This circuit was required because
the VCO integrated circuit’s
characteristics were such that an input
voltage close to zero volts produced
the highest output frequency, and
conversely, the largest input voltage
produced the lowest output frequency.
This was the exact opposite of how it
was desired for the VCO to respond to
the error signal.
Essentially, the VCO signal
conditioning electronics output a high
voltage to the VCO when a small input
voltage is input, and generates a small
output voltage when a large voltage is
input to the VCO signal conditioning
circuit. The NTE987 is a quad, low
power op-amp and can operate at
supply voltages as low as 3V.
Figure 9. Pulse generator.
The VCO provides a square wave output signal where
the frequency is a function of the input voltage. The
magnitude of the C1 capacitor and the R1 variable resistor
determine the frequency range of the output signal that
can be commanded by the input voltage. R1 and C1 are
tuned to match the stepper motor’s maximum achievable
angular velocity. The output frequency is given by:
2((V+) - VINPUT) f0 = R1C1V+
The function of the pulse generator circuitry (Figure 9)
is to take the input square wave from the VCO (nominally
around 300 Hz) and produce two square waves phase-shifted by 90 or 270 degrees, depending on which direction
of rotation is desired for the stepper motor (Figure 10).
The two square waves are produced by the D-type flip-flop
and the 90 or 270 degree phase relationship is controlled
by the ADG888 analog switch integrated circuit. The
switching function is controlled by ε; see the polarity
sensing circuit (Figure 11). The outputs of the pulse
generator circuit are Q1, Q2, Q3, and Q4, nominally around
75 Hz. These signals control the current through the motor.
Figure 10 shows the waveforms generated by the
Figure 10. Waveform phase relationship depending
on direction of rotation.
SERVO 12.2012 71