Available in the UK from measurement solutions supplier Sensors UK, the D-Series laser distance sensors from Swiss manufacturer Dimetix AG employ an innovative measurement method that is particularly effective over long distances.
There are many applications where large distances need to be measured as quickly as possible and with a high degree of accuracy. Typical applications range from rack feeders and automatic storage and retrieval systems (ASRS) in distribution centres, fill‑level controls in large tanks and silos through to mechanical engineering, e.g. in systems for processing metal tubes or in machinery used for cutting beams or planks to length. These are the areas of use for laser distance sensors that can measure distances from 0.05m to 500m quickly and with millimetre precision.
Laser distance sensors usually measure either time of flight or phase shift. For measurement of the time of flight, a short pulse of light is emitted. The pulse propagation time, i.e. the time that the light beam needs to travel from the source to a reflector and back again to the source, is then used to determine the distance. This method is fast but often not precise enough for challenging time measurements. With distances of several hundred metres, the resolution is normally only in the centimetre range. Alternatively, the phase shift of the reflected laser beam relative to the emitted beam can be evaluated instead. The phase shift is distance‑dependent, i.e. can be used to determine the distance travelled. This measurement method is considerably more accurate, but the more complex evaluation means that it is not as fast as a pure time of flight measurement.
Combined measurement
Sensor specialist, Dimetix AG, takes a different approach. In its long‑distance laser sensors, the manufacturer combines the benefits of both measurement methods by evaluating both time of flight and phase shift. To achieve a high measurement speed, the technique uses high‑frequency modulation of the laser amplitude and evaluates the phase position and the spacing of these modulated high‑frequency signals (bursts). To this end, the laser beam is amplitude‑modulated at short intervals. This enables the distance‑dependent shift in propagation time of the individual pulse bundles as well as the phase shift of the individual waves within the modulated bundles to be measured extremely quickly. Therefore, the sensors measure more quickly than would otherwise be possible and return precise values even in the case of large distances.
The D-Series laser distance sensors are suitable for distances of 0.05m to 500m and measure with an accuracy of +/- 1mm with a repeatability of +/- 0.3mm. They usually use orange reflective foil as the reflector. At distances up to 100m, the named accuracy values also apply to natural surfaces. The devices can deliver reliable measurement results even with black target surfaces or in direct sunlight if used outdoors. Their measurement accuracy is specified with a statistical certainty of 95.4% (as defined by ISO 1938‑2015). This is equivalent to +/- 2σ, i.e. twice the standard deviation. This already takes into consideration any distance errors due to the effects of temperature as well as linearity errors. The maximum measurement speed is 250Hz with an output rate of 1kHz. Users can also benefit from all these properties in the extended temperature range from -40°C to +60°C.
Thanks to their compact dimensions of 140mm long, 78mm wide and 48mm high, and with a weight of just 350 g, the distance sensors can be integrated well in the different applications and, for example, attached to the mast of a rack feeder. The robust housing meets the requirements of IP65 and therefore also provides protection in harsh industrial environments and in the case of outdoor use.
The laser distance sensors are available in eight different models that each offer the same properties but cover different requirements for operating range and resolution. For a cost‑optimised solution, it is worth carefully considering whether a maximum accuracy of +/- 3 mm would be enough for the application concerned. Furthermore, a temperature range of -10°C to +50°C is usually sufficient for indoor applications. An analogue output with 0/4mA to 20mA serial interfaces as well as digital inputs and outputs are integrated as standard. PROFINET, EtherNet/IP and EtherCAT are available as options. These are installed by simply changing the interface module of the sensor.
