Page 69 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

When power is applied to a standard electric motor, the rotor begins
turning smoothly. The speed and position of the motor’s rotor are a
function of voltage, load on the motor, and time. Precise positioning
of the rotor is not possible.
A stepper motor, however, runs on a sequence of electric pulses to
the windings of the motor. Each pulse to a winding turns the rotor
by a precise predetermined amount. The incremental movements of
the rotor are often called steps. Hence the name, stepper motors.
Not all stepper motors rotate the shaft (rotor) by the same
amount per step. They are manufactured with different degrees
of rotation per step (or pulse). The optimum degrees per step will
depend upon the particular application. Stepper motor specifica-
tions clearly state the degree of rotation per step. You can find a
variety of stepper motors, with the range of rotation per step varying
from a fraction of a degree (i.e., 0.72 degree) to many degrees (i.e.,
22.5 degrees).
Stepper motor circuit

Figure 4.8B is a schematic of a stepper motor driver circuit. The
stepper motor in the circuit is a unipolar (six-wire) type. IC U1 is
48 a 555 timer that is set up in astable mode to output square-wave
clocking pulses on pin 3. U2 is a stepper motor controlling chip
UCN 5804. The clocking pulses received on pin 11 of the UCN 5804
turn the stepper motor. Each pulse received on pin 11 turns the
stepper motor one step. The faster the clocking pulses, the
quicker the stepper motor turns.
In this sample circuit the clocking pulses are produced by a 555
timer. Clocking pulses can be generated by any number of sources
like a microcontroller (discussed in Chap. 6) or a photoresistive
neuron (discussed in Chap. 5). Switch SW1 is a fast/slow control.
SW2 controls the stepper motor direction.

Stepper motors may be used in making a robotic platform (see
Chap. 10).

Servo motors

Servo motors are geared DC motors with positional control feed-
back. Hobbyist servo motors are commonly used for position control
for radio-controlled (R/C) models. The shaft of the motor can be
positioned or rotated through a minimum of 90 degrees.
Because of their widespread use in the hobby market, servo motors
are available in a number of stock sizes (see Fig. 4.9). While larger
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