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