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 complex joints not accessible freely, automation of the process using robots is better 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.
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Fig. 9.57 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.
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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 and 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 offset displacements are stored in the memory. For subsequent passes, seam tracking is not required because the processor shifts the programmed path data to obtain 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 workpieces 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.
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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 gyro axis 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.