Pulsafeeder PULSAmatic 7120, Installation, Operation, & Maintenance Instruction

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For example, if a pump is operating at 50% stroke in response to a 50% input signal, the
signal must typically increase to 51% or decrease to 49% before the actuator does not
respond, or is “dead”, is called “deadband”. If deadband is too narrow, the actuator will
frequently make slight adjustments in response o small signal variations. In the extreme
case, the actuator will continually “hunt” back and forth over a small range of adjustment.
If deadband is too broad, response will lag and accuracy will suffer. The “Null”
potentiometer near the center of the circuit board adjusts deadband. Clockwise
movement decreases deadband, increasing sensitivity. Counterclockwise movement
increases deadband, decreasing sensitivity.
Deadband adjustment for response to a 1% change in signal (depicted in the example
above) is appropriate to most installations. This can be set approximately by setting the
“Null” potentiometer in the six o’clock position shown in Figure 4. To check deadband
adjustment, cycle the pump automatically, by input signal command, to an approximately
midrange stroke setting. Leaving the override switch in the “in” or automatic position,
slowly adjust the handwheel in either direction until the actuator responds to return the
stroke to the original set point. Care must be taken during this operation, as the
handwheel will move without warning. Deadband is observed on the mechanical stroke
indicator as the difference between the original stroke setting and that at which the
actuator responds.
5.2 Circuit Board Calibration
The PULSAmatic circuit senses all control signals in terms of voltage. A current signal is
converted to a voltage signal measured across a resistor, provided in the circuit board,
through which the current passes. For example, the most commons signal, 4-20 mA DC,
passes through a 470 ohm resistor to generate a 1.88-9.40 volt DC signal, (0-6.3 v with
Ratio Control).
This procedure trims the actuator circuits to the low and high ends of the actual input
control signal.
Without Ratio Control – Coarse Adjustment
1. Place the override switch in the “out” or manual position. The pump need not be
running for coarse adjustment.
2. Set up a voltmeter to read a full scale DC voltage of 10.
3. Connect the positive lead of the voltmeter to TB2-2 and the negative lead to TB2-1.
4. Set the control signal at the low end (0%) and record the voltage.
5. Set the control signal at the high end (100%) and record the voltage.
6. Set up a voltmeter for DC voltage measurement between TB2-5 (positive) and TB2-1
(common).
7. Adjust the “LO” trim potentiometer on the circuit board to the voltage recorded in step
4.
8. Set up the voltmeter for DC voltage measurement between TB2-4 (positive) and
TB201 (common).
9. Adjust the “HI” trim potentiometer on the circuit board to the voltage recorded in step
5.