Sensors allow a PLC to detect the state of a process. Logical sensors can only detect a state that is either true or false. Examples of physical phenomena that are typically detected are listed below.
• inductive proximity - is a metal object nearby?
• capacitive proximity - is a dielectric object nearby?
• optical presence - is an object breaking a light beam or reflecting light?
• mechanical contact - is an object touching a switch?
Recently, the cost of sensors has dropped and they have become commodity items, typically between $50 and $100. They are available in many forms from multiple vendors such as Allen-Bradley, Omron, Hyde Park and Turck. In applications sensors are interchangeable between PLC vendors, but each sensor will have specific interface requirements.
This chapter will begin by examining the various electrical wiring techniques for sensors, and conclude with an examination of many popular sensor types.