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A tape measure
17/02/2006
Precision encoders analyse movement and turn it into electrical signals. At the GTC we will use them to determine the position of the telescope and its components. The encoders have now been installed...and some are more than 50 metres long.
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The tape and LEDs
The tapes that have recently been installed are the ones at the azimuth and elevation axes. The latter is equipped with two encoders, one on either side of the telescope, to correct any inaccuracies caused by twisting. These encoder tapes, which are marked with lines, will tell us how far we have moved and where we are. Position and speed will be read by reader heads fitted with an LED to illuminate the marks on the tape, which will be reflected to photocells and converted into electrical signals.
These encoders were manufactured by the German company Heindenhain, and they had to be carefully installed: at the azimuth axis, which will provide the horizontal movement, the 50-metre long tape had to be fixed into cavities bored in the circular floor. The reader heads, sited in the moving part (the spinning platform that supports the telescope structure), had to be calibrated so that they neither touch nor become too distant from the tape, maintaining a position just 0.7mm from it.
It is "miniaturised" engineering for a giant of 300 tonnes.
Natalia R. Zelman