This is typically done by feeding ε to a power
amplifier that drives a DC motor. Since the “DC
servo” contains a stepper motor, it is necessary
to convert ε to two series of pulses, such that
they are 90 degrees out of phase (270 degrees
out of phase implies the reverse direction) before
being supplied to the motor drive electronics. In
actuality, the implementation of the concept was
Figure 6. Absolute value circuit.
The function of the summing electronics
(Figure 5) is to compare the commanded
position with the actual position of the motor
and develop an error signal
proportional to the
difference. A zero adjust
potentiometer was added to
control any residual offset.
Figure 7. VCO signal conditioning electronics.
Figure 8. Voltage controlled oscillator.
70 SERVO 12.2012
The overall electronic circuitry required to implement
the DC servo using a stepper motor was shown in Figure 4.
The basis for the design consisted of driving the position
error signal, ε, to zero.
The absolute value
circuit (Figure 6) was
required to prepare the ε for
controlling the frequency of
the pulses that were to be
generated to drive the
Depending on the
magnitude of the error
signal, the larger the error
signal, the higher the
frequency of the pulses
generated by the voltage
controlled oscillator (VCO),
i.e., the higher the angular
velocity of the stepper
motor. The VCO used
(NE556) only produces a
frequency proportional to
positive voltage input
signals. This necessitated the VCO signal conditioning circuit
(Figure 7). The goal was to make the frequency of the
quadrature square wave drive signals to the motor from the
pulse generator (Figure 8) proportional to the magnitude
of the error signal. The larger the error signal, the higher
the frequency, and the higher the angular velocity of the