Difference in forward and backward pulsout values

Computeruser

I got a Basic Stamp BS2 Board of Education Rev D Robot kit yesterday afternoon.

I have it running and it seems to work properly. Today I got it running forward and backward in a nice straight line for 10 seconds

Forward Values:
PULSOUT 13, 850
PULSOUT 12, 685

Backward Values:
PULSOUT 13, 650
PULSOUT 12, 810

Question: I do not understand why the forward delta from top speed on Servo 12 is 35 going forward and the delta on Servo 12 is 40 going backward. Should not the backward value be PULSOUT 12, 815? But that value veers of to the right a bit whereas 810 gives me a nice straight line (same floor location in both tests).

.... C

Electrodude

The two servos are tuned differently. On the side there is a hole you can stick a screwdriver through and turn a potentiometer to tune the servos. If you already did this tuning, this phenomenon is probably caused by the fact that the servos have some (or all?) analog circuitry, which always has variations like this. No two capacitors, resistors, or inductors are ever exactly the same, and this is what's causing your problem. This difference is perfectly normal.

Duane Degn

I agree with what Electrodude said.

It's tough to get two motor to move at the same speed without encoders.

I've noticed some servos seem to behave better at lower speeds. I'd try cutting your speed in half and see of the forward and backward values match better.

When I had my last CR servo powered robot perform a figure 8 I used a speed of 100us which would be a speed setting of 50 on a BS2.

Computeruser

Thank you both for your replies. The discussion about analogue circuitry in the servos makes sense and (on a purely ad hoc basis) matches the reason I get calibration differences in two "identical" vertical amplifiers. Interesting that on newer digital devices, I see much smaller differences.

I also did not mean to imply I see a problem. I was merely questioning why this should happen. Electrodude - I did tune the servos prior to final assembly and I made sure both servos were stopped (zero hum or noise) when I calibrated them.

I figured the difference was normal, but I am happy to have the confirmation.

... C

erco

It may start curving after your batteries wear down a bit. Or on a different surface (carpet vs. linoleum, oops, resilient flooring, ack). Or if you add weight or change or CG or the wind shifts... Lots of variables with analog servos.

BTW, welcome to the Forums, Computeruser!