Machine Design

Sensor Sense: Cylinder-Position Sensors

Cylinder-position sensors are used in pneumatic or hydraulic cylinders.

They determine the home and end stroke of the piston head within the cylinder. The difficulty lies in detecting the position of the piston within its cylindrical housing. Magnets placed within the piston head let magnetic sensors detect its position through the metal casing.

Magnetic sensors detect the presence of magnetic fields using numerous sensing techniques. Magnetic-inductive (MI) technology was the dominant method for many years. However, within the last 10 years magnetoresistive (MR) sensing technology has overtaken the MI sensors. MR provides switching speeds up to 10 faster than MI and has a higher resiliency to noise. These factors make MR widely used for cylinder-position sensing.

Magnetic-inductive technology uses a coil that dampens the amplitude of an oscillator when a magnetic field is near the sensing face. A detector circuit analyzes this dampening to trigger the sensor output. A sensor placed at each desired position pinpoints the location of the piston within the cylinder. The most common locations are the home and endstroke positions, though other positions can be monitored.

However, coils can act as antennas picking up electrical interference and noise. The result can be unstable switching frequencies and slowed sensing. Another disadvantage of magnetic-inductive technology is that it can detect only axially polarized magnets, because of the sensor’s small size and physical configuration. That creates problems if the magnet in the piston rotates away from the sensing coil.

Magnetoresistive or MR technology creates a sensing medium through use of nanotechnology to layer ferromagnetic (magnetic) and nonferromagnetic (nonmagnetic) materials. A current passing through the material monitors the resistance between the layers. When a magnetic field is present, the resistance of the material rises. A detector circuit senses the change and triggers the output in response. Unlike MI, MR technology isn’t bothered by electrical interference and noise pickup.

Initially, MR technology detected only axially polarized magnets for cylinder position sensing. However, current MR technology detects both axially and radially polarized magnets. This opens up possibilities for different applications such as level detection and speed sensing. In addition, MR technology can reside on a smaller PCB that makes it easier to produce, reduces lead times, and minimizes failures. Although MR tolerates a narrower band of temperatures, its simpler construction makes it a viable alternative for sensing cylinder position.

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