Case Study P502
Sensor operating in Crosol textile carding machine
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 and damaged.
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 0.5V 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.