When designing a Programmable Logic Controller (PLC) system, the first step is to determine the control scheme. The next step is the selection and engineering design of the PLC. The characteristics of the process flow and application requirements are the main basis for the selection. The PLC and related equipment should be integrated and standardized, and the selection should be based on the principles of easy integration with the industrial control system and easy expansion of its functions. The selected PLC should be a mature and reliable system with operational experience in the relevant industrial field. The hardware, software configuration, and functions of the PLC system should be compatible with the scale of the device and the control requirements. Familiarity with PLCs, function charts, and related programming languages can help shorten programming time. Therefore, during the engineering design, selection, and estimation, the characteristics of the process and control requirements should be analyzed in detail, the control tasks and scope should be clarified, and the required operations and actions should be determined. Then, based on the control requirements, the number of input/output points, the required memory capacity, the functions of the PLC, and the characteristics of external equipment should be estimated. Finally, a PLC with a high performance-price ratio should be selected, and a corresponding control system should be designed.
I. Estimation of Input/Output (I/O) Points
When estimating the number of I/O points, an appropriate margin should be considered. Typically, the estimated number of I/O points is calculated by adding 10% to 20% for expansion. When actually ordering, the number of I/O points needs to be rounded up according to the product characteristics of the programmable logic controller from the manufacturer.
II. Estimation of Memory Capacity
Memory capacity refers to the size of the hardware storage units that the Programmable Logic Controller (PLC) itself can provide. Program capacity refers to the size of the storage units used by the user application within the memory; therefore, program capacity is smaller than memory capacity. During the design phase, since the user application program has not yet been written, the program capacity is unknown and can only be determined after program debugging. To allow for a certain estimation of program capacity during design and selection, an estimate of memory capacity is usually used as a substitute.
There is no fixed formula for estimating memory capacity. Many documents provide different formulas, which are generally based on 10 to 15 times the number of digital I/O points, plus 100 times the number of analog I/O points. This number is used as the total number of words in memory (16 bits is one word), and a margin of 25% is added.
III. Selection of Control Functions
This selection includes options for features such as computing capabilities, control capabilities, communication capabilities, programming capabilities, diagnostic capabilities, and processing speed.
1. Computational functions
Simple programmable logic controllers (PLCs) include logic operations, timing, and counting functions. Ordinary PLCs also include data shifting and comparison functions. More complex PLCs include algebraic operations and data transmission. Large PLCs also include analog PID calculations and other advanced functions. With the advent of open systems, most PLCs now have communication capabilities. Some products communicate with lower-level machines, some with peer or upper-level machines, and some even have data communication capabilities with factory or enterprise networks. When designing and selecting a PLC, the required functions should be chosen based on the actual application requirements. Most applications only require logic operations and timing/counting functions. Some applications require data transmission and comparison. Algebraic operations, numerical conversions, and PID calculations are used when used for analog signal detection and control. Decoding and encoding operations are required to display data.
2. Control Functions
Control functions include PID control calculations, feedforward compensation control calculations, ratio control calculations, etc., and should be determined according to control requirements. Programmable logic controllers (PLCs) are mainly used for sequential logic control; therefore, in most cases, single-loop or multi-loop controllers are used to handle analog quantity control. Sometimes, dedicated intelligent input/output units are used to complete the required control functions, improving the processing speed of the PLC and saving memory capacity. Examples include PID control units, high-speed counters, analog units with speed compensation, and ASCII code conversion units.
3. Communication Function
Large and medium-sized programmable logic controller (PLC) systems should support multiple fieldbuses and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when needed. Communication protocols should conform to ISO/IEEE communication standards and should be used on open communication networks.
The communication interfaces of a programmable logic controller (PLC) system should include serial and parallel communication interfaces, RIO communication ports, and commonly used DCS interfaces. The communication bus (including interface devices and cables) of large and medium-sized PLCs should be configured with 1:1 redundancy, the communication bus should conform to international standards, and the communication distance should meet the actual requirements of the device.
In the communication network of a programmable logic controller (PLC) system, the upstream network communication rate should be greater than 1 Mbps, and the communication load should not exceed 60%. The main forms of communication networks for PLC systems are as follows:
1) A PC is the master station, and multiple programmable logic controllers of the same model are slave stations, forming a simple programmable logic controller network;
2) One programmable logic controller (PLC) is the master station, and other PLCs of the same model are slave stations, forming a master-slave PLC network.
3) The programmable logic controller network is connected to a large DCS as a subnet of the DCS through a specific network interface;
4) Dedicated programmable logic controller network (the communication network of dedicated programmable logic controllers of various manufacturers).
To reduce the CPU's communication workload, communication processors with different communication functions (such as point-to-point, fieldbus, etc.) should be selected based on the actual needs of the network composition.
4. Programming Functions
Offline programming: The programmable logic controller (PLC) and the programmer share a single CPU. When the programmer is in programming mode, the CPU only provides services to the programmer and does not control the field devices. After programming is complete, the programmer switches to run mode, and the CPU controls the field devices but cannot program them. Offline programming reduces system costs, but it is inconvenient to use and debug. In-circuit programming: The CPU and the programmer have their own CPUs. The host CPU is responsible for field control and exchanges data with the programmer within a scan cycle. The programmer sends the online-compiled program or data to the host, and in the next scan cycle, the host runs according to the newly received program. This method is more expensive, but system debugging and operation are convenient, and it is commonly used in medium and large-sized PLCs.
Five standardized programming languages are required: three graphical languages—Sequential Function Chart (SFC), Ladder Diagram (LD), and Function Block Diagram (FBD)—and two text-based languages—Instruction List (IL) and Structured Text (ST). The selected programming language should comply with its standard (IEC 6113123) and should also support multiple programming language formats, such as C and Basic, to meet the control requirements of special control applications.
5. Diagnostic function
The diagnostic functions of a programmable logic controller (PLC) include both hardware and software diagnostics. Hardware diagnostics determines the location of hardware faults through logical judgments, while software diagnostics is divided into internal and external diagnostics. Internal diagnostics involves diagnosing the PLC's internal performance and functions through software, while external diagnostics involves diagnosing the information exchange functions between the PLC's CPU and external input/output components through software.
The strength of the diagnostic functions of a programmable logic controller (PLC) directly affects the technical skills required of operators and maintenance personnel, and also affects the mean time to repair (MTB).
6. Processing speed
Programmable logic controllers (PLCs) operate using a scanning method. From a real-time perspective, the processing speed should be as fast as possible. If the signal duration is shorter than the scan time, the PLC will not be able to scan the signal, resulting in the loss of signal data.
Processing speed is related to the length of the user program, CPU processing speed, and software quality. Currently, programmable logic controllers (PLCs) have fast response times and high speeds, with each binary instruction execution time being approximately 0.2–0.4 ms, thus meeting the needs of applications requiring high control and fast response. The scan cycle (processor scan cycle) should meet the following requirements: the scan time for small PLCs should not exceed 0.5 ms/K; the scan time for medium and large PLCs should not exceed 0.2 ms/K.
IV. Types of Programmable Logic Controllers
Programmable logic controllers (PLCs) are classified into two types according to their structure: integrated and modular; and according to their application environment: field-installed and control room-installed. They are also classified according to their CPU word length: 1-bit, 4-bit, 8-bit, 16-bit, 32-bit, 64-bit, etc. From an application perspective, selection is usually based on control functions or the number of input/output points.
Integrated programmable logic controllers (PLCs) have a fixed number of I/O points, limiting user options and making them suitable for small control systems. Modular PLCs, on the other hand, offer a variety of I/O cards or plug-ins, allowing users to select and configure the I/O points of their control systems more efficiently. They also provide convenient and flexible feature expansion and are generally used in medium to large-scale control systems.
