1 Introduction
Currently, the design, manufacturing processes, and production equipment for spring products in China are developing rapidly, and the performance of spring materials is also improving quickly. Consequently, the performance requirements for applied springs are becoming increasingly stringent. Therefore, the development of intelligent equipment capable of spring performance testing has become a necessity and an inevitable trend in the spring industry. In recent years, with the increasing quality awareness of spring manufacturers, more and more automated equipment, such as sensors, controllers, and intelligent instruments, are being applied to industrial production control. This design involves technologies from various disciplines, including mechanics, electronics, control, and pneumatics. The device consists of three parts: mechanical, electrical, and software, forming a typical mechatronics system. It can achieve precise positioning control of the CNC system and serial communication between the computer and PLC, and monitor the entire testing process in real time using expert control. It possesses intelligent functions, automatically recording and managing data, judging test results, and analyzing fault causes. The design and development of this product can greatly improve the efficiency and accuracy of spring performance testing.
2. Overview of Spring Performance Testing System
2.1 Composition of the detection system [1]
A spring has multiple parameters that need to be tested, such as load, stiffness, and flexibility. Among these, load is the most common test indicator. It indicates the load value generated by the spring under a specified deformation, which is used to control the power required by the automated machinery without generating excessive load. Therefore, this system sorts and tests batches of springs by detecting whether the force output of the springs meets the system requirements when compressed or stretched to a certain extent. The spring performance testing system consists of a host computer and a slave computer, including mechanical parts, electrical control parts, and computer software parts. The schematic diagram of the testing system is shown in Figure 1.
Figure 1 Schematic diagram of the spring detection system
2.2 Working Principle
The testing system can operate intelligently according to the settings. To achieve full automation of the spring performance testing process and meet intelligent control requirements, the operator needs to set parameters for the entire testing process through the host computer software before operation. These settings include spring type, spring compression cycles, stroke per compression, pass/fail criteria, limit settings, and more. The working principle is as follows: a pressure sensor collects the rebound force of the spring under compression and converts it into a 4-20mA current signal, which is then transmitted to the analog signal receiving module of the main controller PLC. The PLC performs A/D conversion, filtering, and rectification on the collected signal, and then communicates serially with the host computer, sending the processed signal to the computer's buffer. Upon receiving the control signal, the servo system controls the forward and reverse rotation, rotation angle, and rotation speed of the servo motor. The synchronous toothed belt transmits the motion back to the lead screw, causing the lead screw and spring seat to move up and down together, thus enabling the spring under test to achieve extension/retraction detection. The working principle of the entire testing process is shown in Figure 2.
Figure 2. Block diagram of the working principle of the spring detection device
The system has three operating modes: calibration, manual, and automatic. Calibration mode: Each time the system is powered on, the height between the test head and the base is calibrated using the set standard length range to ensure test accuracy. Manual mode: Spring parameters can be tested manually during equipment debugging or when the computer system malfunctions. Automatic mode: Based on different spring models, the extension force is automatically divided into N equal parts. During spring compression, at each compression point, the compression force, spring height, and compression amount are stored as a set of data in the corresponding PLC data register.
3 System Hardware Design
3.1 Mechanical Component Design
In the mechanical part, taking into account technical performance requirements, reliability requirements, safety requirements, and standardization requirements, the testing device adopts a dual-station operation design, which can simultaneously test two different types of springs. The mechanical structure design considers the ease of loading and unloading spring products onto the testing platform and features a user-friendly external size. For operation, to allow operators to operate from multiple angles, the control box is connected to the bed by a movable bogie, which can rotate 180 degrees. For safe operation, each spring to be tested is placed on a spring post with grooves to prevent side slippage, and four fixed rods around it form a protective railing to prevent outward bounce; one of these rods can be removed for loading and unloading. For mobility, the testing device is equipped with four braked pulleys for easy movement and fixation.
3.2 Electrical Component Design
The electrical control section of the testing device uses a Siemens S7-200 PLC as its control core. Each workstation is equipped with an independent pressure sensor, motor, servo system, conveyor belt, control buttons, etc., and can perform both automatic and manual control functions. The system mainly includes a computer system (host, monitor, printer), PLC basic unit, analog expansion unit, communication module, text display, magnetic scale, load sensor, etc. The PLC control port definitions are shown in Table 1.
Input port definition (I) | |
I0.0 1# Workbench Automatic/Manual Switching | 1 = Automatic, 0 = Manual |
I0.1 1# Workbench Cylinder Action Status Input | 1 = upward movement, 0 = downward movement |
I0.2 controls the upward movement of the #1 worktable motor. | 1 = Start, 0 = Stop |
I0.3 controls the downward movement of the #1 worktable motor. | 1 = Start, 0 = Stop |
I0.4 1# Workbench Upper Limit Indicator | 1 = Spring has reached its upper limit; 0 = Spring has not reached its upper limit. |
I0.5 1# Workbench Lower Limit Indicator | 1 = Spring has reached the lower limit; 0 = Spring has not reached the lower limit. |
I1.0 2# Workbench Automatic/Manual Switching | 1 = Automatic, 0 = Manual |
I1.1 2# Workbench Cylinder Action Status Input | 1 = upward movement, 0 = downward movement |
I1.2 controls the upward movement of the #2 worktable motor. | 1 = Start, 0 = Stop |
I1.3 controls the downward movement of the #2 worktable motor. | 1 = Start, 0 = Stop |
I1.4 2# Workbench Upper Limit Indicator | 1 = Spring has reached its upper limit; 0 = Spring has not reached its upper limit. |
I1.5 2# Workbench Lower Limit Indicator | 1 = Spring has reached the lower limit; 0 = Spring has not reached the lower limit. |
Output port definition (O) | |
Q0.0 1# Workbench servo position given | HSC0 pulse train output port |
Q0.1 2# Workbench servo position given | HSC3 pulse train output port |
Q0.2 1# Workbench servo orientation setting | 1 = Start, 0 = Stop |
Q0.3 2# Workbench servo orientation setting | 1 = Start, 0 = Stop |
Q0.4 1# Workbench servo enable setting | 1 = Spring has reached its upper limit; 0 = Spring has not reached its upper limit. |
Q0.5 2# Workbench Servo Enable Setting | 1 = Spring has reached the lower limit; 0 = Spring has not reached the lower limit. |
Q0.6 1# Workbench Cylinder Action Status Output | 1 = upward movement, 0 = downward movement |
Q0.7 2# Workbench Cylinder Action Status Output | 1 = upward movement, 0 = downward movement |
Receive buffer: VB100, size 10 bytes | Send buffer: VB200, size 39 bytes |
Table 1 PLC Input/Output Port Definitions
The positioning control of the PLC system is mainly manifested in the following ways: the PLC controls the pulse signals, forward/reverse signals, enable signals, etc. of the servo driver, so that the servo driver can accurately control the speed, angle, direction, etc. of the motor, and the motor drives the lead screw to move to complete the positioning control of the spring seat. In the positioning control process, the main factors affecting the positioning accuracy are: (1) a series of transmission mechanical errors involved in the positioning control process, including lead screw backlash, tension of synchronous toothed belt, etc. (2) the accuracy of the pulse signals sent by the PLC to the servo driver according to the actual situation.
In this detection device, a SIEMENS SS7-200 series PLC was selected. It features high-speed pulse output, generating high-speed pulses at the output terminal to drive the load for precise control. High-speed pulse output includes two methods: High-Speed Pulse Train Output (PT0) and Pulse Width Modulation (PWM). PT0 outputs a pulse train (50% duty cycle), with control over the pulse period and number. The pulse period ranges from 10 to 65535 µs or 2 to 65535 ms, and is 16-bit unsigned data; the number of pulses is represented by a double-word unsigned number, ranging from 1 to 4294967295. PWM outputs a continuous pulse train with an adjustable duty cycle, controlling both the pulse period and pulse width. The pulse period is the same as PT0, and the pulse width ranges from 0 to 65535 µs or 0 to 65535 ms. The PT0/PWM output is unaffected by the PLC scan cycle, thus meeting the system's precise positioning requirements.
3.3 Control Scheme Design
CNC systems can be categorized into open-loop and closed-loop systems based on the presence or absence of a position measurement feedback device in their feed servo system. Open-loop servo systems, lacking position feedback, are the simplest type of servo system in CNC. Their primary driving element is a power stepper motor. Command pulses from the PLC are amplified by the drive circuit and sent to the stepper motor. The motor's output shaft rotates through a certain angle, then moves up and down via a synchronous toothed belt and a lead screw nut, driving the lead screw and spring seat. The angle rotated by the stepper motor shaft is proportional to the number of command pulses, and the rotational speed is proportional to the frequency of the command pulses. Because there is no detection feedback device, errors in various parts of the system, such as the stepper motor's step distance error and mechanical system errors, are combined into the system's position error. Therefore, the accuracy is relatively low, and the speed is limited by the stepper motor's performance, exhibiting instability at low speeds and low torque at high speeds. However, open-loop systems are simple in structure, easy to control and adjust, and are generally used in light-load applications with minimal load variations and low precision requirements. They are frequently used in economical CNC machine tools and the retrofitting of ordinary machine tools. During the spring unwinding test, the required compression dimension accuracy error is 0.1mm, so an open-loop control method is adopted in the servo system control.
4 System Software Design
The software component of the testing device mainly refers to the design of the monitoring software and the lower-level PLC control station software. The functional block diagram of the upper-level monitoring system is shown in Figure 3.
Figure 3 Functional block diagram of the host computer monitoring system
This software was developed using VB6.0 programming software based on the spring testing process. It employs expert control to monitor the entire testing process in real time and features functions such as spring compression parameter setting, test data recording and querying, report printing, and simulated keyboard input. The main control interface of the testing device is shown in Figure 4.
Figure 4. Host computer main control interface
The monitoring software parses and processes the information received in the computer's buffer to perform real-time dynamic display, data recording, limit alarms, and fault diagnosis of the spring detection process. Simultaneously, it sends control information to the PLC to input high-frequency pulse signals and switching signals to the servo system. Serial communication between the PLC and the computer is achieved through the RS-485 serial port on the PLC controller and the RS232 serial port on the computer. The PLC control selects the appropriate operating mode via a "verification, manual, automatic" selector switch. The main program flowchart is shown in Figure 5.
Figure 5 Main Program Flowchart
5 Conclusion
This system boasts powerful functions, employing highly reliable industrial control computers and PLCs for control, achieving intelligent operation of the testing process. Its high degree of automation and reliability not only eliminates the influence of human factors in the original testing process, making the test results more accurate, but also significantly reduces the labor intensity of workers, saves operating time, and improves production efficiency. Since its implementation, the equipment has operated normally, requiring minimal maintenance and repair work, greatly reducing maintenance and repair costs. Furthermore, its user-friendly human-machine interface makes the entire system more intuitive and easy to operate, ensuring safe and reliable operation on-site, and possesses high promotional value in the spring industry.
References
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