1. Introduction
Child car seats are essential protective devices for child occupants in cars, widely used in daily traffic, and fall under the category of mandatory national certification products and passive safety. Their technical standards are numerous, especially in North America, Europe, Australia, New Zealand, and India, where they are quite mature and have formed their own standard systems. my country started relatively late in this field, but has now formulated the national standard GB 27887-2011, which specifies the corresponding inspection requirements and methods. my country's national standard is mainly based on the relevant European standard ECE R44, with most technical requirements and test methods being equivalent. Child car seat adjustment mechanisms are frequently in contact with the operator during daily use, and the mechanisms contain elastic components such as springs. Prolonged, high-frequency use can easily lead to fatigue failure; therefore, their durability is a crucial factor affecting the safety performance of child car seats.
Durability testing of child seat adjustment mechanisms is a crucial test item in child seat technical standards, demanding high reliability and safety from the testing equipment. The equipment must not malfunction, stop working, or produce inaccurate counts during testing, and the positioning accuracy of the actuators must be strictly guaranteed to avoid damaging the samples and affecting subsequent dynamic test data. A survey of major laboratories in the industry revealed that while the working principles of the testing equipment used for this project are basically the same, the equipment construction schemes vary significantly. In terms of control system components, programmable logic controllers (PLCs), motion control cards matching data acquisition cards, and relay control are all involved; in terms of actuator components, electric cylinders, pneumatic cylinders, and specific mechanical mechanisms (such as cam mechanisms and worm gear mechanisms) are all present in different laboratories. Developing durability testing capabilities for child seat adjustment mechanisms requires designing and constructing corresponding testing equipment to achieve the goal of conducting testing activities according to the test methods specified in national standards or other technical standards. In the durability testing of child seat adjustment mechanisms, the relevant test requirements and methods of the national standard are completely consistent with the relevant content of ECE R 44.
Equipment design should adhere to certain principles. Theoretically, it should fully consider technical, standardization, and safety requirements. Key performance indicators in the design, manufacturing, and use of the testing equipment should meet relevant standards and specifications, generally including static and dynamic technical indicators. For example, static technical indicators such as power, strength, service life, and efficiency; and dynamic technical indicators such as stability and wear resistance, should be ensured to avoid adverse effects on the equipment. Simultaneously, standardization of related work should be ensured, paying attention to both conceptual standardization (e.g., units of measurement and symbols) and methodological standardization, strictly adhering to the methods specified in relevant technical standards to ensure the equipment's performance. Furthermore, regarding safety, attention should be paid to component safety to avoid excessive deformation, localized fractures, and other adverse effects, reducing design flaws; operational safety to ensure operator safety and establish a user-friendly human-machine interface; and environmental safety to consider the potential impact of the equipment on the surrounding environment, including but not limited to noise and vibration, and taking appropriate preventative measures. The design and development process of industry laboratory testing equipment should systematically consider the inherent relationship between "people, machines, materials, methods, environment, and testing," creating conditions for seamless integration of the equipment in the later stages of the design phase.
2 Technical Requirements
2.1 Standard Requirements
According to the requirements of clauses 5.2.2.7 and 6.2.7 of national standard GB 27887-2011 or clause 7.2.2.7 of European standard ECE R44: the adjuster directly installed on the child restraint system shall be able to withstand durability testing and shall undergo (5000±5) cycles of testing before dynamic testing.
The specific test methods and test load configurations are detailed in Figure 1. A, B, and C represent force loads, expressed as tensile and compressive forces, respectively. W represents the mass load, a constant weight of 1.25 kg. The durability of the regulating device is directly tested during the cyclical opening and closing of the regulating device and the repeated application of force. Alternatively, this indicator can be indirectly assessed in a dynamic test after the initial test. The test process is as follows:
Install the largest applicable dummy on the child seat to be tested, and position it in the specified slack state as required by the standard. Draw a reference line at the point where the free end of the webbing enters the adjustment device. Remove the dummy and place the child seat on the work platform shown in Figure 1. The webbing should circulate through the adjustment device for a length of at least 150 mm, of which at least 100 mm is from the webbing reference line to the free end of the webbing. On the other side of the reference line, the remaining webbing length for circumduction should be approximately 50 mm. If the webbing length from the reference line to the free end is insufficient to meet the required length, a portion of the webbing should be removed to meet the circumduction length requirement. The test frequency is (10±1) times/min, and the webbing removal rate at point B should be (150±10) mm/s.
Figure 1. Schematic diagram of test method and test load configuration
2.2 Other Requirements and Implementation
Human factors engineering is a discipline that involves human physiological and psychological characteristics, work abilities, cognitive abilities, and behavioral patterns. It focuses on studying the interaction between humans, machines, and the environment, aiming to optimize safety, health, comfort, and work efficiency. As a mechanical product, the durability testing equipment for child seat adjustment devices should appropriately consider human factors in its design process. The development of this equipment fully referenced national standards GB/T 10000-1988 (Anthropometric Dimensions of Chinese Adults) and GB/T 13547-1992 (Anthropometric Dimensions of Workspaces), ensuring that the height of the operating parts and the lateral dimensions of the equipment are suitable for operation by the average Chinese adult. Simultaneously, through the rational selection of electrical components, the overall noise level of the equipment is kept below 55 dB.
The CNAL-CL01:2018 (ISO-IEC 17025:2017) Accreditation Criteria for Testing and Calibration Laboratories stipulates that laboratories should have the necessary equipment to correctly conduct laboratory activities and affect results. Equipment used for measurement should achieve the required accuracy and/or measurement uncertainty to provide valid results. Equipment calibration parameters mainly involve counterweight mass, number of cycles, displacement, etc.
3. Implementation scheme for the mechanical part
Based on the technical requirements described in section 2, to achieve the required workflow, a cylinder can be selected as the actuator, a PLC as the lower-level control unit, and a touch screen as the upper-level control unit to implement relevant function settings. This approach facilitates centralized arrangement of the air circuit in the laboratory while saving costs and floor space. Two implementation schemes for the actuator were constructed using 3D digital modeling technology, as detailed in Figures 2 and 3.
In the scheme shown in Figure 2, the sample installation height is more suitable for actual operation needs, and the operator's normal body posture is sufficient for sample installation. Parameters such as height, lateral distance, and tilt angle can all be adjusted, but the combined adjustment method may be inefficient and time-consuming, making it difficult to optimize the overall test workflow and potentially creating a bottleneck in the full-scale inspection process. In the scheme shown in Figure 3, the adjustment of parameters such as height, lateral distance, and tilt angle is more convenient. Although the fixture height during sample installation may not be perfectly matched to the height of an adult operator, the time spent on sample installation and adjustment is short, and durability cycles constitute a large proportion of the test cycle, so the inconvenience can be ignored. Comparatively, scheme 2 is more advantageous for test efficiency and can be used as the practical implementation scheme.
Figure 2 Mechanical Part Scheme 1
Figure 3 Mechanical Part Scheme 2
4. Electrical Component Implementation Scheme
4.1 Key Component Configuration
According to the selected mechanical part implementation scheme in section 3, the control scheme shown in Figure 4 can be constructed. Based on this control scheme, the key electrical control components are selected to form the key component configuration list in Table 1, and the equipment is then assembled in the later stage using this configuration.
Figure 4. Schematic diagram of information flow in the overall control scheme
Table 1 Key Component Configuration Form
4.2 Circuit and pneumatic scheme
The equipment circuit is divided into a main circuit and a slave circuit. The main circuit operates on 220V AC voltage, connects to a contactor, and supplies power to a DC power source, generating 24V DC. The slave circuit is a 24V DC circuit, connecting to the touchscreen and a series-connected solenoid valve and relay. As a lower-level machine, the control circuit wiring diagram for implementing on/off control is shown in Figure 5, where KA1, KA2, and KA3 are three relays, corresponding to three solenoid valves. The pneumatic circuit diagram is shown in Figure 6, where SV1, SV2, and SV3 are the solenoid valves corresponding to three cylinders.
Figure 5 Control circuit wiring diagram
Figure 6 Gas Path Diagram
5. Conclusion
The equipment was designed and assembled according to the scheme described in section 4, meets the requirements of third-party calibration, and has been running stably in the laboratory for about a year (see Figure 7 for the actual equipment). It has also passed CMA metrological certification and CNAS laboratory review, enabling the expansion of the durability test scope for child seat adjustment devices. The equipment's stability, reliability, noise and vibration, and counting accuracy all meet the design and planning requirements.
The main advantages of the equipment are: simple mechanical structure, easy-to-implement control scheme, no interference with surrounding equipment/environment, easy adjustment and control of parameters such as height, lateral length, and tilt angle, quick adjustment for durability tests under complex postures, and good integration with other links in the entire test process, basically achieving seamless integration. As an industry laboratory testing equipment, it can well meet customer needs.
The main shortcomings of the equipment are that the sample clamping does not adequately meet ergonomic requirements. For example, increasing the sample clamping height makes the cylinder height adjustment handwheel too high, inconvenient to operate. Additionally, height adjustment requires manual operation; in the future, the handwheel can be replaced with a motor-assisted device for automatic adjustment as needed. However, these shortcomings can be overcome with auxiliary methods, and the other technical specifications of the equipment are at an advanced level compared to similar equipment in the industry, resulting in high testing efficiency. By constructing a durability testing equipment for child seat adjustment devices, the equipment perfectly implements the durability testing methods for child seat adjustment devices required by national and European standards, providing a quality verification window and testing and R&D platform for both domestic and export child seat products.
Figure 7. Physical image of the equipment
