Mono Drive Board - Thermo Scientific GENESYS 20 Service Manual

8.4

Mono Drive Board

4001-6054,
Revision B
Low-pass filtering has been applied to the ±12 volt power supplies. A 5.00 volt
precision voltage reference (U6) is used for the D/A converters. The
monochromator wavelength is controlled with a unipolar stepper motor.
Microstepping is employed with 16 microsteps per whole step resolution to
achieve roughly 0.3nm per motor step resolution. The drive current to each coil
is controlled by a pair of latched 8-bit D/A converters (U3, U4) which will allow
motor coils to be partially energized. The complementary current outputs of
these D/A converters sink a total of 500
to supply 250 A mps each.
F
When more than 250 A mps is required by an output, the balance of the current
F
is sunk from the virtual ground summing junction of U5. The output power
transistors and the U5 op-amps form a current amplifier for each motor coil.
When current is sunk from the virtual ground node, the same voltage drop occurs
across the 1.50K ohm resistor and the 0.47 ohm resistor, thus causing a current
amplification of 1500/0.47 = 3191. The current supplied to the summing
junction and the amplified current both flow through the motor coil, turning it
on. The complementary output of the D/A in this case will require less than 250
A mps and will cause the extra current from the 20.0K ohm resistors to be
F
forced into the summing junction, causing a negative voltage across the 0.47
ohm resistor. This turns off that coil.
The maximum output current of the D/A converter is 250
maximum possible motor coil current of 0.8 amps which also flows through the
0.47 ohm resistors. Software can limit this to a lower value to match a motor's
rated current limit. A 0.1
frequency oscillations in the current amplifier by slowing down the amplifier
response time. Diodes D1, D3, D5, D7 keep each current amplifier in a linear
region and prevent reverse biasing the power transistor bases when a channel is
turned off. Diodes D2, D4, D6, and D8 protect the transistors from inductive
voltage spikes when the motor coils are turned off.
8 - 7
Electrical Circuit Description and Adjustment
A mps and use the 20.0K ohm resistors
F
F integrating capacitor is used to avoid high
F
A mps, yielding a
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