Multi-Precision Position Measuring Method with R-HPSD Scheme

A 3-D position determining mean is an indispensable function in the industrial automation system to obtain the spatial information. Optical methods have been used widely for this purpose. Semiconductor position sensitive device (PSD) is used widely because of its simplisity in instrumentation. A new type of PSD called R-IIPSD has invented to realize high precision detection. Relative resolution for full sensing area can be incresed extremely without any serious sacrifice of the simplicity in instrumentation and high speed operation. This scheme makes us possible to realize not only the high precision image position sensing but also a multi-precision sensing. These type of operations are useful such as for the optical stylus used in the 3-D cordinates measuring machine. Tntrodr~ction in order to obtain the spatial information such as positions, movements, shapes, deformations and so forth, a 3-D position determining mean is an indispensable function. It is considered that the optical method can provide an effective mean to detect the spatial infomation in the industrial automation system because of its noncontact, nondetractive and remotely dctecting capabilities. Many types of optical position measuring methods have bccn proposed and examined for the above purposes. Triangulation is the most important principle in the optical 3-D position measrlrement and is applied widel y to obtain quite stable measurement. In order to avoid the problem of correspondence in the triangulation. light beam projecting mcthod is adoped widely: in which the surface of an object to be measured is swrpt by a bright spot and image positions of the bright spot are dct,cctcd by the several image position dctecting apparatuses such as composed with TV cameras, CCDs, PSDs and so on. 17icn the 3-D position of a bright spot is determined based on the t,rinngulation. Requirements for 3-D position determining apparatus in industrial automation systems are s~lmmar i zed as high precision, mi n iatr~r i zed, easy to instrument, high stability and so forth. Semiconductor position scnsi tive device (PSD) [21 is used widely in the optical 3-D position determining system. Measuring accuracy of 3-D position is influenced largely by the precision of the image position dctecting apparatus. One of the problem in the utilization of PSD is low relative resolution of image position sensing. In order to overcome the above problem , a hybrid type position sensitive device ( R-IIPSD: Riken IIybrid Type Position Sensitive Device) has invented[3]. A prototype of the R-IIPSD was fabricated and it was proved that the method is extremely effective to increase image position sensing precision of PSD with ordinary technologies o f electronics. F11rthermore. the possibilities 'of its application for the multi-precision sensing in industrial automation system has been investigated. llybr 1 d Type ~ositjon Sensitive Dctcctor ( i<-Iil'SIl) I'SI) is cosistinn PIN or PN junctions: P type resistance rayer ( R ) N type bias layer ( C ) and high resistance Si insulation layer ( P 1 ; photoelectric current flows between resistance layer ( R ) and bias layer ( C ) at the light incident point and flow to the output terminals through the rcsistance layer. The values of the output current are inversely proportional to the resistance value between light incident point and the output terminals, image position can be cnlculated from the output currents by considering the resistance distribution. Usually, these calculations are performed by the analogue circuit in high speed ( order of 10 us ) . One of the spccial features of PSD is its simplisity of instrumentation. On the othcr side. one of the demerits is the lower relative resolution of image position detcction; it is limited up to the order of one part in a few thousands in its detectable area; it is mainly limited by the stability and the accuracy of the signal processing circuit and the precision of A/I) converter. On the othcr hand, required prccision of 3-D position determination in the field of automation is quite high, for instance, one part in a few ten thousands which corresponding to O.lmm accuracy in lm x lm x lm working space is needed. In order to realize such high prccision 3-D position determination system, one part in hundred thousands prccision is required for image posoition sensing apparatus. It may be difficult and quite expensive to realize such high accuracy signal processing circuit and high resolution A/D convcrtcr with high conversion speed. In ordcr to overcome the above problcms, a hybrid ( analogue and digital ) type semiconductor position scrls i t ive device R-IIPSD was invcntcdi31. Auxiliary output terminals arc provided between the output tcrminals in the conventional type PSD ils shown Fig.1 (a). By selecting appropriate pair of output terminals. each divided interval or several Fig. 2 Conceptual configuration of 1-D Comb-Type PSD fabricated in trial based on R-IIPSD { Ta, Tl,..,T , . . , T : output tcrminals, R: dlvidind resister, C : bins layer, t : conductive teeth. indicated dimensions are in mm 1 Fig. 3 Principle of R-IIPSD: Riken IIybrid l'ypc Position Sensitive Device, (a) conccpt~ial figure showing sectional cbonf i gurntion of R-IIPSD elcmcnt and its rq~~ivalcnt circuit, (b) conceptual graph showing various sensitivity friitrlrcs for different terminal sclcction in R-IIPSD ( c ) schematic tlrilwing of the signal processing circuit for R-IIPSD { R: resistance layer. P: photo-sensitive layer, C: bias layer. L: incident light, T1....T ,..,T ' output tcrrninals. CB: bias terminal. 8;: output tcrminal selecting circuit, S . posi t ion detecting sensitivities for iii th and j-th terminal selection. B: buffer amplifier, A: operational i~rnpl i f icr, D: divider, A/D: analogue to digital convcrtcr, I,,,, IB: output current) continuous intervals can works entirely the same as in the ordinary type PSD. By selecting output terminal pair covering whole intervals, the divided interval on which target image is falling can be detected. Then, the output terminal pair which covers narrower intervals including the target image are selected and the image position in the selected interval is dctcctcd. Finally, image position in full sensing area is determined from the dctcctcd position in the selected interval and the position of the interval. The position detecting accuracy is influenced only by that in the selected interval which is limited by the accuracy and stability of analog signal processing circuit and the prccision of A/D converter. Then, the relative resolution can be increased cxtremly higher than that of an ordinary type PSD without any improvement of the prccision of the A/D converter and the stability of signal processing circuit. Conceptual graph representing variation of the sensitivities for different tcrminal selection in R-HPSD are shown in Fig.1 (b). Prototypes of linear position sensitive dcvice based on R-IIPSD were fabricated and its conceptual configuration is shown in Fig.2. Examples of the characteristics of positional output signal for different output terminal selection are shown in Fig.3 (a). (b) and (c). Then the cffcctiveness of R-APSD has been confi rmcd experimentally. Tn ordcr to make construction of the selcction circuit for number of output tcrminals simplcr, it may be suitable to dividc thcm into several groupes and to provide analog switch circuit for selecting two of these groupes[5]. F i g . 3 Examples of positional output chi~rnctcristics of a prototype of R-IIPSD Tor different Interval selection, (a) r1111 interval (80 intervals: 12mm) sclcctlon and (b) partial interval s(?lcct.ion ( 10 intervals: 1.5mm ) (c) piirtinl interval selection( 2 intervals: F i g . 4 Conceptual configuration of a trial optical range sensing probe based on RORS ( S: light source, B: light beam. L: observation 1ens.T: bright spot, ID: laser diode ) F i g . 5 An example of multi-resolution range sensing characteristics of trial range sensing probe based on RORS{ Vt: characteristics in the whole sensing range, Wm: measuring range. Vm: characteristics in the measuring range ) possible to extend new approaches in the industrial automation systems. In the practical automation system, it is often required to make sensing system so that it can be operate in multlprcc i sion mode : rough sensing mode and measuring mode. In the rough sensing mode, wider detectable range is required but detection precision is lower. On the other hand, in the measuring mode, higher precision is required but . l . . 4 + I . > -..--A 4 " mm"",,,,,n.. ralixc uniform dctection sensitivity it is rcqrlired to use an image position scnsi tive mean which has sensitivity of position dctcction corresponding to the d i stance. One of the application of multiprecision position detecting capability in R-IIPSD is the miniaturized optical range scnsor based on RORS( Riken Opt i cal Range Sensing Scheme) [ 4 ] as shown in Fig. 4. This optical range scnsor is fabricated aiming to use as a probe for 3-D cordinates measuring machine and detecting the distance to an object surface. In the surface following mode, no such high precision is needed but wide detectable range is required. On the other hand, in the case of mc:lsuring mode, detectable range is not so wide but high precision sensing is rcquircd. Measurements are made after confirming that the surface of an object is placed in the measuring range. An optical range sensing probe fabricated in trial is shown in Fig.4. In this casc, it is designed so that detectable range is 0 to 10 mm and the detecting accuracy is around 50 um. On the other hand, it is dcsigned so that measuring range is 4 mm to 6 mm (2 mm) and measuring accuracy is around 10 um. In this case, resolution in measuring mode Is about 5 times higher than that of in the surface following mode. Detecting c:har;lctcristics is shown in Ftg.5. If thc spacing between the additional out,put terminals is more closer, detection precision in measuring mode bccomcs more higher. l:lgure 6 represents a conceptual configuration of robot system in which robot hand has reduldant degrees-off'rccdom is controled in feedback mode by dctccting the position and situation of a object and the robot hand. For instancc, multi-precision sensing such as rough positioning mode ( 1 mm accuracy ) and precise adjusting mode ( 0.1 mm accuracy ) in the 1 m x 1 m x 1 m working space can be realized easily by adopting R-FIPSD for image posi tion dctccting apparatus. Concl usion In this paper, a hybrid type positton sensitive device (R-IIPSD) has invented and a prototypc of R-IIPSD elcment has : A n.. c ~ n r r n c --L--Figurc 6. Conceptual figure of robot arm which is controlcd by detecting its end effector position using the optical 3-D position determining system { A: robot arm, 11: robot hand, 0: object to be handled, S: light beam scanner. T: projectcd bright spot. E: LED targets, I): high precision image position dctccting apparatus with R-IIPSD } of 3-D shape measurement, robotics and all 1 omat l on. Thc author would like to thank Dr. E. Goto and Dr. T. Soma of Rlkcn for their ~ncollragerncnt. Ile also would like to cxprcss his apprcciation to Mr. Koei Yamnmoto and his colleagues in llmnmntsu I'hotonlcs Co. Ltd for their cooperation in fabricating the prototype of R-HI'SD. Hc fcrcnccs 1. Nishikawa Y., Inokuchi S.. " Range data Entry Techniques Using a Laser Spot Sranncr and Two Linear Solid State Image Sensor , " IEEE Trans. Vol . IM-30, No. 4, (1081)270 2. Pctcrsson G. P. and Lindholm L . E.. "Position sensitive detector with high 1 inearity," IEEE J. of Solid State Circuit, Vol. SC13, No.3.(1978)392 / Wang W. and Busch-Vishniac I. J.. "The i.incarity and Sensitivity of Lateral Fffcct Position Sensitive Device An Tmnrovcrl Crnrnrtrv " TCCC T w o -