Phase Selection Timing - HP 7925D Service Manual

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Theory of Operation
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7925
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Figure 1-9. Phase Selection Timing
the average spindle motor current derived from the spin-
dle motor current sampling resistor and regulates the
motor current accordingly. This is achieved by applying
the desired spindle current command to the positive in-
puts of two differential amplifiers and the derived average
spindle motor current to the negative inputs. The unity-
gain inverting amplifier inverts negative current samples,
so that they may be processed as positive current samples.
The actual measured current sample can be observed at
the test point labeled "SMe".
The difference between the desired current and the actual
measured current is applied to the negative inputs of two
comparators. The output from a 22 kHz triangle wave
1-14
generator is applied to the positive inputs. This signal can
be observed at the test point labeled "22 kHz". A pulse
train is produced with a duty cycle determined by the
points at which the smooth dc voltage intersects the slopes
of the triangular wave. If there is a small difference be-
tween the desired current and the actual current, a low
duty cycle will be output from the comparators. Similarly,
a larger difference produces a higher duty cycle output. It
is the duty cycle that controls the pulse selection outputs
which in turn control the application of current to the
spindle motor windings. The output that regulates the
positive phases can be observed at the test point labeled
"P+", while the output that regulates the negative phases
can be observed at the test point labeled "P-".