Familiar with the three major elements of PLC

PLC programming algorithm - an overview

There is nothing more than three in the PLC: the amount of switching, the amount of analog, and the amount of pulse. Only by figuring out the relationship between the three, you will be able to master the PLC.

First, the switch quantity is also called logic quantity, which means 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 the most basic application of PLC.

The purpose of the switch quantity control is to make the PLC generate the 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, it is sometimes called sequential control.

The sequence control is divided into manual, semi-automatic or automatic. The control principles adopted are three types: dispersion, concentration and hybrid control.

Second, the analog quantity refers to some continuous changes in physical quantity, such as voltage, current, pressure, speed, flow and so on.

The PLC was developed after the introduction of microprocessor technology into the microprocessor. It can be conveniently and reliably used for switching control. Since the analog quantity can be converted into a digital quantity, the digital quantity is only a multi-bit switching quantity, so the PLC can also perform the processing control reliably with the converted analog quantity.

Since continuous production processes often have analog quantities, analog control is sometimes referred to as process control.

The analog quantity is mostly non-power, and the PLC can only process digital quantity and power. All the transitions between them are to have sensors that convert analog quantities into quantities. If this power is not standard, it must pass through the transmitter to change the non-standard power into a standard electrical signal, such as 4-20mA, 1-5V, 0-10V and so on.

At the same time, there must be an analog input unit (A/D) to convert these standard electrical signals into digital signals; an analog output unit (D/A) to convert the digital quantities processed by the PLC into analog quantities - 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 and the standard electrical signal. E.g:

The resolution of the PLC analog unit is 1/32767, the corresponding standard power 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. Then calculate the number corresponding to 1 ° C is 327.67. If you want to accurately measure the temperature to 0.1 °C, put 327.67/10.

Analog control includes: feedback control, feedforward control, proportional control, fuzzy control, etc. These are the calculation processes of the digital quantity inside the PLC.

Third, the pulse amount is a digital quantity whose value is constantly changing between 0 (low level) and 1 (high level). The number of times the pulse changes alternately per second is called the frequency.

The control purpose of PLC pulse quantity is mainly position control, motion control, and track control. For example: the application of pulse number in angle control. The stepping motor driver is subdivided by 10,000 per revolution and requires a stepper motor to rotate 90 degrees. Then the pulse value of the desired action = 10000 / (360 / 90) = 2500.

PLC programming algorithm - calculation of analog quantity

  1. -10-10V. -10V - 10V voltage is converted to F448-0BB8He x (-3000-3000) at 6000 resolution; E890-1770He x (-6000-6000) is converted to 12000 resolution.

  2. 0-10V. When the voltage of 0-10V is converted to 0-1770He x (0-6000) at 12000 resolution, it is converted to 0-2EE0He x (0-12000) at 12000 resolution.

  3. 0-20 mA. When the current is 0-20mA, it is converted to 0-1770He x (0-6000) at 6000 resolution; 0-2E0He x (0-12000) is converted to 12000 resolution.

  4. 4-20 mA. When the current is 4-20 mA, it is converted to 0-1770 He x (0-6000) at 6000 resolution; 0-2E0He x (0-12000) is converted at 12000 resolution.

The above is only a brief introduction. Different PLCs have different resolutions, and the range of physical quantities you measure is different. The calculation results may have some differences.

Note: Requirements for wiring for analog input

  1. Use shielded twisted pair, but do not connect shield.
    When an input is not in use, short the V IN and COM terminals.

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

  3. When there is interference on the power line, install a filter between the input section and the power supply unit.

  4. After confirming the correct wiring, first power up the CPU unit and then power up the load.

  5. When the power is off, first cut off the power of the load, and then cut off the power of the CPU.

PLC programming algorithm - calculation of pulse quantity

The pulse amount control is mostly used for angle control of stepping motor, servo motor, distance control, position control, and the like. The following is a stepping motor as an example to illustrate the various control methods.

First, the angle control of the stepper motor

First, we must clarify the number of subdivisions of the stepper motor, and then determine the total number of pulses required for the stepper motor to make one revolution. Calculate "angle percentage = set angle / 360 ° (ie one turn)" "number of angular action pulses = total number of pulses per circle * percentage of angle."

The formula is: the number of angular action pulses = the total number of pulses per circle * (set angle / 360 °).

Second, the distance control of stepper motor

First, the total number of pulses required for the stepping motor to make one revolution is determined. Then determine the diameter of the stepper motor roller and calculate the circumference of the roller. Calculate the running distance of each pulse. Finally calculate the number of pulses to be run at the set distance.

The formula is: set distance pulse number = set distance / [(roller diameter * 3.14) / total number of pulses per circle]

Third, the position control of stepper motor is the combination of angle control and distance control

The above is just a simple analysis of the control method of the stepper motor, which may differ from the actual ones, only for reference by colleagues.

The servo motor operates in the same way as the stepper motor, but considers the internal electronic gear ratio of the servo motor and the reduction ratio of the servo motor.

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