Sensors 101: Control system inputs

Thinking of sensors as a physical input to a control system can help an engineer realize there are a lot more possibilities than they realize in a field that is narrowly focused in its thinking.


Thinking of sensors as a physical input to a control system can help an engineer realize there are a lot more possibilities than they realize in a field that is narrowly focused in its thinking. Courtesy: Frank Lamb, Automation PrimerMost engineers who specify equipment think of sensors as discrete or analog devices, proxies or photoelectrics, flow or pressure sensors, thermocouples and the like. If asked to classify them into categories, they might say analog and digital, machine level or process control, optical or inductive.

There is another way to think of sensors: as any physical input to a control system. Broadening your thinking as to what constitutes a sensor opens up a lot of new possibilities. Is a pushbutton a sensor? Yes. It detects an operator's decision to do something. How about a relay? Yes, it provides the status of some device or machine condition.

Another way to categorize sensors other than the method they use to detect things is by their function. Sensors pretty much all fall into two categories: those that detect the condition of the machine or system (cylinder switches, encoders, some photoeyes, and vibration sensors), and those that detect or evaluate the parts or substance that the system processes (flow and pressure sensors, machine vision, and discrete part presence detectors).

Thinking of sensors this way makes you realize that not all sensors fall into neat categories like analog and digital. A vision system is a good case in point. The actual method of detection involves optics and millions of individual pixels. The actual interface to the control system may be a simple digital pass/fail signal, or involve sending strings of data that need to be decoded. But you certainly need to understand all of the details of the complex topic of machine vision before specifying it.

Another example are encoders and resolvers. The data may consist of discrete pulses or a sinusoidal analog signal, but you can't feed these signals into your digital and analog input cards. These factors need to be taken into account when deciding what kind of hardware is specified along with the sensor's complexity and cost of the interface with a control system.

Thermocouples are also analog, but you can't wire them into an analog card. A thermocouple card contains an algorithm that knows what kind of response curve goes with the type of thermocouple selected. Not only that, but you can't wire the thermocouple into a standard terminal block and run regular copper wire back to the card.

Other atypical sensors include:

  • Eddy current tester: An inductive probe inserted into a machined hole to determine if it had been threaded.
  • Capacitive ion leakage probe: a charged probe and plate placed on the opposite sides of a piece of plastic to determine if a pin sized hole actually allowed ions through to the other side.
  • Mass spectrometer: A customized spectroscopy system intended to detect the presence of a deadly nerve agent.
  • Ultraviolet camera: A machine vision application where we actually coated rubber gaskets with an ultraviolet ink to detect if raised areas had been molded correctly.
  • Floating rocker arm cover measurement: A system with independent linear variable differential transformers (LVDTs) that calculated the intercept of an imaginary plane to determine if a rubber gasket had been mounted correctly.

Whether the focus is on troubleshooting existing equipment or designing new systems and machines, understanding how all of these sensing devices work together with control systems is an important part of a good well-rounded education. 

Frank Lamb is the founder of Automation Consulting LLC. This article originally appeared on the Automation Primer blog. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media,

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