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Encoder Question 
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Rookie

Joined: Fri Jan 02, 2009 9:13 pm
Posts: 10
Post Encoder Question
We only have one encoder (the other broke, it will be warrantied, but not in time for when we need it) and we have the working one attached to one motor. There is another motor powering the same mechanism, and we would like to use PID control. Is there any way to use the encoder value from one motor one another motor?

tl;dr version: is there a method of finding the PID value of a motor?


Fri Jan 02, 2009 9:16 pm
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Professor

Joined: Fri Sep 19, 2008 1:22 am
Posts: 200
Post Re: Encoder Question
In our experiance you need to turn off PID control for both motors if only one of them has an encoder. There are algorithms available on the net to do the PID control manually but I suspect that you won't need them for the purposes of this competition. I suggest trying it without and not spending the short time remaining to you fighting it.

Note: You can still use the encoder to count rotations even if PID control is OFF.

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Jeff McBride
Benson Robotics Club


Fri Jan 02, 2009 10:54 pm
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Post Re: Encoder Question
We have been doing without so far, but it is very difficult because the arm the motors are attached to have a lot of weight on them. I can see the number I need right on the debug window, is there really no way to access it in my program?


Sat Jan 03, 2009 1:13 am
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Professor

Joined: Fri Sep 19, 2008 1:22 am
Posts: 200
Post Re: Encoder Question
Hmm. Anything you can see in the debugger should be accessible to you somehow at runtime. What value are you looking at in which debug window?

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Jeff McBride
Benson Robotics Club


Sat Jan 03, 2009 1:56 am
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Post Re: Encoder Question
The circled one.

Thanks for all your help so far!


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Sat Jan 03, 2009 2:07 am
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Rookie

Joined: Fri Jan 02, 2009 9:13 pm
Posts: 10
Post Re: Encoder Question
So I guess its not possible, then?


Mon Jan 12, 2009 9:40 pm
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Novice

Joined: Mon Oct 13, 2008 6:29 pm
Posts: 75
Location: Florida, USA
Post Re: Encoder Question
try "pm"ing tfriez or Dick Swan and please post the reply here, i would like to know too.

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Tue Jan 13, 2009 2:19 pm
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Joined: Fri Feb 09, 2007 9:21 am
Posts: 615
Post Re: Encoder Question
Youcannot use the encoder value from one motor to control a second.

FOr the LEGO NXT motors, the PID control is implemented in the ROBOTC firmware code. And the values in the debugger are the valid calculation of the values.

The initial implementation of the 12V motors simply enabled the existing ROBOTC firmware (i.e. in the NXT CPU) to control the 12V values. Unfortunately, this did not work as well as expected. There was too much latency in the control loop partially due to I2C messaging delays but mostly due to delays within the HiTechnic controller firmware.

Fortunately, the firmware in the HiTechnic motor control also implements a PID algorithm. It's not as "good" (i.e. less feature rich) as the implementation in the NXT firmware -- for example it does not have the capability to synchronize two motors for driving straight. So the ROBOTC firmware PID control is disabled for the 12V motors and the PID control in the HiTechnic motor control is used.

For the 12V motors, the ROBOTC debugging field in the motor display that shows "PID setting" is not valid. It is the value that the ROBOTC firmware would use if it's PID code was enabled.


Wed Jan 14, 2009 8:31 am
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