NPL has been developing a range of hardware and software to produce a high accuracy, portable coordinate measuring machine (CMM) based on multiple laser trackers and multilateration.

Overview

NPL is researching the use of a network of high accuracy laser trackers and multilateration software to produce a portable Coordinate Measuring Machine (CMM) with a volumetric uncertainty significantly better than any existing CMM. NPL has developed and patented a novel design of laser tracker, which is to be used for this research work.

Research

Multilateration is a way of measuring the position of a target, or targets, relative to several fixed measuring stations. Measurement data associated with the displacements of the target relative to each of the measuring stations is collected as the target is moved through a calibration path. Generally speaking, these positions will cover a large part of the volume encompassing the locations of the measuring stations. From this displacement data the spatial coordinates of the measuring stations can be determined as well as the positions of the target, with no prior knowledge. When a target is located at or near the key features of the component to be measured the coordinates of the measuring stations are used to determine the coordinates of the target and hence the position of the features of interest.

This technique can achieve accuracies significantly better than those of the individual measuring stations and no prior knowledge of station location is required, i.e. the system is self-deterministic. Various measuring stations can be used, however NPL has concentrated on the use of tracking laser interferometers ('laser trackers') since these can already achieve small measurement uncertainties when used individually, hence a multilateration system consisting of a network of such devices can be capable of very small uncertainties.

NPL has developed and patented a novel design of laser tracker, which is to be used for this research work. NPL is also researching the use of laser trackers for error mapping and machine verification of CMMs, machine tools and large-scale metrology systems.

As part of this work NPL has developed a range of sophisticated software tools for solving generic multi-sensor metrology networks and for determining the uncertainty associated with the target locations and other system parameters. The techniques we have developed are applicable to any measurement network involving multiple sensors, be they distance, displacement or angle based, or any combination. This flexibility makes these tools particularly useful for assessing the performance of new classes of large volume dimensional measurement systems such as laser radar, indoor GPS, etc.

These software tools are applicable to many of today's challenging advanced manufacturing and assembly processes in the aerospace, automotive, defence and power generation fields.

NPL and PTB have jointly developed a method of directly measuring the parametric errors of CMMs and machine tools using a single laser tracker and a form of multilateration. This patented technique has the following advantage over other techniques:

• High accuracy
• No physical artefact is required
• The method work for any size of machine
• The technique is much quicker than other techniques

Research for third parties

As well as the government-funded Large Volume research, NPL also undertakes research studies, measurement campaigns and contract-based research in Large Volume for a range of customers.

Contact: Ben Hughes