4-2
4.2 Electronic
Control
4.2.1 Microprocessor
Control
The pump is controlled by hybrid microprocessor circuitry which
(1) provides control signals to the motor drive circuitry, (2) interfaces
with the keyboard/display, (3) receives signals from the refill flag, and
(4) provides external input/output (RS-232) interfacing. Firmware
programming is stored in an EPROM.
An eccentric cam provides refill in a fraction of the full cam cycle.
The remaining revolution of the cam provides piston displacement for
outward flow of the mobile phase. In addition to the rapid refill
characteristics of the drive, the onset of refill is detected by an infrared
optical sensor. The microprocessor changes the refill speed of the
motor to an optimum for the set flow rate. At 1ml/min, the refill rate is
more than five times faster than if the motor operated at constant
speed. The optimum refill rate minimizes the resulting pulsation while
avoiding cavitation in the pump head.
The flow rate of any high pressure pump can vary depending on
the operating pressure and the compressibility of the fluid being
pumped. The Binary Solvent Delivery Module is calibrated at 1000 psi
using an 80:20 mixture of water and isopropanol.
4.2.2 DC Power Supply
Power for the pump is provided by a switching power supply
which accepts voltages from 90 – 240 VAC. Output is 24 VDC for the
pumps and heater. A switching 5 VDC supply is also provided to
power control and display circuits.
4.2.3 Remote
Interfacing
RS-232C modular jacks are provided on the back panel. See
Appendix A for information on operation via this connection.
4.2.4 Motor Stall Detector
The motor can stall and create a loud buzzing sound if the flow
path connected to the pump's outlet becomes plugged, if the pressure
exceeds the maximum pressure rating of the pump, or if the
mechanism jams. If a motor stall occurs, the electrical current being
supplied to the motor is turned off and the fault light is turned on.
The Motor Stall Detector is enabled or disabled during power-up by
pressing and holding the RUN/STOP and the PRIME buttons while
the power is switched on. Release the buttons when the display
displays "SFE". To enable the Motor Stall Detector press the UP-
ARROW button and the display will display "On". To disable the
