Measurement Velocity by the CCD Linear Image Sensor Operating in theTDI Mode.Ludě k Kejzlar, Jan FischerCzech Technical University, Dept. of Measurement, Technicka 2,166 27, Prague 6, Czech Republic, firstname.lastname@example.org, email@example.comSUMMARYThis paper is devoted to description, analysis andpractical verification of a new method of control ofCCD linear image sensor working in TDI (TimeDelay Integration) mode. TDI mode of operation isused when objects observed by CCD sensor move inthe direction of sensor line. The results of experimentswith test patterns moving with different velocities arediscussed. Conclusions and hints applicable forfinding velocity of movement are also included in thepaper.Keywords: CCD linear image sensor, TDI, velocitymeasurementSubject category: Physical sensors (non-magnetic)CCD LINEAR IMAGE SENSOR MODELSThe CCD linear image sensor physical structure ison Figure 1. This structure could by described by theCCD linear sensor signal model, which is on Figure 2.Photo-sensitive elementsShift registerTransfer gateShutter gateΦ XΦ SΦ t1Φ t2SiO 2ElectrodesSemiconductor PDepeletion AreaFigure 1. CCD linear image sensor physical structureThe colours in Figure 2 correspond with colours inFigure 1 and describe the same parts of sensor. Thephotosensitive element is modelled as convector E/Qwith accumulator (green). The transfer gate ismodelled as N switches controlled by one signal (red).Two-phase shift register is represented by delay linewith delay T t (blue). The shutter gate is represented asN switches that are connected to ground (yellow).The signal model was used to develop thesimulation program for CCD linear sensor. Thesignal model was used for mathematics descriptiontoo.Φ SΦ XΦ tE/QΣΣD -TtImage of object1 2 3 NE/Q E/QE/QΣΣD -TtΣΣD -TtΣΣD -TtPhoto sensitiveelementsK rShutterTransfer gateShift registerOutputFigure 2. CCD linear image sensor signal modelCCD linear sensor principle diagram is on Figure3 this scheme will by used to explain the TDIoperation mode.Φ SΦ XΦ tq 1photo sensitiv elementsq itransfer gateTxcharge transp. registerq NOutputUoutFigure 3. CCD linear image sensor principle diagramPRINCIPLE OF CONTROLLING THE CCDLINEAR CAMERA IN TDI MODETDI mode is a mode of the CCD linear sensorcontrol. In this case the TDI mode is implemented inline. In this mode a value of each output pixel isdetermined from values of all N active pixels incorresponding times.The principle of the TDI mode lies in repetition ofa cycle (described below) during control the sensoroperation.During the first phase of operation the charges areintegrated in the CCD photo-elements (’’pixels’’) (q i ),during the time T Xef .q 1v IMv CHq iNImageq NΦ XUoutFigure 4. a) Principle of CCD linear image sensorworking in TDI ; second cycle - second phase
In the second phase (after the end of integration)the charges are transported to the charge shift registervia the transfer gate controlled by signal Φ X (seeFigure 4 a)). Transported charges are added to thecharges, stored in shift register from the previouscycle.In the third phase the charges in shift register aremoved by one position via control signal Φ t (withfrequency f Φt ). Step three of this cycle overlaps withthe step one of the next cycle (see Figure 4 b).q 1v IMv CHq iNImageq NΦ tUoutFigure 4 b) Principle of CCD linear image sensorworking in TDI ; first cycle - third phase andsecond cycle - first phaseIn the TDI mode the charges in the shift registershifts with velocity v CH , determined by the multiple ofthe pixel pitch of the photo elements d PIX [m] and thecontrol frequency of shift register f Φt [Hz] (the controlfrequency of shift register f Φt correspond withtime T X ).vCH=dTXd=2fΦ tWhen the velocity of charge v CH is equal to thevelocity of image of object v IM , coherent accumulationof charges occurs and sharp image of the object isobtained. The equation ( 1) describes chargeintegrated in photosensitive element i for integrationtime T X .this equation correspond with the colours of blocks inthe signal model (Figure 2).SPECIAL CCD LINEAR CAMERAThe TDI mode of operation was realised in specialCCD linear camera, we designed. Its block diagram isin Figure 5. There is used the DSP (Digital SignalProcessor) to control the CCD linear sensor.PCRS232ControlRAMDSPADSP2115Signal or DataLensA/DCCDVideoFigure 5. Special CCD linear camera block diagramAdvantage of this solution is possibility to changeor adjust the control algorithm. This camera can workin many special modes like standard mode, TDImode, FIR mode and so on.PRINCIPLE OF MEASURING VELOCITY BYTHE CCD CAMERA WORKING IN TDI MODEThe special CCD linear camera working inTDI mode is observing the moving object (test tapewith black and white stripes). The measuring set-upwe used for velocity measurement is described inFigure 6.CCDLensdv CHv IMMM = d Dv Oq i(nT t) = k c∫ E i ( t ) dt0T Xef» k cE i( nT t)T Xef( 1),where the E i (t) is i-th pixel radiant intensity (constantfor time T X ), k r represents conversion constant (pixelarea, conversion of watts to coulomb, ...), n is timeindex and T t is the charge shift time T t = 2 / f Φt .The Equation ( 2) describes output voltage of theCCD linear image sensor working in TDI mode.u OUT(i) =( 2)+ N Q( O )k rN( q i( t-i·T t) )i = 1where the q i (t) is charge integrated in i-th pixel attime t, the T t = 2 / f Φt , the N is number of active pixelcells of the sensor and the Q 0 represents the parasiticcharges generated in charge shift register. Colours inDMoving test tapeFigure 6 Set-up of experimentv o= v IMMChoice of coherency criterion is basic for correctfunction of this method. We used the criteriondescribed by the following equation (mean varianceof the image signal):Kef1=NN∑12( s − EX )i( 3) ,where s i are samples of image signal and EX is meanvalue of image signal. When the velocity ofcharge v CH and the velocity of image v IM are equal,then coherency criterion has its maximum value. Thiscriterion was verified using simulation program wedesigned.
In Figure 7 a) is simulated curve of coherencycriterion and in Figure 7 b) is measured curve ofcoherency criterion. The curve is measured(simulated) when the test tape has a constantvelocity v O and the shift time T t is changed. For everyobtained frame (with different T t ) criterion K ef ( 3 ) iscalculated. The graph in Figure 7 b) represents theresult of calculation i.e. K ef as a function of v CH , whichitself is calculated by Equation ( 4 ).0,1K ef[ - ]v CH / v IM [ - ]0,00,5 1 1,5 2Figure 7. a) Simulated coherency criterion curveBoth of the curves are obtained using test tapewith periodically repeated black and white stripes.The test tape has 16 black stripes in camera angle ofview (see Figure 8. a)).6050403020100K ef[ - ]v CH / v IM [ - ]0,5 1 1,5 2Figure 7. b) Measured coherency criterion curveThe detection of maximum of coherence criterionis not easy as the curve is non-monotonous (seeFigure 7 a). This fact should be respected by a properdesign of the measurement algorithm.20010001 251 501 751 1001 1251 1501 1751 2001samplesFigure 8. a) Strip test static frame (standard mode)Frame in the Figure 8 a) is obtained as static byCCD linear camera working in standard mode. Youcan compare this frame with frame in Figure 8 b)where is the same scene obtained by CCD linearcamera working in TDI mode and the test tape movesby velocity v O = 0,92 m/s.200CALCULATING A VELOCITY FROM THECHARGE SHIFT TIME T tVelocity of test tape movement can be calculatedfrom Equation ( 4 ).vdT t= PIX( 4) ,Ot OMwhere d PIX is pixel pitch, T t is the charge shift time forwhich the criterion had maximum value, t 0 is aresolution of T t and M is magnification of lens.Equation ( 5) found by differentiation of ( 4)shows that the resolution of this method depends onthe measured velocity and magnification of lens.dPIX∆vMmin=2tOM Tt1∆Tt( 5).RESULTS AND PARAMETERS OF VELOCITYMEASUREMENTFigure 9 shows the measured transfercharacteristic of this method. The linearity error isequal to δ L =0,06% and maximal relative error ofvelocity v CH is δ CH = 0,13%.4321v O[ m/s ]v CH[ m/s ]00 0,1 0,2 0,3 0,4Figure 9. The dependence of v O on v CH for M = 0.1060,10%0,05%0,00%-0,05%-0,10%δv O[ % ]0 0,1 0,2 0,3 0,4v CH [ m/s ]Figure 9. Linearity error of the function on Figure 9For measured velocity of test tape v O = 1.518 m/sfor time 15 min the maximum relative error of thisvelocity was δ v CH = 0.86% (see Figure 10).Measuring range was 1 : 10 of integrating timedue to the limitations of used camera.1,531,521,51v [ m/s ]01 251 501 751 1001 1251 1501 1751 2001samplesFigure 8. b) Strip test dynamic frame (TDI mode)1,501,491,48t [ m/s ]Figure 10. StabilityVo [ m/s ]Vch [ m/s ]
0,4%0,2%0,0%-0,2%-0,4%δv CH[m/s]t [ - ]SpecialCCD linearcameraRS 232to PCFigure 10. Stability errorLensLIMITATION OF METHOD WHEN ONLY ONECCD LINEAR SENSOR IS USEDA basic condition is movement in the direction ofline sensor axis. For the movement under some smallangle with respect to sensor axis only projection ofthe measured velocity vector to the axis can be found.The next basic condition for correct function isscanning scene the objects of which fulfil the samecoherency criterion. When this is not true moresensors must be used and their output signals shouldbe compared.The next condition of correct function of thismethod is constant or slowly varying illumination ofthe measured scene. When the deviation of velocityshould be measured a constant illumination isnecessary.EXAMPLES OF APPLICATIONExample of measuring test tape velocity is inFigure 11. The velocity v O is measured by IRC sensorand the velocity v CH is measured by CCD linearcamera working in TDI mode.1,761,741,721,71,681,661,64v [ m/s ]Vm v CH [ [ m/s m/s ] ]Vt v O [ m/s [ m/s ] ]Figure 11. Curves of test tape velocityt [ - ]IRCSensorV OStepmotorMovingtest tapeFigure 12. Measurement set-upThe result of this experiment is in Figure 11.There are two graphs, first represent velocity v Omeasured by IRC sensor and second representsvelocity v CH measured by CCD linear camera workingin TDI mode. There is apparent correlation betweenthese curves. The v CH differ from v O in multiplicationconstant, because of the magnification of lens.The method can by used for:- Contact-less measurements of velocity- Regulation and monitoring of velocity- Measurement of velocity deviation from correctvelocity valueThe described method is related to the correlationbased velocity measurement.ACKNOWLEDGEMENTThis work has been supported by the researchproject MSM 210000015 „Research of New Methodsfor Physical Quantities Measurement and TheirApplication in Instrumentation“.1,0%0,5%0,0%-0,5%δvCH[ % ]Figure 11. Centred velocity v CH errort [ - ]REFERENCES[ 1 ] E. Wagner, R. Dändliker, K. Spener ; SENSORSA Conmprehensive Survey, Volume 6, Opticalsensors ; Weinheim , New York 1992 ; pp 234 -252[ 2 ] Chamberlain, S.,G., Washkurak W.,D.,‘ HighSpeed, Low Noise, Fine Resolution TDI CCDImagers‘, in 252 SPIE Vol. 1242 Charge CoupledDevices and Solid State Optical Sensors, 1990http://measure.feld.cvut.cz/groups/videometry