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.Automation in Construction 7 1998 401411 A concept of digital control system to assist the operator of hydraulic excavators L. Ponecki ), W. Tra mpczynski, J. Cendrowicz Abstract A concept of digital control system to assist the operators of hydraulic excavators is presented and discussed. Then, control system based on described ideas was mounted on a special numerically controlled stand, equipped with DrA and ArD converters, where small hydraulic backhoe excavator K-111 fixtures were used. Experimental results shows that it fulfils all described requirements and can be used as the machine operator assist. It enables for precision tool guidance, . automatic repetition of realized movements, realization of specific tool trajectories including energetically optimal paths and automatic improvement or optimization of realized paths. Tool trajectories can also be prescribed using the setting model, making excavator the machine of teleoperator class. Presented system can be used as a basis for real machine control system. q1998 Elsevier Science B.V. All rights reserved. Keywords: Digital control system; Hydraulic excavators; Tool trajectories 1. Introduction The automation of heavy machines, including hy- draulic excavators, began in mid-1970s and was possible due to invention of real time controllers and hydraulic elements with good dynamic properties. The first excavator equipped with several mechatron- ics systems, which was shown as a working model, was the excavator FUTURE prepared by Orenstein and Koppel for BAUMA83 Fairs. Since that time, machines equipped with systems automating the en- gine operation, pumps operation, machine fixtures, machine diagnostic, etc., are presented and offered. Such systems bring real help to the operator and clear economical profit. For example, LIEBHERR R902 excavator equipped with LITRONIC System .has for a trench digging the efficiency 40% higher ) Corresponding author. and unit costs 30% lower, than similar machine without such automatic system. Although automation .in some case, optimization of several machine sys- tems develops quite fast, the main machine process the shoving processhas no proper understanding and description until now. Its automation is quite limitedto systems repeating already performed .movements, laser levelling systems, etc.and sys- tems optimizing such processes are not developed yet. Quite new experimental results show clear idea for energetically optimal tool trajectories in the case of cohesive materials. The tool tip has to be guided along slip lines, which are generated from the tip tool during the previous stage of the shoving process. To realize such trajectories for practical purpose and real machines, it is necessary to build a special control system for the tool motion, which enables automatic realisation of such trajectories as well as realisation of other tasks that help the operator. 0926-5805r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. .PII S0926-5805 98 00045-4 ()L. Ponecki et al.rAutomation in Construction 7 1998 401411402 Taking into account up-to-day heavy machines de- velopment, such system has to fit for digitally con- trolled electro-hydraulic drives. The concept of such control system, and verifying experimental results, are presented in this paper. 2. The optimization of tool trajectories wxIt was experimentally found 1,2 that in the case of earth-moving process due to heavy machine tools, the cohesive material deforms to generate rigid zones .sliding along the slip lines well visible cracks along which the material substantially changes its parame- ters the initial cohesion c decreases to the residual .value close to c s0 . In the case of the simple tool r pushing process perpendicular wall, the force-dis- placement relation shows that the horizontal force grows as the process advances, but in an unstable manner. The moment of reduction of the force coin- cides with creation of a kinematical mechanism orig- inating from the tool end. Such mechanisms are created periodically and can be described theoreti- cally using kinematically admissible mechanisms of wx .the theory of plasticity 48Fig. 1 . A lot of effort was made to describe the soil cutting process within the theory of plasticity, where the problem of active pressure exerted by a rigid wall on a granular medium under plane strain condi- .tion can be assumed as a simplified model for soil shoving. In such case, the method of characteristics wxwas used 3,9 and several theoretical solutions for .statics and kinematics were obtained under the as- sumption of rigid-perfectly plastic soil behavior. Al- though a number of boundary value problems were solved in this manner, there exist several limitations in obtaining complete solutions or even the kinemati- w xcally admissible ones 9 , particularly in the case of more advanced earth cutting processes. Another approach, based on kinematically admis- w xsible mechanisms, was proposed later 5 and applied for the description of more advanced earth shoving wxprocesses 6,7,1012 . Let us discuss the problem of plane strain rigid wall shoving presented schematically in Fig. 1. As- suming the material to be rigid-perfectly plastic and to obey the CoulombMohr yield criterion: 11 sysqsyssinyc coss01. . 1212 22 where: cmaterial cohesion,winternal friction angle. The flow rule takes the form: EGs. i j sl2 .i j Esi j .where Gsrepresents a plastic potential. i j In the case when potential is described by the yield criterion Eq. 1 , the associated flow rule is assumed, when another function is taken, the flow rule is non-associated. Using this approach and assuming a change of wxmaterial parameters within the slip line6,7 , the Fig. 1. Typical deformation of cohesive soil in the case of the advanced shoving process realised by the horizontal tool motion theoretical . solution . ()L. Ponecki et al.rAutomation in Construction 7 1998 401411403 different kinematically admissible solutions for a rigid wall shoving process can be proposed and the solution predicting minimum energy is searched. Kinematically admissible solution for rigid wall shaped as letter L is presented in Fig. 1. It shows main effects observed experimentally. As the process advances, the horizontal force grows in unstable manner and the moment of reduction of the force coincides with creation of a kinematical mechanism originating from the tool end. Such mechanisms are wxcreated periodically. It was shown2,6,7,10that such theoretical predictions describes quite well the main effects observed experimentally. Taking into account experimental observations and theoretical solutions, it is possible to show experi- mentally that as soon as slip lines are created within cohesive material, the energetically most effective way of the tool filling is to follow previously created wxslip line by the tip of the tool 12 . Experiments were wxperformed on a special laboratory stand 1,12 under plane strain conditions, using an artificial material, imitating a clay and its parameters. It consisted of: cement50%, bentonite20%, sand18% and white vaseline12%, and was characterized by the following parameters:ws248 internal friction an- . 3 gle ,gs18.4 kNrm . Application of white vase- line, as one of the components, resulted with obtain- ing a cohesive soil, which parameters were not influ- enced by air humidity and liquid flow. It also en- sured that those parameters were stable during all experimental program. wxTypical experimental results12are shown in Figs. 2 and 4 for equal amount of dug out material .about 600 Nin a following way. The L-shaped .Fig. 2. Experimental program for slope sample: a model of the .tool and slope; b initial stage of the processhorizontal move- .ment;cadvanced stage of horizontal movement and various .trajectories; d the final stage of the process. Fig. 3. Values of specific work for different inclination of the withdraw lines in the case of two-phase piece-wise trajectory. tool, inclined at an angle of 58 simulating the real .process, LAs180 mm was first pushed into slope .to a certain position Fig. 2b . When the tool was advancing, the slip lines were created periodically from the tool tip to the material free boundary, at the .angleas458. In the next phase withdraw phase , the tool tip was moved along three different straight .lines Fig. 2c , with simultaneous rotation of the tool .Fig. 2d to have it filled with the material. Those straight lines were inclined at anglesas308, 408 and 508. The valuesas408 and 508 were close to the inclination of slip lines created during the tool .horizontal pushing process Fig. 2c . It means that in such case, the end of the tool was moved almost along the slip line, where material cohesion c sub- stantially dropped as a result of material softening during the slip process. Specific energy of those processes for different preliminary horizontal displacements, chosen to en- sure similar amount of dug out material in every test .600 N , is shown in Fig. 3. It can be seen, that in the case ofas308, the specific unit energy is much higher than foras408 andas508even over .100% . Hence, conducting the tool tip along the line inclined at the angle similar to the angle of slip lines inclinations, the specific energy of the earth-filling process can be significantly reduced. Experimental results shows that in the case of .cohesive soil earth-moving process: 1 material de- forms as rigid zones sliding along the slip lines where material substantially changes its parameters . .cohesion ; 2 moving the machine tools along pre- viously created slip lines, one can substantially save .energy used for earth-moving processes tool filling . ()L. Ponecki et al.rAutomation in Construction 7 1998 401411404 This observation can be the basis for the optimiza- tion of the filling process. 3. The basic concept of the computer aided con- trol system It was shown before that analyzing the mechanics of the soil deformation during the shoving process, it is possible to determine energetically optimal cutting tool trajectories. Hence, the automatic tool move- ment along slip lines generated in cohesive material has to be a quite important option of proposed system. It should also enable precision tool guidance, automatic repetition of already realized movements .for example, teach-in , realization of some tool movements impossible to realize manually, etc. Taking into account to-day experience with au- tomation of heavy machines, such system should be constructed to assist machine operator, who still plays a main decisive and control role. Hence, the proper separation of tasks, between the control sys- tem and the operator, is necessary. Such control system for excavators was built on laboratory scale. Its basic assumptions can be stated wx .as follows 13 : 1 operation of the central control system is based on cooperation of two digital sys- tems. The first one controls directly the motion of the machine fixture using the control system of the hydraulic cylinders position. The second one works .out control signals for the first one. 2 Under the standard work conditions, action of the proportional hydraulic valves of the fixture cylinders is controlled through the computer. The direct operator control is .possible only in case of emergency conditions. 3 The feedback between the machine environment and control system is realized through the operator. He participates continuously in the process of the con- .trol of machine fixtures motion. 4 For realization of the tool motions which are impossible for manual control, the operator has a possibility to coordinate displacement of separate cylinders by means of hard- .ware or software. 5 The operator has a possibility to switch into automatic control of the fixture motion to realize a special tool trajectories. For example, it can be energetically optimal tool trajectory where tool tip moves along slip lines or specific trajectory .realized and stored previously. 6 The optimal cut- ting tool trajectories can also be realized as correc- tion of trajectories given by the operator. Such cor- rection is done mainly during the time parametriza- .tion of the tool path. 7 The trajectories given by the operator can be corrected by the system to take into account such limitations as geometrical ones, maxi- mal power of the pump, maximal output of the pump, maximal pump efficiency, etc. Presented concept is based on such cooperation between the operator and control system that the fixture movements are controlled by the operator while the control system corrects him or, when ordered, can act automatically. 4. Examples of the control system functioning The control system based on described above ideas was mounted on a special numerically con- trolled stand, equipped with PC computer having CrA and ArC converters, where small hydraulic wxbackhoe excavator K-111 fixtures were used 1417 . The control system of the fixture motions utilizes the control system of the cylinder positions. The fixture cylinder displacement is controlled by the propor- tional hydraulic valves fed by the variable output multi-piston pump. The control system for fixture cylinders is based on three control systems, each to control different cylinder displacement using PID or state controllers wx14 . It enables control of the fixture motions using different methods of the tool trajectory planning, measuring of acting forces and displacements and determining other magnitudes related to the fixture movements. Experimental data acquisition is also possible. One of quite important problems, which should be taken into account when building the control system, is the way of the tool trajectory planning. It is .wxrealized as usually in two steps 15 . In the first one, the trajectory shape is planned and determined. In the second one, the trajectory curve is parametrized in time in a determined manner, what defines the trajectory within the generalized coordinate space. On this basis, the time runs of the generalized coor- dinates describing the configuration space of the machine are determined. In the case of an excavator, lengths of hydraulic cylinders are those coordinates ()L. Ponecki et al.rAutomation in Construction 7 1998 401411405 and then they are used as signals for control system to reproduce planned trajectory. Some system abili- ties are described below. 4.1. The tool moement along prescribed line The control system build for experimental stand wx1517enables, among others, programming the work motion in the excavator work space, or in its configuration space, using point to point technique. In this method, the coordinates of the initial and final points, and sufficient number of the characteristic nodal points, are defined. Values describing this points are then introduced to the system, where remaining points of the trajectory are calculated us- ing interpolation methods. Linear or the third degree polynomial interpolation is used. The trajectory parametrization in time can be realized through: determination of the total trajectory run-time and its division into individual segments of the path. System calculates the velocities of cylinders, determination of the run-time between following nodal points, taking into account some limitations .or conditions for optimization . In the case of standard excavator construction, it is quite difficult to precisely realize trajectories, where simultaneous movement of two or three cylin- ders is necessary. 4.2. The tool moement using the setting model Another method of controlling the fixture motion is to control using the setting model. It is somehow similar to the manipulator unit in robotics. The con- wxtrol is carried out by means of the phantom 18 , understood as a kinematic duplicate or the model of the machine kinematics, equipped with systems mea- suring the motion parameters. The excavator con- trolled in this manner becomes the machine of tele- wxoperator class 19 . The setting model is the model of K-111 excava- tor fixture, situated on the plate, made in scale of 1:10. Three potentiometers are located on the rota- tion axes of the model element. Signals from these potentiometers allow us to determine the configura- tion of the fixture. Mechanical end stops are pro- vided to the model, to limit the rotation angles of individual fixture elements to the values obtained in the K-111 excavator fixture. The special switch acti- vates the system. The setting model is used only for planning of the tool path and during its movement the tool trajectory is registered using the point method. The trajectory points are registered when: the sum of the cylinder length increments, com- paring with former position, is higher than as- sumed, time of data registration is later compared with former registration time. Points of the path are registered at constant time intervals, excluding the fixture stoppage. The path nodal points are defined by the corresponding fixture cylinder lengths. Other path points are calculated by computer applying the linear interpolation in the configuration space. Deviation of the path calculated in this way from that marked by the setting model could be disregarded at the nodal point intervals correspondingtoseveralsamplingperiods. Parametrization of such path is realized on the basis of the assumed output of the hydraulic feeder. Hence, the system operates through nodal point registration and determination on the basis of the already de- scribed nodal points and assumed output of the .feeder of the set points for the control system of the fixture cylinder positions. If the motion of setting model is slow, for the properly assumed feeder output, the real excavator fixture moves like its model. For faster motions, the path planning ad- vances its realization by the real excavator fixture. Experimental results for t