The resolution, accuracy and precision of sensors

In this article we would like to discuss a number of terms related to sensors. These are often used interchangeably – usually incorrectly. These are very important specifications of sensors that are decisive in the functioning of the sensor. It's all about the resolution, accuracy and precision of sensors.

The resolution of sensors

The term resolution is often associated with a measurement. However, when it comes to sensors, the term resolution gains a totally different meaning. Simply put, the resolution is: the smallest possible change that a sensor can perceive. For a laser light grid, for example, this is a shift in position.A sensor with a low(er) resolution will only detect or report displacements in whole centimetres, for example. When a sensor with a higher resolution is used, it is possible to do this down to millimetres. Of course, this is only of use when the application requires it.It can be said as well that the application is decisive for the required resolution. A critical application, such as the monitoring of components on a PCB, as seen in the image, requires a high resolution. In other words, sensors with a higher resolution have a lower minimum voltage to which they switch.

The accuracy and precision of sensors

A frequently asked question when selecting a sensor is: how accurate is it? When it comes to sensor accuracy, one often tends to think about the difference between a measured distance and actual distance. Within sensors, we know two types of accuracy: absolute accuracy and repeat accuracy.Absolute accuracy is what is often meant when talking about precision. So absolute accuracy is the deviation in a single measurement. Think of measuring the distance to an approaching truck in a loading bay. To prevent a collision it is important to know that when measuring 50 cm distance from the truck, the sensor does not indicate that there is still 1 meter distance. If the sensor has an absolute accuracy of ±10 cm then it will indicate a value between 40 or 60 cm in the given example.The repeat accuracy is the difference between two measurements. If the first measurement indicates 101 mm and a second measurement, under exactly the same conditions, yields 102 mm the repeat accuracy is noted as ±1 mm. In many (but certainly not all) applications, repeat accuracy is more important.

But.

However, knowing these two values is not enough to judge the right choice of sensor. People naturally want the best specs, so it's easy to go for the best repeatability and best possible absolute accuracy. To give an example of an application in which this is not the best choice, one can think of measuring the thickness of a product on a conveyor belt using a displacement laser.If a laser is used with an absolute or repeat accuracy of ±0.01 mm, it cannot function along or above a conveyor belt because it is constantly vibrating, thus producing a deviation of, say, at least 1 mm. If all measurements are so accurate to within 0.01 mm it means that you are consistently measuring and detecting error on a vibrating conveyor belt.