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Abstract—Magnetoresistive sensors are not only used for

measuring rotational and linear motion, but also for non‐contact switching applications and furthermore for highly dynamic current measurement. This is largely the result of increasingly complex demands on the sensors for high performance electric drives. Sensors must not only be accurate and dynamic, but also be robust under difficult operating conditions and exhibit very high reliability. Recent developments, such as the trend to electro‐mobility, are generating additional demands, with respect to compact dimensions and energy efficient operation. This combination of demands is leading to the more intensive use of magnetic sensor principles, compared to optical, inductive or capacitive principles.

Index Terms—Sensors, magnetic sensors, motor feedback systems, encoders, current sensors

I. INTRODUCTION HERE is an a growing demand for mechatronic motion systems in a wide range of industrial fields. Not only in industrial automation, but also in the automotive and

aerospace sectors, there is a sharp increase in the number of mechatronic actuators being used. “Decentralized motion control”, “steer-by-wire” or “more electric aircraft” are just some of the terms used to describe the results of this trend in different technological areas.

This development is placing new demands on the technology used for measuring rotational motion, linear motion and electrical current. Fig. 1 shows a simplified structure of a motion control system. This system typically comprises a servo controller with current sensors, a servo motor, speed and position sensors (encoders) as well as mechanical machine elements.

Presently there are number of trends that are changing the technical and commercial requirements for both actuators and sensors. The high performance and flexibility of magnetoresistive (MR) technology is playing an increasingly significant role in helping machine and actuator designers to deal with these new requirements.

The magnetoresistive effect is best known from the read heads of computer hard discs or from magnetic memory

Manuscript received June 15, 2012. R. Slatter is with Sensitec GmbH, 35633 Lahnau, Germany (phone: +49

6441 9788 13; fax: +49 6441 9788 17; e-mail: rolf.slatter@sensitec.com)

(MRAM) applications, but it is also well suited to uses in sensor technology. It has a long history, being first discovered in 1857 by Lord Kelvin. However, the MR-effect did not experience widespread use until the early 1980s, when the first MR-based read heads were implemented in hard disc drives [1], [2]. The first industrial applications for MR-based sensors followed at the beginning of the 1990s, since when the number of applications has increased dramatically. The applications are not only limited to terrestrial use – MR sensors are used to control the electric drives used on “Curiosity”, the Planetary Rover that is planned to land on Mars in August 2012.

Fig. 1. Simplified schematic of a motion control system

II. TRENDS IN MOTION CONTROL The market for motion control technology is characterised

by a number of current trends, which are leading to new requirements for actuator and sensor technologies [3].

a) Higher accuracy The emergence of microsystems- and nanotechnology are setting new targets for the accuracy of robots, machine tools and other forms of industrial automation. Not only are the demands for positioning accuracy increasing, but also the demands for higher resolution to allow better path control. Last, but not least, the required accuracy should be stable over the operating life of the machine and remain consistent under different environmental conditions. This is leading, in turn, to increased use of direct measurement systems.

b) Increased power density

Magnetoresistive Sensors for High Performance Electric Drives

Dr. Rolf Slatter

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There is a trend to more compact machine design, to make best use of available space and to increase energy efficiency. As a result the power density of actuators is increasing steadily, in the form of higher output torques / forces as well as higher speeds without a proportional increase in physical size.

c) Decentralized motion control The increasing computing power of microcontrollers and digital signal processors is leading to a shift in measurement and control tasks down to the level of the individual actuator. Decentralized actuators comprising servo motors with integrated electronics need less wiring, less assembly effort and also demand less computing and storage capability from higher level controllers or PLCs. This significantly reduces the costs for machines featuring a large number of actuators.

d) Increased use of closed-loop control Electro-mechanical actuators are beginning to be used in applications previously reserved for hydraulic or pneumatic actuation. In order to counteract this trend manufacturers of hydraulic and pneumatic equipment are increasingly employing closed-loop control to improve the accuracy and dynamic performance of their actuators. This is leading to increased demand for linear and rotary position sensors.

e) More challenging operating environments There is an increasing number of applications at low or high operating temperatures. There are the well-known examples, such as aerospace, but increasingly actuators and sensors in the industrial field are being operated at extremes of temperature.

f) Faster time to market The life cycle of almost all types of machine is becoming shorter. This means that development times must also become shorter to enable a quick amortisation of the development effort. This is leading machine and actuator designers to look increasingly to intrinsically reliable sensor solutions with a higher degree of integration.

III. NEW REQUIREMENTS FOR ROTARY, LINEAR AND CURRENT MEASUREMENT SENSORS

These trends have an immediate impact on the sensors used for the measurement of linear and rotational motion as well as for the measurement of electrical current. They result in a complex set of requirements, most or all of which apply in many new applications. The actuator or machine designer must search for a sensor solution that offers the following features [4], [5]: - High accuracy, for precise positioning or high control

quality - High resolution, for accurate control - High bandwidth, for dynamic control - Compact dimensions, to allow easy integration in a small

envelope - Contactless, wear-free operating principle, to ensure a

long operating life or provide high isolation strength - High permissible operating temperature, to survive hot

environments

- Robust against contamination, to withstand dust, oil, water or worse

- High reliability, to minimize down-time - High energy efficiency, to optimize running costs - Increased integration and functionality (“ready-to-

measure”), to allow a fast “time-to-market” This complex combination of requirements is best met by

sensor systems based on MR sensor technology. Magnetic measurement systems offer a variety of advantages compared to other measurement priniciples. Optical systems, for example, offer a high measurement accuracy, but due to limited linearity a lower resolution than magnetic systems. The maximum permissible temperature range of optical encoders is often just 85 °C and seldom as high as 100 °C, which is thereby significantly lower than MR sensors, which are qualified for automotive applications at 150 °C. Optical sensors have limited potential for miniaturisation due to their complex construction (LED – glass scale – sensor). Last, but not least, magnetic sensors are much more robust and less susceptible to contamination through dirt or fluids [6].

These advantages are the basis for an increasing market share for magnetic sensors, as demonstrated by the following examples.

IV. MAGNETORESISTIVE SENSORS FOR ACTIVE MEASUREMENT SCALES

In order to reduce the time-to-market for new actuator and machine designs the latest MR sensor chips are increasingly integrated in modules and kits, which reduce the design and assembly effort at the user, by incorporating signal processing electronics and offering a matched combination of sensor module and measurement scale. Modules and kits from Sensitec are opening up new possibilities for machine designers in a variety of different fields.

The trend to decentralized motion control is most evident in the field of woodworking, packaging or paper converting machines. The decentralized actuators consist of servo motors with modular electronics on-board. There is then no need for a complex control network and higher level control computers are not overloaded with the complicated control of individual machine axes. Dunkermotoren is one of the leading manufacturers of decentralized actuators and offers a wide palette of brushless DC-motors with integrated electronics. There are versions featuring simple commutation up to completely autonomous actuators with a high level of on-board intelligence. In all these actuators the sensors used to provide angular feedback play an important role. These sensors enable the actuator to be used in a number of different operating modes – for speed, position and torque control.

Fig. 2 shows a section through the brushless DC-motor with integrated electronics. The encoder module is located directly at the interface between the motor and the compartment for the power and logic electronics. The requirements for the encoder module in this application are complex. What is needed is not only a very compact design, but also high

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resolution, a high bandwidth for highly precise speed and torque control, a high absolute angular accuracy for position control, a high permissible operating temperature due to the high power density of the motor, high stability under different operating conditions, insensitivity to dust, grease and other contamination, a large permissible air gap between sensor and measurement scale and last, but not least, a long operating life.

Fig. 2. Brushless DC-Motor with integrated power and control electronics (Source: Dunkermotoren)

The development team at Dunkermotoren looked long and hard for an easy to integrate module, but the complex combination of requirements made it difficult to find the appropriate solution. Optical encoders could not be used either due to the small available space, the difficult environmental conditions, high operating temperatures or large assembly tolerances. Magnetic encoders based on hall-effect sensors were also not applicable due to the tough requirements regarding resolution, accuracy and high temperature stability. Eventually Sensitec was given the task of developing an extremely compact, robust, yet accurate MR-based encoder module.

Fig. 3. EWS16 MR-Encoder Kit (Source: Sensitec)

Fig. 3 shows the encoder kit, comprising an injection moulded pole ring with 32 poles and an electronic module. The pole ring is mounted directly on the motor shaft and does not require any additional bearing support. There are two code tracks for an incremental angle signal and a reference signal. The electronic module has an area of just 1 cm2 and is mounted directly on the power electronic board inside the

motor housing. The electronic module includes two MR sensor chips as well as an interpolation-ASIC for further signal processing. The incremental angle information is measured using an anisotropic MR (AMR) sensor chip, while the reference signal is generated using a giant MR (GMR) sensor chip.

The key performance data for the encoder kit are as follows: - Resolution 4096 flanks per revolution - Angular resolution 0.088 ° - Absolute accuracy +/- 0.3 ° - Maximum permissible speed of rotation 8500 rpm - Output signals A,B,Z (TTL) - Supply voltage 5 V

The encoder kit has undergone an extensive set of qualification tests under very demanding environmental conditions. There are now several thousands of encoder kits already in the field and further motor sizes will be equipped with this solution.

MR-based solutions are not only suited to integration inside motors, but also are used increasingly for position encoders mounted externally on or within a machine. Many of the requirements are similar to those for the encoder module described above, with respect to resolution, accuracy and bandwidth. However, there are additional demands placed on position encoders that need to be included in the specification. External mounting means that the encoder is more likely to come into contact with dirt, oil, water or other contaminants. Furthermore the shock and vibrational loading can often be higher than for motor-integrated encoders. Sensitec was given the task of designing an encoder module with intelligent diagnosis and adjustment functions for a leading manufacturer of process and factory automation equipment. The module should feature an integrated self-test function, to ensure totally reliable operation over the complete specified range. There was also to be a special assembly function, which should give an optical signal when the encoder module has been correctly assembled and adjusted. This function serves to increase reliability and also allows a fast and safe assembly on behalf of the end-user.

Fig: 4. EBR7911 MR-Encoder Kit (Source: Pepperl + Fuchs) Fig. 4 shows the EBR7911 magnetic encoder module,

which is at the heart of the customer’s encoder kit. Here, too, the kit comprises a pole ring, featuring a vulcanized rubber code track, as well as an electronic module. An AMR sensor chip is used for the incremental angle measurement, while in

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this case a new GMR-based sensor is incorporated for the reference signal. The GF708 sensor features a spin valve design and has an extremely high sensitivity. This makes the reference signal particularly robust under widely varying air gaps and operating temperatures. In order to provide even better protection against difficult environmental conditions both the angle and reference sensors are mounted in SIL6 packages. The electronic module is housed by the customer in a compact package, which provides IP67 protection. Importantly, the module can be reprogrammed after housing and can be easily adapted to the specific requirements of the end-user.

Fig. 5. Encoder Module in Machine Tool Application (Source: Pepperl + Fuchs)

The encoder module is designed to operate at rotational

speeds up to 30000 rpm and can withstand shock loading up to 200 g, as well as vibrational loading up to 40 g. Fig. 5 shows a typical application in a machine tool, where the encoder kit is used to measure the speed and angular position of the main spindle motor. This solution is also well suited to applications in wind turbines, offshore and construction equipment, materials handling equipment or in commercial vehicles.

V. MAGNETORESISTIVE SENSORS FOR PASSIVE MEASUREMENT SCALES

The solutions described in Section 4 all rely on an active measurement scale, that is, a moving magnetic target, in order to provide rotational or linear measurement. However, there are many applications where the use of a magnetic scale is disadvantageous or not possible.

In some cases, where the operating temperature range is very wide, the thermal expansion of the plastic or rubber-based magnetic scale causes an unacceptable loss in accuracy. In other cases the mechanical loading of the scale is too high for magnetic materials to survive. Last, but not least, the machine designer often wishes to use an existing machine element as part of a direct measurement system. This approach can help to reduce assembly effort as well as increase system accuracy, by directly measuring the position of the moving machine element. In order to help machine designers realise compact and accurate contactless encoder solutions for this type of application, Sensitec has developed a new range of tooth sensor modules incorporating GMR sensor chips and an

integrated bias magnet. The basic structure of the GLM module is shown in Fig. 6.

The sensor chip is mounted directly on a bias magnet and enclosed within a housing. The arrangement of the chip and magnet is such, that the strength and homogeneity of the field is ideal. The magnet and sensor chip are subsequently encapsulated to protect them from difficult environmental conditions. The completed module has dimensions of just 3.5 x 5.5 x 13 mm3. The user simply has to mount the module at the right distance from the machine element that is used as measurement scale.

Fig: 6. GLM Tooth Sensor Module

The basic principles of tooth sensors have already been

described in [7], so the description of the operating principle will only be summarised here. Fig. 7 shows how the magnetic flux is distorted by the ferromagnetic toothed structure. This distortion is measured by the GMR sensor chip and converted into a sine and cosine wave for each tooth by the Wheatstone bridge arrangement on the chip. The sensor chip itself, from the GL family, has been optimised to reduce the higher harmonics in the sensor signal. This increases the accuracy of the sensor and provides a very linear and interference-free signal.

Fig. 7. GLM – Principle of Operation

The GMR layer system has been optimised for the use with bias magnets with very high field strengths (up to 300 mT), which makes the module largely insusceptible to homogeneous interference fields and guarantees an excellent signal-to-noise ratio.

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The GLM tooth sensor modules are available in different versions, to allow their use with readily available machine elements, such as gear wheels or racks with different tooth pitches (1, 2 and 3 mm) or tooth modules (0.3 and 0.5). The modules can be used at operating temperatures between -40 and +125 °C (Target: + 150 °C) and deliver a distortion-free sine/cosine signal with a high signal-to-noise ratio. The high signal quality allows a 100-fold interpolation, which allows linear movements with a speed of 10 m/s to be measured with a linear resolution of 10 µm.

These features offer the user the following benefits: - High control quality - High resolution and high positioning accuracy - Very high permissible linear and rotational speeds - Simple, cheap assembly and adjustment - Deployable in difficult operating environments (high or

low temperatures, contamination, high mechanical loads) - High energy efficiency - Can be mounted horizontally or vertically (see Fig. 8) - Allows existing machine components to be used as

measurement scales (see Figs. 9 and 10)

Fig. 8. Variable Orientation of GLM Module The GLM tooth sensor modules enable a precise and highly

dynamic measurement of linear and rotational motion and are ideally suited to encoder applications for hydraulic, pneumatic and electro-mechanical actuators. The use of such controlled actuators is a major contributor to energy saving.

Figs. 9 and 10 are representative of the many different application possibilities for the GLM tooth sensor module. In linear measurement applications the GLM module can directly sense the movement of a cylinder rod.

A similar solution to that shown in Fig. 9 has been applied to the direct measurement of a hydraulic cylinder. In this case the toothed structure machined into the surface of the rod is subsequently filled by a smooth chrome coating and polished, so that the seals of the hydraulic cylinder are not damaged. The chrome coating is non-magnetic, so it does not influence the performance of the tooth sensor.

Fig. 9. Linear Measurement with GLM Module

Fig. 10 shows a typical rotary encoder application, as

already successfully implemented in a hollow-shaft encoder for a high-speed torque motor. In this case the high operating temperature range and high permissible rotational speeds were the reasons for replacing the previous optical encoder.

Fig. 10. Rotational Measurement with GLM Module This cost-effective sensor concept offers the opportunity to

open up completely new applications by reducing or completely eliminating the need for maintenance due to the contactless and wear-free principle of operation. The simple assembly and robustness of construction increase the acceptance of the user and at the same time improve the availability of the machine in which the module is used. The “ready-to-measure” design means that the user needs no prior knowledge about magnetic circuits and can easily implement the module, so enabling a fast “time-to-market” for a new machine design. The tooth sensor modules can be used for the control of actuators in the following applications: - Machine tools - Injection moulding machines - Plastic blow moulding machines - Die casting machines - Glass moulding machines - Continuous casing machines - Tube- and wire-bending machines - Rolling and straightening machines - Woodworking machines - Packaging machines

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VI. MAGNETORESISTIVE SENSORS FOR CURRENT MEASUREMENT

A number of trends, such as increasing demand for highly compact and highly dynamic sensors for applications in the field of electro-mobility, are leading to the growth of applications for MR-based current sensors. Here electrical currents, for example, the currents of the individual phases of motors, must be measured at high bandwidth, with high accuracy, with no hysteresis and with a high isolation strength. All these requirements can be met by MR-based current sensors. Due to the high sensitivity of the MR-effect these sensors do not need any flux concentrators, as typically needed by Hall-based sensors. This means that there is no hysteresis and currents can be measured from DC to the MHz-range with extremely high accuracy at high speed by physically small current sensors [8].

Fig. 11. Motor Controller with MR Current Sensors These sensors operate using a differential field

measurement principle. The current to be measured is carried by a U-shaped conductor and generates a magnetic field. The magnetically sensitive layers of the MR sensor chip mounted in the current sensor have a geometry that allows the gradient of the magnetic field between the two legs of the U-shaped conductor to be measured. This principle strongly reduces the effects of homogeneous stray magnetic fields in practical applications.

There are two basic current sensor designs. Fig. 11 shows a typical application of MR-based current sensors for through-hole assembly. In this case the U-shaped conductor is integrated into the current sensor housing. The small dimensions of these sensors allows a significant reduction in the volume of motor controllers or DC-DC converters

Fig. 12 shows a further type of sensor, where the MR sensor chip and signal processing ASIC are mounted in a automotive-qualified JEDEC compatible housing for surface mounted assembly. In this particular case the current sensors measure the current passing through a stacked bus bar mounted beneath the PCB. This arrangement allows currents up to 1000 A to be measured at bandwidths of up to 500 kHz.

Fig. 12. 3-Phase Module with MR Current Sensors

VII. OUTLOOK The innovative MR-solutions for encoder- and motor-

feedback-systems, as well as for current sensors, are further examples of the potential of magnetoresistive sensors for opening up new opportunities for actuator and machine designers.

Sensitec is working on the development of sensor modules and kits with even higher performance and increased functionality. Miniaturised dimensions, higher resolution, higher absolute accuracy or higher intelligence in the form of extended diagnostic functions are just some of the development themes currently being explored.

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Bristol, 2001 [2] Ripka, P., Magnetic Sensors and Magnetometers, Artech House, Boston,

2001 [3] Slatter, R. and Buss, R., “Innovative MR-Solutions for Encoder- and

Motor Feedback Systems”, Proc. of 10th MR-Symposium, Wetzlar, 2009 [4] Crowder, R., Electric Drives and Electromechanical Systems,

Butterworth-Heinemann, Oxford, 2005 [5] Gißler, J., Elektrische Direktantriebe, Franzis Verlag,

Poing, 2005 [6] Loreit, U., “Neue AMR-Längensensoren mit verbesserten

Offsetwerten”, Proc. of 8th MR-Symposium, Wetzlar, 2005 [7] Traute, J., Abtastung von weichmagnetischen Zahnstrukturen mit MR-

Sensoren“, Proc. of 8th MR-Symposium, Wetzlar, 2005 [8] Slatter, R., “Magnetoresistive Sensoren für die Elektromobilität”, SMT

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