The Universe

          The Universe has no edge and nothing exists beyond it, not even space. In the vastness of the Universe, the Earth, the Sun and planets are tiny dots. The Sun is merely one star in a galaxy comprising 100,000 million stars. At least as many thousands of million of other galaxies, each with its own star system, extend as far as the largest and most sophisticated of optical and radio telescopes can reach.

          As the Universe expands, all galaxies and clusters of galaxies move apart from each other at speeds that depend upon their distances. The each other at speeds that depend upon their distances. The furthest ordinary galaxy yet detected, Abell 1835 IR is 13,230 million light years away and is receding at 45 per cent of the speed of light. The most distant object in the Universe as on 14 July 2004 is in background of Abell 2218. This corresponds to a distance greater than 2 billion light years. Beyond this, astronomers are looking back to a time when the Universe was born, and the detectable Universe fades out.

          Our present view of universe began with the findings of Copernicus, Galileo, and Newton. What they found was very much opposed by others at the time, mainly because established order was based on Aristotle's teachings. These "new" ideas were thought to diminish the role of humans in the universe, to undermine our importance we have expanded our vision since then by enormous effort, painstaking observations, and an ongoing desire to comprehend our surroundings.

Universe theory:

The Big Bang Theory:

          According to this theory, the Universe had its origin in a giant explosion about 18,000 million years ago. The Matter flung out from the explosion condensed into lumps called galaxies, which are still rushing outwards. As the Universe grows old, the matter in it thins out. The expansion continues indefinitely. The Big Bang theory received its strongest confirmation when 'cosmic background radiation' was discovered in 1964 by Amo Penzias and Robert Wilson, who won the Nobel Prize for this discovery. In 2003, NASA's Wilkinson Microwave Antisotropy Probe made measurements of the temperature of this 'cosmic microwave background' radiation to within millionth of a degree. From these measurements scientists were able to deduce that our universe is 13.7 billion years old and that first generation stars began to form a mere 200 million years after the Big Bang.  

Inflation Theory:

          Immediately following the Big Bang, the universe likely began a period of exaggerated outward expansion, with matter flying outward faster than the current speed of light. This is the inflation theory, widely accepted in the astrophysics community.

The Oscillating Universe Theory:

          This theory, a variation of the Big Bang Theory, suggests that expansion of the Universe will eventually slow down and stop, followed by contraction of the galaxies into another Big Bang. The Universe, therefore, continues in endless cycles of expansion and contraction; the laws of nature may differ in each cycle.

The Steady State Theory:

          An alternative view to the Big Bang Theory, this theory says that Universe never originated at any one instant, nor will it ever die. According to this theory, as the Universe expands new matter is created to fill the space left. Therefore, the appearance of the Universe remains constant with time.

Robotic Arc Welding System (RAWS)

     Arc welding and GMAW/GTAW welding processes are at present the major fabrication techniques in most of the industries, world over. Best results are achieved by automating the process, using robots and microprocessor controlled systems. Mechanisation and automation of arc welding techniques results in consistent quality joints. It has now become indispensable where high rates of production are involved and for welding in inaccessible/hazardous environment because the robots can be inverted, suspended over a welding station or supported in any unusual position. It also overcomes labour fatigue and results in overall economy, higher productivity and improved working conditions.

     Where welding of linear, circular or spiral joints is involved, mechanisation of arc welding process is good enough. However, for compex joints not accessible freely, automation of the process using robots is belter suited. Robotic arc welding system (RAWS) is best suited for batch production involving frequent design changes in a component and even where different components are to be handled one after other. This is possible due to the highly flexible system provided by RAWS. However the justification for installation of such a system has to be looked through return on investment by considering all the expenses (on equipment, material handling devices, training, etc.) and the likely savings on account of increased production, improved quality, saving of energy, men-hours and materials due to reduction in reworking of components, lower turn-over of employees in the shop and reduced burden of strikes, etc.

     Fig  shows a schematic arrangement of robotic arc welding system (RAWS) and the various units involved. The robot consists of manipulator (series of mechanical linkages and joints capable of producing all sorts of designed movements), controller and power supply unit (to provide energy to the activators). Each link of manipulator is driven by activators which may be operated either, by hydraulic/pneumatic power cylinder or electrical motor. The forearm of robot can move in a nearly spherical way, thus covering a large working volume and providing greater application flexibility. It is easily possible to reach down into or onto objects placed over the conveyor. Feedback devices are incorporated to sense the positions of the various links and joints. The information from these devices is fed to the controller. The controller initiates and terminates motions of the manipulator in desired sequences and at desired points through interfaces with and manipulator's activators and feedback devices. It also stores position and sequence data in memory and performs complex arithmetic functions to control path, speed and position. The controller is also lined with other auxiliary devices like power source, wire feed unit, conveyor etc.

The control unit has a computer with lot of computational capability. The movement of torch centre point installed at the end of forearm of robot can be controlled either by

(i) co-ordinated axis control motions, or

(ii) controlled path generation.

     Only the end points in case of linear path land three points in case of circular path are specified and the computer automatically generates the controlled path at the desired velocity including acceleration and retardation.

     An important feature of the RAWS is the searching and following of the actual welding seam or groove/ seam tracking in deviation of the pre-planned line. Without this facility, the programmed welding groove and actual welding groove would be different because of errors due to imprecise component clamping and assembly of improper fit-up and inconsistent orientation of the component, etc. However seam tracking system takes care of these problems and ensures the actual welding grooves to be as per programmed welding grooves.

     In the Case of multi-run welding process, the first (root) pass is welded by using seam tracking and the various off-set displacement are stored in the memory. For subsequent passes, seam tracking is not required because the processor shifts the programmed path data of the obtained correct location for the given set up.

     An oscillation movement of torch is required for bridging of the gaps and for welding in difficult positions. The control unit can provide the oscillation movement with programmable frequency and amplitude for the given requirements.

The control unit also incorporates facilities like axes transformation (for welding similar work pieces at different stations by changing the positions of a set programme with regard to one or several coordinates), mirror imaging (welding identical jobs but from opposite sides of a manufacturing line), random access reserve function (to allow welding job to be carried out in any sequence on a number of working stations), etc. For high positional accuracy of the torch typically ≡ 0.2 mm), the arms around the rotating axis should be balanced, employing needle bearings and using feedback sensing systems having high resolving power.

     For consistent welding operations and weld quality, the torch angle with respect to the work is kept constant by employing gyroax is mechanism, etc.

In case of emergency, the system comes to a halt. The control panel has all the facilities for position status and failure displays. The system incorporates built in diagnostic facilities. Sometimes, a backup system is provided to avoid loss of production in case of breakdown of RAWS.

     

     The operations to be performed by the robot should be within its capacity, such as carrying load, work envelope, speed and complexity. To minimise the down time, it is essential that a team of trained personnel to operate and maintain the system along with adequate tools and spare parts is available.

Automatic Welding Process

          Automation, like in other fields, in welding too, increases production, improves quality and lowers cost. Automatic welding employs various automatic power devices (variable speed drive motors), gas controls, coolant fluid, controls, sequence controls, instruments, gauges, timers, limit switches, etc.

Some of the advantages and limitations of automating arc-welding are listed below.

Advantages:

           High weld rate; uniform weld height, width, fusion and penetration ; low electrode stub loss due to the continuous feed from a reel of welding wire; use of high currents; automatic care of arc length, speed and other variables to produce quality weld.

Limitations:

           Correctly fitting weldment components have to be used to obtain all the advantages, no compensation (in form of speed control) for poor fits and alignments is possible, once the automatic controls have been adjusted.

The various components involved in automatic welding are described briefly below :

Adjustable speed drive motors:

          These are used to move the torch/electrode along the work at a controlled rate, and to feed the welding wire from coils into the weld area. In arc welding the adjustment of arc length is important to obtain quality welds. Change in arc length due to poor fits, etc. is sensed by the voltage changes in the welding transformer and a signal is sent to wire drive motor to speed up if voltage rises and vice versa. The best motor for such cases is the low voltage series wound universal motor whose speed varies proportionally to the voltage. These motors are then electrically connected to the arc circuit so that as arc length increases, voltage in the arc circuit increases which automatically increases the feed rate to adjust the arc length. Manual'override controls to feed wire are also provided.

Gas Controls:

           Gas is used in gas welding and in inert gas arc welding. These consist of pressure regulators, flow meters, and solenoid valves for gas flow control. The solenoid valve may bq energizing by pressing a push button, or automatically as follows. When arc is struck, current flows in the secondary winding which can be used to energizing a relay, which in turn energizing a solenoid. Since gas flow is required for a short time on breaking of arc in order to protect the electrode from contamination during cooling, a bimetal strip is utilized to de-energisc the solenoid valve on gas line with time delay. As long as gas arc is on, current through secondary windings also keeps solenoid energizing. When arc is broken, current in secondary winding falls to zero, but bimetal strip takes time in cooling and changing its position. Thus solenoid valve remains open for some extra time on breaking of arc.

Water controls:

           Some coolant (usually water) is required for cooling transformers, electrodes (in resistance welding), and electrode holders (in GTAW and GMAW welding). The pressure and flow oil cooling medium must be properly controlled to reduce wear and increase efficiency of welding. Interlocks to stop welding machine in case of inadequate coolant supply are provided. The solenoid valve in coolant supply line is energizing  only when welding is “on” by energizing it through the current in the secondary circuit.

         Three- way solenoid valves are used to direct the hydraulic pressure to be communicated to either operate a cylinder to perform certain operation or to reservoir of oil.

        The operation phases of a welding cycle arc controlled by timers which may be mechanical type (cam or clock) or electronic type. Sometimes exact timings have to be controlled as in the case of spot welding of aluminium or stainless steel. This can be done by thyratr on controls in conjunction with ignition controls which permit up to 1500 interruptions per minute. It is possible to control current flow in welding operation to produce regulated amount of heat by phase shifting the AC cycle. Sometimes sequence controls are utilized to stagger starting operation of various welding machines in a shop to reduce the instantaneous heat load on AC supply lines. Sufficient instrumentation is provided in automatic welding operation to monitor working of various electrical, electronic, cooling, pneumatic and hydraulic circuits. Parameters like weld pressure, forge pressure weld current, weld time and forge time are monitored and recorded on a recorder for permanent records. It has been possible to fully automate practically all welding processes, viz. gas welding, arc welding, resistance welding, brazing, cutting, metal surfacing etc.

      It may be understood that automatic welding calls for more safety precautions than manual welding. It should be thoroughly ensured before starting automatic operation that various movements are without any obstruction and hindrances and that various devices during their operation do not move in hazardous area.