problem: A Dancing Servo

[gilkeedman]

we plugged in servo (with a jumper) and we plugged in tow FP motors to victors with a Y-cable but when we activate the motors the servo starts to dance.
I've tried to disconnect the pwm cable which come from the SideCar to the victors, and then the servo isn't dancing, so that ruled out a program issue, the problem also disappered when we disconnect the motors from the victors.

[Russ Beavis]

You should confirm that your robot's chassis isn't electrically connected to ground, a signal line, 12V or a motor lead.

Russ

[Yoel2630]

Originally Posted by Russ Beavis

You should confirm that your robot's chassis isn't electrically connected to ground, a signal line, 12V or a motor lead.

Russ

we have
It seems to be some kinda noise on the signal while drawing current.
This happens only when we activate the servo simultaneously with two fisher-price motors which are connected to two Victors

[JamesCH95]

Clearly there's a line noise problem. Try twisting up the servo cable, or shielding it, assuming is an EMF problem. Also try plugging it in to a output thats further away from the FP signal.

[Al Skierkiewicz]

Fisher Price motors are notorious for producing "brush noise" which is in effect RF noise. Try separating the servo cable from the power wiring for the motors. If they must cross do so at right angles. Robot rules allow you to add some filtering to the motor and the servo lines.

<R69> A signal filter may be wired across motor leads or PWM leads. For the purposes of inspection and rules compliance, such filters will not be considered custom circuits, and will not be considered a violation of Rule <R54> or Rule <R68>. Acceptable signal filters are:
• A one microfarad (1 μF) or less non-polarized capacitor may be applied across the power leads of any motor on your ROBOT (as close to the actual motor leads as reasonably possible)
• A resistor may be used as a shunt resistor for the PWM control signal feeding a servo.

I would recommend a good quality 0.1 microfarad capacitor across the terminals of the motor rated for at least 100 volts. I would also suggest making the servo cable as short as possible. Radio Shack has a variety of ferrite cores that can be placed on the servo cable that may help as well.

[JamesCH95]

Al's suggestion is a very sound one. You can solder the capacitor directly to the motor leads on the motor can itself.