When doing PLC training, it is important to see the advantages and disadvantages. Some advantages are: favorable cost, flexible, and it can be applied again to rapid and easy control of other systems.
Their abilities allow sophisticated control, problem identification tools facilitate programming and reduce the downtime. The trustworthy components make the system work for years without problems, the possibility of viewing the operation, operating speed, different programming methods.
PLCs are not so good at:
– using a large set of data, complex data or complicated mathematical functions;
– reading and writing databases;
– generating reports;
– displaying operator data and information.
Another disadvantage is the novelty of technology.
PLCs have a variety of uses including basic control of relays, motion control, process, and can also be used with Distributed Control Systems.In short, a PLC is an automaton that uses inputs to monitor a process and outputs to control a process using a program. More info here.
Principles of operation, block schemes
The operation of a PLC is based on the scheduled scan of its inputs and outputs. The scanning process has 3 basic steps:
Step 1 – Entry Test Status. To begin with, the PLC scans each entry to determine the ON or OFF states that they can present. In other words, check whether sensors or switches connected to the inputs are activated or not. The information collected during this step is stored in the memory and will be used in the next step.
Step 2 – Run the program. At this stage, the PLC executes a sequential program (instruction after instruction). As a result, one or more outputs can be activated, or you can store information in specific areas in memory, and then use them in the next step.
Step 3 – Check and Set Outputs. Finally, the PLC checks the output states and modifies them if necessary. Changes are based on the input states read during the first step and the program execution results from step two. After step execution3, the PLC resumes the 3-step cycle.
The scan time is defined as the time it takes for the 3 steps to be configures. Depending on its complexity, the PL program can also perform the following functions (the first three being identified by the steps above): reading entries;resolving Boolean equations and executing logical calculus sequences;command of outputs;memorizing the partial results or the state of the inputs and outputs;the registration of the values and their transfer to the process;implementation of some arithmetic calculation functions;the realization of the exploitation dialogue;carrying out the supervision dialogue; carrying out the programming dialogue;making the dialogue with the peripheral elements.
This url has useful information: http://www.plcdev.com/how_plcs_work
AP operation involves tracking variations in process signals. To this end, AP inputs are read at well-established time intervals. Each reading is made in a row, but since the time between readings of two different inputs is very small (in the order of microseconds) for a certain category of processes, it can be assumed that readings are made simultaneously for all inputs.
The implementation of the command scheme within the AP is done by modeling it through Boolean equations or logical calculus sequences where the variables are the input signals of the AP. Results are the values to be assigned to AP outputs. This operation is executed in each cycle, taking into account updated input values.
The equations are solved one after the other, but for a certain category of processes it can be considered that the results are obtained simultaneously for all the equations because the time of solving two different equations is very small.
AP operation means that it outputs command signals based on the input values of the program that implements the command function. Each output is ordered one at a time. Also, for a certain category of processes, it can be considered that all outputs are ordered simultaneously because the time between commands of two different outputs is very small.
In the case of high complexity programs involving a series of laborious calculations, CPU registers may not be sufficient for storing intermediate results. To avoid this, AP needs to be able to temporarily store these results in a working memory for later use. It is possible to store input and output values in a previous cycle.
The complexity of the processes ordered via AP makes the command scheme implemented by it to perform a number of operations other than logical operations.