Victors Non-linear!!!
 

I have confirmed a suspicion i have had for a while. Apparently the victor 884's do not have a linear PWM input vs. voltage input.

A DVM was attached to a victor and the values recorded while incrementing the PWM input by a set number every 6 seconds. The reason for the initial values being a bit sketchy is that i was using a cheap digital meter that fluctuated a bit. As time went on, i got a little better at figuring what number it was hovering around.


Here i attached an unloaded motor to a victor and measured its speed vs. victor PWM input. The speeds were determined using a 1024 count optical encoder encoder. The motor was a fisher-price motor w/gearbox.
The test was automated and did the following:
1.set victor output
2.wait 2 secs for speed to stabilize
3.count encoder ticks for x secs. (I think the test was set to wait 3 seconds at the time this data was taken).
4.output data
5.increment pwm output and goto step 2 unless finished

Note that since the motor is unloaded, motor models dictate that speed should be directly porportional to the input voltage of the motor.


I should mention that these tests were done on a single victor 884 and i cannot guarentee that all victors behave similarly although i believe that they do. I also believe that this curve is a feature of their design.

There will be higher resolution data as well as code examples of how to compensate for this to come. It will probably be a while as i am busy with other things and will be out of town all this week.

I hope my imagehost holds up

Re: Victors Non-linear!!!
 

Jim,


Very interesting data. Thank you for sharing. I'm not sure why IFI would make its device non-linear.

I would be tempted to redo the test and monitor the controller output PWM with a scope.

Mike

Re: Victors Non-linear!!!
 

Very interesting information--definitely throws an extra idea or two in for the software crew this year.

And I'm sure we'd all love you to run this test on a few more Victors to see if the numbers hold up.

Re: Victors Non-linear!!!
 

You are suggesting that perhaps this is an issue with the controller PWM output being nonlinear? I didn't think of that. This is something else i will test.

Re: Victors Non-linear!!!
 

I think this is actually good news for most FIRST applications. Power is, of course, proportional to voltage squared and this curve looks to be approximately 1/x^2 so that way, PWM values are close to proportional to power.

Re: Victors Non-linear!!!
 

Jim,

I rather doubt that the controller is non-linear and, the more I think on it, it does not matter. This non-linearity does shed some light on some phenomena I saw working with autonomous mode last year. I'm thinking a lookup table to linearize the output may be in order this year.

Once again, Thanks for sharing the data.

Mike

Re: Victors Non-linear!!!
 

Erm, so I'm confused. I was under the impression that the Vics have a PWM output instead of a variable voltage output. As such, you can't really measure the output with a standard multimeter. This would be why Mike Betts was talking about checking it with a scope.

This has not bearing on if the RPM output is linear, however. I think I'll do a Simulink model tonight and see how RPM varies with duty cycle, if I can get enough data on motors and the PWM output.

Re: Victors Non-linear!!!
 

Exactly,
It's general knowledge that the Victors pulse 12V to the motors to control their speed. Except at the extremes when the FET's give the full 12V. It would be very difficult to measure this on a general purpose multimeter.

I am extremely skeptical of the voltages you are getting. A better test would be to see if the frequency is linear. I plan on checking that when I get back to the lab after the holidays.

Re: Victors Non-linear!!!
 

Wouldn't these numbers make perfect sense in these instances? how at around 0 it went flat.. and untill that point it was near the bottom, and when it was near full 12v it was at the top? It would be taking the pulse average?

And with it fluctuating, it would confuse the meter?

Am I way off here?

Re: Victors Non-linear!!!
 

You can with a true RMS meter.

Actually, the frequency is fixed. The duty cycle is what changes. This too can be measured directly with some higher-end meters, like the Fluke 189.

Edit: I wrote some code that shows how to generate the PWM signals using the PIC's CCP hardware. It's here: http://kevin.org/frc/edu_led_pwm.zip

-Kevin

Re: Victors Non-linear!!!
 

That looks very much like a graph from how Tiddlywinks ratings are calculated.


http://www.cheng.cam.ac.uk/~pjb10/wi...gs/method.html

Re: Victors Non-linear!!!
 

Ah, I didn't think of that. That makes much more sense that the frequency chaging.
Either way, we have an HP scope in the lab that will tell us duty cycle. Thanks for the heads up.

Re: Victors Non-linear!!!
 

At the extremes? Yes. The duty cycle would be 100% or closer and the meter would easilly be able to read the value. His data does indeed show that motor will get full voltage at the extremes.
However, the change from 0 volts to 12 volts is definitely not linear and more exponential. I think most of us have operated under the assumption that the change was linear.

EDIT: If anyone has taken an electronics course, Jim's graph looks very similar to an RC circuit charging and discharging.

Re: Victors Non-linear!!!
 

Actually, if the Victors will work down to five volts, you could feed the waveform back into a robot controller running suitable software to give you the information you want. This is pretty much what last years IR receiver software did.

-Kevin

Re: Victors Non-linear!!!
 

Kevin,

The both the input and the output are PWM. I was asking about the input.

The output is a PWM whose duty cycle varies from 0 to 100% so it can be easily read with a multimeter.

The input is a RC compatible PWM. See (Servo Basics) for a good explanation. A multimeter will not necessarily give good results. That is why I suggested a scope.

Mike