Since the object under test is a micro-torque coil spring, the stiffness measurement is performed as a static stiffness test. Static stiffness is defined as the ratio of torque to angular displacement, expressed as: k = M/H (1). To obtain the stiffness value, multiple points of torque (M) and corresponding angular displacement (H) are measured and then processed through a fitting calculation. For measuring the torque (M), a direct measurement method is used, where the torque sensor directly captures the torque value. This approach minimizes errors introduced by intermediate components and enhances measurement accuracy. The angular displacement (H) is determined by the program based on the step angle set by the stepper motor during its controlled operation. During testing, the motor is driven step by step, and for each step, the corresponding torque (Mi) and angular displacement (Hi) are recorded. The static stiffness is then calculated using linear regression, with the system employing the least squares method for fitting. Let K represent the torsional static stiffness; the fitting formula for K is given as: K = [nΣ(MiHi) - ΣMiΣHi] / [nΣ(Hi²) - (ΣHi)²] (2). The selected measurement scheme is illustrated in <1>.
The overall design of the test system focuses on evaluating the performance of the micro-torsion spring. The automatic test device operates based on the principle shown in the diagram. A test software system was developed using VC++6.0 on the Windows operating system, as shown in <2>. The system automatically collects data from the torque sensor via an RS232 serial port. After processing the collected data, the stiffness of the tested spring is calculated. The software also supports automated torque and angle measurements, as well as automatic application of pure torque, as described in <3>.
In terms of hardware design, an automated test system was developed according to the selected test configuration and specific requirements for the torsion spring being tested. The system controls a precision rotary table via a computer-controlled stepper motor, achieving angular positioning accuracy up to 0.00125°. Under the influence of the load torque, the spring undergoes rotational deformation, which is captured by a high-precision torque sensor and sent to the torque meter chromatography workstation software. The system achieves a torque measurement accuracy of 2×10â»â¶ Nm. The experimental curve obtained is shown in the figure.
The single fourth measurement torque curve and five torque-angle curves demonstrate the system's high test accuracy. The micro-spring test data shows that the linearity of each measurement is excellent, and the repeated measurements exhibit strong consistency. The system is fully automated and meets all required testing standards.
In conclusion, by addressing the challenges associated with micro-spring torsional stiffness testing, the system successfully integrates a high-performance control device with advanced measurement and control software. This results in an efficient and accurate testing process. The system has already been implemented in practical production environments.
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