Sensors protect power station cranes

Edited by the Engineeringtalk editorial team Jun 12, 2007

Sick|Stegmann's Pomux KH53 Linear encoder system allows the Dinorwig hydroelectric power station to avoid collisions between its three maintenance cranes.

The manmade caverns of the mountain of Elidir, only a few miles from Snowdon, contain a hydroelectric power station capable of generating sufficient power to supply the electricity needs of the whole of Wales for five hours.

The Dinorwig hydroelectric power station, which has Sick|Stegmann to thank for a new maintenance crane safety system, 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 m3 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, when we all switch on our electric kettles, creating a demand for additional generating capacity, the water is allowed to flow back from the top lake to the bottom lake through the turbines - generating the electricity required to satisfy the additional demand.

In generating mode, the power station can produce 1728MW of electrical power for a period of up to five hours.

In as little as 12 seconds - about as long as it takes to read this paragraph - Dinorwig can go from zero to 1320MW.

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 used 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 could be possible for the cranes to collide, prompting them to look at solutions 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 - a noncontact 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.

2m 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 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 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 represent in the UK.

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