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Problems with the motor encoder

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Problems with the motor encoder

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Koldo

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Joined: Sun Aug 12, 2007 3:18 pm
Posts: 38

Problems with the motor encoder

Hi

I want to develop a program that moves the motor to diverse positions in forward and backward. I do not have problems when it is in forward but yes in backward. I am trying to correct the difference when the nEncodertarget is <0 , but I have some problems with the encoders.

In he program: when the nEncoderTarget is -200, the result is Tach User is -281 but nMotorEncoder [motor] read -200 (it isn’t the real position), so I can’t correct the diference.

Witch tach read nMotorEncoder [motor]?

Thanks


	Code: //!!Motor, motorC, motor__, tmotorNxtEncoderClosedLoop, !!// //!! !!// //!!Start automatically generated configuration code. !!// const tMotor motor__ = (tMotor) motorC; //tmotorNxtEncoderClosedLoop //!!!!// //!!CLICK to edit 'wizard' created sensor & motor configuration. !!// task main() { int position,difference; nMotorEncoder[motor__]=0; nMotorEncoderTarget[motor__]=-200; motor[motorC]=-100; while(nMotorRunState[motor__] !=runStateIdle); eraseDisplay(); position = nMotorEncoder[motor__]; //nMotorEncoder[motor__]=0; The result is the same with this line and wihtout it nxtDisplayString(4, "%d",position ); wait10Msec(200); nMotorEncoderTarget[motor__]=200; motor[motorC]=80; while(nMotorRunState[motor__] !=runStateIdle); eraseDisplay(); position = nMotorEncoder[motor__]; nxtDisplayString(4, "%d",position ); wait10Msec(500); }

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Fri Jul 04, 2008 5:02 am

Abbeyjean

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Joined: Thu Jun 26, 2008 11:27 am
Posts: 21
Location: Johannesburg, South Africa

Hi

If you change the line:
nMotorEncoderTarget[motor__]=-200;
to be
nMotorEncoderTarget[motor__]= 200; (no negative sign)
you will get a more accurate result. Setting the nMotorEncoderTarget to a negative value means the motor will coast or float rather than braking after it reaches the desired target.

Sat Jul 05, 2008 4:42 am

elemes

Expert

Joined: Fri Nov 09, 2007 4:51 am
Posts: 121
Location: Hungary, Europe


	Abbeyjean wrote: Setting the nMotorEncoderTarget to a negative value means the motor will coast or float rather than braking after it reaches the desired target.

wow, how did you find it out?

is that a bug or a feature?

Sat Jul 05, 2008 7:48 am

Abbeyjean

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Joined: Thu Jun 26, 2008 11:27 am
Posts: 21
Location: Johannesburg, South Africa

It's a feature I guess as it is mentioned in the documentation.

Sat Jul 05, 2008 1:23 pm

Ford Prefect

Senior Roboticist

Joined: Sat Mar 01, 2008 12:52 pm
Posts: 936
Location: a small planet in the vicinity of Beteigeuze

Actually, to my opinion, this "feature" is a big fat BUG ! :evil:

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Sat Jul 05, 2008 1:44 pm

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Joined: Mon Oct 29, 2007 4:25 pm
Posts: 7
Location: Potsdam, Germany


	Ford Prefect wrote: Actually, to my opinion, this "feature" is a big fat BUG !

I knew that bugs simply are undocumented features.
Now, documented features turn into bugs ;-)

?

I stopped to judge the motor control by my expectation as how motor control should work. For my application, precise position control is required. Initially, I expected the nMotorEncoderTarget being of use for that. However, even after finding out that these values are not absolute targets (encoder positions that the PID tries to reach) but count from the current encoder position, there still is the drift due to overshoot.

Still, the nMotorEncoder value is my friend, based on which a simple PD position control can be implemented (works fine for me and has been posted if I remember correctly).

MK

Sat Jul 05, 2008 5:03 pm

starwarslegokid

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Joined: Wed Jan 31, 2007 3:39 am
Posts: 298
Location: San Diego, California. USA


	Quote: It's a feature I guess as it is mentioned in the documentation

where in the documentation is this stated?

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Fri Jul 11, 2008 12:05 am

Abbeyjean

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Joined: Thu Jun 26, 2008 11:27 am
Posts: 21
Location: Johannesburg, South Africa

Hi

It is in the section 'Motors Overview' under 'Moving to a Specific Encoder Value'


	Quote: You can move a motor a specific amount with the variable nMotorEncoderTarget[]. This indicates the number of encoder ticks to move. If you specify a positive value the motor will slow to a stop at this position. A negative value will leave the motor in coast / float mode when the encoder position is reached.

Fri Jul 11, 2008 3:47 am

Koldo

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Joined: Sun Aug 12, 2007 3:18 pm
Posts: 38

My last solution:


	Code: if (Turngrades> 0) { nMotorEncoderTarget[motorA] = Turngrades; motor[motorA] = 100; while (nMotorRunState[motorA] != runStateIdle); } else { bMotorReflected[motorA] = true; nMotorEncoder[motorA]=0; nMotorEncoderTarget[motorA] = abs(Turngrades); motor[motorA] = 100; while (nMotorRunState[motorA] != runStateIdle); bMotorReflected[motorA] = false; }

With "bMotorReflected[motorA] = true" the encodertarget always is positive. It runs fine.

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Fri Jul 11, 2008 5:55 pm

Abbeyjean

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Joined: Thu Jun 26, 2008 11:27 am
Posts: 21
Location: Johannesburg, South Africa

Hi

You could cut this code down to 3 lines if you wanted.


	Code: nMotorEncoderTarget[motorA] = abs(Turngrades); motor[motorA] = sgn(Turngrades) * 100; while (nMotorRunState[motorA] != runStateIdle);

Sat Jul 12, 2008 3:02 am

Ford Prefect

Senior Roboticist

Joined: Sat Mar 01, 2008 12:52 pm
Posts: 936
Location: a small planet in the vicinity of Beteigeuze

or forget about the nMotorEncoderTarget and runstates etc and do it this way, simple and stupid:


	Code: int target; int speed=100; while (target!=nMotorEncoder[motorA]) { motor[motorA] = sgn( target-nMotorEncoder[motorA])*speed; } else motor[motorA]=0;

maybe you wish to make the motor a little slower if is it's getting close to the target, you can add to the while loop:


	Code: if (abs(target-nMotorEncoder[motorA])<10) speed=30; // or sth like that

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Ford Prefect

Never purchase release 1.x ! (ancient programmer's wisdom)
"Don't argue with idiots. They'll drag you down to their level and then beat you with experience."

Sat Jul 12, 2008 5:11 am

Koldo

Rookie

Joined: Sun Aug 12, 2007 3:18 pm
Posts: 38

Thanks

it runs fine

Abbeyjean wrote:

Hi

You could cut this code down to 3 lines if you wanted.


	Code: nMotorEncoderTarget[motorA] = abs(Turngrades); motor[motorA] = sgn(Turngrades) * 100; while (nMotorRunState[motorA] != runStateIdle);

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Sat Jul 12, 2008 11:29 am

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