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 [DPRG] Motor control Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: [DPRG] Motor control Next message: [DPRG] Motor control Subject: [DPRG] Motor control From: Dale Wheat dale at dalewheat.com Date: Sat Jun 7 16:16:00 CDT 2003 ```David, Thanks for the good information about motor control. Thanks also for explaining it to me in agonizing detail last Tuesday. Although I'm chomping at the bit to understand PID motor speed control algorithms (which will be fun, because I barely know what an integral (The I in PID) is and until last Tuesday had no clue what a derivative (The D in PID) was), I have to make sure I completely understand what has been covered so far. As usual, I'd like to go over a few points in some more detail, mostly concerning units of measure. Please correct me if my assumptions are incorrect. The "arbitrary scale" of -100 to +100 in the first method expresses a value of power applied to the motors. This is "open-loop power control" and does not take into account variations in motors/gears/wheels, friction, voltage, slopes, etc. Applying an equal amount of power to both wheels does not guarantee driving in a straight line. The next step involves dealing with the abstract quantites velocity and rotation. Velocity is defined as distance over time. For our purposes, the distance can be measured in shaft encoder counts (which hopefully can be accurately translated into some more traditional linear measurement like feet, inches, furlongs, parsecs, etc.), while the time is broken down into the units between each measurement made. On SR04, for example, the time is 50ms, since measurements are taken 20 times a second. So (linear) velocity is expressed as encoder counts over timer ticks. Rotation (or more accurately rotational velocity) is defined as angular displacement over time. To use the equations presented, where rotation is added or subtracted from the left and right velocities, respectively, the units of measure would of course have to be the same, namely encoder counts over timer ticks. If the wheelbase (the effective distance between the wheels) is expressed in encoder counts, then the angular displacement can also be expressed in radians, using one of the odometry equations, specifically "theta = (right - left) / wheelbase". The rotational velocity can then be expressed as the angular displacement (either in radians or encoder counts) per timer tick. If the observer (me) is looking down on the robot from a height (and until I build a flying/hovering robot, I will be), rotation to the left (counter-clockwise) would be expressed as a positive angle and rotation to the right (clockwise) would be a negative angle. Since turning to the left necessitates slowing down the left motor, or speeding up the right motor (or both), shouldn't the equations be: Left_motor = Velocity - Rotation Right_motor = Velocity + Rotation instead of the other way around? This next step seems like it will make things much simpler when traveling in a straight line (I wish!) or turning in place, but what about describing an arc? It seems to me that it would be easier or at least more accurate to make a turn about a point other than the center of the robot, since the measured distances would be larger and suffer less from round-off error in odometry, or would these benefits be outweighed by the increased cumulative navigational error? It occurs to me that there should be a simple way to express this given the level of abstraction already in place. The exact translation escapes me at the moment. Also, since all my robots seem bound and determined to travel in arcs, anyway, I think it would be wise to "go with your strengths" and find a way to let them do what they do best, and still do what I want. Any thoughts would be appreciated. All you other nav-geeks feel free to pitch in. The last couple of days' discussion on "dead reckoning" have also been enlightening. Thanks, Dale Wheat http://dalewheat.com (972) 486-1317 (800) 330-1915, access code 00 ----- Original Message ----- >From: "David P. Anderson" To: Sent: Saturday, June 07, 2003 11:18 AM Subject: [DPRG] Motor control > > Howdy > > As per our discussion of motor control from last week's > RBNO, I thought I'd try to summarize the scribblings on > the whiteboard and perhaps clarify a couple of points. > (omitted) > Left_motor = Velocity + Rotation > Right_motor = Velocity - Rotation > (omitted) > > Next week, the PID Speed Controller... > > hope this helps! > cheers, > dpa > > _______________________________________________ > DPRGlist mailing list > DPRGlist at dprg.org > http://nimon.ncc.com/mailman/listinfo/dprglist > > ``` Previous message: [DPRG] Motor control Next message: [DPRG] Motor control Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list