Digital Reference Unit Manual 82006
4 Woodward
To determine and preset the Digital Reference Unit output values, set up the
speed, load, or process control as it would be used without the Digital Reference
Unit. Replace the reference unit connections to the control with wiring, switches,
or potentiometers, as indicated on the plant wiring diagram for the control. Adjust
the control for the speeds, loads, or process values which the reference unit will
set. For each operating condition (low limit, high limit, and intermediate set point)
measure and record the voltage on the terminals to which the output of the
reference unit will be connected. (See Chapter 4 for wiring suggestions between
the reference unit and various controls.)
It is sometimes difficult to use this method to determine the voltages of the
reference unit output on load-sharing lines. Since the load-sharing lines voltage
is one-half of the load gain voltage, it is easy to calculate the reference unit
output voltage proportional to the load-gain voltage. Zero load is zero volts. One-
half load is 1/2 by (1/2 x load gain). With the typical 6 volts load gain, this would
be 1.5 volts. Full load for the same system is then 1 x (1/2 x load gain) or 3 volts.
1. Attach a voltmeter capable of reading 0 to 8 Vdc to terminals 10(+) and
11(–).
NOTE
The reference unit can produce a negative voltage. If a negative voltage
output is desired, the voltmeter at terminals 10 and 11 should be able to read
both + and – outputs.
2. Connect the power supply and appropriate switches to terminals 1 through
5. Close the switch between terminals 2 and 5. (Set the FIRST ramp time
counterclockwise to minimize the time required to reach the limits or set
point.) When the output stops ramping down, adjust the Low Limit
potentiometer through the cover of the reference unit for the desired output
as indicated by the voltmeter. (Clockwise on the potentiometer will increase
the low-limit position.)
3. Open the switch between terminals 2 and 5, and close the switch between
terminals 2 and 3. Adjust the Range pot to provide the maximum desired
output from the reference unit. Clockwise will increase the High Limit
setting.
4. Open the switch between terminals 2 and 3, and close the switch between
terminals 2 and 5. The output should decrease to the desired low limit. If
readjustment is necessary, ramp back to High Limit and readjust the Range
potentiometer (clockwise is increase.)
5. Open all the switches. Close the switch between terminals 2 and 4. The
output will ramp to the set-point position. Adjust the set-point potentiometer
up or down as necessary.
NOTE
When selecting two inputs at the same time, Set Point has priority over the
Raise, and Lower has priority over the Set Point. If Raise is permanently
closed, the unit will automatically go to the High Limit when neither Lower
nor Set Point switches are closed.
To Next Page
Loading...
