9. plc - PLC PDF

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INTRODUCTION TO PROGRAMMABLE LOGIC CONTROLLERS WHAT IS PLC? A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes. HISTORY:  Developed to replace relays in the late 1960s  The PLC was invented in response to the needs of the American automotive manufacturing industry(primarily General motors).  PLC began in the 1970s, and has become the most common choice for manufacturing controls.  Costs dropped and became popular by 1980s  Now used in many industrial designs PLC COMPONENTS:

POWER SUPPLY - the component that provides power to a PLC CPU and I/O. The power supply must convert the line voltage to around 5VDC for the PLC to use. TYPES: A) 24 volts DC B) 100 - 240 volts AC C) 110/220 volts AC

CENTRAL PROCESSING UNIT (CPU) - the component that processes information and carries out instructions in a PLC. This is also known as the processor or controller.

INPUT MODULE - the component where input devices are physically connected to a PLC. The input module transfer signals to the internal relay. TYPES: A) DC - 12 volts b) DC - 24 volts C)

110/220 volts AC

OUTPUT MODULE - the component where output devices are physically connected to a PLC. The input module transfer signals from the internal relay. TYPES: D) Transistor type (24 VDC) e) Relay/Contact Type(24 VDC/220 VAC) F)

TRIAC Type (110/220 VAC)

INPUT ADJUSTMENT INTERFACE Adjustment interface also called an interface is placed between input lines and a CPU unit. The purpose of adjustment interface to protect a CPU from disproportionate signals from an outside world. Input adjustment module turns a level of real logic to a level that suits CPU unit (ex. input from a sensor which works on 24 VDC must be converted to a signal of 5 VDC in order for a CPU to be able to process it). This is typically done through opto-isolation.

OUTPUT ADJUSTMENT INTERFACE Output interface is similar to input interface. CPU brings a signal to LED diode and turns it on. Light incites a photo transistor which begins to conduct electricity, and thus the voltage between collector and emitter falls to 0.7V, and a device attached to this output sees this as a logic zero. Inversely it means that a signal at the output exists and is interpreted as logic one. Photo transistor is not directly connected to a PLC controller output. Between photo transistor and an output usually there is a relay or a stronger transistor capable of interrupting stronger signals.

SINKING-SOURCING CONCEPT PLC has input and output lines through which it is connected to a system it directs. Input can be keys, switches, sensors while outputs are led to different devices from simple signalization lights to complex communication modules. This is a very important part of the story about PLC controllers because it directly influences what can be connected and how it can be connected to controller inputs or outputs. Two terms most frequently mentioned when discussing connections to inputs or outputs are "sinking" and "sourcing". These two concepts are very important in connecting a PLC correctly with external environment. The most brief definition of these two concepts would be: SINKING = Common GND line (-) SOURCING = Common VCC line (+)

TYPES OF PLC: 1. Unitary 2. Modular 3. Rack Mounted

Unitary The unitary PLC is one of the earliest types used back in the 1970's and is very compact with a simplistic design and is the least expensive of PLC types on the market. It houses all of the basic features such as, the CPU and I/O boards and are commonly used to run small manufacturing processes or fixed machines. Advantages  Less scanning time  Usually small, have less memory so cost is less  Less troubleshooting time so less downtime Disadvantages  Limited number of inputs and outputs  Not suited for future expansion  If part of unit fails, the entire unit may need replacement

Modular The modular PLC followed on from the unitary PLC and is very similar in size and design but is slightly more expensive. They consist of 3 basic modules namely, the input, output and CPU with the power source being provided separately. Their advantage is allowing a range of modules to slot together to expand the system by usually mounting them on a DIN(Deutsches Institut für Normung) rail and by simply adding another unit and connecting the processor of the existing unit via a plug. However this expansion is limited due to the capability of the CPU and scan speeds. These are also commonly used for small manufacturing process requirements but with a possibility of expansion if processes were to increase. Advantages  The main advantage is that the number of input and output terminals can be expanded to cope with the changes to the hardware system  Modules can be easily installed or removed without affecting other modules Disadvantages  Scanning time increases with the increase in input and output module  Limited expansion is allowed depending on type of brand

Rack Mounted The rack mounted type of PLC is similar to the modular concept, but is implemented differently. Whereas each module in a modular PLC connects to the base unit directly, a rack mounting PLC keeps each module separate. All extra modules are connected through a network, and modules are held in organized racks. This approach allows for larger systems to be built without becoming overly cluttered and complicated. Modules are well organized on the rack and can be removed and reinserted as needed. The rack mounted PLC is more commonly used where large automation processes are required such as production lines. Advantages  Allow large extensions to the program  Slot and memory cards can be interchangeable or reprogrammed and easily replaced. Disadvantage  Overwhelming initial cost

TYPES OF PLC ACCORDING TO NUMBER OF I/O’s: 1. 2. 3. 4.

Micro PLC Small PLC Medium PLC Large PLC -

less than 100 I/O’s 101 to 526 I/O’s 527 to 1028 I/O’s 1029 to 5120 I/O’s

INPUT DEVICES Push-button switch Limit switch Timer Sensing devices Rotary encoder Proximity switch Pressure switch

OUTPUT DEVICES Alarms Lights Valves Solenoid Heater LED display Motor starters Motors

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PLC OPERATION A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps.

Step 1.CHECK INPUT STATUS First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step. Step 2.EXECUTE PROGRAM Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step. Step 3.UPDATE OUTPUT STATUS Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above. Scan time depends upon the speed of CPU and the length of the program. PLCs are generally programmed in ladder diagram which is a symbolic representation of electric circuits.

PLC PROGRAMMING LANGUAGES 1. INSTRUCTION LISTS(IL) An instruction list gives programs as a series of instructions, with each instruction on a new line. Each instruction consists of an operator followed by one or more operands, that is, the subjects of the operator. Thus we might have: LD A to indicate that the operand A is to be loaded, LD being the operator used to indicate loading. In terms of ladder diagrams, an operator may be regarded as a ladder element, and LD is equivalent to starting a rung with open contacts for input A. Another instruction might be: OUT Q to indicate that there is to be an output to Q.

Example: LD AND OUT

X001 X002 Y001

2. STRUCTURED TEXT(ST) ST b0rrows its syntax from PASCAL, enforces data typing and support structured programming. The structured text consists of a series of instructions which, as determined in higher level language, (“IF..THEN..ELSE”) or in loops (WHILE..DO) can be executed. Example: IF value...