Laser distance sensors enable fast and highly accurate measurement of objects for reliable inline quality control, they provide measured values for high-precision positioning of objects or the y-axis positioning of a robot arm, or they measure the thickness for double layer control. Triangulation-based solutions are particularly suitable due to their almost surface-independent mode of operation. Compared to mechanical or tactile solutions, measurements are made without contact, which prevents both wear on the measuring device and damage to the product. But what should be considered in detail when choosing the right sensor?

Select measuring range and focus

Comparison of the signal changes of a laser distance sensor based on triangulation at short or long distances: The shorter the measuring range, the greater the changes on the receiving line.

© Baumer

It is generally advisable to select the measuring range as small as possible, as the precision of triangulation sensors decreases with increasing distance to the measurement object. Triangulation sensors determine the distance to the measurement object using an angle calculation: The sensor projects a laser dot onto an object. This object reflects the laser dot back to the sensor, where it hits a receiving element at a certain angle depending on the distance. The distance to the target in the sensor is calculated from the position of the light spot on the receiver element and the distance from the transmitter to the receiver element.

The change in position of the light spot on the receiver and therefore also the differentiation of distances are significantly lower at long range than at close range, which reduces the precision of the sensor.
However, the sensors cannot always be mounted as close as actually necessary to the
measuring object as necessary. One limitation could be the installation situation, for example, which prescribes a certain distance to the measurement object.
To optimize the performance of the laser triangulation sensor, work should be carried out as close as possible to the focus (or sweet spot) of the laser beam. The minimum diameter of the light spot minimizes interfering optical effects and therefore ensures the best local repeatability and precision of the measurement result.

Line or dot?

In many cases, the portfolio of laser distance sensors includes both point and line beam shapes, which should be used depending on the type of application. Line sensors offer a clear advantage in terms of measured value stability when measuring metals, wood or rough and structured surfaces, as an averaged measured value is output over the structure of the surface of the measurement object. Point sensors measure with an extremely small, focused light spot and are used for the precise measurement of the smallest objects, such as electronic components, or for the precise positioning of components.

No additional expense for protective devices

Different beam shapes for different surfaces: The laser line beam shape is suitable for metallic surfaces, for example, while the focused laser point is suitable for high-precision positioning.

© Baumer

Due to automatic adaptation to the optical properties of the measurement object, most laser distance sensors also provide reliable results within the specifications of laser class 1. The laser radiation from laser class 1 sensors may achieve a maximum power of <25.5 µW at a wavelength between 400 nm and 700 nm and is therefore harmless to the human eye. The sensors are quick and easy to install, without any additional protective measures. For this reason, they are the preferred choice in industrial applications.

Nevertheless, there must also be sensors with laser class 2: With extremely dark materials, such as matt rubber conveyor belts, most of the light is absorbed. As a result, only a small amount of light is reflected back to the receiving element of the sensor. If sensors with laser class 1 are used in such cases, they require a significantly longer emission time for a reliable signal. This reduces the measuring speed and the entire measuring cycle would be significantly longer. A laser class 2 sensor can provide a remedy here, as these can emit a larger amount of light and therefore the measuring cycles remain short. As a rule, no further protective measures are necessary here either, as the laser beam is harmless for short exposure times. However, looking into the beam or reflections over a longer period of time should be avoided.