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Three Masses in Siemens PLC Programming
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Three Masses in Siemens PLC Programming I. Three Masses in PLC
There are only three large quantities in PLC: switching quantity, analog quantity and pulse quantity. Only by clarifying the relationship between the three, you can master the PLC skillfully.
1. Switching quantities are also called logical quantities, which refer to only two values, 0 or 1, ON or OFF. It is a common control. Controlling it is the advantage of PLC and the basic application of PLC.
The purpose of switching quantity control is to make PLC produce corresponding switching quantity output according to the current input combination and historical input order of switching quantity, so that the system can work in a certain order. Therefore, it is sometimes called sequential control. Sequential control can be divided into manual, semi-automatic or automatic. The control principles adopted include decentralized, centralized and mixed control.
2. Analog quantities refer to some continuously changing physical quantities, such as voltage, current, pressure, speed, flow, etc.
PLC is developed after relay control is introduced into microprocessing technology. It can be used for switching control conveniently and reliably. Because analog can be converted into digital, digital is only multi-bit switching, so the converted analog, PLC can also be completely reliable for processing and control. Analog control is sometimes called process control because there are often analog quantities in continuous production process.
Analog quantity is mostly non-electric quantity, while PLC can only deal with digital quantity and electric quantity. In order to realize the conversion between them, sensors are needed to convert analog quantities into digital quantities. If the power is not standard, the non-standard power will be transformed into standard electrical signals through transmitters, such as 4-20mA, 1-5V, 0-10V and so on.
At the same time, there must be analog input unit (A/D), which converts these standard electrical signals into digital signals, and analog output unit (D/A), which converts the digital quantity processed by PLC into analog quantity - standard electrical signal.
Therefore, the conversion between standard electrical signals and digital quantities requires a variety of operations. This requires a clear understanding of the resolution of analog units and standard electrical signals. For example:
The resolution of the PLC analog unit is 1/32767, the corresponding standard power is 0-10V, and the temperature value is 0-100 C. Then 0-32767 corresponds to the temperature of 0-100 C. Then the number corresponding to 1 C is 327.67. If you want the temperature ** to 0.1 C, 327.67/10 is enough.
Analog control includes feedback control, feedforward control, proportional control and fuzzy control. These are the calculation process of the internal digital quantity of PLC.
3. Pulse quantity is the digital quantity whose value always alternates between 0 (low level) and 1 (high level). The number of alternating pulses per second is called frequency.
The main purpose of PLC pulse control is position control, motion control, trajectory control and so on. For example, the application of pulse number in angle control. Stepper motor driver is divided into 10,000 cycles, requiring the stepper motor to rotate 90 degrees. Then the impulse value of the action is 10000/(360/90)=2500.
Ink Blooming Segmentation Line
2. Computation of analogue quantities
1. - 10-10V. - When the voltage is 10V-10V, it is converted to F448-0BB8he (-3000-3000) at 6000 resolution and E890-1770he (-6000-6000) at 12000 resolution.
2, 0-10V. When the voltage is 0-10V, it is converted to 0-1770he (0-6000) at 12000 resolution and 0-2EE0he (0-12000) at 12000 resolution.
3, 0-20 mA. The current of 0-20mA is converted to 0-1770he (0-6000) at 6000 resolution and 0-2EE0he (0-12000) at 12000 resolution.
4, 4-20 mA. The current of 4-20mA is converted to 0-1770he (0-6000) at 6000 resolution and 0-2EE0he (0-12000) at 12000 resolution.
Just for a brief introduction, different PLCs have different resolutions, and the range of physical quantities you measure is different. There may be some differences in the calculation results.
Note: Wiring requirements for analog input
1. Use shielded twisted pair, but do not connect shielding layer.
2. When an input is not used, shorten the V IN 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 supply line, a wave filter is installed between the input part and the power supply unit.
5. After confirming the correct connection, first power the CPU unit, and then power the load.
6. Cut off the power of the load first, and then cut off the power of the CPU.
Ink Blooming Segmentation Line
3. Calculation of Pulse Volume
Pulse control is mostly used for angle control, distance control and position control of stepper motor and servo motor. The following is an example of stepping motor to illustrate the various control methods.
1. Angle control of stepping motor. Firstly, the fraction of stepper motor should be defined, and then the total number of pulses needed for a turn of stepper motor should be determined. "Angle percentage = set angle / 360 degree (i.e. one circle)" and "Angle action pulse number = total number of pulses in one circle * angle percentage" were calculated.
The formula is: the number of angular action pulses = the total number of pulses in a circle* (set angle/360 degrees).
2. Distance control of stepping motor. Firstly, the total number of pulses needed to turn a step motor is defined. Then the diameter of the stepper motor rollers is determined and the circumference of the rollers is calculated. Calculate the running distance of each pulse. * The number of pulses to be operated at the set distance is calculated.
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