An analog signal is an instantaneous reading from a device to a PLC or from a PLC to a device. You can read an earlier post that defines a PLC if necessary.

The signal to or from the PLC can be in many forms . The typical signal would be 4-20MA (could also be 0-20MA, 0-10VDC). Other examples would be an RTD, a thermocouple, millivolts, etc.

When this signal is an input to a PLC it would be used to monitor devices in the field, such as a flow meter, a pressure transducer, level transmitters, feed back from drives, or any other of the many devices used to measure a process.

When the signal is an output from a PLC it would be used to give a speed reference to a device in the field, such as an inverter or dc drive, a positioning control, regulating valve or any other device that can accept an analog signal as a input to control it.

Analog signals are usually used in a PLC with a PID loop which is a closed loop control.

A typical example of a PID closed loop control would be an input to the PLC from a flow meter and an output from the PLC to an ac drive. The operator or PLC programmer would have entered a set point for the flow into the PLC program. The PLC will compare the flow measurement from the flow meter with the set point and depending upon whether the flow is above or below the set point, will increase or decrease the signal to the drive. The drive in this case is activating a pump and the increase or decrease in signal will result in and increasing or decreasing of flow to bring it closer to the desired flow rate. The further the input is from the set point, the greater the change in output will be.

More about PID loops in another discussion. If you need help with any control challenges, why not ASK JIM?