1 System Composition
1.1 System Functions
The entire process of a certain type of twisting machine is driven by 5 motors .
According to the process requirements, the operating speed of the entire equipment can be adjusted as needed, and the speeds of each motor must maintain a certain ratio, i.e., the motors must operate synchronously. Therefore, the core function of the twisting machine control system is to control each motor in the equipment, ensuring that the speeds of each motor always maintain a certain ratio and operate synchronously according to the process requirements. At the same time, the system is also required to have functions such as logic control, process parameter setting, system debugging, finished product length measurement, equipment status monitoring, and fault alarm handling.
The implementation methods for the various functions required by the control system are as follows: a. Motor control. The starting, stopping, forward and reverse rotation, and dynamic speed adjustment of the five motors are all controlled by the PLC through digital communication with the frequency converters, thereby realizing motor control. The synchronous control of the motors is calculated by the PLC program based on the process parameters, and then the corresponding frequency converters are controlled separately.
b. Logic Control. The system's logic control is achieved by the PLC detecting various switch signals and sensor signals from the equipment. These signals are used as conditional contacts in the PLC program for start-up, stop, various faults, and working states. These contacts are then used to set the trigger conditions of the ladder diagram to realize logic control.
c. Process parameter settings. A touchscreen is configured as the system's human-machine interface, and the setting and display of various process parameters are all completed through the touchscreen.
d. Equipment status monitoring and fault display. Various status parameters are displayed on the touchscreen, including the real-time speed of the motor flywheel; and system fault information when equipment malfunctions.
1.2 System Structure
As shown in Figure 1, the control system of this twisting machine consists of the following parts:
a. Frequency converters. Five Siemens MICROMASTER 440 general-purpose frequency converters drive five motors. Each frequency converter is equipped with a PROFIBUS bus interface module. The MICROMASTER 440 is Siemens' fourth-generation high-performance vector frequency converter.
It has a variety of control methods and functions, and uses its low-speed, high-torque output to ensure the synchronization of the five motors when starting.
b. Controller. The controller used in the system is a Siemens E7300.
The PLC system configuration is as follows: The CPU is a CPU3152DP with a DP bus interface, supporting PROFIBUS and MPI network communication modes for communication with the frequency converter and touchscreen respectively; the power supply module is a PS307; two SM321 input modules with a total of 64 input points are selected based on site conditions; one 32-point output module SM322 outputs to an intermediate relay, which controls the motor contactor. A high-speed counting module FM350 is used to implement pulse meter counting for the wire length. The touchscreen is the cost-effective TP170B.
c. Fieldbus Network
The system employs PROFIBUS bus communication technology to connect the PLC and five frequency converters into a bus network. Based on the system's high-speed data transmission requirements, a master-slave PROFIBUS configuration is used. This optimized high-speed communication connection method is employed for high-speed data transmission in distributed control systems. Its topology is bus-type, with a communication rate of 1.5 Mbit/s. The PLC, acting as the master station, is the core of the control system, while each frequency converter acts as a slave station, controlling the operation of the variable frequency speed-regulating motors. The control system uses an LCD touchscreen as the human-machine interface. The touchscreen communicates with the PLC via an MPI bus to set process parameters.
2 System Software Design
The system uses Siemens' standard configuration and programming software STEP7 for configuration and software programming.
2.1 Hardware Configuration and Communication Settings
Based on the system composition, as shown in Figure 2, the system is configured with hardware: the data communication between the frequency converter and the bus network is completed through data defined as "Parameter Process Data Object" (PPO). The data structure of PPO consists of two parts: parameter area PKW and process data area PZD. In this system, the data communication structure between each frequency converter and the fieldbus network is defined as PPO type 1, that is, PKW is 4 words and PPO area is 2 words.
2. System Hardware Configuration Additionally, set the following main parameters on the inverter: P003=3 (parameter access level); P1080=0 (minimum frequency); P1082=110 (maximum frequency); P0700=6 (inverter command source: communication); P1000=6 (inverter operating frequency source: communication); P0719=50 (communication parameters); P0918=37 (inverter slave address number).
With the above settings, each frequency converter can establish communication with the PLC and exchange data. The communication method settings are shown in Figure 3.
2.2 Software Design
The system control software adopts a modular design. The program consists of a main program OB1, function modules (FC), and data modules (DB). The main program OB1 sequentially calls each function module (FC) to control the system. Data module DB1 contains the parameters required for system operation; additionally, five data modules DB3 and DB7 are created corresponding to five frequency converters, and the relevant parameters of the five frequency converters are set in DB3 and DB7.
During program execution, the system state is first determined, and then different actions are executed based on the different states. When controlling the motors, the given speed for each motor is first calculated based on the state, and the speed is converted into frequency form through data conversion. Then, the function modules FC40 and FC44 are called to control the five motors respectively. An important function is to implement communication operations on the bus, which is achieved in the program through the system function modules: Read Bus (SFC15) and Write Bus (SFC14).
2.3 Motor Synchronization Control
The core function of this control system is to ensure the synchronized operation of five motors. Specifically, during startup, shutdown, and speed adjustment, each motor drives its respective flywheel to rotate, and the rotational speeds of these flywheels must maintain a specific proportional relationship according to process requirements. Because the power of each motor in this twisting machine varies significantly—motor 1 is 18.5kW, motor 2 is 11kW, motor 3 is 5.5kW, and the other two are 1.5kW—simultaneous synchronization by setting identical start-up and stop-down times for each frequency converter is difficult to achieve, as demonstrated by field tests. This is mainly due to the different loads on each motor and the performance differences between the frequency converters. Therefore, this system proposes using small increments to increase or decrease the frequency values to achieve the synchronization requirement.
Taking the starting process of motor 1 and motor 2 as an example: If the required starting time is 30 seconds, and the flywheel speed of motor 1 is 4500 r/min and the flywheel speed of motor 2 is 10000 r/min in stable operation after starting, the corresponding AC voltage frequencies output by the inverters of motor 1 and motor 2 are 54 Hz and 100 Hz, respectively. Taking 1000 steps, the time corresponding to each step is 0.03 seconds.
The step size of motor 1 is 54Hz/1000=0.054Hz, and the step size of motor 2 is 0.1Hz. Every 0.03s, the flywheels of both motors increase by one step size simultaneously. The speeds of the flywheels of motor 1 and motor 2 increase by 4.5r/min and 10r/min per step, respectively. The frequency converter of each motor increases the frequency value of the output voltage simultaneously with small steps corresponding to the speed ratio. By gradually increasing the speed of the five motors simultaneously with equal steps and small steps, the operation synchronization during the startup process is achieved.
The same method is used for synchronous control during motor stopping.
3. Conclusion
The use of fieldbus technology for synchronous control of multiple motors offers advantages over traditional methods, including a clearer system structure, higher reliability, and greater efficiency. Field operation demonstrates that the system is stable, safe, and reliable, fully meeting the functional and accuracy requirements. The speeds of each motor are highly accurate and reliable, with excellent synchronization. No malfunctions due to industrial interference have been observed, fully illustrating the convenience of accurately controlling multiple motors using fieldbus technology and providing valuable insights for the control of similar equipment.
