3。1。2。2Current Shunt

The principle is to let the current flow through a resistor and measure the voltage drop across it, as if measuring a current with a multimeter。 The method is simple but has some drawbacks。 The resistor must be kept low and hence the voltage signal measured will be small and sensitive to noise。

3。1。3Wire Feed Speed

The wire feed speed is a major parameter to control to achieve a stable welding process。 The welding power source is in most cases controlled to produce a constant voltage and the preset parameters are usually voltage and current。 However, in reality, a current will represent a certain wire feed speed and the common method is to apply a constant value of voltage and wire feed speed and let the current adjust itself accordingly。 A higher wire feed speed will produce a higher arc current that will deposit the wire faster and vice versa with a smaller value of the wire feed speed。 Modern welding power sources have various  control

schemes, but the general principle is to keep the voltage at a preset constant value。 As a result, the current will vary during the process as a result of changes in the environment of the arc welding process, like changes in the distance from the weld torch to the work-piece。

These changes can occur as a result of the movement of the robot or geometrical variations in the weld joint。 However, changes of speed can also originate from the wire feeder system and in the end produce poor results with respect to quality。

The wire is fed through a conduit and the wire feed speed is generated by a feeding unit。 The unit usually has driving wheels that feed the wire through the conduit at a specified speed。 In normal robot systems, the wire feed unit is mounted  on the robot arm rather close (the order of 1 m) to the weld torch, giving a reliable push- feed of the wire during the welding process。 However, in some cases, longer conduits must be used and since the wire diameter must be slightly less than the inner diameter of the conduit, a varying wire speed will result during welding when the conduit is bent and twisted。 In practice, a push-pull wire feed system should be used to counteract this problem。

Measuring the wire feed speed is a big problem and for laboratory purposes custom-made solutions can be build that measure the speed at the contact tube。 A more realistic approach in a production system is to measure the controlled speed of the drive wheel of the feeder unit。 However, this must be complemented with securing the functionality and reliability of the feeder system as its robustness is important for the resulting quality of the welds。

3。2Sensors for Geometrical Parameters

Sensors for geometrical parameters must be able to obtain information about the weld that relates to the geometry of the weld joint。 This information is of great importance in order to both perform seam tracking and use this information for quality control of the weld。 This can be done in many ways but in most cases, a seam tracker is able to extract some information about the weld joint besides the target positions for the weld joint during welding。 Examples of such data include deviations from a nominal path, orientation changes and gap size。

To apply sensors in robotics welding means, in general, to use the sensors during welding。 In some cases sensors can be used to measure the position and orientation of the weld joint or work-piece before welding。 In such cases, different location techniques can be applied in a similar way as for locating any work-piece such as image recognition or binary sensors to detect the position of some plates of the work-piece。

The challenge, however, is to use sensors during welding。 Due to the harsh environment with high temperature, intensive light and high currents, purpose built

sensors must be applied。 The most common use of sensors are (i) optical sensors that use a laser light source on the weld joint under study and a sensor with a narrow bandwidth filter to extract the information of interest, and (ii) through-arc sensing that uses the electrical parameters from the arc together with knowledge about the motion of the weld torch which is controlled by the robot。