Precise and flexible 3D measurement from transportable measuring arm

Citation

(1999), "Precise and flexible 3D measurement from transportable measuring arm", Aircraft Engineering and Aerospace Technology, Vol. 71 No. 1. https://doi.org/10.1108/aeat.1999.12771aab.009

Precise and flexible 3D measurement from transportable measuring arm

Precise and flexible 3D measurement from transportable measuring arm

Keywords Disc GB, Measurement, Software

Bringing new levels of accuracy with a resolution within three microns to portable three dimensional measuring, Disc GB of Hatton in Derbyshire claims to have broken new ground for measuring tasks such as determining points, prismatic dimensions and contours, comparing a component to original CAD data and reverse engineering (Plate 4).

With the launch of Polar, an articulated measuring arm, managing director Andrew Fulton maintains: "Conventional 3D co-ordinate measuring machines (CMM) can be restrictive because there is always a time delay between manufacture and measurement. They are relatively inflexible because of the need to transport and queue any part to be measured at the machine and traditional portable arms are low accuracy having band widths of around 150 microns".

Plate 4 Polar developed and manufactured by Disc (GB) brings precision to potrtable measurement

Polar comes into its own when a company has, say, a tooling or fixturing problem, a large mould or die component on a machine or when platen location points, a press panel or an assembly needs to be measured. Because Polar is easily transportable it can be moved to a different site and check multiple points on a production flow line, for instance. By measuring directly on site, the need for lengthy strip-downs, transportation, delays and costs can be eliminated.

Polar will, it is believed, prove invaluable in any manufacturing or measuring environment and will improve the way industry can inspect and measure. It introduces high orders of micron accuracy through its portable articulated arm, which can be used in a multitude of different positions. The measuring volume is specified as a sphere which is three metres in diameter and the resolution is within three microns.

The design and development of Polar was carried out by Disc GB with DTI funding. It is of modular format with six degrees of freedom comprising a base unit for easy attachment to a stable surface and three articulated joints connecting two tubular arms to the probe holder.

The measuring head incorporates an ergonomically designed operator hand set which is thermally isolated from the probe shaft, which is said to eliminate any possibility of heat transference from the operator. The handset can be used by either left or right-handed operators and carries the controls for a trackball/mouse and probe, which can be either Renishaw touch trigger or a solid probe.

Contributing to the high accuracy, repeatability and ensuring radial stability, Polar's articulated joints have been developed around twin-race, high precision pre-loaded aerospace bearings to minimise friction. The bearing arrangement provides a consistent feel for the operator throughout the measuring envelope and any tendency for axial instability is overcome through the use of thrust bearings shimmed between hardened steel races.

A new design of analogue encoder working to 1,250,000 pulses per revolution and utilising an advanced "divider" chip is set in each joint to record and transmit position. This level of positioning enables the probe tip resolution to be maintained within a three micron band.

Digital data from probe, handset operating controls and encoders are transmitted using a network interface which sends and receives signals via a central processing unit (CPU) along a single multiplex cable forming an "intelligent" electronic data highway. This reduces the size of the normal wiring harness or bundle, creates far lower friction at the joints and increases reliability.

Within the package is a portable laptop PC preloaded with Metromec, windows-based Metrosoft CM professional 3D software package. The software covers all 3D geometric elements such as point, plane, circle, sphere. Surface functions are available to provide true CAD analysis of free form surfaces and will interface with IGES, VDA, DXF and DMIS.

The CPU is able to perform high-speed calculations of X, Y and Z data and provide the geometric information in either graphical or data screen format. Included in Polar specification is thermal compensation software to stabilise captured data in line with the measuring environment.

Designed for easy transportation, the lightweight Polar arm is housed in its own purpose designed carrying case. A smaller case secures and protects the PC and printer. Service, recalibration and repair is by "return to base", in which case, a loan machine will be supplied to ensure continuity of measurement for the customer and to maintain repair standards.

Further details can be obtained from DISC (GB) Limited. Tel: +44 (0) 1283 520127.