Operation and design of magnetic sensors
Hall sensors host a current-carrying semiconductor element which is exposed to permanent magnetic prestress built by a stationary permanent magnet. Any change in the magnetic field intensity caused by a ferromagnetic object penetrating the field will be identified by the semiconductor element as voltage change. The sensor’s integrated electronics will evaluate the generated sine voltage in an amplified square wave signal.
The magnetoresistive element is made of a specialized material which will only respond to magnetic fields, such as generated by a permanent magnet, by outputting a digital signal. Capable of detecting even very weak magnetic fields, the material is about ten times more sensitive than a Hall element and thus allows for substantially extended sensing distances. Magnetoresistive proximity switches are omnipolar and therefore capable of detecting both north and southpole of the permanent magnet.
Detection through tank walls
The sensor can detect magnetic fields through non-ferromagnetic materials. This is particularly useful if sensor or permanent magnet require protected or isolated mounting, or when being separated from each other by a tank wall.
Shielded (flush) installation
Magnetoresistive proximity switches are conceived for shielded (flush) installation, meaning the sensing face being plain to the carrier surface. The carrier material must be non-ferromagnetic. Shielded installation in a ferromagnetic material will reduce the sensing distance by up to 25%, unshielded or non-flush installation (sensing face protruding by its diameter from the ferromagnetic carrier surface) will enhance the sensing distance by up to 25%.
The piston inside the cylinder carries a permanent magnet which builds a magnetic field to penetrate all non-ferromagnetic materials. The sensor is tripped when detecting the magnetic field. To mount the sensor it is introduced in the slot provided at the cylinder and secured. By aid of clamps and bolts available as accessories, cylinder sensors can be attached to all conventional cylinders.
The magnetic fields of the permanent magnets inside the cylinders scatter and will differ in their specifications according to the cylinder type. Thus, it is not possible to specifiy the exact tripping point of the sensor. To ensure the cylinder sensor is placed in the proper position proceed as follows:
1) Get the piston into the required switching position
2) Move the cylinder sensor in the slot in opposite direction to step one*
3) After LED light up, withdraw the sensor a little to ensure reliable switching operation
4) Secure the sensor
5) Verify the switching point by aid of the LED
*Round cylinders or Tie-Rod cylinders: First attach the sensor loosely to the cylinder using a clamp or bolt.
The heart of a magnetic angular sensor is the integrated dual differential Hall element which builds an electrical parameter related to the flux direction of an exterior magnetic field. This magnetic field rotating about the element’s center axis generates two sinusoids shifted by 90° which are utilized to detect the rotation angle for output as an absolute value. The integrated electronics evaluates the sinusoids into a linear voltage or current signal. The absolute detection principle ensures output of the correct rotation angle even after power failure.
Example of an output signal provided by a sensor with a sensing angle throughout 360° and voltage output: