A belting good idea

Production managers across the many industries that use high-speed band-type conveyor belts are only too aware of the problem of belts coming off their rolls.

This is caused when the rolls are not perfectly aligned — something that is virtually impossible to achieve when the rolls are typically many metres apart in a harsh manufacturing environment. Each turn of the rolls will move the belt or band a small degree to one side, with a cumulative effect (over anything from five minutes to five days) until the belt runs off the rolls and production is brought to a halt.

Various remedies are already in use, but are far from perfect. The rolls can be slightly crowned, so that they are of greater diameter in the middle than at the edges. There are some 'deadeye' systems available that stop the conveyor when the belt has displaced beyond a set limit, allowing the belt to be manually recentered. And there are a couple of 'ping-pong' devices that constantly bat the belt to and fro from one extreme position to an opposite one.

None of these are ideal, however, and a major manufacturing company recently asked pneumatics specialist Hoerbiger-Origa to investigate the problem and propose a solution for a new set of conveyors it wanted to install at Southampton Docks. Hoerbiger-Origa had several ideas and brought in development company ATI to evaluate them and develop one or more into a solution. In the event one idea has been developed into two slightly different finished products, which Hoerbiger-Origa is now planning to offer to all band conveyor users.

The core principle of the system is a small diameter 'correction roll', over which the return (non-conveying) run of the band or belt passes. This is mounted in an adjustable bracket, so that it can be pivoted about its centre point to form a slightly non-perpendicular angle to the length of the conveyor. When angled slightly out of true, the correction roll will have the effect of forcing the belt to track across the main rolls, either left or right depending on whether it is a positive or negative angle.

Correction rolls in themselves are not new, but the Hoerbiger system takes the idea further by adding an interactive closed-loop control system. An optical sensor tracks the belt's lateral displacement and this information is used to produce a proportional signal for an electronic controller that changes the angle of the roller to compensate for the offset and bring the belt back towards the centre.

The sensor is set to constantly track the position of the edge of the belt, sampling at a regular time base and in effect building up a graph of its displacement. An early issue the development team had to overcome was the fact that the edge of any working belt is likely to be uneven due to wear and tear. To overcome this ATI wrote some code that effectively damps out recurring patterns from the graph. This code resides in a Hitachi single chip controller at the heart of the system.

The chip is constantly calculating a new angle for the roll to optimise belt position and feeds this out to the adjustment drive mechanism.

Having designed and perfected the system, Hoerbiger-Origa decided to develop it into a standard product by rationalising the system design into discrete elements that could be easily assembled for volume production. As a result a four-part system was conceived: controller, correction unit/drive mechanism, belt tracker head and tracker cable.

Ian Bannister is with Hoerbiger-Origa and Alex Wilson is with Automatic Technologies International.