1. PROGRAMMABLE LOGIC CONTROLLERS
1.1 INTRODUCTION
Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been ud for control and early electrical control was bad on relays. The relays allow power to be switched on and off without a mechanical switch. It is common to u relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls. PLC have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is becau of the advantages they offer. gateshead
. Cost effective for controlling complex systems.
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. Flexible and can be reapplied to control other systems quickly and easily.
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Computational abilities allow more sophisticated control.
numb歌词. Trouble shooting aids make programming easier and reduce downtime.
. Reliable components make the likely to operate for years before failure.
npo1.2 Ladder Logic
Ladder logic is the main programming method ud for PLC. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to u the relay logic diagrams was a strategic one. By lecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. Modern control systems still include relays, but the are rarely ud for logic. A relay is a simple device that us a magnetic field to control a switch, as pictured in Figure 2.1. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that clos when the coil is energized is called normally open. The normally clod contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to reprent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally clod contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally clod contacts will be clod (conducting).
西南财经大学自考Relays are ud to let one power source clo a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure 2.2. In this system the first relay on the left is ud as normally clod, and will allow current to flow until a voltage is applied to the input A. The cond relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and clo the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.
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1.3 Programming
The first PLC were programmed with a technique that was bad on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLC today. An example of ladder logic can be en in Figure 2.5. To interpret this diagram imagines that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or clod in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a nsor, switch, or any other type of nsor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are normally open and normally clod, which means if input A is on and input B is off, then power will flow through the output and activate it. Any other combination
of input values will result in the output X being off.
The cond rung of Figure 2.5 is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct the diagrams. There are other
methods for programming PLC. One of the earliest techniques involved mnemonic instructions. The instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure 2.6. In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input 00001. This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or fal). The next line us an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the rever of the LD). The AND statement recalls the last two numbers remembered and if they are both true the result is a 1; otherwi the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered. The process is repeated for lines 00003 and 00004, but when the are done there are now three numbers remembered. The oldest number is from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remem
babyfacebered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwi the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on; if it is 0 the output will be turned off.
