Anomalous Interaction

[dkinzer]

[liam.zbasic]

The motors have a separate power supply (11.1v). The ZX-40N device and motor Driver share the same regulated 5V power supply. I enclosed some hardware pictures for clarification.

[spamiam]

Its hard to say for sure from the pictures, but it also appears that the motor grounds go back to the motor battery via heavy wires. Where is the common ground hooked to the motor batteries. The common ground connection should be connected as close as possible to the motor batteries.

If, for instance, there is a 10 milliohm resistance between the common ground connection point and the motor batteries, then when the motors pull 21A (each!), the common ground could be (V=IR) 42A * 0.010R = 0.42V above true ground!

I presume that the motors do have a common ground with the rest of the electronics, though they could be completely isolated. I suspect that with the motor driver powered by the ZX power supply, that there is a common ground.

With use of optoisolators, it may well be possible to completely isolate the ZX from the motors and motor controller.

-Tony

[liam.zbasic]

[spamiam]

[liam.zbasic]

I have not implemented the optoisolated circuit. As the project now stands, the ZX and motor battery likely share the same ground somewhere inside the motor driver (schematic below) - that I can't undo.

http://www.pololu.com/picture/view/0J411

Once I optoisolate the project, the motor driver will have its own motor battery and its own +5V regulated power source. The ZX circuit will also have its own battery. So that's a total of 3 batteries! I may consider a 4th for the IR range sensors, which are quite noisy. The 1,000uF low ESR filters at each IR sensor does help significantly.

[spamiam]

[GTBecker]

> ... 3 batteries...

One can take isolation to an impractical and likely unnecessary
extreme. The objective is to reduce the noise from heavy loads that's
induced onto logic lines and, perhaps, minimize brownout droop - not to
isolate loads from one another. And, while it is possible to
galvanically isolate your controller from the load it controls - no
conductive path whatsoever from any point on one side to any on the
other side - it is probably not required, and I think makes
troubleshooting the circuit more difficult since there is no single
reference point from which to measure levels.

Motors can probably share a common supply with other inductive actuators
and with positive-temperature-coefficient loads like incandescent lamps,
and other dynamic high-demand loads. A simple method has been discussed
here - a common ground point at or very near the supply return (a star
ground) - which assures that logic ground is as close to zero volts as
possible - which has served many projects for many decades.

Ground noise is not a new problem; a number of books have been written
about it alone and it has probably been experienced since about 1890.

[liam.zbasic]

I wired up a PS2501-4 optoisolator to my project. The motor-driver logic signals that control fwd/reverse motion function correctly. The PWM signal does not work. The ZX-40n is sending a 20khz PWM to the optoisolator. My voltmeter detects that frequency on the output, but the output RMS voltage is near 5V regardless of the input duty cycle. The enclosed picture shows the optoisolator circuit used for both logic signals and PWM. Is this circuit appropriate for PWM? Is there a better optoisolator for this application? I'm driving a VNH2SP30 motor driver module from pololu. Appreciate your comments. Thanks.

[spamiam]

I am not an electrical engineer, but usually I put the resistor on the (+) side of an NPN transistor. The reason for this is that the (-) side is common to the base and the 10K resistor will limit base current. Limiting base current will limit the gain of the transistor. It is harder to get the transistor to turn on.


But for you moving the 10K resistor would make the transistor act as an inverter. As a result you would have to reverse the duty cycle logic or add another (regular NPN, not opto) transistor to act as a second inverter. It is probably easier to change the PWM logic at the ZX.

-Tony

EDIT: This is not to say that you can't use your circuit as shown. Look up "common emitter" and "common collector" in wikipedia and other sources. then you can select whichever design you prefer.

[dkinzer]

[liam.zbasic]