A Primer on Sensors for Machine Automation
Machines need sensors and transmitters to operate reliably and consistently. All it takes is a small drop in air pressure or a slight jam to cause a machine to crash; these sorts of problems can often be detected before they lead to bigger issues.
Machine Design Magazine takes a look at the top 10 physical properties detected by machine automation sensors and transmitters—temperature, pressure, flow, level, current, voltage, presence, position, distance, and speed—and one or more common applications for each property.
In machine automation, monitoring and controlling temperature is often essential, and there are many temperature switches or transmitters that can make these measurements. Temperature switches work well in overtemperature, set and reset hysteresis, and dual-temperature high and high-high temperature applications. In these last two types of applications, temperature measurement methods provide increased reliability where exceeding a temperature must be avoided, such as in boilers, compressors, and large motors. Switches are also suitable for simple temperature control.
For more complex temperature monitoring and control, thermocouples and RTDs connected to temperature transmitters provide more granular feedback to close the control loop.
Thermocouples are good choices for measuring temperature, especially if the distance from the thermocouple to the controller is less than 100 ft. Thermocouples come in a wide variety of types, fit into small areas, have wide temperature ranges, and provide fast responses.
However, temperature sensors’ millivolt signals are sensitive to electrical noise, require matching extension wires, and are not as linear or accurate as a two-, three-, or four-wire RTDs. The millivolt signals are also not scaled, so the PLC must do this calculation.
When distance and electrical noise are a concern, temperature transmitters should connect the thermocouples to the PLCs.
Temperature transmitters convert millivolt input signals from thermocouples or RTDs to 4 to 20mA analog outputs or digital fieldbus signals. Temperature transmitters are commonly used in machine processes, tanks, and ovens. Their scaled analog outputs ease integration to PLCs and simplify calibration with their pre-configured temperature ranges and other configuration capabilities.