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Theory of Operation 2-11
NovaJet 500/630/700 Series Service Manual
THEORY OF
OPERATION
Each circuit contains four main functions (see Figure 2-7):
1. Reference waveform generator
The microprocessor uses a D/A (digital to analog) converter to set
the desired level for the current in the stepper motor winding. The
output of the D/A converter varies in time to create a reference
waveform. This reference waveform is centered around 10V.
2. Motor current sense
The voltage across a series current sense resistor is measured and
level shifted so that it is centered around 5V.
3. Comparator
This portion divides the output of the reference waveform generator
by two and compares it to the output of the motor current sensor.
Logic inside the gate array generates the control signals for the
power driver that applies voltage across the motor winding in order
to make the actual current match the reference waveform.
4. Power driver
An H-bridge allows the supply voltage to be applied across the
winding in either polarity used to drive the current level to the
desired value.