When you are developing a PLC program, you need to ensure that it is tested and validated properly before showing it to the customer. This is because a PLC program has many parts in it, like logic, configuration, settings, etc.
So, it is necessary to test and validate each and every part, regardless of which program will malfunction. If there are bugs in configuration or logic, then the system will malfunction at the site.
To avoid this, most programmers spend hours testing the PLC logic and it is a very important step done by them. In this post, we will learn the testing and validation process of PLC development.
Some of the points related to PLC testing and validation are mentioned below.
The above steps provide a simple guide for preparing testing and validation in a new PLC development project.
Let us have a look at some important steps one by one which need to be followed till the end for proper functioning.
PLC functions according to inputs and outputs. So, the very first step in testing the program is checking IO mapping in PLC. You have to either force each digital input on the simulation or give actual hardware input to the PLC.
One by one, if the mapping is proper, then the same will be reflected in your graphics and program. This verifies digital inputs. Follow the same process for analog inputs. But, for analog inputs, you have to give multiple raw counts rather than just one count.
A wide range of counts gives you a proper idea of whether the channel is functioning properly or not. Then, for digital outputs, you have to force them one by one. If the PLC outputs are turning on and off according to the sequence, then your DO mapping is proper.
Follow the same process for analog outputs and give a wide range of raw counts instead of a single count. You will get a proper idea of whether the AO channel is functioning properly or not.
Once the IOs have been verified in the PLC program, the next step is to check communication and network addresses. Suppose a PLC has an Ethernet port and a Modbus RTU port. You have connected an HMI to the Ethernet port and three VFDs to the Modbus port. Modbus port is being used to transfer and receive data with VFD, like current, frequency, voltage, etc.
These mappings have been done by you in the PLC logic. You have to first check the Ethernet port by testing whether the IP is pinging or not; and whether it is communicating with HMI or not. Then, you have to establish Modbus communication and check whether the data is being communicated properly with PLC or not.
This clears your hardware part completely because you can now communicate data properly to the field; either through hard IO or soft IO. These two basic steps are the first step in your validation.
Some systems have manual mode without critical interlocks and some systems have manual mode with critical interlocks. To check manual mode, you have to turn on each and every output step by step and check whether the actual physical output is turning on or not.
This first step verifies whether the physical DO or AO has been properly linked with manual mode buttons or not. Then, you have to generate corresponding alarms and check whether the output is turning off or not.
Only the linked outputs should turn off. The remaining outputs should not be impacted. This ensures that all the outputs have been linked properly in the program with proper interlocks. Because, once the manual mode has been cleared, it will be confirmed that the physical outputs can be turned on manually and auto mode can then be checked more easily.
Directly checking auto mode logic will confuse the programmer as to what output is getting on or off. Verifying manual mode will ensure that individual outputs are being turned on or not.
The next big step is checking auto mode. The auto mode comes as a verifying sequence with interlocks. Every system is given a control logic document that shows how the sequence works with proper interlocks and output matrix.
The PLC programmer has to ensure that when any sequence is running, the corresponding outputs are turning on / off accordingly or not. Also, the sequence is running properly with interlocks or not?
Auto mode logic when made must be mostly divided into four parts
This technique makes the program flow very easy to look at and troubleshoot.
One should try to avoid the constant use of set–reset coils and ladder logic. Ladder logic is easy to use, but when run in simulation online, it consumes time to troubleshoot.
Also, set-reset coils are difficult to manage, because once a set coil is used, then care must be taken to reset it somewhere. Otherwise, the bit will remain set if the condition is not written properly.
In this way, we saw the testing and validation process in PLC development.
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