Study Of Gear Sensor Based On Magnetic Tunnel Junctions

Gear sensor has been widely used in the automotive and industrial field, which is important to the measurement of velocity, angel, angular velocity, direction of rotation. Compared with Hall device and other magnetoresistance device, TMR device has higher sensitivity, better temperature characteristics, better frequency characteristics and cheaper cost. Although Hall sensors and other magnetoresistance sensors are often used at present, TMR sensors will have the potential to be the next generation sensors. The aim of this thesis is to study on the TMR gear sensor. Based on the optimal design of magnetic circuit, TMR chip and electric circuit, a series of gear sensor that with high signal to noise ratio (SNR), high frequency response, good temperature characteristics and large airgap were prepared. As a result, these sensors can be used for detecting speed of gear, movement direction of gear and rack missing of gear accurately. The main conclusions are as follows:Ⅰ. Design of magnetic circuit for TMR gear sensorStudy on the magnetic signal of gear sensor among magnet sizes, resistance pitch of TMR chip, periodic pitch of gear and airgap with COMSOL multiphysics, which is a simulation software, establishing correspondence relationship between the various parameters and magnetic signal. The main contents include:(1) Compared the magnetic field distribution of magnet in different shapes that include cylinder, cuboid and cuboid with a groove. We found that the cuboid magnet with a groove has great advantages in many aspects, such as:reduce the bias magnetic field along the sensitivity direction of TMR chip, increase the work space of TMR chip, decrease the magnetic field perpendicular to the direction of TMR chip.(2) Groove will affect magnetic field distribution of magnet, to optimize the groove of each magnet that this thesis referred, will make sure TMR chip works on the best condition..(3) With the increase of the height of magnet, both the peak to peak value of magnetic signal that TMR chip output and the bias magnetic field that TMR chip sensed are decreased; With the increase of the length and width of magnet, both the peak to peak value of magnetic signal that TMR chip output and the bias magnetic field that TMR chip sensed are increased. (4) The maximum output signal appears when the pitch between TMR resistors is half of the periodical pitch of gear.(5) The output signal of TMR chip decreased exponentially as the airgap between TMR chip and gear increased. The large airgap will affect the normal operation of gear sensor.Ⅱ. Design and Performance analysis of TMR chipAccording to the analysis result of simulation, we determined the basic structural design of TMR chip. Measurement and analysis on the performance of TMR chip was also finished. There are many structural designs with different pitch between resistors depend on the basic structural design of TMR chip. The main contents include:(1) Comparative analysis on the designs of V01, V02, V03. Considered the assembly error, the magnetic field perpendicular to the sensitivity direction of TMR chip and the shield to the external magnetic field in the same direction, we found that V02is the most reasonable one, although the output of V02is related to the pitch of TMR resistors.(2) TMR chip was designed to be a gradiometer. There are four resistor area each TMR chip, and every resistor area includes two resistors, the spacing between each resistor area is0.25mm..(3) Based on the structure of TMR chip, we can get the single wheatstone bridge and dual wheatstone bridge respectively, and the pitch between two resistors is adjustable that the minimum pitch is0.25mm.(4) Combined the result of simulation, the Performance of TMR device and the application requirement of gear sensor, TMR chip what we used in this thesis has these characteristics, such as: large linear range, small hysteresis, great sensitivity and high frequency response. The more details are also measured.Ⅲ. Design of electric circuit for TMR gear sensorDepend on signal processing scheme, signal acquisition circuit, amplifier circuit and digital circuit, the design of electric circuit for gear sensor was finished and optimized. The main contents include:(1) Signal processing scheme in this thesis include:signal acquisition, analog amplification, digital output.(2) The signal processing scheme with wheatstone full bridge is better than half bridge. The full bridge can suppress common-mode noise and temperature shift, and the output is twice as large as half bridge. (3) Resistance-capacitance coupled amplifier circuit was adopted. The voltage follower can provide accurate bias voltage for differential amplifier. Blocking capacitor and filter capacitor played an important effect on isolating DC signal and filtering high frequency noise respectively.(4) The digital circuit used retardation voltage comparator with inverting input to convert analog signal into digital square wave signal.Ⅳ. Preparation and Testing of TMR gear sensorThe test platform of TMR gear sensor was built for testing and analyzing the main performance of sensors, and verifying the function of sensors. The main contents include:(1) The noise of analog signal was about70mV when rotary motor is in running, so it’s important to filtering the noise in signal processing circuit.(2) The test result of analog signal is consistent with the simulation result. We can get the maximum signal when the pitch between TMR resistors is half of the periodical pitch of gear; the output will decrease immediately when the airgap increased.(3) The TMR gear sensor prepared in this thesis can be used for detecting speed of gear, movement direction of gear and rack missing of gear accurately...