Subject:
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Re: RCX Output
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 15 Dec 1999 19:05:02 GMT
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Original-From:
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Nick Taylor <ntaylor@iname.com>
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Viewed:
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577 times
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Hello Joel,
Attempting to drive an RC servo directly from the RCX is NOT a good
idea! The servo is a three wire device; +4.8V to +6.0V for power
(usually the red wire), an approximately 40Hz pulse train of 1.0 to
2.0ms pulses at 0V and +5V logic levels (white or yellow wire), and
ground (black wire). The pulse train does NOT power the servo motor,
it only tells the chip in the servo where to go ... the red wire
supplies power to drive the motor until an internal pot is turned
to the point where the position of the servo equals the position
commanded by the pulse width on the white wire.
The RCX delivers a two wire output which changes polarity when a
command is sent to reverse the motor ... and the servo's logic chip
would not be very happy.
I'm sure that you could build external circuitry to let the RCX
control a servo, but I'll leave that exercise to the reader.
Play well,
- Nick -
Joel Shafer wrote:
>
> So if you need a 1, 1.5 & 2ms pulse for controlling a servo and you said
> the low setting in NQC = a 1ms pulse, are there other matching motor speed
> settings to get a 1.5 or 2ms pulse?
>
> At 05:14 PM 12/15/99 +0000, you wrote:
> > Hi F'Amos,
> > I just now put a scope on the motor output ran an NQC motor test
> > program. At OUT_FULL the motor output is about +7V DC. At OUT_LOW
> > the motor output is a pulse train of 1ms 7V pulses at about 125Hz.
> > The low level depends on the motor load ... almost 7V with no motor
> > connected and approaching 0V with a stalled motor. NQC provides
> > seven motor speeds ... and by increasing the duty cycle of the pulse
> > train the average DC level to the motor is changed.
> >
> > RC servos are also controlled by varying the pulse width. For most
> > servos a 1.5ms pulse tells the servo to go to the center position.
> > Increasing the PW toward 2.0ms turns the servo CW and decreasing
> > toward 1.0ms turns the servo CCW. The exact pulse width determines
> > the exact stopping position. Servos that are modified for use as
> > motors can never find a null position so they continue to turn
> > seeking the nonexistent null. Unless the PW sent to the servo is
> > very close to the null PW, the servo will turn at full speed. If
> > the null PW is 1500us, then a 1550us PW will result in a slower
> > speed.
> >
> > Hope this helps,
> > - Nick -
> >
> > "Avery, Amos E." wrote:
> > >
> > > --- Nick Taylor <ntaylor@iname.com> wrote:
> > > > If I'n not mistaken, the RCX output is pulse width
> > > > modulated to control motor speed, while the train controller
> > > > varies the DC voltage.
> > >
> > > Can anyone confirm the output characteristics of the RCX?
> > > It is my understanding that PWM is used to control RC servos
> > > and not to power them. Has anyone put a scope on the RCX's
> > > outputs to see what they look like? If the info is available,
> > > I'd be interested to know the exact output specs (eg. 0-9v 200ma).
> > > F'amos
>
> Joel Shafer joel@connect.net
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Message is in Reply To:
 | | Re: RCX Output
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| So if you need a 1, 1.5 & 2ms pulse for controlling a servo and you said the low setting in NQC = a 1ms pulse, are there other matching motor speed settings to get a 1.5 or 2ms pulse? (...) Joel Shafer joel@connect.net (25 years ago, 15-Dec-99, to lugnet.robotics)
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