The presentation IDAASC’2003 LvivLet me introduce myself. I am Ludě k Kejzlar. I am a PhD student at the CzechTechnical University in Prague, Faculty of Electrical Engineering, Department ofMeasurement. I work in laboratory of videometry. The filed of my study is the non-standardmethod of control of the CCD image sensors and I am focusing to the Linear CCD imagesensors and usage for contact-less measurement.S1Ladies and Gentlemen let me present here one of non-standard method of control ofthe linear CCD image sensor, which can be used for signal processing or pre-processingwithin the CCD sensor. The topic of this article is Inherent Signal Processing in the LinearCCD Image Sensor.S2I would like to describe here the principle of this method, show some exampleapplications and in the end I want to summarise advantages and disadvantages of this mode ofoperation.S3If we want measure some parameter by way of (through) contact-less methods thanlinear CCD image sensors are often used. Signal processing is often required to find out someparameters. The digital signal processing is usually used for signal modification but I want topoint out to the method where the signal processing is occurred directly in the CCD sensor.Base of this method is controlling the CCD sensor in FIR mode of operation. Themode is called the FIR mode because the signal processing in CCD is actually a parallelcalculation of Non Recursive Digital Filter with Finite Impulse Response.S4Here are two equations. The first equation describes the output voltage of the CCDsensor controlled in FIR mode. We can compare this equation with the equation two, whichdescribes calculation of the NRDF with FIR. Here we can see that illuminance values ofphoto-elements correspond to the samples of signal and the lengths of integration timescorrespond to values of the filter coefficients. Following from this confrontation, the length ofintegration times must be controlled to achieve a filtration in the CCD sensor.S5Now I want to describe the principle of the FIR mode of operation. Here we can seethe physical structure of Linear CCD image sensor where each functional block isdistinguished by different colour. At the bottom (of slide) is shown the principle scheme ofcontrol of CCD sensor in FIR mode. Colours of blocks in this scheme correspond to the partsof CCD sensor, which have to be activated in the time. To control the CCD sensor in FIRmode the following cycle must be repeated N F times for realisation of filter with N Fcoefficients.
The sensor must be cleared before the beginning of each frame. It can be done by theelectronic shutter, as we can see here.After the clearing the first cycle is started by the integration. Duration of the firstintegration time corresponds to value of the first coefficient of the filter. After a lapse ofintegration time the charges are transported from the photocells to the charge shift register,where they are added to the charges stored there in previous cycle. After this the charges inthe shift register are moved by one position towards the output. This is the end of the firstcycle. The next cycles follow. The last cycle starts by the integration time N F , whichcorresponds to value of the last filter coefficient. After the lapse of the last cycle the read outis executed. After read out we have the image of object filtered by the implemented FIR filter.S6Now I am showing two examples of FIR mode applications. As first I am shovingexample of Movement compensation, method which is useful for scanning of moving objects.As second I am showing example of One-Frame filtration, method which is useful forinherent signal processing.S7Movement compensation : When a measured object is moving the image of this objecthas velocity v IM. This situation is shown in this picture. The velocity is given by the objectvelocity and a lens magnification M. The object must not significantly change its positionduring the integration time when we want to acquire the sharp image. Due to this, only shortintegration times must be used for acquisition. However, when we use the FIR mode with thesame length of coefficients (integration times), the charges in CCD shift register have velocityv CH given by the integration time per coefficient t INT and pixel pitch d PIX . Now, when we wantto acquire sharp image the velocities v CH and v IM must be equal.S8Optimum number of coefficients N F can be calculated by Eq. 4 from requiredintegration time T X and from parameters of measurement set-up. The image acquisition inFIR mode contains N F integrations and all of them have integration time sorter or equal tomaximum convenient time for scanning of moving objects. Due to this we acquire sharpimage with N F times longer integration time then in usage of Standard mode.S9There we can see results of one experiment. There are shown 3 acquired waveforms ofthe test image with black and white stripes. The first waveform is the reference image of thetest. It is static image acquired in Standard mode. In this case average width of edges is 3pixels. Second waveform shows image of the same test, which is moving with velocity 0,44m/s. This image was obtained by standard method using the same integration time as in theprevious case. The average edge is 23 pixels wide in this case. Third waveform shows imageof the test tape, which is moving with velocity 0,44 m/s. However in this case the FIR modewith 32 coefficients is used. The average width of edge is 5 pixels in this case. It is almost thesame value as in reference image. We can see that the using of FIR mode decreased themotion blur from 20 pixels to 2 pixels.
S10Now I want to show example of One-frame filtration. One-frame filtration means thatthe FIR filter is calculated during obtaining of the one frame in the line CCD sensor. Thisexample shows realisation of Low-Pass anti-aliasing filter, which is used for decreasingfrequencies in image before re-sampling is applied. Here is shown common principle ofdecimation where the Digital Low-Pass filter and digital decimator are used. In this case, theA/D conversion must be done with maximum sampling frequency. Unlike this, the principleusing the FIR mode is shown. Here the Low-Pass filter is calculated in CCD sensor. Thesample decimation is done by decreasing the sample frequency of the A/D converter.S11Here we can see the transfer function of realised filter. The filter has only 23coefficients what is clear from this impulse response. This filter was designed for decimationby factor 8 (, it means that the frequencies are decreased 8 times). At the bottom we can seeimage of test paper with growing frequency. Left picture shows the acquired waveform andright picture shows dependence of relative frequencies (red circles) on position in thewaveform. The green line corresponds to the amplitude of the signal. We can see there thefrequencies grow from left to right and the amplitude is almost constant.S12Here are shown two acquired waveforms. Left waveform corresponds to re-sampledsignal without the filtration and right waveform corresponds to re-sampled signal, which wasfiltered in CCD sensor. As we can see, in the left waveform, the aliasing frequencies arise pastthe position of the 150 th pixel. In figure below, we can see, many different frequencies arisebetween the 100 th and the 200 th pixel and this signal has relatively high amplitude. On theother hand in the right waveform no aliasing frequencies arise. As we can see in the bottomfigure only a few frequencies with very small amplitude arise around the 160 th pixel. Theband from the 100 th to the 200 th pixel, where the aliasing frequencies may arise, is suppressed.This application was used for simple opto-gate with single-chip AVR micro-controller. It wasused for rough finding of edges.S13In the end I am summarising the advantages and disadvantages of this mode ofoperation. The main advantage is that the standard linear CCD image sensor can by used forFIR mode implementation.In case of movement compensation the variable integration times can be used foracquiring of the image. The times can be longer than maximum time for Standard mode.Following from this the sharp images with higher output level can be obtained.In case of One-frame filtration we can achieve filtration of image in CCD sensorwithout the complex circuits. The calculation is executed parallelly and during the imageacquisition, which save the time. The good agreement between the simulations andrealisations of filters are achieved. But there are some problems with designing of the filters,because only the positive coefficients of filter can be implemented. It can be solved by Twoframefiltration, the method where first frame includes positive coefficients and second frame
includes absolute value of negative coefficients. External subtraction is needed in this case.The fact that smarter control unit is required needn’t result to more complex circuits becauseeasier computation circuits are required.One-frame filtration is suitable for simple measurement systems, which are controlledby single-chip micro-controllers or as pre-processing to systems with DSP.Thank you for your attention.