Pomux linear encoder monitors three gantry cranes

Manmade caverns in the mountain of Elidir, near Snowdon, contain a hydroelectric power station capable of generating sufficient power to supply the electrical needs of the whole of Wales for five hours.

The Dinorwig hydroelectric power station is officially referred to as a pump storage scheme – an installation that can be likened to a very large rechargeable battery. During the night, the country's coal-fired and nuclear power stations generate excess electrical power, which Dinorwig uses to pump as much as seven million cubic metres of water from lake Llyn Peris at the foot of the mountain over 600m up to Marchlyn Mawr, the lake at the top of the mountain. In the morning, the water flows back from the top lake to the bottom lake through turbines – which generate the electricity required to satisfy the daytime demand.

In generating mode, the power station can produce 1728MW of electrical power for a period of up to five hours. Even more remarkable is its response time: in as little as 12 seconds, Dinorwig can ramp up its power output from zero to 1320MW.

Cranes for maintenance

Opened in May 1984, Dinorwig has been designed to have an operational life of 40 years. To keep the power station's six generators in reliable working condition a thorough maintenance schedule is essential. Every three years each generator must be dismantled and maintained. Each generator, weighing over 450 tonnes, requires large cranes to be utilised for their dismantling. For this purpose, running across the 180m long generating hall are two 250-tonne gantry cranes and one 10-tonne gantry crane.

There are two tracks on two different levels – the higher track for the two 250-tonne cranes and the lower track for the 10-tonne crane. With this arrangement, it is possible for the 10-tonne crane to travel under the two 250-tonne cranes. With all three cranes being manually controlled, engineers at Dinorwig were concerned that it was possible for the cranes to collide, prompting them to look at systems that would semi-automate the cranes and incorporate anti-collision protection.

After a long search, Dinorwig's engineers decided to install a positioning system onto each crane, with a PLC to read the positional information and communicate with the other cranes' PLCs via a radio link. The positioning system chosen for this task was Sick|Stegmann's Pomux KH53 Linear encoder system – an innovative non-contact absolute positioning encoder that enables the position of the cranes to be calculated with millimetre accuracy. The system comprises just two component parts – the read head and the measuring elements.

180m measurement length

Two-metre long measuring elements, incorporating a number of permanent magnets, are placed end to end along the full 180m length of the track. The magnets within the measuring elements are fixed in place such that the separation between them is unique and never repeated. When installed in a specific order, the magnet separations form an absolute code, which is read by the read head. The read head, mounted on the crane, can detect at least three magnets at any position along the travel. The read head can therefore detect two unique magnet separations and hence identify its absolute position.

The construction of the system enables multiple read heads to be used with a single run of measuring elements. Therefore in this application it was only necessary to install one set of measuring elements to serve all three cranes. The read head produces an SSI or Profibus DP digital output with a resolution of 0.1mm. In this application it was desirable to feed the positional data into the PLC in an analogue format. To facilitate this, the SSI data was converted to a 0-10V analogue signal using an IV251 interface module, manufactured by MKS in Germany whom Sick represents in the UK.

Now the positional system is installed and commissioned, engineers at Dinorwig can operate the cranes safely, confident that collisions will not occur.