9 - AHP Merkle GmbH

3 Switches and sensor systems
!
The possible switches for control and position monitoring of hydraulic cylinders differ fundamentally in their
physical mode of action, their construction, their robustness and their operational limits.
Typical position detection systems are:
● Inductive switches integrated into the cylinder up to 120 °C (248 °F) (standard 80 °C (176 °F))
● Inductive, externally mounted switches up to 120 °C (248 °F) (standard 80 °C (176 °F))
● Mechanical switches up to 180 °C (356 °F) (standard 80 °C (176 °F))
● Magnetic fi eld sensors up to 130 °C (266 °F) (standard 80 °C (176 °F))
● Linear position transducers up to 75 °C (167 °F)
When selecting a hydraulic cylinder, it should be clarifi ed at an early stage
whether a sensor is needed. Subsequent installation of switches is not possible.
Electronic position switches have a certain voltage drop in „idle“. This means that it is
not possible to operate an unlimited number of electronic switches in series from a single
voltage source. In contrast, mechanical position switches have no voltage drop.
3.1 Inductive proximity switches
!
The mode of action of inductive sensors is based on a magnetic fi eld generated by a coil (winding). When an
electrically conductive material comes near the sensor, eddy currents are generated in the magnetic fi eld. An
oscillator detects the change in the magnetic fi eld, and the sensor switches. This simple sensor principle can
be used to detect positions in a non-contact, and thus wear-free, manner.
Inductive proximity switches have high switching accuracy (0.1 mm (0.0039 inches)), and can be used up to 80 °C
(176 °F) (in special cases up to 120 °C (248 °F)). This means that they are ideal position sensors for hydraulic
cylinders. They can be integrated into hydraulic cylinders in a pressure-tight manner, and are used for end
position sensing.
If a cylinder with a high pressure inductive sensor is selected, the sensing point can be defi ned up to 5 mm
(0.2 inches) before the stroke end position. Subsequent relocation of sensing point is not possible.
A special option for cylinders with inductive proximity switches is the fi tting of an external sensor; this is implemented
by means of an actuating rod opposite the piston rod. This allows easy adjustment of the sensing points.
If the ripple in the electronic system is too high,
malfunctions may occur in inductive sensors.
Switches and sensor systems en
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