1 General Description

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1 AN534-1 AS534, AS536 Magnetic Sensor Circuits Multi-pole Magnet requirements APPLICATION NOTE 1 General Description This document provides a detailed explanation of the underlying principles for high resolution contactless linear motion sensing as well as off-axis rotary motion sensing using integrated Hall sensor technology. Furthermore, it describes the requirements for the associated multi-pole ring or strip magnets. The AS534 and AS536 are pin-compatible Hall-ICs suited for such applications. Both IC s can be used for linear motion sensing as well as off-axis rotary motion sensing. In either application, a multi-pole magnet, sliding over the IC is required. The magnet must have a fixed pole length of 2mm (AS534) or 1.2mm (AS536) respectively. Fig. 1 shows the required magnet for linear motion sensing. It is a multi-pole strip with alternating north and south poles of equal length. Fig. 1: AS536 (AS534) with magnetic multi-pole strip magnet for linear motion sensing Fig. 2 shows the required magnet for offaxis rotary motion sensing. In this case, a multi-pole magnetic ring with alternating north and south poles is used. The pole length of either magnet must be fixed. Hence, the number of poles increases with increasing ring diameter. At the same time the resolution increases linearly with the number of pole pairs. Fig. 2: AS534 (AS536) with multi-pole ring magnet for off-axis rotary motion sensing Revision 1. Page 1 of 1

2 1.1 Increasing the resolution of multi-pole magnets In low resolution applications, a simple Hall switch is typically used. The Hall switch has a digital output that changes state with each change of magnet polarity. The resolution of this setup is therefore equal to the number of poles on the magnetic ring or strip. As the magnet scans over the Hall switch, it generates one pulse for every pole pair on the magnet. However, there is a practical limit to the number of poles that can be fabricated on a given length of magnetic material. There is a rule of thumb saying that the distance between magnet and sensor should be less than one half pole width. In most applications, a typical gap between sensor and magnet is around.25..5mm, thus the pole length is also limited to ~.5.1.mm. Consequently, the resolution of a multi-pole magnet + Hall switch arrangement is typ. 5µm or more. Many applications require much higher resolution, at least 1 times better than the resolution obtained with simple Hall switches. In this case, it is necessary to electrically divide one pole pair of the multipole magnet into fine steps of equal length. This is accomplished by using interpolation. 1.2 The interpolation principle In order to interpolate a multi-pole magnet into many equally spaced steps, a minimum of 2 linear Hall sensors (ideally 4) is required. As opposed to Hall switches that have a digital output, linear Hall sensors provide an analog voltage that is proportional to the strength of the magnetic field perpendicular to the sensor surface. In the case of a multi-pole strip as shown in Fig. 3, each sensor generates a sinusoidal output signal as the magnetic strip scans over the sensor array. If the linear Hall sensors are spaced at ½ of the pole length, the generated sinusoidal signals are phase shifted by 9 electrical degrees. S N S N Sin -Sin Sin Sin DSP Angle Cos Cos Magnitude -Cos Cos Frontend Amplifier ADC Digital Filter Fig. 3: Angle measurement block diagram In mathematical terms, the four Hall sensor signals generated from a sliding multi-pole magnet are sin(x), cos(x), -sin(x) and cos(x). Opposite Hall sensor signals are combined by differential amplification, generating sin(x) and cos(x) signals of double amplitude: sin(x) [-sin(x)] = 2 sin(x) cos(x) [-cos(x)] = 2 cos(x) These signals are then digitized by A/D-converters and fed to a dedicated DSP performing a vector coordinate transformation: the coordinate transformation by the DSP converts the rectangular coordinates Sin(x) and Cos(x) into the polar coordinates angle (=phase) and magnitude (see Fig. 3). The angle information is a value between and <36 and is proportional to relative position of a magnetic pole pair with respect to the Hall sensor array. It repeats from <36 for each subsequent pole pair. In the AS534 and AS536 the angle information is resolved into 16 steps (per pole pair) and further decoded into 4 quadrature pulses. Hence, the resolution is: Resolution = pole length / 16 For the AS534: resolution = 4. mm / 16 =.25mm = 25µm For the AS536: resolution = 2.4 mm / 16 =.15mm = 15µm Revision 1. Page 2 of 1

3 The magnitude information is proportional to the magnetic field strength. This magnetic field strength is influenced by the following factors: the gap between magnet and IC surface: the smaller the gap, the stronger the magnetic field the magnetic strength (remanence Br) of the used magnet material: recommended= rubber or plastic bonded Ferrite or NdFeB magnets the temperature of the magnet magnetic field decreases with rising temperature 1.3 The Automatic Gain Control (AGC) The underlying principle of coordinate transformation also suggests that the angle information is independent of the magnetic field strength! In other words, if the magnet is locked at a certain position and at the same time the distance between magnet and IC is changed, both the Sin(x) and the Cos(x) amplitude will change simultaneously so after coordinate transformation only the length of the vector (= the magnitude) will change but the angle will remain unchanged. This effect enables a very accurate angle output despite temperature changes of the magnet as well as vertical distance changes. In practice however, a weak magnetic field (e.g. due to a large vertical gap or a weak magnet material) incurs a poorer signal to noise ratio compared to a strong magnetic field. As a result, the higher noise on the sin(x) and cos(x) signal inputs of the DSP will lead to more jitter of the angle and magnitude outputs. To tackle this effect, an automatic gain control (AGC) is implemented in the AS534 and AS536 encoders: it stabilizes the sin(x) and cos(x) signals by changing the sensitivity of the Hall sensor front-end, trying to maintain a constant magnitude output. This way, the magnetic field may change over a wide range (typ. ~5.6mT 1) ) while the magnitude output of the DSP will remain at a stable value. 1) The magnetic field strength refers to the Z or vertical vector of the magnetic flux density (also known as induction) B r, measured in in mt (millitesla). It is often referred to in other measurement units as well: 1 mt (millitesla) = 1G (Gauss) = 796A/m (amperes per meter) =.796kA/m (kiloamperes per meter) Magnetic field out-of-range indicators If the magnetic field gets too weak and the AGC is at the upper regulation limit at maximum gain, the AS534 and AS536 outputs indicate an out-of-range condition by switching the incremental outputs to a state that does not occur in normal operation: A = B = low Index = high 2 Locating the Hall sensor array As shown in Fig. 4 below, the Hall sensor array is located 1.2mm above the horizontal centerline of the TSSOP-2 package, or 3.2mm + 1.2mm = 4.22mm above the edge of pins #1.1 (top view, pin#1 at bottom left). Die C/L ± ±.1 3.2±.235 Package Outline 3.475± ±.15 Fig. 4: Location of Hall sensors in the TSSOP-2 package Revision 1. Page 3 of 1

4 3 Multi-pole strip magnets for the AS534 and AS536 Fig. 5 shows the proper placement of a multi-pole strip over the AS534 / AS536. The centerline of the magnetic strip is located over the Hall array (see Fig. 4). For proper operation, the magnet should fully cover the Hall array. The AS534 / 6 will generate pulses at the A-, B- and Index outputs as soon as the magnetic field is stronger than the out-of-range threshold (see 1.3.1). This may already happen when the magnet does not fully cover all Hall elements. It is therefore recommended to advance to the second index position (see 3.1.1) before a valid reading is taken. lp w Hall sensor scan path l Fig. 5: Proper placement of a multi-pole strip in linear motion sensing applications Multi-pole strip parameter SYMBOL MIN TYP MAX UNIT NOTE Pole length lp 2. AS534 mm 1.2 AS536 Strip length l 2 Not limited Pole pairs Strip width w 1 2 Not limited Pole length lp Strip thickness d not limited mm Field strength drops with thin magnets Magnetic Amplitude Amag 5 6 mt Measured at IC surface Magnetic Offset Offmag ±.5 mt Magnetic Temperature Drift Tdmag -.2 %/K Depending on material Soft stop feature for linear movement measurement When using long multipole strips, it may often be necessary to start from a defined home (or zero) position and obtain absolute position information by counting the steps from the defined home position. The AS534 / 6 provides a soft stop feature that eliminates the need for a separate electro-mechanical home position switch or an optical light barrier switch to indicate the home position. The magnetic field warning indicator (see 1.3.1) together with the index pulse can be used to indicate a unique home position on a magnetic strip: 1. First the AS534/6 moves to the end of the strip, until a magnetic field warning is displayed (Index = high, A=B=low) 2. Then, the AS534/6 moves back towards the strip until the first index position is reached (note: an index position is generated once for every pole pair, it is indicated with : Index = high, A=B= high). Depending on the polarity of the strip magnet, the first index position may be generated when the end of the magnet strip only covers one half of the Hall array. This position is not recommended as a defined home position, as the accuracy of the AS534/6 is reduced as long as the multipole strip does not fully cover the Hall array. 3. It is therefore recommended to continue to the next (second) index position from the end of the strip (Index = high, A=B= high). This position can now be used as a defined home position. Revision 1. Page 4 of 1

5 4 Multi-pole ring magnets for the AS534 and AS536 Fig. 6 shows the proper placement of a multi-pole ring over the AS534 / AS536. The centerline of the magnetic ring (median Hall sensor scan path) is located over the Hall array (see Fig. 4). For proper operation, the magnetic ring should be designed such that the pole length lp matches the required length at the scan path (typically the center) of the ring. The IC is oriented in perpendicular with respect to the rotation center. l p r o r i r m median Hall sensor scan path Fig. 6: Proper placement of a multi-pole ring in off-axis rotary motion sensing applications Multi-pole strip parameter SYMBOL MIN TYP MAX UNIT NOTE Pole length lp Inner ring diameter ri ~2 Not limited mm 2. AS534 mm 1.2 AS536 Stronger curvature of small diameters generates additional errors Median ring diameter rm ri + (ro-ri)/2 mm Centerline of magnet; must be a multiple of (2x lp) Outer ring diameter ro ri + lp Not limited mm width = >1 pole length Strip thickness d not limited mm Field strength drops with thin magnets Revision 1. Page 5 of 1

6 Magnetic Amplitude Amag 5 6 mt Measured at IC surface Magnetic Offset Offmag ±.5 mt Magnetic Temperature Drift Tdmag -.2 %/K 5 XY-scans of multipole strip magnets Depending on material Fig. 7 shows a 3-dimensional scan of the vertical field vector Bz over a rubber bonded strontium ferrite (SrFe) multipole strip magnet with 2mm pole length and a width of 5mm. The measurement was taken at a vertical distance of mm (referenced to the IC package surface). As the scan data shows, the peak amplitude is ~4mT Bz [mt] Y [µm] X [µm] Fig. 7: 3D-graph of a multipole strip magnet Fig. 8 shows the same magnet, this time the scan is taken at the center of the magnet along the x-axis at three different vertical distances between magnet and IC package: mm,.25mm and.5mm. The peak amplitude at mm is again ~4mT (see Fig. 7), at.25mm gap, the peak amplitude is ~25mT and at.5mm, the peak amplitude is still close to 2mT, which is well above the lower limit of 5mT. Both Fig. 7and Fig. 8 show that the peak magnetic field at the edge of the strip is lower than over the poles inside the strip. Therefore it is recommended to operate the magnet only in areas where it fully covers the encoder IC (see 3.1.1). Fig. 9 shows a scan in the (shorter) Y-axis of the magnet strip. This scan is important to determine the available misalignment range in respect to the Y-axis of the magnet. The used magnet is also shown on the graph as a cross section, showing width (w) and thickness (d). The width (w) of the used magnet is 5 mm, which is 2 ½ pole lengths and the thickness (d) is.76mm. The scan data is taken at mm and.5mm vertical distance between magnet and IC package. As the scan data shows, the magnetic field along the center of the magnet is fairly stable for ~1 1 ½ pole lengths, depending on the vertical distance. This stable area is the available range for magnet misalignment in the Y-axis. The magnetic field starts to drop at about ½ pole length from inside the magnet. This leads to the conclusion that the width of the multi-pole strip should be a minimum of 1 pole length. Revision 1. Page 6 of 1

7 scan X-axis (strip length) Bz [mt] X-axis [mm] Z=µm z=25µm z=5µm Fig. 8: Scan data along the X-axis of a multipole strip magnet at different vertical distances Scan Y-axis (strip width) Bz [mt] w Magnet z=µm z=5µm Y-axis [mm] d Fig. 9: Scan data along the X-axis of a multipole strip magnet at different vertical distances Revision 1. Page 7 of 1

8 6 Substrate material for multipole magnets Multi-pole magnetic strips or rings can be mounted on both non-magnetic (e.g. plastic) and soft magnetic substrates (e.g sheet metal). In fact, as Fig. 1 shows, mounting the magnetic strip / ring on a soft magnetic plate results in ~2% increase of the magnetic field strength Bz. The graph shows a lateral scan over a 1-pole magnetic strip mounted on a non-magnetic substrate (blue line) and the same scan with the magnet mounted on a soft magnetic substrate (yellow line). Field Bz with and without soft magnetic substrate Bz [mt] non-magnetic substrate X-scan [mm] soft magnetic substrate Fig. 1: 1-pole strip magnet with and without magnetic carrier plate Revision 1. Page 8 of 1

9 7 Calculations Based on the fixed chip parameters for pole length (2.mm or 1.2mm), interpolation factor (16) and maximum input frequency (5kHz) the system parameters for maximum rotatonal speed, resolution, ring diameter and linear travelling speed can be calculated: multipole magnet AS534 AS536 Max. rotational speed of a ring magnet (rpm) 3. rpm / number of pole pairs on the ring magnet Resolution of a multi-pole ring magnet (steps per rev.) 16 x number of pole pairs on the ring magnet Resolution of a multi-pole ring magnet (bit) log (resolution [steps]) / log (2) Ring diameter at scan area (mm) 4.mm x (number of pole pairs) / π 2.4mm x (number of pole pairs) / π Linear travelling speed of a multi-pole strip magnet (m/sec) 5 X pole pair length (mm) 7.1 Resolution and maximum rotating speed When using the AS534/6 in an off-axis rotary application, a multi-pole ring magnet must be used. Resolution, diameter and maximum speed depend on the number of pole pairs on the ring Resolution The angular resolution increases linearly with the number of pole pairs. One pole pair has a resolution (= interpolation factor) of 16 steps or 4 quadrature pulses. Resolution [steps] = [interpolation factor] x [number of pole pairs] Resolution [bit] = log (resolution[steps]) / log (2) Example: multi-pole ring with 22 pole pairs Resolution = 16x22 = 352 steps per revolution = 4x22 = 88 quadrature pulses / revolution = bits per revolution =.123 per step The length of a pole pair across the median of the multi-pole ring must remain fixed at either 4mm (AS534) or 2.4mm (AS536). Hence, with increasing pole pair count, the diameter increases linearly with the number of pole pairs on the magnetic ring. Magnetic ring diameter = [pole length] * [number of pole pairs] / π for AS534: d = 4.mm * number of pole pairs / π for AS536: d = 2.4mm * number of pole pairs / π Example: same as above: multi-pole ring with 22 pole pairs for AS534 Ring diameter = 4 * 22 / 3.14 = 28.1mm (this number represents the median diameter of the ring) For the AS536, a 22-polepair ring would have a diameter of: 2.4 * 22 / 3.14 = 16.8mm Maximum rotation speed The AS534 / 6 uses a fast interpolation technique allowing an input frequency of 5kHz. This means, it can process magnetic field changes in the order of 5 pole pairs per second or 3. revolutions per second. However, since a magnetic ring consists of more than one pole pair, the above figure must be divided by the number of pole pairs to get the maximum rotation speed: Maximum rotation speed = 3. rpm / [number of pole pairs] Example: same as above: multi-pole ring with 22 pole pairs: Max.speed = 3. / 22 = rpm (this is independent of the pole length) Maximum linear travelling speed For linear motion sensing, a multi-pole strip using equally spaced north and south poles is used. The pole length is again fixed at 2.mm for the AS534 and 1.2mm for the AS536. As shown in above, the sensors can process up to 5 pole pair changes per second, so the maximum travelling speed is: Maximum linear travelling speed = 5 * [pole pair length] Example : linear multipole strip: Max. linear travelling speed = 4mm * 5 1/sec = 2.mm/sec = 2m/sec for AS534 Max. linear travelling speed = 2.4mm * 5 1/sec = 12.mm/sec = 12m/sec for AS536 Revision 1. Page 9 of 1

10 8 Recommended multi-pole magnet suppliers Multipole magnets are available from many vendors. For a list of recommended magnet suppliers, please refer to application note: AS5 Magnet Selection Guide, available for download at the austriamicrosystems website ( Note that austriamicrosystems does not explicitly restrict the use of magnets from only the listed suppliers, nor can austriamicrosystems be held accountable for the quality of magnets delivered by the recommended suppliers. It is the responsibility of the customers to ensure adequate quality of the magnets used in their applications. 9 Contact Headquarters austriamicrosystems AG A 8141 Schloss Premstätten, Austria Phone: Fax: Copyright Devices sold by austriamicrosystems are covered by the warranty and patent indemnification provisions appearing in its Term of Sale. austriamicrosystems makes no warranty, express, statutory, implied, or by description regarding the information set forth herein or regarding the freedom of the described devices from patent infringement. austriamicrosystems reserves the right to change specifications and prices at any time and without notice. Therefore, prior to designing this product into a system, it is necessary to check with austriamicrosystems for current information. This product is intended for use in normal commercial applications. Copyright 28 austriamicrosystems. Trademarks registered. All rights reserved. The material herein may not be reproduced, adapted, merged, translated, stored, or used without the prior written consent of the copyright owner. To the best of its knowledge, austriamicrosystems asserts that the information contained in this publication is accurate and correct. However, austriamicrosystems shall not be liable to recipient or any third party for any damages, including but not limited to personal injury, property damage, loss of profits, loss of use, interruption of business or indirect, special, incidental or consequential damages, of any kind, in connection with or arising out of the furnishing, performance or use of the technical data herein. No obligation or liability to recipient or any third party shall arise or flow out of austriamicrosystems rendering of technical or other services. Revision 1. Page 1 of 1