The capacity of plc includes two aspects: I/O points and user storage capacity.
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(1) Choice of I/O points
The average I/O point of the PLC is still relatively high. Therefore, the number of I/O points of the PLC should be reasonably selected. The I/O points that strive to be used are the least, but must have a certain margin. .
Usually, the number of I/O points is determined according to the actual needs of the input and output signals of the controlled object, plus a margin of 10%~15%.
(II) Selection of storage capacity The storage capacity required by the user program is not only related to the function of the PLC system, but also related to the method of implementation of the function and the level of programming. An experienced programmer and a beginner, when completing the same complex function, the program size may be as much as 25%, so for beginners should be more margin in the storage capacity estimation.
The number of I/O points of the PLC reflects the functional requirements of the PLC system in a large program. Therefore, based on the determination of the number of I/O points, the storage capacity can be estimated by the following formula, and then 20%~30% is added. Margin.
Storage capacity (bytes) = number of switching I/O points × 10 + number of analog I/O channels × 100
In addition, while selecting the storage capacity, pay attention to the selection of the type of memory.
PLCI/O module selection steps and principles The price of general I/O modules accounts for more than half of the PLC price. PLC I/O modules include switching I/O modules, analog I/O modules, and various special function modules. Different I/O modules have different circuits and functions, which directly affect the application range and price of the PLC, and should be selected according to actual needs.
(1) Selection of switching I/O modules
1. Switching input module selection The switching input module is used to receive the switching signal of the field input device, convert the signal into a low voltage signal accepted by the PLC, and realize the electrical isolation of the internal and external signals of the PLC. The following should be considered when choosing:
1) Input signal type and voltage level The switch input module has three types: DC input, AC input and AC/DC input. The selection is mainly based on the on-site input signal and surrounding environmental factors. The DC input module has a short delay time and can be directly connected to electronic input devices such as proximity switches and photoelectric switches. The AC input module has good reliability and is suitable for use in harsh environments with oil mist and dust.
The voltage level of the input signal of the digital input module is: 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. Words V, 12V, and 24V are used for transmission distances. For example, the 5V input module must not exceed 10 meters. For modules that are farther away, modules with higher input voltage levels should be used.
2) Input wiring mode The switch input module mainly has two types of connection methods: sink point type and group type.
The sink-type digital input module shares a common terminal (COM) for all input points; the grouped digital input module divides the input points into groups, each group (several input points) has a common end, each The groups are separated. The price of the grouped switch input module is higher than that of the sink point type. If there is no need to separate the input signals, the sink type is generally used.
3) Pay attention to the number of input points that are connected at the same time. For the selection of high-density input modules (such as 32 points, 48 ​​points, etc.), it should be considered that the number of points simultaneously connected to the module should not exceed 60% of the input points.
4) Input threshold level In order to improve the reliability of the system, the input threshold level must be considered. The higher the threshold level, the stronger the anti-interference ability and the farther the transmission distance. For details, please refer to the PLC manual.
2. Switching output module selection The switching quantity output module converts the internal low voltage signal of the PLC into a switching signal for driving the external output device, and realizes the electrical isolation of the internal and external signals of the PLC. The following should be considered when choosing:
1) Output mode The switch output module has three modes: relay output, thyristor output and transistor output.
The relay output is inexpensive, can be used to drive both AC and DC loads, and has a wide voltage range and small on-state voltage drop, while being able to withstand transient overvoltages and overcurrents, but It belongs to the contact element, and the action speed is slow (when the inductive load is driven, the contact action frequency must not exceed 1HZ), the life is short, and the reliability is poor. It can only be applied to occasions with infrequent switching.
For loads that are frequently switched on, thyristor outputs or transistor outputs should be selected, which are non-contact components. However, the thyristor output can only be used for AC loads, while the transistor output can only be used for DC loads.
2) Output wiring mode The switching output module mainly has two types of wiring methods: grouped and separated.
The grouped output is a group of several output points, one group has a common end, each group is separated, which can be used to drive external output devices of different power sources respectively; the split output is one for each output point. On the common side, the output points are isolated from each other. The selection is mainly based on the power type and voltage level of the PLC output device. General integrated PLCs have both grouped and split outputs.
3) Drive capacity The output current (drive capability) of the switch output module must be greater than the rated current of the PLC external output device. The user should select the output current of the output module based on the current output of the actual output device. If the current of the actual output device is large, the output module cannot be directly driven, and the intermediate amplification step can be increased.
4) Pay attention to the number of output points that are simultaneously turned on. When selecting the switching output module, you should also consider the number of output points that can be turned on at the same time. At the same time, the accumulated current value of the output device must be smaller than the current value allowed by the common terminal, such as a 220V/2A 8-point output module, each output point can withstand 2A current, but the output common terminal allows the current to pass. Not 16A (8 x 2A), usually much smaller than this value. In general, the number of points that are simultaneously turned on should not exceed 60% of the number of points on the same common end.
5) The maximum current output is related to the load type, ambient temperature and other factors. The technical specifications of the switch output module are closely related to different load types, especially the maximum output current. In addition, the maximum output current of the thyristor will decrease as the ambient temperature increases, and should also be noted in actual use.
(II) Selection of analog I/O module The main function of the analog I/O module is data conversion, and is connected to the PLC internal bus, and also has electrical isolation function for safety. The analog input (A/D) module converts the continuous analog signal generated by the sensor detection into an acceptable digital quantity in the PLC; the analog output (D/A) module converts the digital quantity inside the PLC. Output for analog signals.
The range of typical analog I/O modules is -10V~+10V, 0~+10V, 4~20mA, etc., which can be selected according to actual needs, and factors such as resolution and conversion accuracy should also be considered.
Some PLC manufacturers also offer special analog input modules that can be used to directly receive low-level signals (such as RTDs, thermocouples, etc.).
(III) Selection of special function modules At present, PLC manufacturers have introduced some I/O modules with special functions, and some have also introduced intelligent I/O modules with their own CPUs, such as high-speed counters, cam simulators, Position control module, PID control module, communication module, etc.
