[Abstract] Based on the relevant technical requirements of GB16987-2010 and strictly following the test methods specified in the national standard, this paper introduces the development scheme of a test bench for the negative pressure resistance performance of motor vehicle brake hoses, used for testing the negative pressure resistance performance of motor vehicle vacuum brake hoses. Through the rational selection and integrated innovation of electrical components, and after practical application and verification, all technical indicators of the equipment meet the requirements, and the accuracy and reliability of the test data have significant advantages over similar domestic equipment.
1 Introduction
Motor vehicle brake hoses, commonly known as brake lines, are one of the most important components of a motor vehicle's braking system. Their main function is to transmit braking force during braking, ensuring that braking force is transmitted to the brake shoes or calipers to generate braking force, thus making braking effective at all times. This product is included in the national mandatory product certification catalog and is subject to the CCC product certification system. Currently, common motor vehicle brake hoses are mainly divided into three categories: hydraulic, pneumatic, and vacuum. Among them, vacuum brake hoses are widely used in the braking systems of various trucks, buses, and passenger cars, and are the most common type of brake hose.
According to the provisions of Clause 7.1 Performance Requirements and Clause 7.2 Test Methods in GB16897-2010 "Structure, Performance Requirements and Test Methods of Brake Hoses", 11 tests should be performed on vacuum brake hose assemblies (8 tests for vacuum brake hoses made of plastic material, excluding the three items of inner hole throughput after necking, bonding strength, and deformation resistance). Among them, the test of the change in outer diameter after resisting negative pressure is a very important mandatory test item, which is crucial for the evaluation of the comprehensive performance of vacuum brake hoses. The national standard requires [1]: Take a brake hose with a length of 300mm±6mm and one end sealed, and measure the outer diameter of the brake hose; connect the brake hose to a vacuum pressure source, apply a vacuum of 85KPa±3KPa and maintain it for at least 5 minutes; under this vacuum, measure the size of the part of the brake hose with the greatest deformation, which should be ≤1.6mm.
Based on national standards for testing the change in outer diameter of vacuum brake hoses after they withstand negative pressure, the China Automotive Certification Center Laboratory has developed a new type of negative pressure resistance test bench for automotive brake hoses. This equipment is PLC controlled, uses a vacuum pump to achieve vacuum levels, employs solenoid valves for function switching, utilizes a vacuum pressure reducing valve to match the vacuum expansion tank for pressure stabilization, and a digital display shows the system pressure. The test bench has a high degree of automation.
2 Equipment Construction Scheme
The equipment design consists of four core functional modules: control module, function start/stop execution module, vacuum degree generation module, and display module. Their interrelationship is shown in Figure 1.
The control module is implemented using a PLC. Due to the limited number of control points in this equipment, the commonly used Omron CP1H/L series is selected, and the algorithm is programmed using a ladder diagram. The function start/stop execution module uses an SMC brand two-position three-way solenoid valve, which is controlled by the PLC and connected via a relay to achieve the control function and also protect the PLC transistor. The vacuum generation module uses a vacuum pump, along with a vacuum pressure reducing valve, vacuum tank, and vacuum expansion tank, to ensure constant pressure. The display module uses a vacuum pressure gauge to monitor the system vacuum level in real time. All core modules are mechanically connected via pipes, connectors, cables, and other peripheral accessories.
Figure 1. Relationship diagram of core modules of the equipment
2.1 Equipment Principle
The device's functionality is achieved by fully utilizing existing, mature pneumatic control technologies to establish a basic pneumatic circuit. The device's construction scheme is shown in Figure 2.
1. Vacuum pump 2. M12x1.25-Ø10 lock nut 3. Vacuum pressure reducing valve 4. Vacuum gauge 5. Lock nut tee 6. Straight-through terminal 7. Pressure relief valve 8. Silencer connector 9. Silencer 10. Straight-through terminal 11. Vacuum tank 12. Vacuum expansion tank 13. Silencer G1/4″ 14. Solenoid directional valve 15. Pressure sensor 16. Sensor 17. Lock nut straight-through 18. Specimen connector 19. Specimen
Figure 2. Equipment Construction Scheme Diagram
2.2 Selection of Key Execution Components
2.2.1 Vacuum Pump
Based on their working principles, commonly used industrial gas transfer pumps can be divided into variable displacement vacuum pumps and momentum transfer pumps. Variable displacement vacuum pumps are further divided into reciprocating and rotary vane types, while momentum transfer pumps include molecular vacuum pumps, jet vacuum pumps, diffusion pumps, diffusion-jet pumps, and ion transport pumps. Considering both cost and performance, a rotary vane variable displacement vacuum pump is selected. This type of vacuum pump has advantages such as compact structure, small size, light weight, low noise, and low vibration, making it more suitable for precise control in laboratories.
2.2.2 Vacuum container
The vacuum chamber was chosen primarily to maintain a stable vacuum level and reduce pressure fluctuations, acting as an accumulator for the system's pressure. Since the internal volume of the sample is not significantly different from the internal volume of the system piping, factors such as changes in the volume of the flexible sample, initial evacuation/purging, and external disturbances to the vacuum source can all cause pressure fluctuations within the piping. Adding a vacuum chamber effectively mitigates these issues. The main technical requirements are leak-free operation and easy replacement; a large volume is not necessary, as long as it is effectively matched to the system.
Introducing λ=
Where: λ—the ratio of the volume change caused by sample deformation to the maximum capacity limit of the entire system;
V<sub>sample</sub>—the internal volume of the sample;
V-system—the internal volume introduced by piping and vacuum source;
V-can — Internal volume of a vacuum can.
In a closed system, according to the ideal gas law, the change in internal volume is the largest factor affecting the gas pressure in the system, and the two are inversely proportional. By adding a vacuum chamber, if the aforementioned chamber size V is large enough, theoretically the value of λ can be reduced to zero, meaning the subsequent change in vacuum caused by the sample volume can be ignored. Typically, the value of λ is predetermined, and its impact on experimental data should be acceptable. Blindly increasing the size of the vacuum chamber only increases the equipment's size, affecting its appearance and user experience.
2.2.3 Solenoid directional valve
The electromagnetic directional valve primarily controls the opening and closing of functions, enabling the connection and disconnection between the vacuum source and the sample, and effectively maintaining the vacuum level inside the sample even when the vacuum source is cut off. This solution uses a two-position three-way directional valve to achieve the designed functional requirements. The main technical requirements are low pressure loss, no leakage, rapid switching, smooth operation, and reliability. Electromagnetic directional valve technology is mature, and commonly available products from brands such as SMC and FESTO can effectively achieve this function.
3. Experimental process control scheme
The technical solution uses a PLC as the control module, with instruction programming via ladder diagrams. The vacuum pump and solenoid directional valve are directly controlled via relays. After the vacuum level is set, if the system pressure changes, the PLC implements corresponding control through feedback, ensuring the actual pressure remains within the acceptable range of the set pressure. For this test project, the process control scheme shown in Figure 3 is adopted.
Figure 3. Process Control Scheme
Among these, the PID control of the PLC is crucial for ensuring the accuracy of the vacuum value. Due to external disturbances, the control action must remain continuously effective to maintain stable field control parameter values. If a disturbance causes a change in the field control parameter value, the monitoring components, such as sensors, will transmit this change to the PID controller. This alters the process variable value, which is then sent to the input of the PID controller via a transmitter. The deviation value is compared with the setpoint to obtain the deviation. The controller then issues a control signal based on this deviation and a pre-set tuning parameter control law to change the controller's opening degree. This increases or decreases the controller's opening degree, thereby changing the field control parameter value and bringing it closer to the setpoint, thus achieving the control objective. This is the basic principle of PID control.
The discrete form of the ideal PID control algorithm can be obtained from its continuous form. It is divided into three types: position algorithm, incremental algorithm and speed algorithm. The ideal algorithm needs to be adjusted before it can be applied in practice. The selected Omron PLC has built-in PID control instructions [2], as shown in Figure 4. PID is an abbreviation for the combined action of proportional operation (P), integral operation (I) and derivative operation (D). Among them, the proportional action is a proportional band operation based on the set value (SV). Within this band, the control variable (MV) is proportional to the deviation, providing a smooth control process without oscillation; the integral action refers to the automatic correction process of step deviation; both the proportional action and the integral action are corrected through the control result, so response lag is inevitable. The derivative action makes up for this defect. It controls by making the operation variable proportional to the slope (derivative coefficient) formed by the deviation, which can accelerate the response to disturbances. In the PLC instructions, PID (190) and PIDAT (191) can both be used for PID control. Selecting the latter allows the PLC to automatically calculate the P, I, and D parameters as needed.
Figure 4. PLC's built-in PID control instructions
4 Conclusion
4.1 This paper presents a feasible solution for testing the negative pressure resistance of automotive brake hoses, and a novel negative pressure resistance testing device was developed based on this solution. Currently, this device has been practically applied in the central laboratory, with over one hundred samples tested, and the results have been excellent.
4.2 This paper uses a PLC as the control module, which demonstrates good reliability, stability, and availability in environments with strong electromagnetic interference and industrial settings. However, for laboratory applications, there are multiple solutions, and the solution presented in this paper is only one of them. Given the complexity of the control requirements, from a cost perspective, a solution based on FPGA or microcontroller supplemented with peripheral circuits can still achieve good results. Alternatively, while a data acquisition card-based implementation may increase costs, it is expected to improve the human-machine interface, control accuracy, and response speed. The proposed solution fully considers the possibility of future technology adoption to better adapt to large-scale factory production and testing environments.
4.3 The solution also strictly adheres to national standards, with all parameter settings strictly meeting or exceeding the standard requirements. It also fully considers future revisions to national standards; the equipment, through a quasi-modular design, allows for the replacement of modules or changes to parameters at any time, exhibiting good responsiveness.
References
[1] Standardization Administration of China. GB16897-2010 "Structure, performance requirements and test methods of brake hoses" [S]. Beijing: China Standards Press, 2010.
[2] OMRON Automation (China) Co., Ltd. SYSMACCP Series Programming Manual [M]. Shanghai: OMRON Automation (China) Co., Ltd., 2014
