Introduction

The emergence of the magnetoresistor gave rise to the use of a semiconductor element with a magnetically controllable resistor for a wide range of industrial applications, and one that subsequently became indispensable for decades. Using the technology, magnetic field modulation could be easily transformed to an electrical change in resistance and evaluated using a subsequently connected electronic circuit. Although the magnetoresistor was deployed in many industrial branches, the automotive sector remained the most economically dominant. As a consequence of the rapid rate of new developments in diesel technology, magnetoresistors are no longer required in this field and therefore their production became unprofitable. As a result, after announcing phasing-out of magnetoresistors, their production ceased in 2003. Intensive searching and development for alternative sensor elements then began. In automation technology, a product spectrum ranging from the simplest switching elements to high-spec sensor system for precise engine speed and position recording is affected by the developments.

Sensor elements

The magnetoresistor represented the key semiconductor element for contactless magnetic sensor technology in transforming the modulation of a magnetic field to an electrical resistance.

The electrical resistance of a magnetoresistor increases with increasing magnetic field strength due to the Lorentz force (see Magnetoresistors - GMR elements). The electron stream flows increasingly on curved paths and therefore longer distances through the semiconductor element. The disadvantage of this is that the non-linear, relatively slight change in resistance makes it necessary to purchase technically usable values with relatively broad, meander-shaped structures on the semiconductor surface.

The Anisotrope-Magneto-Resistance (AMR) effect in ferro-magnetic metals is fundamentally a different physical process. It describes the angular dependency of an electrical resistance on the current to magnetisation direction. The technically employable change in resistance in optimised AMR sensors is however limited to about 3%. Moreover, the metallic structures can be deployed for greater temperature ranges. AMR sensors are manufactured using a thin-film method, and are based on functional layers measuring only a few nanometres in thickness. Due to the orientation of the magnetic field of an external magnet, the direction of magnetisation, and therefore the resistance in the direction of the current, can be directly modulated. Particularly for recording rotational movements, this direct angular dependency has the notable benefit that no additional solid measure is required, and that extremely cost-favourable absolute measuring systems can be established.

The Giant-Magneto-Resistance (GMR) effect occurs in ultra-thin layer sequences, comprising at least two anti-ferro-magnetic layers separated by an intermediate non-magnetic layer (see magnetoresistors - GMR element). Magnetisation with anti-parallel alignment in a fieldless case will orient itself spontaneously parallel to an external magnetic field. The resistance of the GMR sensors drops in the direction of magnetisation. The GMR effect is based on magnetic coupling effects in multi-layer systems and about 10 years ago revolutionised computer hard disks with a new type of miniature GMR-based read heads. GMR elements exhibit a high degree of linearity for a technically usable change in resistance from 5 to 25%. The clearly preferred direction of magnetisation within the layer levels must however be considered by intensive magnetic field simulation when setting up sensor systems so that symmetrical and homogeneous magnetic field control is assured and that the effects of leakage fields can be minimised.

The GMR elements are available in full-bridge circuits and as a consequence demonstrate considerably less temperature drift than previously employed magnetoresistors in half-bridge circuits.

Based on the magnetic measurement principle with auxiliary magnet applied here, Lenord + Bauer have developed a new sensor generation and technological basis with GMR elements.

Solid measures

Along with the selection of a sensor element, the availability of a suitable solid measure also plays a decisive role in the construction of a robust and flexibly applicable sensor system. A solid measure that can be variably dimensioned offers marked competitive advantages for the integration of sensors into complex machines and systems in the most confined spaces.

The measuring toothed-wheels are produced with purely mechanical finishing by self-generating milling during which involute toothing is milled without impact into a ferro-magnetic steel blank. No further steps are required in principle and a simple visual inspection for damage to individual teeth often serves sufficiently for quality assurance. The tooth spacing a, and therefore also the number of teeth z and the outer diameter da are related to the so-called module m as expressed by the relationship a = m * ( and da " m * z.

Optimum scanning for generation of low-noise sine-wave signals is obtained using a sensor element layout that is precisely designed for the tooth geometry. With typical module values of 0.3 to 1, high-resolution sensor systems are realised with correspondingly high numbers of teeth or sine-wave periods per revolution.

Precision measuring toothed-wheels of this kind are suitable for constructing incremental encoders with their own bearing (see Speed sensors), which are employed as extremely robust industrial sensors in the steel sector, for wind power applications, machine tool manufacture and other industrial areas.

They are just as well-suited for direct machine integration for the purpose of utilisation of the existing bearing, which is able to withstand high engine speeds. The toothed wheels can be integrated by easy installation into a system design by clamping, shrinking or bolting. With typical inside diameters ranging from 5 mm to 600 mm and outer diameters from 20 mm to 650 mm for high resolution and high revolutions, more than 700 customised measuring toothed-wheels are a permanent feature in system designs. The high-resolution MiniCODER scans the measuring toothed-wheels with a typical air gap of 150 µm to 250 µm at revolutions of up to more than 70,000 min-1 in high-speed cutting spindles. A ceramic inlay protects the sensor surface and serves as an adjustment mark. Based on the GMR sensor technology in the GEL 2442, high quality in the sine/cosine signals and reference signal is achieved (see SIN/COS signals).

The graph illustrates amplitude synchronism of 99% for the sine/cosine traces (trace A/B), a high degree of symmetry and a low distortion factor, a phase error of less than one degree at an offset voltage of lower than 5 mV. The reference signal is located precisely between the sine/cosine incremental signal and has a high amplitude level.

Analogue signals with this quality can be interpolated upwards (2048-fold) and underline the optimum design of the GMR-based new MiniCODER generation.

In the case of ships' drives, toothed wheels constructed in an analogous manner are however implemented with inside diameters of up to 7500 mm and mounted directly on drive shafts. These toothed wheels with extremely large outer diameters are produced with a module value greater than 1. Hall elements are particularly suitable for the associated coarser teeth structures.

Speed sensors

The new GMR elements have also already parted company with magnetoresistors in the incremental sensor series SensorLine. Precision measuring toothed-wheels with module values of 0.3 to 1 are likewise employed in the series. The integrated encoder electronic circuitry has been transformed to the new GMR elements in a re-design, in which the output signal patterns and mechanics are compatible to the previous standard.

The illustrated high quality of the sine-wave incremental signals of the MiniCODERs is, of course, best achieved with self-supporting measuring systems since the design of the scanning system, comprising a GMR element, supporting magnet and a precision measuring toothed-wheel, is first installed permanently in its final position and then adjusted. The incremental signals of the high-resolution speed sensors can be interpolated with up to 10 bits, thereby enabling very high numbers of pulses (up to 270,000) to be attained.

Length scales

A toothed or slotted structure can also be introduced to linear solid measures. The linear scales are scanned magnetically without making contact, but it is also possible to use a gliding sensor system. To do so, the principle is that only the ceramic inlay (see MiniCODER) is enlarged to cover the whole sensor surface to protect the GMR sensors. This design provides significant benefits for GMR-based sensors because the geometrical relationships are precisely defined and are extremely insensitive to shocks and vibrations. Sensor systems of this nature are already used in their thousands in industrial applications.

Contour disc for absolute value measurements

Based on an extended Nonius principle, an absolute position is determined by scanning a new kind of solid measure, the so-called contour disc (see Solid measures). GMR elements scan the shown traces and the absolute position is determined from the clear phase relationship of the incremental signals to each other. Instead of involute toothing with dimensions in the millimetre range, a height profile of only a few 100 µm is scanned on the contour disc. The scanning system with GMR elements could be flexibly adapted to the new geometry and, applying this principle, for the first time a new type of high-resolution absolute value encoder was able to be implemented.

Technology after magnetoresistors

With the introduction of the new sensor elements based on the AMR and GMR effects the magnetoresistor has now already been successfully phased out of many applications and critical performance data of the sensor systems once more considerably improved. However, the development of the new sensor elements demanded intensive consideration of the magnetic field orientation in combination with an adapted installation and connection technique. The results demonstrate that Lenord + Bauer, with more than three decades of experience in magnetic measurement technology, were able to maintain their market leadership. A new technology base was able to be established with the launch of the new AMR and GMR sensor elements, and thousands of linear and rotatory sensor systems have already been positioned in the market. Magnetoresistors have been substituted and the era after their demise has already commenced.

Graphs and figures:

Magnetoresisor - GMR elements: A typical characteristic curve for a magnetoresistor (left figure) illustrates that the electrical resistance does not rise linearly with increasing magnetic field strength, while GMR elements (right figure) exhibit linear stretches with falling resistance for increasing magnetic field strength.

AMR effect (left): The change in resistance directly follows the orientation of the magnetic field WITHOUT an additional solid measure or modulator.

GMR effect (right): Without an external magnetic field, the magnetisation in adjacent layers is aligned in an anti-parallel pattern, but orients itself spontaneously parallel to an external field.

Magnetic field modulation: The permanent magnetic field of the supporting magnet is modulated by means of a precision measuring toothed-wheel (illustrated in the left figure) or contour disc (in the right figure). The field lines become more tightly compacted in the vicinity of a tooth or contour (indicating a strong magnetic field).

True-scale image of solid measures for recording position and speed by means of magnetic scanning. The precision measuring toothed-wheel (bottom right and second from the right) have involute toothing and a reference groove. A slotted disc and a contour disc with three tracks are shown in the centre of the image. The slotted belt is suitable for linear sensor systems.

SIN/COS signal: MiniCODERs based on GMR elements provide SIN/COS signals with 1 Vss (trace A/B) and a reference signal (trace N) of high signal quality.

MiniCODER Gel 2442: A compact magnetic scanning head with GMR elements, supporting magnet and integrated evaluation electronic circuitry in fully encapsulated housing. A ceramic inlay measuring approx. 4 x 4 mm provides effective protection for the sensor surface.

Lenord, Bauer & Co. GmbH, Oberhausen

Author: Marcus Staszeweski