PLCs play a vital role in production. To maximize the effectiveness of a PLC, its capacity must be fully considered during the design process, and the most appropriate selection should be made based on the actual situation.
The capacity of a PLC includes two aspects: the number of I/O points and the user storage capacity.
(a) Selection of I/O Points
The average price of PLC I/O points is still relatively high. Therefore, the number of I/O points of the PLC should be selected reasonably. While meeting the control requirements, the number of I/O points used should be minimized, but a certain margin must be left.
The number of I/O points is usually determined based on the actual needs of the input and output signals of the controlled object, plus a margin of 10% to 15%.
(II) Selection of Storage Capacity
The storage capacity required for a user program depends not only on the functions of the PLC system, but also on the method of function implementation and the programmer's skill level. An experienced programmer and a beginner might produce a program with a 25% difference in size when completing the same complex function. Therefore, beginners should allow for a larger margin when estimating storage capacity.
The number of I/O points of a PLC largely reflects the functional requirements of the PLC system. Therefore, based on the determined number of I/O points, the storage capacity can be estimated using the following formula, and then a margin of 20% to 30% can be added.
Storage capacity (bytes) = Number of digital I/O points × 10 + Number of analog I/O channels × 100
In addition, when choosing storage capacity, pay attention to the choice of memory type.
Selection steps and principles for PLCI/O modules
Generally, the price of I/O modules accounts for more than half of the price of a PLC. PLC I/O modules include digital I/O modules, analog I/O modules, and various special function modules. Different I/O modules have different circuits and functions, directly affecting the application range and price of the PLC; therefore, they should be selected according to actual needs.
Selection of digital I/O modules
1. Selection of digital input modules
Digital input modules are used to receive switching signals from field input devices, convert the signals into low-voltage signals acceptable to the PLC, and achieve electrical isolation between internal and external signals of the PLC. The following aspects should be considered when selecting a digital input module:
1) Type and voltage level of input signal
Digital input modules come in three types: DC input, AC input, and AC/DC input. The selection is primarily based on the input signal and surrounding environmental factors. DC input modules have shorter delay times and can be directly connected to electronic input devices such as proximity switches and photoelectric switches; AC input modules offer high reliability and are suitable for use in harsh environments with oil mist or dust.
The input signal voltage levels of digital input modules include: DC 5V, 12V, 24V, 48V, 60V, etc.; AC 110V, 220V, etc. The selection is mainly based on the distance between the field input device and the input module. 5V, 12V, and 24V are used for short-distance transmission; for example, a 5V input module should not be used more than 10 meters away. For longer distances, modules with higher input voltage levels should be selected.
2) Input wiring method
Digital input modules mainly have two wiring methods: point type and group type.
In a point-type digital input module, all input points share a common terminal (COM); while in a group-type digital input module, the input points are divided into several groups, each group (of input points) has a common terminal, and the groups are separated from each other. Group-type digital input modules are more expensive than point-type modules. If separation between input signals is not required, point-type modules are generally chosen.
3) Note the number of input points connected simultaneously.
When using high-density input modules (such as 32-point or 48-point modules), it should be considered that the number of points connected simultaneously by the module should generally not exceed 60% of the total number of input points.
4) Input threshold level
To improve system reliability, the input threshold level must be considered. A higher threshold level results in stronger anti-interference capabilities and a longer transmission distance; refer to the PLC manual for details.
2. Selection of digital output modules
A digital output module converts the low-voltage signals inside the PLC into switching signals to drive external output devices and achieves electrical isolation between signals inside and outside the PLC. The following aspects should be considered when selecting one:
1) Output method
The digital output module has three output methods: relay output, thyristor output, and transistor output.
Relay outputs are inexpensive and can be used to drive both AC and DC loads. They are also suitable for a wide range of voltages, have low on-state voltage drop, and are strong in withstanding instantaneous overvoltage and overcurrent. However, as a contact element, they have a slow operating speed (the contact operating frequency must not exceed 1Hz when driving inductive loads), short lifespan, and poor reliability, and are only suitable for applications where switching is infrequent.
For loads that require frequent switching, thyristor outputs or transistor outputs should be selected, as they are contactless components. However, thyristor outputs can only be used for AC loads, while transistor outputs can only be used for DC loads.
2) Output wiring method
The output modules mainly have two wiring methods: grouped and isolated.
Grouped outputs consist of several output points grouped together, with each group sharing a common terminal. These groups are isolated from each other and can be used to drive external output devices powered by different power supplies. Separate outputs, on the other hand, have each output point with its own common terminal, and the output points are isolated from each other. The choice depends primarily on the power supply type and voltage level of the PLC output devices. Generally, integrated PLCs offer both grouped and separate outputs.
3) Driving capability
The output current (driving capability) of the digital output module must be greater than the rated current of the external output device connected to the PLC. Users should select the output current of the module based on the actual current of the output device. If the actual current of the output device is large and the output module cannot drive it directly, an intermediate amplification stage can be added.
4) Note the number of output points that are connected simultaneously.
When selecting a digital output module, the number of output points that can be simultaneously activated should also be considered. The cumulative current of simultaneously activated output devices must be less than the current allowed through the common terminal. For example, a 220V/2A 8-point output module may have each output point capable of handling 2A of current, but the current allowed through the common terminal is not 16A (8×2A), and is usually much smaller. Generally speaking, the number of points activated simultaneously should not exceed 60% of the number of output points with the same common terminal.
5) The maximum output current is related to factors such as load type and ambient temperature.
The technical specifications of a digital output module are closely related to different load types, especially the maximum output current. Additionally, the maximum output current of a thyristor decreases as ambient temperature increases, which should also be considered in practical applications.
Selection of Analog I/O Modules
The main function of analog I/O modules is data conversion, and they connect to the PLC's internal bus. For safety, they also have electrical isolation. Analog input (A/D) modules convert continuous analog signals generated by sensors in the field into digital signals acceptable to the PLC. Analog output (D/A) modules convert digital signals from the PLC into analog signals for output. Typical analog I/O module ranges are -10V to +10V, 0 to +10V, 4 to 20mA, etc., and can be selected according to actual needs, while also considering factors such as resolution and conversion accuracy.
Some PLC manufacturers also offer special analog input modules that can be used to directly receive low-level signals (such as RTD, thermocouple, etc.).
Selection of special function modules
Currently, PLC manufacturers have successively launched a number of I/O modules with special functions, and some have even launched intelligent I/O modules with built-in CPUs, such as high-speed counters, cam simulators, position control modules, PID control modules, and communication modules.
