Sensors for automation and control
A single gauge in a bridge mistakes 20 °C of heating for 100 microstrain. With two or four gauges in adjacent arms, the temperature cancels and the signal doubles or quadruples.
01Presence and position sensors
An automation system needs to know where the parts and mechanisms are. Presence sensors give a binary output; position sensors, a value:
| Sensor | Principle | Detects |
|---|---|---|
| Limit switch | Mechanical contact | Any object that actuates it |
| Inductive | An oscillator loses energy through eddy currents in the target | Metals, at a few millimeters |
| Capacitive | The object changes the capacitance of an electrode | Almost any material, even liquids through a container |
| Optical | Through-beam, reflection from a mirror or from the object | Objects at distances of meters |
| Ultrasonic | Time of flight of a pulse | Distance, level, transparent objects |
| Encoder | Optical or magnetic disk with marks | Angular position and speed |
Three-wire industrial sensors have a PNP output, which connects the load to the positive, or NPN, which connects it to ground; the one that matches the controller input has to be chosen. An incremental encoder delivers two channels in quadrature, A and B, which give the position by counting pulses and the direction by which one leads; it is lost when power is cut. An absolute encoder delivers a code, often Gray, that identifies each position.
02Temperature, force and pressure
Thermocouples generate a few microvolts per degree between two different metals, about 41 μV/K in type K, and need cold-junction temperature compensation. Platinum RTDs, such as the Pt100, have 100 Ω at 0 °C and a coefficient of 0.00385 per degree according to IEC 60751; they are connected with three or four wires so as not to measure the resistance of the cables. Force, weight and pressure are measured almost always with strain gauges: a metal trace on an insulating backing, bonded to the part, that changes resistance when it deforms:
03The strain-gauge bridge
In a Wheatstone bridge excited with \( V_{ex} \), one active gauge in one arm (quarter bridge) gives a nonlinear output and cannot tell strain from heating, because the gauge changes resistance with temperature. With two gauges in adjacent arms that deform in opposite directions, like the two faces of a beam in bending (half bridge), the output is doubled, linear, and the temperature effect cancels. With four (full bridge), it is quadrupled:
Gauges with GF = 2 in a bridge excited with 5 V, with an apparent thermal output of 5 μm/m per degree. The strain is measured by dividing the output by \( k V_{ex} GF \). The blue curve is the chosen configuration; the gray one, the ideal straight line.
04Industrial interfaces
The 4–20 mA current loop transmits an analog measurement over two wires for hundreds of meters: the current does not depend on the cable resistance, the same loop powers the transmitter, and the live zero of 4 mA makes it possible to detect a broken cable, which gives 0 mA. The HART protocol superimposes digital data on that loop. IO-Link (IEC 61131-9) connects sensors and actuators with point-to-point communication over the same three-wire cable, which makes it possible to configure and diagnose them. The connection of these sensors to controllers is covered in introduction to control and in the control subjects.
05In the lab
Bond gauges on both faces of a cantilever beam, load it with known weights and compare quarter and half bridge, before and after heating it with air.
Measure the switching distance of an inductive sensor with steel, aluminum and brass targets, and compare with the reduction factors in the datasheet.
Build a loop with a transmitter, a 24 V supply and a 250 Ω resistor, and measure the voltage at the input for the whole range.
06Common mistakes
- Measuring with a single gauge without compensation. Temperature shows up as strain.
- Connecting a PNP sensor to an input that expects NPN. The input never sees the change.
- Connecting a Pt100 with two wires over a long cable. The cable resistance adds as temperature.
- Reading 0 mA as the bottom of the scale. In a 4–20 mA loop it is a broken cable.
07Self-assessment
What output does a full bridge give with GF = 2, 5 V and 1000 μm/m?
10 mV.
And a quarter bridge?
About 2.5 mV, slightly less because of the nonlinearity: 2.4975 mV.
Why does the half bridge compensate temperature?
The two gauges change equally with temperature and are in adjacent arms, where equal changes cancel.
What current corresponds to 50 % of the scale in a 4–20 mA loop?
12 mA.
08Further reading
- Ramón Pallás Areny. Sensores y acondicionadores de señal. 4th ed., Marcombo, 2003 (in Spanish). Strain gauges, bridges and position sensors.
- Jacob Fraden. Handbook of Modern Sensors. 5th ed., Springer, 2016. The physics and application of each family of sensors.
- Antonio Creus Solé. Instrumentación industrial. 8th ed., Marcombo, 2010 (in Spanish). Transmitters, the 4–20 mA loop and sensors for the process industry.