The most recent Encoder newsletter of
the Seattle Robotics
Society includes two articles on speed control of R/C servos. The
first, by Ringo
Davis, describes how to modify a Futaba S148 servo so that its
speed can be directly controlled with 2 pwm signals. The second
article, by Theodore Johnson, presents an alternate method of
controlling speed by varying the "off" duration of the pulse train.
Many servos already have speed control. If you give it a wide
distance to travel, it will go faster than if you give it a
smaller distance to travel. So, for example, if you give it
a 1.5ms pulse train, the hacked servo will stop. A 1.6ms pulse train,
the hacked servo will turn slow. A 2ms pulse train, the hacked servo
will turn fast. Perhaps their particular servo doesn't have this
feature that they need to go through all the trouble to change
the pulse train window length?
I think they removed the servo guts.
Then they have to use a H-bridge motor controller to run the servo.
Thus they can use two wire PWM in that case.
Considering the number of RPM's you can get out of a servo. Most of the
time if you gut the servos, you only need to provide 1's or 0's as PWM
doesn't get you much of anything speedwise.
The problem with gutting a servo is you now need 4 I/O lines to control
two servos, wheras if you leave the guts in, you only need two I/O
Now some guys gut the servos, and then carefully squeeze in tiny
quadrature encoder disks and sensors. Then they can measure RPM and
distance effectively. But this method is hard to do as the servos are
so small. I prefer to come off the output of the servo, using encoder
discs and sensors externally.
On some servos, you can bypass the servo IC and use the little h-bridge
inside the servo directly. So you sort of, semi-gut out the servo in