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Motor "Responsiveness" 
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Joined: Fri Feb 09, 2007 9:21 am
Posts: 616
Post Motor "Responsiveness"
Several recent posts have asked questions about the responsiveness of both the NXT and 12V motors. You may find the attached post in a different forum useful

NOTE: I recall using a 7.2V VEX battery to control the HiTechnic controller and 12V motors. I did this for the measurements and wanted to use approximately same battery voltage for all three motor types (NXT, VEX and TETRIX 12V).

Tue Dec 15, 2009 10:27 pm

Joined: Sun Nov 16, 2008 3:07 pm
Posts: 45
Post Re: Motor "Responsiveness"
Without knowing the moment of inertia of the load, it seems premature to be shocked, but offhand, those numbers seem horrible, even for a toy.

Was that just the Tetrix motor and gearbox with nothing attached?

Have you looked to see how long from the program commanding a stop to the message being transmitted on I2C or a change on the output of the Motor Controller? (that would factor out the mechanical issues).

Thu Dec 17, 2009 9:13 pm

Joined: Fri Feb 09, 2007 9:21 am
Posts: 616
Post Re: Motor "Responsiveness"
All measurements were done with unloaded motors.

I've mentioned previously the HiTechnic TETRIX motor controllers have a "speed ramping" function embedded in their (non-modifiable) firmware. I believe it adjusts speed a maximum of 10 "steps" per clock "tick". So:
  • Starting with speed 100 and the motor is commanded to go to speed -100.
  • On the next clock tick the speed is set to 90.
  • On the next clock tick the speed is adjusted to 80.
  • And so on until the motor speed gets to -100.
So it will take 10 clock "ticks" simply to get the speed to zero. And another 10 ticks to reach the final speed of -100.

Clock "ticks" are the internal processing in the HiTechnic controller. The firmware in the controller recognizes a speed change on a periodic basis which is either 25 or 50 milliseconds. I can't remember which is used.

I believe this is the root cause of the slow response from the 12V TETRIX motors.

Fri Dec 18, 2009 12:14 pm

Joined: Mon Nov 03, 2008 1:45 pm
Posts: 14
Post Re: Motor "Responsiveness"
Last year we did some experiments and made a motor function that checks whether the user has done a reverse direction.
If so, the function would stop the motor, wait 20 msec then start it in the opposite direction.
That seemed to give us a much more responsive system.

David Burnette
Tigard High School Robotics

Tue Feb 16, 2010 7:56 pm
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