Sensor Selection: Laser Sensor Specifications Explained

One of the first specifications you might expect to see is accuracy.  Accuracy represents the maximum difference between the measured value and the actual value; the smaller the difference between the measured value and actual value, the greater the accuracy.  For example, accuracy of 0.5 mm means that the sensor reading will be within ± 0.5 mm of the actual distance.

However, accuracy is often not the most important value to consider for industrial sensing and measurement applications.  Keep reading to see why and to learn the most important specifications to take into consideration based on the type of application.

For discrete laser measurement sensors, Banner provides two key specifications: repeatability and minimum object separation.  While these are both useful for comparing products for discrete sensing, minimum object separation will be the most valuable for helping you select a sensor that can perform reliably in a real world application.

Repeatability (or reproducibility) refers to how reliably a sensor can repeat the same measurement in the same conditions.  Repeatability of 0.5 mm means that multiple measurements of the same target will be within ± 0.5 mm.

This specification is commonly used among sensor manufacturers and can be a useful point of comparison; however, it is a static measurement that may not represent the sensor’s performance in real world applications. 

Repeatability specs are based on detecting a single-color target that does not move. The specification does not factor in variability of the target, including speckle (microscopic changes in target surface) or color/reflectivity transitions that can have a significant impact on sensor performance.

Minimum Object Separation (MOS) refers to the minimum distance a target must be from the background to be reliably detected by a sensor.  A minimum object separation of 0.5 mm means that the sensor can detect an object that is at least 0.5 mm away from the background.   

Minimum object separation is the most important and valuable specification for discrete applications.  This is because MOS captures dynamic repeatability by measuring different points on the same object at the same distance.  This gives you a better idea of how the sensor will perform in real world discrete applications with normal target variability.

Repeatability, or how reliably the sensor can repeat the same measurement, is a common specification for IO-Link sensors.  However, as with discrete applications, repeatability is not the only or most important factor for IO-Link applications.  

Accuracy also becomes more important here.  As mentioned previously, accuracy is the maximum difference between the actual value and the measured value. When using IO-Link, the measured value (shown on the display) is directly communicated to the PLC. Therefore, it is important that the value be as close to “true” as it can be. 

The best-case scenario for an IO-Link application is a sensor that is both accurate and repeatable.  However, if the sensor is repeatabile but not accurate, it is still possible for the user to calibrate out the offset via the PLC.

In this example application pictured to the right, a Q4X laser measurement sensor detects the presence of dark colored inserts on a dark colored automotive door panel.  Learn more about this application.

The IO-Link process data shows distance to where the insert should be to determine if the insert is present.  Measurement must be accurate regardless of the target color. 

Total Expected Error is the most important specification for IO-Link applications. For IO-Link sensors, Banner calculates the Total Expected Error a bit differently than for analog applications.  For IO-Link sensors, the Total Expected Error represents the combined effect of accuracy, repeatability, and temperature effect.   Again, since the factors are independent they can be combined using the Root-Sum-of-Squares method to calculate the Total Expected Error.  

See below for an example of how this is calculated for IO-Link sensors.