Handheld Gaussmeter. Robert Ito Michael Wong Faculty Advisor Professor Henry Lee Graduate Student Mentor Owen Finch

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1 Handheld Gaussmeter Robert Ito Michael Wong Faculty Advisor Professor Henry Lee Graduate Student Mentor Owen Finch

2 Table of Contents I. Introduction II. Background Hall Sensor III. Design Objectives Hall Sensor and AD22151 A3516 Voltage Regulators Design and LM78M05 Microcontroller Design Chosen IV. Results Schematic and Layout PCB Finished System Functioning Gaussmeter Pictures V. Conclusion

3 I. Introduction From what we learned up to this point in our academic careers, it s clear that when there is a magnetic field there will always be a corresponding electric field. The two come hand in hand as a pair. Our device, the gauss meter, uses the Hall Effect to track the surrounding magnetic field. To sum up the theory behind the Hall Effect or Hall voltage; it is the potential difference between opposite sides of an electrical conductor, created by a perpendicular magnetic field and flowing electrical current. The most important device is our hall sensor, device A3516, because this is the part that can actually track the magnetic field and output a voltage. From there we send that voltage to the microcontroller, where it looks up the appropriate gauss value reading and sends it to the LCD screen. Those three parts, the Hall sensor, microcontroller, and the LCD screen make up the meat of our device. All those parts are connected with the proper circuitry, including resistors, capacitors, and voltage regulators. We will go into more detail in the next few sections of our paper. Our gauss meter might not be the most sensitive or the handiest, but in real world hall sensing devices serve very important purposes. Since the hall sensor can track a magnetic field, it is easy to detect mechanical pieces (by attaching a magnet to the part) that are moving at high rates. There is also a satellite GOES that can measure the Earth s magnetic flux in different parts of the world quickly and efficiently. These are only some of the possible applications for the hall sensor. In our case, we simply used the Hall sensor to make a hand-held device that can measure the magnetic flux in your everyday life. Our main goal was not to simply make a device and use it to solve problems in our lives, it was more important for us to learn from the overall process of going from a theory to actual manufacturing of a prototype. In essence, we were aiming at 1) Understanding the Hall Effect 2) Learning about design flow/process 3) Learning about basic circuit design and 4) and getting in introduction into microcontrollers. Lastly, it was important that we make a working device from scratch. II. Background Hall Effect

4 A charge moving with velocity given by,, moving in a magnetic field will feel a Lorentz force. This force is orthogonal to the magnetic field and the velocity of the particle. A current applied to a conductor will experience the Lorentz force in a similar manner. This leads to the electrons accumulating on an edge of the conductor, while a positive charge accumulates on the opposite edge. This can lead to an unbalanced charge distribution which will cause an electric field and a resulting force. This force skews the equipotential lines [1]. The diagram below shows this effect. Fig 1 Diagram of Hall Effect [2] The hall sensor outputs a voltage based the magnetic flux and current. III. Design The project at hand is to build a handheld Gauss Meter, which is a device that measures the magnetic flux. This is accomplished by utilizing a Hall Effect sensor which will output a voltage proportional to the magnetic flux if the current is held constant. Following this section, the Hall Effect is described in more detail. Since the device must be portable, the power must come from a battery source. The voltage regulated and brought down to the proper bias voltage of 5V for the Hall Sensor. To do this we will utilize a voltage regulator. Finally to store data, interface with testing equipment, and perform a basic averaging function, a microcontroller is used. The resulting output after the calculations will be viewed on a LCD screen.

5 Fig 2 Block diagram of the planned system Objectives We will design a handheld gaussmeter to measure the magnetic flux. Since the device will be handheld it has to be powered by a battery source. We will utilize a hall sensor that will output a voltage to a microcontroller. The microcontroller will then either perform an averaging function or output the voltage directly to an LCD. The time for the averaging function can be determined by the user utilizing a switch which will allow start and stop the function. The LCD controller will be display when the device is in the average or normal modes, output voltage, and the magnetic flux in gauss. Device will be handheld and portable Measure the magnetic flux accurately Utilize a microcontroller to perform computational and functional tasks Perform averaging function Display voltage and magnetic flux readings on an LCD screen Hall sensor

6 Fig 3 Diagram of Hall effect [2] The voltage equation for the hall sensor is given by: One can see that polarity changes are reflected on the voltage, and that if current is current is constant the flux can be measured as well. Hall Sensor Characteristics For our design we will be looking for the following characteristics to choose the appropriate Hall sensor for our application. To have a high output voltage the Hall coefficient must be high. The material should have a low resistance to avoid over heating and noise Common materials used are Gallium Arsenide and Indium Arsenide. Have a reasonably low gauss to voltage ratio for the sensitivity to increase accuracy

7 Analog Devices AD22151 Hall Sensor: We have orders all the parts, except the microcontroller, to perform assembly and testing. The Hall Sensor, AD22151, requires some analysis to decide upon the calibration of the compensation. The following will examine some of those results. The AD22151 has a built in temperature and gain compensation, and by adding external resistors one can change this compensation based on the application. From the documentation from Analog Devices we were able to determine these values. The figure below shows the diagram of the Hall Sensor and where the external components need to be placed. Figure 4 Hall Sensor [3] As we are assuming we will be testing the device with a ferrite magnet, from the graph below we determined a 10kOhm resistor for 2000ppm is needed in between node 1 and 3, as those are the temperature compensators.

8 Figure 5 Drift Compensation [3] The gain equation was provided by Analog Devices is as follows: [3] Further testing has to be done in order to determine how much the gain needs to be compensated. If we desire to set the sensitivity to a certain value we can utilize the equation provided to us in the documentation for the AD But as we have the voltage regulator, which has a fixed output, and therefore requires no calibration or external devices. Allegro 3516 Hall Sensor: The Allegro 3156 is a calibrated device, meaning that the sensitivity, temperature offset, and gain are all determined by the manufacturer. This device, although lacking the degrees of freedom of the Analog Devices Hall sensor carries a similar [3] Voltage Regulator Design We have decided to make one of our options for a voltage regulator to supply 5V to the hall sensor to be a self-designed configuration. This design was chosen from a lab from Carnegie Mellon University s lab manual. This design is a standard power supply voltage regulator consisting of a NPN transistor, 5.6V Zener diode, a decoupling capacitor, and load resistors. The following diagram shows the design that we implemented. The operation of the regulator is as follows: When the transistor is on, the resistors values are chosen to make sure the diode is in the reverse break down region. Therefore the voltage at the base node is held at 5.6V. So Vbe and the load is also at 5.6V. Since 5.6V is larger than 0.7V the diode between the base and emitter is now forward biased. So the voltage across the load is now V = 4.9V. The current being supplied to the Hall sensor supply is 4.9/780Ohms = 6.28mA. The current is held constant because the current is split between the base emitter diode and the Zener diode. The base current is proportional to the collector current by a factor of β and the emitter current is equal to (1+ β)ib. Therefore any change in the load is reflected in the output current of the Zener diode so the output is always 5V. The capacitor serves the purpose of maintaining the stability of the voltage [4].

9 Fig 6 Voltage Regulator Design [4] Voltage Regulator LM78M05 The LM78M05 is a variable voltage regulator. We have chosen to use it in its default configuration which outputs 5V with a maximum of 0.5A of current. Fig 7 LM78M05 Top View [5] We are considering this to run both the microcontroller, LCD, and switches and Hall Sensor. Microcontroller Upon discussing the microcontroller with our faculty advisor and graduate student mentor, we have decided to use the Freescale MC68711E20. Since almost all the programming will be handled by our graduate student mentor, we came to the consensus to use something the most similar to the Motorola 68HC11 family microcontroller which the mentor is most familiar with. The following diagram shows the flow chart of how the program on the microcontroller will operate.

10 Fig 8 Microcontroller Block Diagram We will be responsible to create the lookup table. The lookup table is a table of values in hexadecimal format. Each number sampled from the ADC is a 16-bit number. This numbers value will be stored in memory and then the program will lookup the value according to the address of the corresponding value in the table. Since the values can only be stored in 8-byte sections, the address being looked up is multiplied by two in order to output the full 16-bit number. The following diagram shows the pin layout of the microcontroller. Fig 9 Pin Layout of the MC68711E20 [6] Design Chosen We chose to use the Allegro A3516, a component based voltage regulator, the Motorola 68HC11 microcontroller, an LM78M05 voltage regulator, and the DMC16204 LCD screen from Optrex.

11 Fig 10 Block diagram of the design chosen We chose to use the Allegro 3516 calibrated hall sensor. Since there are time constraints on this project, the Allegro is much easier to use and integrate in the system. Although we lose some degrees of freedom in the design parameters such as temperature compensations, gain, and sensitivity, we found that these were negligible since this is a prototype and we will only be operating in a controlled environment. We also chose to implement our designed voltage regulator and the LM78M05. The designed regulator will be used for the hall sensor. Since this design does only outputs 6mA and the microcontroller and other parts need upwards of 25mA we will used the LM78M05 for these devices because of the greater maximum current output. The averaging function will be carried out by the microcontroller and the gaussmeter will have the function of being able to be toggled between single sample mode and averaging mode. The averaging mode will take 100 samples per second and output that average. The LCD could have been any compatible device, but the DMC was chosen because the documentation that was available provided more detail in the connection and programming of the LCD controller. IV. Results We implemented the above block diagram using OrCad Capture. The following diagram shows the schematic.

Fig 11 OrCad Capture Schematic From this schematic we created the netlist and imported it into OrCad Layout Plus. Before we imported is we created the footprints for each device.

12 Fig 11 OrCad Capture Schematic From this schematic we created the netlist and imported it into OrCad Layout Plus. Before we imported is we created the footprints for each device. The following is the layout that resulted using the autorouting tool. Fig 12 Layout from Orcad We then shipped the layout to Advanced Circuits where they manufactured the PCB. Below a picture is shown of the devices connected to the PCB in the case.

Fig 13 PCB and Devices in Case Fig 14 Finished Gaussmeter in Averaging Mode Fig 15 Gaussmeter outputting a reading We also took readings of horseshoe

13 Fig 13 PCB and Devices in Case Fig 14 Finished Gaussmeter in Averaging Mode Fig 15 Gaussmeter outputting a reading We also took readings of horseshoe magnets to test if the device properly outputted the change in polarity. The device successfully outputted the change as well as the magnitude of the magnetic flux.

14 V. Conclusion Non-Technical Issues The non-technical constraints that we looked at was manufacturability and ethical. Regarding manufacturability we were able to see that our product only needed a couple basic parts and the amount of parts were relatively little. As we were ordering our PCB we noticed that as you order more, the price per board became cheaper. We assume that s the case in the real world as well. The whole process of assembling the board was relatively easy and didn t require any special skills. From an ethical standpoint, our product is not exactly environment friendly due to the microcontroller and other parts being non-biodegradable. Other than the possible harm to the environment, it is perfectly safe for everything else. The actual uses for the gauss meter is very helpful because it tells you extra information you wouldn t normally know. Final Plans Upon reflection on our design and techniques used throughout this product, it is apparent we could have made significant improvements. One obvious improvement would be to not make some easily preventable mistakes. Examples of this would be our problem with the hole sizes of some of our parts. This could easily be prevented if we were to make the hole sizes with extra clearance when making the footprints with Orcad Capture. Another preventive measure we could of taken was to proofread our schematic before submitting it to be manufactured. In essence, we should have taken a finer approach during each step of the process. Concerning our actual device and how well it functions, we could definitely make improvements on the overall project. Our particular Hall sensor was not as sensitive as we would have liked. As mentioned in the previous sections, there are different hall sensors we could have bought, but it would make the overall design more complex. The range of our hall sensor was as high as what we would have liked, and this can be attributed again to the particular model that we chose for our design. In terms of cost, time, safety, and quality, we were fairly satisfied with our design. We can see that if we were to produce more quantities rather than a single prototype, the cost and time per device would decrease with the amount of devices. Regarding safety and quality, we could make some minor improvements in workmanship. What We Learned: In the beginning our objectives were to first and foremost produce a working gauss meter device. Other objectives would include learning about the process of creating

15 a working product. The main programs we used to design our gauss meter were Orcad Layout and Orcad Capture. Lastly, we were looking to gain some experience in physically assembling the circuit board and corresponding case. Reflecting on our overall process, we can say we met most of our stated objectives. Most importantly, our gauss meter works in the way we originally intended it to. We were able to gain essential information on the Do s and Don ts of PCB design and can learn from the mistakes we made. Overall, this experience was a success when looking at all that we accomplished. To wrap up, we were able to gain a first hand look into the field of electrical engineering. In our academic and professional career, we will probably never need to make a gauss meter, but we can certainly draw upon what we learned and apply it to similar situations. For instance, PCBs are a huge part of electrical engineering design and it is very likely we will encounter them again in the future. When designing and making our gauss meter, we had to make quick decisions concerning the problems that arose and it is a satisfying feeling to know that we were able to come up with solutions to get to our end product. This project was something we both have never done before and certainly will never forget.

16 References [1] SYPRIS Test and Measurment, An Introduction to the Hall Effect, F.W. Bell Datasheet, Available [Accessed Nov 8, 2007] [2] National Institute of Standards and Technology, Hall Effect, Evolution of Resistance Concepts, National Institute of Standards and Technology, NIST. Available [ Accessed March 5, 2008] [3] Analog Devices, AD22151 Datasheet Rev A, Analog Devices, Available [Accessed Nov 8, 2007] [4] Sullivan, T., Introduction to Electrical and Computer Engineering, Chapter 5 Voltage Regulators, Carnegie Mellon University, Available Chapter5.pdf, [Accessed Nov 10, 2007] [5] National Semiconductor, LM7805 Datasheet, National Semiconductor, Available [Accessed Feb 15, 2008] [6] Freescale Semiconductor, M68HC11, Freescale Semiconductor, Rev 5.1 July Available M68HC11E.pdf, [Accessed Dec. 21, 2007]

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