Chief Delphi - Encoder for Rpm help!

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Ro13star

02-04-2012 15:09

Encoder for Rpm help!

Hi, i am a new programmer for my team and i am having problems with using the data from the encoder in the program. i have confirmed the encoder works with a multi-meter. the encoder is a us Digital e4p.
this is the relevant part of my code please help:

class RobotOscar : public IterativeRobot
{
// RobotDrive myRobotDrive; // robot drive system

Jaguar shooter_motor;
Jaguar shooter_motor2;
Encoder left_shooter;
double count;

public:
RobotOscar(void):

shooter_motor(3),
shooter_motor2(4),
left_shooter (3,4,false)

...
...
void TeleopInit(void) {
printf("Telop Init Entry\n");
left_shooter.Start();
left_shooter.Reset();
}
...
...
void TeleopContinuous(void)
{

while(IsEnabled() && IsOperatorControl()){
count=left_shooter.Get();

printf("count= %f \n", count);

}
}
also can you use one pwm to a digital imput to get just the A channel from the encoder for rpm only. << is the current set up

Ether

02-04-2012 15:14

Re: Encoder for Rpm help!

Quote:

Originally Posted by Ro13star (Post 1152998)

How did you confirm that the E4P encoder works by using a multi-meter?

Ether

02-04-2012 15:17

Re: Encoder for Rpm help!

Quote:

Originally Posted by Ro13star (Post 1152998)

can you use one pwm to a digital imput to get just the A channel from the encoder for rpm only. << is the current set up

The E4P encoder output is not PWM.

You can use just one channel, but then you would need to use the Counter class, not the Encoder class, to process the signal.

Jay Meldrum

02-04-2012 15:26

Re: Encoder for Rpm help!

If you are using a digital I/O/PWM encoder here is an example of code.

(Note: we use pointers)

Code:



class BuiltinDefaultCode : public IterativeRobot

{

        

        //Declare Encoders

        Encoder *m_Encoder1;



public:



        BuiltinDefaultCode(void)

        {

                //Initialize encoders

                m_Encoder1 = new Encoder(1, 2, true);

                m_Encoder1 -> SetDistancePerPulse(1);

                m_Encoder1 -> SetMaxPeriod(1.0);

                m_Encoder1 -> Start();



                double m_Encoder1Distance;

                double m_Encoder1Rate;



                void TeleopSensorUpdater(void)

                {

                        m_Encoder1Distance = m_Encoder1 -> GetDistance();  //Returns the amount of counts since the last reset

                        m_Encoder1Rate = m_Encoder1 -> GetRate();  //Returns the rate in pulses/second

                }

       }

}

rbmj	12-04-2012 10:53

Re: Encoder for Rpm help!

We do something similar to jsmeldru...

Code:

#include <PIDSource.h>

#include <Counter.h>



class freq_pid : public PIDSource {

private:

   Counter& count;

public:

   freq_pid(Counter& c) : count(c) {}

   double frequency() { return 1/count.GetPeriod(); }

   double PIDGet() { return frequency(); }

};

The nice thing for this is that it lets you use PID with a jag and such.

We take everything by reference since we centralize all of our sensor port data in one file.

Note that that frequency is in hz. If you want rpm you can just change the 1 to a 60.

bob.wolff68

12-04-2012 16:05

Re: Encoder for Rpm help!

This looks nice. Now, my question is how to integrate this with a PIDController and Jaguar to make a speed-based/rpm-based PID Control. We played with modifying PIDOutput() in a super-class of Jaguar such that it 'accumulated' rather than just sending the total_error to the Jag which effectively stops the Jag when you get to an error of zero (hence a positional PID controller as it is today)...

Seems that LOTS of people would want to be able to do this, but I wasn't able to find such a thing this year and so we tried creating our own and didn't have luck.

Anyone have experience here?

bob

Ether

12-04-2012 16:11

Re: Encoder for Rpm help!

Quote:

Originally Posted by bob.wolff68 (Post 1157346)

For speed control of a shooter wheel, try this.

mikets

12-04-2012 16:37

Re: Encoder for Rpm help!

We have created a generic PID controller based on the WPI PID controller that, among other things, can handle speed control.
http://proj.titanrobotics.net/hg/Frc...b/TrcPIDCtrl.h

rbmj	12-04-2012 17:43

Re: Encoder for Rpm help!

It's pretty easy actually. WPILib makes this easy for you.

We actually had two motors, so...

Code:

template<class T>

T coerce(T val, T min, T max) {

    if (val < min) {

        return min;

    }

    else if (val > max) {

        return max;

    }

    else {

        return val;

    }

}



class two_jags : public PIDOutput {

private:

    Jaguar& jag1;

    Jaguar& jag2;

    float speed;

public:

    two_jags(Jaguar& a, Jaguar& b) : jag1(a), jag2(b), speed(0.0) {}

    void PIDWrite(float output) {

        speed = coerce(output + speed, -1.0f, 1.0f);

        jag1.PIDWrite(speed);

        jag2.PIDWrite(speed);

    }

    void reset() {

        speed = 0.0;

    }

};

Some integration code (note that we were working with a counter hooked up to a hall effect sensor detecting a single magnet in the wheel, so one tick was one rotation):

Code:

two_jags jags(jag1, jag2);

freq_pid freq(cnt);

PIDController shooter_ctl(0.005, 0.0, 0.002, freq, jags);



shooter_ctl.SetTolerance(4.0); //accept a 4% steady state error

shooter_ctl.SetInputRange(0.0, 75.0); //our max freq was 75 hz

shooter_ctl.SetOutputRange(-0.25, 0.25); //only ramp a little at a time



//...

void enable_shooters(float freq) {

    jags.reset(); //zero out the accumulator

    shooter_ctl.SetSetpoint(freq);

    shooter_ctl.Enable();

}

You can do more stuff, like adding in a startup burst to get it close and then enable the PID controller to fine tune the speed. We ended up with a quarter second burst of full power before we started the PID controller. Make sure you set the starting speed (two_jags::speed) to your startup speed before, otherwise you'll put the motors back to zero power and defeat the whole purpose of the start speed.

mikets

12-04-2012 18:28

Re: Encoder for Rpm help!

Quote:

Originally Posted by rbmj (Post 1157384)

It's pretty easy actually. WPILib makes this easy for you.

We actually had two motors, so...

Since you are using two motors, you should define the two motors in a sync group so you can synchronize them. BTW, I think you have a typo in the parameter of two_jags:: PIDWrite(float output);

rbmj	13-04-2012 10:04

Re: Encoder for Rpm help!

Quote:

Originally Posted by mikets (Post 1157399)

Since you are using two motors, you should define the two motors in a sync group so you can synchronize them. BTW, I think you have a typo in the parameter of two_jags:: PIDWrite(float output);

Yeah, that (typo) is what happens when you're being lazy :P edited.

And I haven't heard of a sync group...

Anyway, what we had worked very well for us.

Joe Ross

13-04-2012 10:09

Re: Encoder for Rpm help!

Quote:

Originally Posted by mikets (Post 1157399)

Since you are using two motors, you should define the two motors in a sync group so you can synchronize them. BTW, I think you have a typo in the parameter of two_jags:: PIDWrite(float output);

sync groups only apply when using CAN. PWM is always synced.

Ken Streeter

13-04-2012 22:03

Re: Encoder for Rpm help!

Quote:

Originally Posted by Ro13star (Post 1152998)

public:
...
shooter_motor(3),
shooter_motor2(4),
left_shooter (3,4,false)

...

also can you use one pwm to a digital imput to get just the A channel from the encoder for rpm only. << is the current set up

I think that the problem you are having is a side effect of wiring only the A channel. I think that in order to get the encoder to work with just the A channel, you will need to ensure that it is a "1x" encoder. (Note that I've never tried this, we always hook up both the A and B channels, even if we are only using "1x" counting.) To use 1X counting, when you initialize the Encoder, you will need to use a line like the following:

Code:

left_shooter(3,4,false,CounterBase.EncodingType.k1X)

Ether

13-04-2012 22:15

Re: Encoder for Rpm help!

Quote:

Originally Posted by Ken Streeter (Post 1157701)

To get the encoder to work with only one channel connected, you should use the Counter class instead of the Encoder class.

In the Counter class, you can specify either 1x or 2x decoding. I think it works this way: With 1x, it counts rising edges only. With 2x, it counts both rising and falling edges.

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