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Advantages of MR sensors

Advantages of MR sensors in challenging new applications AUTOMATION The increasing demand for Magnetoresistive (MR) sensors is largely the result of increasingly complex demands on the sensors for high performance electric drives. The sensors must not only be accurate and dynamic, but also be robust under difficult operating conditions and exhibit very high reliability. Recent market developments are generating additional demands. This paper describes the principle of operation and benefits of MR sensors. There is a growing demand for mechatronic motion systems in a wide range of industrial fields. Not only in the field of industrial automation, but also in the automotive and aerospace sectors, there is a sharp increase in the number of mechatronic actuators being applied. “Decentralized motion control”, “X-by-wire” or “more electric aircraft” are just some of the terms used to describe the results of this trend in different technological areas. 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 requwirements. The MR effect is best known from the read heads of computer hard discs or from magnetic memory (MRAM) applications, but it is also well suited to uses in sensor technology. Trends in Motion Control The market for motion control technology is characterized by a number of current trends, which are leading to new requirements for actuator and sensor technologies: higher accuracy, increased power density, decentralized motion control, increased use of closed-loop control, more challenging operating environments and faster time to market. These trends have continued in the meantime and inten-sified in the past 5 years. There is, however, a number of new, additional trends that are playing an increasingly important role in the selection of sensors for angle, position and speed measurement: 1) The “embedding” of sensors within motors and actuators is becoming ubiquitous, leading to stronger demand for compact sensors. 2) Demand is growing also for sensors with extended functionality (e.g. for condition monitoring) as well as for sensors that exhibit functional safety. 3) In new applications e.g. E-boosters for automobiles, motors are being developed with ever higher speeds and higher dynamic performance, which is increasing demand for sensors with extremely high bandwidth and low inertial. 4) The overall trend to higher energy efficiency is increasing the demand for low-power sensors. Author: Dr. Rolf Slatter, Managing Director, Sensitec GmbH, Lahnau, Germany New requirements for rotary and linear measurement These trends have an immediate impact on the sensors used for the measurement of linear and rotational motion. 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. These include high accuracy for precise positioning or high control quality, high resolution for accurate control, high bandwidth for dynamic control, compact dimensions, contactless, wearfree operating principle, high permissible operating temperature to survive hot environments, robust against contamination, high reliability, high energy efficiency and 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 principles. For example, the maximum permissible temperature range of optical encoders is often just 85 °C and seldom as high as 100 °C. This is significantly lower than MR sensors, which are qualified for automotive applications at 150 °C. Furthermore, optical sensors have limited potential for miniaturization 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. MR sensors exhibit similar performance benefits in comparison to inductive and capacitive position sensors as well. Basics of magnetoresistive sensor technology Magnetoresistive (MR) sensors are firmly established in numerous applications in automobiles, mobile telephones, medical devices, wind turbines, machine tools or industrial robots: be it for the measurement of path, angle or electrical current, or as an electronic compass. Originally developed for data storage applications, the different MR effects open up new measurement possibilities for sensors, not only in terrestrial applications, but also in aerospace applications. MR sensors are robust, reliable, precise and miniaturized. This combination of features is leading to continuous growth in the application field of MR sensors. The extremely low power consumption of MR sensors makes them ideal for wireless, autonomous sensor applications. They present the developers of many dif-ferent types of mechanism or instrument with completely new possibilities to measure angle, path, electrical currents or 01 Special feature of the humanoid robot “Rollin’ Justin” is the integration of the complete electronics into the hand 26 WORLD OF INDUSTRIES 2/2018

GMR Tooth Sensor Module Motor spindle with air bearings Levicron GMBH 02 Encoder kit for high speed air-bearing spindle 03 Intelligent legged endoscopic capsule robot magnetic fields. As mentioned the range of applications for MR sensors in increasing steadily and each of the following examples demonstrates how current requirements can be best met with this type of magnetic sensor MR-Sensors support lightweight robots A good example for the “embedding” of sensors within a machine is given by the latest generation of lightweight robots. The Robotics and Mechatronics Center of DLR plays a major role in the field of applied robotics. Numerous generations of lightweight robot arms have been developed. A further development stage was the design of an anthropomorphic hand arm system for future service robotics. This competes with the kinematic, dynamic and force properties of the human arm. This highly integrated mechatronic system consists of 52 drives and no less than 112 position sensors, many of which utilise MR sensor technology. The mobile humanoid robot “Rollin’ Justin”, shown in Figure 1, is one of the latest creations at DLR and provides a research platform for autonomous dexterous mobile manipulation in human environments. The mobile base of the robot, which allows the long range autonomous operation of the system also features numerous MR-based encoder kits. High speed air-bearing spindle High speed machining is particularly relevant where the requirements regarding workpiece toleranc-es and surface quality are particularly high, such as in mould- and tool-making. GMR-based tooth sensors open up new possibilities for high-speed spindles with air bearings. As can be seen in Figure 2 a fine tooth-like structure with a module of 0.3 mm is machined directly into the surface of the spindle shaft. This provides the measurement scale for an encoder module for the commutation of the motor and positioning of the spindle shaft. There is no need for an additional encoder disc and the imbalance of the shaft can be reduced to a minimum. Furthermore this solution has no additional inertia, which enables highly dynamic acceleration and also reduced energy requirements. This solu-tion allows speeds of up to 90.000 rpm with an angular resolution of < 0.1°. This application is for incremental angle measurement, but absolute measurement is also possible using two toothed structures and a vernier principle. Mars Rover „Curiosity“ MR sensors have been used successfully in space robotic applications for more than 10 years. The „Mars Exploration Rover Mission“ (MER) is a space mission initiated by NASA, which began in 2003 with two launch vehicles to transport two automatic „rovers“ – called „Spirit“ and „Opportunity“. With this mission the geological conditions on Mars should be explored to find evidence of water. Each rover is equipped with 39 magnetoresistive sensors used as components of the magnetic encoders of special motors supplied by the Swiss company Maxon Motor AG. In November 2011 another Mars Mission called „Mars Science Laboratory“ (MSL) started. The rover named “Curiosi-ty” landed in August 2012 to search for organic material. The rover shall find out if Mars ever offered the living conditions for small living organisms. For this purpose the rover is equipped with a highly complex collection of scientific instruments. MR sensors are used to control the motors of almost all the moving mechanisms. The sensors must not only withstand high mechanical loading during take-off and landing, but also must withstand temperature swings of 190 °C and also withstand high levels of radiation. Miniaturized robotic pill for advanced diagnostics Sensitec has participated in the EU-funded project Vector with the objective of developing intelli-gent endoscopic capsules using innovations in micro- and nanotechnology. The Vector project aimed at investigating and developing a miniaturized robotic pill for advanced diagnostics and therapy in the human digestive tract. The project mission was to make a significant contribution to the diagno-sis and treatment of digestive cancers and their precursors. Figure 3 shows the legged endoscopic capsule robot, which is capable of mesoscale locomotion within the gastrointestinal tract. The robot has similar dimensions to commercial pill cameras (11 mm diameter by 25 mm length). Sensitec’s role in the project was therefore to develop a miniaturized magnetic motor encoder to control the position of the legs (Figure 3). Outlook The innovative MR-solutions for encoder- and motor-feedbacksystems, as well as for current sensors, are further examples of the potential of magnetoresistive sensors for opening up new opportunities for the designers of motors, actuators or machines. Sensitec is working on the development of sensor modules and kits with even higher performance and increased functionality. Further miniaturized 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. Photographs: 01 DLR, 02 Levicron, 03 Sensitec WORLD OF INDUSTRIES 2/2018 27


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