Design and analysis of industrial sewing machine control system based on DSPIC

In recent years, my country's clothing industry has developed rapidly, and the performance of industrial sewing machines has been increasingly demanded. Industrial sewing machines with brushless DC motors as power units are occupying the original sewing machine market. Compared with clutch motors, brushless DC motors have the advantages of small size, good dynamic performance, and convenient control. Most of the new generation of industrial sewing machine control systems have photoelectric encoders as position feedback to calculate the position and speed of the motor. The advantage of this system is that it can accurately obtain the current position of the motor, but the disadvantage is that the cost is relatively high.

This paper takes the brushless DC motor as the control object and proposes a high-performance and low-cost industrial sewing machine control system that does not require a photoelectric encoder and uses Hall signals as feedback.

1 System Hardware Design

1.1 System performance indicators

The realization of most functions of industrial sewing machines ultimately depends on the servo motor control system, so the performance of the servo system is the key to affecting the performance of the controller. Its main performance index requirements are as follows:

(1) Quick start and stop. Because the start and stop time is related to the production efficiency of garment processing, sewing machine operators have high requirements for the start and stop time of the equipment, which should be within 200 ms and 120 ms.

(2) Accurate positioning. After the sewing process is completed, the needle needs to stop automatically, which requires accurate positioning of the needle. This affects whether operations such as thread cutting and thread pulling can be carried out smoothly. Generally, the positioning accuracy is required to be controlled within ±5°.

(3) The system starts and stops frequently, needs to work for more than 16 hours a day, and the working environment is very dusty, which places high demands on the reliability of the controller hardware circuit.

(4) Wide speed regulation range and high speed accuracy. It realizes stepless speed change, with a speed regulation range of 200 to 5,000 r/min and a speed control accuracy of <±5 r/min.

1.2 Hardware Design

In view of the performance indicators of the industrial sewing machine servo system, the hardware control unit adopts the 16-bit DSC control chip DSP IC33F newly launched by Microchip Company, which is mainly for motor control. It has a 16-bit CPU and a DSP core. In addition to common peripherals, the chip has a 6-channel motor-specific MCPWM controller. This device greatly simplifies the control software and external hardware for generating pulse width modulation (PWM) waveforms, and can generate complementary three-phase 6-channel PWM waveforms through programming. The dead time can be programmed to prevent the two power tubes on the same bridge arm from being directly connected and causing a short circuit. The chip has both a fast DSP computing engine and the interface driving capability of a PIC microcontroller, and can run up to 40 MIPS. The chip also has special functional units such as 8-channel PWM drive, orthogonal encoder interface and 12-bit ADC for three-phase motor drive control. The power circuit uses the intelligent power module IRAMS10UP60B as the main circuit. In view of the characteristics of industrial sewing machines with single control functions, few pin requirements, strong real-time performance and complex calculation of DC brushless motor drive, the above-mentioned DSC chip can be used to build a simple drive control system. The system requires few interfaces and few auxiliary lines. Therefore, the cost can be effectively reduced. The principle block diagram of the entire hardware system is shown in Figure 1:

As shown in Figure 1, the AC 220 V voltage provides DC power to the IPM module after rectification and filtering. The DSP determines the current position of the rotor and calculates the motor speed based on the captured Hall position signal. The output PWM turns on the corresponding MOS tube through the intelligent power module IRAMS10UP60, and the inverter generates a three-phase voltage to supply the brushless DC motor. The motor drives the head of the industrial sewing machine to work. The motor speed can be changed by adjusting the speed control box. The IPM module itself has a circuit for detecting over-temperature and over-current. If over-temperature and over-current are detected, the PWM waveform is immediately blocked and the fault signal is sent to the DSP for processing.

2 Control strategy and system software design

2.1 Control strategy of motor operation

PWM control is the control core of the brushless DC motor vector control system. The final implementation of any control method is almost completed by various PWM control methods. In this system, we use square wave control, which is based on the ideal flux circle of the symmetrical motor stator when the three-phase symmetrical sine wave voltage is supplied. The actual flux vector generated by the different switching modes of the three-phase inverter is used to approximate the reference flux circle to achieve high-performance control and improve the power supply voltage. Utilization efficiency Since this control method considers the inverter and the motor as a whole, the model structure is relatively simple and easy to implement digitally.

As shown in Figure 2, this is a typical three-phase voltage source inverter structure. The voltage output of the inverter bridge circuit is controlled by 6 switch signals. When a power transistor in the upper half of the inverter bridge is turned on, the corresponding power transistor in the lower half is turned off. The switching states of the three power transistors T1, T3, and T5 determine the size of the corresponding voltage output by the inverter.

In Figure 2, the Hall sensor outputs 6 states during one rotation of the motor rotor. The details are shown in Table 1.

Each state lasts for π/3 electrical degrees, so it is divided into 6 intervals in space. Correspondingly, the rotor rotates in the corresponding interval. In each cycle, each power tube conducts for π/3 electrical degrees. The stator winding conducts positively and negatively for each cycle: π/3 electrical degrees, and the positive and negative current intervals are: π/3 electrical degrees. At any time, the stator has two-phase windings energized and generates stator magnetomotive force. As the rotor position signal changes, the stator synthetic magnetomotive force rotates step by step with a step length of π/3. The rotor magnetic field rotates synchronously with the stator magnetomotive force.

2.2 Overall design of system software

Software design directly affects the overall performance of the system. According to the development process of DSP control system, the top-down design method mainly includes the following modules:

(1) System initialization module.

(2) The main program is a process of waiting for interrupts in a loop. Once an interrupt occurs, it will switch to executing the corresponding interrupt service program.

(3) Motor speed control module: The motor speed is adjusted according to the user settings and the control signal of the foot pedal combined with the improved integral separation PI algorithm.

(4) Fault handling procedure: The fault monitoring procedure is an important part of protecting the safety of the system. Once a fault occurs, the PWM output is immediately blocked and the protection of the intelligent power module is activated.

3 Experimental results and analysis

With the above system design and control strategy, we conducted actual tests on the circuit board. The test conditions were a DC bus voltage of 380 V, a PWM frequency of 10 kHz, a dead time of 2 μs, and a complementary working mode. Since PWM is a center-aligned mode, the dead time will be inserted at both ends of the effective duty cycle. Figure 3 shows the PWM waveform. The detailed diagram of the hardware test platform is omitted. From this, we can see the feasibility and stability of the entire system. Figure 4 shows the waveform of the motor's back electromotive force.

4 Conclusion

This paper designs and analyzes the control system of an industrial sewing machine, and shows that the system has a good control effect, and all performance indicators meet the design requirements. Since photoelectric encoders or Hall sensors are no longer used as feedback elements, the cost of the entire system is reduced, while reliability is improved, and it has a high cost-effectiveness. This design is not only suitable for industrial sewing machines, but also for other servo control systems.