Magnetic sensorsThere are many ways of sensing magnetic fields. Optical sensors can be based on crystals that exhibit a magneto-optic effect, or specially doped optical fibres. Coils can be used, although microfabricated coils are generally two dimensional, which often isn't useful for many applications. The continuing development of high-temperature superconductors is also broadening the possibilities for sensors based on superconducting quantum interference devices (SQUIDs), which are capable of detecting the the magnetic fields the of heart or brain. There are also a variety of other devices.Many measurements can be made, however, using Hall effect sensors. These are very common, and are outlined below. A Hall effect sensor is shown diagrammatically in figure 5. The sensor consists of a conducting material, usually a semiconductor, and a current is passed between two contacts on opposite sides of the device. Two sensing contacts are placed on two other sides of the device, opposite each other, perpendicular to the current flow. A magnetic field perpendicular to the plane of the contacts is causes a deviation in the current flow across the device. This in turn is detected as a potential difference between the two sensing contacts.
 Figure 5. Hall effect sensors operate typically in the range 0.1 milli-Tesla to 1T (the earth's magnetic field is about 0.05mT). Hall effect IC packages are available which typically give an output of about 10mV per mT. |