Camera \ EMF problems...

[zachriggle]

I have a very peculiar situation, involving the camera servo's dancing when the Fisher-Price motor turns on.

We are using a Fisher-price motor for a ball launcher mechanism, mounted on a turret-style (pan only, no tilting mechanism) plate. The camera is also mounted on this plate.

With the servo's set to center (to rule out programming as a potential source of the problem), and the camera unplugged, the servo's go absolutely NUTS as soon as the Fisher-Price motor turns on. I mean disco dancing on steroids. The FP motor is connected to a Victor, as required, and is controlled via a PWM cable, as required.

However, when the FP motor is given direct voltage from a second battery... there is no such effect. Something in the electrical system is allowing voltage to creep and spike on some portion of the PWM cables, bet it the signal or +7.2V wire, I don't know.

None of the other PWM-controlled devices seem to be effected. The power wires for the FP motor run near the camera... but I doubt that *that* has anything to do with it. The wires from the camera also run between the battery leads, between BOTH drive (CIM) motors (3 cm proximity) under a window motor (2 cm proximity), and is equally centered between the victor for the FP motor and the victor for the Van Door motor... and with ALL of the other motors on, this phenomena is not replicated. We are using 12ga (10ga?) wire for all motors.

Additionally, the camera's terminal interface seems to work just fine, because Kevin's code can lock onto targets for short periods of time with the FP motors on, until the servo's craziness causes the camera to lose track of its target... until it's reaquired.

Also, can anyone else pick up voltage between any parts of their chassis and the common ground? We're getting between 0 and 3.2v readings. We physically removed the camera from the chassis to remove that as a reason for the servo weirdness, but it's still something we don't like. There is conductance between the casing of the FP motor and the plate that the entire firing mechanism rests on. Does the FP motor create a magnetic field so large that it would treat a 0.75cm rubber spacer more like a resistor than a block to conductance (the motor is not physically touching the chassis, but it conducts to the top, rotary plate. The top, rotary plate does not show conductance when tested against other parts of the chassis, pecularly).

Can anyone explain this phenomena? All that we can think of is that the FP motor is either generating a HUGE EMF, or there is some kind of back-feed down the PWM signal cable attached to the Victor controller.

[steven114]

I can't help you on your servo problems, but when you're checking for a conducting path between your frame and electrical system, it's probably better to use the "continuity" setting on your multimeter, or at least measure resistance instead of voltage difference. If your frame is isolated, as it should be, the potential difference between that and the electrical system could be almost anything (correct me if I'm wrong there) but the resistance should be effectively infinite (usually displays as a 1 in the leftmost column on the multimeter).

[zachriggle]

We also measured continuity, and measured 46 ohms resistance between the FP motor and the rotary chassis. We measured no continuity between the negative lead and the chassis.

[steven114]

If that's the case, better start checking for touching wires... :/

[Manoel]

Are you, by any chance, using PWMs 13-16?

If that's the case, you just have to change them to some other output. A great rule of thumb is to never use those fast outputs because if you're using other interrupts, they can interfere with the output, causing this kind of spastic operation you are seeing.

A better discussion can be found here

If that's not your problem, I'd suggest removing power wires and stuff like that away from the camera.

God luck and let us know how it works out!

[zachriggle]

Nope, I'm not using those PWM's for anything, actually. I assumed that might be an issue from the get-go, and decided against using them for anything.

We're thinking the EMI is coming from the Victor controller itself, rather than the motor. We found out why the chassis conducts with the motor, is through the metal shaft, that goes through a metal bearing, to a metal mounting bracket, to a metal chassis.

Regardless, that doesn't explain the spazz-ness of the camera :'(.

[Al Skierkiewicz]

Zach,
What you are describing is likely caused by mounting screws that are too long for the FP motor. One of the screws may be touching the wiring inside the motor hence the 46 ohms you read to chassis. If you were to measure the resistance on an FP motor, you will likely find the same value. (turning the shaft will produce different resitance readings but one will be close.) You might have a solderless connector pushed onto the motor contacts that is "leaning" far enough to allow the connector to contact the frame of the motor. This is an illegal condition and is likely the source of interference with the other systems. The higher the main battery voltage, the worse it becomes is due to higher voltage available in the charged battery. ( i.e. more current into the frame) Remove the FP from it's mount and/or remove the wiring to the motor and see if your resistance reading goes away.
al

[Kevin Sevcik]

Also, the servo spazzing could be caused by running the servo PWM near the FP power leads if you run them together for any significant distance, you could have a problem. You wouldn't notice anything running the motor off another battery because it's getting a constant DC voltage. Unless the Victor is running at 100%, the voltage will be jumping between 0V and 12V. If your PWM is running alongside the power leads for long enough, it could be slightly coupled with the leads, and will pick up some of those 0V to 12V jumps. Especially because the FPs can draw large currents.

Try running a temporary PWM completely detached from your chassis and away from all other wiring and see if that fixes the problem.

[btgdaniel]

Originally Posted by Kevin Sevcik

Also, the servo spazzing could be caused by running the servo PWM near the FP power leads if you run them together for any significant distance, you could have a problem. You wouldn't notice anything running the motor off another battery because it's getting a constant DC voltage. Unless the Victor is running at 100%, the voltage will be jumping between 0V and 12V. If your PWM is running alongside the power leads for long enough, it could be slightly coupled with the leads, and will pick up some of those 0V to 12V jumps. Especially because the FPs can draw large currents.

Try running a temporary PWM completely detached from your chassis and away from all other wiring and see if that fixes the problem.

1. Our camera TILT servo motor *always* gets the jitters when we have the RC disabled (with the dongle).

2. It *almost always* get the jitters while we are downloading code to the RC.

It has always behaved this way, even before it was mounted on the robot.

Anyone else seen this behavior?

[Al Skierkiewicz]

Originally Posted by btgdaniel

1. Our camera TILT servo motor *always* gets the jitters when we have the RC disabled (with the dongle).

2. It *almost always* get the jitters while we are downloading code to the RC.

It has always behaved this way, even before it was mounted on the robot.

Anyone else seen this behavior?

I think that the problem is that the RC when disabled has an open circuit output with no loading. This leaves the servo to pick up crosstalk from any wiring around it and interpret that noise as a valid PWM command.

[ahanktcd]

Originally Posted by Al Skierkiewicz

I think that the problem is that the RC when disabled has an open circuit output with no loading. This leaves the servo to pick up crosstalk from any wiring around it and interpret that noise as a valid PWM command.

Our cam servos did this same thing. Our's did it's antics only when disabled and not when motors were running. We had the camera data and servo signals running in the same SHIELDED wire (no outside interference). The serial data is +-15V and the servo ports are open (high impedance) during disable. The +-15V was 'jumping' from the serial data to the open servo wire and the servo had no idea what it should do this this random data. Electrical engineers, please excuse my terminology!

We solved the problem by running separate individually shielded wires up to the camera. Problem solved.