When planning a PLC system, the control plan should be determined first, and the next step is PLC engineering design and selection. The characteristics of the process flow and the application requirements are the primary basis for design and selection. The PLC and related equipment should be integrated and standardized, selected according to the principles of easy integration with the industrial control system and easy expansion of its functions. The selected PLC should be a proven and reliable system with operational experience in the relevant industrial field. The PLC's system hardware, software configuration, and functions should be compatible with the equipment design and control requirements. Understanding programmable logic controllers, function tables, and related programming languages can help shorten programming time. Therefore, during engineering design, selection, and budgeting, the characteristics of the process flow and control requirements should be analyzed in detail, the control tasks and the required operations and actions should be clearly defined, and then, based on the control requirements, the number of input/output points, the required memory capacity, the functions of the PLC, the characteristics of external equipment, etc., should be budgeted. Finally, a PLC with a high performance-to-price ratio and a corresponding control system should be designed.
I. Budgeting for Input/Output (I/O) Points
When budgeting I/O points, an appropriate margin should be considered. Generally, based on the calculated number of input/output points, an additional 10% to 20% expansion allowance should be added.
After accounting for the margin, this is used as the budget data for input/output points. Constant pressure water supply equipment is widely used in enterprises, residential areas, and villages for production, daily life, and operational water supply. It is suitable for locations with fewer than 5000 households, a daily water supply volume of less than 3000 m³, and a water supply height of over 100 meters. When actually ordering, the number of input/output points also needs to be rounded up according to the product characteristics of the PLC from the manufacturer.
II. Memory Capacity Budget
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 planning phase, since the user application has not yet been written, the program capacity is unknown and can only be determined after program debugging. To allow for a budget for program capacity during the planning and selection process, a budget for memory capacity is generally used as a substitute.
There is no fixed formula for budgeting memory capacity. Many documents provide different formulas, but they 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 other margins are considered at 25% of this number.
III. Selection of Control Functions
This selection includes options for features such as computing power, control power, communication power, programming power, diagnostic power, and processing speed.
(I) Operational Function
The basic PLC functions include logic operations, timing, and counting. General PLC functions also include data shifting and comparison. More complex PLC functions include algebraic operations and data transmission. Large PLCs also include PID calculations for analog signals and other advanced functions. With the advent of open systems, PLCs now generally have communication capabilities. Some products can communicate with lower-level machines, some with peer or upper-level machines, and some can even communicate with factory or enterprise networks. When designing and selecting a PLC, one should start with the requirements of practical application and rationally select the necessary computational functions. In most applications, only logic operations and timing/counting functions are needed. Some applications require data transmission and comparison. Algebraic operations, numerical conversions, and PID calculations are used only when used for analog signal detection and control. Displaying data requires decoding and encoding.
(II) Control Function
Control functions include PID control calculations, feedforward compensation control calculations, ratio control calculations, etc., and should be determined according to control requirements. PLCs are primarily 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 also used to complete the required control functions, improving the PLC's processing speed and saving memory capacity. Examples include PID control units, high-speed counters, analog units with speed compensation, and ASCII code conversion units.
(III) Communication Function
Large and medium-sized 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 required. The communication protocol should conform to ISO/IEEE communication standards and should be an open communication network.
The communication interfaces of a PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication ports, industrial Ethernet, and common DCS interfaces. The communication bus (including interface devices and cables) of large and medium-sized PLCs should be redundantly configured at a ratio of 1:1. The communication bus should conform to international standards, and the communication interval should meet the actual requirements of the equipment.
In the communication network of a PLC system, the communication speed of the upstream network should be greater than 1Mbps, and the communication load should not exceed 60%. The primary methods for PLC system communication networks are as follows: 1) A PC is the master station, and multiple PLCs of the same type are slave stations, forming a simple PLC network; 2) One PLC is the master station, and other PLCs of the same type are slave stations, forming a master-slave PLC network; 3) The PLC network is connected to a large DCS through a specific network interface as a subnet of the DCS; 4) Dedicated PLC network (dedicated PLC communication networks of various manufacturers).
To reduce the CPU's communication workload, communication processors with different communication functions (such as point-to-point, fieldbus, and industrial Ethernet) should be selected based on the practical requirements of the network configuration.
(iv) Programming Functions
Offline programming method: The PLC and programmer share a single CPU. During programming, the CPU only provides services to the programmer and does not control the field devices. After programming ends, the programmer switches to operating mode, where the CPU controls the field devices but cannot program. Offline programming reduces system costs, but it is inconvenient to use and debug. Online programming method: The CPU and programmer have their own CPUs. The host CPU handles field control and exchanges data with the programmer within a scan cycle. The programmer sends the online-compiled program or data to the host. In the next scan cycle, the host operates based on 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—Sentence List (IL) and Structured Text (ST). The selected programming language should conform to its standard (IEC 6113123) and should also support multiple programming language methods, such as C and Basic, to meet the control requirements of special control applications.
(V) Diagnostic Function
The diagnostic function of a PLC includes both hardware and software diagnostics. Hardware diagnostics identifies the location of hardware defects through logical judgments. Software diagnostics includes both internal and external diagnostics. Internal diagnostics involves diagnosing the PLC's internal functions and capabilities through software, while external diagnostics involves diagnosing the information exchange functions between the PLC's CPU and external input/output components.
The strength of a PLC's diagnostic capabilities directly affects the technical skills required of operation, control, and protection personnel, and also influences the average repair time.
(vi) Processing speed
The PLC uses a scanning method for operation. From a real-time perspective, the processing speed should be as fast as possible. If the signal duration is less than the scanning 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, PLC contacts are fast and efficient, with each binary instruction execution time approximately 0.2–0.4 ms, thus meeting the needs of applications requiring high control and fast response. The scan cycle should meet the following requirements: scan time for small PLCs should not exceed 0.5 ms/K; scan time for medium and large PLCs should not exceed 0.2 ms/K.
IV. Model Selection
(I) Types of PLCs
PLCs are classified into two types according to their structure: integrated type and modular type; according to their application environment: field device type and control room device type; and according to their CPU word length: 1-bit, 4-bit, 8-bit, 16-bit, 32-bit, 64-bit, etc. From an application perspective, the selection can generally be based on the control function or the number of input/output points.
A fixed number of I/O points in a standard PLC limits the user's choices, making it suitable for small control systems. Modular PLCs, on the other hand, offer a variety of I/O cards or plug-ins, allowing users to more effectively select and configure the I/O points for their control system. This provides convenient and flexible function expansion, and they are generally used in medium to large-scale control systems. Water supply equipment is a new type of environmentally friendly and energy-saving specialized equipment specifically developed to address situations where insufficient pressure prevents water from reaching users at the required depth or flow rate.
(II) Selection of Input/Output Modules
The selection of input/output modules should be considered in conjunction with the application requirements. For example, for input modules, application requirements such as signal level, signal transmission distance, signal isolation, and signal power supply method should be considered. For output modules, the type of output module to be selected should be considered. Generally, relay output modules have the characteristics of low price, wide operating voltage range, short lifespan, and long service life; thyristor output modules are suitable for applications with frequent switching and low power factor loads, but they are more expensive and have poor overload capacity. Output modules also include DC output, AC output, and analog output, which should be selected in conjunction with the application requirements.
Intelligent input/output modules can be selected appropriately according to application requirements to improve water flow control and reduce operating costs. For applications where water flow frequently changes, variable frequency water supply equipment uses pump speed regulation to achieve zero energy loss. Its energy-saving effect is far superior to other water supply methods, making it a recognized and highly effective energy-saving method.
Consider whether there is a need for rack expansion or long-distance I/O racks.
(III) Power Supply Selection
For PLC power supplies, in addition to being introduced into the PLC along with the equipment and planned and selected according to the product manual, a 220VAC power supply should generally be selected to match the domestic power grid voltage. For critical applications, a discontinuous power supply or a regulated power supply should be used.
Assuming the PLC has its own power supply, the supplied current should be checked to ensure it meets the operational requirements; otherwise, an external power supply should be planned. To prevent external high-voltage power from being introduced into the PLC due to misoperation, isolation of input and output signals is necessary. Sometimes, simple diodes or fuses can be used for isolation.
(iv) Memory Selection
Due to advancements in integrated circuit technology, memory prices have decreased. Therefore, to ensure the normal operation of projects, PLCs generally require at least 8KB of memory for 256 I/O points. For complex control functions, larger capacity and higher-level memory should be selected.
(V) Selection of Redundancy Functions
1. Redundancy of the control unit
(1) Important process units: CPU (including memory) and power supply should both be 1B1 redundant.
(2) When required, a hot standby redundant system composed of PLC hardware and hot standby software, a dual or triple redundant fault-tolerant system, etc. can also be selected.
2. Redundancy of I/O interface units
(1) Redundant I/O cards should be provided for the control loop.
(2) Redundant I/O cards can be equipped for important detection points. (3) Depending on the requirements, dual or triple I/O interface units can be selected for important I/O signals.
(vi) Economic considerations
When selecting a PLC, the price-performance ratio should be considered. When considering economic efficiency, factors such as scalability, operability, and return on investment should also be taken into account. A comprehensive comparison and overall assessment should be conducted to ultimately select the most satisfactory product.
The number of input/output points directly impacts the price. Each additional input/output card incurs a cost. As the number of points increases to a certain value, the corresponding memory capacity, rack space, motherboard, etc., must also increase. Therefore, increasing the number of points affects the selection of the CPU, memory capacity, and control function design. These factors should be fully considered during budgeting and selection to ensure a reasonable performance-to-price ratio for the entire control system.
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