Hall-Effect Semiconductor Sensors - Lab Report Example

Hall-Effect Semiconductor Sensors and Section # of TABLE OF CONTENTS Introduction 2. The Hall-Effect Phenomenon 3. In Situ Hall-Effect Sensor 4. Drive System with Hall-Effect Sensor 5. Magnetic Micro bead Detection 6. Hall-Effect Sensors in Trackball Sensing 7. Space Shuttle Hyperbolic Thrusters 8. Hall-Effect Pressure Devices 9. Hall-Effect Photo Sensor 10. Split-Drain MOS Devices LIST OF FIGURES Reference Title Page Ramsden, Edward. (2006). Hall-effect Sensors. Second Edition. Burlington. Newnes. Figure 1 - The Hall-effect in a conductor 1 Ali, A. Et al. (2008). “A new contactless trackball design using Hall Effect sensors.” Institute of Petroleum Engineering, Heriot-Watt University. Figure 2 - Schematics of one of the electromagnets 3 Ali, A. Et al. (2008). “A new contactless trackball design using Hall Effect sensors.” Institute of Petroleum Engineering, Heriot-Watt University. Figure 3 - Effects of the ball movement and the Hall Effect 4 INTRODUCTION This paper is about the Hall-effect and the way it is used to detect magnetic fields. The report discusses the operation of Hall-effect devices and gives specific examples such as proximity sensors, speed sensors, pressure and photo sensors with PMC and the application in the detection of magnetic micro bead. Hall-effect trackball which is the center of attention for many industries is also discussed and its operations are described in details. THE HALL-EFFECT PHENOMENON Hall-effect is demonstrated in a straightforward set up in which a thin plate of conductive material [1] such as iron core or copper carries DC current. Two probes of a voltmeter can be connected to the plate in opposite directions to the other sides of the plates [1]. When there is no magnetic field perpendicular to the plate, as shown in situation (a) in Figure 1, the voltage that will be measure in the voltmeter will be zero [1]. Figure 4 - The Hall-effect in a conductor In case where there is a magnetic field present, the voltmeter will show a reading as shown in the (b) part of the diagram. This phenomenon is known as the Hall-effect. A Hall probe can be used to measure the Hall-effect that was demonstrated above [2]. When an electric current runs in a conductor in a magnetic field, a traversal force is exerted [2] on the charge carriers by the magnetic field. Therefore the charge carriers are pushed on one side of the conductor [2] as shown in the (b) part of the diagram. This phenomenon was discovered by E. H. Hall in 1879 [2]. This is the basic principle used in the operations of Hall-effect devices. IN SITU HALL-EFFECT SENSOR A sensor used for measuring thickness or the compactness based on Hall-effect has been developed; this sensor is called in situ sensor [3]. Whenever there is a change in the magnetic force being applied, the Hall sensor, which is works as a transducer [3], generates an output voltage. This output is variable and is dependent upon the thickness of the material being measured and the force of magnetic field being applied. The distance between the sensor and the magnetic source is the main determinant of the output voltage [3]. Vacuum failure and vacuum bag leaks can be detected with the use of the sensor [3]. This sensor is primarily used in the quality control departments and cure cycle [3] of the factories producing different material which need a reliable sensor. Most of the times, the sensor is paired with a monitoring device to demonstrate the readings. For the purpose of online monitoring and compaction of a curing composite, the sensor is quite successful. This device is being used in the quality control departments and the manufacturing departments of many businesses. They ensure a reliable and low cost solution to the quality control problems. They are used to ensure the thickness of material being monitored is in the range. As they have an ability to measure and detect very compact material, they are used widely in the leather, CD manufacturing and clothes manufacturing industries. DRIVE SYSTEM WITH HALL-EFFECT SENSORS This device uses two Hall-effect sensors for position and speed estimation without current measurement sensors. The current sensors are removed from the device but two Hall-effect sensors are placed for the estimation of the position and speed [4]. The drive system is simpler and does not employ rotary encoder because of which it can be easily implemented in a 16-bit microcontroller [4]. The sensor has a resolution of only 90 degrees [4]. The current position and the current and the speed are estimated with the help of Hall-effect sensors. Single reading is taken from each sensor and then sent to the Permanent Magnet Synchronous Motors (PMSM) [4]. This device follows an algorithm to calculate the average speed between the two Hall-effect sensors. The magnetic field produced with the change of speed is used by the sensors to estimate the speed. The measurement of the rotating shaft’s speed and position is essential for the operations of a variety of machinery. Automobile ignition controls, exercise equipments and other tools make use of speed sensors and time sensors [1]. An example is Geartooth sensor which is rotated past its flat end in the use of gears in automobiles [1]. MAGNETIC MICROBEAD DETECTION Thousands of nanometer sized super-magnetic particles are used in the commercial magnetic micro beads for bio-chemical applications [6]. Different coating are applied to these beads, therefore they can be attached to the target [6]. These targets can be cells, bacteria or nucleic acids. The detection of these beads is performed by micrometer sized Si Hall Sensor with CMOS technology. With these devices, it is possible to detect the presence of even a single magnetic micro bead. Planar Hall effect is used in the device to detect the magnetic micro beads. Whenever the device is immersed into the fluid, the device comes in contact with a magnetic field. The output signals from the device were zero volts when it was not immersed into the fluid. When it comes in contact with the magnetic field, the signal increases and gives an increased output. This sensor can be used in the detection of bio-molecules and study the interaction of molecules especially the DNA module interaction. HALL-EFFECT TRACKBALL SENSOR This trackball design is based on a motion detector which detects motion without contact [7]. In the design, electromagnets are placed near to the ball surface outside the ball. As a magnetic field detector, a Hall sensor is placed inside each electromagnet [7]. The magnetic field is recorded on the ball’s surface as the ball moves. Figure 5 - Schematics of one of the electromagnets The movements cause instant changes in the output of the sensor. In order to determine the direction and the speed of the ball [7], the output is converted to different format understandable by the output machine. This ball tracking is also used similarly in wheel rotation sensing. Anti-lock brake systems mainly use this device to sense the wheel rotation. The function has also been extended to provide a more refined system of anti skidding. It has many industrial uses also to control the movement of mechanic arms [7] and robotic paint sprays. The trackball sensor has many advantages over the existing optical trackballs. It does not require many components such as encoders [7] and frame. As there is a contact of the ball in optical sensing trackballs, there is a lot of wear and tear [7] and can cause many problems. As the trackball with Hall sensors is contact-less [7], it is more accurate, there are no noise errors and there is no wear and tear of the assembly parts when trackball is used. Figure 6 - Effects of the ball movement and the Hall Effect SPACE SHUTTLE HYPERBOLIC THRUSTERS To estimate the main stage open-time of a valve, Hall-effect sensors are used in the high pressure gaseous nitrogen conditions. The Hall-effect sensors are used to in the laboratory while testing the external temperature derivations and electrical current in the shuttle. In the actual thrust firings also, the sensors are used to estimate the temperatures and current and to relate then with opening the valve. Changes in temperatures take place on the thrust firings and these temperature changes cause the output voltage to change accordingly. The sensor has a type-k thermocouple. HALL EFFECT PRESSURE DEVICES In this Hall device, polymer material composite (PMC) [8] is used as the magnetic film. Whenever the magnetic flux changes due to the changes [8] in the pressure, the readings on the output of sensor also change. The voltage across the PMC changes because of the changes in air gaps or air pressure between the PMC and magnetic field generator. With the increase in reluctance, the Hall voltage in the PMC material falls [8]. This is an indirect sensor as the readings of the voltage and the magnetic field being generated are used to deduce the pressure in the atmosphere. PMC is used in this sensor as a semiconductor because it has a high sensitivity and responds to stress changes sharply. Even very small pressures are measurable with this device. The indirect pressure sensor can be employed where there is an air gap between the PMC and the magnetic field generator. This device is used in harsh environments of the industries in processing looms to measure the pressure accurately. It is also used in pistons in automobiles to control the piston’s position and detect the new position if changed. HALL EFFECT PHOTO SENSOR For a Hall sensor to be made as a photo sensor, the PMC being used should be coated with radiant flux absorbent material such as black colored platinum or carbon. This sensor works when light falls on the coated material and because of its color, the light energy is transferred to the PMC [8]. Therefore the temperature of the PMC rises and the reluctance of PMC increase which decreases the permeability of the material. Therefore the output voltage increases with the rise in the light intensity. Through this way, the conversion of light to magnet is achieved [8]. This is because the change in the permeability of the material brings changes in the Hall voltage. This light sensor can be used to detect the distance of the source form the device, given that the reflection of the surface is constant. Photo sensor can be used as a reflective sensor to detect the features drawn on the surface of a wheel to encode the shaft rotations. However the light used in this measurement should be of a constant wavelength. SPLIT-DRAIN MOS DEVICES The device performs the four functions of sensing the magnetic field, working point control, provision of gain and the voltage conversion [1]. This device is uses a PNP transistor [1] and operates on current deflection. This doped semiconductor sensor has an array of split drain transistors which are connected in parallel. This device is primarily used to measure the strength of the magnetic field. This device can be used in many secondary sensors to sense the pressure, proximity etc. BIBLIOGRAPHY 1. Ramsden, Edward. (2006). Hall-effect Sensors. Second Edition. Burlington. Newnes. 2. Hyper Physics. (2009). Hall Effect. Available from http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/hall. [November 27, 2009] 3. Saliba, T. Et al. (1994) “Development of an in situ Hall-effect sensor for online monitoring of thickness and compaction during composite curing.” Journal of Composite Science and Technology. 4. Olarescu, N. V. (2008). “Enhanced Current-Sensorless Drive System for PMSMS using two Hall-effect sensors for wide speed range.” OPTIM 2008. 5. Faglia, G. Et al. (1998). “Square and collinear four probe array and Hall measurement on metal oxide thin film gas sensors.” INFM and Department of Chemistry and Physics for Engineering and for Materials. 6. Boero, G. Et al. (2003). “Micro-Hall Devices: performance, applications and technologies.” Swiss Federal Institute of Technology Lausanne. 7. Ali, A. Et al. (2008). “A new contactless trackball design using Hall Effect sensors.” Institute of Petroleum Engineering, Heriot-Watt University. 8. Seki, K. Et al. (1993). 29: 3189-3191. “Pressure/Photo sensor utilizing polymer magnetic composite.” IEEE Transaction on Magnetics. Read More