Subject:
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Re: r/c servos
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 20 Aug 1999 11:53:27 GMT
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Original-From:
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John Barnes <barnes@sensors^StopSpammers^.com>
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Viewed:
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821 times
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If the pwm is really being controlled by an 8 bit counter, that might be fine
for the servos. The current rcx output has a cycle time of 8mS and uses 1mS
increments. If those are the top 3 bits, then the resolution is 5 bits worth
or 0.03mS. This would set the average servo, which needs a pulse from about
0.7 - 2.0 mS for 180 degress to one of about 45 steps or every 5 degrees. That
could be useful, but note that a geared down lego motor and the position
sensor
can do a lot better than that!
JB
> I love the idea of controlling an R/C servo with the RCX, but I don't • think it
> is possible with only software. On the other hand, a relatively simple
> circuit could be made to convert a duty cycle to a R/C servo signal. Is
> anyone out there interested in something like this? If there is enough
> interest I will take the time to draw up a circuit and post it somewhere. • Let
> me know.
>
> A few comments on the questions asked.
>
> > Since RC servos use pulse width as the input, there is already potential
> > here. Does any one, with a deeper knowledge of the insides of the RCX code
> > know if the squarewave duty cycle is being controlled by software or a
> > hardware based PWM?
>
> I am almost positive that the PWM is done in hardware. There are three • output
> driver chips, seperate from the microcontroller. The microcontroller sends
> duty cycle commands to these chips, but I believe the PWM frequency is fixed
> by an RC network extenal to the driver chip.
>
>
> > In response to the question about RC servos, I noted, while probing
> > the signals on the I/O ports with a scope, that the motor power settings
> > result in varying duty cycle square waves from 1:7 through 7:1 and finally
> > full on as the parameter value sweeps 0 -7.
>
> I believe these output controllers are also capable of much more than 3 bits
> of resolution. I just got my RIS and have not done much programming yet, but
> I have been doing lots of research. If you look at the documentation for
> LegOS (an alternate firmware for the RCX, found at • http://www.noga.de/legOS/),
> the motor speed can be set from 0 to 255. That's 8 bits! Does anyone else
> have more experience with LegOS?
>
>
> Zeke
>
>
> --
> Did you check the web site first?: http://www.crynwr.com/lego-robotics
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message has 2 Replies:
 | | Re: r/c servos
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| (...) To get rid of the speculation here, the PWM is controlled by a programmable timer that the code in ROM configures to generate an interrupt every 1 ms. Output for each of 8 power levels is stored in a small table of waveforms, with the 1 ms (...) (25 years ago, 20-Aug-99, to lugnet.robotics)
| | | Re: r/c servos
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| (...) I can't understand this. Can you correct your grammar so it is clear what you're saying the average servo needs? - Robert Munafo (URL) LEGO: TC+++(8480) SW++ #+ S-- LS++ Hsp M+ A@ LM++ YB64m IC13 (25 years ago, 20-Aug-99, to lugnet.robotics)
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