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800961-C
Nov. 2000
区 JALLINCKRODT
The other inputs of the comparators, Level 1 and Level 2, in these
two circuits are set to a level corresponding to the current pressure
limit value. This value is determined during pressure calibration
and is set by the digital potentiometer, U8. When the pressure
sensed exceeds the pressure limit value two actions occur.
First
the hardware pressure limit control will turn off the PWM control,
thereby reducing the power output to the motor and reducing pres-
sure. Second, the CPU board is notified through the comparator
output and the A/D that a pressure limit condition has occurred.
For example, assume that the pressure limit is set to 200 psi and
an injection is enabled.
Assume that during pressure calibration it
was determined that at 200 psi the gained up voltage level equals
1.5V. The digital potentiometer, U8, will output 1.5V to the two
comparators used for pressure limit detection on the secondary
side of the servo transformer, U10 pin 5 and U10 pin 7. Assume
the injection starts and the pressure approaches 200 psi. When
200 psi is exceeded, the gained up voltage from the sense resistor
R19 will exceed 1.5V. At this point the hardware pressure limit
comparator will go low thereby disabling the PWM controller, U16.
In addition, the 1.5V is sensed by the A/D on the CPU board and
the comparator input to the CPU board also goes low.
The hardware pressure limit circuitry provides the primary pressure
limit control with the software providing the backup pressure limit
function. A circuit similar to the circuitry that provides information to
the GPU board (i.e. gained up voltage level and pressure limit
comparator output) from the secondary side of the servo trans-
former also exists on the primary side of the servo transformer.
This circuitry is not currently used for pressure limit control.
3-13
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