Design & Development of Digital Panel Meter

Transcription

1 e-issn Volume 2 Issue 5, May 2016 pp Scientific Journal Impact Factor : Design & Development of Digital Panel Meter Gangadhar Shinde 1, Vinodpuri Gosavi 2 1 P.G. Student, 2 Assistant Professor, 1,2 Department of Electronics & Tele communication Engineering, Maharashtra Institute of Technology, Aurangabad, India Abstract Measurement of electrical parameter is an essential aspect in today s world, as electricity has become an indispensible part of our lives. Earlier electrical parameters used to be measured using analog meters but later several manufacturers started developing digital meters to overcome the drawbacks of analog meters. Today with several manufacturers in market Cost and also Accuracy have become major parameters to be considered before taking up design of any project. Digital Panel Meter for short DPM s are absolutely necessary in any electrical industry we find. Digital Panel Meters to be designed in proposed project will be Cost effective, efficient and also accurate in measurement, which are the features found desirable in any DPM that customer wish to purchase. Keywords True RMS, DPM, SMPS, Accuracy Class, MCU, ADC I. INTRODUCTION Digital Panel Meters (DPM s) are electronic devices used to measure the electrical parameters such as voltage, current, frequency, power, etc. Digital Panel meters are generally used in electrical industries or wherever there is a need to measure aforementioned parameters. These meters are usually mounted on panel where values of electrical parameters are to be measured ergo they are called as Panel meters. Digital Panel Meters are most accurate and highly efficient compared to their analog counterparts. When any electrical machine is working it is desirable to keep track of inputs given to the machine and outputs given by machine and this task cannot be better accomplished by anything other than Digital Panel Meters. Digital Panel Meter comes in two standard size 96mmx96mm and 48mmx96mm. While designing any digital panel meter the size of the printed circuit board should be planned according to the standards of size of the structure. As stated earlier digital panel meters are mounted on a panel and those panels are designed in such a way that it will have standard size spacing available to mount the meter. Digital Panel Meters are very useful and take very less space for mounting. With the help of digital panel meter one can have a good understanding of the electrical system by just looking at it. User will be able to tell the voltage present at particular points like Line to Neutral voltage of Mains supply or if supply is three-phase it is also possible to know the Line to Line voltage status. If energy Meters are used one can easily tell the energy consumed by particular machine. Microcontrollers (MCU) are the widely used components in every electronics design due its convenience. Constantly improving performance at low price makes MCUs more and more attractive for variety of applications. Most of the MCUs have the built-in analogue to digital (AD) converters (ADC) which is used to convert Analog data into Digital type. In built ADCs have the 10-bit resolution that is sufficient for the general purposes but where higher accuracy is desired higher bit ADC s are available in advance versions of controllers or external ADC can also be interfaced to Microcontroller(MCU) for conversion purpose. Where cost is a constraint internal ADC can be used and with the help of firmware (Software) one can increase the accuracy to certain All rights Reserved 544

2 Paper is organized as follows. Section II lists the Technical Input Specifications for Digital Panel Meter. Section III describes the Block diagram of proposed system with explanation of each block in brief. Section IV Presents results of designed system s performance. Lastly section V states the conclusion. II. TECHNICAL INPUT SPECIFICATIONS Technical input specifications are the specifications that are desired to be present in a proposed system and should always be defined before actuating the design process. Some of the important specifications are described below: 1. Measuring principle: This is first and the foremost specification for most of the measuring devices like DPM. Measuring principle employed in proposed DPM system is True RMS i.e. True Root Mean Square. While measuring the Analog quantities like voltage, current, etc. if shape of waveform is not perfect sine wave then simple Root Mean Square principle would not work to measure exact value of voltage. Hence Measurement of electrical quantity with accuracy with disregard of its shape can be possible only with True RMS principle. RMS principle based measurement will show measured parameter values less than actual value if waveform of measuring signal is not having perfect sine waveform nature. True RMS calculates the equivalent DC value of the parameter to dissipate same amount of power as AC. 2. Accuracy Class: Accuracy Class defines the error limit of the system. If a device claims to have accuracy class of 1.0 then it means the maximum error limit for that device will be +/- 1. Accuracy class of 0.5 is better than 1.0 and so 0.2 is better than 0.5. Lesser is the class higher will be the accuracy. Component selection for system design will be highly affected due to selection of class as to achieve higher accuracy, component used should also be accurate. 3. Power Supply: This is also one important specification as it defines the flexibility of device i.e. its range of operation. Power supply for proposed system is Switch Mode Power Supply with universal auxiliary voltage (voltage to turn on the device) 40V to 340V AC/DC. With this supply user will have the flexibility to operate the device on whatever the supply voltage available of any nature either AC or DC. 4. Display Type: Two types of displays are available liquid crystal Display (LCD) or Light Emitting Diode (LED) Display. In proposed system LED displays are used as they are inexpensive. 5. Measurement Range: Parameters to be measured in this design are Voltage and current. Measuring range of the voltage is expected to be between 30V to 300V AC. For current it is from 50mA to 6A and also1a to 20A. For DC voltage measurement range is 5V to 500V. III. BLOCK All rights Reserved 545

3 Data International Journal of Current Trends in Engineering & Research (IJCTER) SMPS Power Supply 3.3VDC Input Sense & Filter Circuit Atmel AVR 8- bit Microcontroller Unit (MCU) Four Digit Seven Segment Display Control Lines Data Fig. 1 Block Diagram of Proposed Digital Panel Meter Basic Blocks in DPM Circuit: 1. SMPS (Switch Mode Power Supply) i. Rectification ii. Transformer iii. Link switch IC iv. Regulator IC 2. Input i. Voltage divider circuit/current Transformer 3. Controller 4. Display Bridge Rectifier As voltage required for microcontroller is of DC type bridge network of diode is used to convert the AC supply to DC type. In bridge rectifier when input is positive, two diode will be forward biased and remaining two will be reverse biased. In same way for negative cycle of input the diodes which were conducting in positive cycle will be reverse biased and non conducting diodes in earlier case will be in forward bias. Thus converting AC supply to DC type of voltage. (a) (b) (C) Fig. 2 Voltage Rectification (a) Full wave rectification Circuit (b) Input Voltage Waveform (c) Output Voltage Waveform Transformer Transformer is the second device in power supply circuit as it will generate voltage at secondary terminal. Transformer in switch mode power supply scheme does not step down the voltage. The function of transformer is more like the function of All rights Reserved 546

4 When voltage is applied at the primary terminal of the transformer, current flows through the primary winding and then abruptly path for current conduction will be cut off which causes inductor (primary winding) to generate back EMF as inductor has a property of opposing the change in current flowing through it. This back EMF will generate voltage in secondary winding which is the output of the transformer. The transformer used in SMPS is not linear in nature so it does not get heated. Link switch IC This link switch IC is manufactured by Power Integrations, the company which provides simplified power supply solutions. This IC merely works as a switch to connect or disconnect Drain and Source of MOSFET in the path of primary winding to generate sufficient amount of back EMF to get the required output voltage. Besides secondary winding there is also a feedback winding voltage of which is given to the Link switch IC at FB pin. According to the feedback voltage this IC changes its frequency with which it turns ON and OFF the MOSFET. Link switch IC has a very major role to play in this switch mode power supply. Regulator IC Output of Secondary of transformer is given to the regulator IC because though the output of the transformer is constant 5V it may very when input Aux voltage changes from 230V to 300V or from 230V to 40V. This 5V input will be converted into 3.3V stable and constant output which will be given to the controller after separation of Analog and Digital VCC and Ground. Voltage divider circuit / Current transformer Analog voltage to be measured cannot be applied to the controller directly. It first needs to be reduced using a suitable voltage divider circuit. It depends upon the reference value of the voltage of analog to digital converter in the micro controller. Maximum voltage should be greater than reference voltage value. Values of resistances are selected in such a way that output voltage does not exceed the limit. Reference voltage of analog to digital converter is around 1V so 1V is top limit for voltage divider circuit. If the values of Resistances are considered as 1K and 1M-ohm i.e. ratio is approximately if output is taken across 1K resistance. If the applied input voltage is 230V AC only 0.230V will be given as input to analog to digital converter. Similarly for current input current transformer is used. In DPM current transformer of 2500 turns is used so the output current will be 2500 times less than input current which can be dropped across appropriate resistor to get the voltage. Once signal is converted into the voltage form it will be given to ADC after filtering for measurement purpose. Controller: Atmel AVR controller of 8-bit is selected which has sufficient Flash memory and EEPROM Memory for programming. Internal Analog to Digital converter is used which has resolution of 10- bit. Using port pins of controller the measured value of analog quantity will be displayed on four digit seven segment display which is connected to controller with its data lines time multiplexed. Internal two special timers are used. One timer is used to drive four digits seven segment displays in cycle of 2msec. And another timer is used to take samples of Analog signal to convert it into digital signal. Required calculations will be done in firmware and result will be displayed on four digit seven segment All rights Reserved 547

5 IV. RESULTS In DPM for current Measurement reference input is applied from a highly accurate source with Accuracy class of 0.02 And corresponding output shown by the prototype designed of proposed system is shown below in table with percentage error. SN Songyang Reference Input(A) Measured value by Proposed system (A) Percentage Error Table. 1 Results for DPM for AC Current Measurement Here error observed is less than 1.0 so the Accuracy class of this Digital Panel Meter can be claimed as Class 1.0. In similar fashion just by replacing Currrent Transformer used for current sensing in this DPM by a voltage divider circuit and by making required changes in firmware Voltage can also be measured. Results for Voltage Measurement are shown in below table: SN Reference Input(V-AC) Measured value by Proposed system (V-AC) Percentage Error Table. 2 Results for DPM for AC Voltage Measurement Only few readings have been mentioned in the table but system can be subject to any input value within the specified range. Accuracy of class 1.0 can be claimed for voltage measurement too.similarly Values of DC voltage can be measured without making any change in hardware or firmware of the proposed system and still the error is within the limit of Class 1.0. V. CONCLUSION Here system hardware is designed in order to meet the customer requirements such as Accuracy, Affordability and bright display. Hardware designed for project is tested on lose-circuit board and it works completely fine as performance analysis shows. Accuracy class of designed system is Class 1.0. Proposed system will be helpful to measure electrical parameters accurately. With proper component selection and advanced software writing techniques cost is reduced significantly without hampering the accuracy of the proposed system. This DPM will be an affordable, reliable and also user friendly when it comes to electrical parameter measurement. REFERENCES [1] Using Microcontrollers for High Accuracy Analogue Measurements by M. Jaanus A. Udal, V. Kukk, K. Umbleja, Tallinn University of Technology [2] 4.pdf [3] [4] All rights Reserved 548