1. The switching quantity is also called the logical quantity, which means that there are only two values, 0 or 1, ON or OFF. It is the most commonly used control, and controlling it is the advantage of PLC, and it is also the most basic application of PLC.
The purpose of switching control is to make the PLC produce the corresponding switching output according to the current input combination of the switching quantity and the historical input sequence, so that the system can work in a certain order. Therefore, it is sometimes referred to as sequential control. Sequence control is divided into manual, semi-automatic or automatic. There are three types of control principles adopted: decentralized, centralized and mixed.
2. Analog quantities refer to some continuously changing physical quantities, such as voltage, current, pressure, velocity, flow, etc.
PLC is developed from the introduction of microprocessing technology in relay control, which can be easily and reliably used for switching control. Because the analog quantity can be converted into digital quantity, and the digital quantity is only a multi-bit switching quantity, the converted analog quantity can also be reliably processed and controlled by PLC. Since continuous production processes often have analog quantities, analog quantity control is sometimes referred to as process control.
The analog quantity is mostly non-electric quantity, while the PLC can only handle digital quantity and electric power. All the conversions between them need to be sensors, which convert analog quantities into digital quantities. If the amount of electricity is not standard, it must go through the transmitter to turn the non-standard amount into a standard electrical signal, such as 4-20mA, 1-5V, 0-10V and so on.
At the same time, there is an analog input unit (A/D) to convert these standard electrical signals into digital signals; Analog output unit (D/A) to convert the digital quantity processed by the PLC into an analog quantity - a standard electrical signal.
Therefore, the conversion between standard electrical signals and digital quantities requires various operations. This requires a clear understanding of the resolution of the analog unit as well as the standard electrical signal. For example:
The resolution of the PLC simulation unit is 1/32767, the corresponding standard electricity is 0-10V, and the temperature value to be detected is 0-100°C. Then 0-32767 corresponds to a temperature value of 0-100°C. The number corresponding to 1°C is then calculated to be 327.67. If you want to be accurate to 0.1°C, you can use 327.67/10.
Analog control includes: feedback control, feedforward control, proportional control, fuzzy control, etc. These are the calculation processes of the digital quantities inside the PLC.
3. The pulse quantity is a digital quantity whose value always changes alternately between 0 (low level) and 1 (high level). The number of times a pulse alternates per second is called frequency.
The purpose of PLC pulse quantity control is mainly position control, motion control, trajectory control, etc. For example, the application of the number of pulses in angle control. The subdivision of the stepper motor driver is 10,000 per revolution, which requires the stepper motor to rotate 90 degrees. Then the pulse value to be operated = 10000/(360/90) = 2500.
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