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Cascade loop


Amar
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I am trying to implement cascade loop with analog feedback in PPMAC.

Resolution of the analog feedback is +/-10 volts mapped into -32767 to 32767 counts.

I am giving this signal to a virtual axis through encoder conversion table.

I am closing the virtual loop by giving command #14j=32767. I want to make error zero. Somewhere i am making mistake. After some time Motor[14].IqCmd is reaching at 32767 and it will stop motion of inner loop if still error is there.

Please suggest me if i am making any mistake.

 

Below is the setting for encoder table, outer loop motor and gearing with inner loop.

 

ptr ainError->Acc28E[4].AdcSdata[3]

 

Sys.Idata[1501] = ainError + 32767; //(Error will be +/- 32767)

 

//Encoder table settings

EncTable[12].type = 1

EncTable[12].pEnc = Sys.Idata[1501].a

EncTable[12].pEnc1 = Sys.pushm //Sys.Idata[1501].a

EncTable[12].index1 = 0

EncTable[12].index2 = 0

EncTable[12].index3 = 0

EncTable[12].index4 = 0

EncTable[12].index5 = 0

EncTable[12].ScaleFactor = 1

 

EncTable[14].type = 1

EncTable[14].pEnc = Sys.Idata[3001].a

EncTable[14].pEnc1 = Sys.pushm

EncTable[14].index1 = 0

EncTable[14].index2 = 0

EncTable[14].index3 = 0

EncTable[14].index4 = 0

EncTable[14].index5 = 0

EncTable[14].ScaleFactor = 1/65536

 

//Cascade motor setup

Motor[14].pDac = Sys.Idata[3001].a

Motor[14].pEnc = EncTable[12].a

Motor[14].pEnc2 = EncTable[12].a

Motor[14].Servo.Kvfb = 0

Motor[14].Servo.Kp = 0.1

Motor[14].Servo.Ki = 0.0001

Motor[14].Servo.BreakPosErr = 0

Motor[14].Servo.Kvff = 0

Motor[14].Servo.Kaff = 0

Motor[14].pAmpEnable = 0

Motor[14].FatalFeLimit = 0

Motor[14].PosSf = 1

Motor[14].Pos2Sf = 1

Motor[14].AbortTa = 256

Motor[14].InvAmax = 256

Motor[14].JogTa = 256

Motor[14].JogTs = 32

Motor[14].JogSpeed = 16384

Motor[14].PwmSf = 1

Motor[14].MaxDac=32767

Motor[14].InPosBand = 100

Motor[14].FatalFeLimit = 32000

Motor[14].WarnFeLimit = 16000

Motor[14].pAmpFault=0

Motor[14].pLimits=0

 

//PLC for controlling inner loop using gearing

 

open plc0

Motor[1].pMasterEnc = EncTable[14].a

Motor[1].MasterPosSf = 1

Motor[1].MasterCtrl = 1

 

CMD "#14j=32767"

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You know that the PLC (0 in this case) keeps on executing over and over again... At RTI rate. That is NOT good. I would never write a PLC with motion until I have full control over the system. You do NOT need a PLC for this function. You can turn it on/off with MasterCtrl.

 

Do you need to do cascaded loop or just electronic gearing? What is it physically that you are trying to achieve? What does the input represent? Typically, force, height, or tension work better with integrating the cascaded motor output in the ECT.

 

You do need to make sure that your cascaded motor output direction matches your physical motor direction or else you are creating a runway condition.

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You know that the PLC (0 in this case) keeps on executing over and over again... At RTI rate. That is NOT good. I would never write a PLC with motion until I have full control over the system. You do NOT need a PLC for this function. You can turn it on/off with MasterCtrl.

 

Do you need to do cascaded loop or just electronic gearing? What is it physically that you are trying to achieve? What does the input represent? Typically, force, height, or tension work better with integrating the cascaded motor output in the ECT.

 

You do need to make sure that your cascaded motor output direction matches your physical motor direction or else you are creating a runway condition.

 

Thanks..

Here i haven't described whole plc0. In plc0 i am taking care of, it will execute once and other task will execute over and over again.

We are using same scenario in UMAC for Auto Mode in Antenna system for satellite tracking. Error will generate at continuous rate from integrated tracking receiver. I want to know what i missing in PPMAC. Why DAC is reaching at maxdac value? Till reaching at maxdac value of outer loop inner loop motor is following correctly. once outer loop dac output reach at maxdac value it will stop motion.

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Is the following error building up? Does it look like it is speed dependent? If it saturates faster with higher speed, this could mean that your gains are not suitable. Once you have control over both loops, you should be able to perform tuning moves on the cascaded (outer loop) motor to see the response.

 

On a side note, usually best results with cascaded loops is obtained by integrating the virtual motor output in the ECT (using index4, typically =1) before feeding it into the master position of the inner loop. If you apply this, tuning gains will change. Make sure you have a reasonable fatal following error limit set on the inner loop.

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Hi Richard,

I am Kaustubh, working along with Amar.

I agree that it could be the gains who are creating a problem with cascade loop.

To verify the same we created a user written servo implemented through either PLC0 or through the user written algorithm.

We used the template given in the PowerPMAC user manual page no. 763.

 

The template is as shown below -

 

double user_pid_ctrl(struct MotorData *Mptr)

{

double ctrl_out;

if (Mptr->ClosedLoop) {

// Compute PD terms

ctrl_out=Mptr->Servo.Kp*Mptr->PosError-Mptr->Servo.Kvfb*Mptr->ActVel;

Mptr->Servo.Integrator+=Mptr->PosError*Mptr->Servo.Ki; // I term

ctrl_out+=Mptr->Servo.Integrator; // Combine

return ctrl_out;

}

else {

Mptr->Servo.Integrator=0.0;

return 0.0;

}

 

 

 

But, here the error we are talking about is coming from an analog source. It is not coming from a feedback device like encoder for e.g., so the error doesn't mean following error.

 

Hence, what i am doing right now is subtracting the error value read at the last RTI from the present error value so as to get the value equivalent to following error. This value i m feeding to the servo.integrator.

Now, i m getting a proper servo output with integration, it is not saturating.

 

But, still the system oscillates maybe due to the integral error value i m putting.

Slight change is making it unstable.

 

My question now is that according to the template the first line ctrl_out calculates the servo out based on Kp, should i be doing the same thing to generate "following error" kind of error to be multiplied with Kp to generate ctrl_out as well in the first place before integration?

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Click on your user name. Upper right hand side, click on private messages

 

When i click on private messages it says -

You cannot use the private messaging functionality as it has been disabled by the Administrator.

 

Can you email me?

kaustubh@intelmotion.com or

kaustubh.hunswadkar@gmail.com

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