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Microprocessors and PLCs

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Microprocessors and PLCs

 ABSTRACT

In everyday life, we have been using electronic scales or weighing machines. In every grocery store, vegetable shop or a jewellery shop, weight measurement system is used. The weight of chemicals can be measured in laboratory and these items are weighed using electronic weighing machines. Electronic weighing technology presents management with rapid, timely and accurate information that provides quick turnaround times for customers. Its popularity can be attested by its use in all the sectors of the industry. In fact, its spectrum of use spans from the traditional retail industry, to manufacturing and and warehousing, to postal, health and transport industry. A load cell is used to sense the weight of object, so here load cell works as a sensor. When load is applied the load column gets compressed and its length changes. This column acts as a primary transducer since it converts applied force into change in length. This change in length is not measured directly. At the same time strain gauge connected to load column gets compressed. The strain gauge acts as a secondary transducer since it records the displacement of the load column. As the strain gauge is compressed, its length gets changed, which depends on magnitude of the applied force on the top of the load cell. The resistance of strain gauge changes when its length changes. This change in resistance can be measured in terms of change in output voltage and amplifiy using differential amplifier.If the voltage comes to be negative, the inverter makes it positive. Thus, a load cell gives us a voltage level equivalent to the weight applied. Key words: Kitchen scale, Altium, Proteus, Simulation, Load cell.

Colour Sensor Signal Conditioning-Weight Sensor Signal Conditioning:This work proposes a fully-digital interface circuit for the measurement of inductive sensors using a low-cost microcontroller (μC) and without any intermediate active circuit. Apart from the μC and the sensor, the circuit just requires an external resistor and a reference inductance so that two RL circuits with a high-pass filter (HPF) topology are formed. The μC appropriately excites such RL circuits in order to measure the discharging time of the voltage across each inductance (i.e. sensing and reference) and then it uses such discharging times to estimate the sensor inductance. Experimental tests using a commercial μC show a non-linearity error (NLE) lower than 0.5%FSS (Full-Scale Span) when measuring inductances from 1 mH to 10 mH, and from 10 mH to 100 mH.

Design notes which may include:
Weight Measurement System Design Notes

The Weight Measurement Using Inductive Sensing Reference Design is a sub-system design which converts a distance measurement to a weight measurement. This design is intended as a reference design for building automation and weighs scale applications..Features

Calibrated sensor output from microcontrollerLow power consumption of 37.8 mWOutput resolution: 2 gTypical System Error: 4.2 g (600 g to 650 g)IEC61000-4-2: ESD: Air Discharge: ±8 kV Class AIEC61000-4-4: EFT ±2 kV Class A

 

LOAD CELL : Load cell is a passive transducer or sensor which converts applied force into electrical signals. They are also referred to as “Load transducers”.  DIFFERENTIAL AMPLIFIER: Amplifies the output of Load Cell.

 INVERTER: It inverts the output of differential amplifier.

 MICROCONTROLLER: The output from differential amplifier will be given microcontroller

.POWER SUPPLY:Supply is given to differential amplifier,PIC and LOAD CELL.

LCD : The output voltage is converted into digital form and displayed on LCD

Load Cell: A load cell is described as a “weight measurement device necessary for electronic scales that display weights in digits.” However, load cell is not restricted to weight measurement in electronic scales. Load cell is a passive transducer or sensor which converts applied force into electrical signals. They are also referred to as “Load transducers”. The load cells based on strain gauges. Hence, the term ‘load cell’ means ‘strain guage-based load cells’. The sensing or spring element is the main structural component of the load cell. The element is designed in such a way that it develops a strain, directly proportional to the load applied. Sensing elements are normally made of high strength alloy steels (nickel plated for environmental protection), precipitation-hardened stainless steels, heat-treated aluminum alloys, or beryllium copper alloys. 

 CHARACTERISTICS OF LOAD CELL : 

1. Highly precise and linear measurements 

2. Little influence due to temperature changes.

3. Long operating life due to lack of moving parts or any parts that generate friction.

4. Ease in production due to small number of components.

5. Excellent fatigue characteristics The Load cell which we are using is for Kitchen scale i.e we can measure weight of vegetables,fruits etc.

Available load cells are 6 Kg., 10 Kg., 20 Kg., 40 Kg., 80 Kg and others. Out of these available load cells we will choose 6 Kg.load cell as it satisfies all the requirements. We have used the load cell which is available in local market.

Load cell selected – CZL601-6Kg.

2.2 MICROCONTROLLER The output from differential amplifier will be given microcontroller. We can use an 8051 microcontroller, PIC microcontroller or AVR microcontroller. The 8051 microcontroller does not have in built analog to digital converter i.e. ADC block. Hence, a separate A to D circuitry will be required along with microcontroller, which will increase number of components. PIC microcontroller has in built 8 bit or 10 bit ADC. Hence with use of PIC microcontroller, circuit will become compact.

 OBSERVATION Table1. Comparison for various PIC microcontrollers is as given in following table Parameter PIC18F4520 PIC16F685 PIC18F452 Operating frequency 8MHz 20MHz 8MHz Program memory 32768 words 4096 words 32 Kbytes Data memory 1536 bytes 256 bytes 1536 bytes I/O Ports A, B, C,D,E A, B, C A, B, C, D, E 10 Bit ADC channels 12 channels 12 channels 8 channels.

PLC Design Notes 

callMyProcedure   -- calls "MYPROCEDURE"call 'MyProcedure'  -- calls "MyProcedure"Some steps of the function and subroutine search order are case sensitive, so some care may need to be exercised that the correct name form is used: Pseudocode outline for your program  
Microcontroller Code :

PLC stands for Programmable Logic Controllers. They are basically used to control automated systems in industries. They are one of the most advanced and simplest forms of control systems which are now replacing hard wired logic relays at a large scale.Flowchart or Sequential Function Chart
  I/O list In I/O list, the following information should be stated but not limited to:·         Tag number·         Loop Number·         Service description·         P&ID Number·         Type of Instrument·         Location·         I/O Type·         Control System·         Range or set pointPLC Program:

Demonstration of required functionality: Read of output of weight measurement System

BLOCK DIAGRAM Given below is the general block diagram of ‘Weight Measurement System’. It shows basic building blocks of the project.
CODE FOR WEIGHT MEASUREMENT SYSTEM
·         sbitlcd_RS at RD2_bit; ·        sbitlcd_EN at RD3_bi·         sbit lcd_D4 at RD4_bit; ·         sbit lcd_D5 at RD5_bit; ·         sbit lcd_D6 at RD6_bit; ·         sbit lcd_D7 at RD7_bit; ·         sbitlcd_RS_direction at TRISD2_bit; ·         sbitlcd_EN_direction at TRISD3_bit; ·         sbit lcd_D4_direction at TRISD4_bit; ·         sbit lcd_D5_direction at TRISD5_bit; ·         sbit lcd_D6_direction at TRISD6_bit; ·         sbit lcd_D7_direction at TRISD7_bit; ·         intadc_rd; float adc_val; ·         float wt; char finaltxt[8]; ·         float quant=5.0/1024; ·         void main() { //char res[5]; ·         PORTD=0xFF; TRISD=0xFF; ·         ADCON1=0Xc0; ADC_Init(); ·         LCD_Init(); ·         lcd_cmd(_LCD_CLEAR); ·         lcd_cmd(_LCD_CURSOR_OFF); ·         while(1) { adc_rd=ADC_Read(0); ·         adc_val=adc_rd*quant; ·         wt=(adc_val-0.361)/0.079; ·         sprintf(finaltxt,"%1.2f",wt); ·         lcd_cmd(_LCD_CLEAR);·         Lcd_Out(1,1,"WEIGHT: "); ·         Lcd_Out(1,9,finaltxt); ·         Lcd_Out(1,14,"KGs"); ·         Delay_ms(1000); } }Structure of PLC program: Earlier computing systems were classified into mainframes, mini-computers, and microcomputers. The microcomputers were desktop machines, supporting a single user. The "brain" of a microcomputer was a single IC (integrated circuit) chip. It is usually called the CPU.
 

 
 
REFERENCES [1]. “PIC Microcontroller and Embedded Systems”, By Mazidi, Mckinlay and Causey. [2]. “Mechanical and Industrial Measurements”, By R. K. Jain, Khanna Publishers. [3]. Belove, C. (1986), Handbook of Modern Electronics and Electrical Engineering, John Wiley and Sons Inc. U.S.A. [4].www.fairchildsemi.com [5].www.microchip.com [6]. Belove, C. (1986), Handbook of Modern Electronics and Electrical Engineering, John Wiley and Sons Inc. U.S.A., pp 2-19. [7]. Bentley, J. P., Measurement Systems, Addison Wesley Longman Ltd., UK, 2000. [8]. Borer, J. (1991), Microprocessor in Process Control, University Press Great Britain, pp 21- 30. [9]. Braby, P. W., Electronics: A Practical Introduction, John Wiley & Sons, Canada Limited, 1983. [10]. Brophy, J.J.(1990) Basic Electronics for Scientists, McGraw-Hill International Editions, fifth Edition,pp 462. [11]. Chandhuri, N. (1985), A Technique for Simultaneous Measurement with a Microcomputer: IEEE Transactions on Industrial Electronics, Vol.32 No.2, p9. [12]. Commer, D.J (1987) State Machine versus Microprocessor Controller: Digital System Design. Vol. E 30 NO.2, pp 5-8. [13]. David, et al, Using the 8051 Micro controller with Resonant Transducers, IEEE Transaction on Industrial Electronics, Vol. IE-32 No. 2, November 1985. [14]. Dorf, R (1993), Electrical Engineering Handbook, CRC press, U. S. A. Downtown, A. C. (1980), Computers and Microprocessors: Components and Systems. Nostral Van Reinhold (UK) CO.LTD, England, pp 1-23. 



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