Application Note Magnets for MLX90316 Angle Sensor Scope The aim of this Application Note is to give useful information about magnets for angular sensing with the MLX90316 Triaxis sensor. Applications – Magnetic Design Introduction The IMC Angle Sensor measures two orthogonal components of a magnetic field (20…70mT) in the sensor plane which represent the sine and cosine of a rotating magnetic field. With the integrated DSP (Digital Signal Processing) circuit these signals are transformed into a linear angle signal 0° .. 360°. Mechanical Set-up The mechanical alignment between axis of rotation, magnet position and sensor position strongly determines measurement accuracy. Mechanical alignment errors (Fig. 1) can result in additional offset, phase shift, amplitude change and nonlinearity of the ideal Sin and Cos output curves. Whereas offset, phase and amplitude may be easily trimmed and compensated, non-linearity due to off-axis shift between sensor and rotating magnet in the XY plane (eccentricity) are not so easy to compensate with. In most cases the best solution is to choose a magnet big enough to limit linearity to a tolerable value for the predefined mechanical tolerances. Magnet Diameter S N X LX M 316 90 Z Radial Off Axis Axial Distance Magnet-Sensor Y Fig. 1: Mechanical Setup for Angular Sensing with MLX90316 The axial working distance between Magnet and Sensor is defined by the saturation effects (electrical or magnetic) for the lower limit and by the required signal-to-offset or signal-to-noise ratio for the higher limit. When one increases the distance between sensor and magnet it decreases the output signal amplitude; if the offset stays at the same level, the angular error is increased). Off-axis and magnet diameter Off-axis position due to production tolerances, mechanical play, etc. will lead to non-linearity of the angle output signal. The following graph shows the non-linearity for a given setup with a disk shape magnet 90316200611 Rev 006 Page 1 of 6 Nov-2006 Application Note Magnets for MLX90316 Angle Sensor (15mm magnet diameter, 4mm height). The airgap between magnet surface and sensitive area of MLX90316 is 5mm. Angle error simulation: Angle error vs. mechanical angle for different off-axis positions; Diametral D15H4 magnet 0.25 0.2 angle error [°] 0.15 0.1 0.05 -180 -135 -90 0 -45 -0.05 0 45 90 135 180 -0.1 -0.15 -0.2 -0.25 mechanical angle [°] 0mm error ° 0.2mm error ° 0.5mm error ° 1mm error ° Fig. 2: Angle Error over 360° for Off-axis Positioning Angular errors due to a given off-axis misalignment will become smaller with an increasing diameter of the magnet. The figure below helps to estimate the required magnet diameter for given production tolerances plus lifetime wear-out (eccentricity) if a predefined non-linearity (angle error) shall not be exceeded. Magnet Homogenity Error due to excentricity (radial off-axis position) of Sensor; max. error over 360° rotation 2 angle error [°] 1.75 1.5 1.25 1 0.75 0.5 0.25 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 eccentricity (off-axis) [mm] D6 H2.5 Error ° D15 H4 Error ° 10x10 H5 Error ° D6d1.5 H2.5 Error ° Fig. 3: Angle Error vs. Eccentricity for different Magnets 90316200611 Rev 006 Page 2 of 6 Nov-2006 Application Note Magnets for MLX90316 Angle Sensor Example: maximum expected eccentricity due to production tolerances plus lifetime wear-out is 0.5mm and maximum admitted non-linearity is 0.2° (0.05% of 360° full scale). Then a magnet of 10mm diameter is a good choice. However: To choose a magnet as big as possible will not lead to the best result: • Large magnets can be less homogeneous (‘hot spots’ on the magnet surface will create angular errors) • Strong fields need a big distance between sensor and magnet or cause saturation effects • Big magnets are more expensive Typically the magnet diameter has to be 10x bigger than the max. eccentricity for less than 1° nonlinearity error and 20x bigger for less than 0.3° nonlinearity. Axial distance between sensor and magnet Using a small distance between sensor and magnet will increase the danger of either electrical or magnetic saturation. Furthermore imperfections of the magnet material may create magnetic ‘hot spot’ on the magnet’s surface which cause local field deflection and result in angular errors; For this reason we advise our customers not to use for example bonded ferrite material with large diameters. The magnetic saturation level of the MLX90316 is approx. 80mT; there is no damage for fields >80mT Using a big distance (small amplitude) will decrease the signal to noise ratio. S M 6 S mCo m agnet: m easured field strength vs. axial distance 90 Operating distance range 2 ... 5mm 80 field strength [mT] 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 11 axial distance from m agnet [m m ] Fig. 4: Horizontal Flux Density vs. Axial Distance from Magnet (Airgap) The fig. 4 shows a typical relation between the field strength and the distance. It is obtained from a measurement on a 6mm SmCo magnet of 2.5mm height. The optimum flux density (20mT…70mT) is reached with 2mm to 5mm airgap between magnet surface and the sensor’s sensitive spot. Related Melexis Products MLX91204; MLX90251; MLX90215 90316200611 Rev 006 Page 3 of 6 Nov-2006 Application Note Magnets for MLX90316 Angle Sensor Magnets and Material Properties Some typical magnets used in this application note: !"#$ 6 2.5 Diametral 2.5 Diametral 10*10 5 Diametral 15 4 Diametral Outer Ø:6 Inner Ø: 1.5 Material properties: Sintered SmCo Br=900…1100mT Sintered SmCo Br=900…1100mT Anisotropic Ferrite Br=300…500mT Bonded NdFeB Br=350…450mT (Please refer to your suppliers material documentation) % & '( NdFeB 1300 -0.1 SmCo 1000 -0.03 AlNiCo 900 -0.02 Ferrite 300 -0.2 Bonded NdFeB 450 -0.1 Aging: Has to be specified by your supplier. 90316200611 Rev 006 Page 4 of 6 Nov-2006 Application Note Magnets for MLX90316 Angle Sensor Application table: *++ - + ) & small (0 – 2mm) 0.5mm 1mm middle (2 – 5mm) 0.5mm 1mm big (5 – 8mm) 0.5 mm 1mm Magnets for MLX90316 Angle Sensor + , / - + ." # , ( 0 + , .+ 0.3° D8 h2 NdFeB SmCo sintered or plastic bonded 1° D6 h3 NdFeB SmCo sintered or plastic bonded 0.3° D10 h2 NdFeB SmCo sintered or plastic bonded 1° D8 h2.5 NdFeB SmCo sintered or plastic bonded 0.3° D10 h3 NdFeB SmCo sintered or plastic bonded NdFeB D12 h3 1° D6 h3 NdFeB SmCo sintered, hard ferrite or bonded NdFeB D6 h3.5 SmCo D6 h2.5 0.3° D20 h4 NdFeB SmCo sintered or plastic bonded 1° D10 h3 NdFeB SmCo Sintered, hard ferrite or bonded 0.3° D12 h4 NdFeB sintered or plastic bonded 1° D8 h4 NdFeB sintered or plastic bonded 0.3° D25 h5 NdFeB sintered or plastic bonded 1° D12 h5 NdFeB sintered or plastic bonded NdFeB D6 h3.5 SmCo D6 h2.5 NdFeB D12 h3 *1) For better eccentricity (< 0.5mm) can obviously be used smaller magnets (smaller diameter) 90316200611 Rev 006 - - , Page 5 of 6 Nov-2006 Application Note Magnets for MLX90316 Angle Sensor Material Advantage NdFeB Neodym best magnetic characteristic SmCo Samarium-Cobalt best magnetic characteristic over a wide temperature range HF hard ferrite Bonded plastic bonded Magnetproducer cheap all magnet shapes can be easy produced Currently, most used magnet type in the automobile industry. (Melexis does not take any responsabilty for the magnet quality of the herein listeds) , 1. + + . , . & Magnetfabrik Bonn D - 53119 Bonn www.magnetfabrik.de Dr. M. Grönefeld +49 (0) 228 72905-13 HF / SmCo / NdFeB / Plastic Bonded Magnetfabrik Schrammberg D-78713 Schramberg-Sulgen www.magnete.de Herr R. Rapp +49 (0) 7422 519-226 HF / SmCo / NdFeB / Plastic Bonded Herr Stettbacher +41 (0)44 936 60 30 HF / SmCo / NdFeB / Plastic Bonded Herr Bohny +41 (0)62 836 90 56 HF / SmCo / NdFeB / Plastic Bonded www.precisionmagnetics.com Herr M. Albert +41 (0)56 464 21 23 HF / SmCo / NdFeB / Plastic Bonded www.bomatec.ch +41 (0)1 872 10 00 HF / SmCo / NdFeB / Plastic Bonded / AlNiCo Maurer Magnetic CH-8627 Grüningen BBA CH-5001 Aarau Precicsion magnetic CH-5242 Lupfig Bomatec CH-8181 Höri 90316200611 Rev 006 www.maurermagnetic.ch www.bba.ch Page 6 of 6 Nov-2006