Medrad ProVis Mark V, Service Manual

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Mechanical Stop Drive Card
11 - 1
Mechanical Stop
Drive Card
Refer to Figure 11.1 for a block diagram of the Mechanical Stop Drive
Card.
Differential Amplifier The inputs to differential amp U2A are: Mechanical Stop Position
Command (BMSPOSCMD) and the Mechanical Stop signal (MSPOS)
from the Mechanical Stop Pot in the Injector Head. BMSPOSCMD
input represents the desired position of the Mechanical Stop. MSPOS
input represents actual position of the Mechanical Stop. The output of
U2A will be near zero volts when the Mechanical Stop is in position,
and will swing positive or negative when there is a difference between
BMSPOSCMD and MSPOS.
Absolute Value
Amplifier Regardless of the polarity of the signal from U2A, the absolute value
circuit surrounding U2B provides a positive “error” signal to compara-
tor U2C.
Comparator One input to comparator U2C is a triangular waveform (generated by
U2D), the other, an error signal from U2B. U2C compares these
inputs, generating a pulse train which actuates drive transistor Q4.
The width of the pulses, which determine the duty cycle for Q4, vary
proportionally to the amplitude of the error signal from U2B.
Drive Transistor Motor power is controlled by Q4, through the contacts of relay K1.
Direction Relay
Control Circuit (Card revision D and below)
Relay K1 determines the polarity of the voltage applied to the motor. If
output of differential amplifier U2A is negative (meaning that the stop
should move back), Q7 energizes K1 and reverses motor direction.
Direction Relay
Control Circuit (Card Revision E and above)
Relay K1 determines the polarity of the voltage applied to the motor.
The output of U2A is sent to the non-inverting input of U3, a high gain
comparator circuit. A comparator will switch between positive and
negative saturation, depending upon the polarity at the input. A hys-
teresis circuit provides positive feedback through R37 to prevent the
output of U3 from switching when the input is near 0 volts.
The output of U3 is then sent to Q7, a 2N3906 transistor that deter-
mines the directional control for the Mechanical Stop motor by switch-
ing K1 on the MSD card. When Q7 is OFF (output of U3 is at positive
saturation), the Mechanical Stop will move toward the rear of the
injector head (when the injector is armed). When Q7 is ON (output of
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