D-Series Syringe Pumps - Isco

D Series Syringe Pumps
Section 4 Theory of Operation
The current control circuit includes U202A and the associated
resistors. They form a linear differential amplifier which is the
controlling element in the current control circuit. The circuit subtracts
the current feedback signal from the current setpoint
signal and amplifies the resulting error signal. The current error
signal at pin 1 of U202 drives the gate of Q01 through R207. This
prevents the motor current from exceeding the amount set by the
current setpoint potentiometer. When the current setpoint is
exceeded, the voltage on the drain of Q01 rises. This signals the
CPU (via chip U206) to turn off the motor. R205 with C203 connected
to the gate of Q01 provide high frequency stability.
To ensure valve opening, the maximum drive current is
increased when the valve open direction is selected. The opening
current maximum is about 1.5 times the closing current
maximum. Q202 is turned on when valve open direction is
selected. This places R206 in series with R209. The resulting
voltage divider reduces the current feedback signal to 68% of the
voltage at R204. The current required to match the feedback
signal to the setpoint will be about 1.5 times greater than with
Q202 off.
Standby
Circuit Failure Detection
4.3 Pump Power Supply
and Motor Drive
Power amplifier
One section of the 2803 is used to insure that the motor drive is
turned off when the microprocessor is not operating. The
STANDBY signal from the front panel switch turns the open collector
2803 off when the switch is set to RUN. When the switch is
set to STANDBY, R214 turns the 2803 on. The output pulls the
gate of Q1 low, which turns the motor drive off, regardless of the
state of the 82C55 output.
U202C functions as a comparator to monitor the motor drive
current. The threshold is set at 1.14 amps. This current value
should never be exceeded in normal operation. If a failure occurs
in the current control circuit, which results in excess current, the
output (pin 8) of U202C will go high. This signal will be read by
the microprocessor and the user will be notified of the failure.
The motor drive circuit, located in the pump module on the pump
power circuit board, consists of an input signal isolation circuit, a
current limiting control circuit, and a power amplifier circuit. An
LC filter, which converts the pulse-width modulated (PWM)
power signal to a DC motor drive voltage, is located near the
motor off the circuit board.
The PWM signal and the direction signal from the controller are
buffered by U102, a high speed optocoupler circuit. The outputs
at pins 6 and 7 are active high when the respective inputs are
flowing current (controller outputs low).
The two control signals then pass through the current limiting
and direction steering circuit. The PWM signal (labeled PULSE)
is gated by U102C to turn off when the motor drive current
exceeds 5 amps. R104 with R105 divide the +12 volt supply to
provide a 0.5 volt reference. Comparators U101A and U101B
compare the reference to the voltage across 0.1 ohm current
sense resistors R112 and R116 respectively. R116 senses the
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