Introduction to PLC
1 A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program usually via software or programmer, that gives the desired results.
PLCs are used in many “real word” applications. If there is
industry present, chances are good that there is a PLC present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries, you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a PLC.
2 For example, let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on foe. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.
As you can see, the bigger the process the more of a need we have
for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified tome.
We will take a look at what is considered to be the “top 20” PLC
instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence.
That’s right, more than 80%! Of course we’ll learn more than just
these instructions to help you solve almost ALL your potential PLC applications.
The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data, as shown in Figure 17-1. We can actually consider the PLC to be a box full of hundreds or thousands of separate replays, counters, timers and data storage locations. Do these counters, timers, etc really exist? No, they don’t “physically” exist but
rather they are simulated and can be considered software counters, timers, etc. These internal replays are simulated through bit locations registers.
Input Counters Output
Internal Timers Data
Utility Storage What does each part do?
Relays INPUT RELAYS-(contacts)
These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc . typically they are not relays but rather they are transistors.
INTERNAL UTILITY RELAYS-(contacts) these do not receive
signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. These are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are only once during power-on and are typically used for initializing date that was stored.
COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pluses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down.
TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from lms through ls.
OUTPUT RELAYS-(coils) These are connected to the outside
world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen.
DATE STORAGE-Typically these are registers assigned to simply store data. They are usually used as temporary storage for math or date manipulation. They can also typically be used to store date when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!
A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps, as shown in Figure17-2.There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values
CHECK INPUT SYATUS
Steps 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 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. 3 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 PLC goes back to step one and repeats the continuously. One scan time is defined as the time it takes to execute the 3 steps listed above. Thus a practical system is controlled to perform specified operations desired.