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Re: Meaning of "speed" for Differential Motor Controller?Than

10/15/2007 2:22:29 AM

rpg

35 posts

Re: Meaning of "speed" for Differential Motor Controller?Than

Hi Jason,

This is great news--and very timely too since I justed got my quad encoders working with the BaneBots motors. I'll wait for the official release to try it out.

In the meantime, I'm wondering if you could answer a question about the PID calibration. I've read through the instructions about setting up the parameters but I'm not quite getting it. I'm hoping you can give me a sample set of sensible numbers if I give you my wheel dimensions. They are:

WheelDiameter: 7.0 inches
WheelTrack: 12.6 inches
EncoderResolution = TicksPerRevolution = 44
GearReduction = 1.0 (since I don't have any external gears between the axle and the wheel--the internal ratio is 100:1).
RPM (max measured): 110 RPM

Where I'm getting stuck is on the VelLoop and VelocityDivider. I tried the test suggested in the docs using HyperTerminal. I set VPID to 10:0:0:100, then got the wheel going at max (pwm 1:127 because of the firmware bug) and then did a vel command and got back only 9. So I went to the max value of L (255 it looks like) and still got back only 25. The documentation recommends a value between 50-250 so I'm wondering if I can proceed with a number as low as 25.

One other question: I was playing around with the Rotate and TravelAtSpeed commands and it occurred to me: what if I want the bot to make a turn as it is moving? In other words, suppose the robot is going along in a straight line at 75% speed and wants to turn 20 degrees to the right while maintaining speed? Is there a way to do this? It seems that the Rotate command causes forward motion to stop. Though I'm testing all of this with the wheels off the ground because I don't have my wireless setup in place yet so apologies if I'm missing something obvious.

Many thanks!
patrick

10/16/2007 9:05:36 PM

ringo

36 posts

Re: Meaning of "speed" for Differential Motor Controller?Than

110 RPM is 1.83 RPS. At 44 ticks per revolution that is about 81 ticks per second.

With an L of 100 that is an interrupt every 160ms, or about 6 time per second. So every interrupt you would see a max of 44/6 or 7.3 ticks per interrupt (which is what vel returns).

So you are doing everything correctly, the problem is that you need more encoder counts.

To do the turn you would need to set the speed of each wheel independently of each other. the Serializer does not handle turns like that for you, you will have to do the calculations to determine the correct speed settings.

Hope this helps,

Ringo

Ringo Davis, Hardware Technical Lead RoboticsConnection.com

10/17/2007 11:09:34 AM

rpg

35 posts

Re: Meaning of "speed" for Differential Motor Controller?Than

Thanks Ringo. Yes that helps a lot. Unfortunately, one of the encoders doesn't seem to be working correctly (only one of the LEDs flashes and the vel command returns 0 on that channel) so I'll have to wait for the replacement to arrive before continuing to experiment.

--patrick

10/17/2007 1:25:52 PM

jywilson

35 posts

Re: Meaning of "speed" for Differential Motor Controller?Than

Patrick, Make sure that you check the distance between the encoder and the disk. If the distance is not correct, the LED's will not flash.

James Y. Wilson
http://www.learningce.com
Fortudine Vincimus!

10/18/2007 10:15:32 AM

rpg

35 posts

Re: Meaning of "speed" for Differential Motor Controller?Than

Hi Jason,

Yes, I played with the gap distance extensively. Here is the full story: I am using your Quadrature Encoder Combo kit but since I got them through Trossen Robotics, I absolve you of all responsibility :-). Anyway, one of the encoders came with all 10 LED pins un-soldered to the PCB. So I soldered the pins and that encoder works great--both LEDs flicker as they should. The second encoder was fully soldered, but one of the LEDs was permanently on but at a dim output level. No matter what gap I chose between the LED and encoder wheel, it wouldn't blink, even though the other LED blinked fine. So I resoldered the pins on the faulty LED and, lo, the LED brightened, but it still wouldn't flicker. To save time, I simply ordered a new QEC kit from you guys directly and I'm guessing it'll show up to day or tomorrow.

--patrick

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