If hydraulic cylinders are operating in a closed circuit position control system , it makes sense to integrate the associated position measuring system within the cylinder, since it often proves very difficult to arrange this on the outside (hydraulic axis).
Integration does mean, however, that the measuring system can operate in oil and is resistant to pressure if the idea is to avoid very intricate designs. The following systems are proven designs (Figure I 17):
- systems involving changes in resistance (potentiometers),
- inductive position sensors,
- systems involving changes in capacity,
- ultrasonic systems based on the magnetostrictive principle.
Whereas the first three use measurement lengths of 500 or 1000 mm, ultrasonic sensors can be manufactured for lengths of several metres. They may also feature a digital output.
Figure I 17: Integrated position measuring systems