Subject:
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Servo Motor Control
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 27 Dec 2004 01:39:28 GMT
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Viewed:
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971 times
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"danny" <daniele_benedettelli@yahoo.it> wrote in message
news:I9BszE.1KF0@lugnet.com...
> In lugnet.robotics, Dick Swan wrote:
> > First "official" version of Mindstorm replacement firmware can now be
> > <<snip>>.
>
> With new timer resolution and fine motor power control, will RCX
> control SERVO
> MOTORS with precision?
> I tried to learn PBforth, but I prefer NQC.
> It would be great!
No. It doesn't appear that it will be possible to directly control a
servo.
Servos are controlled by a single variable width pulse repetitively sent
about every 20 milliseconds. The neutral position is usually 1-msec on
the popular servos. The servo measures the difference in pulse width
from the neutral value. It uses this "difference" value to calculate the
servo position. A quick web search indicates that the usual control
range is from 1 to 2 milliseconds. Without some pretty specific
interrupt support this is simply not possible on the RCX.
I suspose you could build a (simple?) circuit that averaged the motor
output over a time period; 16 msec would give you 32 steps if the PWM
was configured at the 0.5 msec rate. The circuit would use this average
motor output value as a input to a pulse generator to determine the
pulse width. The pulse generator would have to trigger one pulse every
20 msec.
You don't need to use servos. If you've got a rotation sensor and a
spare sensor port, you can get extremely fine position control with the
new firmware. I've got a small PID control demo using the new firmware
using a motor and rotation sensor. The PID program smoothly accelerates
and decelerates the motor to reach any desired rotation count value. It
works without any overshoot! I've geared the rotation sensor 1:5 so
that I get 80 counts per revolution of the main shaft. The motor is
connected to the shaft with 1:1 gearing. I get position accuracy within
one count with the shaft rotating at 5 revolutions per second (i.e. full
motor speed). A practical application would surely gear the shaft down .
I suspect this is far finer resolution than you can get with a SERVO. I
hope to post a video of this soon.
The new firmware does allow very fine control of the motor control
output but the resolution is at 1-msec or (optionally) 0.5 msec range.
It has some currently undocumented features to bypass the PWM software
and set a user supplied bit stream to use on the motor control output.
This bit stream will be output to the appropriate motor control register
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Message has 1 Reply:
 | | Re: Servo Motor Control
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| (...) Dick, I am very eager to see this! For a long time, I've been trying to do exactly this with limited success. Can you elaborate a bit on what you did code-wise to achieve this? Iain (20 years ago, 29-Dec-04, to lugnet.robotics)
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Message is in Reply To:
| | Re: Replacement Firmware Released
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| (...) With new timer resolution and fine motor power control, will RCX control SERVO MOTORS with precision? I tried to learn PBforth, but I prefer NQC. It would be great! (20 years ago, 26-Dec-04, to lugnet.robotics)
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