Topographic point welding automatons ( SWR ) are one of the most technically advanced and most used automatons in the present twenty-four hours industry. Speed & A ; preciseness are two of the major extremely desirable demands during the welding procedure the absence of which can do mechanical failure of the constituents welded about immediately or before the predicted failure time..
From the kineticss perspective these automatons can be defined as 2R or 3R operators. For our MAE 547 undertaking we have chosen to pattern a SWR as a 3R operator to acquire a thorough apprehension of constructs taught in category and to visualise of import constructs such as remarkable constellations, flight planning and many others which remain abstract when merely studied
Topographic point Welding
Spot weldingis a procedure in which reaching metal surfaces are joined by the heat obtained from opposition to electric current flow. Work-pieces are held together under force per unit area exerted by electrodes. Typically the sheets are in the 0.5-3.0mm thickness scope. The procedure uses two shapedcopperalloyelectrodesto dressed ore welding current into a little “ topographic point ” and to at the same time clamp the sheets together. Coercing a big current through the topographic point will run the metal and organize the dyer’s rocket. The attractive characteristic of topographic point welding is a batch of energy can be delivered to the topographic point in a really short clip ( ten to one hundred msecs ) that permits the welding to happen without inordinate warming to the remainder of the sheet.
The sum of heat ( energy ) delivered to the topographic point is determined by the opposition between the electrodes and the amplitude and continuance of the current. The sum of energy is chosen to fit the sheet ‘s stuff belongingss, its thickness, and type of electrodes. Applying excessively small energy wo n’t run the metal or will do a hapless dyer’s rocket. Applying excessively much energy will run excessively much metal and do a hole instead than a dyer’s rocket. Another attractive characteristic of topographic point welding is the energy delivered to the topographic point can be controlled to bring forth dependable dyer’s rockets.
Till day of the month the figure one application of industrial robotics happens to be welding. Spot welding with aid of automatons is a mature application in the car industry. The grounds for this are truth, repeatability, preciseness, efficiency of the topographic point welding automaton. The usage of topographic point welding automatons in an car fabricating unit increases the rhythm clip and besides reduces the figure of culls from the works.
Topographic point welding is the most common application of Industrial Robotics which motivates us to plan a topographic point welding automaton for our undertaking.
The demands of the Spot Welding Robot to be designed must carry through are:
- Precise Positional Accuracy of the End-Effector.
- Link Weights must be every bit minimum as possible to cut down effects of inactiveness.
- Actuator of the automaton must be capable to get the better of torsions, forces and other external tonss moving on the automaton.
The 3R operator has three revolute articulations and three links as shown in the figure below. This is better than 2R in footings of an excess grade of freedom. The ground for taking a 3R operator could besides be attributed to the fact that kinematic analysis of a 3R is more complex than 2R and therefore increases the proficient competency of our undertaking
The 3R operator chosen has the undermentioned nexus lengths
- A1 = 8 units
- A2 = 8 units
- A3 = 6 units
The above dimensions where obtained from TVS motors pvt. Ltd, Hosur, India. They use four topographic point welding automatons in their motorcycle fabrication works for TVS Apache.
Positional Accuracy is the most of import demand the topographic point welding automaton must carry through, so it is really of import to set up the relationship between the work infinite and the joint infinite. In car assembly workss the work infinite co-ordinates are given as input and the automaton is positioned to make that point by altering its joint angles. Thus the demand arises to deduce the relationship between joint infinite and work infinite, this is done by utilizing reverse kinematics technique that is cognizing the work infinite coordinates the joint angles for the links are determined as shown below:
The terminal effecter of the welding automaton carries the welding gun. The speed with which the terminal effecter moves happens to be an of import parametric quantity for consideration as it determines the quality and efficiency of the welded articulation produced. If the speed is to high, the clip for runing the metal at the part to be welded will be less and it would ensue in improper bonding of the parts. On the other manus if the terminal effecter speed is excessively less, the sum of stuff deposited at the articulation will be more than required which consequences in bad visual aspect of the joint and thereby bad expressions of the car. Thus it is really indispensable to find the speed with which the terminal effecter travels, this is done taking the first derived function of the place equations as shown below:
From the above equations the speed, the terminal effecter needs to go with can be obtained. Typical speeds for the terminal effecter happens to be in the scope of 120 & A ; deg ; /s to 200 & A ; deg ; /s [ 1 ] .
Remarkable Configurations are those constellations of the automaton where the terminal effecter looses one or more grades of freedom. Specifying Manipulability Ellipsoids aid to geometrically visualise the different remarkable constellations. At an unmanipulable uniqueness, the spheroidal becomes degenerate ( the lengthof one or more axes become zero, connoting that the ellipsoid has zero volume ) . When the mechanism is near to an unmanipulable constellation, the ellipsoid would besides be severely conditioned, since the length of one or more of the axes will be near to nothing. Hence, a step of the intimacy ” to uniqueness may be chosen to be the status figure of J. [ ]
A simulation was carried out to expose the different ellipsoids for different joint angles, the snapshots of the simulation are shown below.
PROGRAM LOGIC AND RESULTS
3R TRAJECTORY PLANNING SIMULATION
Trajectory planning is required when the operator has to travel to a desired point in a given clip following certain conditions. Trajectory planning is of usage when the operator has to avoid hitting an obstruction or when there is a infinite restraint. There are normally four conditions which any operator has to follow. They are
- Initial place ( in joint infinite co-ordinates )
- Concluding place
- Initial speed
- Final speed
The simplest equation which solves the above four restraint is a three-dimensional multinomial because a three-dimensional multinomial has four constants.The following snapshot shows how the terminal effecter reaches a concluding point through a user defined via point. The via points are marked in bluish
- Simulation shows how the operator moves to make a coveted place while go throughing through the via points which are given by the user. Gives a really good visual image of the overall construct.
- The simulation plots the following variable of involvement
- Theta Vs Time
- Velocity Vs Time
- Acceleration Vs Time
The above graphs help us in see how the initial conditions are depicted diagrammatically. The first secret plan shows the three splines ( three-dimensional multinomials ) that govern the three joint angles. The initial conditions are
3R SIMULATION WITH WELD SPACE CONTROL
This codification when executed asks the user to specify the work infinite by come ining the upper limit and minimal distance of the terminal effecter from the beginning ( in polar co-ordinates ) .
Once it is done the user can seek snaping anyplace outside the weld infinite and notice that the operator does non travel. Shown below is a snapshot from our simulation which shows how weld infinite control is achieved
- Real clip simulation. The terminal effecter reaches to place that is defined by the user
- The user can specify weld infinite. This characteristic is to cut down mistake when the operator is used as a “ Manual handled automaton ”
3R VELOCITY AND FORCE MANIPULATED ELLIPSOIDS
When one tries to utilize a chalk on a board with his/her cubitus articulation directly, presume he/she writes his name on the board. It is clearly noticable that it is an uncomfortable undertaking. This is a layman account of force manipulated ellipsoids. In welding automaton finding the eclipsiss help us to place the following dyer’s rocket topographic point such that the terminal effecter makes it to that place with less attempt ( joint torsions ) .Thought the placement of weld musca volitanss in the above mode may be tough it is a good pattern to do certain that force and speed manipulability ellipsoids are studied.
The figure displayed following shows the speed manipulabilty ellipsoids at clip T = 0 sec. From the oval it is clear that the operator can travel along the way indicated in the figure. This is about a remarkable constellation as described in the old subdivision
In the following figure the oval has still been expanded along the minor axis demoing possibilities of traveling through the minor axis way. At the ulterior portion of the simulation shapes about similar to circle occur bespeaking equal moving potencies along the both axis.
- Mark W. Spong, S. Hutchinson, M. Vidyasagar, Robot Modeling and Control, Wiley Publishers, 2006
- John J. Craig, Introduction to robotics: Mechanicss and control ( Addison-Wesley, Reading, MA, 1986 ) .
- Saeed B. Niku, An debut to robotics analysis, systems, applications ( Upper Saddle River, N.J. : Prentice Hall, 2001 ) .
- John F. O’Brien and John T. Wen, Passive Joint Braking: A Solution to Unstable Singularity www.mathworks.com Mathworks Forums