Multilateration

Application for the calibration of precision machines

Behind the bulky term multilateration hides a simple geometric principle: If you know the distance of a point in space to at least three other known points, then its coordinates can be determined.

The method is used every day by all of us since the Global Positioning System (GPS) is based on this principle. The receiver (e.g. in your navigation system or mobile phone) is calculating from time of flight of microwave signals to at least three satellites the distance to the satellites and its position on the earth. The more satellites are received, the higher is the accuracy of this position measurement. Because distances can be measured very accurately the position can be determined with an accuracy of five meters at a distance of 20,000 km to the satellites! This would be equivalent to 0,5 microns on a distance of two meters.

Etalon has now made this method available for the calibration of precision machines. Instead of time of flight measurement of microwave signals Etalon technology uses high-precision laser interferometry that can measure changes in distance with a resolution of 1 nanometer (one millionth of a millimeter).

The satellite is formed by an Etalon LaserTracer with its patented reference sphere. The receiver is the reflector, which is mounted to the spindle or the probe head of the machine and which the Lasertracer automatically follows with its measuring beam.

Real-time multilateration

A system with four LaserTracers is an ideal implementation of the multilateration principle. Four measuring beams follow the same reflector and continuously measure the distance change. The fact that four distance measurements are carried out, but only three coordinates need to be calculated, makes it possible to determine also the location of the “satellites” after a certain number of measured points. Therefore, the system is self-calibrating.

The real-time multilateration today is used especially by Etalon customers in research in applications where high accuracy and at the same time the highest dynamic resolution of a motion measurement are necessary. Examples are the high-precision analysis of machine tool or robot movements or as a reference system for the measurement of large structures. Customers are, for example, the Physikalisch-Technische Bundesanstalt, the German Aerospace Center, the University of Aachen and the Chinese State Metrological Institute in Beijing.

Sequential multilateration

Much higher importance for industry has a modification of this method, the sequential multilateration. In this case, only one LaserTracer is used that measures the changes in distance to the reflector for a given set of points from one Lasertracer position, which is then changed manually. After at least three positions highly precise spatial information can be derived and then be used e.g. for machine calibration. The Etalon software is optimized for this purpose, it guides the user through the measurement process and directly calculates the geometric deviations of different axes configurations. As also in this case only the length information is used provided by the high-precision laser interferometer with nanometer resolution, this method achieves extreme accuracies, which is even sufficient to serve as a reference for the most accurate coordinate measuring machines in the world.

Therefore, this method is now used in many national metrology institutes, as well as by manufacturers and users of coordinate measuring machines and machine tools.

Check with us about the possibilities of the method.