Properly Selecting PLC for Your Application
Choosing the right PLC platform for an application and ensuring that it meets the requirements of the process can often be a challenge. Whether it is a new process or an existing one have fundamentally the same selection process.
First, understanding the process, or machine, is essential and drawing a block diagram of the process will help identify the control devices and their locations. This will help in developing a PLC system design plan and calculating the level of I/O.
Inputs and Outputs
Input and outputs are the PLC’s bread and butter. They are integrated with the PLC circuit in order to provide vital information for monitoring and controlling the process. Outputs to actuators allow a PLC to cause something to happen in a process. Output actuators include the following:
- Solenoid Valves
- Motor Starters
- Servo Motors
Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters.
Inputs come from sensors that translate physical data into electrical signals. Typical input sensors include:
- Proximity Sensors
Inputs for PLC come from a few basic varieties, the simplest are AC and DC inputs. In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type of card.
Output modules rarely supply power, but instead act as switches. External power supplies are connected to output cards that switch the power on or off for each output. These cards typically have 8 to 16 outputs of the same type, but can be purchased with different voltage ratings.
The I/O requirements are the very first consideration for electrical engineers when selecting PLCs to be used. The PLC system must have enough termination points to connect all signal and control lines for the process. These termination points must conform to the system specification regarding:
- Voltage levels and current loadings
- The number and type of I/O points required per module
- Controller isolation from target process
- Future needs of processing plant, expansion and spare I/O points
- I/O points power supply requirements
Understanding what inputs, outputs, and capacity requirements are needed for your system is the biggest part of finding the right PLC. The only thing left is essentially the price and type you are looking for. Let look at why documentation is just as important as selecting the right PLC.
PLC Documentation Systems
When considering a development tool for PLC systems proper documentation can go a long way and save time and money when quick error corrections and revisions are necessary. Documentation systems are computerized alternatives to producing PLC documentation by hand. The most basic of these systems involve software packages that include both programming and documentation tools. More sophisticated systems provide programming, documentation, and design aid for I/O wiring and other drawing documents.
The primary output of documentation systems is the control program. Other outputs include print date, program name, job number, PLC model, and cross-reference of inputs and outputs. An advantage of such listings is that it shows a single document with practically all information for the control program. This ability eliminates the need for several documents during troubleshooting.
The system also generates a variety of reports and documents. Address usage reports, for example, list real and internal I/O, and data table registers specifying the use of each location, as well as identifying used and unused addresses.
Advanced Developmental Tools
The most basic form of PLC documentation support system is the following:
- Program development/documentation of PLC families
- Upload and download PLC programs
- Off-line program editing
- Program rung and page numbering
- Labeling of all program instruction elements
- Ladder rung commenting
- I/O and memory address usage reports
- Cross-reference listings of I/O and memory addresses
- Fully annotated program listings
These systems are designed primarily to provide commenting and cross referencing of the control program. Or a more sophisticated system provides design/documentation systems that serve as word and graphic processors for program development and documentation. The advantage of this type of sophisticated system is that it offers complete design and development tools that help reduce overall design time. Here are some features of PLC design/documentation systems.
- Program development/documentation for different PLC models
- Full upload/downloading of programs and documentation
- Extensive text editing
- Generic addressing
- Address reassignment utilities
- Export documentation to standard database formats
- Program/address conversions between different PLC models
- I/O signal and power wiring diagrams
- PLC program software simulation and modeling
- File-to-file and file-to-PLC comparison
A complete development system, design/documentation system would typically include all of the following:
- Documented program printout
- Program title and version format
- Comments on each program rung or statement
- Symbolic addressing
- Symbolic address editor
- Descriptor on each program element
- Free format text description
- Forced I/O table
- CPU memory and I/O configuration
- Timer/Counter/Internal coil usage report
- I/O address usage/cross-referencing report
- Data table usage report
Generic addressing eliminates the need for specific PLC addressing allowing the design to progress without actual or valid addressing. Address reassignment allows actual addresses to be automatically substituted later. With the copy feature can provide the user the ability to re-apply the same logic several time over.
Typically, PLC design systems are based on software modules that can be purchased based on what you need at that time, instead of a huge bundle purchase of everything. Like a system configuration package would allow for various system components for a specific PLC model to be configured. For instance the I/O drawing package would construct power diagrams and the I/O device connections to the module , showing addresses and wiring numbers. Built-in libraries for I/O modules for different PLC models and of standard JIC device symbols would allow automatic symbol generation. Lastly, a software simulation package would allow the program to be tested and validated without the actual PLC. This definitely has genuine applications in industrial automation.
E3.Schematic is core to all E3.series modules and provides electrical engineers with an ease to use solution for designing and documenting electrical control systems including schematic diagrams, terminal plans, and of course programmable logic controllers. It is an object oriented architecture that provides an integrated approach to design, where is helps eliminate errors, improve quality, and reduce design time.
PLC data held in schematic is read into excel, where changes are applied and fed directly back into E3.Series. PLC functions, addresses and location information can be managed centrally and then updated with a simple easy to use interface. Additionally, the tool checks for duplicated addresses and allows users to export data in the required PLC format.
E3.Schematic manages all design data including affiliated documentation, such as bill of materials and connection lists or assembly instructions and datasheets. Its object oriented structure ensures manufacturing instructions always match the design data.
The new innovation from the creators of E3.Series have developed the E3.PLCBridge platform for easy-to-use bidirectional PLC data exchange in formats such as B&R Automation Studio, which is widely used in the machinery sector. B&R Automation Studio allows the controller, drive, communication, and visualization to be configured in one environment, reducing integration time and maintenance costs. With bidirectional transfer data, interaction between electrical design and software development teams are accelerated and data quality is improved.
The generation of user-defined reports for manufacturing is simple. Any data stored in E3.Series can be turned into any format of report, as predefined by the user. The reports either configure using E3.Series API or defined with E3.Reports or alternative report generators. Allowing users to keep detailed information of the processing lines.
End The End
It is important to consider the needs of the facility and the PLCs expected operations. Understanding the system and its limitation will serve better than expecting the solve all solution. Determining the right PLC for the right job is key and understanding the importance of proper PLC documentation will save you trouble down the road. Remember to properly research and prepare for specific applications.What do you currently use for your manufacturing or supply systems? DCS, PAC, or already using PLC? Let us know below.