I. Three Major Elements in PLC
There are essentially three types of large quantities in a PLC: digital signals, analog signals, and pulse signals. Once you understand the relationship between these three, you can master PLC operation.
The purpose of digital input control is to cause the PLC to generate corresponding digital outputs based on the current input combination and the historical input sequence, so that the system can operate in a certain sequence. Therefore, it is sometimes called sequence control. Sequence control is further divided into manual, semi-automatic, or automatic. The control principles used are distributed, centralized, and hybrid control.
PLCs evolved from relay control with the introduction of microprocessor technology, and can be conveniently and reliably used for on/off control. Because analog signals can be converted into digital signals, and digital signals are simply multi-bit switching signals, PLCs can reliably process and control the converted analog signals. Since continuous production processes often involve analog signals, analog control is sometimes also called process control.
Analog signals are mostly non-electrical quantities, while PLCs can only handle digital and electrical quantities. To convert between them, sensors are needed to convert analog signals into digital electrical quantities. If this electrical quantity is not standardized, a transmitter is required to convert it into a standardized electrical signal, such as 4-20mA, 1-5V, 0-10V, etc.
In addition, there should be an analog input unit (A/D) to convert these standard electrical signals into digital signals; and an analog output unit (D/A) to convert the digital quantities processed by the PLC into analog quantities—standard electrical signals.
Therefore, various calculations are required to standardize the conversion between electrical signals and digital quantities. This necessitates understanding the resolution of analog signal units and the standardized electrical signals. For example:
The PLC analog unit has a resolution of 1/32767, corresponding to a standard voltage of 0-10V, and the temperature value to be detected is 0-100℃. Therefore, 0-32767 corresponds to a temperature value of 0-100℃. Then, the digital value corresponding to 1℃ is calculated to be 327.67. To obtain a temperature value accurate to 0.1℃, simply divide 327.67 by 10.
Analog signal control includes: reactive control, feedforward control, proportional control, fuzzy control, etc. These are all digital signal calculation processes within the PLC.
The main purposes of PLC pulse quantity control are position control, motion control, and track control. For example, the application of pulse count in view control. If the stepper motor driver has a microstepping of 10,000 per revolution and requires the stepper motor to rotate 90 degrees, then the required pulse value is 10,000 / (360 / 90) = 2,500.
II. Calculation of Analog Quantities
The above is only a brief introduction. Different PLCs have different resolutions, and the measurement range of the physical quantity you are measuring may also differ. Therefore, the calculation results may vary.
Note: Wiring requirements for analog inputs
1. Use shielded twisted pair cable, but do not connect the shielding layer.
2. When an input is not in use, short-circuit the VIN and COM terminals.
3. Analog signal lines are isolated from power lines (AC power lines, high-voltage lines, etc.).
4. When there is interference on the power line, a filter should be installed between the input section and the power supply unit.
5. After confirming that the wiring is correct, power on the CPU unit first, and then power on the load.
6. When powering off, first disconnect the power supply to the load, and then disconnect the power supply to the CPU.
III. Calculation of Pulse Quantity
Pulse quantity control is widely used for angle control, interval control, and position control of stepper motors and servo motors. The following uses a stepper motor as an example to illustrate each control method.
The formula is: Number of viewpoint motion pulses = Total number of pulses per revolution * (Set viewpoint / 360°).
The formula is: Set interval pulse count = Set interval / [(roller diameter * 3.14) / Total pulse count per revolution]
The above is just a simple analysis of the control method of stepper motors, which may differ from reality. The operation of servo motors is the same as that of stepper motors, but the internal electronic gear ratio and reduction ratio of servo motors must be considered.
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