Potentiometric Application Example - ABB AV3 Instruction Manual

Direct-acting cam rotation of 0 to 100 percent produces increasing
resistance between terminal block TB1-1 and TB1-2. Reverse-acting
cam rotation of 0 to 100 percent produces decreasing resistance
between TB1-1 and TB1-2.
The resistive change of the potentiometer is approximately 9.9 ohms
(nominal) per degree of cam rotation, with a total resistance of 2000
ohms (nominal).
Application of the potentiometric option must consider the possibility
of end resistance wrap-around.

POTENTIOMETRIC APPLICATION EXAMPLE

A customer has a Type AV positioner with a 90 degree, direct-acting
cam, potentiometric position transmitter, along with a 24 VDC supply.
The customer wishes to output a five-VDC signal when the Type AV
positioner output is 80 percent.
1. Determine the angular rotation of 80% of 90 degrees:
0.80 x 90 degrees = 72 degrees
The 72 degrees of cam rotation is 80% of 90 degrees.
2. Determine the resistive change of the potentiometer through 72
degrees of rotation. For every degree of rotation the resistive change
is 9.9 ohms (nominally):
72 degrees x 9.9 ohms = 712.8 ohms
The change of resistance for this rotation is 712.8 ohms, and appears
between TB1-1 and TB1-2.
3. To guard against wrap-around, the minimum resistance at the
potentiometer wiper at 0% cam rotation is set to 200 ohms. There-
fore, the resistance between TB1-1 and TB1-2 should vary between
200 and 912.8 (712.8 + 200 = 912.8) ohms for a cam rotation of 0 to
80%.
4. Determine the required current in the potentiometer that develops
5 volts at the wiper at 80%:
5 VDC ÷ 912.8 ohms = 5.48 mA
5. Determine the nominal voltage to produce 5.48 mA across the
potentiometer:
5.48 mA x 2000 ohms = 10.96 VDC
POSITION TRANSMITTERS
CALIBRATION
A - 3