Programmable Logic Controllers are electronic digital devices – meaning they are discrete systems that only understand electrical signals in the forms of 1′s and 0′s. So, how do you get a real-world measurement – like pressure – into a form that a PLC can understand? That is the function of the PLC Analog Input System.

Although “PLC people” might refer to an analog input as a single item, it takes several steps and sub-systems to convert a real-world measurement into a discrete digital signal.  Several steps need to occur and several devices will be involved. The sub-systems are: sensor (transducer), transmitter, and analog input module.

A transducer converts a real world physical variable into a very low-level electrical current or voltage.

COMMON PHYSICAL VARIABLES TO BE MEASURED

• Flow
• Pressure
• Temperature
• Vibration

A transmitter takes the low level electrical signal and amplifies it so it can be connected to an analog input module.  Since there are many different types of physical variables (pressure, flow, vibration, etc) and many types of transducers available, analog input modules have a fairly finite list of standard electrical input ratings that most manufacturers provide.

Standard Currents

• 0 – 20 mA
• 4 – 20 mA

Standard Voltages

• 0 to 1 volts DC
• 0 to 5 volts DC
• 0 to 10 volts DC
• 1 to 5 volts DC
• -5 to +5 volts DC
• -10 to +10 volts DC

The standard electrical signal from the transmitter is usually wired to an analog input module or card on the PLC. The Analog Input Module then converts these continuous real-world current or voltages into a multibit series of 1′s and 0′s that can be understood by the PLC, and writes that value to the proper memory space during every input scan of the PLC. The module takes the analog signal and transforms it through an (A/D) analog-to-digital converter (A/D). The analog signal will be translated to a corresponding series of bits to be stored in a PLC register (memory). The number of bits used will determine the range and resolution of the signal conversion.