Programmable Logic Controller

Programmable Logic Controller

INTRODUCTION :PLC is a digitally operating electronic apparatus which uses a programmable (through dedicated language such as Relay Ladder Programming) memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog input output modules, various types of machines and processes. The programmable controllers (PC) have developed as the heart of the automation of the industries. The PCs offer a number of advantages over the conventional relay control logic in many aspects like reliability, repeatability, programmability, compact design etc. Small controllers play an important role in the automation where such controllers are implemented in huge and complicated processes and plants. In such situations the communication among the PCs is of vital importance along with other aspects.

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Programmable Logic Controller

TYPICAL SPECIFICATIONS OF THE PROJECT :1. Main module based around 8031 microcontroller. 2. 16 digital Inputs and digital outputs with proper isolation and signal conditioning. 3. Twenty-five hex keyboard and 7-segment display interface. 4. Closed loop PI controller for DC motor of 20 HP.

HARDWARE DESIGN :The general block diagram of the system is shown in fig1. The system is divided in following different cards: 1. 8031 CPU card 2. Display and Keyboard card 3.Digital input card

4.Digital output Card 5.Closed Loop PI Controller which

includes ADC and DAC. These cards are fitted on the back plane through 96-pin EURO connector. Thus it is completely modular and open ender for future expansion. 1. 8031 CPU CARD Its general block diagram is shown in fig. 1.1 SELECTION OF PROCESSOR The EPROM requirement for monitor program is less than 4K bytes but additional 4K bytes of ROM memory space are given for future expansion of the system. No. of digital inputs and outputs is 16. This has caused to select Intel 8031 microcontroller as the CPU. 1.2 CONFIGURATION OF THE CARD The Intel 8031 microcontroller is interfaced to two EPROMs (total 8K bytes), a SRAM 6116 (2K bytes), keyboard and display controller 8279 two

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Programmable Logic Controller

programmable peripheral interface chips 8255 alongwith necessary Reset, Clock, and Chip Selection logic circuit. 2. DISPLAY AND KEYBOARD CARD Its block diagram is shown in fig.2.2 . Five keys hex keyboard a common cathode 7-segment displays are interfaced to corresponding control signals of 8279 from EURO connector. 3. DIGITAL INPUT CARD Its block diagram is shown in fig.2. Digital inputs are sensed through limit switches and carried on the card through PUT 2.5 to opto isolators MCT2E. Its output is connected to port pins of 8255 via 96-pin EURO connector. Total no. of digital inputs are 16. 4. DIGITAL OUTPUT CARD Its block diagram is shown in fig.2.The digital outputs are taken from 8255 ports. These are used to make on and off the relevant 10 VA solenoid valve through proper signal conditioning circuit. These outputs are also used to change the armature voltage of 24 VDC motor (Lens rotation motor) in the step of 24 V and 15 V through proper signal conditioning. Total no. of digital outputs are 16.

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Programmable Logic Controller

CLOSED LOOP PI CONTROLLER CARD :Its block diagram is shown in fig.2. Characteristics of the DC motor are shown in fig.3. Power developed by motor = Ta*2*PI*N watt

(1)

where, Ta =Armature torque in Nm. N = Motor speed in rps. PI = 3.14 From this it is clear that the motor rotates at constant torque till rated speed and it enters in the constant power mode above rated speed. The back EMF Eb is given by, Eb = V – la*Ra

(2)

and also N = K* (Eb/field flux)

(3)

Therefore speed of motor is directly proportional to Back EMF and inversely proportional to Field flux. Speed of the DC motor can be sensed by different methods such as. Tacho meter Stroboscopic method, Back EMF sensing method etc. Back EMF sensing method is suitable in noisy industrial environment and therefore it is selected to sense the speed of DC motor. V, Ia, Ra are sensed and given through proper signal conditioning circuit as input to the ADC 0808 (8 channel successive approximation ADC). Overload current is also sensed through proper signal conditioning and given as input to ADC. Control signals of ADC. Control signals of ADC are connected to port 1 of

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Programmable Logic Controller

8031 and controlled through software. The DAC input is connected to the port of 8255 and its output is connected to ADC input through PI controller circuit.

SELECTION OF CONTROLLER :PI controller has following properties : 1) It contains properties of P and I elements. 2) It can be used to compensate one time constant of the system. 3) It has the sluggish response. Due to motor inertia the sluggish operation is desired to control the speed of motor. Here PI controller is selected as against P,PI,PID.

OPERATING MODES :The system works in six different modes of the operation. These modes can be selected through the keyboard . These modes are : i)

Configure mode,

ii)

Report mode,

iii)

Programming mode,

iv)

Execution mode,

v)

Development mode and

vi)

Self test mode. These modes are explained in the following paragraphs. In the

Configure mode, the set points for the analog inputs, the look-up tables are loaded into the memory. Also the initialization of the High Speed I/O section is done in this mode. In the Report mode, the recorded data is sent to the computer or the MODEM

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Programmable Logic Controller

over the serial communication link. Programming mode enables the user to load the program (set of the instructions) into the program memory of the PC. This mode is further explained in the following section. In the Execution mode , programs loaded in the above mode are executed. For the execution program number and the starting step number is required. The Development mode is useful for moving a block of memory from one place to another communicating with a computer(down- loading and up-loading of the program or the data) and modifying the memory contents. The Self-test mode carries out the I/O checking for any malfunctioning of the channels or discontinuity of the channel and indicates the fault on the display.

AFTER THE POWER ON :After the power on of the system (or after the power-on reset) the following procedures are carried out: 1

CPU test,

2. Memory test, 3. Register initialization, 4. Peripheral Device Initialization, 5. The I/O Initialization If all the tests and initializations are successful, then the system enters into the 'mode selection' phase in which it accepts only the mode selection keys, otherwise, it displays the error message and halts. During the mode selection, it waits

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Programmable Logic Controller

for a mode selection key to be pressed. These keys are CONFIG, REPORT, PROG. EXEC, DEVP and TEST keys. After selecting the mode, the mode name is displayed on the display of the program terminal and then further functions within that mode can be selected through the 'NXT' (Next) and 'CR' (Carriage Return) keys. This will be clear from the following example. Power-on Display

: MINI PLC

Key pressed

: Display

CR

: GET MODE

CONFIG

: CONFIGURE

NXT

: LD SETPT

NXT

: INIT HSO

ESC

: GET MODE

( LD SETPT : Loads the set points for the analog input channels LDTBLS : Loads the look-up tables etc.) Thus the modes and the subfunctions within the modes are displayed on the LED displays. And to select the subfunction , CR key is used.

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Programmable Logic Controller

HOW TO PROGRAM ? The PC is programmed using the statement list programming. The instructions for the PC cover the arithmetic, Logic, transfer and I/O type of the operations. In all there are fifty instructions. The direct logical ANDing/Oring operations of the digital inputs is also provided which simplifies the programming for the ladders. Delay instructions gives the time delays in micro- or milli-seconds which are useful in the ladder solutions. These instructions are placed into the program memory at certain step numbers. While loading the instructions, the display shows the step number (first two digits). Then the instruction and its operand ( if necessary). The program mode shows the previously loaded instructions and also enables to change these instructions. An instruction to be loaded is selected by an instruction key (e.g.RD, WR, LD, ST etc.) and then the actual instruction is obtained by pressing the ‘NXT’ key till the display shows the desired instruction, the instruction field, e.g. The actual instructions for 'RD' are RDAI (read analog input) and RDDI (read the digital input. The desired instruction is confirmed by pressing the 'CR' key. To cancel the selection 'ESC' key is used.

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Programmable Logic Controller

-: APPLICATION OF PLC : LSM Controller Fig. Shows how the control action is achieved I/P devices (e.g. mechanical switches, proximity sensors) and O/P devices (e.g. motors, solenoids) to be controlled are connected to the LSM I10 interface card. A user has to write a program to operate the I/P and O/P devices.

The LMS Controller continuously

monitors the status of the I/P and control the O/P’s according to user program. The programmable motion controller is an intelligent module designed to control position, velocity, acceleration and Jerk of high speed servomotors. The transition from analog electric to digital is an swing in modern industry and the digital intelligent controllers are having many advantages like. 1) Peruse control is possible. 2) Adaptive control techniques can be used to compensate problems like noise, temp, drift. 3) Features previously implemented using mechanisms, can how be implemented in electronics such as  Electronic gearing  Indexing  Cut-to- length application.  Programmability leads to flexibility.  Report generation and built in diagnostics are possible.

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Programmable Logic Controller

LSM servo motion controller card is used to control DC Servo motor. It is build up of digital signal processor (DSP). These processor are very fast which can perform tens of millions of operation per second. LSM Servo motion controller is used to control DC Servo motors. processor is the host computer.

It is a Pc based system, where the Pentium

The actual complications (PID control, velocity

profile) is done by a digital signal processor (DSP) and application specific integrated circuity (ASIC). In addition to very sophisticated control capabilities, it can also has extensive communication capabilities. So in turn it can provide and interface between controllers to control several motors of at high speeds for simultaneous movement of multiple areas.  LSM Servo Cards The LSM Servo Cards is the heart of the system, that can control four DC Servo motors simultaneously. It provides closed loop digital servo control bar large variety of servomotors. It uses optical incremental encoders feedback and a DAC or PWM compatible O/P drive. According to the reference command given by the user it generates the reference voltage (DAC Voltage). Reference voltage will be directly proportional to the defined velocity. These voltage can range between +/- 10v. It also generates drive enable voltage to enable the desired drive.  4 Quadrant DC Servo Drive. The reference voltage and drive enable voltage of LSM Servo Card are fed to the DC Servo drive through the LSM servo interface card. Dc Servo drive 

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Programmable Logic Controller

amplifies the reference voltage +/-10v to +/- 24v DC Servo drive gets the velocity feedback from the tachogenerator and compares the defined velocity to maintain the desirable velocity.  DC Servo Motor:The amplified voltage from the drive is fed to the DC servomotor through the LSM motor. Tacho generator interface card motor can be coupled to any mechanical device.  Encoder :Optical incremental encoder gives the position f/B to the LSM Servo Card. The signal from the encoder tell the controller whether the motion has properly occurred.  Tachogenerator :Tachogenerator generates the velocity feedback to the DC Servo drive. These gives out the voltage proportional to the speed of the dc servo motors shaft.

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Programmable Logic Controller

CONCLUSION :The desired PLC is useful in areas like special purpose machines lab instruments, process control and motion/position control systems. The powerful instruction set enables the user to program the PC in many different configurations and control methods to suit various applications. The ability to communicate with other controllers or the main computer facilitates its roll in the large automation systems using discrete, small size controllers. The effort has been made to keep the product cost to a minimum. The product is based on a compact, modular and expandable structure, with the facility of modifying the system's programs to enhance its capability and the number of the I/Os to three to four times its current capacity.

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Programmable Logic Controller

REFERENCE :1. Robert Wilhem Jr., Programmable Controller Handbook, Hayoen Book Company, 1984. 2. Douglas Considine, Process Instruments and Control Handbook, 3 rd edition, McGrawHill. 3. 16-bit Embedded Controller Handbook, Intel Corp., USA. 4. Embedded Control Applications Handbook, Intel Corp., USA.

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