piclist 2000\12\15\032332a >
Thread: Robotics with Dan's PIC SBC
face BY : Dan Michaels email (remove spam text)

Roman wrote:
>> Analog pickup amps --> LM324, 2 each 2-stage, single-supply,
>> ac-coupled, inverter followed by non-inverter, gain selectable
>> by R values, designed to handle any of phototransistor,
>> electret mike, or ultrasonic rcvr. Pretty much std ckt.
>> Any specific suggestions here?
>I almost always use some type of RC filter etc before
>any opamp, maybe a couple of stages. I suppose a few pads
>there to patch in R,C,L etc without having to cut tracks??

There are currently caps across the opamp feedback resistors,
but I'll cook up the schematic for the opamp stages tomorrrow
and send a .GIF - there are several components on the front-end
for different arrangements/gains/etc. Be good to have some
feedback. Want to have it work for electret, phototransistor,
and ultrasonic receiver transducer, all three.

>> Separate IR LED / optologic detector [QSE156QT] channel.

I designed a really cool ckt today which I "think" can pulse
an LED at up to 40Khz and "also" control its intensity by varying
the width of the 40Khz pulses. 5 components, including 2 R's,
1 Cap, NPN, and LED. Requires only 1 PIC pin.

Think you can duplicate it? [just try :) - hint - the tricky part
is the cap - you have to be able to get a graded current in the
LED, but still interrupt the current at a 40khz rate - tricky
time constants]. I estimate I can get at least 6-8 intensity levels
running off a 4 mhz xtal - 1 usec cycle time.

>For small stepper bots (ie micromouse competition) the
>normal steppers are about 100mA to 400mA per motor, so that's
>fine. Being able to drive a 1A motor (two 0.5A coils per motor
>with small heatsinking will allow a pretty tough bot. My bot
>pushes 1kg and uses 200mA total per motor. DC motors similar,
>most micromouse dc motor/gearbox are under 500mA on average.

Luckily, the L293 chips can handle motors that use large
voltages [and lower currents] - unluckily, the Tamiya is
low-volts and hi-mA.

I jigged up the L293 to drive the Tamiya twin-motor tonite.
Oh boy, did I learn a lot. I PWM'ed the enable inputs to
control motor speed, and purposely didn't use any bypass caps,
just to see how it ran, and also whether the PIC would burp
or not. No burps.

At full tilt boogie, the Tamiya was pulling >1.2A with both
motors grinding hard. No heat sinking on the L293 at present,
except for the pinchers on the ZIF socket.

You lose 2+ volts in the chip, so it was dissipating >2W,
and running pretty hot [I made popcorn]. I figure with 2 L293s
on the board, and long motor run times, it's probably best to
wire each motor to a different chip to spread the load.
The final SBC will have copper trace heatsinking, of course.

[note I typically like to also stress and qual test during my
"initial" design/proto phase - I managed to smoke the SBC
main v.reg - I smelled it in the other room!!].

Question --> ever try using PWM for controlling speed with
this kind of chip? I noticed you get really nice 40+ volt
spikes on the motor supply bus at the points where the
PWM turns on. Note this is with the catching diodes present
in the chip - but no caps present anywhere. Adding a bypass
cap does reduce the spikes to just a few volts, however.
And interestingly, the spikes don't mess the PIC.

BTW, big spikes not present with PWM = 0FFh. 100% duty cycle.

At any rate, I think it will work better if I filter the
PWM before applying it to the enable input, and was wondering
if you have done this with your chips too? [as it is, you really
get some retched looking waveforms on the motor windings].

>Since the price of your SBC was quite reasonable, I am sure
>you could sell half-kits (board, PIC, L293 etc) to schools
>and colleges for micromouse competition. Having one board
>that does the lot makes it more a programming exercise than
>a major construction one.

Yeah, I always take the easy way out. Do it all in one
place and all at the same time. Still need to proto the IR
driver and design the ultrasonic transceiver.

>Might I suggest the ability to measure battery voltage?
>Two little resistors on-board would be nice! ;o)

Aww, come on - those are already on there. Whatyaexpect?
But none of those fancy dodads people were designing last
week. Just 2 resistors.

>PS. Maybe you sponsoring a mouse competition might
>be cool??

First prize, a burned up L293.

best regards,
- Dan Michaels
Oricom Technologies

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