A Microcontroller-based scheme for measurement of magnetic field position using Giant Magnetic Resistor (GMR) in Contactless
Measurement and Robotics
Omidreza Esmaeili Motlagh
Faculty of Electronic Engineering, Mazandaran University ( UMZ ), Babol, Iran
Magnetic sensors in contactless applications:
Magnetic sensors can be used for all sorts of applications where a contactless measurement of position and orientation is required. For instance in cars, where a large number of sensors are employed, magnetic sensors obviously have great advantages over their counterparts which involve the use of moving parts.
Also they are variety of Magnetic sensors used in industry to simplify designs in different cases. For instance the current measurement using shunt resistor is in some cases impractical or impossible. For large currents the shunt resistors are heavy and they cause voltage drop and dissipate heat. They are not insulated and the conductor should be disconnected for mounting. Contactless current sensors may be used for remote conductors at high potentials, underground cables etc. Today a wide range of AC and DC contactless current sensors is produced. Also precise contactless magnetic sensors are always required by car industry, chemical industry and many other industries, for measurement of power and many other applications.
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Solid-state magnetic field sensors are moving into a growing range of new contactless applications because of their small size, low power consumption, and relatively low cost.
The first aim of the project therefore will be to become familiar with different kinds of magnetic sensors, especially new GMR family and to investigate their characteristics and behavior in presence of applied magnetic field. [1]
Giant Magnetic Resistor (GMR):
Structure:
The sensor consists of stack of magnetic and non-magnetic layers and Giant Magnetic Resistivity or GMR effects occur when these ultra-thin magnetic layers are set in close proximity to each other. Alternately magnetic and non-magnetic layers can be built up with the magnetic fields either parallel or anti-parallel, creating very sensitive low to high resistance components. The resistance of a GMR sensor as a measure for the applied field varies with the angle between the hard internal magnetization and the soft magnetic covering layers which follows the magnetic field. [2]
The Giant Magnetic Resistor is a resistive device whose Ohmic value changes gradually only when a nearer external magnetic field changes its relative orientation and the originated field intensity or the distance between magnet and GMR are not relevant within certain limits. Any rotary button or knob containing a magnet on it, may work as a contactless switch when its rotation is detected by a GMR. The resistance of the GMR with a typical value of about 700 ohm changes in a cosine form with orientation of the magnetic field. This variation of the resistance is registered as change of the voltage drop over a single sensor. To gain the sensitivity and to avoid offset and temperature drift of the GMR two different bridge configurations, B6 and C6, are used. The GMR devices contain two half bridges. Each half bridge consists of two sensors with opposite hard magnetization. The B6 device is formed by two half bridges in anti-parallel configuration.
The differential output signal allows angle detection up to 180 degrees. The two half bridges of the C6 device are perpendicular to each other, which provide full 360 degrees detection or advanced resolution up to 90 degrees. [3]
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Applications:
The generic nature of the sensor and its very small dimensions make for a wide variety of potential applications, promising increased activity and competitiveness for manufactures.
This project also aims to begin an exploration into applications of this kind of magnetic sensors family and find the best method to convert sensors analog output into digital code using an A/D converter. Then a microcontroller will be used to process digital data and to execute the program that will be developed for selected application.
Current applications of GMR sensors in automotive industry, contactless measurement and control, Medical industry and robotics are for example:
1. Contactless Potentiometers
2. Contactless Current Measurement
3. Over current and Short Circuit Detection
4. Current Sensing in PCB Traces and Wires
5. Detection of Spin, Speed, Angle and X-Y Position
6. Speed and Position detection for Electric Motor Shafts
7. Position Sensing for Shock Absorber Feedback Control
8. Precise detection of Position of Pneumatic Cylinders
9. Wheel speed Sensing for ABS Break Applications
10. Speed of Wheels and Engines
11. Transmission Gear Speed Sensing for Shift Control
12. General Field Detection in Implantable Medical Devices
13. low field Detection in Currency Applications
14. Vehicle Detection for Traffic Counting Applications
15. Magnetic Encoder Detection for Secure Systems
16. To detect the exact position in Robotic Applications
17. Earth’s magnetic field Detection
18. Compass Applications
[4]
Future of GMR applications:
1. NVE sensor products use GMR materials to provide advantages in magnetic sensitivity, temperature performance, speed, and power consumption.
2. NVE isolator products use GMR technology to provide significant advantages in speed, size and reliability.
3. MRAM has the potential for faster access times than today’s RAM devices. MRAM chips could also provide advantages in write-time and write-cycle endurance, compared to semiconductor nonvolatile memory technologies.
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4. These materials have application in big-medical areas where fields generated by neurological or muscular activity must be detected.
5. Magneto-resistive read heads are already being applied to disk and tape drives. Over the next few years, GMR materials are likely to enhance the reliability of such drives by delivering higher signal levels than the AMR materials in first generation magneto-resistive drives.
6. High sensitivity low field GMR materials will be used in high accuracy compasses and geophysical applications such as magnetic field anomaly detection in the earth’s crust.
[5]
Procedure:
In almost all of the applications, analog signals produced by the sensors, are processed to figure out the direction, angle or speed of rotation of applied magnetic field. For a digital system an Analog to Digital Converter (ADC) is used to convert analog signals to digital code. The processor of the system which is used to process the ADC results might be a simple microcontroller or a large mainframe depends on application and its operating program. The program should be developed to process the situation and to result desired outputs from system inputs. For any microcontroller-based structure in addition a chip programmer is needed to transfer the program into EEPROM code memory of microcontroller chosen by designer. The best scheme to minimize the size of hardware and even the program to be executed is to use an AVR microcontroller. This way there will be no need to external ADC and there are so many other advantages in such structure as mentioned below.
AVR Microcontrollers Architecture and Advantages:
Today there are several AVR products to choose from when preparing for a project. The Atmel AVR® family has been growing rapidly since its debut in the late 1990s and it is widely being used in industry for real-time applications and industrial control. Atmel’s AVR microcontrollers have a RISC core running single cycle instructions and a well-defined I/O structure that limits the need for external components. Internal oscillators, timers, UART, SPI, internal pull-up resistors, Pulse Width Modulation (PWM), internal ADC and analog comparator and watch-dog timers are some of the features that can be found in such devices.
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AVR instructions are tuned to decrease the size of the program whether the code is written in C or Assembly. With on-chip in-system programmable Flash and EEPROM, the AVR is a perfect choice in order to optimize cost and get product to the market quickly. [6]
For all Atmel AVR microcontrollers using In-system Serial Programmer (ISP) is the best method to transfer operating programs into code memory. AVR-ISP is a low cost serial programmer to perform in-circuit programming without removing the microcontroller from the target board. It sends and receives data through serial port so that it simplifies the programming task.
After research requirements are developed, finally the end result of the project will be to investigate future of GMR applications then design and construct a prototype of a system that will detect the direction, angle or speed of rotation of applied magnetic field and consequently will perform a desired reaction according to its microcontrollers program. The emphasis is mostly given on a robot with the ability to detect and process the position of magnetic field of the earth. This robot can follow magnetic patterns to find its own path automatically or control the motion of a vehicle according to a certain magnetic pattern. Such system can be used in intelligent systems and robotics, compass applications, automatic navigation systems and many other applications. For instance in compassing applications, this system can be used to detect the components of the Earth’s magnetic field to determine direction relative to magnetic north. [7]
Time Plan:
From the start of the project it will be spend respectively, 2 month on studying and gathering the references, exploration of new applications and developing project requirements. 2 month on familiarization with different magnetic sensors and microcontrollers to choose appropriate devices and for designing the hardware. 2 month on programming and optimizing the algorithm. 3 month on construction of the prototype and testing. It should be noted that above time periods may have conflict on each other.
Conclusion:
Magnetic sensors can be used for all sorts of applications where a contactless measurement of position and orientation is required.
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The Giant Magnetic Resistor is a resistive device whose Ohmic value changes gradually only when a nearer external magnetic field changes its relative orientation.
The generic nature of the sensor and its very small dimensions make for a wide variety of potential applications, promising increased activity and competitiveness for manufactures.
This project aims to begin an exploration into applications of magnetic sensors and find the best method to convert sensors analog output into digital code using an A/D converter. Then a microcontroller will be used to process digital data and to execute the program that will be developed for selected application.
The best scheme to minimize the size of hardware and even the program to be executed is to use an AVR microcontroller.
After research requirements are developed, finally the end result of the project will be to investigate future of GMR applications then design and construct a prototype of a system that will sense the direction, angle and speed of rotation of applied magnetic field and consequently will perform a desired reaction according to its microcontrollers program. The emphasis is mostly given on a robot with the ability to detect and process the position of magnetic field of the earth. Such system can be used in intelligent systems and robotics, compass applications, automatic navigation systems and many other applications.
