LINEAR POSITION MEASUREMENT USING A MAGNETORESISTIVE SENSOR
Abstract: A smart transducer used to measure the linear displacement of an object in a magnetic field is presented. The application is to be safely used for demonstration purposes in the Playpen laboratory. The system uses an array of five HMC1501 Magnetoresistive (MR) sensors to sense the position of a magnet attached to the moving object. The system also has the ability to monitor the ambient operating temperature and detect if it is exposed to an external disturbing magnetic field. Several other components such as the differential amplifier, a band-pass filter and a temperature compensation circuit are used to process sensors signal and compensate the errors. The system offers a high resolution of 0.00616 mm, a bandwidth of 9 KHz and a measured distance of 60 mm. Keywords: MR sensors, band pass filter, magnet, smart transducer. IntroductionLinear position sensors are very useful in applications where precision positioning is required CITATION Tam08 \l 7177 . There are several methods for measuring the position of moving objects and for each method a specific sensor type can be usedCITATION Placeholder1 \l 7177 . In measurement systems where MR sensors are used, the measurement range of each element is limited to a maximum of 25 mm, however some systems uses a combination of multiple sensors to increase the measurement range to a full scale range above 2 m with a non-linearity less than 0.005 % CITATION Nyc04 \l 7177 . This report details the design of a linear displacement measurement system using an array of five MR sensors. The operating principles of displacement measurement using MR sensor is explained. The designed system is based on the Bentley model CITATION Ben05 \l 7177  of a measurement system. The design of each component of the system is illustrated and the system response and error analysis are presented and analysed. Background
MR sensors are Wheatstone bridges that are mostly used to measure magnetic field CITATION Hon10 \l 7177 . The resistance of the bridge elements changes when a magnetic field is applied across the MR ferrous material that form the resistive elements CITATION Hon10 \l 7177 . The change in the resistance of the ferrous material of the bridge causes a variation of the output voltage. Most MR sensors have a common bridge resistance of 1 Kilo Ohms (KΩ) and a bandwidth in the 1-5 MHz range with a fast reaction effect which is not limited by coils or oscillating frequencies CITATION Mic \l 7177 . When a MR sensor is used for detecting the position of moving objects, it is usually combined with a source of magnetic field which originates from a magnet attached on the moving object. Figure 1 CITATION Hon12 \l 7177  below illustrates the principle of linear position detection. Sensor
Direction of movement
Magnetic field lines
Direction of movement
Magnetic field lines
Figure 1: Linear position detection of moving objects
Using MR sensors for linear position measurement have many advantages over other electrical and mechanical methods. These advantages include low cost implementation, high sensitivity, reduced size, noise reduction and reliability CITATION Hon10 \l 7177 . The common errors in MR sensor circuit are: the offset voltage, the change in material constant with change in temperature, and the part to part tolerance in material constant which occurs when multiple sensors are being used CITATION Mic \l 7177 . Honeywell AN211 application note CITATION Hon12 \l 7177  implements a linear position sensing using an array of four sensors to measure the position of a moving object. The measured distance is determined by using the output signal of the nearest three sensors. The proposed system uses an array of five MR sensors to...
References: Bratland Tamara and Hong Dr. Wan, Linear Position Sensing Using Magnetoresistive Sensors, Honeywell Solid State Electronics Center, 2008.
David S. Nyce, Linear Position Sensors: Theory and Application: John Wiley & Sons, Inc., 2004.
Microchip, PIC18F2423/2523/4423/4523 Datasheet, 2007.
Philips Semiconductors, General Magnetic fields sensor DataSheet, 1998.
Philips Semiconductors, KTY82-210 Series Silicon temperature sensors datasheet, 1998
Microchip Thermal Management, TC1046, TC1047/A and MCP9700/1 Voltage Output Temperature Sensor Familly, 2005.
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