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Hall Effect

The Hall effect refers to the potential difference (Hall voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a 'Hall bar' (or a van der Pauw element) through which an electric current is flowing, created by a magnetic field applied perpendicular to the Hall element. Edwin Hall discovered this effect in 1879.
 
The ratio of the voltage created to the amount of current is known as the Hall coefficient and is a characteristic of the material that the element is composed of.
 
 
Hall effect devices are digital On/Off sensors constructed of semiconductor material used to sense the presence of magnetic fields. In brushless servomotors they are used as position feedback when six-step commutation is employed.
 
 

MR (Magnetoresistive)

A magnetoresistive (MR) sensor comprising of a layered structure formed on a substrate includes a first and a second thin film layer of magnetic material separated by a thin film layer of non-magnetic metallic material such as Cu, Au, or Ag, with at least one of the layers of ferromagnetic material formed of either cobalt or a cobalt alloy. The magnetization direction of the first ferromagnetic layer, at zero applied field, is set substantially perpendicular to the magnetization direction of the second ferromagnetic layer which is fixed in position. A current flow is produced through the sensor, and the variations in voltage across the MR sensor are sensed due to the changes in resistance produced by rotation of the magnetization in the front layer of ferromagnetic material as a function of the magnetic field being sensed.
 
 

Giant Magnetoresistance Effect (GMR)

Electron scattering at the magnet/non-magnet interface in a magnetic layered structure depends on whether electron spin is parallel or antiparallel to the layer magnetic moment. It is observed that the resistance of the structure is much higher when the magnetic moments of the adjacent magnetic layers are aligned antiparallel than when they are parallel. Switching from the antiparallel to the parallel configuration can be achieved by an applied magnetic field. The effect is called giant magnetoresistance (GMR) and is illustrated in the enclosed figure.
 
The GMR Switch integrates GMR sensor elements with digital onboard signal processing electronics. The GMR Switch offers unmatched precision and flexibility for magnetic field sensing. The GMR Switch accurately and reliably senses magnetic fields with less error than any other magnetic sensor available. There is little shift in the magnetic field operate point of the GMR switch over voltage and temperature extremes. This enables high precision, tight tolerance magnetic sensing assemblies.
 
The GMR Switch can operate over a wide range of magnetic fields, and is the most precise magnetic sensor on the market. It is the clear choice for a digital output magnetic sensor.  
 

AMR 

The AMR sensor chip works as a "strong field" sensor; sensor magnetization follows the cogwheel's stronger magnetic field. Since sensor signals are dependent only on the resulting angle between the direction of magnetic field and current, the amount of magnetization is not critical. The sensor chip, therefore, measures a mere 0.5 × 1.8 mm². The strong field principle also produces a signal that is widely independent of mechanical tolerances.
 
To create a fixed 90-degree phase relationship between channels A and B, the AMR sensor chip comprises two sets of four ferromagnetic metal strips. A Wheatstone's Bridge arrangement shifts one against the other by a quarter of the cogwheel's pole pitch. Each magnetic pole, consequently, gives a complete and practically harmonic-free sinusoidal signal that is suitable for multiplying signals using interpolation. The index signal is produced digitally, prompted by the signal of an additional AMR sensor in the magnetic disc's index pole.    
 

VR (Variable Reluctance) 

Variable reluctance sensors are used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel.
 
As the wheel rotates, the reluctance of the flux path through the coil changes, and the flux linkage through the coil changes, which results in a change in voltage that is measured by an external circuit. The path of the flux generated by the permanent magnet varies as the toothed ring rotates in the field of the VR sensor.
 
The flux linkage varies periodically as the teeth pass the sensor. The flux linkage variation converts to a voltage signal. For example, the rotational speed of the ring of 600 RPM with 36 teeth around the ring could translates to a period of 2.8 ms for the sensor's output signal.
 
The major disadvantage of variable reluctance sensors is the decreasing signal strength as wheel rotation slows and approaches lockup.   
 

Reed Switch 

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Elwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied.
 
A magnetic field (such as from an energized coil around the glass tube or a permanent magnet moved towards it) will cause the contacts to pull together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, upon removal of the magnetic field. A more complicated configuration contains a nonferrous normally closed contact that opens when the ferrous normally open contact closes. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds.   
 

Inductive 

The inductive sensor is a simple enough device at the fundamental level. It is simply a coil of wire with a current passing through it. It completely ignores most objects which pass near this coil. However, when a metallic object passes near it, it acts as a core material in the inductive loop which increases its inductance significantly. Most non-metallic objects have a negligible effect on its inductance. The complex part of the sensor is the sensing circuitry which detects this change in inductance by monitoring the electric current in the loop. When the inductance changes enough, it triggers the sensor's output, which sends a signal to some other machine to do it's intended function when a metallic object is near the sensor. 
 

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