Abstract: This design presents a comprehensive control system for an asynchronous motor based on a programmable logic controller ( PLC ) hardware platform. The system uses a PLC to control a frequency converter , ultimately achieving closed-loop control of the asynchronous motor's speed. The results are displayed intuitively on an HMI panel. Specifically, the design uses P and I inputs to regulate the motor speed. After multiple parameter setting comparisons, the system achieves satisfactory control performance, with a maximum overshoot of only slightly more than two degrees, and stabilizes within ±10 r/s. The speed can quickly reach the set speed after a load change.
Keywords: asynchronous motor; speed control; PLC; PID
Foreword
A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. Generally, a device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency is called a "frequency converter." The uses of frequency converters are: to change the power supply voltage by changing the frequency of the power supply, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. Using frequency converters can improve labor productivity, product quality, equipment automation, quality of life, and the living environment; secondly, it can save energy and reduce production costs. Users choose different types of frequency converters according to their actual process requirements and application scenarios. The advantages of frequency converters in speed control systems are: 1. Smooth speed regulation and a wide speed range. Through the control of the controller, the output frequency of the frequency converter can be continuously adjusted to achieve stepless speed regulation, resulting in low starting current, low dynamic load, and smooth speed regulation without impact. 2. High speed regulation accuracy. Motors have a hard external characteristic when operating under natural conditions, and their speed changes little with the load. III. Excellent Dynamic Quality. It minimizes the time required for starting, braking, reversing, and speed regulation of the hoist, resulting in excellent dynamic quality. IV. Easy Motor Reversal. Reversing can be achieved once the frequency drops to zero, and reversal can be achieved by changing the phase sequence using a controller, thus allowing for a smooth transition within the four quadrants. V. Significant Energy Savings. Variable frequency speed control saves 20% to 40% of energy compared to speed control methods using series resistance in the rotor circuit. This design uses a Siemens MM420 frequency converter.
A Programmable Logic Controller (PLC) is an industrial control computer that integrates computer, automatic control, and communication technologies. It features strong anti-interference capabilities, low cost, high reliability, simple programming, and ease of learning and use, making it popular among engineering operators in the industrial field. Therefore, PLCs are widely used in various areas of industrial control.
With the continuous development of variable frequency speed control technology, the performance of AC drive systems has improved dramatically. AC asynchronous motors, with their low cost and robust structure, are increasingly widely used. In many applications of AC drives, high requirements are placed on the speed control and positioning performance of the motor. Asynchronous motors have secured their place due to their unique advantages of high power and cost-effectiveness, but their speed control and positioning performance are not perfect and still need improvement. This experiment presents a comprehensive asynchronous motor control system based on a programmable logic controller (PLC) hardware platform. This system uses a PLC to control the frequency converter, ultimately achieving closed-loop control of the asynchronous motor's speed. The results are then displayed intuitively on the HMI panel.
1. System Hardware Structure Diagram
For controlling motor speed, an open-loop system has a simple structure, with the control voltage directly controlling the motor trigger circuit. However, the system has poor static characteristics, and the speed changes significantly with torque variations. A closed-loop system has a more complex structure, but better speed regulation performance, a stiffer system characteristic curve, and torque changes have almost no effect on the system speed disturbance. In many industrial applications, it is necessary for the motor speed to accurately follow a given speed; therefore, a closed-loop control system is used. This design is based on a PLC-based frequency converter to implement a closed-loop speed regulation system.
The system mainly consists of three parts: a programmable logic controller (PLC), a frequency converter, and a motor. First, a given input is set for the PLC, then the PLC controls the frequency converter, which in turn controls the motor. Subsequently, the motor speed is fed back to the PLC, compared, and then output to the frequency converter, thus achieving zero steady-state error speed regulation.
Hardware structure block diagram of a closed-loop speed control system:
PID controller structure diagram:
PID controllers, with their simple structure, high stability, reliable operation, and convenient adjustment, have become an irreplaceable key technology in industrial process control. PID control consists of three parts: proportional, integral, and derivative.
Proportional adjustment responds to system deviations proportionally. Once a deviation occurs, proportional adjustment immediately takes effect to reduce it. A large proportional adjustment can speed up the settling time; however, an excessively large proportional adjustment can decrease system stability and even cause instability.
Integral control aims to eliminate steady-state errors and improve system accuracy. Because of the presence of error, integral control is implemented until the error is eliminated. The strength of the integral action depends on the integral time constant Ti; the smaller Ti is, the stronger the integral action, and vice versa. Adding integral control can slow down the system's dynamic response. Integral action is often combined with two other control laws to form a PI controller or a PID controller. The PI controller is the most commonly used controller.
The derivative control action responds to the rate of change of the system deviation signal, possessing predictive power and the ability to foresee the trend of deviation changes. Therefore, it can produce proactive control action, eliminating the deviation before it even materializes. This improves the system's dynamic performance. With appropriate selection of the derivative time, overshoot and settling time can be reduced. However, the derivative action amplifies noise interference; therefore, excessively strong derivative control is detrimental to the system's anti-interference capabilities.
Variable frequency speed control principle: The speed n of an asynchronous motor can be expressed as...
In the formula, n2 is the synchronous speed, Δn1 is the speed at which slip loss occurs, p is the number of pole pairs, s is the slip rate, and f is the frequency of the power supply. It can be seen that changing the power supply frequency can change both the synchronous speed and the motor speed.
A decrease in frequency leads to an increase in magnetic flux, causing magnetic circuit saturation, increased excitation current, decreased power factor, and overheating of the core and coils. This is clearly unacceptable. Therefore, it is necessary to reduce the voltage while reducing the frequency. This requires coordinated control of frequency and voltage. Furthermore, in many cases, to maintain the maximum torque of the motor during speed regulation, it is also necessary to maintain a constant magnetic flux. This is also achieved through coordinated control of frequency and voltage, hence the term variable frequency and variable voltage speed regulation, or simply variable frequency speed regulation. The device that implements variable frequency speed regulation is called a frequency converter. A frequency converter generally consists of a rectifier, filter, drive circuit, protection circuit, and controller (MCU/DSP).
2. Hardware wiring
System wiring diagram:
The motor unit consists of an asynchronous motor and a DC generator connected coaxially. For the squirrel-cage asynchronous motor, the stator winding is delta connected, and AZ, BX, and CY need to be short-circuited respectively. The separately excited DC generator is excited by the DC220V power supply of the experimental platform, and the armature winding is connected in series with a 0-1000 ohm variable resistor of the experimental platform.
3. Software Design
3.1 Touchscreen Configuration Screen
3.2 PID Wizard
1. Specify Loop Number: Specify which PID loop to configure. If the project contains existing PID configurations created using STEP7Micro/WIN3.2, you must select to edit one of the existing configurations or create a new configuration before continuing with step 1.
2. Set Loop Parameters: The loop setpoint is a parameter provided to the subroutine generated by the wizard. Specify how the loop setpoint (SP) should be calibrated. Select any real number for "Lower Range Limit" and "Higher Range Limit"; specify the following loop parameters: proportional gain, sampling time, integral time, and derivative time.
3. Loop Input and Output Options: The loop process variable (PV) is a parameter you specify for the subroutine generated by the wizard. It specifies how the loop process variable (PV) should be calibrated. Options include: unipolar (0 to 32000), bipolar (-32000 to 32000), and 20% offset (6400 to 32000, cannot be changed). The lower limit of the loop setpoint (SP) must correspond to the lower limit of the process variable (PV), and the upper limit of the loop setpoint must correspond to the upper limit of the process variable so that the PID algorithm can scale correctly. The upper limit calculated for this design is 23040.
4. Specify a storage area for the calculation: The PID instruction uses a 36-byte parameter table in the V storage area to store parameters used for control loop operation. PID calculation also requires a "temporary storage area" to store temporary results. The byte address of the V storage area at the beginning of this calculation area needs to be specified. Generally, a suggested address is sufficient.
5. Specify initialization subroutines and interrupt routines: If the project contains an active PID configuration, the names of existing interrupt routines are set to read-only. Because all configurations in the project share a common interrupt routine, any new configuration added to the project must not change the name of the common interrupt routine. The wizard specifies default names for initialization subroutines and interrupt routines. These default names can be edited.
6. Generate Code: This screen displays a list of POUs generated by the PID wizard and provides a brief explanation of how to integrate them into the user program.
7. After configuring the PID wizard as described above, click "Finish." The S7-200 instruction wizard will generate program code and data block code for the specified configuration. The subroutines and interrupt routines created by the wizard become part of the project. To enable this configuration in the program, call the subroutine from the main program block using SM0.0 during each scan cycle. This code configures PID0. The subroutine initializes the variables used by the PID control logic and starts the PID interrupt routine "PID_EXE." The PID interrupt routine is called cyclically according to the PID sampling time.
The code using the PID wizard method is as follows:
3.3 PID Instructions
Main program:
4. Conclusion
During system operation, there is a significant difference between the actual rotational speed and the calculated current rotational speed. To address this, the upper limit of the range in the loop input options needs to be changed to the calculated accurate value. Additionally, the maximum output voltage of the encoder should be measured and input into the program during the proportional calculation to ensure synchronization between the output and display. When adjusting the P and I parameters via the PID control panel, automatic adjustment can be used initially, followed by manual fine-tuning once the system stabilizes.
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