Optical encoders used in crane application

As the exclusive UK distributor for Wachendorff Automation, Variohm Eurosensor presents the following technical note on high-performance optical encoders used in a crane application.

Geared cam limit switches have for a long time been an integral part of cable winches.

They are constructed in such a way that they reproduce numerous turns of a shaft on the angle of rotation of the integral cam-disc or camshaft.

As they move, the individual lobes operate mechanical switch contacts, which, for example, ensure the emergency shutdown of the winch before the end of the cable is reached, or for slower unwinding of the winch shortly before this point.

Geared limit switches are essential component parts of power-operated hoisting equipment, their use being stipulated by accident prevention regulations.

They are required for independent limiting of travel on cranes and conveying equipment.

However, movement in the opposite direction must still be permitted after operation of the limit switch.

For some time, cam gears have also been used in the construction of theatre and stage equipment, as modern stages become ever more mobile and versatile.

For 75 years, the Stromag Company from Unna in Germany has been manufacturing clutches, brakes, flexible couplings and geared limit switches.

It is one of only a few German companies that has obtained the certification required for geared limit switches for the stage construction industry.

Experience in the manufacture of geared limit switches has been built up over the years.

Stromag's design and development department faced a new challenge as a result of the fact that in modern power-operated hoisting equipment geared limit switches are not only required to fulfil their traditional functions, there is now also the requirement that electronic detection of the winch position has to be provided to the crane operator.

This is particularly helpful, because the human eye is not able to accurately estimate large distances and so the crane operator also has to rely on external help.

By using a digital display it is possible to see the exact length of cable that has been paid out or the height to which the crane arm has been raised.

This small addition makes operating a hoist much easier and improves the ease of operation with heavy loads.

Stromag has integrated various combinations of geared limit switches with position feedback systems into a flexible choice of housings.

The immediate goal for Stromag was to find a system that would be easy to retrofit to existing systems.

Retrofitting incremental measuring systems to existing geared limit switches can be achieved using an assembly bell with integrated encoder systems.

Even the first of these requirements posed a problem, as the hollow-shaft encoder, which was to collect the desired data, together with all the necessary connections, had to fit into the small housings of the various cam gears.

Furthermore, it also had to be capable of accommodating a relatively large 14mm diameter shaft with spring coupling, for retrofitting to older existing systems.

In the face of stiff competition, the Wachendorff Company from Geisenheim managed to secure the order.

The company was able to fully comply with the requirements of Stromag by customising the encoder model WDG 58H, so that the new dimensions of the encoder met Stromag's specifications, with a diameter of 58mm and an overall length of 44.5mm.

The accuracy of the incremental encoder, with up to 5,000 pulses per revolution, is sufficiently high enough for use in the corresponding limit switches.

The functionality of the encoder is assured even under adverse conditions, thanks to its IP65 protection level.

The maximum allowable operating temperature is in the range of - 40C to +80C.

The encoder's permissible maximum shaft loadings are 80N radial and 60N axial.

The encoder is double supported.

Two precision ball bearings provide backlash-free support for the encoder.

The service life is 109 revolutions at 100 per cent of full rated shaft load and is extended to 1,010 revolutions at only 20 per cent bearing load.

The infrared LED that is used has a life of around 100,000 hours - a figure normally only rarely achieved in limit switches applications.

WDG58 encoders also come equipped with an early warning output that transmits a warning signal if the light intensity of the diode falls to more than 10 per cent below the original value, which signals an imminent failure.

Despite this, the encoder will go on working without any problems for more than 1,000 hours, with the advantage that it can be exchanged during a normal maintenance period.

As a general rule, all encoders are wired up via a radial connector; however, in this case, this would have taken up too much space in the housing.

So for the application with Stromag limit switches, this was replaced by a cable output.

This cable now leads to a connector that is attached externally on the cam gear.

The cable length with Wachendorff encoders can be up to 100m.

In reality, only a few centimetres are necessary.

If Stromag now receives an order for a cam gear with a position feedback system then the encoder is simply slipped onto the shaft of the cam gear.

The separate housing that accommodates and fixes the WDG 58H is then screwed to the gear and wired up - the rest of the limit switch remains unaltered.

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