Principle Of Auto-collimation Research Articles

REAL-TIME ON-ORBIT CALIBRATION OF ANGLES BETWEEN STAR SENSOR AND EARTH OBSERVATION CAMERA FOR OPTICAL SURVEYING AND MAPPING SATELLITES

Abstract. On space remote sensing stereo mapping field, the angle variation between the star sensor’s optical axis and the earth observation camera’s optical axis on-orbit affects the positioning accuracy, when optical mapping is without ground control points (GCPs). This work analyses the formation factors and elimination methods for both the star sensor’s error and the angles error between the star sensor’s optical axis and the earth observation camera’s optical axis. Based on that, to improve the low attitude stability and long calibration time necessary of current satellite cameras, a method is then proposed for real-time on-orbit calibration of the angles between star sensor’s optical axis and the earth observation camera’s optical axis based on the principle of auto-collimation. This method is experimentally verified to realize real-time on-orbit autonomous calibration of the angles between the star sensor’s optical axis and the earth observation camera’s optical axis.

A PD-edge method associated with the laser autocollimation for measurement of a focused laser beam diameter

For measurement of a focused laser beam diameter, a new method, referred to as the PD-edge method, in which an edge of an active cell in a photodiode (PD) is directly employed in the same manner as a knife-edge or a knife-pad in the conventional methods, is proposed with the enhancement of the principle of laser autocollimation. In the proposed PD-edge method associated with the laser autocollimation, a focused laser beam diameter can be estimated by using the gradient of output signal from the PD with respect to the change in the angle of incidence of a collimated laser beam made incident to an objective lens. In this paper, the feasibility of the proposed PD-edge method is at first verified in computer simulation. After that, the edge sensitivity characteristic of an active cell in a commercially available PD is investigated in experiments as well as in computer simulation based on the simple edge-transmission region model to search for the possibility of directly employing a commercially available PD for measurement of a focused laser beam diameter. Furthermore, an optical setup is constructed, and some experiments as well as the measurement uncertainty analysis are carried out to verify the feasibility of the proposed PD-edge method.

Reflector automatic acquisition and pointing based on auto-collimation theodolite

An auto-collimation theodolite (ACT) for reflector automatic acquisition and pointing is designed based on the principle of autocollimators and theodolites. First, the principle of auto-collimation and theodolites is reviewed, and then the coaxial ACT structure is developed. Subsequently, the acquisition and pointing strategies for reflector measurements are presented, which first quickly acquires the target over a wide range and then points the laser spot to the charge coupled device zero position. Finally, experiments are conducted to verify the acquisition and pointing performance, including the calibration of the ACT, the comparison of the acquisition mode and pointing mode, and the accuracy measurement in horizontal and vertical directions. In both directions, a measurement accuracy of ±3″ is achieved. The presented ACT is suitable for automatic pointing and monitoring the reflector over a small scanning area and can be used in a wide range of applications such as bridge structure monitoring and cooperative target aiming.

柱透镜组在小角度测量中的应用

In order to realize three-dimensional( 3D) angle measurement,a photoelectric method using cylindrical lens is presented. First,based on the principle of auto-collimation,a pair of cylindrical lens and a tetrahedral reflector are used instead of a plane mirror to indicate the change of pitch,yaw and roll. Then after the analysis with matrix theory,the relationship between shape and position of the light beam and the angles to be measured is revealed,followed by the algorithm and parameter analysis. Finally,an experiment for roll angle measurement is conducted to test and verify this method. Experimental results show that the system has a range of 20' and an accuracy of error RMS( Root Mean Square) superior to 8″ at a working distance of 1. 2 m,which can basically satisfy the requirements of non-contact small angle measurement,and has certain practical value in engineering.

Development of a Sensitive Two-Axis Micro-Angle Sensor

This paper presents a two-axis micro-angle sensor for measurement of surface profiles and error motions. The sensor, which is based on the principle of laser autocollimation, employs a laser diode as the light source. The reflected beam from the target surface is received by an autocollimation unit consisting of an objective lens and a quadrant photodiode (QPD) located at the focal position of the lens. It is verified that the sensor sensitivity does not depend on the focal length of the objective lens, which is important for reduction of the sensor size. It is also confirmed that applying an offset to the QPD position along the optical axis is effective for avoiding the influence of the gaps between the PD cells. A prototype micro-angle sensor with a dimensional of 26 mm (W)×22 mm (L)×12 mm (H) is designed and constructed. Experimental results have shown that the sensor has a resolution of 0.05 arc-seconds.

A compact and sensitive two-dimensional angle probe for flatness measurement of large silicon wafers

A two-dimensional (2D) angle probe was developed to realize a new scanning multi-probe instrument employing 2D angle probes for flatness measurement of large silicon wafers. Each probe, which utilizes the principle of autocollimation, detects the 2D local slope components of a point on the wafer surface. The 2D local slopes (angles) are obtained through detecting the corresponding 2D positions of the reflected light spot on the focal plane of the object lens using position-sensing devices (PSDs). To make the probe compact in size, it is more effective to improve the sensitivity of angle detection by selecting proper PSDs than using an objective lens with a larger focal distance. Two kinds of photo devices, linear lateral effect PSDs and quadrant photodiodes (QPD), for sensing 2D positions were discussed theoretically and experimentally. It was shown that a QPD is the best for highly sensitive 2D angle detection. In the experiments, a compact prototype angle probe with dimensions 90 (L) mm ×60 (W) mm ×30 (H) mm employing a QPD as the PSD was confirmed to have a resolution of approximately 0.01 arc-second.

A new multiprobe method of roundness measurements

This paper presents a new multiprobe method for roundness measurements called the mixed method. In this method, displacements at two points on a cylindrical workpiece and an angle at one of the two points are simultaneously monitored by two probes. The differential output of the probes cancels the effect of the spindle error, and deconvolving the differential data yields the correct roundness error. The mixed method is compared to the traditional 3-point method with respect to the transfer function and resolution. Unlike the 3-point method, the mixed method can completely separate the roundness error and the spindle error, and can measure high-frequency components regardless of the probe distance. Resolution can also be improved throughout the entire frequency domain by increasing angular separation of the probes. An optical sensor specifically suited to the mixed method is designed and used to make roundness measurements. A fiber coupler and single-mode fibers are used in the sensor to divide a light beam from a laser diode into two beams, resulting in a compact sensor with good thermal drift characteristics. The displacement meter of the sensor is based on the imaging system principle and has a resolution of 0.1 μm. The angle meter is based on the principle of autocollimation and has a resolution of 0.5 in. A measurement system is constructed to realize measurements of roundness by using the optical sensor. Experimental results confirming the effectiveness of the mixed method for roundness measurements are also presented in this paper.