With the widespread adoption of PLCs , the variety and types of PLC products are increasing, and their functions are becoming more comprehensive. There are dozens of series and hundreds of types of PLC products imported from countries such as the United States, Japan, and Germany, as well as domestically assembled or independently developed PLC products. Their structures, performance, capacity, instruction sets, programming methods, prices, etc., vary, and their applicable scenarios also differ. Therefore, the rational selection of PLC products plays a crucial role in improving the technical and economic objectives of PLC control systems. Generally speaking, products from various manufacturers are reliable; the selection of the model mainly refers to how to meet one's functional needs without wasting the machine's capacity. PLC selection mainly includes several aspects such as model selection, capacity selection, input/output module selection, and power supply module selection.
1. Programmable Controller (PCC) Control System I/O Point Count Calculation
The number of I/O points is an important indicator for determining the size of a programmable controller (PCC). Based on the total number of input and output signals of the controlled object, select a PCC with an appropriate design and allow for a 10%–15% I/O margin. After estimating the number of I/O points on the controlled object, a PCC with an appropriate number of points can be selected. If the goal is stand-alone automation or mechatronics, a small PCC can be chosen. If the control system is larger, with more input/output points and distributed controlled devices, a large or medium-sized PCC can be selected.
2. Memory estimation
The memory capacity required by a user program is affected by several factors: memory utilization; number of digital input/output points; number of analog input/output points; and the user's programming skill level.
(1) Memory Utilization: User-written programs are entered into the host computer via a programmer and finally stored in memory in machine language form. For the same program, different manufacturers require different amounts of memory to convert the program into machine language. We call the ratio of the number of nodes in a program segment to the number of memory words required to store the machine language represented by that program segment the memory utilization rate. High utilization brings benefits to users. For the same program, it can reduce memory usage, thus lowering memory investment. Furthermore, for the same program, it can shorten the scan cycle time, thereby improving system responsiveness.
(2) Number of Digital Input/Output Points: The total number of digital input/output points in a programmable controller is an important basis for calculating the required internal memory capacity. In a typical system, the ratio of digital inputs to digital outputs is 6:4. The empirical formula in this regard is given based on the total number of digital inputs and outputs.
Required memory words = Total number of digital inputs (inputs + outputs) * 10
(3) The total number of analog input and output points: A system with analog control needs to use digital transmission and operation function instructions. These function instructions have low memory utilization, so the memory occupied needs to be increased.
In systems with only analog inputs, analog signals typically require reading, digital filtering, transmission, and comparison operations. When both analog inputs and outputs exist, more complex operations are required, usually closed-loop control, necessitating larger memory requirements than in systems with only analog inputs. In analog signal processing, analog input, filtering, and output are often programmed as subroutines, significantly reducing memory usage, especially when there are many analog channels. The memory required for each channel is noticeably reduced. The following is a general empirical formula:
As long as analog input:
Memory word count = Analog signal points * 100
When analog inputs and outputs coexist:
Memory word count = Analog signal points * 200
The algorithm for these experience formulas is based on around 10 analog points. When the number of points is less than 10, the number of words in memory should be increased appropriately, and when the number of points is large, it can be decreased appropriately.
(4) Program quality: The quality of the program written by the user has a significant impact on the program length and execution time. For the same system, programs written by different users may have large differences in program length and execution time. Generally speaking, beginners should leave more memory allowance, while experienced programmers can leave less.
In summary, the following experience calculation formula is recommended:
Total memory words = (Number of digital input points + Number of digital output points) * 10 + Number of analog input points * 150. Then, a margin of 25% is considered based on the calculated number of memory words.
3. Responding Moment
For process control, the scan cycle and response time need to be carefully considered. The sequential scanning operation of the programmable controller (PLC) means that it cannot reliably receive input signals with a duration shorter than the scan cycle. For example, if the effective detection width of a product is 5cm and the product conveying speed is 50m per minute, in order to ensure that no product is missed during detection, the scan cycle of the PLC must not exceed the time distance of the product passing through the detection point by more than 60ms (T=5cm/50m/60s).
The system response time refers to the time interval between the occurrence of the input signal and the moment when the output signal state changes as a result. System response time = input filtering time + output filtering time + scan period.
4. The function and structure should be reasonable.
Standalone control typically uses a single programmable logic controller (PLC) to control one device, or one PLC to control several small devices, for example, to modify or improve the functionality of an existing system. Standalone control avoids communication issues between PLCs; however, it requires comprehensive functionality. A box-type PLC is preferable. If only on/off control is needed, models such as F1, F2, FX, GE-1, C-20, S5-101, TI100, and EX-40 are suitable. Additionally, domestically produced models such as CKY-40H, D-40, CF-40, PCZ-40, and ACMY-S256 are comparable to imported models.
If the controlled target shares both digital and analog signals, a programmable logic controller (PLC) with corresponding functions should be selected. Modular structures offer flexible system configurations and are easy to expand, but they are expensive and suitable for large and complex industrial environments.
5. Selection of Input/Output Modules
Programmable controller input modules detect and convert high-level signals from field devices (buttons, limit switches, proximity switches, etc.) into internal machine-level signals. Module types include DC 5, 12, 24, 48, and 60V; and AC 115V and 220V. The voltage level is selected based on the distance between the field device and the module. Generally, 5, 12, and 24V are considered low-level, and the transmission distance should not be too long. For example, a 5V input module should not be used more than 10 meters away. In other words, for devices at greater distances, using a higher voltage module is more reliable. Other high-density input modules, such as 32-point and 64-point modules, have a simultaneous active point count depending on the input voltage and ambient temperature. Generally, the simultaneous active point count should not exceed 60%. To improve system stability, it is necessary to consider the threshold level (the difference between the on and off levels). A higher threshold level provides stronger anti-interference capabilities and allows for longer transmission distances.
The output module's task is to convert internal machine signal levels into control signals for external processes. For loads with frequent switching, high electrical instability, and low power factor, thyristor output modules are recommended, although their disadvantages include higher price and slightly lower overload capacity. Relay output modules offer advantages such as a wide applicable voltage range, low on-state voltage drop, and low price, but their disadvantages include shorter lifespan and slower response time. The cumulative current across all connected points of the output module must be less than the current allowed through the common terminal. The output module's current rating must be greater than the load current rating.
6. Structural considerations
PLCs are classified into two types: integrated and modular. The integrated structure places the PLC's I/O and CPU on a single large printed circuit board, eliminating the need for interfacing. This results in a compact structure, smaller size, and lower average cost per I/O point compared to modular PLCs. Therefore, small PLC control systems often adopt the integrated structure. Modular PLCs, on the other hand, offer greater flexibility and ease in expanding functionality, increasing or decreasing the number of I/O points, and adjusting the input-to-output ratio compared to integrated PLCs. During repairs, module replacement and troubleshooting are quick and convenient. Therefore, for more complex and demanding systems, the modular structure is generally preferred.
7. Requirements for user memory
PLCs typically use CMOS RAM as user memory, which features low static current consumption (1/A). To protect user programs and field data during power outages, lithium batteries are usually used as backup power.
If the process requirements of the controlled system remain unchanged, and the program has been debugged and is relatively perfect, it does not need to be modified frequently. In order to prevent others from arbitrarily modifying the control program, the user program can be solidified using EPROM (optional).
8. Is a communication network function required?
Most small PLCs are designed for stand-alone automation and generally lack interfaces for communication with a host computer. If the user requires the PLC to be integrated into a factory automation control network, a PLC with a communication interface should be selected. Generally, large and medium-sized PLCs have communication capabilities. In recent years, some high-performance small PLCs (such as FX, C40H, S5-100U, etc.) also have communication interfaces, allowing connection to a host computer or another PLC via an RS-232 serial interface. They can also connect to external devices such as printers and CRTs.
The above briefly introduces the basis for PLC selection and several issues that should be considered. Users should comprehensively consider various factors based on the needs of production practice and select products with a suitable performance-price ratio to fully satisfy the control requirements of the controlled object and give full play to the functions of the PLC.
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