Functionality and technology of optical distance sensors
Functionality and technology of laser distance sensors
How does a laser distance sensor work?
A laser distance sensor, also known as a laser displacement sensor, measures distances to detect objects within a certain measurement range. Laser distance measurement sensors may be based on a variety of technologies. One of the most important ones is the triangulation principle. Here, a laser point or a laser line is projected onto the object whose position needs to be determined. The object reflects part of the incident light, which is in turn detected by a receiving element within the sensor. The position of the reflected light on that receiving element depends on the distance between the sensor and the object. Hence, when the distance changes, so does the position of the signal on the receiving element. With proper calibration, the sensor can measure the object's distance with high precision.
Triangulation is a geometric method used to measure distances by forming a triangle between known reference points and the target object. The key element is a laser source which emits a beam of light toward a target. The light reflects off the target and is captured by a receiving element which is sensitive to the position of the signal. The angle of incidence changes as a function of object distance, and hence, the position of the signal on the receiving element changes accordingly. Using pre-calibrated data, it is then possible to extract the distance between sensor and object, based on the position of the signal on the receiver.
With ever-improving performance of the critical components, the measurement performance of the sensors improves as well, in particular on very challenging surfaces. Examples include very smooth, shiny surfaces which result in a very low signal on the receiving element. Triangulation has long been a well-established technology, and still represents a very precise and reliable method for distance measurement. It is especially advantageous for measuring distances ranging from tens of millimeters to a few meters. Beyond that, other technologies are preferred, as precision and reliability using triangulation are reduced at very large distances.
Modern triangulation-based sensors offer a variety of interfaces which often allow easy, seamless integration into the control system. IO-Link is ubiquitous, as are other serial-based interfaces. Higher-end products also offer Ethernet-based protocols like Profinet, EtherNet/IP, EtherCAT or OPC UA.
In the case of distance measurement, the sensor is ready to use immediately and gives the distance from the sensor to the object. The measured value can, for example, be used for the precise positioning of objects or for controlling a system. A digital output can also be parameterized as an option.
If, for example, the dimensional accuracy of objects is to be checked, a direct tolerance measurement can be made by teaching-in a reference, thus allowing the deviation from the nominal dimension to be determined directly. Here too, a digital output can be parameterized accordingly.
Beam shapes
Besides different dimensions and ranges also beam shapes are an important factor. Thanks to continuous further development, Baumer can offers three different beam forms in its portfolio:
Powerful algorithms integrated in the sensor make laser distance sensors very insensitive to external light sources. This guarantees reliable, robust operation.
Optical distance sensors by Baumer automatically adapt to different object colors and brightness levels by varying their transmission intensity and optimizing their exposure time. This they are not affected by the reflectivity of an object. It is also possible to measure objects with a reflectivity of up to 2%.
The measurements of several sensors can be synchronized through the sync-in input. For thickness measurements, two sensors can be triggered simultaneously in synchronous mode through the sync input. In asynchronous mode, on the other hand, several sensors that interfere with each other in an application can deliberately be operated one after the other.
The noise of the output signal can be reduced by activating filtering, thus increasing the resolution. The filter is used to suppress measurement errors. The output changes only after a defined number of measured values. The measuring frequency is not affected by this filter, but the response time is. The filter function can be parameterized through the selection of predefined precision modes.
The measuring range can be adjusted by the user within the maximum measuring range with the teach-in button, the teach-in line or through the display. The analog output has its full stroke within this taught-in area and thus higher measuring accuracy. The factory setting is the maximum measuring range.
A switched output should switch as soon as a defined measured value exceeds or falls below the set level. For a reliable switching signal, the hysteresis (difference between the switching point and the return switching point) can be parameterized in millimeters in absolute terms. The safe operation of your system is guaranteed, regardless of the position of the object in the field of view.
Time-of-Flight / Run time measurement
With the time-of-flight measurement method distances are measured indirectly by measuring the time required by a signal to travel the length of the range to be covered. This translates into the real world as follows: a sender unit is emitting a burst signal which, when reflected by an object, is picked up by the sensor's receiver. The sensor's electronics evaluates the time elapsed and/or the phase-shift encountered which is then converted into distance information. By applying the run time technology objects can be detected precisely and reliably even at long distances.
