Piston Position Monitoring

In pneumatic applications, cylinder sensors are commonly used to monitor piston position.

A cylinder sensor is mounted on the housing of a pneumatic cylinder, precisely at the point where feedback on the piston position is required.

The piston is fitted with a magnetic strip that detects the sensor when both components are aligned. The magnet activates the position sensor, which in turn emits a signal. This signal is passed on to a higher-level control system or PLC. Depending on the program, the system then initiates the next step or activates safety measures.

Reed Switch: Operation Based on Magnetism

Inside a housing filled with inert gas or vacuum, two thin overlapping metal reeds are drawn together under the influence of a magnetic field.

When the piston’s magnetic field reaches the sensor position, the reeds move towards each other, creating contact and generating an output signal.

Reed switches are suitable for voltages from 5 to 230 V and typically have a service life of 20–50 million switching cycles.

These sensors are not protected against overload, polarity reversal, or short circuits.

Reed switches provide an efficient and cost-effective solution for detecting piston position.

Solid-State Sensors: Operation Based on Magnetic Field Resistance

The operation of a solid-state sensor is based on a change in resistance within the sensor caused by a magnetic field. A solid-state sensor is typically equipped with four so-called magneto-resistive sensors connected to a Wheatstone Bridge.

When the piston’s magnet reaches the solid-state sensor, the resistance of the individual sensors changes, altering the bridge voltage. This voltage change generates an output signal from the solid-state sensor.

A key advantage of a solid-state sensor is the absence of moving mechanical components. The entire circuit consists of solid-state semiconductor elements that are protected against overload, reverse polarity, and short circuits.

As a result, there is no wear, and these solid-state sensors achieve a much longer service life, with an average MTBF of around 4,077 years.

Pneumatic Sensors: Signal at End Position

These pneumatic sensors are designed to provide a pneumatic signal when a pneumatic cylinder reaches its end position.

When the piston of a pneumatic cylinder reaches its end position, the exhaust side of the cylinder becomes depressurised.

As the pressure drops, the pressure from port P of the two push-in fittings moves a diaphragm back, creating a connection between port P and port S. The pressure signal from port S confirms that the cylinder has reached its end position.