Basic knowledge points of PLC programming

Calculation of switch quantity

Switching quantity, also known as logic quantity, has only two values, 0 or 1, ON or OFF. It is the most commonly used control. Controlling it is the advantage of PLC and also the most basic application of PLC.

The purpose of switch quantity control is to make the PLC generate corresponding switch quantity output according to the current input combination of the switch quantity and the historical input sequence, so that the system can work in a certain order. Therefore, the control of switch quantity is also called sequential control.

The advantages of sequential control are as follows:

First, it can distribute the program actions more clearly and concisely, making programming easier;

Secondly, when monitoring a program, you can clearly monitor the operation of the program, quickly find the fault point, and facilitate maintenance.

Calculation of analog quantities

Analog quantities are frequently used in industrial automation, and are generally used to detect and control digital quantities such as voltage, current, pressure, speed, temperature, and force. Advanced ones are also used in combination with PID. Since continuous production processes often involve analog quantities, analog quantity control is sometimes also called process control.

01. Characteristics of analog quantity

Analog quantities are mostly non-electrical quantities, while PLC can only process digital quantities and electrical quantities. Therefore, to achieve the conversion between them, a sensor is required to convert the analog quantity into a digital quantity. If the quantity is not standard, it must pass through a transmitter to convert the non-standard quantity into a standard electrical signal, such as 4-20mA, 1-5V, 0-10V, etc.

02. Calculation of analog quantity

1. -10—10V. When the voltage is -10V—10V, it is converted to F448—0BB8Hex (-3000—3000) at 6000 resolution; it is converted to E890—1770Hex (-6000—6000) at 12000 resolution.

2. 0-10V. When the voltage is 0-10V, it is converted to 0-1770Hex (0-6000) at a resolution of 12000; it is converted to 0-2EE0Hex (0-12000) at a resolution of 12000.

3. 0-20mA. When the current is 0-20mA, it is converted to 0-1770Hex (0-6000) at 6000 resolution; it is converted to 0-2EE0Hex (0-12000) at 12000 resolution.

4. 4-20mA: When the current is 4-20mA, it is converted to 0-1770Hex (0-6000) at 6000 resolution; it is converted to 0-2EE0Hex (0-12000) at 12000 resolution.

Of course, different PLCs have different resolutions, so we still have to set them according to actual conditions.

03. Requirements for analog input wiring

1. Use shielded twisted pair cables, but do not connect the shielding layer.

2. When an input is not used, short-circuit the V IN and COM terminals.

3. The analog signal line is isolated from the power line (AC power line, high-voltage line, etc.).

4. When there is interference on the power line, install a filter between the input part and the power unit.

5. After confirming the correct wiring, first power on the CPU unit and then power on the load.

6. When powering off, first cut off the power to the load, and then cut off the power to the CPU.

Calculation of pulse quantity 01. What is pulse quantity

Pulse quantity is a digital quantity whose value is always changing between 0 (low level) and 1 (high level). The number of pulse changes per second is called frequency.

02. Characteristics of pulse quantity

The control purpose of PLC pulse quantity is mainly position control, motion control, trajectory control, etc.

03. Pulse quantity control method

The control of pulse quantity is mostly used for angle control, distance control, position control, etc. of stepper motors and servo motors. Take servo motors as an example:

1. Basic control idea: The total amount of pulses determines the motor displacement, and the pulse frequency determines the motor speed. Pulses are used to control the servo motor. When you open the user manual of the servo motor, you will generally find a table like the following:

2. Servo motor pulse control implementation method:

(1) The driver receives two high-speed pulses (A and B), and the phase difference between the two pulses determines the direction of rotation of the motor.

(2) The driver still receives two high-speed pulses, but the two high-speed pulses do not exist at the same time. When one pulse is in the output state, the other must be in the invalid state.

(3) Only one pulse signal is needed to be given to the driver, and the forward and reverse running of the motor is determined by one direction IO signal.