Subject:
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Re: Motor power? (LONG! with code example)
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 18 Feb 1999 04:55:04 GMT
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Viewed:
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928 times
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(My apologies if this is a duplicate. I tried to send it to the lugnet news
server, but that derver doesn't seem to like HTML in news message)
Y'all,
Yesterday, I asked how to use motor power to control the motor's speed. The
answer that I got was that the RCX firmware uses an internal PWM (pulse
width modulation) technique to control power and possibly speed. I observed
that I didn't see much change in motor speed even under load when I played
with motor power.
So I created a little program with NQC (thanks Dave Baum!) that illustrates
my concept. However, to do it effectively, I need to pulse between motor ON
and motor OFF. If I cycle betweem motor ON and motor FLOAT, I see basically
what I get with RCX.
With a higher speed execution environment such as legOS, the pulsing that
you see with RCX shouldn't even be noticeable. legOS provides a 1 ms
interrupt rate. Assuming that the motor runs at 6000 RPM, that is 100
rotations per second, or 10 interrupts (to change motor power/speed) every
rotation.
Enjoy.
Cheers,
danm
---------- Cut here ---------------------------------------
// if MY_PWM is defined, my own PWM motor control is used
// otherwise, use standard RCX motor power control
#define MY_PWM
int powA; // power setting for motor A
int powC; // power setting for motor C
// move robot in an ess shaped curve by varying the power to motors
// motors are controller with PWM (pulse width modulation)
//
// -
// / \
// | |
// / \
// - - -
// \ /
// | |
// \ /
// -
// |---|---|---|---|
// | | | |
// | | | |
// | | | -> Ramp A down and up
// | | -----> Ramp C down and up
// | ---------> Ramp C down and up
// -------------> Ramp A down and up
//
task main
{
Fwd(OUT_A + OUT_C, OUT_FULL);
// start with some power settings
powA = OUT_FULL;
powC = OUT_FULL;
#ifdef MY_PWM
// start motor PWM task
start Motor;
#endif
// for always, ...
while (true) {
// ramp motor A down to 0
while (powA != 0) {
#ifndef MY_PWM
Fwd(OUT_A, powA);
#endif
Sleep(100);
powA -= 1;
}
// ramp motor A up to OUT_FULL
while (powA != OUT_FULL) {
#ifndef MY_PWM
Fwd(OUT_A, powA);
#endif
Sleep(100);
powA += 1;
}
// ramp motor C down to 0
while (powC != 0) {
#ifndef MY_PWM
Fwd(OUT_C, powC);
#endif
Sleep(100);
powC -= 1;
}
// ramp motor C up to OUT_FULL
while (powC != OUT_FULL) {
#ifndef MY_PWM
Fwd(OUT_C, powC);
#endif
Sleep(100);
powC += 1;
}
// ramp motor C down to 0
while (powC != 0) {
#ifndef MY_PWM
Fwd(OUT_C, powC);
#endif
Sleep(100);
powC -= 1;
}
// ramp motor C up to OUT_FULL
while (powC != OUT_FULL) {
#ifndef MY_PWM
Fwd(OUT_C, powC);
#endif
Sleep(100);
powC += 1;
}
// ramp motor A down to 0
while (powA != 0) {
#ifndef MY_PWM
Fwd(OUT_A, powA);
#endif
Sleep(100);
powA -= 1;
}
// ramp motor A up to OUT_FULL
while (powA != OUT_FULL) {
#ifndef MY_PWM
Fwd(OUT_A, powA);
#endif
Sleep(100);
powA += 1;
}
}
}
#ifdef MY_PWM
// variables for motor PWM task
int powCur; // current power setting for motor task
int accumA; // power accumulator for motor A in motor task
int accumC; // power accumulator for motor C in motor task
// Motor task
// This task works by modulating the motor power by the ratio
// power / powerMax
// This ratio is computed by successively subtracting
// the numerator (power) from an accumulator. When the accumulator is
// positive, the motor is turned off. When the accumulator underflows
// (becomes negative), the denominator (powerMax) is added to the
// accumulator and the motor is turned on. When this happens very
// rapidly, the motor appears to be moving at the appropriate fraction
// of full speed.
//
// Due to the limited speed of RCX code, you can actually see the motor
// speed pulse or throb. Running on a faster system (such as legOS),
// no pulsing should be perceivable.
task Motor
{
// initialize task variables
accumA = 0;
accumC = 0;
while (true) {
// motor A is current motor
powCur = powA;
// if motor A is supposed to go forwards, ...
if (powCur > 0) {
accumA -= powCur;
OutputDir(OUT_A, OUT_FWD);
}
else { // motor A goes backwards
accumA += powCur;
OutputDir(OUT_A, OUT_REV);
}
// if accumulator A underflows, ...
if (accumA < 0) {
accumA += OUT_FULL;
OutputMode(OUT_A, OUT_ON);
}
else {
OutputMode(OUT_A, OUT_OFF);
}
// motor C is current motor
powCur = powC;
// if motor C is supposed to go forwards, ...
if (powCur > 0) {
accumC -= powCur;
OutputDir(OUT_C, OUT_FWD);
}
else { // make motor C go backwards
accumC += powCur;
OutputDir(OUT_C, OUT_REV);
}
// if accumulator C undeflows, ...
if (accumC < 0) {
accumC += OUT_FULL;
OutputMode(OUT_C, OUT_ON);
}
else {
OutputMode(OUT_C, OUT_OFF);
}
}
}
#endif
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Message has 2 Replies:
 | | Storage for RCX - and other Technic
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| I have just updated my website and added a new page outlining PLANO Tackle boxes and how to modify them for that few Technic parts that just won't fit. Have a look at <www.bmts.com/~rhempel> and follow the clearly marked path on the left to the (...) (25 years ago, 18-Feb-99, to lugnet.robotics, lugnet.storage)
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Message is in Reply To:
| | Motor power?
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| When I set the motor power (both in RIS and nqc), I expected the speed of the motor to be proportional to the power setting (i.e., higher power setting makes motor turn faster and vice versa). However, what I see is that the power setting appears to (...) (25 years ago, 17-Feb-99, to lugnet.robotics)
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