Subject:
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DC motors vs. servo motors
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 6 Jun 1998 12:51:07 GMT
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Original-From:
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Fred G. Martin <fredm@/nospam/media.mit.edu>
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Viewed:
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1230 times
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The nature of the PWM (pulse width modulation) is entirely different
for servo's vs. plain DC motors.
For a DC motor, it has a two wire connection. All drive power is
supplied over these two wires -- think of a light bulb. When you turn
on a DC motor, it just starts spinning round and round. Most DC
motors are pretty fast, about 5000 RPM (revolutions per minute).
With the DC motor, pulse width modulation refers to the idea of
controlling the power level by STROBING THE POWER ON AND OFF. The key
concept here is DUTY CYCLE -- the percentage of "on time" versus "off
time." If the power is on only 1/2 of the time, the motor runs with
1/2 the power of its full-on operation.
If you switch the power on and off fast enough, then it just seems
like the motor is running weaker -- there's no stuttering. This is
what PWM means when referring to DC motors. The Handy Board's DC
motor power drive circuits simply switch on and off, and the motor
runs more slowly because it's only receiving power for 25%, 50%, or
some other fractional percentage of the time.
A servo motor is an entirely different story. The servo motor is
actually an assembly of four things: a normal DC motor, a gear
reduction unit, a position-sensing device (usually a potentiometer --
a volume control knob), and a control circuit.
The function of the servo is to receive a control signal that
represents a DESIRED OUTPUT POSITION OF THE SERVO SHAFT, and apply
power to its DC motor until its shaft turns to that position. It uses
the position-sensing device to determine the rotational position of
the shaft, so it knows which way the motor must turn to move the shaft
to the commanded position. The shaft typically does NOT rotate freely
round and round like a DC motor, but rather can only turn 200 degrees
or so back and forth.
The servo has a 3 wire connection: power, ground, and control. The
power source must be constantly applied; the servo has its own drive
electronics that draw current from the power lead to drive the motor.
The control signal is pulse width modulated (PWM), but here the
DURATION of the positive-going pulse determines the POSITION of the
servo shaft. For instance, a 1.520 millisecond pulse is the center
position for a Futaba S148 servo. A longer pulse make the servo turn
to a clockwise-from-center position, and a shorter pulse make the
servo turn to a counter-clockwise-from-center position.
The servo control pulse is repeated every 20 milliseconds. In
essence, every 20 milliseconds you are telling the servo "go here."
To recap, there are two important differences between the control
pulse of the servo motor versus the DC motor. First, on the servo
motor, duty cycle (on-time vs. off-time) has NO MEANING whatsoever --
all that matters is the absolute duration of the positive-going pulse,
which corresponds to a commanded output position of the servo shaft.
Second, the servo has its own power electronics, so VERY LITTLE POWER
flows over the control signal. All power is draw from its power lead,
which must be simply hooked up to a high-current source of 5 volts.
Contrast this to the DC motor. On the Handy Board, there are specific
motor driver circuits for four DC motors. Remember, a DC motor is
like a light bulb; it has no electronics of its own and it requires a
large amount of drive current to be supplied to it. This is the
function of the L293D chips on the Handy Board, to act as large
current switches for operating DC motors.
Plans and software drivers have been given to operate two servo motors
from the HB. This is done simply by taking spare digital outputs,
which are used to generate the precise timing waveform that the servo
uses as a control input. Very little current flows over these servo
control signals, because the servo has its own internal drive
electronics for running its built-in motors.
Typically, the term PWM (pulse-width modulation) refers to power
control on a DC motor, not the servo control waveform. I can see why
it makes sense to call the servo control signal a pulse-width
modulated signal (since the width of the pulses confers a positional
meaning!), but it is not the traditional usage of the term.
Fred
In your message you said:
> For servo, there is a certain threshold duty cycle (say 50%) of the PWM
> signal, where anything above this threshold will make it to run forward,
> and less will make it to run backward. So, you still need to apply the PWM
> even if the servo is not moving.
> And this is because servo is originally used for moving at certain amount
> of angle, once it is modified as a driving unit, that angular feedback
> device within the servo will also be modified. And the detail of how it
> works has somthing to do with the threshold duty cycle. Please correct me
> if I am wrong, as I am also thinking about using dc motor or servo.
>
> John
>
> At 06:25 PM 6/5/98 -0500, Chris Rhodes wrote:
> > That did answer my question. I am not familiar with servo motors, I
> > have only used plain DC motors. I am also familiar with PWM. I was
> > just unable to determine if the same PWM signal that drives plain DC
> > motors could also drive servo motors. From what I understand a servo
> > motor and DC motor use PWM to control the speed, and if that is the case
> > I did not see the difference in using the same PWM signal that drives a
> > DC motor to also drive a servo motor. Thank you for all of your help.
> > If I stated anything incorrectly in this paragraph please correct me.
> >
> > Chris
> >
> >
> >
>
>
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