In this important historical period, focused on the environmental care and in the waste recycling process, engeneering of efficient mechanical systems assume an important role maintaining however an ecological conformation. The object of this work is the modelling and the simulation of a system for emptying rubbish containers for small city compactors.
Fig. 1 Vehicle with bin lift system
The aim of the project is to virtualize the process of anchoring and emptying the containers for domestic use in the tank of a small-sized eco-compactor, with attention to the dynamics of the loading-unloading cycle, the forces involved and the simulation of bodies in the passage from the box to the emptying tank:
The key steps of the work are the following:
- 3D modelling of the entire system (Porter, Tank, bin-emptying-system, box and various bodies of elementary geometry) on SolidWorks;
- Importing in ADAMS of all components;
- Positioning of components;
- Definition of component properties (masses, moment of inertia) and setting of reference frames with relative orientations;
- Selection of joints;
- Simulation of emptying process;
- Creation of simple geometry bodies and contacts (body-to-body, body-to-bin, body-to-box);
- Bodies motion simulation and criticity solving;
- Comparison with same mass tank emptying;
- Evaluation of the effort on the cylinders.
Most of multibody sistem components have been designed by Solidworks 3D software. The images below show the parts created.
Fig. 2 Sistema Alza Cassonetti
Fig. 3 Assembly
The kinematic chain is composed by a cylinder for lifting the bin and a 4 bar linkage that allows, thanks to a second cylinder, the rotation of the box in order to carry out the emptying. For educational purposes, It was chosen to study the case of only two cylinders, instead of 4 (2 pairs), which still allow the correct kinematics to the system.
Fig. 4 Kinematic scheme
For each component was defined a material in order to know exactly important properties like mass, moments of inertia and center of gravity position. Once 3D modelling process has been completed, it has been imported into ADAMS. All the components have been put in the correct position and orientation, coordinates of position and orientation of “markers” have been verified and, if necessary, corrected with exact mass properties of such part (extract by SolidworksÒ).
Fig. 5 Example of mass property
One of the most important step in the multibody model development is surely selection and correct positioning of joints, for a correct cinematic of motions between the rigid bodies involved. Type and number of joints affect also the system’s degrees of freedom, while redundant constraints can cause integration problems. According to Grubler’s formula, in fact, each kinematic joint removes a certain number of d.o.f. In this specific case, there are 2 d.o.f. for the kinematics (1 for lifting, 1 for rotation) and 1 for the cover (simply connected to the box and free to rotate), while all others are due to the bodies which are inside the box.
Components involved in this projects are:
- Porter Frame called “TELAIO_PORTER”
- Moving arm called “COMP_2″
- Press called “PRESSA”
- Bin cover called “COPERCHIO”
- Bin box called “CASSONETTO”
- 42 mm Cylinder composed by “CIL_42_A” and “CIL_42_B”
- 60 mm Cylinder composed by “CIL_60_A” and “CIL_60_B”
- Cube called “CUBO”
- 2 Spheres called “SFERA_1″ and “SFERA_2″
Joints involved in this project are:
- 1 revolute joint between bin cover and bin box
- 2 cylindrical joint between pistons and cylinders
- 1 fixed joint between bin and press
- 2 revolute joint between cylinders and porter frame
- 2 spherical joint between cylinder-moving arm and cylinder-porter frame
- 1 revolute joint between moving arm and press
- 1 revolute joint between moving arm and porter frame
Fig. 6 Example of info window
Before studying the motion of elementary-shape bodies that are emptied in the Porter’s tanks, the correct kinematics of the system have been verified and it has been analyzed the motion of the container due to cylinders: it has been verified that the bin is initially lifted vertically thanks to the parallelism of the two arms of “quadrilatero” (one of which arm is the second cylinder), and then rotated by the second cylinder, until the bin is parallel to the rear wall of the tank, very inclined; This shape of porter box has multiple functions: it allows waste not to fall too quickly into the bottom of the box and it lets the positioning of the emptying system inside the vehicle size.
Fig. 7 Simulation
Basical geometry bodies were created to simulate the waste: 2 spheres of equal diameter and different masses (to analyze the differences) and a cube. A cylinder was also created but the whole system was too computationally complex for the available hardware performances. This criticity has been shown by excessively long simulation times and completely unlikely results. Simulation problems have been found even in the case of 3 bodies. That issues has been solved acting on stiffness and damping parameters of the imposed contacts. The complexity of this type of simulation implies possible negative effects on the numerical integration made by software, which graphically shows body rebounds and improbable trajectories, as well as, in the worst cases, the “failure” of the simulation. Also the “step size” simulation parameter affects the success of the simulation. The time needed to complete a total cycle was estimated to be 20 s. It must be added the time needed to attach/disattach the box to the Porter frame. Once the best compromise was found, measurements were made of the force exerted on the cylinders by the moving load, obtaining the following results:
Fig. 8 Simulation
Fig. 9 Stress and angle as a function of time
It is evident that these results allow the designer to accurately evaluate the forces that the cylinders must exert on the system in order to complete the process of emptying the box and the power supplied by the hydraulic circuit to carry out the work in reasonable time. It is much evident that the graph above shows an improbable trend in the forces on the cylinders: this problem is caused probably by whole system stiffness, non-linear emptying of bin and peaks due to movement discontinuity (STEP function caused by). So it was thought to compare these stresses with the case of small (more realistic) objects, idealizing the whole load in a tank that gradually empties itself. To simulate this scenario, it was choosen to express the weight force of the discharged volume (assuming a total mass equivalent to the previous case) as a function of the emptying angle of the box, i.e. the angle between the horizontal axis and the inclination of the box. This force point of application is a sphere of negligible size and mass that moves according to the coordinates of the center of gravity of the tank, again according to the emptying angle. The following results were obtained.
Fig. 10 Fluid and angle as a function of time
Fig. 11 Forces on actuator as a function of time
Fig. 12 Position of the CM of fluid and related weigh force
Fig. 13-14 Table with position of CM and weight force as a function of angle
The analysis shows a more fluid distribution of loads in the case of the tank, which is not affected by shocks due to the contacts between the bodies in the rotation and slamming on the walls of the bin.The phenomena of “packing” have been neglected: It often enforces the waste not to move off and caused an opposite reversal moment in the last phase of rotation of the box: this last aspect influences the choice of the cylinders and the design of the whole system.
The project, which aimed to modelling a cinematic system for everyday use in urban centers, showed the versatility and usability of a multibody software like ADAMS, which allowed to study the behavior of the system and the stress transmitted to the frame, in the case of solid rubbish and in the case of same mass emptying tank.
PIEMME ALLESTIMENTI (Ponzano V.to) drawings archive.