simulation of the resistance forces of bulk media to bucket in a loading process 2007

7/27/2019 Simulation of the Resistance Forces of Bulk Media to Bucket in a Loading Process 2007

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24 th International Symposium on Automation & Robotics in Construction (ISARC 2007)Construction Automation Group, I.I.T. Madras

SIMULATION OF THE RESISTANCE FORCES OFBULK MEDIA TO BUCKET IN A LOADING PROCESS

Ahmad Hemami and Ferri Hassani Dept. Mining, Metals and Materials Eng.

McGill University, Montreal, QuebecCanada H3A 2A7

{Ahmad.hemami, Ferri.hassani}@mcgill.ca

ABSTRACT

In this work the process of penetration of a bucket into bulk media (for loading or excavation) issimulated based on the forces that are involved in the process. The main objective is to simulate theresistance force of medium to cutting by a tool (bucket/blade) during a loading or excavation process.The work stems from an initial purpose to evaluate the effectiveness and performance of a strategy for automation of loading of bulk media (autoloading) by an excavator. In addition to serving for that

purpose, the results of this work can be used in the development of training simulators for constructionand excavation machines that include animation of the motion of a bucket, but lack the behaviour of thematerial during the process.

The encountered resistance by a tool (bucket) dictates its motion. The medium resistance can changesignificantly even within a small area and it depends on the tool orientation. If not controlled, the cuttingtool follows the path with the least resistance. For this simulation a planar motion is considered for thesake of simplification of the complicated process. As can be depicted by a number of examples, the paper shows that the simulation is valid and can be successfully used.

KEYWORDS

Simulation, Resistance Force, Bulk Media

1. INTRODUCTION

The problem of automation of loading by a cyclicexcavator, such as a front-end-loader, has been thesubject of research for more than a decade. Yet, nocommercial product has been introduced to themarket so far, nor a breakthrough has beenreported on the subject. One reason for this is thecomplexity nature of the process and the fact thatthe essential research is time consuming andcostly.

As a necessary part of research in this matter onehas to examine the effectiveness of any controlstrategy as well as comparing various proposedcontrol techniques. As a tool to facilitate such anevaluation and comparison, simulation softwarecan speed up the process and determine the

functionality of a control strategy at the primarystep.

This work is related to what becomes necessary for simulation of the loading/excavating by a machineto be automated. It simulates the interaction

between a medium and a cutting tool or blade thatmust overcome the resistance from the mediumduring loading (cutting, penetrating and scooping).Simulation of properties of materials has become a

popular way for research on many different studiesto be performed. Nevertheless, the scope of application is so vast and the results for oneapplication can be far from what is required inanother. These are mostly offered in the form of commercial simulation packages for a particular application. For instance, modeling and simulationsolutions for studying chemicals and materials can

24th International Symposium on Automation & Robotics in Construction (ISARC 2007)Construction Automation Group, I.I.T. Madras


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Hemami,. A & F. Hassani

be at the level of molecular bonding and crystalstructure, the process of crystallization, andstructure-activity relationships, although all theseare indeed material simulation. Or the work canfocus on the formulation of deformation and fluidflow. Also, it could be concentrated on metals, atemperature treatment on metals, or the emphasiscould be on certain class of material, such as bio-materials. There are plenty of referencesassociated with simulation and materials, twokeywords of this paper, but there is no pointreferring to any publication that is not directlyrelated to this work. As for the objective of this

paper not much previous work can be addressed.

2. APPLICATION OF THIS WORK

The results of this work can be used for applications that call for simulation of a tool or

bucket interfacing with bulk media. As mentioned before, the main purpose has been using this as atool for examination/ verification of the effect of acontrol policy for loading particulate media.Another very important application is in thesimulators that are employed for training of operators for large excavating mashines. Thesedays, similar to a flight simulator for training of

pilots, for many such machines, like a backhoe or grader, it is much more cost effective to train anoperator on a simulator, first, before he gets on tothe real machine. The results of this work can beintegrated with such software. In this way, whenthe operator trainee runs the cutting tool (blade,

bucket, etc.) through the soil or fragmented rock,the software makes a distinction between movingthrough a medium and moving through the air.

Furthermore, based on this work, the trainingsimulator can be enhanced through feedback of theforces exerted to the loading element to the levers,

pedals and the like. In this way the trainee feelsthe effect of his actions in a more realistic manner.

3. LOADING RESISTANCE FORCE

When a tool is in contact with a medium thecutting edge of the tool has to interface with themedium (Hereafter the world tool will be used for the cutting element in an excavator, whether a

blade, a bucket or other, depending on themachine). So, the cutting edge encounters a forcethat must be overcome in order for the tool to

penetrate into or cut through the medium. This

force must be provided by the drivingelements/actuators on the excavating machine.This, however, is not the only force that must beovercome and must be provided by the actuators.Depending on the machine and the cutting toolform and size a number of resisting forcecomponents exist which have a resultant at theactuator level. The actuator, thus, must providedan active force superior to the total resistive force.For a bucket in a front-loader or a loader typeexcavator these force components are shown infigure 1 and are as follows [1]:r 1: The weight of the material in the bucketr 2: the force of the bucket body pushing (andcompressing) the materialr 3: Friction forces of the material moving into the

bucketr 4: Cutting force

Figure 1 Forces From Media of a Bucke t

In addition to these, the active forces of theexcavating machine must move the bucket, itscontents and the material above the bucket thatmoves with the bucket. Analysis, formulation, andexperimental measurement of excavation forcecomponents has been vastly studied by Zelenin etal [2] in earlier decades and more recently anumber of researchers have worked on the subject(see for instance [3][4])

When the active force from the actuator and theresistive force from the medium come to aninteraction then the behaviour of the machine andthe medium has to follow the physical laws. Inthis respect, the machine has stiffness and dampingthat can lead to its deformation. So has themedium that undergoes a physical change. Figure2 shows the model for the tool-medium interaction.For the medium, nevertheless, the stiffness is

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Simulation of the Resistance ForcesProcess

1 8 4

, 1 4

3 0 1 , 8 9

7 0 6 , 8 4 4 0 5 , 3 1

6 0

Cylinder 1 (lower):Min. 404.9mmExtended 706.8mmstroke 301.9mm

Cylinder 2 (upper):Min. 797.7mmExtended 1101.7mm

stroke304.0mm

5 0 0

2 0 0

300,63

2 9 9

, 1 3

5 1 2 , 4 8

879,71

6 9 0, 0

2

6 9 0 ,0 2

relatively very low and damping is relatively veryhigh.

The model shown in figure 2 is for a simple systemcomprising a single actuator. In practice, usuallythere are more than one actuators and the

necessary treatment must include the wholesystem. As a result, the simulation becomes moreinvolved, as well as more meaningful. As for example, when the bucket of a back-hoe or aloader moves the motion is not necessarily astraight line and the direction of all the forcecomponents vary at each instant. For this reason,the simulation code is machine dependent. That is,the simulation code (this software) should beintegrated with the code for moving the variousactuators in a machine.

Figure 2 Tool-Medium Interaction Model

In this simulation a small front loader is used as

the device the bucket of which interacts with thesoil. The conceptual design of this machine isshown in figure 3, depicting the extreme lower andupper reach positions of the bucket and thedimensions. For simplicity the loading motion of the bucket is assumed to take place in a plane. Infact, such a machine has three degrees of freedomwhich lead to a planar motion of the bucket. Theschematic of a model of this machine is illustratedin figure 4. Figure 4 is a model of the loading gear in a front-end loader excavator consisting of a

prismatic joint representing the advance motion of the vehicle and two linear actuators leading torotating motion of the bucket. Thus, the loader isrepresented by a robotic arm having one prismaticand two revolute joints. All the definitions for various dimensions are shown also in figure 3.The associated modeling definitions, kinematicsand force relationships for this class of excavator are given in [1].

Figure 3- Schematic of a Model Loader

Figure 4 Robotic Model of a Loader

4. SIMULATION ANALYSIS

In the simulation we are introducing a force (either fixed or variable) to each actuator and also wedefine the medium properties in the form of a forcevector, then the movement of the bucket under theeffect of this force is considered and recorded.

Out of the five force components that werementioned in the previous section and are shown infigure 1, the component r4 (representing theresistance to the cutting blade) is the dominantforce. Since the simulation at the current stage(the objective of this paper) is for the medium

resistance, considering this force component, only,is acceptable. Otherwise, the effects of the weightof the loaded material and the friction forces mustalso be included. The effect of the weight changecan be included in the mass parameter that is usedas a rigid body on which the resultant of actuatorsforces acts.

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Simulation of the Resistance ForcesProcess

displacement in meters, in the horizontal andvertical directions. All the examples have the samestarting configuration. As is depicted by negativevalues shown in figure 5, the bucket has stayed

below the vehicle platform, near the ground. Inother words, the active forces have not beensufficient to move the bucket through the medium(for loading).

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1.24 1.26 1.28 1.3 1.32 1.34 1.36 1.38 1.4 1.42 1.44

Figure 5 Bucket Motion for Example 1

5.2 Example 2

For this example all the data are the same as beforeexcept for the active forces provided to theactuators that are increased five times by 20% ineach step. The bucket movement and itsorientation at a few points are shown in figure 6. Inthis case the bucket has maneuvered a complete

loading cycle by the time it is in the forth positionshown.

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0

0.2

0.4

0.6

0.8

1

1.25 1.3 1.35 1.4 1.45 1.5 1.55

or=15

or=25

or=60

Figure 6 Bucket motion for Example 2

In this position, the linear actuators have reachedtheir limits and, thus, the orientation has notchanged afterwards, but the bucket has been

pushed further forward. At the level of this paper,we are not concerned about if the motion is corrector if the bucket is filled. Obviously, the forces arenot determined beforehand and the process is notcontrolled. What we are interested in is that if the

simulation of the behaviour of the material isreasonable. From the figure we may realize thatduring the first portion of the motion the forward

push of the bucket has not been sufficient or,saying it differently, the forces of the actuators 2and 3 have been more than necessary.

5.3 Example 3

In this example it is assumed that the same forcescenario as in example 2 is used, with theexception that the material is heavier. The variablerepresenting the material mass are five (5) timeslarger than those in example 2. Everything else(except the random values that determine the finalresistive force) is the same as in example 2. Theresults are shown in figure 7. The bucket hasstopped at the final point shown, meaning that for

the period (simulation duration) the bucket could be moved only halfway. The motion has to becontinued for completion.

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0

0.1

0.2

0.3

0.4

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1.24 1.26 1.28 1.3 1.32 1.34 1.36 1.38 1.4 1.42

or=1 1.46

or=31.2

Figure 7 - Bucket Motion for Example 3

5.4 Example 4

Two changes have been introduced here comparedto the previous example. The stepwise increases inthe actuators and mass have been modified from20% and 10%, respectively, to 33% and 20%. Theoutcome is depicted in figure 8.

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0

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0.3

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0.5

0.6

0.7

0.8

1.24 1.26 1.28 1.3 1.32 1.34 1.36 1.38 1.4 1.42

or=27.5

or=48


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Hemami,. A & F. Hassani

5.5 Example 5

The material density related values (massrepresentations as well as damping coefficients)have been further increased from its values inexample 4; they have been doubled, for a heavier substance. The results are depicted in figure 9,which show a reasonable change (note that themotions for all the three degrees of freedom aresmaller).

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0

0.1

0.2

0.3

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1.26 1.28 1.3 1.32 1.34 1.36 1.38 1.4

or=9

or=30

Figure 9 Bucket Motion for Example 5

5.6 Example 6

For the final example, we have kept all thecharacteristic data of example 5, except that theinitial forces have all been increased by 200 units.As a result, the bucket has been moved further,

particularly, the actions of the linear actuators aremore pronounced.

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0

0.1

0.2

0.3

0.4

0.5

1.26 1.28 1.3 1.32 1.34 1.36 1.38 1.4

or=13. 5

or=48.4


6. SUMMARY AND CONCLUSIONS

The process of interaction of a tool with a mediumis complicated. The resistive force componentsinvolved depend on the tool orientation when a

blade or bucket is moved.

These facts are brought into consideration in asoftware developed for simulation of the behaviour of a medium to cutting in a loading or excavation

process. The simulation software is machinedependent. The paper discusses the moreimportant points and the general approach. Itshows that the simulation is valid and can besuccessfully used, as can be depicted by a number of examples. As such, the methodology and thesimulation code may be incorporated with thetraining simulator software for any type of machine interfacing with bulk or particulatematerial. As well, it can serve for the purpose of evaluating the control strategies for loadingautomation and so on.

7. REFERENCES

[1] Hemami, A. (1994), Modelling, Analysis andPreliminary Studies for Automatic Scooping,Journal of Advanced Robotics, Vol. 8, No. 5, 1,511-529.

[2] Zelenin, A. N., Balornev, V. I. and Kerov, I. P.,

(1985), Machines for Moving the Earth,Amerind Publishing Co., New Delhi.

[3] Blouin, S, Hemami, A and M. Lipsett, (2001)Review of Resistive Force Models for Earthmoving Processes, Journal of AerospaceEngineering, Vol. 14, No. 3, 102-111.

[4] Takahashi, H., Hasegawa, M. and Nakano, E.,(1999), Analysis of the resistive forces actingon the bucket of a Load-Haul-Dump machineand a wheel loader in the scooping task.,Advanced Robotics, 13(2), 97-114.

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simulation of the resistance forces of bulk media to bucket in a loading process 2007

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