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UNIVERSITY OF NAIROBI FACULTY OF ENGINEERING DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING ELECTRONIC BALANCE FOR PRECISION WEIGHING. PROJECT INDEX: PRJ 016 BY NJUGUNA GICHUKI SAMUEL F17/1794/2006 SUPERVISOR: PROF. ELIJAH MWANGI EXAMINER: DR. MBUTHIA Project report submitted as a partial fulfillment of the requirement for the award of the degree Of Bachelor of Science in Electrical and Electronic Engineering of the University of Nairobi Submitted on: 18th May, 2011
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DEDICATION To the almighty God, for the many gifts you have bestowed upon me, I can never thank You enough. To my parents for teaching me valuable life lessons.
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ACKNOWLEDGEMENTS I gratefully acknowledge the support of my project supervisor PROF. ELIJAH MWANGI for his great help and motivation all through my project. To all the lecturers in the department of electrical and information engineering for instilling knowledge in us. My friends Dancan Waiharo, Adam Olalo and Kiiru Nelson for their constructive criticism.
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DECLARATION AND CERTIFICATION I certify that the information presented in this report is my original effort and has not been presented before for a degree award in this or any other university to the best of my knowledge.
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NJUGUNA GICHUKI SAMUEL F17/1794/2006
Date:………………………
This report has been submitted to the Department of Electrical and Information Engineering, University of Nairobi with my approval as supervisor:
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PROF. ELIJAH MWANGI.
Date:……………………… iv
Table of Contents DEDICATION ........................................................................................................................................ ii ACKNOWLEDGEMENTS .................................................................................................................... iii DECLARATION AND CERTIFICATION ............................................................................................ iv LIST OF FIGURES ................................................................................................................................ vi ABSTRACT.......................................................................................................................................... vii CHAPTER 1 ........................................................................................................................................... 1 INTRODUCTION ............................................................................................................................... 1 Background ..................................................................................................................................... 1 OBJECTIVE ....................................................................................................................................... 2 MAIN OBJECTIVE ........................................................................................................................ 2 SPECIFIC OBJECTIVES ................................................................................................................ 2 PROJECT SCOPE ........................................................................................................................... 2 CHAPTER 2 ........................................................................................................................................... 3 LITERATURE REVIEW .................................................................................................................... 3 ELECTRONIC BALANCE ............................................................................................................. 3 BLOCK REPRESENTATION......................................................................................................... 3 SENSORS ....................................................................................................................................... 4 SIGNAL CONDITIONING ............................................................................................................. 6 PROGRAMMER ........................................................................................................................... 15 PARALLEL PORT........................................................................................................................ 15 Power supply ................................................................................................................................. 18 CHAPTER 3 ......................................................................................................................................... 19 PROJECT DESCRIPTION AND IMPLEMENTATION ................................................................... 19 DESIGNING A POWER SUPPLY.................................................................................................... 19 MOUNTING THE STRAIN GAUGE ON A STEEL RULE .......................................................... 20 DESIGNING THE SIGNAL CONDITIONING CIRCUIT ............................................................. 21 A DAPA PROGRAMMER CONNECTED TO ATmega32L INTERFACED TO THE LCD ...... 22 ELECTRONIC BALANCE DESIGN ............................................................................................ 23 DESIGNING THE SYSTEM SOFTWARE ................................................................................... 26 CHAPTER 4 ......................................................................................................................................... 35
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RESULTS AND DISCUSSIONS ...................................................................................................... 35 Calibration ..................................................................................................................................... 35 Output on the LCD screen............................................................................................................. 37 CHAPTER 5 ......................................................................................................................................... 39 CONCLUSIONS ............................................................................................................................... 39 RECOMMENDATIONS AND SUGGESTIONS FOR FURTHER WORK ....................................... 39
LIST OF FIGURES Figure 1: block diagram of the electronic balance .................................................................................... 3 Figure 2 : Bonded metallic Strain Gauge.................................................................................................. 5 Figure 3: signal conditioning block representation ................................................................................... 6 Figure 4: power supply .......................................................................................................................... 19 Figure 5: mounted strain gauge .............................................................................................................. 20 Figure 6: signal conditioning circuit....................................................................................................... 21 Figure 7.interface of the microcontroller to the LCD and DB-25 ............................................................ 23 Figure 12: LCD displaying .................................................................................................................... 33 Figure 13: Output on the LCD screen .................................................................................................... 38
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ABSTRACT Accuracy in weight measurement in the business world especially in the market places today has become a crucial issue. Analog scales can measure accurately but some approximation has to be done when the needle pointer is between two calibrated measurements. The springs also suffer from wear and tear leading to declining accuracy. This has called for introduction of the digital scales. This project describes the development and use of an electronic balance using the Atmel ATmega32L microcontroller. The designed scale is to perform weight measurements to a high degree of accuracy. A high-resolution Analog-to-Digital Converter (ADC) resident in the microcontroller is used to accurately measure the load cell output. The measured weight is to be in digital format and is displayed on the LCD.
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CHAPTER 1 INTRODUCTION Background An electronic scale provides an excellent measurement of weight of given objects. These scales are capable of measuring weights in an accurate manner. Unlike the conventional spring-based measurement system these devices age gracefully rather than gradually. Digital display makes the interpretation of the reading free from ambiguities and also the fact that these scales often have a great look and are also easy to use. Problem definition and Justification
The traditional scales have measured weights and served most of our purposes since time immemorial. People have been happy with the weights to a good extent and practical problems have been addressed using the analog weighing machines. However there are significant benefits of using electronic weighing scales over analog ones. The primary benefits make it clear enough why most people today have less preference to the old spring-and-needle weighing scales and have started switching to the digital electronic system. The advantages include; a)
Accurate measurement: the electronic scale measures the weight accurately. These scales measure weights with smaller increments compared to analog scales, thereby significantly enhancing the accuracy. This has the advantage over analog scales that employs delicate springs that wear out with time.
b) Accurate reading: The reading provided to the scale user is in exact grams, and not an
approximate reading left for the user to interpret based upon the movement of some needle. c)
Heavy-duty measurements: These scales can measure heavier weights compared to analog ones and are accurate over a higher weight range.
d) Long-term precision: Digital scales do not lose accuracy with time, unlike an ever-
loosening spring in the traditional weighing machine. So the machine reads accurately over a long period. 1
OBJECTIVE MAIN OBJECTIVE The main objective of this project was to design an electronic balance for precision weighing using foil type strain gauges and the Atmel atmega32L microcontroller and to display the result on the LCD. SPECIFIC OBJECTIVES a) Mount the strain gauge on a suitable beam. b) To design the signal conditioning circuitry for the strain gauge so that it can be used as a measurement device. c) Perform analog to digital conversion using the inbuilt ADC module in the microcontroller. d) Interface the conditioning circuitry to the microcontroller and also between the microcontroller and the LCD. e) Design and build a dapa (direct avr parallel access) programmer for loading the program into the controller. f) Develop a program using AVR studio and PonyProg2000 and load it to the microcontroller using the dapa programmer. g) Perform calibration of the system (weighing scale). h) Do some weight measurements and compare them with those from a commercial scale. PROJECT SCOPE The Atmega32L based electronic balance was to be used to measure weights between 0 and 1 kg and display the mass on the LCD. The high resolution of the 10 bit ADC resident in the microcontroller enables the measuring with a sensitivity of 1 gram for every 4.833mv.
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CHAPTER 2 LITERATURE REVIEW ELECTRONIC BALANCE The sensing element is placed as one arm of the Wheatstone bridge. With no load on the sensing element the bridge is balanced. On placing some load on the sensing element , the resistance of the sensing element changes causing some imbalance of the bridge resulting in some output voltage in the order of a few milivolts. The small signal is then amplified using suitable amplifiers to levels compatible to the analog to digital converters. The ADC converts the analog signal to a digital format which is required by the microprocessor, for precision measurements a high resolution ADC is employed. The data sent is read by a microprocessor and is finally displayed on a suitable display device such as a seven segment display or the LCD. The above constitute the building blocks of the electronic balance. BLOCK REPRESENTATION
SENSOR
ANALOG TO DIGITAL
SIGNAL CONDITIONING
CONVERSION
MICROCONTROLLER
LCD DISPLAY Figure 1: block diagram of the electronic balance
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SENSORS A sensor is a device that receives a signal or a stimulus and responds with an electrical signal. Sensors and their associated circuits are used to measure various physical properties such as temperature, force, pressure, flow, position, light intensity and others. These properties act as stimulus to the sensor, the sensor output has to be conditioned and processed to provide the corresponding measurement of the physical property. Sensors are usually part of a larger system consisting of signal conditioners and various analog to digital signal processing circuits. The system can be a measurement system, data acquisition system or process control system. Sensors may be classified as active or passive. An active sensor requires an external source of excitation. These comprise of resistor-based sensors such as thermistors, resistance temperature detectors (RTDS) and strain gauges. They are called active because a current must be passed through them and the corresponding voltage measured to determine the resistance value. Passive sensors on the other hand generate their own electrical outputs signals without requiring any external excitation. Passive sensors include thermocouples and photodiodes which generate thermoelectric voltages and photoelectric currents respectively. Another logical way to classify sensors may be by the physical property that the sensor is designed to measure. This means grouping them as pressure sensors, temperature sensors, force sensors and motion sensors. It is however important to note that sensors that measure different physical properties may have the same type of electrical output. An example is the strain gauge and RTDS. [REF. 3] STRAIN GAUGE AS A SENSOR A strain gauge is a conducting wire whose resistance changes by a small amount when it is lengthened or shortened. Strain gauges are made from metal alloy such as constantan, nichrome V or platinum alloy. For high temperature work they are made of wire. For moderate temperature applications they are made by forming the metal alloy into very thin sheets by a photo etching
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process, the resultant product is called a foil type strain gauge. The strain gauge can be used can be used as a sensor for pressure, force, torque, and as a position sensor. The majority of strain gauges are foil type available in a wide range of shapes and sizes to suit a variety of applications. They operate on the principle that as the foil is subjected to stress the resistance of the foil changes in a defined way. The metallic strain gauge consists of a metallic foil arranged in a grid pattern. This grid pattern maximizes the the amount of metallic wire subject to strain in the parallel direction. The cross-sectional area of the grid is minimized to reduce the amount of shear strain. The grid is bonded into a thin backing which is attached directly to the test specimen. This implies that the strain experienced by the test specimen is transferred directly to the strain gauge which responds with a linear change in electrical resistance.
Figure 2 : Bonded metallic Strain Gauge
A fundamental parameter of the strain gauge is its sensitivity to strain expressed quantitavely as the gauge factor (GF). Gauge factor is defined as the fractional change in the electrical resistance to the fractional change in length. GF= (∆R/R)/ (∆L/L) The ratio ∆L/L is called unit strain. [REF. 1]. It is important to note that the resistance change for the gauge for the full force range is small, thus to use the gauge as a measurement device calls for a way of measuring very small resistance 5
changes with high accuracy. Such precisition measurement applications call for the use of a bridge measurement circuit. The wheat-stone bridge is therefore employed.
SIGNAL CONDITIONING Conditioning the signal means that some characteristic of the signal is being changed. Strain gauge measurement involves sensing extremely small resistance changes with high accuracy. This means that there must be proper selection and use of the bridge, good signal conditioning, reliable wiring and data acquisition.
BRIDGE COMPLETION
AMPLIFICATION
FILTERING (LPF)
Figure 3: signal conditioning block representation
Signal conditioning includes; a) Bridge completion. b) Excitation. c) Amplification. d) Filtering. e) Bridge balancing. Signal conditioning is primarily utilized for data acquisition in which sensor signals must be normalized and filtered to levels suitable for analogue to digital conversion so that they can be applied to processors. Whatever form conditioning takes circuitry and performance is governed by the electrical characteristic of the sensor and its output. Factors such as sensitivity, voltage and current levels, linearity, maximum electrical ratings and stray impedances can spell the difference between a substandard and a successful application of the device. To some degree some sensor outputs are
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non-linear with respect to the applied stimulus and as a result their outputs must be linearized in order to yield correct output.
Bridge completion A simple method of measuring resistance is to force a constant current through the resistive sensor and measure the voltage output. Any change in current will be interpreted as a resistance change. The wheat-stone bridge offers an attractive alternative for measuring small changes in resistance accurately. It consists of four resistors, a source of excitation voltage and a voltage detector connected across the other diagonal. The strain gauge is used as one arm of the bridge ( quarter bridge circuit) and the output voltage change is an indication of the resistance change. The sensitivity of the bridge is the ratio of the maximum expected change in the output voltage to the excitation voltage. Bridge sensitivity= (full-scale bridge output) / (excitation voltage). Since more than one arm of the bridge can be made variable, sensitivity increases as more elements are made variable. This also implies that the bridge output can be increased by improving the sensitivity of the bridge, i.e. the bridge output can be doubled by doubling the number of the working gauges. An unfortunate characteristic of the strain gauge is its resistance change with temperature. This means that the bridge may be unbalanced even when the gauge is unstrained giving an output. This can be alleviated by using a “dummy gauge “ to cancel the effects of temperature. power dissipation in the resistive sensor must be kept low so that self heating does not produce errors. Excitation voltages Although large excitation voltages yield proportionally larger full-scale output voltages, they also result in higher bridge power dissipation and thus raise the possibility of sensor resistor selfheating errors. On the other hand low values of excitation voltages require more gain in the
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conditioning circuit and also increase susceptibility to low level errors such as noise and offset voltages. The stability of the excitation voltage or current is also an important factor to consider. This is accomplished by designing a regulated power supply to be the excitation voltage.
Amplification The output of strain gauges and bridges are relatively small. For a bridge whose sensitivity is 10mv/V the full-scale output with an excitation voltage of 10V is 100mv which is still small. This calls for the use of amplifiers to boost the signal level, increase the measurement resolution and improve the signal to noise ratio. The output of a single element varying bridge may be amplified using a single varying op-amp, this method although attractive has poor overall performance. This is because the RF resistor must be chosen carefully and matched to maximize the common mode rejection. It is difficult to maximize common mode rejection while at the same time allowing different gain options since gain depends on the bridge resistances and RF. The differential amplifier has better performance than the single input amplifier. For the single input amplifier noise voltages act as if they are in series with the input signal, this means that both the noise voltages and the input signal are amplified equally. This may be undesirable where the noise voltage level is equal to or greater than the input signal. For the differential amplifier case the noise voltage is not amplified and is therefore eliminated from having any significant effect on the output voltage. This condition is guaranteed as long as the resistors are matched. The two disadvantages of the basic differential amplifier are low input impedance and changing gain. The first disadvantage is eliminated by buffering (isolating the inputs with voltage followers). The gain can be adjusted by one resistor. The setback of this buffered differential amplifier is that it can only drive floating loads. A much better approach to amplify the bridge output is to use an instrumentation amplifier, the instrumentation amplifier has better gain accuracy usually and high common mode rejection ratio. 8
Instrumentation amplifier An instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement. Additional characteristics include very low DC offset, low drift low noise, very high open-loop gain, very high common rejection ratio and very high input impedances. Instrumentation amplifiers are used where great accuracy and stability of the circuit both short- and long-term are required. The electronic instrumentation amp is almost always internally composed of 3 op-amps. These are arranged so that there is one op-amp to buffer each input, and one to produce the desired output with adequate impedance matching for the function. The instrumentation amplifier is preferred for bridge amplification because of its stable gain and high common mode rejection. In this project the µa725 instrumentation amplifier was used. Filtering Filters are used to separate signals passing those of interest and attenuated the unwanted frequencies. In data conversion, filters are mostly used to eliminate aliasing in A/D systems. Filters are mostly employed after the signal has been amplified to increase its amplitude. Then a selection of particular signal frequencies is accomplished by passing the signal through a frequency selection filter. A low pass filter was employed before the signal was applied to the analog to digital converter (ADC).
Analog to digital conversion Once the signal has been amplified it is taken to the ADC module of the microcontroller. A microcontroller is used to process information from the natural world, decide on the course of action based on the information collected and then issue control signals to implement the decision. Since the signal from the sensor is analog in nature and the controller is a digital processor therefore an analog to digital converter is required.
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Converter performance is important and maintaining that performance in a system application is important. When dealing with low level signals and low frequency applications resolution is an important factor. Resolution = 2^n where n is the no of digital outputs. Increasing the no of bits results in a finer resolution and therefore reduces the quantization error where the quantization error is the difference between the the input analog signal and the quantized digital value. ADCs must also have a wide dynamic range, high sampling frequencies and generally excellent ac specifications. Other desirable characteristics include, low power, single supply operation, low cost and smaller input/output signal swings. ADC conversion technologies The four different types of technologies used in ADCs are the successive approximation conversion, the integration conversion, the counter-based conversion, and the parallel conversion. Successive Approximation The successive approximation technique uses a DAC, a controller, and a comparator to perform the ADC process. Starting from the MSB down to the LSB, the controller turns on each bit at a time and generates an analog signal, with the help of the DAC, to be compared with the original input analog signal. Based on the result of the comparison, the controller changes or leaves the current bit and turns on the next MSB. The process continues until decisions are made for all available bits. Integration The integration technique uses an integrator, a comparator, and a controller to convert analog signals to digital signals. A sampled analog signal is integrated over a fixed period, say n clock cycles of the digital system. Another fixed reference signal is integrated over time and compared with the input analog signal integrated. Although the value of the reference signal integrated is smaller than the input analog signal integrated, the reference signal is continuously integrated, and the time for the integration is measured. When the two integrated values equal, the measured time is converted to a digital encoded value.
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Counter-Based Conversion This conversion is performed with the help of a counter, a DAC, and a comparator. The counter starts at 0 and counts up. As the counter counts up, the corresponding value is converted to an analog value and compared with an input analog signal. As long as the input analog signal is greater than the signal generated by the DAC, the counter counts up and the process continues. The Atmega32L ADC module uses the successive approximation technique. After the signal has been converted to digital format it is applied to the microcontroller for processing so that it can be successfully displayed. [REF. 2]
MICROPROCESSOR A microprocessor is a device that receives data or information processes it in accordance with a stored software program and outputs signals in digital form. It accesses memory units, fetches instructions and executes them, performs arithmetic and logical calculations, monitor external events, keep track of where information is to go and controls timing and operation sequence. The main constituents of a microprocessor are; -
Central processing unit (CPU).
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Primary memory that include RAM and ROM.
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Input and output interfaces to handle communication the microprocessor and the outside world.
The advantage of the microprocessor in circuit design is that one can change functionality by changing the software while keeping the hardware the same. This reduces the cost of production since different systems can use the same hardware base. Microprocessors can be categorized into several architectural forms namely; -
Von-Neumann architecture
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Harvard architecture
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CISC and RISC architecture
Microcontrollers based on Von-Neumann architecture have a single data-bus that is used fetch both instructions and data program. Instruction and data are stored in a common main memory. When addressing main memory, the controllers first fetches an instructions and then it fetches data to support the instructions. The two separate fetches slow down the controller operation. Microcontrollers based on Harvard architecture have a separate data bus and instruction bus. An instruction is pre-fetched while the current instruction is being executed. Once the current 11
instruction is completed, the instruction is ready to be executed. This pre-fetch allows for faster execution of instruction than the Von-Neumann architecture. CISC - processors have a bigger instruction set with many addressing modes. It has an easy compiler design but decoding of an instruction is complex and execution time is much slower. An example is the Intel 8086. RISC - processors have a smaller instruction set with few addressing mode. It has a complex compiler design but its executions are faster. An example is the Intel 8085. In this project I used the Atmel AVR Atmel ATmega32L. The AVR is a modified Harvard architecture 8 bit RISC single chip which was developed by Atmel co-operation in 1996. The AVR was one of the first microcontroller’s families to use on-chip flash memory for program storage as opposed to one time programmable ROM, EPROM or EEPROM. Some of the reasons why the Atmel ATmega32L is preferred include; -
High performance coupled with low power consumption.
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Outstanding flash memory technology.
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Reduced instruction set computer Harvard architecture.
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Single cycle execution.
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Wide variety of operating voltages (1.8V-5.5V).
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Architecture designed for C language.
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One set of development tools for the entire AVR line.
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In system programming, debugging and verification.
In this project the Atmel ATmega32L microcontroller was used mainly because it has an inbuilt ADC module, internal selectable RC oscillator and the ability to support in system programming (ISP).
The Atmel ATmega32L The ATmega32L provides the following desirable features: 32Kbytes of In-System Programmable Flash Program memory with Read-While-Write capabilities, 1024bytes EEPROM, 2Kbyte SRAM, 32 general purpose I/O lines, 32 general purpose working registers, On-chip Debugging, support and programming, three flexible Timer/Counters with compare modes, Internal and External Interrupts, a serial programmable USART, a byte oriented Two12
wire Serial Interface, an 8-channel, 10-bit ADC with 8 single ended channels, a programmable Watchdog Timer with Internal Oscillator, an SPI serial port, and six software selectable power saving modes that include the Idle mode , the Power down mode ,Power-save mode , the ADC Noise Reduction mode , In Standby mode and extended Standby mode.[datasheet] ATmega32L pin description VCC- Digital supply voltage. GND- Ground. Port A (PA7-PA0) - Port A serves as the analog inputs to the A/D Converter. Port A also serves as an 8-bit bi-directional I/O port, if the A/D Converter is not used. The PortA output buffers have symmetrical drive characteristics with both high sink and source capability. Port B (PB7-PB0) - Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers have symmetrical drive characteristics with both high sink and source capability. Port C (PC7-PC0) - Port C is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up resistors are activated. Port D (PD7-PD0) - Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up resistors are activated. RESET- Reset Input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. XTAL1 - Input to the inverting Oscillator amplifier and input to the internal clock operating circuit. XTAL2 - Output from the inverting Oscillator amplifier. AVCC - AVCC is the supply voltage pin for Port A and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. AREF - AREF is the analog reference pin for the A/D Converter.
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Time base The microcontroller is a complex synchronous state machine. It responds to program steps in a sequential manner as dictated by a user-written program. The microcontroller executes a command through a predictable fetch, decode and execute sequence. The speed at which a microcontroller performs these actions is controlled by a precise time base called the clock. The clock source is routed throughout the microcontroller to provide a time base for all peripheral subsystems. The ATmega32L may be clocked internally, using a user-selectable resistor capacitor (RC) time base, or externally using a crystal. The RC internal time base is selected using programmable fuse bits. One may choose an internal fixed clock operating frequency of 1, 2, 4, or 8 MHz To provide for a wider range of frequency selections, an external oscillator may be used. The external time sources, in order of increasing accuracy and stability, are an external RC network, a ceramic resonator, or a crystal oscillator. [REF. 2].
Serial Communications When choosing controllers to be used for various applications and circuits it is important to consider the various communications subsystems that the controller is equipped with and even to consider how to load a program in the controller. The ATmega32L is equipped with a host of different serial communication subsystems, including the Universal Synchronous and Asynchronous Serial Receiver and Transmitter (USART), the Serial Peripheral Interface (SPI), and the Two-Wire Serial Interface (TWI). What all of these systems have in common is the serial transmission of data. In a serial communications transmission scheme, data are sent a single bit at a time from transmitter to receiver. Serial USART - The serial USART is used for full duplex (two-way) communication between a receiver and transmitter. This is accomplished by equipping the ATmega32L with independent hardware for the transmitter and receiver. The USART is typically used for asynchronous 14
communication. To maintain synchronization between the transmitter and receiver, framing start and stop bits are used at the beginning and end of each data byte in a transmission sequence. Two-Wire Serial Interface - The TWI subsystem allows the system designer to network a number of related devices (microcontrollers, transducers, displays, memory storage, etc.) together into a system using a two-wire interconnecting scheme. The TWI allows a maximum of 128 devices to be connected together. Serial Peripheral Interface - The SPI may be viewed as a synchronous 16-bit shift register with an 8-bit half residing in the transmitter and the other 8-bit half residing in the receiver. The transmitter is designated the master because it provides the synchronizing clock source between the transmitter and the receiver. In the SPI system, the transmitter and receiver share a common clock source. The serial peripheral interface (SPI) method of communication was employed to send code to the ATmega32L. This was accomplished using a dapa (direct avr parallel acces) programmer and a PC with a parallel port. PROGRAMMER A programmer is a hardware device that downloads (writes) code onto the controllers ROM or Flash memory. the code is written and compiled using suitable software , the executable set of instructions obtained are sent to the controller using either the parallel port ,serial port or USB port using various programmers. A popular method to program a controller is to use in system programming (ISP). The Atmel Avr line of controllers support ISP ,this allows one to test and debug code without interfering with the hardware set up every time one needs to write a new version of code. Programmers that use ISP tend to be cheaper and simpler compared to methods like parallel or high voltage programming. For this project a dapa programmer was used (direct avr parallel access). When using ISP the data is send serially even though the port being used is the parallel port. PARALLEL PORT Parallel Port interfacing is the simplest method of interfacing. Parallel Port’s are standardized under the IEEE 1284 standard first released in 1994. It has data transfer speed up to 1Mbytes/sec. 15
Parallel port is basically the 25 pin Female connector (DB-25) in the back side of the computer (Printer Port). It has 17 input lines for input port and 12 pins for output port. Out of the 25 pins most pins are Ground and there is data register (8 bit), control register (4 bit) and status register (5 bit). The following Pins of the parallel port are used Table 1: DB25 – pins used for serial communication
Pin No.
Description
7,8,9
Data pins
10
Status pin
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Ground
After the program has been sent to the controller the next step is to interface the controller with an output device such as a display device. Display devices available today include LEDs seven segment displays and LCDs. Display devices A number of display devices are available today as output devices, these are; a) LEDs. b) Seven segment displays. c) LCDs In this project the LCD display was used.
LCD display LCDs are used as output devices to display text information. A 16x2 LCD is the most common and has the capability of displaying 2 lines of 16 characters. Data paths and control lines are required between the LCD and the microcontroller. A small controller mounted on the back panel of the LCD translates the ASCII characters and control signals to properly display the characters. 16
LCD consists of rod-shaped tiny molecules sand- witched between flat pieces of glass and an opaque substrate. The rod shaped molecules align into different positions based on the electric charge applied to them. Light passing through makes the desired images appear. In recent times the LCDs have found popularity replacing LEDs and seven segment displays. This is due to; a) Ability to display numbers characters and graphics. b) They are thin and flat. c) They consume very little amount of power. d) The declining prices of LCDs. e) Presence of a refreshing controller in the LCD relieving the CPU the task of refreshing. The HD44780U standard allows for easy communication with the majority of LCDs regardless of their manufacturer. The control chip receives data from the external source and communicates directly to the LCD. This standard requires three control lines and 4 or 8 I/O lines. The enable line (EN) - This line is used to indicate the start of a transmission of a data byte to the LCD controller. To indicate the start of transmission, this line is brought high. When transmission is complete, the EN line is brought low. The register select line (RS) - This line indicates to the LCD controller whether the data byte is to be treated as a command or as text data to be displayed on the screen. If the RS line is high, the data byte is treated as text to be displayed. If the RS line is low, the data byte is treated as a command. The read/write line (RW) - When this line is low, the information on the data bus is written to the LCD controller. If this line is high, the LCD controller can be read to check the status of the LCD.
Every electronic circuit is designed to operate with some supply voltage which is usually assumed to be constant. If the supply is not constant unpredictable happenings may occur to the microcontroller and their associated circuits. This makes a power supply a very crucial part of any electronic circuit.
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Power supply In normal operation, microcontrollers and their associated circuits are sourced by a regulated and somehow stabilized power supply. Whenever in such cases the supply voltage is below the minimum specified level, the microcontroller's behavior cannot be predicted, which can lead to critical situations of the application. Unwanted effects include: - Wrong execution of the application program. - Outputs changing state. - Unintended accesses to internal or external memory. A voltage regulator provides this constant DC output voltage and contains circuitry that continuously holds the output voltage at the design value regardless of changes in load current or input voltage. The LM7805 and regulator was used in this project to design a regulated power supply for the circuit.
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CHAPTER 3 PROJECT DESCRIPTION AND IMPLEMENTATION DESIGNING A POWER SUPPLY A regulated power supply was designed due to the fact that a.c line voltage suffers from considerable variations which may result in variations in the power supply output voltage. This varying voltage is not healthy for the proper operation of the microcontroller and other components of the circuit. Thus the LM7805 regulator was used to design a 5V power supply. A transformer was used to step down the 240V ac to 12V ac. A rectifier was then employed to convert the ac voltage to dc. A full wave rectifier was made using the IN4001 diodes and then a 100uf capacitor was used for smoothening. The output capacitor was made to be 10uf. The output of the rectifier was then connected to the LM7805 regulator. The output voltage is thus stable regardless of the load variations or variations in the ac input voltage from the mains. The LM7805 regulator employs internal current limiting, thermal shutdown and safe area compensation making them essentially indestructible. Diode D5 is a protection diode connected output to input. If the input voltage is accidentally shorted to ground the input is pulled to ground potential while the output remains near the initial output voltage due to stored charge in the output capacitor. This diode thus shunts most of the capacitors discharge current around the regulator to ground.
TR1
D5 BR1 U1
DIODE
7805 VI
VO
3
GND
1
R1
2
470R DF02M TRAN-2P2S
C1
C2
100u
10u
D1 LED
Figure 4: power supply
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MOUNTING THE STRAIN GAUGE ON A STEEL RULE The strain gauge is a sensitive sensor and proper and careful handling during mounting was observed so as not to damage the sensor and make it unusable. The following procedure was observed; - Cutting the gauge to size – the gauge was cut to size using a new blade as recommended - Cleaning and preparing the sensor block – the surface of the steel rule was scrubbed using rough sand paper and then softened using a soft sand paper. The surface was then wiped using a clean sponge. - A suitable adhesive (super glue) was then used to mount the gauge on the clean surface with the foil type side up. - The terminals of the strain gauge were soldered on the steel rule to avoid the contacts weakening or being loose or even breaking from the gauge. - The gauge resistance was then measured to ensure that its resistance is 120 ohms and its resistance has not been tampered with.
Figure 5: mounted strain gauge
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DESIGNING THE SIGNAL CONDITIONING CIRCUIT The balanced bridge circuit was designed so as any change in the gauge resistance can unbalance the bridge and cause a voltage change which can then be amplified and make it suitable to be used for measurement purposes. The ua725 instrumentation amplifier was used due to its high common mode rejection ratio, low input offset current and low input voltage drift. The instrumentation amplifier was used to boost the signal level to levels compatible to analog to digital converters, amplification also improves the measurement resolution and signal to noise ratio.
R7
330R
120R
RV1 R4
R5
3
10k
2
6
R6
1M
U1
1M 7
R9
4 1 5
R1
741
10k
POT
R3 330R
R10
R8
R11
120R
1M
470R
C1 100u
Figure 6: signal conditioning circuit
21
A dapa programmer connected to Tmega32L interfaced to the LCD This simple programmer is a parallel port interface programmer. This programmer uses the parallel port of the PC to communicate with the controller. Requirements include male DB_25 and connecting wires. The pins of male DB-25 and Atmega32L were connected as below.
Table 2: pins of DB25 corresponding to those of ATmega32L
DB-25
ATMEGA32L
7
RESET (9)
8
SCK (8)
9
MOSI(6)
10
MISO(7)
18
GND(18)
The parallel port is a very sensitive device, the power supply should never exceed 5V. for additional protection 1k resistors were used on SCK,MOSI and MISO wires. a) SCK (Serial clock) - This is used for synchronization between the PC and the microcontroller. b) MOSI (master out slave in) – the master refers to the pc and the slave is the controller this line is used by the pc to send data to the controller. c) MISO (master in slave out) – Used for verifying the code on the chip. d) RESET – This is used to reset the controller to signal the start of transmission. e) GND – A common ground connection.
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RV1 POT
R1 10k
U1 9
J1 1 14 2 15 3 16 4 17 5 18 6 19 7 20 8 21 9 22 10 23 11 24 12 25 13 CONN-D25M
13 12
SW1 SW -SPST-MOM
40 39 38 37 36 35 34 33 1 2 3 4 5 6 7 8
RESET XTAL1 XTAL2 PA0/ADC0 PA1/ADC1 PA2/ADC2 PA3/ADC3 PA4/ADC4 PA5/ADC5 PA6/ADC6 PA7/ADC7 PB0/XCK/T0 PB1/T1 PB2/INT2/AIN0 PB3/OC0/AIN1 PB4/SS PB5/MOSI PB6/MISO PB7/SCK
LCD1 PC0/SCL PC1/SDA PC2/TCK PC3/TMS PC4/TDO PC5/TDI PC6/TOSC1 PC7/TOSC2 PD0/RXD PD1/TXD PD2/INT0 PD3/INT1 PD4/OC1B PD5/OC1A PD6/ICP PD7/OC2
AVCC AREF
22 23 24 25 26 27 28 29
1 2 3 4 5 6
14 15 16 17 18 19 20 21
7 8 9 10 11 12 13 14
30 32
VSS VDD VEE RS RW E D0 D1 D2 D3 D4 D5 D6 D7
LM016L
ATMEGA32
C1 100n
Figure 7.interface of the microcontroller to the LCD and DB-25
ELECTRONIC BALANCE DESIGN The electronic balance was made up of two parts the hardware part and the software part. Hardware part This involved putting together all the individual parts that had previously been designed and doing a good interface to come up with the whole unit. The parts were the sensor, signal conditioning circuit ,the microcontroller and the LCD display.
23
Block diagram of the hardware part
SENSOR
SIGNAL CONDITIONING
ATmega32L
LCD display Figure 8.hardware part. Sensor When the strain gauge has been mounted on the steel rule, the rule was attached firmly to a holding position. Applying a force at the end of the rule caused it to bend thus stretching the strain gauge. When a strip of a conductive material is stretched it becomes longer and skinnier resulting in an increase in the electrical resistance. If the gauge is placed in a balanced Wheatstone bridge that has some excitation , applying a force on the rule an output voltage results which is small and which depends on the strain gauge sensitivity.
Signal conditioning Bridge completion and balancing These involve bridge completion, bridge balancing and amplification. The bridge was completed using two 330 ohms resistors and and one 120 ohm resistor. Resistor RA 1 mega ohm and RB 10k potentiometer were used to balance the bridge so that at zero load the bridge output was zero. µa725 instrumentation amplifier The µa725 is a monolithic instrumentation amplifier constructed using the Fairchild planar epitaxial process. It is intended for low level signal amplification where low noise, low drift and accurate closed loop gain are required.
24
Considering that the output of the bridge is in the order of a few milivolts, the instrumentation amplifier was used to raise the signal level so that it can be useful for measurements applications. The µa725 is lead compatible to the µa741 and has other desirable characteristics such as ; -
High common mode rejection ratio – 120 db.
-
High input voltage range - ±14V
-
Wide power supply range - ±3V to ±22V
Once the signal has been amplified the signal must first be passed through a low pass filter before it can be applied to a data conversion device. The low pass filter removes noise that may have been added during signal amplification.
ATmega32L ADC module The ADC feature available on PORTA of the ATmega32L is powerful and flexible and has the following desirable characteristics; -
10 bit resolution.
-
2 LSB absolute accuracy
-
13 ADC clock cycle conversion time.
-
8 multiplexed single ended channels.
-
Selectable right or left justification.
-
0 to VCC ADC voltage range.
The full-scale input voltage range of the ADC is generally determined by the reference voltage VREF. The reference voltage (maximum) value can be taken from the AREF pin or an internal reference level of 2.56V which is provided and which can be accessed via the software. The AVCC may also be used as AREF by simply decoupling It (connecting to ground through a capacitor). The reference voltage was selected to be 5V. Since 10-bit resolution is available. This means that you can distinguish between two readings which have a difference of value equal to the resolution. This means that the max value is broken up into 1024 parts and for our reference voltage of 5V, the resolution is 4.88mV. The absolute accuracy was therefore found to be 9.76mv. The ADC can be used in free running mode(which means it will keep on converting continuously) or single conversion mode. An interrupt may be generated once a conversion is complete, so that the MCU need not keep on checking for completion of conversion. 25
There is a ADC Noise Cancelling mode, which allows the user to switch off the other digital circuitry which might affect the conversion. Since the result is 10 bit, the result is stored in 2 registers, ADCH and ADCL. If the ADLAR bit is not set, the result is right aligned. In that case, the 2 MSBs of the result are the 2 LSBs of the ADCH register and the remaining 8 LSBs of the result are in the ADCL. In this case, it is necessary to read the ADCL first and the ADCH second, only then will the ADC registers be updated with the next result. Accuracy of the ADC result depends on the prescaler setting, its best between 50 and 200 KHz clock frequency. Running at 8 MHz the prescaler was set as 64, which gives a clock frequency of 8/64, which is 125 KHz. DESIGNING THE SYSTEM SOFTWARE
The software tools used were; a) WinAVR– WinAVR is open source package in which two sub-programs are used, Programmers Notepad & Mfile. b) AVR STUDIO c) PONYPROG2000 The system program was written in C language in the programmers’ notepad in WinAVR and the compiling and debugging done in AVRstudio. AVRstudio was set to generate a Hex file when the program was run. Burning code to the ATmega32L The programmer circuit pins of parallel port have to interface with pins of ATmega32L microcontroller which are responsible for in-system programming as described above. The generated hex file was burnt into the microcontroller using the dapa programmer and the Ponyprog2000. The controller was placed in the veroboard and the programmers pins and the
26
controllers pins connected as described above. The hex file was then opened in PonyProg2000 and a write command issued.
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PROGRAM FLOW CHARTS AND DISCUSSION
START
LCD INITIALIZE DISPLAY A WELCOME MESSAGE
ADC INITIALIZE
PERFOM ADC CONVERSION
WAIT FOR CONVERSION
CONVERT ADC RESULT TO WEIGHT
DISPLAY RESULT ON LCD SCREEN Figure 9: the whole program flow chart
28
ADC INITIALIZE FLOW CHART
SELECT CHANNEL
SET ADC PRESCALER
PERFOM A DUMMY CONVERSION
Wait for conversion to complete
Clear the conversion
Figure 10: ADC initialization
29
ADC CONVERSION
SELECT CHANNEL
SET ADC PRESCALER
START ADC CONVERSION
WAIT FOR CONVERSION
DO LEFT OR RIGHT ADJUST AND STORE RESULT IN ADCH OR ADCL
READ ADC RESULT IN EITHER ADCH OR ADCL Figure 11: ADC conversion
30
ADC initialization and conversion flow chart analysis The key registers for the ADC system are ADC Multiplexer Selection Register (ADMUX), ADC Control and Status Register A (ADCSRA) and ADC Data Registers (ADCH and ADCL). ADC Multiplexer Selection Register (ADMUX) - The ADMUX register contains the ADLAR bit to select left or right justification and the MUX[4:0] bits to determine which analog input will be provided to the ADC for conversion. The REFS [1:0] bits of the ADMUX register are also used to determine the reference voltage source for the ADC system. ADC Control and Status Register A (ADCSRA) - The ADCSRA register contains the ADC Enable (ADEN) bit. This bit is the ‘‘on/off’’ switch for the ADC system. The ADC is turned on by setting this bit to a logic 1. The ADC Start Conversion (ADSC) bit is also contained in the ADCSRA register. Setting this bit to logic 1 initiates the ADC. The ADCSRA register also contains the ADC Interrupt flag (ADIF) bit. This bit sets to logic 1 when the ADC conversion is complete. The ADIF bit is reset by writing a logic 1 to this bit. ADC Data Registers (ADCH and ADCL) - The ADC Data Register contains the result of the ADC. The results may be left (ADLAR=1) or right (ADLAR=0) justified. Select channel - The ADC is equipped with a single successive approximation converter. That means that only a single ADC channel may be converted at a given time. The input of the ADC is equipped with an eight-input analog multiplexer. The analog input for conversion is selected using the MUX [4:0] bits in the ADC Multiplexer Selection Register (ADMUX). Set ADC prescaler – The ADC requires 13 analog-to-digital clock cycles to perform an ADC conversion. The ADC system may be run at a slower clock frequency than the main microcontroller clock source. The main microcontroller clock is divided down using the ADC Prescaler Select (ADPS [2:0]) bits in the ADC Control and Status Register A (ADCSRA). Perform a dummy conversion – The first ADC conversion value is not accurate and should be discarded. The next consecutive values are accurate. Read ADC result - The 10-bit result from the conversion process is placed in the ADC Data Registers, ADCH and ADCL. These two registers provide 16 bits for the 10-bit result.
31
FLOW CHART ANALYSIS OF THE LCD MODULE LCD INITIALIZE
Set LCD to 4 bit or 8 bit mode
Write command to command register (clear LCD)
Write data byte to data register
Display a welcome message Figure.12. LCD initialization
32
DISPLAYING THE ADC RESULT ON LCD
Send a low to high pulse on the enable pin
Select command (RS=0) or instruction (RS=1) registers
Set RW to low (to write to LCD)
Give a delay to enable the LCD to Interpret the instructions
Display instruction or command Figure 8: LCD displaying
LCD flow chart analysis The three control lines are enable(EN), read/write(RW) and register select(RS). The Enable line (EN) - This control line is used to tell the LCD that we are sending it data. To send data to the LCD, our program should make sure this line is low (0) and then set the other two control lines and/or put data on the data bus. When the other lines are completely ready, bring EN high (1) and wait for the minimum amount of time required by the LCD (delay), and end by bringing it low (0) again.
33
The Register Select line (RS) - When RS is low (0), the data is to be treated as a command or special instruction (such as clear screen, position cursor, etc.). When RS is high (1), the data being sent is text data which should be displayed on the screen. The Read/Write control line (RW) - When RW is low (0), the information on the data bus is being written to the LCD. When RW is high (1), the program is effectively querying (or reading) the LCD. Only one instruction ("Get LCD status") is a read command. All others are write commands so RW will almost always be low. The LCD was set to 4 bit mode and therefore only four of the 8 data lines were used.
34
CHAPTER 4 RESULTS AND DISCUSSIONS Calibration Table 3: Test 1
Weight in grams
ADC value
50
211
100
213
150
217
200
226
250
243
300
255
A graph of adc value vs weight in grams 230 A D C
225
V A L U E
215
220
Series1
210 205 200 50
100
150
200
weight
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Table 4: Test 2
Weight in grams
Adc value
50
212
100
218
150
224
200
236
250
241
300
255
300 a d c
250
v a l u e
150
200
Series1
100 50 0 50
100
150
200
250
300
weight
The above variations may have been due to the fact that once the rule was stretched by the weight being applied it never went back to its original position. The sensor used was also locally mount and not accurate enough leading to some non-linearity in the bridge output.
36
The gradient of the graph plotted using the results of test 2 was determined and used to calibrate the scale. Adc_value=M x weight +207 Weight=(adc_value -207)/M Where M is the gradient and from the best line of fit it was found to be 2. A comparison of the scale and a commercial one
Commercial scale
Designed scale
250
252
200
207
150
157
100
103
50
45
The variations of the result from the real values may have been caused by; a) When weight is applied on the beam the beam never comes back to the original position. b) Vibrations on the beam as weights are applied. c) The non-linearity of the strain gauge sensor. d) Temperature effects since only one sensor was employed and there was no temperature compensation.
Output on the LCD screen
37
Figure 9: Output on the LCD screen
38
CHAPTER 5 CONCLUSIONS A dapa programmer was successfully made and was useful in loading the program to the ATmega32L using AVR studio for a compiler and PonyProg2000 for writing and verification. The signal conditioning circuit was successfully made using suitable resistors and amplifiers raising the signal level to improve the measurement resolution. The power supply was successfully designed and constructed and the output voltage was a stable 5V. The interface between the microcontroller and the LCD was made and tested using a simple program and the results expected were displayed on the LCD screen. Calibration of the system was done by plotting adc_values vs weight and the best line of fit was drawn. A relationship between the adc_value and the weight was established. The relationship was was done in software. Some test measurements were done, the results were close to those of the ideal weight scale. The project objective was therefore met. RECOMMENDATIONS AND SUGGESTIONS FOR FURTHER WORK Accuracy and precision depends on the resolution of the analog to digital converter, future work on can be done using ADCs of higher resolutions.
39
40
REFERENCES 1. Robert F. Coughlin and Frederick F. Driscoll. Operational amplifiers and linear integrated circuits. Sixth edition. Copyright © 2001 by Prentice-Hall. 2. Steven F. Barret and Daniel J. Pack. Atmel AVR microcontroller Primer – programming and interfacing. Copyright © 2008 by Morgan and Claypool. 3. Walt Jung. Operational amplifiers applications. 4. Ron Mancini. OP-amps for everyone. Copyright © 2002 Texas instruments incorporated. 5. Fernando E. Valdes-Perez Ramon Pallas- Areny. Microcontrollers, Fundamental and applications with PIC. © 2009 by Taylor & Francis Group, LLC. 6. http:www.hitachi-displays-eu.com. 7. http:www.liquidcrystaltechnologies.com 8. Allen Mottershed. Electronics devices and circuits. Copyright © 1973 by Goodyear publishing company.
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APPENDIX PROGRAM #define F_CPU 1000000 //define the cpu speed #include
#include #define dataport PORTC #define commport PORTD #define en PORTD4 #define rs PORTD5 #define rw PORTD6 #define LCD_RS 0 //define MCU pin connected to LCD RS #define LCD_RW 1 //define MCU pin connected to LCD R/W #define LCD_E 2 //define MCU pin connected to LCD E
#define LDP PORTD //define MCU port connected to LCD data pins
#define LCP PORTC //define MCU port connected to LCD control pins
#define LDDR DDRD //define MCU direction register for port connected to LCD data pins
#define LCDR DDRC //define MCU direction register for port connected to LCD control pins 42
int ADC_init(void);
int LCD_init(void);
int LCD_senddata(void);
int wrcomm(void);
int wrdata(void);
int new_PORTC;
int old_PORTC;
int ISR(ADC_const);
int ADC_result;
43
int main(void) { DDRA=0X00;//SET PORTA as input port PORTA=0X00;//PORTA initialize low DDRC=0XFF; //set portC as output port PORTC=0X00;//portC initialize low DDRD=0X70; //set portD as output port PORTD=0X00;//PORTD initialize low ADC_init();//ADC initialize LCD_init(); //initialize lcd LCD_senddata();//write to lcd return 1; } int LCD_init() { dataport=0x38;//set lcd to 8bit 2lines 5x7matrix wrcomm();//write the command byte to command register dataport=0x01;//clear lcd wrcomm(); dataport=0x80;//cursor on line 1,position 1 wrcomm();
44
return 1; } int LCD_senddata(void)//sending data to lcd display { ADC_result=PINC; if(new_PORTC==old_PORTC) { new_PORTC=PINC; wrcomm(); } else if(new_PORTC != old_PORTC) { new_PORTC=PINC; wrcomm(); } return 1; } int wrcomm() { commport&=~(1<
