Development of Magnetic Position Sensor for Unmanned Driving of Robotic Vehicle

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1 Development of Magnetic Position Sensor for Unmanned Driving of Robotic Vehicle Dae-Yeong Im, Young-Jae Ryoo, Soon-Gil Park Department of Control System Engineering Mokpo National University Jeonnam, Korea Hyun-Rok Cha Automotive Components Center Korea Institute of Industrial Technology Gwangju, Korea Abstract In this paper, a position sensing system for magnet based autonomous vehicle using 1-diemnsional magnetic field sensor array was described. In advanced vehicle control, position sensing is an important task for the identification of their locations, such as the lateral position within a trajectory. The magnet based autonomous vehicle was identified position using magnetic materials. In the magnetic sensing system, the Earth field is a disturbance. It should be estimated a real-time. And, the memory space should be reduced in implementation. To solve the above problems, this paper proposes the magnetic field sensor array system included a vertical component of magnetic field, a linear region of the sensor output, method of position determination using a simple equation with a microcontroller. The proposal is verified practicability for magnetic position sensing system by the experimental results. I. INTRODUCTION Although the autonomous vehicles we see in science fiction movies appear to navigate with effortless precision, in reality vehicle and robot navigation is a difficult research problem. Indeed, simply answering the question Where am I? in truly autonomous fashion is a serious challenge for today vehicle and robot[1]. One type of critical information used by vehicle and robot control systems is the measurement of lateral position relative to a trajectory[3]. To determination of a vehicle or robot position, it uses the information from external sensors. The magnet based autonomous vehicle and robot senses the position via magnetic materials that generates magnetic field, as a magnetic marker or a magnetic tape. There are many technologies for position sensing of vehicle electrically powered wire, computer vision, magnetic sensing, optical sensing, inertial navigation and global positioning systems. Among the technologies, this paper focuses on the magnetic sensing system for autonomous vehicle and robot. There are several researches of position sensing for magnet based autonomous vehicle; (1) averaging, (2) peak-valley identification, and (3) differentials between dual sensor measurements[4-5]. These techniques has a weakness for the below problems. The Earth has a magnetic field with north and south poles. The magnetic field of the Earth[6-7,11] is surrounded in a region. And, a the Earth field by magnetic field sensor varies with direction of the sensor. It is a basic external disturbance on the magnet based position sensing system. The magnetic sensing system should be freely performs from the Earth field effect. This disturbance should be estimated a realtime. And, the memory space should be reduced in implementation. Therefore, the position sensing system with a 1- dimensional magnetic field sensor array is proposed. To solve the problems and for precise position sensing, the proposed system includes that the magnetic field sensor array[2,8,10], the vertical component of magnetic field, the linear region of the sensor output and the algorithm of position determination using a simple equation with a microcontroller. The proposed system in this paper is verified practicability for magnet based position sensing system through the analysis of experimental results. In this paper, the background of this research introduced the section 2. For the proposed sensing system; the vertical component of magnetic field, sensors array and the method of position determination are described in the section 3. The experimental results and analysis using the proposed system is reported in the section 4. Then, in the section 5, this paper concludes with a discussion of this research. II. ACKGROUND A. Field patterns of sample magnetic marker In this section, the experimental data are presented for a comprehensive analysis of the magnetic fields. The data was from a static test on a bench table. The sample magnetic marker is made of neodymium material in a cylindrical shape with a diameter of 2.5cm and a length of Identify applicable sponsor/s here. (sponsors) /09/$ IEEE 1618 IEEE SENSORS 2009 Conference

2 0Magnetic Field 4cm. The long axis of the marker is placed perpendicular to the surface of the bench table. Figure 1 shows the lateral(y) and vertical(z) fields generated by a sample marker. The longitudinal axis is not shown because of it is similar to the lateral axis. It should be noted that the magnetic field of the magnetic marker could be modeled as a magnetic dipole. We use the experimental data for an actual representation. Figure 1. Lateral and Vertical Field of Sample Marker In the three-dimensional space, the lateral field is opposite in symmetry in magnitude on two sides of the marker. The lateral component rises from zero at the center of the marker and reaches its peak at a distance about ±10-15cm from the marker, then gradually weakens farther away from the marker. The vertical field is the strongest just above the marker, and decrease to zero at about 40cm away from the marker.. The Earth magnetic feld The Earth field is approximately a magnetic dipole, with one pole near the north pole and the other near the geographic south pole. The notice point in a magnetic sensing system is the Earth field in all places. The Earth field may be fixed or varying, depending on the specific locations and the geological features. C. A magnetic position sensing for lateral distance There are several methods for magnet based position sensing system; (1)averaging, (2) peak-valley identification, and (3) differentials between dual sensor measurements. The first method is based on an average of longitudinal data measurements over a number of markers. This is based on the assumption that the longitudinal fields averaged from the magnetic markers will approach zero, thus the remaining average represents the Earth disturbance. While it is simple to implement, it is slow to respond to a quickly changing field. The second method utilizes the identification of valleys in the vertical field between markers, thus reflecting the fields. To current the Earth field estimate, it is use the previous valley. Since the Earth field changes with the location and direction as well as the vehicle/robot orientation, it does require a real-time estimation. The last method uses two nearby sensors to eliminate the common background components by taking the differential between dual sensors. It is based on the assumption that the background fields are approximately the same at the two sensor locations. If using this method, the Earth field can be directly removed. However, this is use a lateral displacement table look-up. The table lookup is a one-to-one mapping relationship of magnetic field versus distance. The table look-up has some drawback; no data makes accuracy decrease, interpolation is required, and more accuracy, more memory space. In practice, this method requires too many memories to store the vast relationship data. Therefore, the position sensing system with 1-dimensonal magnetic field sensor array is proposed. III. PROPOSE POSITION SENSING SYSTEM efore you begin to format your paper, first write and save the content as a separate text file. Keep your text and graphic files separate until after the text has been formatted and styled. Do not use hard tabs, and limit use of hard returns to only one return at the end of a paragraph. Do not add any kind of pagination anywhere in the paper. Do not number text headsthe template will do that for you. A. A vertical component of magnetic fied Magnetic Field of the Earth Linear Operating Region Figure 2. Variation of the Earth Field and Direction Figure 3. Fig. 3. Vertical Magnetic Field and Magnetic Maker In the figure 2, the Z-Axis(vertical component) of the Earth field is no variation with relation to directions. The 1619

3 vertical field varies when the sensor is located with different height. An important fact is that the Earth field has a stationary direction at some point on the Earth. Also, the vertical component has always same direction quality in the horizontal plane. Thus, the vertical field is chosen for the proposed system. Figure 3 illustrates a variation of the vertical field and location of magnetic marker. As seen the previous section A, the vertical field has the strongest value just above the marker and decrease more and more away from the marker. The data of a magnetic field versus a marker position has a non-linear characteristic. ut, there is a region which is reasonably linear between spot of the above marker and before the Earth field. In this region, we can be obtaining the information of regular relationship on the magnetic field and marker position. 0Magnetic Field Magnetic Field of the Earth Linear Operating Region Figure 4. Vertical Magnetic Field and Magnetic Maker. The 1-dimensional sensors array In the figure 3, the linear operating region is two areas on two sides of the marker. Since these linear regions are bilateral symmetry, such as an equilateral triangle, it can be not distinguish from only vertical field. In this paper, the Z-Axis(vertical field) sensor array is chosen for discrimination of two linear region. data of the sensor 2 to reference, it can be distinguish with bilateral region of the sensor 1 and the sensor 3. For the precision positioning and a good operating condition, this position sensing system is required some consideration; (1) the space between the sensors, (2) the height between the sensor and surface of the magnetic marker, (3) operating magnetic field range. The three considerations are closely connected. The space between the sensors in the array is dependent on the magnitude of the marker field and desired system accuracy. The height between the sensors array board and marker is finalized from the each sensor linear output range. If the height is varied, the magnitude of the applied field varies too. Also, the space reduced to high accuracy, it wastes many sensors. Thus, the space and height should be determination according to required system accuracy The number of sensor in the array is determined by positioning length, once the space between each sensor is decided. Adding more sensors to the array increases the sensing length. C. Position determination Figure 2 showed that the vertical field is no varying according to direction in the Earth field. In principle, the vertical component of the Earth field has unique value on the Earth surface. However, real the Earth field strength varies with location and covers the range from about 300[mG] to 500[mG]. Thus, the Earth field should be removed for the magnetic position sensing system. The magnetic field by the sensor includes the marker field, the Earth field and an field. The field at any place, such as the point C in the figure 5 can be expressed as: = ker + + (1) mar In the field by each sensor, if it takes subtract the smallest field from other fields, the marker field remains. For example, let us think about point C in the figure5. = + + (2) Magnetic Field Sensor Array Sensor1 Sensor2 Sensor3 = + + sensor2 sensor 2 = + + (3) (4) A Moving = (5) sensor2 sensor 2 Figure 5. Magnetic Field Sensor Array Figure 4 depicts the position sensing system using a 1- dimensional magnetic field sensors array. The stationary each sensor measures to magnetic field at different displacement areas. The nearby two sensors has a common area, such as A and in the figure 4. The magnetic field at common area is the linear operating region in the figure 3; A is the linear region of the, and is the sensor 3. If it uses the = (6) The Earth field is directly removed. In addition, the field is eliminated too. Figure 5 presents sensor array output versus magnetic marker position at the as figure 4. When the marker is moved below each sensor 1, 2, 3, the sensors output is a minimum value(peak). We can know the linear region of the sensor 1( A ) and sensor 3( ) via the comparison of each 1620

4 sensor output; when the sensor 2 is peak, if the sensor 1 smaller than the sensor 3, the region A (sensor 1) is usable section for position sensing. The regions A and has a same linear characteristic on a different inclination. 0Magnetic Field Position The Earth Magnetic field or the height between the sensor and marker(vehicle or robot suspension). A change of vehicles/robots heading angle vary a little magnetic field value. In other words, since irregular condition on a vehicle/robot trajectory, the height between the sensors array and marker can be change. Figure 6 shows the variation of magnetic field according to the height. Thus, to more accuracy, it requires the data of each m and b in the equation (7) according to the variation of applied field(height). Sensor 1 A IV. EXPERIMENT AND ANALYSIS Sensor 2 Sensor 3 A. Composition of the position sensing system C Magnetic marker Move Fig. 5. Sensor Array Output Versus Marker Position To determination of the marker position, we use a simple equation of the linear region, as below formula. y = mx+ b (7) In the equation (7), the y is the magnetic field of usable linear region. The x is the position data. The m is the inclination of straight-line on the linear region. And, the b is a constant of stationary sensor position update value. The value of m and b in the equation (7) can be compute using magnetic field data. Then, the position data can be obtained from the sensors array output. This method is required that only data of the m and the b saved in an electrical memory. Thus, it can be reduce memory space. Figure 7. Composition of Proposed Sensing System Figure 7 shows composition of the proposed position sensing system. The system consists of quintuple 1- dimensional sensors array, signal conditioning circuit with amplifiers and the magnetization circuit, the positioning algorithm with a microcontroller for the position determination and a magnetic marker that provides a magnetic field. A stationary sensor converts the magnetic field strength into a differential voltage, then amplifiers and other discrete components provide signal conditioning for the sensor signal. The sensor output is proportional to the position of the marker in the sensor linear range. The algorithm is based on the characterization of the magnetic field sensor responding to the magnetic marker; takes the data from all the sensors, determines the linear region of usable sensor, and then calculate the position. In the sensors array, each sensors spacing is 4cm. Thus, a sensing range of the system in figure 7 is ±80mm(left and right) from the center of sensors array. Figure 6. The Height etween the Sensor and Marker Versus Measured Magnetic Field However, a variation of the applied field is changed to the equations inclination(m) and position constant(b). The variation of applied field is caused by change of the Earth field. Experimental results The output of magnetic field sensor has an output offset that is caused by setting angle error and gain error. In this paper, it is compensated using the Earth magnetic field[9]. Figure 8 is magnetic Field to independent system for height variation, it have been magnetic field measurement according to variation of height. And it is obtained each the simple equation of the linear region from field that is from -700[mG] to -2300[mG], such as figure

5 removed a real-time. In implementation, the memory space should be reduced. To solve the problem in this paper, it is used the vertical component of 3-dimensional magnetic field sensor, sensors array, the linear region of the sensor output and a simple equation for position determination. The result of experiment, the proposed system is verified that it has precision capability to magnetic position sensing, with average error of 1.4mm. ACKNOWLEDGEMENT Figure 8. Result of Magnetic Field of Different magnitude. Figure 9 is the experimental results for applied magnetic field of different magnitude(height). It shows that the proposed magnetic position sensing system has a maximum error of 5.6mm and an average error of 1.5mm in the situation of changing height. Therefore, the proposed position sensing system is a good candidate for magnet based autonomous vehicle and mobile robot. V. CONCLUSION This paper presented an approach for the position sensing system for magnet based autonomous vehicle and robot. This proposal utilizes an array of 1-dimensional magnetic field sensors, the method of position determination using a simple equation. Other circuit includes signal conditioning electronics and a microcontroller. The proposed sensing system represents the position of the magnetic marker along the long axis of the sensors array. There are several sensing systems designed to detect the position between the magnetic marker and the sensor. These systems has some drawback to solve the below problems. The Earth field is an external disturbance on the magnet based position sensing system. The disturbance field should be This research was supported by the Program for the Training of Graduate Students in Regional Innovation which was conducted by the Ministry of Commerce Industry and Energy of the Korean Government. REFERENCES [1] John. J. Leonard, Hugh F. Durrant-Whyte, Directed Sonar Sensing for Mobile Robot avigation, Kluwer Academic Publishers, USA, [2] David S. Nyce, Linear Position Sensors: Theory and Application, Wiley-Interscience, USA, [3] Y.-Y. Jung, Y.-J. Ryoo, Intelligent position sensing system for magnet based autonomous vehicle, ITSWC2005, CDROM, [4] Ching-Yao Chan, A system review of magnetic sensing system for ground vehicle control and guidance, California PATH Research Report, UC-ITS-PRR , [5] C.-Y. Chan, H.-S. Tan, Evaluation of magnetic markers as a position reference system for ground vehicle guidance and control, California PATH Research Report, UC-ITS-PRR , [6] Thomas Stork, Electronic compass design using KMZ51 and KMZ52, Application ote, AN00022, Philips Semiconductors, [7] M. J. Caruso, Application of magnetoresistive sensors in navigation systems, Sensors and Actuators 1997, SAE SP-1220, pp , [8] ratland Tamara, Wan Hong, Linear position sensing using magnetoresistive sensor, Application ote, A2.4, HoneywellSSEC. [9] Y.-Y. Jung, D.-Y. Lim, Y.-J. Ryoo, A method and system to compensate vertical component of 3-dimensional magnetic field sensor using the s field, Journal of Fuzzy Logic and Intelligent Systems, Vol. 16, No. 3, pp ,