.....
>Preliminary info at:   http://www.oricomtech.com/bot40.htm
>

Here is a little update on my botboard project. I received the pcbs
and have been testing them. Everything looks fine so far.

Classically, the 68HC11 Handyboards, which also use the L293D H-bridge
chips, are said to not work well with little 3vdc type motors - too
much current and too noisy - so of course, these are precisely what
I have been concentrating on in my testing - get it to work for the
worst case :).

I have also tested a beautiful 12vdc, 100mA high-torque gear motor,
and it is quiet as can be - both sound-wise and noise-wise. No problem.

However, regarding the little 3v motors, I have been PWM'ing them up
to 600 mA, and the faster they go the more electrical noise they make,
of course. The noise that is bothersome is slow - large pulses in the
0.1-0.2 msec range, which I assume are due to brush bouncing/jitter,
and is not correlated with the PWM. The much faster noise, related to
PWM'ing, is well-filtered by the bypass and reservoir caps on the board.

The question is ---> is there any good way to get rid of the slow motor
[brush bounce?] noise?

Things done so far:

- the L293's have internal catch diodes, of course.
- also have a 20v transzorb and .047uF/250V bypass cap on the motor
  power, and have tried reservoir caps up to 1000uF.
- also tried a snubber cap on the motor directly.
- also tried a series R in the motor power lead.
- the pcb is laidout with motor drivers on one side, cpu and analog
  ckts on the other side, and a single point gnd connection between
  logic and motor gnds, made thru a ferrite bead.
- motor and logic power are separate.

Given the nature of this noise, I don't think shielding the wires
would help much.

What else can I try?

Interestingly, the noise generated by the 3v motors is no problem for
microcontroller operation, even when PWM'ing 2 motors simultaneously
- maybe PICs are less sensitive than HC11's. ??????!!

Also, it looks like the pcb is fairly well laidout, because I have
an op amp ckt with 100X gain on the board amplifying an electret
mike, and the motor electrical noise is not apparent in the output
of the amp.

Any advice about dealing with these motors is appreciated.

best regards,
- Dan Michaels
www.oricomtech.com
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