Dionex AXP/AXP-MS Manual
To be effective, the pulse damper requires a backpressure of
approximately 500 psi or greater. If the system does not generate 500
psi, a length of small bore tubing (restrictor) can be used between the
pulse damper and the application.
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 1.00 mL/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 Dionex AXP/AXP-MS pump 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 an isolation transformer which has
taps to accommodate voltages of 110-120 or 220-240 VAC. Selection
is accomplished by changing the transformer jumpers. A different
transformer is supplied for use at 100 VAC. The transformer input is
provided with two fuses for line current. A linear rectifier circuit
provides 36 VDC, which also has a fuse, to drive the stepping motor.
A switching 5 VDC supply is provided to power control and display
circuits.
4.2.3 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 the motor stalls, the electrical current being
supplied to the motor is turned off and the fault light is turned on.
15
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