Abstract: This paper explores the application of servo systems in the tilt locking test of motor vehicle seat belts, introducing components such as PLCs, servo motors, and encoders, and proposing an implementation method for the testing equipment. The test requirements and methods stipulated in Chinese national standards are elaborated in detail, and feasible solutions for the mechanical parts of this type of testing equipment and the design of the measurement and control system are discussed. The measurement and control process involved in PLC programming and the communication relationships of key components are systematically explained.
1 Introduction
Motor vehicle seat belts are protective devices used to restrain vehicle drivers and passengers, effectively improving the passive safety of vehicles. Since seat belts were first installed in passenger cars in the United States in 1955, their use has become increasingly widespread. Countries and regions such as the United States, Europe, and Japan have successively enacted legislation or regulations requiring car occupants to wear seat belts. my country's Ministry of Public Security also issued a notice on November 15, 1992, stipulating that from July 1, 1993, all drivers and front-seat passengers of passenger cars (including sedans, jeeps, vans, and microcars) must use seat belts. Furthermore, Article 51 of my country's Road Traffic Safety Law stipulates that drivers and passengers of motor vehicles shall use seat belts as required when the vehicle is in motion.
According to national standards, a seat belt is an assembly consisting of webbing, buckles, adjustment mechanisms, and connectors that secure it within a motor vehicle. It is used to reduce the severity of injury by limiting the wearer's body movement during sudden deceleration or collisions, including energy-absorbing or retracting devices for the webbing. Currently, there are many common types of seat belts. Based on their structure, seat belts can be categorized as lap belts, shoulder belts, three-point seat belts, S-belts, and full-back seat belts. Among these, seat belts with retractors offer superior performance but require more sophisticated manufacturing processes. As a key component of seat belts, the retractor can be classified according to its performance and adjustment method as follows: non-locking retractor (Type 1), manually adjustable retractor (Type 2), self-locking retractor (Type 3), emergency locking retractor (Type 4), and high-response emergency locking retractor (Type 4N). The most common seat belt retractor types are Type 4 and Type 4N, both of which feature belt-feel and/or vehicle-feel locking and tilt locking capabilities. Vehicle-sensing locking and belt-sensing locking are used to evaluate the seat belt protection performance of a vehicle and its occupants during emergency actions, respectively, while tilt locking is used to evaluate the seat belt protection performance of a vehicle in pitch and roll positions. This article focuses on the tilt locking characteristics of seat belts and their detection methods.
Clauses 4.2.5.3.1c) and 4.2.5.3.1d) of the national standard GB14166-2013, "Seat Belts, Restraint Systems, Child Restraint Systems and ISOFIX Child Restraint Systems for Motor Vehicle Occupants," stipulate that the retractor must not lock when the sensitive device is tilted 12° or less in any direction from its manufacturer-specified installation position. For Type 4 retractors, the retractor should lock when the sensitive device is tilted greater than 27° in any direction from its manufacturer-specified installation position. For Type 4N retractors, the retractor should lock when the sensitive device is tilted greater than 40° in any direction from its manufacturer-specified installation position. Clause 5.6.2.3 of the national standard specifies the tilt locking test method. To check whether the requirements of 4.2.5.3.1c) and 4.2.5.3.1d) are met, the retractor should be installed on a horizontal platform, and the platform should be tilted at a speed not exceeding 2°/s until locking occurs. The test should be repeated in other directions to meet the requirements.
2. Key Considerations for Experimental Procedures and Equipment Construction
To implement the test process specified in the standard, a relevant procedure needs to be planned. After the retractor is installed according to the specified installation method, it should be able to simulate the pitch and roll posture of the vehicle, monitor the locking status of the retractor in real time in the four flip directions of front, back, left and right, and record the locking angle.
Currently, seat belt manufacturing plants equip their production lines with tilt and locking testing equipment to achieve 100% inspection in COP testing. However, to adapt to the production line's pace and improve production efficiency, the tilt speed specified in clause 5.6.2.3 of the national standard is generally not considered or met by this type of online equipment. Furthermore, the retractor clamping method typically uses specially designed fixtures adapted to the retractor's shape and size, employing a pneumatic clamping mode. Laboratory testing equipment, however, cannot be simply equated with online equipment. The former has higher requirements for installation positioning accuracy, tilt angle, and locking angle measurement to comply with the provisions of ISO/IEC 17025:2005 "General requirements for the competence of testing and calibration laboratories," and even after the 2017 version of this requirement was revised, the relevant requirements for testing instruments have not been weakened.
To construct this testing equipment, the following systems are essential for the testing process: a precise tilt speed control system, a retractor locking recognition system, a 90° reversing system, and a webbing extraction system, all meeting the standard requirements. The precise tilt speed control system primarily utilizes a servo motor and a PLC to form a precise speed control mechanism. The retractor locking recognition system incorporates a pressure sensor; if the retractor locks, the pressure applied by the webbing to the sensor increases sharply, and a reliable value is set to achieve the locking recognition function. The 90° reversing system switches between the two vertical rotation axes (front/back and left/right) using a servo system composed of a servo motor and a PLC. The webbing extraction system uses rollers to clamp and rotate the webbing; the clamping is achieved using a cylinder, and the rotation is achieved using a small servo motor.
3. Experimental Equipment Construction Method
The testing equipment consists of a mechanical part and a measurement and control part. The mechanical part is the execution part, which directly contacts the sample and is composed of the aforementioned systems and related accessories. The measurement and control part mainly consists of a PLC, a touch screen, and a rotary encoder. The touch screen serves as the human-machine interface and also as the host computer, while the PLC is the slave computer. The host computer sets parameters for the slave computer, and the slave computer has a built-in ladder logic program that communicates directly/indirectly with the encoder and servo drive.
3.1 Mechanical Part
The mechanical design directly utilizes UG to create 3D models, followed by digital prototype virtual assembly and interference checks. This involves all the aforementioned systems and fully considers human factors engineering. This process is paperless and significantly shortens the product production cycle. The equipment assembly drawing is shown in Figure 1, and the physical equipment drawing is shown in Figure 2.
Figure 1. Three-dimensional assembly diagram of the seat belt tilt locking test equipment
Figure 2. Actual picture of the seat belt tilt locking test equipment.
3.2 Measurement and Control System
The measurement and control system mainly realizes four functions: tilt speed control, 90° rotation control, lock-up discrimination, and lock-up angle measurement. Tilting speed control is executed by motors I and II, and can be activated at the start of the test, with a maximum rotation range of 180°. 90° rotation is executed by the steering motor, activated upon completion of the front and rear rotation axis tests, and upon the start of the left and right rotation axis tests. Webbing extraction is executed by the webbing extraction motor, taking effect when the cylinder presses down and drives the rollers to clamp the webbing. A total of four servo motors are used. The lock-up discrimination condition is that the pressure sensor's withstand force value exceeds the limit, and subsequent actions are executed based on this condition. Lock-up angle measurement uses an absolute high-precision rotary encoder, eliminating the need for zero-point finding upon startup, monitoring the absolute position information of the shaft, and exhibiting strong anti-interference capabilities without the zero-point accumulation error problem of incremental encoders. Table 1 shows the corresponding wiring relationships of the selected Omron NPN output absolute type E6C3-AG5C encoder, CP1H Omron PLC. This type of encoder can output Gray code, which is a reliability code that minimizes errors and helps reduce encoder errors.
Table 1. Wiring Relationships Between Absolute Encoder, PLC, and DC Power Supply
4. Implementation methods for key control components
The host computer uses a Kunlun Tongtai touch screen, which can perform graphical programming and background operations, and serves as a human-machine interface to realize relevant parameter settings and power on/off functions.
Reliable control is achieved through ladder logic programming of the lower-level PLC. PLC outputs can be categorized into transistor and relay outputs; this solution employs transistor outputs, with additional relays to control the servo drive and solenoid valves, thereby controlling the motor and cylinder movements. The detailed control flow is shown in Figure 3. In the "pulling the belt motor back to position & forward rotation" process, the dividing point between back to position and forward rotation is when servo motor I/II rotates to its initial position (zero degree position).
The host computer and the slave computer communicate via DP, based on the optional RS-232C port of the PLC. Figure 4 shows the pin definitions of the corresponding port on the Omron PLC side. Figure 5 shows the wiring diagram of the selected Kunlun Tongtai touch screen and PLC.
Figure 3. Equipment Measurement and Control Flowchart
Figure 4. PLCRS232C Port Pin Definitions
Figure 5. Wiring diagram between touch screen and PLC
5 Conclusion
This paper explores the application of a servo system in seatbelt tilt locking tests and constructs a high-precision automatic testing scheme. The prototype equipment has been put into practical use, accumulating over 1000 tests with no adverse feedback. The overall performance of the equipment meets national standards and plays a positive role in improving laboratory equipment for testing purposes.
During the equipment development process, the wiring between the four servo motors, drivers, PLC, touch screen, and various buttons, switches, relays, and other auxiliary components was quite complex. Although a technical solution was achieved, there are still areas for improvement and optimization. Introducing a bus module could greatly alleviate the current complex wiring situation and potentially further shorten the equipment development cycle. This feasibility is particularly evident for large and complex equipment.
Furthermore, the accuracy of the equipment system is a comprehensive accuracy derived after fully considering the accuracy of all relevant subsystems of the equipment, based on factors such as usage and operating conditions. The weakest link effect is particularly applicable to the accuracy evaluation of this type of equipment. Although encoders have high accuracy, reasonable matching should be considered, and mechanical accuracy, such as reducer gear backlash, photoelectric switch installation, adaptability to different samples, and external disturbances, should also be taken into account. This process can be considered a systematic engineering project; if all aspects are considered, the cost-effectiveness can be optimized.
