1 Introduction
Hydraulic transmission systems in automobiles, aircraft, and construction machinery are subjected to vibration and impact under various operating conditions. With increasing demands for product reliability and the evolving needs of various industries, the shock and flexural strength of pipelines is receiving growing attention, making shock resistance a crucial indicator of quality and reliability. Furthermore, the rapid development of my country's automotive industry necessitates the use of hydraulic pulse testing equipment to assess the performance of hoses under diverse environmental and operational conditions.
The hydraulic pulse testing machine is used for pulse pressure life testing of hoses such as automotive brake lines, fuel lines, steering lines, coolant lines, radiator hoses, and heater hoses. This testing machine can conveniently and stably detect whether the hoses used in the equipment meet the requirements of the standards.
The hydraulic pulse testing machine control system is a two-level hybrid control system based on PLC. The lower-level machine uses Rockwell Automation's SLC500 as the core processor and a real-time controller, while the upper-level IPC uses a human-machine interface written in LabVIEW software, which is easy to operate and maintain. Connecting the upper and lower-level machines via Ethernet gives the pulse testing machine excellent real-time performance, strong anti-interference capabilities, and greater stability and reliability.
2. Test System Requirements
The testing system requires the test tubes to undergo life tests under different environmental conditions, ranging from -40 to 160°C and from 10 to 30 MPa. New test tubes are subjected to pressure impact and flexural tests until they burst to test whether the products meet the relevant standard requirements.
Hydraulic pulse testing machines primarily control pressure and pulse waveforms through a hydraulic servo system. Waveform error, pressure application method, response time, and accuracy directly affect the accuracy of the testing system; relevant standards require an absolute control error of 2% for the waveform. Since the acquired and processed data needs to be uploaded to the host computer in real time, the control system must have fast data transmission speed and strong anti-interference capabilities to ensure the testing system has excellent real-time waveform curves. During the test, the test sample tube will deform and expand, causing changes in pipe diameter and hydraulic servo system parameters. How the control system adjusts according to these variables presents a significant challenge in both hardware and software design.
3. Test methods
The pulse testing machine mainly uses two methods: pressure impact testing and flexural testing. The two methods are performed simultaneously, which effectively simulates the actual road conditions of a car driving under different environmental and working conditions.
3.1 Pressure shock
The pressure of the hydraulic pulse testing machine is controlled by the pulse waveform. The control waveforms are mainly divided into four types: trapezoidal wave, convex wave, sine wave, and square wave. The control system can select a predetermined pulse waveform from the host computer according to the test process requirements to achieve the test objective. The user can freely select the interval pressure relief according to the structure of the hydraulic servo system. For waveforms with interval pressure relief, such as trapezoidal waves and convex waves, the pressure relief valve is opened at the trough of the pulse to allow the medium to flow in the test sample tube. The specific requirements for the waveform are shown in Figure 1.
Figure 1 Trapezoidal wave curve requirements
(1) Trapezoidal wave (square wave, triangular wave). The trapezoidal wave (square wave, triangular wave) is designed according to the German standard TL82415. The pressure increase rate in the t1 segment is adjustable between 1 MPa/s and 300 MPa/s; the holding time in the t2 segment is adjustable between 0 and ns, where n is any value greater than 0; the t3 segment is basically symmetrical with the t1 segment; the holding time in the t4 segment is adjustable between 0 and ns, where n is any value greater than 0; pmin is controlled between 0 and 10% of pmax; pmax has different requirements depending on the pipe, but currently it will not exceed 60 MPa.
Figure 2 shows the waveform curve requirements for the convex character.
(2) Convex wave. Referring to Figure 2, the requirements for the convex wave are: the pressure increase rate in segment t1 is required to be 100 MPa/s, t1 = 2 s; the pressure holding time in segment t2 is required to be set from 0 to ns, where n is any value greater than 0, and is generally set to 2 s; segment t3 is required to be basically symmetrical with segment t1; the pressure holding time in segment t4 is required to be set from 0 to ns, where n is any value greater than 0, and is generally set to 4 s; pmin = 0; first pressure step = 0.35 MPa; maximum pressure = 1.05 MPa.
(3) Sine wave (half-sine or full sine). A sine wave is relatively difficult to achieve. In actual experiments, it is mainly simulated by generating cam waveforms using dedicated cam commands from a PLC. A full sine wave refers to a wave drawn in a closed state. The frequency requirement is to draw a complete sine wave in 1 second, with a maximum peak value of 60 MPa and a trough value of 0. For half-sine waves, a half-sine wave is drawn in 0.5 seconds, with the remaining 0.5 seconds designed for pressure relief, where the pressure is 0 during relief.
3.2 Flexural Test
The control system controls the up-and-down vibration of a servo cylinder in the y-direction, and uses a frequency converter to control a motor to drive the test sample tube to rotate, controlling the deflection speed and angle in the x-direction, forming a two-dimensional combination of vibration and deflection. The maximum frequency of the deflection test is 17 Hz, the maximum amplitude is ±35 mm, and the amplitude at the maximum vibration frequency is ±4 mm.
4 Control System Hardware Design
The control system consists of a host computer (IPC), a slave computer (PLC), and external circuit control. The host computer and slave computer are connected via Ethernet to complete the hydraulic servo system. Changes in environmental conditions are controlled by instruments, which communicate with the host computer via RS485. The control system block diagram is shown in Figure 3.
Figure 3 Block diagram of the control system of the hydraulic pulse testing machine
4.1 Pressure transmitter and AD module
The pressure transmitter is located at the booster outlet, with a range of 0–30 MPa, an output of 4–20 mA, and a frequency response of 200 Hz. The accuracy of the pressure transmitter is 0.75% of full scale . To achieve higher full-scale accuracy, the hydraulic system employs a combination of switching two pressure transmitters. All sensors sample at a frequency of 500 Hz, and the pressure sampling provides 3–5 16-bit data points per transmitter cycle for smoothing and improved measurement accuracy, ensuring an absolute error requirement of ±2%.
The system employs a 4-channel differential input 16-bit AD module with an input current of 4-20mA, a total conversion time of 600μs, a upload time of 1ms, and a sampling frequency equivalent to 500Hz; it also has 2 channels of voltage/current output, with an output of ±10V, a total conversion time of 600μs, and a download time of 1ms. The servo valve is adjusted at 100Hz, which takes 10ms . The PID adjustment algorithm time is approximately 0.6ms, and the PID instruction execution time is less than 400μs, enabling 5 PID adjustments, thus ensuring real-time performance and stability.
4.2 Hydraulic Servo System Control
The response frequency and adjustment accuracy of a hydraulic servo system depend entirely on the system's natural and resonant frequencies, making servo system simulation analysis crucial for servo system design. Due to space limitations, this article omits relevant content on hydraulic servo systems.
The control system outputs a 0-10V voltage signal through the AO module, which is amplified by a servo amplifier to control the opening of the servo valve. The opening of the servo valve determines the flow rate of the liquid, thereby controlling the pressure on the test sample tube. The hydraulic servo system enables the system's output quantities, such as displacement, velocity, or force, to automatically, quickly, and accurately follow changes in the input quantity, while simultaneously amplifying the output power significantly. The hydraulic servo system boasts unique advantages such as fast response speed, high load stiffness, and high control power.
The servo control employs the ad-da method, using a pressure transmitter as the feedback element. The servo refresh cycle is 1000μs. The two servo valves in the servo system are controlled using the same method, differing only in pressure.
4.3 High-speed counter and other circuits
The HSC module provides four 50kHz high-speed input pulse counters and connects to the encoder of the flexure motor. The flexure motor controls the flexure speed and angle. The system's DI and DO modules are used for on/off control, such as oil pump, liquid level, hydraulic valve, frequency converter, medium temperature and agitation, hydraulic system protection and alarm control, etc.
4.4 Ethernet
The SLC500/L533 processor has a built-in Ethernet port, which offers higher reliability and faster transmission speeds compared to RS232/RS485 Ethernet, with data transmission rates reaching 10-100 Mbps. Therefore, this system connects the host computer and the slave computer via Ethernet, reducing the impact of data transmission lag on the waveform curve, giving the pulse testing machine excellent real-time performance, strong anti-interference capabilities, and greater stability and reliability.
5 Control System Software Design
The main challenge in software design is controlling the real-time pulse waveform curve, that is, ensuring that the actual waveform curve is always within the upper and lower allowable error range of the given waveform curve, as shown in Figure 4.
Figure 4 Real-time waveform curve
5.1 Host Computer Software
The entire control system uses LabVIEW to program and implement the human-machine interface, communicating via Ethernet to send test commands to the lower-level computer and receive data uploaded by the lower-level computer. LabVIEW is a graphical programming language and an open environment for quickly creating flexible, scalable test, measurement, and control applications. LabVIEW allows for easy acquisition and analysis of actual signals to derive useful information, and the measurement results can then be shared through intuitive displays, reports, and networks.
The host computer is responsible for the interface design of the entire control software, including the dynamic display of data such as temperature, pressure, amplitude, and speed, presented in numerical and graphical formats. Test data is stored in a database, including equipment hardware information (hydraulic system capacity, booster ratio, servo valve model, etc.) and current test information (test standards, specimen specifications, test parameters). Users can extract data from the database and generate reports from the measured test data. All waveforms are stored in corresponding waveform files, containing data on medium temperature, ambient temperature, given pressure waveform, and actual pressure waveform. The software can replay historical curves and adjust the playback speed for quick browsing of historical pulse curves. The control system also implements an alarm function; if the tank temperature, medium temperature, ambient temperature, level float, ruptured float, filter blockage, or cylinder tipping occurs, an alarm signal is immediately output. The host computer connects to the environmental instrument control via RS485 communication protocol. The ambient temperature is controlled by an independent environmental chamber; the host computer can write temperature control values or temperature control curves and read the ambient chamber temperature in real time.
5.2 Lower-level software
The lower-level computer is responsible for the real-time servo control and logic control design of the entire control software. This includes receiving the given pressure waveform curve, vibration frequency, amplitude, and flexure speed and angle from the upper-level computer, adjusting the three closed-loop control systems of the two servo cylinders and the flexure motor, and performing logic control of the switching quantities. The logic control function is omitted here.
Since the system response time must be at least 4 system time constants, the lower-level computer controls the servo cylinder and booster according to the given pressure waveform curve to ensure that the pressure rise ramp time is less than 50ms, the adjustment cycle is 5-10ms, and the interface waveform display lags by about 1 real-time waveform.
The pulse pressure setpoint curve and servo signal adjustment are affected by factors such as the test pressure magnitude, the expansion amount of the test sample tube, the booster ratio, and the servo valve amplifier gain. The lower-level computer should automatically adjust the setpoint pressure curve and amplifier gain according to the aforementioned influencing factors, and control the pulse pressure by controlling the servo valve and booster. Optimal matching of the AD module sampling frequency and the servo valve response frequency ensures that the absolute error between the actual pressure curve and the set pressure curve does not exceed 2%. Real-time data from the control system is uploaded to the upper-level computer via Ethernet, enabling real-time monitoring of the pressure waveform curve and ensuring the real-time performance and high reliability of the control system.
6. Conclusion
Due to the complexity of this test system, the control system and hydraulic servo system were first simulated in the laboratory. Then, during the joint debugging of the test system, the electromechanical coupling issues were addressed. Preliminary results have been achieved in the laboratory simulation of the control system, initially resolving issues such as matching the sampling frequency of the AD module and the response frequency of the servo valve, and the staged changes in the given pressure curve and amplifier gain. Further verification during the joint debugging of the test system is needed to confirm whether the errors between the actual pressure curve and the set pressure curve meet the relevant standard requirements.
