Finding RPMs with encoder

[Ether]

Originally Posted by Sparx030

Correct me if I'm wrong but would the 480 counts be for 4x, 240 for 2x, and 120 for 1x?

That's my understanding of the WPILib code. But I'm not a WPILib guru.

Also, I have been told that 4x deciding is needlessly complicated for what I am doing (maintain RPM on a shooter with a PID) would you agree with that and if so what do you recommend?

To answer that, I need more information.

What make and model encoder are you planning to use?

Where do you intend to mount the encoder?

How fast will the encoder be spinning at your shooter operating point?

Is the motor output shaft directly connected to the wheel? If not, describe the transmission.

What motor and motor controller are you planning to use?

Will you be connecting the encoder to the RIO, or to the motor controller (if SRX)?

[Sparx030]

Where do you intend to mount the encoder

On the shooter wheel

How fast will the encoder be spinning at your shooter operating point?

This may change but let's just say 3000 RPM

Is the motor output shaft directly connected to the wheel? If not, describe the transmission.

It will be directly connected

What motor and motor controller are you planning to use?

AndyMark RS775-5 with a Victor controller

Will you be connecting the encoder to the RIO, or to the motor controller (if SRX)?

The RIO

Also, if possible, would you be able to provide a brief generalization of what each decoding type is used for?

[Ether]

Originally Posted by Sparx030

On the shooter wheel

This may change but let's just say 3000 RPM

It will be directly connected

AndyMark RS775-5 with a Victor controller

The RIO

Also, if possible, would you be able to provide a brief generalization of what each decoding type is used for?

Not that it matters that much, but Victor SP or 888 ??

And did you consider using bang-bang instead of PID? This is the perfect application for it. And there's no tuning required.

A 250 CPR encoder spinning at 3000 RPM is easily decoded by the roboRIO at 4X or 2X or 1X. It samples for edges much faster than the cRIO did, which removes many of the considerations which came into play in the cRIO days with high-RPM high-CPR encoder use. But, there are still some things you need to know to be successful.

I will expound on that and answer the rest of your questions in a few minutes.

EDIT: Oh, and what is the period of your control loop (e.g. 20ms, 10ms, etc).

[Sparx030]

Originally Posted by Ether

Not that it matters that much, but Victor SP or 888 ??

They are Victor 888.

And did you consider using bang-bang instead of PID? This is the perfect application for it. And there's no tuning required.

I have heard of it and got it to work very well but a PID might be used for other robot systems that I can't test right now and while those would be very different from this shooter I would still like to be comfortable with PIDs.

[Ether]

What is the period of your control loop (e.g. 20ms, 10ms, etc)?

[Sparx030]

Originally Posted by Ether

What is the period of your control loop (e.g. 20ms, 10ms, etc)?

20ms

[Ether]

Originally Posted by Sparx030

20ms

OK, doing the calculations for 1X decoding:

rpm=3000; CPR=250; X=1; secondsPerControlLoop=0.020;

edgesPerControlLoop: rpm * (1/60) * (X*CPR) * secondsPerControlLoop = 250

So 250 edges fly by every 20ms, which should be plenty enough to get a nice accurate and clean speed signal at 1X decoding. I recommend setting the FPGA sample size to something greater than the default value of "1"... say maybe 64.

If you like your code to be super efficient you can compute the edge period time once outside your control loop:

secondsPerEdge: float((1/rpm) * 60 * 1/(X*CPR)) = 80e-6

... and use that as the setpoint in your PID to compare to the getPeriod() process variable which returns the edge period in seconds.


When using PID to control shooter wheel speed there are some additional considerations to be aware of in order to be successful. Things like do you want to apply dynamic braking or reverse motor commands when wheel speed exceeds setpoint. I will leave it to shooter-wheel-PID gurus to fill in the details and recommendations.

You may be able to get better speed regulation at 10ms.

[Sparx030]

alright, thanks for the help. I don't have means for testing anything right now but all of this was very helpful.

[Sparx030]

Originally Posted by Sparx030

alright, thanks for the help. I don't have means for testing anything right now but all of this was very helpful.

Update: everything is working perfectly, thanks again for the help.

[Ether]

Originally Posted by Sparx030

Update: everything is working perfectly

Not to be too pedantic, but what do you mean by "working perfectly"?

Could I talk you into posting a trace of your wheel speed vs time?

Others can maybe learn from it.

Thanks.

[Sparx030]

Originally Posted by Ether

Not to be too pedantic, but what do you mean by "working perfectly"?

Could I talk you into posting a trace of your wheel speed vs time?

Others can maybe learn from it.

Thanks.

I managed to get the encoder to show me values in RPM as well as allow inputs to be in RPMs, my big issue was not getting values I wanted as well as not knowing what stood for what in the code. so my result was this...

        RPM = 5000.0;

    	eShooter = new Encoder(1, 2, true, EncodingType.k1X);
    	eShooter.setDistancePerPulse(3);
    	eShooter.setPIDSourceType(PIDSourceType.kRate);
    	pidShooter = new PIDController(0, 0, 0, eShooter, mShooter);

        pidShooter.setSetpoint(((RPM/60)*360)/3);

        SmartDashboard.putNumber("Motor RPM ", ((eShooter.getRate()*3)/360)*60);

So to explain things best I can (because this is the actual code for this and most of what I wrote from this thread is on a piece of paper that would be less easy for me to explain) this allows you to put a desired RPM to set the wheel to that speed as well as tell you on the computer (SmartDashboard) what RPM it is moving at.

        RPM = 5000.0;

    	eShooter = new Encoder(1, 2, true, EncodingType.k1X);
    	eShooter.setDistancePerPulse(3);
    	eShooter.setPIDSourceType(PIDSourceType.kRate);
    	pidShooter = new PIDController(0, 0, 0, eShooter, mShooter);

This has 'RPM = 5000;', what RPM you want the wheel to move at. 'eShooter = new' is creating the Encoder in the code with 1x decoding (EncodingType.k1X). 'setDistancePerPulse(3);' is the distance in degrees the wheel will move per pulse which with 1x decoding and a 120cpr encoder will be 1 pulse (or count) per 3 degrees. setPIDSourceType is to make the encoder read the rate as opposed to distance. 'pidShooter = new' creates a speed controller with P, I, and D variables and the encoder and the motor it is powering (this also has no values for P, I, or D these would have something in place to properly tune it).

That might have been lengthy and possibly unimportant for those who might look to this for learning about the RPM calculations but the 'setDistancePerPulse' was a big trouble of mine when trying to find what to put in there and is important to the code. 3 would not be the distance per pulse if this was not 1x decoding.

        pidShooter.setSetpoint(((RPM/60)*360)/3);

        SmartDashboard.putNumber("Motor RPM ", ((eShooter.getRate()*3)/360)*60);

setSetpoint wants an input in counts per second. To turn counts into pulses (with 1x decoding) will need you to turn desired RPM into RPS (into second) with 'RPM/60'(RPM divided by 60) then turn it from revolutions to degrees '*360'(multiply by 360) and finally degrees per second to counts per second '/3'(divide by 3).

now for what the encoder tells you. getRate() in also in counts per second so it's the exact opposite. the rate in counts per second multiplied by 3 to turn it into degrees, then divide by 360 to turn it into RPS(revolutions per second), and finally multiply by 60 to get RPMs.

This might have been a bad explanation or included too much rambling and not enough detail, especially with be just putting in the exact code, if that is the case I'm sorry I'm not much of a teacher, so if you want me to include more specific details let me know and I will say what I did, how I did it, and what value everything (related to this thread) intakes and outtakes.