How a cylinder sensor works
A cylinder sensor can be mounted on the housing of an electric or pneumatic cylinder, precisely where feedback of the piston position is required. The piston is equipped with a magnet (tape) that is detected by the sensor when both components are in the same position.
The magnet triggers the position sensor, which generates a binary signal. This signal is sent to a higher-level controller or PLC. Depending on the programme, a next step is taken, or safety measures are initiated.
Types of cylinder sensors
Cylinder sensors or position sensors are available in two main variants. The two most common in pneumatic and electrical drive technology are reed and solid-state sensors.
Reed sensor
The operation of a reed sensor or reed contact is based on magnetism. In a housing containing an inert gas or vacuum, there are two thin overlapping metal strips, one fixed and one sprung open, that moves under the influence of a magnetic field. . . When the piston magnet reaches the sensor position, it attracts the moving metal strip into contact with the fixed strip. This creates a closed circuit, and the reed sensor generates an output signal.
Features of a Reed Sensor
An effective method of detecting a position, but due to the moving mechanical components, it is also subject to wear and tear. Reed sensors have a limited life span of 20 - 50 million cycles, and their performance deteriorates with time due to the 'sticking' of the metal strips. The inert gas or vacuum reduces the risk of internal micro sparking and oxidation but being a mechanical device with moving parts; there will be a finite life expectancy. In addition, this sensor is not protected against overloading, reversed polarity and short circuits. The maximum current is 500 mA, and the maximum cable length is 10 m.
Solid-state sensor
The operation of a solid-state sensor is based on a change in resistance in the sensor due to a changing magnetic field. A solid-state sensor is typically equipped with four so-called magneto resistive sensors connected to a Wheatstone Bridge. When the magnet in the cylinder piston or on a moving shaftreaches the solid-state sensor, the resistance of the individual sensors will change and thus change the bridge voltage, the voltage change will - after amplification and processing - lead to an outgoing signal from the solid-state sensor.
Properties of solid-state sensors
An important advantage of a solid-state sensor is the absence of movable, mechanical components. The entire circuit consists of solid-state semi-conductor elements that are protected against overload, reverse polarity and short circuits. This means that no wear and tear occurs. Solid state sensors, therefore, have a long life span and typically a mean time before failure MTBF of 4077 years (!). The maximum current is 100 mA, and the maximum cable length is 30 m.
When to use a reed and when a solid state cylinder sensor?
Because of several favourable features of the solid-state cylinder sensor and its much longer life span, in 90% of the applications, this version will be the preferred option. Due to properties such as:
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Festo solid state cylinder sensors:
The Festo product range includes the following solid state cylinder sensors covering 90% of industrial applications.