... from www.positek.com
To control the tension on a delicate bundle of cotton fibres
as passing through processes at high speed requires a sensor
that can measure very small angles with high resolution over
a long time. The cotton fibre does not have a very consistent
cross section as it is drawn through the processes and therefore
it has to be pulled with a variable load. This pulling load
has to be controlled accurately so the fibre bundle is not overstretched
The sensor chosen for this measurement and control system was
the Positek P502 small angle rotary sensor. The sensor measures
a maximum angle of +/-2.5° rotation and produces a 05V to
9.5V output over the range from a 24V supply.
Potentiometers were originally rejected for the application
because of the very high number of very small angular movements
that the sensors move through. These small movements are normally
described as dither cycles and are the Achilles heel for the
lifetime expectancy of a potentiometer. The engineers were concerned
that a potentiometer would soon fail and with machines all over
the world there would be a high service cost.
The P502 sensor has no rubbing contacts used for producing
the output signal and therefore the sensor has a much higher
cycle life than a potentiometer. Tests have been carried out
to over 100 million dither and full travel cycles without difficulty.
The P502 sensor incorporates technology developed by Positek
and used in its other rotary displacement sensor products. This
PIPS® technology is based on inductive coils and uses a
unique custom integrated circuit to produce an accurate analogue
output signal. In the P502 sensor a specially designed printed
circuit board is laid out with tracks to make an array of coils
which allow the sensor to produce very high resolution and stability
over a small angle of rotation.