Absolute magnetic sensors for large diameter through-shaft applications

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1 Sensoren und Messsysteme in Nürnberg Paper 8 Absolute magnetic sensors for large diameter through-shaft applications Dr. Didier Frachon, Dr.-Ing. Gerald Masson, Thierry Dorge, Dipl.-Ing. Michaël Delbaere, Dr.-Ing. Stephan Biwersi Moving Magnet Technologies SA, Rue Christiaan Huygens, 5 Besancon, France Contact : info@movingmagnet.com Abstract This paper introduces application of a through shaft 6 magnetic position sensor technology developed by MMT to the case of large diameter shafts (typically > 4 mm and up to mm ore more) and the specific solutions that are required to get an efficient signal processing and an accurate output (typically < +/-.5% of the full stroke). Introduction Through shaft rotary position sensors are a strong requirement for certain applications where a location of the sensor s moving element at the end of a shaft is not possible. Examples are automotive steering angle sensors or absolute detection of shaft position of various electric motors over 6. Such a requirement corresponds to specific challenges, especially in the demanding field of automotive applications: simple structures, high reliability, high accuracy, compactness In that scope, MMT has developed a contactless 6 through shaft absolute angular position sensor using a probe which measures the angle of the magnetic field generated by a diametrically magnetized magnet [, ]. However, this solution meets some limits when the shaft diameter is getting large (typically > 4 mm and up to mm ore more), because of significant differences between the two magnetic field components used by the sensor, and therefore can t be used as it is. Such cases are currently getting frequent in applications like absolute position sensing of high power electric drives dedicated to electric or hybrid electric vehicles. The current state of the art for such sensors is the resolver []. However, magnetic position sensors based on Hall or GMR ICs may offer an attractive solution due to their performances and cost. In this paper, after some reminder on the basic principle of our through shaft sensor, we will explain the solution proposed by MMT to adapt this technology to the constraints of large diameter shafts, while retaining the merits of a simple and accurate structure. Then, we will also discuss possibilities to reach higher accuracies in this configuration. Prototype measurement will be provided. 6 through shaft sensor solution.. Overall principle The through shaft 6 position sensor developed by MMT relies on a sensitive device placed on an outer diameter of a diametrically magnetized magnet ring and able to measure at least two components of the magnetic field at a single point (Figure ). Figure : Through shaft 6 position sensor principle In opposition to standard end-of-shaft configurations [4], in that case the angle of the magnetic field does not follow the rotational angle of the shaft due to the fact that although magnetic field components theoretically have a sine and cosine shape, they do not have the same amplitude. It has been shown however [] that through the use of an adjustment parameter before the computation of the field angle, this difference can be balanced and that an accurate output can be reached for various sizes and strokes of sensor. Figure provides the example of the two magnetic field components in a steering angle sensor prototype having an inner shaft diameter of 5 mm. The ratio between the amplitudes is of approximately. ISBN VDE VERLAG GMBH Berlin Offenbach

Induction [G] Induction [G] Sensoren und Messsysteme 8. 9.5. in Nürnberg Paper 8 Figure shows its measurement curve over the - 4/+5 C temperature range. As one can see, the linearity is below +/-.

2 Induction [G] Induction [G] Sensoren und Messsysteme in Nürnberg Paper 8 Figure shows its measurement curve over the - 4/+5 C temperature range. As one can see, the linearity is below +/-.5% of the full stroke. Use of flux concentrators to adjust field components.. Principle Figure : Magnetic field components over 6 of a through shaft position sensor (shaft diameter 5 mm ) In order to allow a proper signal processing, it is required to significantly lower the differences between both field components or if possible to make them equal, while keeping the signal amplitude within a useful range (for example to 7 G for a MLX96). Considering the challenge to retain the simple design of the basic 6 through shaft sensor (one magnet, one Hall IC), MMT has developed a very simple solution based on flux concentrators [5], as depicted for example hereafter (Figure 5). The small ferromagnetic parts are modifying the field lines so that it drastically reduces the ratio between the field components. Figure : Non-linearity on -4 C/+5 C of the 5 mm shaft diameter position sensor Figure 5 : Through-shaft sensor with flux concentrators.. Case of a larger diameter shaft When the shaft diameter is getting higher, the difference between the components is also increasing, with amplitude ratio that will typically be comprised between 5 and for shaft diameters in the range of to mm, if one doesn t want to significantly increase the magnet height. Figure 4 illustrates the case of a 4 mm shaft diameter. This is especially due to the tangential component getting critically low, which makes it very difficult to process for the Hall ICs that are typically used for such sensors. Also, this low component will be more likely to be affected by external magnetic fields Magnet position [ ] Figure 4 : Magnetic field components over 6 of a through shaft position sensor (shaft diameter 4 mm) Br Bt As an example, Figure 6 shows the modification of the field components provided in Figure 4 thanks to the use of flux concentrators Magnet position [ ] Figure 6 : Magnetic field components over 6 of a large through shaft position sensor (shaft diameter 4 mm mm) with flux concentrators. We have to notice from Figure 6 that even if in the scope of the large diameter application we are looking for bringing both components within a reasonable range from each other, this solution even enables to get equal amplitudes for both field components... Influence of geometrical tolerances One could be slightly concerned by the increase of sensitivity due to positioning tolerances of the collectors and probe. If one considers the distance between the two collectors as an important parameter, one can see on Figure 7 that Br Bt ISBN VDE VERLAG GMBH Berlin Offenbach

3 Btangential [G] Amplitude of field components [G] Amplitude of field components [G] Bradial [G] Sensoren und Messsysteme in Nürnberg Paper 8 for a given angle between collectors there is a value for which the influence of this parameter on the radial and tangential is minimal even for typical tolerances on positioning of such parts on a PCB Bradial Angle between collector [ ] Figure 7 : Influence of the distance between collector On the figure 8 below, we show the influence of the angular width of the collector. As one can see the radial component of the induction is quite insensitive to this parameter but the tangential component increases almost linearly with this parameter Btan Bradial Figure 8 : Influence of the angular width of the collector If we consider the sensitivity to the distance between the probe and the collector and the sensitivity to the radial position of the probe, one can see in the figures below that they are two important parameters. If one considers the influence of the axial distance between the collector and the probe, one can see that for the tangential component, the closer the probe the larger the induction but for the radial component the closer the probe the smaller the induction. If one considers the radial position of the probe, one can see that the tangential component is relatively insensitive and the radial component increases as the probe gets closer to the magnet. Btan 4 5 Collector angle [ ] R = 56. mm R = 56.5 mm R = 57. mm R = 57.5 mm R = 58. mm Axial position [mm] R = 56. mm R = 56.5 mm R = 57. mm R = 57.5 mm R = 58. mm Figure 9 : Influence of the axial and radial position of the probe on radial and tangential field components As a summary, these effects can be minimized by design, but also relate mostly to assembly tolerances that can be handled by programming... Prototype measurement This principle has been successfully validated through prototype measurement as depicted in Figure 9 using a 9 mm shaft diameter. It has to be noted that it is typical of the target applications like position control of an electric machine that the sensor only has to provide sine and cosine signals to the ECU that handles the motor control. Therefore, linearity plot provided here is obtained through post-processing on a computer after acquiring the raw values of the both magnetic field components. One can see that the base non-linearity is in the range of +/-.5% of the full scale at 5 C..5.5 Figure 9 : Prototype sensor measurement at 5 C (shaft diameter : 9 mm) 4 Higher accuracy solutions Axial position [mm] Br Bt 4.. Principle A general concern for through shaft sensors, especially under the requirements of automotive steering sensor application, is to increase the accuracy. While a typical of our through shaft sensor features +/-.5% of the full-stroke of non-linearity, requirements down to +/-. or.% of the full stroke can be met. In that scope, MMT has developed a solution based on signal combination of the components of two Hall ICs placed at 9 (Figure ) as described in [6, 7] ISBN VDE VERLAG GMBH Berlin Offenbach

4 Sensoren und Messsysteme in Nürnberg Paper 8 B n B t described in paragraph... and with the same processing conditions, but under principle described in 4... Figures to 4 display B n and B t deduced from measurement of B n, B t, B n and B t B n Br+Bt Bt-Br.5.5 B t Figure : Principle of higher accuracy through shaft sensor Using this configuration and the following signal combination : B B n t n t t n it is then possible to get proper sine and cosine signals having same amplitude that enable to deduce (after processing) the position with a non-linearity below +/-.% of the full stroke. An other feature of this solution is to drastically reduce effects of an homogeneous external magnetic field, which can be very interesting in the case where the sensor is closely coupled to an electric motor in order to reduce the effect of the field generated by coils. 4.. Application to large shaft diameter sensors In a large shaft diameter sensor, this principle can be combined with the flux concentrators as shown on Figure. The flux concentrators will help providing base field components (B n, B t, B n, B t ) within a reasonable window before measurement by the magnetosensitive elements. Figure : Principle of higher accuracy through shaft sensor Figure : Measured B n and B t signals as well as non linearity Figure : Measured B n and B t signals as well as non linearity at -4 C Br+Bt Bt-Br Br+Bt Bt-Br Figure 4 : Measured B n and B t signals as well as non linearity at 5 C. As a summary, one can notice that the non-linearity is remarkably low at 5 C and -4 C (typically +/-.% of the full stroke), and is still very good at very high temperature Hysteresis In such a structure using ferromagnetic concentrators, it is important to check the hysteresis. Figure 5 shows linearity measurement on the sensor for 6 displacement in the clockwise and counterclockwise directions with very small difference which illustrates the hysteresis is extremely small Prototype results In this paragraph we will detail measurement results on a prototype using the same magnet and shaft diameter as ISBN VDE VERLAG GMBH Berlin Offenbach 4

5 Sensoren und Messsysteme in Nürnberg Paper 8 It may also be used to insure compatibility of a through shaft sensor of any size with specific ICs or magnetic field measuring principles that are not able to cope with an adjustment of the magnetic field components, through direct matching of both field components before they are measured. Figure 5 : CW and CCW linearity measurement 4..5 Effect of an external magnetic field One of the feature of the signal combination solution described here above is to theoretically provide a direct cancellation of an homogeneous external field (see [7] for a complete description), keeping in mind however that in reality this field may be more or less homogeneous. It is therefore interesting to look at the realistic case of a field generated by the coils of an electric machine and driven to the sensor through its shaft, which in general will be somehow ferromagnetic. We have therefore built up an experimental set-up with a large coil axially placed over the sensor to simulate such an axial perturbating field. Figure 6 shows results obtained with an equivalent field of Gauss which is a significant value because it represents approximately a large part of the useful magnetic field of the sensor (for example amplitude of B r + B t is of 5G) We can notice that the overall linearity of the sensor stays however well below +/- % of the full stroke. 6 Literature [] Magnetic angular position sensor for a course up to 6, European patent application EP9496 [] Jerance, N. et al.: Through-shaft contactless magnetic sensor with a stroke up to 6, Proc. of SENSOR+TEST Conference 7 [] Kitazawa, K., Principle and application of resolvers for hybrid electric vehicles, Proc. of Innovative Automotive Transmissions Conference 9 [4] et_484.aspx [5] Capteur de position magnétique à mesure de direction de champ et à collecteur de flux, French patent application, not published yet [6] Capteur de position magnétique angulaire ou linéaire présentant une insensibilité aux champs extérieurs, French patent application FR99 [7] Masson G. et al., Multi-turn and high precision through-shaft magnetic sensors, Proc. of SENSOR+TEST Conference 9, pp Figure 6 : Effect of an axial external magnetic field of G. 5 Conclusion In this paper, we have described a simple way to adapt a through shaft magnetic position sensor to the conditions of very large shaft diameter applications, by adjusting magnetic field components and enabling therefore a proper signal processing. Possibilities to improve accuracy have also been discussed and prototype results have illustrated the good potential accuracy of such sensors, as well as low hysteresis and capability to withstand external disruptive fields. A direct application of such principle is absolute position of the shaft of large electric drives for electric or hybrid electric vehicles. ISBN VDE VERLAG GMBH Berlin Offenbach 5

Scalable linear magneto resistive sensor arrays

Summary Scalable linear magneto resistive sensor arrays Andreas Voss, Axel Bartos TE Sensor Solutions, MEAS Deutschland GmbH, 447 Dortmund, Germany Andreas.Voss@te.com 031-9740 560 The advancing digitalization