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Interlock System PLC Analog Scales 
for Telescope Drive Motor Voltage and Current  

John Anderson
October 4, 2000

The Allen-Bradley Programmable Logic Controller (PLC) analog input modules are configured to read +/- 10 VDC inputs in engineering units of 1 mV per count for the motor amplifier voltage and current read backs.

The motor amplifiers are capable of producing +/-60 volts at +/-20 amps. To scale the amplifier outputs to the PLC input module, the motor amplifier internally reduces the signals sent to the PLC for voltage by a factor of 6 and current by a factor of 2.

As an example:

if the motor voltage reading is 4500, the actual motor voltage is

4500/1000*6 = 27 volts.

if the motor current reading is 6000, the actual motor current is

6000/1000*2 = 12 amps.

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