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Actually, most servos used in small robots are radio control
servos (airplane, car, boat, etc.). Of the many brands
Futaba, Airtronics(Sanwa), JR, Hitec, GWS, Multiplex, etc.
They are all compatible with typically the same three wire
connector and control signals.
One difference that you should be aware of:
Type 1) Internally the servo may have limited motion about a
center position. Typically plus or minus sixty degrees for
radio control yet possibly more for the same servo when
driven by computer control.
Type 2) A Type 1 modified internally for continuous rotation.
Internally the servo will have no rotational constraint.
Thus it is just a motor that can be used to drive wheels.
The three wire connector must be correct, else you may burn out
the servo. The ,usually, red wire in the center is +4.8 VDC to
+6.0 VDC for most servos. Less than 4.8 volts direct current may
cause "chatter" and mispositioning. More than 6VDC and a heavy
load on the servo may burn out the servo or shorten servo life.
Some servos handle heavier loads at 7.2 volts.
One of the end wires on the connector is ground. O VDC. This is
usually a black wire yet may be brown or other dark color.
This would be called the negative side if connecting to a battery.
The other end wire on the connector is the signal that controls
the movement of the servo. This may be white, blue, orange, yellow,
or some other color.
The servo is a small electric motor with gears. When disconnected
you may carefully turn the shaft of the servo and hear the motor
spin. Turn it slowly and carefully, else you may strip one of
the gears. The motor control signal is short pulses of voltage
near the 4.8 to 6.0 volt range.
The control signal sending a 1.5 millisecond long pulse about
50 times per second causes the servo motor to not rotate.
(Well, if the servo is internally set up to have a center
position, Type 1, and the servo is not at the center position,
the servo first moves to the center position, then stops.)
Longer pulses, typically up to 2.0 milliseconds make the servo
rotate in one direction while shorter pulses down to 1.0
millisecond make the servo rotate in the other direction.
See: How Servos Work
For a Type 2 motor servo, the farther the pulse is from 1.5
milliseconds long, the faster the wheel turns. This is speed
control. Depending on physical orientation, shorter pulses
may be reverse and longer pulses forward.
For a Type 1 positioning servo, the rotation angle of the
servo shaft is proportional to the pulse length over or
under 1.5 milliseconds. The servo will move to the position
commanded by the pulse length and stay there. Depending on
physical orientation, clockwise for shorter pulses and counter
clockwise for longer pulses. Exceeding the maximum angle
the servo can move by excessive pulse lengths shortens the
life of the servo. It makes a buzzing sound.
Pulse repetition rate may typically vary from 30 to 60
times per second. Stopping the sending of pulses causes the
servo to stop. But, if there is a force being applied to
the servo, it may move when pulsing is stopped.
Some servos advertise 2 microsecond accuracy. Considering the
pulse going from 1.5 milliseconds to 2.0 milliseconds, a 500
microsecond range, 2 microsecond accuracy means about 250
possible servo positions or speeds. Digitally, 8 bits of accuracy.
Do not count on this much physical accuracy. The servo arms
and connections can have "slop". Yes, slop is a technical term
that means small errors occur due to many mechanical causes. e.g.
Stretching, bending, loose connections, misalignment, etc.
Servos can be tested with a radio control. This is a transmitter
that has controls movable by fingers, a receiver that has the
connections to the three wires of the servos, and a battery that
runs the receiver and servos.
Autonomous robots have an on board controller, computer, that
may drive the servos, yet typically the controller drives a
secondary controller that connects to the servos.
One such secondary servo controller is the Parallax Servo Controller.
This servo controller drives up to 16 servos. Two of these servo
controllers can drive up to 32 servos. When using more than
about four servos, a separate battery should be used for just
the servos. It would be unfortunate if the computer lost power
while there was still enough power to drive the servos.
The shaft of the servo rotates. The speed of a S03N is .23 seconds
to travel 60 degrees. The torque available to the servo arm on
the servo shaft is 47 ounce-inches. In other words, about three
pounds of force at one inch from the center of the servo shaft.
A one inch servo arm can move linearly about 0.86 inches
each side of center.
If you have a serial port but no IIC bus, then you need a GP-7. The IIC bus or I2C bus is used by many robot components.
Details of how a servo works A place to buy servos ServoCity.com A place to buy servos TowerHobbies.com There are many more places to buy servos, do a Google search. Parallax 16 Servo Control details Parallax 16 Servo Control Parallax other controllers
Last updated 2/25/05