Customised encoder improves portable measuring arm

Disc (GB) of Hatton in Derbyshire was able to reduce the cost of joint assembly in its Polar articulated measuring arm by some 1,300 pounds using a custom encoder system

By calling upon the expertise of Heidenhain's encoder development team, Disc (GB) of Hatton in Derbyshire was able to reduce the cost of joint assembly in its Polar articulated measuring arm by some 1,300 pounds.

In addition, Disc (GB) now has a 'direct from the box' encoder assembly with fast referencing and users of Polar can recalibrate the measuring arm on-site.

Following bench marking trials, using new measuring algorithms developed by the National Physical Laboratory, Disc (GB) now claims that Polar has a higher volumetric accuracy than similar measuring arms.

As part of the marketing strategy for Polar, Disc (GB) maintains that one of the disadvantages of a co-ordinate measuring machine (CMM) is that anything requiring inspection or measurement has to be moved to where the machine is sited.

While the CMM is acceptable for smaller components, large parts such as aerospace structures, automotive body panels, moulds, on-machine tool inspection requirements and the checking for positioning and location of fixturing and assemblies can create associated problems of logistics, delay in obtaining results and put unnecessary cost on the operation.

Maintains Andrew Fulton, who's company also markets the Talon CMM: "The CMM, due to its construction, can be relatively inflexible when required to measure larger components, structures and assemblies.

The advantage of taking the measuring equipment to the point of working, which is the concept behind portable articulated measuring arms, the flexibility, time delay and logistic problems are overcome.

However, an important consideration has to be given to the accuracy and repeatability of results taken on-site by this type of device.

A portable measuring arm such as Polar comprises three articulated or universal joints which connect two tubular lightweight arms from the base to the holder for the measuring probe.

This assembly creates six degrees of freedom allowing the probe to be manipulated by the operator through a multitude of different positions.

As the probe is touched against the target to be measured, the separate trajectories of the joints have to be recorded with the respective data transmitted to the central processing unit in the control system to determine the point measurement data.

At Disc (GB) the development of its Polar transportable measuring arm focussed on the mechanical stability of the universal joints.

According to Managing Director Andrew Fulton, the joints were critical and had to work with a concentric movement because any loss of concentricity would cause a build-up of error from joint to joint.

As Andrew Fulton maintains: "If you just take a one metre arm, an error of just one arc second would magnify to an error of five microns at the probe tip." Each joint of Polar comprises twin-race, high precision pre-loaded aerospace bearings.

These bearings minimise friction and provide a smooth consistent feel to the operator throughout the measuring envelope which is specified as a sphere some three metres in diameter.

In order to achieve the three micron design resolution of the arm, Disc (GB) investigated different encoder devices and soon found that mounting of the encoder could itself induce errors.

As Andrew Fulton describes: "If you hold the probe static in a cone and then move the arms and joints, errors in each joint encoder at the point of measurement begin to influence the measurement taken.

Andrew Fulton called in Heidenhain (GB) of Burgess Hill in West Sussex to see if they could provide a solution.

Once the problems were understood, Heidenhain confirmed that an analogue encoder would provide the best solution but had nothing suitable in its standard range.

Heidenhain engineers also confirmed that the need to interpolate and change the signal to digital format, in order to interface with the control system, would not be practical due to the size of the card required at each joint interpolator board and already, with seven cables in the arm, space was at a premium.

Heidenhain's solution was to then develop an electronic interpolation device for incorporation into each encoder.

This device had the ability to communicate with the Disc (GB) interface electronics and turn the analogue signal into digital format.

However, repeatability was still a concern and so Andrew Fulton decided to redesign the joints in the arm to improve the situation.

In the meantime, Heidenhain in Germany had borrowed a Polar arm and agreed to work on the problem at its technical centre in Traunreut.

The final Heidenhain developed solution was to introduce flexibility into the ERN 1370 encoder via a 'wobble plate' which ensured the desired concentricity was maintained.

The encoder was also able to achieve a single count repeatability better than + 5 arc secs per joint while the resulting encoder to joint assembly is also far quicker and easier.

The Heidenhain ERN 1370 rotary encoder is a bottomed hollow shaft encoder with an outside diameter of just 58 mm.

To assemble the encoder into each joint of Polar, the hollow shaft is located on the shaft of each joint and secured by two screws.

The wiring is then connected and the appropriate reference position set which takes only a matter of a few minutes.

As Andrew Fulton outlines: "The developments by Heidenhain also reduced the cost of each joint by £430 and when bench marking Polar against competitor machines it demonstrated a volumetric accuracy advantage of 600 microns.

We also have an advantage that any user can recalibrate Polar on-site by following a predefined routine.

Competitor measuring arms have to be sent back to the agent," he says.

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