?
* different winding inductance?
* different magnetic fields/positions?
* physical variations in the rotors?
* differing bearing loads?

If is because of different resistances, fixing the
currents might work... Might create an equal field
in both motors. If it is because of the other factors,
fixing the currents would have an unknown effect.

It is a clever idea since the magnetic factors
are generally proportional to current, but in a simple
bot the usual system is to just run the motors both
at battery voltage when at full speed. Constant
current regulators get pretty lossy. :o(
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\01@202758 by hard Prosser

flavicon
face
Having worked briefly in a factory that produced magnets for (admittedly
cheap) loudspeakers I suspect the difference between motors is due to
differing strengths of the permanent magnet field. This variation could be
due to material differences as well as initial magnetisation and subsequant
use/abuse history.
Since the back emf is due to the product of the armature speed and the mag
field strength, the speed of the motor will be closely dependent on the
strength of the field - particularly under light-medium loading.

Richard P





Peter L. Peres wrote:
>
> Roman, permanent magnet DC motors are also generators and unless they are
> very wildly different they have similar characteristics ;-). This will
> make them run at nearly the same speed if they are connected to the same
> voltage with sensibly equal loads. Now, if they are connected directly in
> parallel and the source is connected through a resistor that is as high
as
> possible (a few ohms or less -- the internal resistance of older
batteries
> f.ex.) then the motors have a fair chance to act one upon the other and
> try to synchronize, to within 5% or so. If they happen to run in N-1
> phases (one collector wire cut on rotor in each, or one rotor magnet
wound
> with more or less turns than the others in each) then they will sync real
> good and try to lock in phase too. Not that I recommend this as a syncing
> method for serious purposes. Some el cheapo dc motors are wound sloppily
> anyway and have this asymmetry built in. Perhaps they do this on purpose
> ;-). The syncing effect is better, the higher the speed of the motors.


I understand what you're saying, but having purchased
many small DC motors for hobby robot projects it annoys
me that they almost never match speeds for the same voltage.

Even decent quality Mitsubishi (etc) VCR motors can be
off by 5% or worse if both driven from the same regulated
voltage. OK, so it's not a big difference, but when your
robot travels in a curve instead of a line it is an extreme
pain in the behind! :o)

Sometimes a simple trim (series) resistor fixes this
pretty well, if you have one main forward speed. The
other option is code the bot's behaviour so it moves in
short spurts with lots of turning and looking around...
This looks ok to an observer but is still a pain.

Going to encoders and fixing the speeds is the nice way
of doing it. I just use steppers lately, haven't bothered
with small DC motors for a while. Steppers make very
impressive little bots, eveyone loves the whine and
the "industrial" looking motion. :o)
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\02@064128 by mike

flavicon
face
On Fri, 2 Feb 2001 14:25:37 +1300, you wrote:

{Quote hidden}

VCR motors are hardly 'good quality' - consumer products like this are
built to save every cent.
I agree that difference in magnetisation is the main issue. A while
ago I was dealing with expensive swiss-made precision motors, and
these had significant variations, caused by differing magnetisation.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\02\05@043651 by Mark Hull

flavicon
face
DC motors have the characteristics of speed being proportional to applied
voltage and Torque being proportional to applied current.
This is because current produces the magnetic flux that reacts to the
permanent magnetic flux, so stronger current, stronger flux, more torque.
With voltage, as the motor spins, it generates a voltage opposite to that of
the applied voltage.
If the motor spins too fast, then the opposing voltage increases to the
point where it's almost equal to the applied voltage, with the result that
no more current flows. The DC motor thus self regulates it's speed depending
on winding resistance, internal magnetic flux, bearing friction and all
sorts of other contributing factors. It's almost impossible to get two DC
motors to run at the same speed without closed loop feedback.

One ingenious feedback method we used once was to use a pair of plastic
fibre optic light pipes (like the optical connections for Hi-Fi's). An IR
emitter shines down one pipe, and an IR detector looks at the end of the
other pipe. The opposite ends were glued together and passed through a hole
drilled in the side of the motor and directed at the commutator. The system
give a pretty decent pulse for each commutator segment.

On small toy/hobby motors you can direct the light pipe between the magnets
to detect the rotor poles instead, since brush arcing interferes with the IR
light. It works even better if you paint the rotor poles white.

Cheers
Mark

-------------------------------------------------------------------
Mark Hull
Engineering/Technical
EMS (Africa) (Pty) Ltd
PO Box 1026, Melville, 2109, South Africa
Ph +27 11 482 4470 Fax +27 11 726 2552

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu


2001\02\06@073639 by MegaBolt

flavicon
picon face
Hi Tom,

Do you have a web page on your PIC based Robot Bozo?

Tom

----- Original Message -----
From: "Tom Handley" <spamBeGonethandleyspamBeGonespamTELEPORT.COM>
To: <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Sent: Monday, January 29, 2001 7:16 AM
Subject: [EE]: PIC Robot Update


>    This is an update on my effort to design a PIC-based Robot. For those
of
> you following the earlier dialog with Dan, Steve, Roman, and others on
this
> subject, I finally have a good (re: great) mechanical platform. After
> tearing apart VCRs, tape recorders, and little green men with no shoes and
> no hair, I finally decided to just buy a decent gear box from Tamiya. Dan
> and Steve already bought the dual motor Twin Gear box which offers ratios
......................

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\02\06@094419 by Alejandro Fubini

flavicon
picon face
I haven't seen the original post, so this may be completely irrelevant, but
have can you try using servo motors instead of standard DC motors? You can
easily modify them so they are not limited to the turns and you drive them
with PWM.

Just a though.

--Alex

{Original Message removed}

2001\02\07@072432 by Tom Handley

picon face
  Tom, I haven't had time to put anything up on my web page yet. My
ISP is also going through major problems since Earthlink bought them.
Mail has been down for a few days and I may have to switch.

  - Tom

PSBS: Again I've been haveing mail problems...

  Someone mentioned drilling a hole in the motor and using optical
detection. I was considering that but with Hall-Effect sensors.

  Someone else mentioned servo motors. I wanted to keep the cost
down but normally I would use a servo motor with built-in tach and
gear reduction.

At 08:40 PM 2/6/01 -0800, you wrote:
>Hi Tom,
>
>Do you have a web page on your PIC based Robot Bozo?
>
>Tom


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


2001\02\07@145012 by Alejandro Fubini

flavicon
picon face
Tom,

If you can get hold of inexpensive servos, I think that would be the best
way to go, you should be able to find good deals on servos at
http://www.towerhobbies.com/

To convert them so they rotate 360 degrees, you can go to
http://www.stampsinclass.com and go to the DOWNLOAD area and then to the
'Robotics Experiments' documents, in it they give you detailed instructions
on how to convert the servos, alternatively you can go to
http://www.hvwtech.com/servos.htm for the same type of information.

While I was at it I referenced a book 'Robot Builder's Bonanza, 99
Inexpensive Robotics Projects' by Gordon McComb. I guess this is considered
the bible for amature robotics and it goes into some depth on feedback
control for DC motors. Unfortunately, you're kind of stuck either using
open-loop control which will not be accurate or closed-loop which will mean
adding an shaft encoder of some sort. If you want me to send you some more
detailed information from the book let me know and I'll email it to you.

Alternatively you could use one motor only to drive the robot forwards and
backwards and then have a separate motor to control the turning wheel.

--Alex



{Original Message removed}

2001\02\08@164721 by Tom Handley

picon face
  Alex, thanks for the info but I've been down that road and I really have
a very nice mechanical platform. I'm real impressed with Timaya's products.

  - Tom

At 02:51 PM 2/7/01 -0500, Alejandro Fubini wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...