1 Introduction
Machine vision systems utilize machines to replace human vision for measurement and judgment. The basic principle is to convert the captured target into an image signal through an image acquisition device (either CMOS or CCD), which is then transmitted to a dedicated image processing system. Based on pixel distribution and information such as brightness and color, this signal is converted into a digital signal. The image system performs various calculations on these signals to extract the target's features, and then controls the on-site equipment based on the judgment results. Machine vision has a history of nearly 20 years since its inception. As a branch of automation technology, its functional characteristics have gradually improved and developed along with the overall development of industrial automation. In Europe and America, machine vision has been widely used in industry. In my country, with the improvement of supporting infrastructure and the accumulation of technology and capital, the demand for intelligent control using image and machine vision technologies has begun to emerge widely in various industries.
A domestic cemented carbide group company's profile plant is currently the largest bar production base in China. Its imported Sandvik extrusion press (D-120/250) can extrude various bars: 0.5mm and 1mm bars, double-hole 1mm, double-hole 1.5mm, and double-hole 2mm bars, 7×2.0, 6×2.5, 6×0.4, and 5.5×0.5 tubes, and 3×10 and 1.5×4 flats, etc. The performance of its speed control system is crucial to the quality of the bars; poor speed control can easily lead to breakage or bar stockpiling. Beijing Hollysys Electric Co., Ltd. has upgraded the extrusion press's speed control system using machine vision and a servo motor-based motion system, significantly improving its performance and thus enhancing the factory's product quality and output.
2 Hardware Introduction
The principles for system hardware configuration are as follows:
(1) The speed regulation system must be highly sensitive and have a wide speed regulation range.
(2) Simple and reliable operation.
(3) Facilitates system upgrades and expansions.
(4) It has a self-protection function in emergency situations.
(5) High performance-price ratio.
Based on the above overall considerations, the system hardware structure is shown in Figure 1, which mainly includes: an embedded image processing system with sub-pixel accuracy, an AC servo motor system with excellent fast response, a PLC control system that is flexible in programming and easy to upgrade, an industrial touch screen for easy viewing and modification of system parameters, and an overvoltage and overcurrent protection system.
3. Software Part
3.1 Control Principle
The main purpose of the speed control system is to stabilize the extruded bar within a certain area. Figure 2 shows a schematic diagram of the speed control system, which means controlling the edge points of the bar detected by the image processor within the equilibrium zone. This ensures that the bar can be extruded normally without breaking or piling up. It also guarantees the uniformity of the extruded bar.
The extrusion speed of an extruder is greatly affected by the strength of the hydraulic system, the amount of material loaded, the quality of the material, and the thickness of the bar. It is difficult to establish a linear model, and simple PID control is unlikely to achieve satisfactory control results. To address this, we employed three control methods to adjust the extruder's extrusion speed: fuzzy modeling of the extruder, differential control, and equilibrium zone control.
(1) First, through on-site speed adjustment tests of the extruder, we summarized its fuzzy model-parabolic model. The closer to the ideal position, the smaller the speed adjustment; the farther away from the ideal position, the larger the speed adjustment. Moreover, the speed adjustment is exponentially related to the difference from the ideal position, as shown in Figure 3.
(2) In the field experiment, the author found that the speed would change drastically in the event of a sudden situation, which seriously affected the stability of the system. The addition of a differential element improved the stability of the system. For example, the extrusion speed suddenly increased due to the hydraulic system. At this time, the system required acceleration, but according to its parabolic model, the acceleration was far from meeting the system's needs. At this time, the system would accumulate material. The addition of a differential element predicted this situation and accelerated the system by a factor of two, thereby improving the system's sensitivity and meeting the requirements.
(3) The author adopted equilibrium zone control. The equilibrium zone is defined as a certain range near the ideal position, and it is believed that the system meets the control requirements at this time. When the bar enters the equilibrium zone from the acceleration zone or deceleration zone, the author adjusts the speed in the opposite direction according to the change in the edge position. This can have a buffering effect, so that the bar can stay in the equilibrium zone in the shortest time. For example, when the edge point is at the upper limit of the equilibrium zone (see Figure 2), the speed control system will reduce the speed to make the edge point of the bar drop. When it enters the equilibrium zone, the equilibrium zone control starts to take effect. The edge position drops, and the speed value is increased according to the change in the edge position (the specific situation here is that when the edge position drops, the change is negative; when the edge position rises, the change is positive. At the same time, the magnitude of the change can reflect the distance of the position change). This prevents the speed from dropping too quickly and rushing through the equilibrium zone, thereby causing the system to oscillate.
3.2 Overall Design
The main function of this system is to follow the extrusion speed of the extruder, thereby ensuring the normal operation of the extruder. The software design is divided into three main parts:
(1) Machine vision component: This involves obtaining the position information of the extruded bar through an image processor;
(2) PLC control section: Write the control program according to the control principle in Section 3.1;
(3) Touch screen section: Allows users to easily view and modify system parameters and monitor system operation.
3.3 Visual Software Design
As shown in Figure 2, the position of the extruded bar is obtained by using the grayscale edge detection function of image software. The obtained position (digital value) is then transmitted to the PLC via serial communication at a baud rate of 115200bps.
3.4 PLC Software Design
The PLC software design is divided into two main parts: manual mode and automatic mode. In manual mode, the speed control system is operated manually and is not affected by the control laws described in Section 3.1. Speed parameters are modified via the touchscreen to change the operating speed. In automatic mode, the speed control system adjusts the speed according to the control principles described in Section 3.1. Users can freely switch between automatic and manual modes.
3.5 Touchscreen Interface Design
As shown in Figure 4, the interface design includes four main parts: manual, automatic, parameter viewing, and alarm.
(1) Manual: The interface for manual mode. When using manual mode, this interface displays the system's operating status and allows you to set the system's operating speed;
(2) Automatic: The interface for automatic mode. When using automatic mode, this interface displays the system's operating status and allows modification of the control parameters of the automatic control law;
(3) Parameter View: Allows you to view the values of various system operating parameters;
(4) Alarm: When the system is not running properly, an alarm message is given, and some faults can be cleared online.
4. Conclusion
This system achieves a perfect combination of motion and vision. It has been debugged and run at the user's site, and the on-site operation is stable, the operation is simple, and the speed control has reached the performance of actual production.
