1998-07-09 , E/DS-1024/11
Product News From Philips Semiconductors

A few years ago, Philips Semiconductors, the largest European semiconductor company, developed a unique design of rotational speed sensor that is more accurate and robust than the conventional Hall effect sensor. Initially targeted for ABS brakes in the automotive industry, its Magneto Resistive (MR) sensors have been sold in their millions. The design is optimised for the automotive industry and uses just two wires to reduce cable and connector costs. Philips Semiconductors has now launched a version, called the  KMI16, that uses an industry-standard, three wire or open-collector interface. This opens up the potential usage for these devices into areas, such as washing machines, dishwashers, and industrial machinery, which need feedback on the speed or position of a rotating component.

The sensor works by detecting the changes in a magnetic field when a ferrous material moves through it. For example, a toothed wheel passing through a magnetic field will result in the sensor detecting a change every time a tooth passes. This means that the sensor can work accurately right down to very low rotational speeds. The conditioning electronics cleans this signal and generates a distinct digital pulse on the 5V output for each change so that the frequency of the signal is proportional to the rotational speed of the wheel. By contrast, inductive effect sensors have output levels that are proportional to the speed of the rotating object and so are not very accurate at slow speeds. In addition, the output can vary from milliVolts to several Volts, requiring additional electronics to turn this into a usable signal.

Another important advantage over Hall effect sensors is that Philips Semiconductors' MR sensors are not pressure sensitive and so can be injection moulded to form complete sub-assemblies containing the electronics and magnet. This complete system approach makes it very easy to design in and install as the configuration of the magnet and sensor is taken care of already.

The MR sensor uses a nickel and iron alloy that changes resistance under a changing magnetic field. The changes are detected using a Wheatstone bridge circuit and are very distinct, being some 10 to 100 times larger than Hall effects. This enables the air gap between the sensor and the rotating object to be much greater and Philips Semiconductors specifies that the sensor will work with an air gap of up to 2.5mm. As a result, lower specification mechanical components can be used as the sensor can cope with larger tolerances, for example oscillating by the rotating part. This greater sensitivity also means that low cost ferrite magnets can be used for the sensors, instead of the high cost, high specification Semarium/Cobalt ones required for the Hall effect.

The KMI16 is priced in the region of $1.50 in 10,000 volumes and is available now.

Philips Semiconductors, a division of Koninklijke Philips Electronics NV, headquartered in Eindhoven, The Netherlands, is the ninth largest semiconductor supplier in the world. Discrete semiconductors are central to the company's operations and it is the third largest discretes supplier in the world, with the target of becoming the world's largest. It has a very broad and deep product range grouped into three specific competences: Power, RF and General Application products which cover all application areas. Philips Semiconductors is supported by a worldwide network of systems laboratories and product application groups to develop complete, dedicated solutions to specific customer requirements.