Camera tracing and image processing system for soil deformation

Journal of Terramechanics, Vol. 29, No. 4/5, pp. 423-431, 1992. Printed in Great Britain.

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TRACING AND IMAGE PROCESSING FOR SOIL DEFORMATION*

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Summary--From the experience of the Terramechanics research group at Beijing Agricultural Engineering University, the authors developed a camera tracing and image processing system for soil deformation. In illustration of this new system, determining soil deformation during cone penetration is demonstrated.

CAMERA TRACING SYSTEM FOR SOIL DEFORMATION

Accurately sprayed block method AT THE present time, methods for measuring soil displacement and strain can be classified into three general classes [1]: (1) Marker [2-4]. Markers are buried at designated points in the soil, and the variation of position of these markers is measured after the soil is loaded. (2) Strain gauge [5, 6]. Strain gauges are arranged at specific points in the soil, and soil deformation is measured after loading the soil. (3) Volumeter [7]. After the soil is loaded, the volume or pressure of gas is measured with volumeters, and, according to that, the volume strain at the designated points is judged. The weak point of the strain gage and volumeter is that they can measure strain at only few points, and cannot determine displacement. In addition, the arrangement of these two types of devices significantly disturbs the soil. Markers can be used to measure displacement and strain at many points. However, the measuring accuracy is very low, and the disturbance to the soil is also very large. In order to overcome these two shortcomings of marker method, the authors invented a new marker, a chemical mixture of A L O , emulsion and water, which can be sprayed to a vertical surface of soil in the form of white rectangular blocks. Because the thickness of this chemical mixture film on soil surface is about 0.1 ram, the disturbance to the soil can be considered to be very little. Owing to large colour contrast of the white blocks and the soil, the boundary line of the white blocks can be thought to be infinitely fine. Hence, the measuring accuracy should be improved.

*This article is subsidized by the China National Education Committee. tDepartment of Vehicle Engineering, Beijing Agricultural Engineering University, Beijing 1{)0083, P.R.C. ~Wuhan Machinery and Electronics Research Institute, Wuhan, P.R.C. 423

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H a l f cone method The basic idea of the half cone is to penetrate a halt" cone along a glass plate which is one side of soil container, as shown in Fig. 1. When the cone is penetrating, soil deformation on the soil surface in contact with the glass plate is recorded by camera. The size of soil container if 75 x 30 x 40 cm, and its one side is removable. When the container is being filled with soil, the removable side is a wooden plate. At the time of cone penetration, the wooden plate is replaced by a glass plate with a thickness of 10 mm. The section of half cone in contact with the glass plate is well polished so as to reduce friction between the glass plate and the section of the half cone. Test-bed The test-bed includes three parts: (1) penetration system, (2) hydraulic system and (3) control system. The penetration system is the execution part of the test bed, which involves the frame, penetration rod, half cone, soil container and so on, as in Fig. 2. The hydraulic system is an open oil circuit, as in Fig.3. Its executing cell is a single-rod hydraulic cylinder. A metering pump and an overflow valve form a pressure-regulating return circuit to secure the required pressure and avoid overload. The regulating valve of the oil admission circuit controls speed by the throttling method. The pressure doubling valve of the return circuit improves the working stability of the oil cylinder. The maximum stroke and load of oil cylinder are 800 mm and 600 kg respectively, and the speed range is from 2 to 50 mm/s. The control system includes two electric circuits: (1) controlling the start of the motor (Fig. 4), and (2) controlling overtravel by a limit switch. The second circuit includes a main loop (Fig. 5) and an auxiliary loop (Fig. 6). In the auxiliary loop, overtravel-limit switches 2ck and lck, as shown in Fig. 2, are mounted at the proper position on the frame in order to control the highest and lowest positions of the cone. The normally closed contact Jcz of the intermediate relay for forward stroke is in tandem with the coil loop of the intermediate relay for background stroke, while the normally closed contact Jcf of the intermediate relay for backward stroke is in tandem with the coil loop of intermediate relay for forward stroke. Consequently, when one intermediate relay is working, the coil loop of the other intermediate relay is automatically cut off. This arrangement ensures the accurate action of oil cylinder.

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I M A G E PROCESSING SYSTEM

Choice of image processing board The authors chose a HYIPB1 image processing board, developed by the HuaYe company in April 1989, as one of the hardware. This board is suitable for any type of IBM PC or compatible computer. Its functions are characterized as follows: (1) real-time image collection (50 frames/s), 64 levels of resolving power of collecting gray, and adjustability of input brightness and contrast. (2) 256 x 256 x 8 bit high-speed image memory, 256 × 256 resolving power of image.

Image processing software for soil deformation (1PSSD) Image processing software, provided by the computer company along with the image processing board, is in general incapable of being directly used to fulfil tasks for measuring soil deformation. Therefore, the authors developed an Image Processing Software for Soil Deformation (IPSSD), which is written in C language and compiled by MS C5.0. The name and basic function of some programs in IPSSD are briefly introduced as followings: (1) CLIP program. In order to obtain full information of about the soil displacement and strain field during cone penetration, the vertical soil profile is sprayed with the authors' white chemical mixture in the form of many white blocks. The penetrating cone may only influence a certain number of white blocks. For the purpose of concentration on the area in which white blocks were distorted by the

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penetrating cone, the CLIP program is designed, by using a rectangular frame, to eliminate other white blocks which deliver no useful information for further analysis of soil displacement and strain. After CLIP operation, the remaining image is rectangular. (2) T R A N S L A T E Program. The authors' method to determine soil displacement field is to compare two frames of images. One image is picked before cone penetration, while another image is picked up at a specified moment during cone penetration. In order to analyse two above frames of images on a common basis, a T R A N S L A T E program is designed. When T R A N S L A T E is running, a permanentbright location marker first appears on the top-left corner of the monitor screen, which is used as a common reference point for locating the two above frames of images on the monitor screen. Numeric pads handle image movement in eight directions (up, down, right, left, up-right, up-left, down-right, down-left). (3) P O I N T program. The determination of the angular point position of the white blocks is crucial to calculate soil displacement and strain at the angular points of the white blocks. There are two different approaches (i.e. automatic and manual methods) which can be adapted to this task. With the automatic method, the angular points of the white blocks are automatically identified by computer through a certain algorithm. Unfortunately, the authors found that there were many unsolved technical problems in using this method. For instance, it is hard to identify the four angular points of a white block which has been split into two separate blocks by penetrating cone. Hence, the authors chose the manual (or interactive) method and designed the POINT program to determine the angular point positions. When POINT is running, a measuring marker, a dot cursor, appears at the center of the monitor screen. The numeric key pad is used to move the dot cursor accurately to angular points of the white blocks, and pressing the H E R E key to mark its location. Then, the dot cursor is fixed at that point in permanent-bright form to prevent repetition in data collection, and the values of the x and y coordinates are automatically transferred to an opened stream data file. (4) L I N E program. The main function of the LINE program is to draw a soil displacement isopleth during cone penetration. Because the white blocks are discontinously arrayed in the vertical profile, the authors did not use ordinary algorithms to draw the isopleth. In the light of cone penetration, a rather simple algorithm was designed, which carries out a U-shaped search around the cone and interpolates displacement values between angular points. (5) S L I N E program. According to elastic mechanics, for an originally rectangular block, the shear strain at an angular point is defined as the angular change, after deformation, at that point. Therefore, for an originally rectangular block, if the x and y coordinates of four angular points at one moment during cone penetration are known, the shear strains at these four points can be approximately determined by the angular difference between 90 ° and an angle formed by two straight lines out of four straight lines which connect the four points in turn. A simple algorithm, a U-shaped search around the cone, was adopted to draw the shear strain isopleth.

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(6) O t h e r p r o g r a m s . Other programs dealing with the shape parameters of soil displacement and strain isopleth during cone penetration will be discussed in another paper.

EXAMPLES OF ISOPLETHS DRAWN BY IPSSD Figure 7 shows two vertical displacement isopleths for displacements of 4 and 12 mm respectively, at a cone penetration depth of 240 mm. Here, vertical displacement stands for the m o v e m e n t along the penetrating direction. The t e s t soil is silty loam at 12% water content and 1.3 g/cm 3 dry density. The cone angle is 30 ° and base area 15 cm 2. Figure 8 represents two shear strain isopleths for the shear strains of 33 ° and 50 ° separately at the penetration depth of 240 mm. The type, water content and dry density of the t e s t soil are the same as above. The friction angle of the test soil is 33 ° which equals the shear strain value t h a t one isopleth, mentioned above, stands for. If their exists a unique shear strain threshold t h a t defines the beginning of soil failure for a certain soil type, water content and dry density, or if this threshold varies little with soil stress state, the authors may have an opportunity to use the shear strain isopleth to separate approximately the soil elastic and plastic zones during cone penetration, and furthermore to draw and analyse the shape of the soil failure zone by computer.

LIMITATIONS AND POTENTIAL APPLICATION The limitations of the authors' camera tracing and image processing system for soil deformation mainly include: (1) The accurately sprayed block method is not suitable for very wet soil and for soil without any cohesion. For very wet soil, the water film on the soil vertical surface for observation will blur the white blocks, and make the images indistinct. In relation to the soil without any cohesion, it is difficult to spray the chemical mixture onto the soil vertical surface without causing the collapse of the soil body in a container with one removable side. (2) The authors have to use a glass plate as one side of the rectangular container so that a camera can pick up images of soil deformation on the vertical surface in contact with the glass plate. Even though there may be some limitations on this new method, its potential applications will still be inspiring in the following aspects: (1) Establishing a computer measuring and analysing system for wetting front diffusion in soil from a point water source, which will become a powerful tool to carry out fundamental research on soil drip irrigation. (2) Measuring the large deformation of grain or other solid particles. (3) Measuring and analysing the soil displacement and strain field under a tire or track. (4) Determining soil failure patterns caused by farm implements. (5) Combining with F E M and making F E M much more powerful in determining the displacement and strain fields of various media.

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REFERENCES D. R. FREITAG, Compaction of agricultural soils. Methods of Measuring Soil Compaction, Chap. 3, pp. 47-103. American Society of Agricultural Engineers Monograph (1971). [2] W . J . CHANCELLOR, R. H. SCHMn)T and W. H. SOEHNE, Laboratory measurement of compaction and plastic flow. Trans A S A E 5(2), 235-239 (1962). [3] C. W. B o v o and S. J. WIND~SCn, A technique for measuring deformations within a sand under controlled wheel loads. Proc. 2nd Int. Conf. ISTVS, pp. 183-197 (1966). [4] W . R . GILJ~, Soil deformation by simple tools. A S A E Paper No. 68-145. [5] E . T . SELICJ, K. E. HOFER and N. A. WEre, Elastic response of soil to tracked vehicles. Mechanics of Soil-Vehicle Systems, Proc. Ist Int. Conf. 1STVS, pp. 97-107 (1961). [6] W. B. TRUESDALL and R. B. SCHWAB, Soil strain gage instrumentation. Proc. Int. Syrup. Wave Propagation and Dynamic Properties of Earth Materials, pp. 931-941(1967). [7] J. D. HOVAN~:;SIAN and W. R. BUCHELE, Development of a recording volumetric transducer for studying effects of soil parameters on compaction. Trans. A S A E 2(1), 78-81 (1959).

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