Nevertheless, an audit of the PLC control system integrator or system supplier is required to determine the level of quality and structural testing built into the standard product and to examine system quality. A good CSDS will provide the end-user a certain level of confidence about the justification expectation. The CSDS shall cover the design standard on instruments; device controls system controls, structure or software, standards approach or programming and the like.
Another area of CSDS includes the software structure and general programming standard. The importance of producing a CSDS is clear when a programmer has to reverse engineer the control functions of a system only from the several hundred lines of source code found in the controller itself. Certainly, anyone who has attempted such a task will agree that it is a frustrating exercise and difficult to completely understand the concepts behind the code. However, if the end-user knows that the system integrator has SOPs for software development methodology, and a justification programme, he will feel confident about the ability of the product to achieve its design requirements. As such, the PLC software structure layout should follow process order and data process. For example, the main tree in a code shows the order of data flow and the process area order as well. Regarding the detail code development, the IEC 61131-3 standard should serve as a reference. In practice, a good rule of the thumb is the development of reusable code wherever possible since this reduces large amounts of system programming and simplifies troubleshooting and justification effort.
Programming tools for justification
The following tools are used for programming a PLC in IEC 61131-3:
- Function Block Diagram (PBD) is a graphical language using defined function blocks from the system library or user-defined function blocks from the system library or user-defined library and writing them together on the screen to accomplish certain functions. There are two types of FBDs, where the predefined FBDs are developed by the programmer to accomplish a certain project- oriented function
- Ladder Diagram (LD) is the traditional method of representing logical equations and simple actions as if these are relay switches. LD and FBD may be freely mixed
- Structured Text (ST) is a high-level structured text language similar to Pascal but more intuitive to the automation engineer
- Instruction List (IL) is a low level language that is highly effective for smaller applications or optimizing parts or an application
Plan of PLC application software
PLC application softwares strategy depends on the size of the target process and its complexity. The strategy referred to here is not for simple machine control with few devices, but rather a large size application with various functional process areas. First, all devices or control components are stand-alone based on their function. A typical classification in a plant comprises control valves, solenoid valves (2-way/3-way), On/Off valve, constant pump/motor variable speed pump/motor, variable speed pump/motor, conveyer (single direction/bi-direction) and others. The ladder logic should serve to programme the basic control function for each classified device, and this D should be packaged as a generic user-defined FBD named after each type of device. Also in FBD, there should be one separate section for mode control, programmed by using either LD or FBD.
The next step is to classify the process into functional areas. There is also a good time to define the software structure. Once process classification into areas/units/cells takes place, the programming on the process level may tart. The programming approach depends on the nature of the functions. If sequential controls are heavily involved, FC can be applied, otherwise FBD should be considered as a general way because it provides the programmer - who is not necessary knowledgeable about the process - a clear way to trace process, troubleshooting and more important justification.
Overall, the concept of using modular software blocks dominates application software development. The software modules/FBDs are the basic cell, and go together in a human readable hierarchy organisation. The work of troubleshooting, debugging, testing and justification can fit into different levels like device control level or process control level or system control level. This way a huge software development breaks down into modules and FBDs, sets up the foundation for the development, integration and testing. More important, it defines a clear path to software documentation.
Case study
Application of Micro PLCs in a packaging machine - The object passes through the machine and the weight is measured using weighing systems. After the scanner records necessary parameters like barcode, he object is transferred to the next stage by robotic controls. The speed of the drive is also controlled during the machine operation. The entire sequence of operations is controlled by the micro PLC.
The micro PLC can perform bit logic, compare, timer, counter, pulse output and simple math instructions. The micro PLCs are installed in the machine with some operator interface for indicating the reading of few parameters like low; pressure and temperature in case of pump controls. This enables the operator to take note of the readings for recording purpose as well as adjust the setpoint of critical parameters that affect the operation of the machine.
The logic for controlling the machine operations is written in the PLC using IEC 61131-3 standards. The programming steps are documented as per the requirements of the above standards to enable the operator to make changes, if required in the future. The programming approach depends on the nature of the functions. For example, for sequential controls, SFC should be applied, and the logic should be implemented; otherwise, FBD should be considered as a general way because it provides the programmer - who is not necessarily knowledgeable about the process - a clear way to trace process, troubleshooting and most important, justification.
Conclusion
Thus, using engineering or software development methodology that supports the requirements for qualification and compliance to GMP requirements, pharmaceutical manufacturers can achieve USFDA justification of automation systems. This will enable them to enter regulated markets.
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