Application Note Magnets for MLX90333 Linear Position Sensor

Transcription

1 Scope This application note provides some guidelines to select a magnet to be used in combination with the MLX9333 Tria is TM 3D-Joystick Position Sensor as a linear position sensor. Related Documents, Products and Tools The documentation and information on the products and tools listed below can be found on Melexis website Related Products MLX9333 Tria is TM 3D-Joystick Position Sensor MLX936 Tria is TM Rotary Position Sensor MLX934 Tria is TM Linear Position Sensor MLX924 Sine/Cosine Tria is TM Rotary Position Sensor Related Documents Application Note Front-End Calibration Application Note Back-End Calibration Application Note Hall Applications Guide Related Tools PTC4 Programmer for Melexis PTC devices Figure MLX9333 Introduction The MLX9333 is a monolithic sensor IC featuring the Tria is TM Hall technology. Conventional planar Hall technology is only sensitive to the flux density applied orthogonally to the IC surface. The Tria is TM Hall sensor is also sensitive to the flux density applied parallel to the IC surface. This is obtained through an Integrated Magneto-Concentrator (IMC ) which is deposited on the CMOS die (as an additional back-end step). The MLX9333 is sensitive to the 3 components of the flux density applied to the IC (B X, B Y and B Z ). This allows the MLX9333 to sense any magnet moving in its surrounding and it enables the design of novel generation on non-contacting linear position sensors, which are often required for both automotive and industrial applications (e. g. man-machine interface). In combination with the appropriate signal processing, the magnetic flux density of a small magnet (axial magnetization) moving above the IC can be measured in a non-contacting way. The linear position information is computed from the three vector components of the flux density (i.e. B X, B Y and B Z ). The output formats are selectable between Analog, PWM and Serial Protocol. By & Bz (mt) Atan(By/Bz) / Nonlinearity Position (mm) By[mT] Bz[mT] Atan(By/Bz) Figure 2 BY, BZ and the arc tangent of the ratio BY/BZ for Figure 9333-LP-AP-287 Page of 2 August-27

2 Mechanical Description The mechanical alignment between axis of movement, magnet position and sensor position strongly determines measurement accuracy. Mechanical alignment errors (Figure 3) can result in additional offset, amplitude change and non-linearity vs. the ideal output curve. Whereas offset and amplitude are easily trimmed and compensated at the IC level (see the Application Note on the Front-End Calibration of MLX936), linearity errors due to mechanical tolerances (between sensor and the moving magnet) are ideally compensated through a linearization of the output transfer characteristic. The MLX9333 allows tapping the full potential of the Tria is TM technology. The capability of the IC in sensing flux densities in all three directions (X, Y and Z) offers different measurement ways. Axially magnetized magnet with magnetization axis parallel to the sensor surface (Figure 4). Figure 3: Axial parallel magnet-sensor adjustment The magnet is positioned above or around the sensor with the magnetization axis parallel to the IC surface. In the zero position should the magnet be placed above the magnetic center of the sensor. The movement axis should be kept parallel to the IC surface. One of the two planar components and the vertical component of the flux density are used to calculate the position of the magnet in reference to the sensor. The information is obtained through the arc tangent operation of the ratio between two components. The big advantage of this magnet-sensor adjustment is that a rotation of the magnet does not influence the measurements. It is ideal to detect the linear displacement of a rotating shaft. The 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. Note: The MLX9333 features an automatic gain control (AGC) loop to adapt to the amplitude of the available field i.e. the higher the gain, the higher the noise. The MLX924 does not feature such an AGC loop LP-AP-287 Page 2 of 2 August-27

3 Working Principle Axial parallel The axially magnetized cylindrical shaped magnet is placed above or in the surrounding of the sensor with the magnetization axis parallel to the sensor surface. The big advantage of this principle is that the magnet can rotate without influencing the flux density perceived by the sensor (Figure 4). MLX9333 Figure 4: Sensor-Magnet adjustment (topview) axial parallel This allows to measure the direction of the magnetic field by a two axis sensor with the components Bx(or By) and Bz (Figure 5) and therefore determine the geometrical position of either sensor or magnet. Figure 5: Measured field components Bz and Bx(By) (sideview) axial parallel For a travel along X-axis the position is determined as: X Pos By = m arctan( ) ( ) B z with m: slope of the output transfer curve The linear position information can also be obtained by the arc tangent function of Bx/Bz LP-AP-287 Page 3 of 2 August-27

4 Extend the Range with Multiple Magnets This section shows the magnet-sensor adjustment of more that one magnet if bigger linear strokes are needed. The combination of two or three magnets allows increasing the linear usable range. Axial parallel with two magnets The same working principle as for a single magnet is used. The main difference is that additional magnets are used to keep the flux density in the desired range. Various arrangements of the magnets are possible, the showed method (Figure 6) has been evaluated for the axial parallel arrangement and the obtained results are excellent. Distance Figure 6: Magnets-sensor arrangement with two magnets axial parallel It is important that the magnets are arranged anti parallel to each other. The exact mechanical adjustment is strongly dependent on the used magnets and on the desired measurement range. The sensor perceives the flux densities generated by the two magnets; Figure 7 shows the side view of the sensor-magnets system. With a movement of the sensor the direction and strength of the flux density is changing. Figure 7: Magnets-sensor arrangement with two magnets axial parallel (side view) 9333-LP-AP-287 Page 4 of 2 August-27

5 Axial parallel with three magnets The same principle is also usable with three magnets (Figure 8). It is important that the magnets are adjusted anti parallel to reach a rotation of the measurable field components. The optimal mechanical adjustment is strongly dependent on the magnet material and geometries. Distance Distance +Pos -Pos Zpos X Z Y Figure 8: Magnets-sensor arrangement with three magnets axial parallel Figure 9 shows the side view of the sensor-magnets adjustment with three magnets. Figure 9: Magnets-sensor arrangement with three magnets axial parallel 9333-LP-AP-287 Page 5 of 2 August-27

6 Accuracy To minimize errors due to assembly or lifetime wear-out, the Z position of the sensor should be chosen as big as possible, or the placement should be more accurate. The next table shows the calculated position errors for a variation of the air gap between sensor and magnet(figure ) for the magnets used in this Application Note. Figure : Variation of the air gap between magnet and sensor Magnet(s) (Cylinder, axially magnetized) Size (D; H) [mm] Error [% FS] Zoffset ±.mm (±.5 mm) Z Position [mm] D6 H8 ±.4(±2.2) 5.5 D6H2 ±.28(±.5) 6. D2H24 ±.5(±.8). Figure : Errors due to a variation of the Z position 9333-LP-AP-287 Page 6 of 2 August-27

7 Simulated Xerror due to a constant Zshift of +/-. and +/-.5 mm Magnet Size D6H8, Stroke (+/- 5) mm, Nominal Zpos 5.5 mm Error [+/- % FS] Pos [mm] Zpos = 5.4 mm Zpos = 5.6 mm Zpos = 5. mm Zpos = 6. mm Simulated Xerror due to a constant Zshift of +/-. and +/-.5 mm Magnet Size D6H2, Stroke 2 (+/- ) mm, Nominal Zpos 6. mm 2.5 Error [+/- % FS] Pos [mm] Zpos = 5.9 mm Zpos = 6. mm Zpos = 5.5 mm Zpos = 6.5 mm 9333-LP-AP-287 Page 7 of 2 August-27

8 Simulated Xerror due to a constant Zshift of +/-. and +/-.5 mm Magnet Size D2H24, Stroke 4 (+/- 2) mm, Nominal Zpos. mm Error [+/- % FS] Pos [mm] Zpos =.9 mm Zpos =. mm Zpos =.5 mm Zpos =.5 mm 9333-LP-AP-287 Page 8 of 2 August-27

9 Application Table Some typical magnets used in this application note: Magnets for MLX93333 Linear Position Sensor Axial parallel adjustment with one, two and three magnets Magnet Z Position Distance (Cylinder, axially Size (D; H) Stroke (magnet-center to (magnetmagnet) magnetized) [mm] [mm] sensor) [mm] [mm] x D6 H8 2x D6 H8 (±5) 24 (±2) x 4 Material* (Temperature - 4 C +5 C) SmCo, NdFeB Br=9 3mT x D6 H2 2x D6 H2 2 (±) 3 (±5) 6 8 x 9 SmCo, NdFeB Br=9 3mT x D2 H24 4 (±2) 2x D2 H24 6 (±3) 2 x 36 SmCo, NdFeB Br=9 3mT *: SmCo and NdFeB are used as an example, but all magnet materials can bu used, as long as a flux density between 2 an 7 mt is garanteed. The dimension of the magnet increases with decreasing magnet material strength LP-AP-287 Page 9 of 2 August-27

10 Simulations Stroke mm (± 5) Linear Position with the MLX9333 Axially magnetized magnet NeFeB, Br = 42 mt Magnet D6H8, Air gap = 5.5 mm, Range = +/- 5 mm 6 3 B [mt] Atan(Bz/By), Nonlinearity Stroke 2 mm (± ) Position [mm] By[mT] Bz[mT] Atan(Bz/By) Nonlinearity (% FS) Figure 3: Simulated By, Bz and the Atan of the ratio Bz/By magnet D6H8 Linear Position with the MLX9333 Axially magnetized magnet NeFeB, Br = 42 mt Magnet D6H2, Air gap = 6 mm, Range = +/- mm 6 3 B [mt] Atan(Bz/By), Nonlinearity Position [mm] By[mT] Bz[mT] Atan(Bz/By) Nonlinearity (% FS) Figure 4: Simulated By, Bz and the Atan of the ratio Bz/By magnet D6H LP-AP-287 Page of 2 August-27

11 Stroke 4 mm (± 2) Linear Position with the MLX9333 Axially magnetized magnet NeFeB, Br = 42 mt Magnet D2H24, Air gap = mm, Range = +/- 2 mm B [mt] Atan(Bz/By), Nonlinearity Position [mm] By[mT] Bz[mT] Atan(Bz/By) Nonlinearity (% FS) Figure 5: Simulated By, Bz and the Atan of the ratio Bz/By magnet D2H LP-AP-287 Page of 2 August-27

12 Material properties (Please refer to your suppliers material documentation) Material Strength Br [mt] Drift [%/ C] Advantages NdFeB Neodymium Best magnetic characteristic SmCo Samarium- Cobalt -.3 HF Hard Ferrite cheap Bonded NdFeB Plastic Bonded Aging: Has to be specified by your supplier. Best magnetic characteristic over a wide temperature range all magnet shapes can be easy produced good magnetic characteristic cheap Magnetproducer (Melexis does not take any responsabilty for the magnet quality of the herein listeds) Company website phone types of magnet Magnetfabrik Bonn D Bonn Magnetfabrik Schrammberg D-7873 Schramberg-Sulgen Maurer Magnetic CH-8627 Grüningen () () () Plastic Bonded Plastic Bonded Plastic Bonded BBA CH-5 Aarau Precicsion magnetic CH-5242 Lupfig Bomatec CH-88 Höri SURA MAGNETS AB 64 3 Söderköping Energy Conversion Systems Cary, NC () () () () Plastic Bonded Plastic Bonded Plastic Bonded / AlNiCo Plastic Bonded / AlNiCo Plastic Bonded / AlNiCo 9333-LP-AP-287 Page 2 of 2 August-27