Rotation measurement with a disk magnet (I 2 C-Mode)

Transcription

1 Rotation measurement with a disk magnet (I 2 C-Mode) KMA36 / Demo board Version: 1.0 October 2012 Please use also KMA36 white paper and KMA36 data sheet Hauert 13, D Dortmund, Germany 1 of 15 fax: +49-(0)

2 Rotation measurement with a disk magnet (I 2 C-Mode) Fig. 1: Setup for rotation measurement with a disk magnet and magnet holder You can use the KMA36 demo board in combination with a disk magnet measurement. This document describes the operation in I 2 C-Mode (supplementary instructions can be found in the document KMA36 white paper ). The measurement was done with a disk magnet (series magnet / produced by Magnetfabrik Bonn GmbH ) and a magnet holder (for more details see appendix 1.) KMA36 technical key data Max. resolution 0.04 Max. absolute range of measurement 360 Max. incremental range of measurement 2^32 x 360 = 1,54619E+12 Digital resolution 13 Bit Operation power supply range V Operating temperature C Average current ma Sleep current 1.2 ma Data Update rate Hz I2C Clockrate Up to 100 kbit/s Hauert 13, D Dortmund, Germany 2 of 15 fax: +49-(0)

3 Setup For setting the standard configuration (s. white paper) please apply two solder connections on the back of the board (Fig. 2.) Fig. 2: Position of junctions. For contacting the circuit board (PCBA) you can solder a double-row pin header (pitch 2.54 mm) (Fig 3). Please use the white paper for detailed information about the pin configuration. Fig. 3: Position of contact pins. A normal disk magnet (we recommend using a magnet with a specified field strength of 25 ka/m at a distance of 5mm from magnet surface) can be used for this measurement, but for comparable result we recommend to use a series magnet from Magnetfabrik Bonn GmbH (s. Fig 4). front site (facing to KMA36) Fig. 4: Series disk magnet for rotation measurement from Magnetfabrik Bonn GmbH. Hauert 13, D Dortmund, Germany 3 of 15 fax: +49-(0)

4 Magnet holder (non-magnetic) Magnetic sensor KMA36 Axis of rotation (though magnetic center of sensor and magnet) Demoboard PCBA Disk magnet Fig. 5: Rotation Setup The air gap between sensor surface and the magnet surface should be between 0,1 mm and 6 mm. Please note that the magnetic center of the sensor is not equal to the center of the chip (supplementary information can be found in the datasheet). The disk magnet must be placed perpendicularly over (or under) the magnetic center of the sensor (s. Fig 5, 6). Hauert 13, D Dortmund, Germany 4 of 15 fax: +49-(0)

5 Axis of rotation Pin 1 mark Direction of Rotation Fig. 6: Ideal measurement setup. Please note that the front site of this magnet must face the sensor because the magnetic field on the front site is much stronger than on the back site (s. Fig 6). Measurement Please configure the sensor via I²C at the beginning of the measurement (supplementary instructions can be found in the data sheet and the white paper). To perform a rotation measurement the linear mode has to be deactivated (clear LIN-Bit in KCONF register). Furthermore we recommend to work with an oversampling of 32 (OVCS-Bits in KCONF register) and a resolution of (KRES register). If required, the incremental counter can also be activated (set CNT-Bit in KCONF register). With activated incremental counter the number of full rotations can be readed out (supplementary information is given in the data sheet and the white paper). Hauert 13, D Dortmund, Germany 5 of 15 fax: +49-(0)

6 The previously described configuration leads to a measurement result as shown in Fig. 7. Fig. 7: Measurement result. Accuracy With the described configuration and an air gap of 1mm a linearity error of less than ±0,15 and absolute error of less than ±1 can be reached (Fig 8, 9). Hauert 13, D Dortmund, Germany 6 of 15 fax: +49-(0)

7 Fig. 8: Linearity error (air gap = 0,1mm). Fig. 9: Absolute error (air gap = 0,1mm). Hauert 13, D Dortmund, Germany 7 of 15 fax: +49-(0)

8 If the air gap is increased the linearity error will increases while the absolute error decreases (Fig. 7). For distances over 6 mm both errors will increase. Fig. 10: accuracy vs. air gap. External look up table Using an external look up table (LUT), for example a error correction algorithm in the I2C Master microcontroller, the accuracy could further increase to ±0,2 (Fig. 11, 12). Hauert 13, D Dortmund, Germany 8 of 15 fax: +49-(0)

9 Fig. 11: Absolute error without external LUT. Fig. 12: Absolute error with external LUT. Hauert 13, D Dortmund, Germany 9 of 15 fax: +49-(0)

10 Rotation speed The maximal rotation speed the KMA36 is able to work with is limited by its maximal sample rate of 720 Hz. The sample rate can be configured via SPD-Bit (KCONF register) (supplementary instructions can be found in the data sheet and the white paper). Increase of the rotation speed decreases the resolution. This correlation is given by this formula: R := Resolution ( ) U := Rotations per second (1/s) S := Sample rate (Hz) The maximum rotation speed is determined by the Nyquist Shannon sampling theorem. This means if the rotation speed reaches a certain level it incapacitates every measurement. The limiting rotation speed is given by this formula: To illustrate this theoretical correlation some examples are given: Example 1: U = 200 RPM = /s S 0 S 1 S 2 S3 In case of low rotation speeds an accurate position measurement is possible. By increasing the rotation speed the accuracy decreases (Tab.1). Hauert 13, D Dortmund, Germany 10 of 15 fax: +49-(0)

11 Example 2: U = RPM = /s S S 1 For higher rotation speed the accuracy will decrease until a proper position measurement is not possible anymore. But in case of measured. the direction of the rotation and the rotation speed can still be Example 3: U = RPM = 400 1/s S 0 S For or the limiting frequency is reached (Nyquist Shannon sampling theorem). This makes any measurement impossible. Hauert 13, D Dortmund, Germany 11 of 15 fax: +49-(0)

12 Explanation of Nyquist Shannon sampling theorem in case of Example 3: Assuming the axis can turn in both directions (clockwise and counter clockwise) there are two possible ways how the measured magnetic angle could be realized: 1. Turning fast clockwise (marked green) 2. Turing slow counter clockwise (marked red) S 0 S Only if you can ensure that the axis is turning in one direction only the limiting frequency is increased to ). S 1 S 0 Hauert 13, D Dortmund, Germany 12 of 15 fax: +49-(0)

13 The following table contents calculated resolution values for typical rotation speeds. In case of the limiting frequency for rotations in both directions is reached (marked red) and in case of the limiting frequency for rotations in one direction is reached (marked blue). U (RPM) R ( ) U (RPM) R ( ) , ,3 Tab. 1: Rotation Speed vs. Resolution Hauert 13, D Dortmund, Germany 13 of 15 fax: +49-(0)

14 Appendix 1 1. KMA36_PCB-A 350BZ200_0003A_TK Hauert 13, D Dortmund, Germany 14 of 15 fax: +49-(0)

15 CONTACT NORTH AMERICA EUROPE ASIA Measurement Specialties, Inc Lucas Way Hampton, VA United States Phone: Fax: Web: MEAS Deutschland GmbH Hauert 13 D Dortmund Germany Phone: +49-(0) Fax: +49-(0) Web: Measurement Specialties China Ltd. No. 26, Langshan Road High-tech Park (North) Nanshan District, Shenzhen China Phone: Fax: Web: The information in this sheet has been carefully reviewed and is believed to be accurate; however, no responsibility is assumed for inaccuracies. Furthermore, this information does not convey to the purchaser of such devices any license under the patent rights to the manufacturer. Measurement Specialties, Inc. reserves the right to make changes without further notice to any product herein. Measurement Specialties, Inc. makes no warranty, representation or guarantee regarding the suitability of its product for any particular purpose, nor does Measurement Specialties, Inc. assume any liability arising out of the application or use of any product or circuit and specifically disclaims any and all liability, including without limitation consequential or incidental damages. Typical parameters can and do vary in different applications. All operating parameters must be validated for each customer application by customer s technical experts. Measurement Specialties, Inc. does not convey any license under its patent rights nor the rights of others. Hauert 13, D Dortmund, Germany 15 of 15 fax: +49-(0)