spark controller dropping voltage in reverse

[Ether]

Originally Posted by dlsmith

I don't recall for sure the models but we tried a couple of different meters. I think one was a Fluke. While it was measuring you could see that it was trying to "lock onto" the signal somehow (showed moving bars constantly) and it wasn't doing a good job averaging the voltage I suspect. Could be that we didn't have all the right modes selected as it seemed like "average voltage" should be possible somehow.

With a cheap $3 DVM from Harbor Freight I am able to accurately measure the PWM signal voltage and the PWM output voltage. So I would imagine your Fluke (a very reputable brand) ought to have a way to do the same.

Cheap DVMs use averaging; expensive are selectable averaging or true RMS.

Cheap DVMs on AC scale use a diode to rectify the signal, average the rectified half wave, and then multiply that by some fudge factor (based on an assumed sine waveform). It works OK for sine waves but gives nonsense readings for square waves or PWM.

Expensive DVMs (your Fluke might be one such) with true RMS do not assume a sine wave, do not throw half the wave away by rectifying, and can measure the actual rms value of any periodic AC waveform (within reason).

[Ether]

Originally Posted by dlsmith

Cheap DVMs on AC scale use a diode to rectify the signal, average the rectified half wave, and then multiply that by some fudge factor (based on an assumed sine waveform). It works OK for sine waves but gives nonsense readings for square waves or PWM.

If you have one of those $3 Harbor Freight DVMs and a battery, you can see this in action:

1) using the DC mode, measure the battery voltage and write it down.

2) using the AC mode, measure the battery voltage (with red lead to +) and write it down.

3) using the AC mode, measure the battery voltage (with black lead to +) and write it down.

If the voltage in (3) is zero, you very likely have a "average of half-wave rectification multiplied by fudge factor" meter, and you probably got a reading in (2) of about 28 volts !!

Now let's do some math to see why.

The true RMS of a sine wave is peak/sqrt(2)

The half-wave-rectified average of a sine wave is peak/pi

The ratio is the fudge factor: f = (peak/sqrt(2)) / (peak/pi) = pi/sqrt(2) = ~2.2

So a AC voltage in (2) will be approx DC*2.2

[fmercier]

Thanks for all the comments.
We have reset the Spark. LED still has intermittent blinking in reverse. The actual motor speed is slow, matching the low output signal. Per our program, we are sending a value "1" for forward and "-1" for reverse. The result is the same with a second Spark. We have also tried a jaguar, and the drop off is less, but there is still a clear difference between forward and reverse. ???

[Ether]

Originally Posted by fmercier

Thanks for all the comments.
We have reset the Spark. LED still has intermittent blinking in reverse. The actual motor speed is slow, matching the low output signal. Per our program, we are sending a value "1" for forward and "-1" for reverse. The result is the same with a second Spark. We have also tried a jaguar, and the drop off is less, but there is still a clear difference between forward and reverse. ???

Do you have, or can you borrow, a DVM to measure the PWM signal at the Spark's PWM input terminals when you are commanding forward and reverse? See posts 9 and 10 below.

[Greg Needel]

Originally Posted by fmercier

Thanks for all the comments.
We have reset the Spark. LED still has intermittent blinking in reverse. The actual motor speed is slow, matching the low output signal. Per our program, we are sending a value "1" for forward and "-1" for reverse. The result is the same with a second Spark. We have also tried a jaguar, and the drop off is less, but there is still a clear difference between forward and reverse. ???

If it is doing the same thing on a different SPARK, I am not suspecting that there might be something else wrong with your setup.

Here are a few things to try.

* Check that your battery is fully charged
* Change your PWM cable
* Change the port your PWM cable is plugged into on the roboRIO
* Ensure that your power wires are secured to the SPARK and your Power distribution board.

Please let me know if you need additional help, send me a PM or email me at Contact [at] REVrobotics [dot] com

[Al Skierkiewicz]

fmercier,
We need to know how you are generating the full reverse throttle. If you are simply using a joystick then you have to calibrate to that joystick. If you are using a code generated full throttle reverse then I would double check your code.
What color is your controller blinking? There is a whole list of possibilities if you examine the manual. If it is blinking Red then that is an indication that you are not at full throttle but are commanding a reverse proportional output. That sounds like what you describe. The full on LED Green would indicate a full throttle forward output.

[Alan Anderson]

I'm going to make a wild guess that you're using buttons to run the shooter motor. One button runs the motor forward when pressed, and stops it when released. The other button runs the motor in reverse when pressed, and stops it when released. Is this what you're doing?

If so, you need to change it. Have one button run the motor forward. Have the other button run it in reverse. Stop the motor only when neither button is pressed.