(Institute of Autonomous Navigation and Intelligent Control, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China)
Abstract: This paper introduces the meaning and advantages of electronic cams; it describes the DSP-based electronic cam control system and its implementation steps, and uses a three-servo packaging machine as an example to introduce the application of the electronic cam control algorithm; the design requirements were met through practical application, and good results were achieved.
Keywords: electronic cam, three-servo packaging machine, control system
Chinese Library Classification Number : TP302 Document Identification Code: B
The Research and Implementation of Electronic Cam of Three-servo Packaging Machine Based on DSP
Yu Lei, Wei Guang
(Autonomous Navigation and Intelligent Control Institute, Qingdao University of Science and Technology, Qingdao 266042, China)
Abstract: The paper introduces the meaning and advantages of the electronic cam and electronic cam control system and implementation steps based on DSP; Takes three-servo packaging machine for example to introduce the control algorithm of electronic cam; At last the machine meets the design requirement and achieves good result.
Keywords: Electronic cam, Three-servo Packaging machine, Control system
1 Introduction
Cam mechanisms are widely used in various automatic machinery and automatic control devices. They can convert rotary motion into desired intermittent linear reciprocating motion or reciprocating oscillation. Theoretically, as long as the cam profile curve is properly designed, the follower can achieve various desired motion laws, and the structure is simple and compact. However, cam mechanisms also have many limitations in practical applications:
(1) The cam mechanism is a high-pair point-line contact mechanism with large contact stress, which cannot transmit large power. In addition, the cam runs at high speed during operation, so the cam wears quickly. Long-term use will cause the motion of the follower to be distorted, so it cannot be used in situations with high repeatability requirements.
(2) A cam can only achieve a predetermined motion law, and the output motion lacks flexibility. When the motion law of the follower changes, the cam manufacturing process needs to be readjusted or completely replaced [1] .
(3) The mechanical structure is complex, the requirements for mechanical installation personnel are high, the production cost is high, and the maintenance and debugging are inconvenient.
(4) There is a lot of noise during the transmission of the cam mechanism.
2. Meaning and advantages of electronic cams
2.1 Meaning of Electronic Cam
With the advancement of science and technology and the emergence of digital servo technology, we can use electronic cams to replace traditional mechanical cams in transmission and control systems to achieve various complex reciprocating motions. Electronic cams are based on servo control technology and combined with advanced microprocessors to realize the function of simulating mechanical cams through digital systems [2] .
2.2 Advantages of Electronic Cams
(1) Electronic cam systems do not have the inertial force, elastic deformation, rigid impact and other factors of mechanical cams, so they have a fast response speed and can be used in high-speed motion transmission devices.
(2) The electronic cam does not wear and the cam curve shape will not change. Therefore, the follower can repeatedly achieve the expected motion with high precision and good stability [3] .
(3) The motion curve can be easily changed. Different motion curves can be achieved by simply changing the corresponding motion parameters, which greatly reduces production and installation costs.
(4) The transmission is smooth, with little mechanical vibration and low noise.
3. DSP-based Electronic Cam Control System
This paper uses the Motorola DSP (Digital Signal Processing) 56F807 chip to design a control system for an electronic cam, coordinating the processor with the servo driver and servo motor. This DSP chip can process 40×106 instructions per second at an 80MHz clock frequency. It has functional modules such as timer module, phase detection module, and PWM output, and has rich peripheral interfaces, which are sufficient to meet the various curve requirements of the electronic cam.
3.1 DSP Control of Electronic Cam
The electronic cam in this paper continuously reads the position of the spindle servo motor through the phase detection module of the DSP, and calculates the corresponding speed and position of the slave axis based on the position of the spindle to realize the correspondence between the spindle and slave axis cam curves. The function of the servo motor is to convert the electrical pulse signal into the corresponding angular displacement. That is, when one electrical pulse signal is given, the motor will rotate through one fixed angle (the fixed angle can be achieved by setting the servo motor axis resolution). Since the angular displacement of the servo motor is proportional to the input pulse, the speed of the servo motor can be adjusted by adjusting the frequency of the electrical pulse issued by the DSP [4] .
3.2 Implementation steps of electronic cam
(1) Determine the correspondence between the speed curves of the master and slave axes.
(2) Divide the displacement of the driven shaft into segments within one cycle, and determine the velocity of the driven shaft in each segment based on the correspondence between the master and slave shaft velocity curves.
(3) Convert each displacement into the number of pulses of the servo motor.
(4) Calculate the electrical pulse frequency required by the DSP based on the speed requirements of the slave axis.
(5) The electric pulse frequency calculated above can be used as the modulus output of the DSP's PWM module to control the speed of the servo motor and realize the electronic cam function.
4. Application of DSP-based electronic cam control system
This article uses a three-servo automatic packaging machine as an example to introduce the specific application of DSP-based electronic cams. Figure 1 shows the working principle diagram of the new three-servo automatic packaging machine.
As shown in the diagram, the machine's horizontal sealing and cutting roller, packaging film shaft, unloading conveyor belt, and feeding fork shaft are each powered by a separate servo motor. This machine integrates automatic feeding, packaging, sealing, and cutting, making it a highly efficient continuous packaging machine. The key and challenging aspects of its control lie in the synchronization of the position and speed of the three axes, as well as the realization of cam motion. Position synchronization requires that the cutting point of the horizontal seal must be within the color mark of the plastic film, ensuring that the horizontal seal does not cut the material. Speed synchronization requires that the cutting speed of the horizontal seal blade be equal to the speed of the plastic film and the material at that moment. Cam motion requires that, since the bag length is variable within a certain range, the distance traveled by the horizontal seal blade in one revolution is generally not equal to the bag length. This necessitates that the horizontal seal complete a cam motion process from synchronized speed to variable speed and then back to synchronized speed within a certain time.
Figure 1. Working principle diagram of a three-servo automatic packaging machine
With the packaging film shaft as the main axis and the horizontal sealing knife shaft as the secondary axis, their speed curves have three relationships based on the bag length and the circumference of the horizontal sealing knife:
(1) The circumference of the horizontal sealing knife is greater than the length of a bag.
(2) The circumference of the horizontal sealing knife is less than the length of one bag.
(3) The circumference of the horizontal sealing knife is equal to the length of a bag.
The following calculation uses an example where the circumference of the horizontal sealing knife is greater than the length of one bag.
The speed curve is shown in Figure 2: the speed synchronization segment is between t1 and t2; t3 is the moment when the horizontal sealing knife cuts.
Figure 2. Velocity-time diagram when the circumference of the horizontal sealing knife is greater than the length of a bag.
The calculated electrical pulse frequency is used as the modulus of the DSP's PWM module. By controlling the modulus, the speed of the servo motor is adjusted to realize the speed curves of the horizontal sealing knife shaft and the membrane shaft.
5. Conclusion
This paper introduces a DSP-based electronic cam control system and its implementation method, using a three-servo automatic packaging machine as an example to illustrate the specific implementation of the electronic cam algorithm. The use of electronic cams in the three-servo automatic packaging machine control system offers the following advantages:
(1) It greatly simplifies the mechanical structure of the packaging machine, with the transmission part directly driven by chain or synchronous belt, saving costs.
The mechanical cam structure was eliminated, reducing the weight of the machine and minimizing the mechanical components.
(2) The blade speed ratio can be set electronically to meet different needs.
(3) It can achieve the functions of preventing empty bags, preventing material cutting, and stopping the machine.
(4) The packaging machine is easier to operate. Users only need to set the bag length, cutting offset position and stop angle to start and adjust the speed.
References
[1] Wang Cheng, He Wei. Research on electronic cam system based on single-chip microcomputer [J]. Mechanical Design and Manufacturing, 2006(11): 4-6.
[2] Zhang Ge, Zou Huijun, Yao Yanan, Guo Weizhong. Concept and design of electronic cams [J]. Mechanical Design and Research, 2000 Supplement 89-91.
[3] Wang Anmin, Lu Hu: Implementation of Electronic Cam Based on C8051 Microcontroller [J]. Mechanical Transmission, 2010(10):85-87.
[4] Yu Lei, Chang Jun, Huang Cunzhu. Software design of automatic loading control system based on DSP [J]. Microcomputer & Application, 2010, (1).
About the author:
Yu Lei (1969-), male, Han nationality, associate professor, Qingdao University of Science and Technology, master's supervisor, research interests include visual robots and digital servo systems.
Wei Guang (1983-), male, Han nationality, Master's student at Qingdao University of Science and Technology, research interests include motion control systems and intelligent control.
Brief introduction of the author:
Yu Lei (1969-), male, Han nationality, Qingdao University of Science and Technology, master tutor, working in vision robot, figure servo system
Wei Guang (1983-), male, Han nationality, Qingdao University of Science and Technology, the master graduate student, working in motion control system, intelligence control
