Anssi Mäkynen

Tracking laser radar for 3-D shape measurements of large industrial objects based on time-of-flight laser rangefinding and position-sensitive detection techniques

A tracking laser radar device is proposed for the 3-D measurement of the shapes of large industrial objects. The measuring system includes a servo-controlled measuring head and small reflectors attached to the object. The sensor consists of a tracking sensor, a pulsed time-of-flight rangefinder and angle encoders. The implementation of the tracking sensor and its test results are explained in detail. Tracking is accomplished by illuminating the reflectors of the target object and focusing the reflected light on the surface of a four-quadrant (4Q) position-sensitive detector, the signals of which are used to drive the servo motors of the measuring head. The test results show a tracking sensor resolution of about 0.003 mm (/spl sigma/ value) and bias of about /spl plusmn/0.1 mm when the distance and angle of the target reflector vary in the ranges 2-5 m and /spl plusmn/45/spl deg/, respectively. The pointing accuracy of the tracking system was better than /spl plusmn/0.3 mm. >

Positioning resolution of the position-sensitive detectors in high background illumination

The effect of high background illumination on the positioning resolution of the LEP and 4Q detector is studied. High background illumination lowers the noise sensitivity of the LEP, whereas the noise sensitivity of the 4Q detector remains unchanged. The noise sensitivity drop roughly from 0.5 to 0.3 was demonstrated in the case of the LEP. This means that by using an LEP instead of a 4Q detector, better resolution in high background illumination could be achieved despite the higher inherent noise level of it.

CMOS photodetectors for industrial position sensing

The properties of a CMOS-compatible pn-photodiode, phototransistor, and one-dimensional lateral-effect photodiode (LEP) for position-sensing applications are characterized. The photodiode and phototransistor seem to have properties that are comparable to typical commercial photodetectors despite the relatively large variations in their spatial and spectral responses and the lower responsivity in the near-infrared band. In addition to the above properties the LEPs show excellent linearity, but 3-4 times larger NEP than corresponding commercial LEPs due to low resistance of the current dividing layer. The responsivity variations have no effect on the linearity of the LEP, and the slightly lower responsivity at near-infrared has only a negligible effect on the achievable resolution (SNR). These properties, usually considered as weak points of CMOS-compatible photodetectors, are believed to have little or no effect on the properties of a position sensor, if the diameter of the light spot is small ( >

A high-resolution lateral displacement sensing method using active illumination of a cooperative target and a focused four-quadrant position-sensitive detector

A method for high-resolution lateral displacement sensing is proposed. The sensor includes an optical transceiver and a piece of reflective sheeting fixed to the object to be measured. The reflector position is measured by illuminating the reflector and focusing the reflected light on a four-quadrant (4Q) position-sensitive detector (PSD). The method provides true lateral displacement in a large working volume without calibration due to the property of a focused 4Q detector in which the size of the measurement span is determined solely by the size of the reflector, thus providing inherently accurate, constant scaling independent of distance. Experimental results obtained with a laboratory prototype show an air turbulence-limited resolution of a few micrometers (s value) and integral and differential nonlinearities better than /spl plusmn/1% and /spl plusmn/7%, respectively, when a square reflector of 1 cm/sup 2/ is used at a distance of 2.5 m. The main systematic source of error seems to be the spatial nonuniformity of the reflector specific radiance. >

Random target method for fast MTF inspection

A random target method for fast MTF inspection is proposed. The setup includes a random target, lens under test and a CCD camera with focus adjustment. The target consists of a random black and white pattern of a flat spectrum. The MTF of the lens is acquired by imaging the random target on the CCD using the lens under test, and then analyzing the spatial frequency content of the image. Frequency range up to about 50 cycles/mm is possible using commonly available CCD imagers. Measurement speed and precision depend on the sample matrix size used in calculation. A matrix of 128*128 samples per measured field point provides better than 2% precision and a few seconds total execution time (ordinary PC-computer) per lens including best focus evaluation and the measurement of tangential and sagittal MTF curves of 5 field points. Thus fast MTF inspection of low to medium quality lenses seems possible.

A binary photodetector array for position sensing

Abstract The implementation and test results of a 16 × 16 CMOS photodetector array for position-sensing applications are presented. Unlike conventional position-sensitive detectors (PSDs), it can be used for simultaneous multiple spot detection with high accuracy. To decrease the signal-processing overhead characteristic of area arrays, binary detection and random access readout are used. The fill factor and pitch of the array are 30% and 50 μm, respectively. Spatial-quantization-limited position-sensing accuracy of 4.3 μm is achieved using a uniform spot of 280 μm in diameter, total signal power of about 8 nW and pulse width of 8 ms. At lower signal levels, the accuracy is decreased by the spatial noise caused by the mismatch of threshold-setting current mirrors operating in weak inversion. The array outperforms a typical largearea (~ 100 mm 2 ) lateral effect PSD (LEP) when a measurement bandwidth less than 10 kHz is used.

Displacement sensing resolution of position-sensitive detectors in atmospheric turbulence using retroreflected beam

The position sensing resolution of a lateral-effect photodiode (LEP) and a four-quadrant (4Q) photodetector used as the position sensitive detector (PSD) in a sensor which measures the lateral displacement of a corner cube reflector (CCR) by illuminating it and detecting the position of the reflected light spot with the PSD is studied. An LEP is much noisier than a 4Q detector, but in an outdoor environment the sensitivity of the 4Q detector to atmospheric turbulence due to defocusing makes its resolution worse than that of an LEP. Submillimeter resolutions at the target distances of several hundreds of meters are electrically achievable with an LEP. Outdoors, however, its resolution is also bounded by the atmospheric turbulence. In turbulent measurement conditions, standard deviations of better than 1 cm were typically achievable up to the distance of 300 m and about 10 cm up to half a kilometer.

CMOS-compatible position-sensitive devices (PSDs) based on photodetector arrays

Abstract This paper reports five different constructions of optical position-sensitive devices (PSDs) implemented using standard CMOS technology. It is found that despite the non-idealities of CMOS-compatible photodetectors, CMOS technology provides a means of implementing PSDs with relatively high performance. This can be achieved, for example, by using an array of discrete photodetectors instead of the continuous single element structure used in conventional lateral effect PSDs (LEPs). The results show that, relative to a conventional LEP manufactured with dedicated technology, the linearity of an array-type two-axis CMOS PSD can be two to eight times better, and that its precision in low bandwidth (

A CMOS binary position-sensitive photodetector (PSD) array

Implementation and test results of a 16 by 16 binary CMOS photodetector array for position sensing applications is presented. Unlike conventional position sensitive detectors (PSDs), it is capable of simultaneous multiple spot detection with high accuracy. To decrease signal processing overhead characteristic of this kind of area arrays, binary detection with on-site thresholding and random access readout is used. The fill factor and pitch of the array are 30% and 50 /spl mu/m, respectively. Quantization limited position sensing accuracy of 4.3 /spl mu/m is achieved using spot of a 280 /spl mu/m in diameter, total signal power of about 8 nW and pulse width of 8 ms. The sensitivity is limited by the spatial noise caused by the mismatch of threshold setting current mirrors operating in weak inversion.

Digital optical position-sensitive detector (PSD)

A digital position-sensitive detector (PSD) is proposed and its performance compared with a conventional lateral effect PSD (LEP). Comparison shows that the digital PSD provides equal accuracy but better sensitivity and lower power consumption at low measurement rates. It is also anticipated that the ongoing decrease in minimum feature size of CMOS technology will further improve the performance of the digital PSD.