Help: First Robot Project - Turning on Carpet Using Tracks

Assuming a sign-magnitude PWM implementation (one line for direction
control, one line for pwm):

Check first to be sure your motor has sufficent power to turn the robot
on the carpet. max pwm is normally a 100% duty cycle pulse the
equivalent of full-on, or 50% for locked-antiphase, but there could be a
problem with your implementation. Try hooking the batteries up directly
so you're certain that you are getting full power.

If you can turn with the batteries hooked up directly, then you should
check the pwm output with a scope, since it may be that the actual PWM
output isn't what you expect it is. Normally, "max" shold be a steady
signal (again, assuming sign-magnitude).

If the pwm signal is as expected, try lowering the frequency to see if
you get any improvement, although with the Tamiya this is probably not
the issue, especially if if you see the problem at 100%.

If the robot won't turn at full power, then the motor output is
inadequate and you should go with the higher torque option. My guess is
that this is your problem, but it's hard to know without seeing the
platform itself.

Hope that helps -- m

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Oops -- 50% for locked antiphase would be full off -- sorry bout that.
Must... review... before posting...

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(Replies: cleanse my address of the Mark of the Beast!)

Teleoperate a roving mobile robot from the web:
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Coauthor with Dennis Clark of "Building Robot Drive Trains".
Buy several copies today!

Hi everyone.

OK.
Thank you for everybody's kind suggestions, here is what I did
so far.


( As I am only a recent student in electrotechnology, I haven't
implemented some of the other more advanced solutions - I simply am not at
that level yet. For me this is a KISS project : Keep It Simple Stupid ! )


I powered the Tamiya ( ~3-6V ) twin gearmotors directly ( no MCU, no
L293DNE motor
driver, no software ). Here are the test results for testing motor speed
and direction ( turning both
directions ) on carpet:

Observations & Results:
------------------------------------------------


Voltage ( V )            Turning Speed ( rev/s)            Remarks
---------------------            --------------------------                  --------------

~ 7.2                    ---                    Class 5 beserk robot tornado ( can't measure it! )

~ 4.8                        ---                    Class 4 beserk robot tornado  "        "    "

~ 2.4                    1                    Still pretty quick, but a lot better



1. Robot turns easily on carpet, even when motors supplied at low voltages.


Conclusion:
------------------

Robot will turn on carpet OK, even at low voltages. Not a mechanical
cause for not turning on carpet ( turns OK on smooth surface )
when driven by MCU and L293DNE motor driver via 60 - 100% PWM.

Any more observations or suggestions please are welcome.

:-]

Cheers

Dale

You're likely either not actually getting 100% pwm, or the L293DNE isn't
giving you full power out. Try hooking up the PWM output to a scope and
  see what you get.

You can also try applying a steady signal to the 293 to see if it really
gives you full power. Check the output voltage and just try it out with
the motors hooked up.

       --------------

 Class 5 beserk robot tornado ( can't measure it! )

 Class 4 beserk robot tornado  "         "       "

 Still pretty quick, but a lot better

First problem is the voltage in column 1 is your problem. Though Tamiya
never really specs the voltage, the RE260 motors are designed for 1.5 to
3 volt operation. IMO, the mechanics of this kit also don't allow for
very high RPMs of the gearings. You're talking about two complete
gearmotors for $10 total. Nice price, but don't expect mechanical
miracles.

Keep the operating voltage to 3 volts. When it comes time to use a motor
bridge, use a bipolar type like the LM293 (or make your own with
discrete transistors), and remember the voltage drop.

As to why it's not turning on carpet, can you be more specific what that
means? Are the motors stalling out, or do the treads just slip over the
carpet without making any traction? Big difference there...

-- Gordon

Hi Gordon.

Thanks for your reply ( by the way I have bought one of your books ).


Sure thing Gordon. I bought these because they were handy
( just  5 minutes ride on the bicycle ), cheap, and
flexible for my first project.
I am running 4.8V of juice through the motor driver, but will soon be
retesting different% PWM duty cycle
with the goal to lower the voltage through motors.


I have now resolved the problem. Yes I should have been more specific,
the motors were stalling on carpet using either the L293DNE ( 600mA per
driver channel ) or SN754410 ( 1A/driver channel ) half H-bridge drivers.

The problem was that in my test program, at the lower % duty cycle of
PWM  the motors stalled, causing a fault in the motor driver, then
causing a flow on effect to the next block of code of my test program.

It was hypothesised ( by me ) that  the current draw was too much for
the motor driver chip, triggering the chip's thermal shutdown feature,
causing the rest
of the turning  test code not to execute. My test program was to loop
through 60-100% PWM duty cycle, but because of the thermal shutdown
at 60% PWM duty cycle, the other code that was to execute
( 70-100% PWM duty cycle sequences ) -- didn't.

The 60% PWM duty cycle was eliminated and substituted for 90-100%. Guess
what? - it worked fine and the robot turned on the carpet
via either of the motor driver chips mentioned above!!! So I guess the
hypothesis was true!

However, then one of the tracks kept coming off! When tightened, this
cause too much tension. Result? - the motors stalled!

So I then decided to go with the HI TORQUE option gear ratio of 203:1. This
meant pulling aprt the gearbox and rebuilding it, then having to take
out a linkin each in the tracks. Anyway, to cut a long story short, it
now turns a
breeze at the higher torque, the tracks stay on, and it turns at about
0.4 rev/s
( maybe a little too fast ).

The next phase of testing the motor drive system is to see what the
lowest % PWM duty cycle/voltage is with the HI TORQUE grear ratio
and to see if the robot will turn at a lower % PWM/voltage to place less
' voltage stress ' on the motors.


I AM VERY HAPPY I SOLVED THE TURNING ISSUE. :-]

THANKS HEAPS TO EVERYBODY WHO REPLIED TO THIS THREAD!!!



No doubt I will be bugging you blokes once I finish testing the motor
drive system and moving on to other systems!

ps Can anyone please recommend the minimum specs to look for in a
digital camera
to take pics of my project? It would need to take close-ups of the
breadboard circuit, and eventually  my own PCB. I plan to publish my
project on
my web site so it will benefit beginners like myself.

Also will be a beginner's perspective on programming PICmicro MCUs using
CCS C
as a complete project.


: -]

Cheers

Dale