Objective  In systems such as airborne photoelectric turntables and multi degree of freedom swing tables, three-dimensional angle measurement is often required. The methods of angle measurement are divided into contact measurement and non-contact measurement, and different measurement methods need to be selected based on actual application scenarios. For some flexible supports, parallel support platforms, and uncertain rotation axes between the moving object and the base, non-contact measurement methods need to be considered. At present, the common non-contact three-axis angle measurement schemes are complex and occupy a large space, which cannot meet the volume and weight requirements of airborne and spaceborne payload. Therefore, it is necessary to develop a non-contact three-axis angle measurement method with a simple structure and small footprint to meet the needs of different usage environments. Therefore, a non-contact three-dimensional angle measurement system based on two position sensitive detectors (PSD) has been proposed.

  Methods  A three-axis angle measurement system based on dual PSD has been established. The system mainly consists of two parts of an autocollimation measurement unit and a double-sided reflection wedge (Fig.2). The autocollimation measurement unit includes a light source, PSD1, PSD2, autocollimation lens, and subsequent processing circuits. The light beam emitted by the light source converges into parallel light through a collimating lens. And PSD1 and PSD2 receive the light spot converged by the reflected beam and perform signal processing calculations through a processing circuit. The double-sided reflective wedge is designed with a semi-reflective and semi-transparent front surface and a fully reflective rear surface. Its function is to disperse the incident collimated parallel light into two beams and reflect them back into the self-collimating lens, which converges onto the target surfaces of two PSDs to form a light spot. According to the principle of angle measurement, the calibration method of two PSDs is designed to compensate for welding errors, and the FIR filtering algorithm is used to filter the simulated collected signal to improve accuracy.

  Results and Discussions   A three-axis angle measurement system based on dual PSD has been designed, and a calibration experimental system (Fig.5) has been established to calibrate the relative position relationship between two PSDs. The welding error of the relative positions of the two PSDs is compensated through the rotation matrix and translation matrix, and the compensation result is great. A 34th-order FIR filter was designed and simulated, and the experimental results show that the designed filter has a good filtering effect on the actual collected noise signals. The filter is applied to the actual processing MCU for experiments, and the phase frequency response characteristics of the selected filter are analyzed. The test results show that the response bandwidth of the filter is 1.31 kHz, which can effectively filter out high-frequency noise signals in the analog voltage signal. The angle measurement experimental system (Fig.13) has been established, and the three-axis angle measurement function of the system has been verified. The system also has high accuracy.

  Conclusions  A non-contact three-axis angle measurement system based on dual PSD is designed. This system has advantages such as simple structure, small size, high accuracy, large measurement range, high bandwidth, non-contact, and insensitivity to axial translation. The rotation matrix, translation matrix, and the designed 34th-order FIR filter obtained from calibrating two PSDs are coded and written into the STM32F4 series microcontroller, and the filter delay is approximately 525 μs, which is within an acceptable range. The processing circuit and selected devices designed according to the actual requirements of the project have been experimentally verified. Within a measurement range of ± 2°, the accuracy of yaw angle measurement reaches 0.006°, pitch angle measurement accuracy reaches 0.009°, and roll angle measurement accuracy reaches 0.021°. The overall autocollimation measurement unit weighs 230 g and has a size of 50 mm × 50 mm × 50 mm square box. The response frequency of the measurement system can reach 1.15 kHz. This system can measure the three-axis angle in real-time at high speed with high accuracy and small volume, and is suitable for various engineering applications, providing stable and high-speed three-axis angle measurement solutions for airborne, spaceborne, and other conditions.