cases several possibilities to control the welding process and fulfill requirements defined in the WPS。

It should be noted here that the concept of specifying the weld to be produced within a WPS document is a method to define a procedure to, in a consistent and reliable way, work out the welding process。 It should include information about the actual weld to produce, like joint geometry and material, but also joint preparation, consumables like shielding gas composition and flow, and welding wire, and nominal operating parameters and the productivity and quality to achieve。 In most cases, however, we can only measure some of the parameters needed and from the available observations make judgments on how to control the process to reach the specifications defined in the WPS。

In this way, the WPS is both a specification of the functional specifications of the weld (quality, productivity) and operating data (nominal operating parameters) to reach the specifications。 To use sensors, however, means to measure the real process, extract features from these measurements and, through a control action, override these pre-set parameters in order to achieve the functional specifications, where quality and productivity issues have to balance each other on a case by case basis。 For a typical case, nominal operating data is defined based on specified weld quality and productivity。 This is used to pre-set data of the welding power source and to instruct the robot to generate the trajectory in accordance with defined velocity, welding torch orientation with respect to the weld joint and the distance to the weld joint。 If a sensor is used which through a feed-back loop will alter one or several of these settings, the WPS should include allowed tolerances for all nominal data。 This is also the case if sensors are used for the purpose of monitoring the robotic welding operation。

3。1Sensors for Technological Parameters

Technological parameters include voltage, current and wire feed speed。 In this section sensors to measure those parameters are reviewed。

3。1。1Arc Voltage

The measurement of the arc voltage should, in principle, be made as close to the welding arc as possible。 The current is delivered to the wire at the contact tube and this could be assumed to be a good measuring point for arc voltage。 However, there is a voltage drop between the contact tube and the wire tip where the arc starts of about 0。3 V, depending on the process characteristics [1]。 In practice, it is difficult, if not impossible in a production environment, to measure the true arc voltage。 This is also the case for measuring the voltage at the contact tube in the weld torch,  and a better and more reliable way is to measure the voltage on the wire inside the wire feeding system。 As the wire does not carry a current between the feeder and the

contact tube the voltage at the wire feeder will be the same as that at the contact tube。 It must be noted that measurements are made within an environment that uses high currents, usually in the range of 150-500 A, and if wires used for sensors are placed in the wrong way, this can result in substantial induced voltages with corresponding reading errors。

3。1。2Welding Current

Basically there are two types of sensors for measuring the welding current: Hall Effect and Current Shunt。

3。1。2。1Hall Effect Sensor

The Hall Effect sensor consists of a circular core of cast iron through which the cable that carries the current flows。 The device itself is placed in the gap in the iron core which in turn consists of a doped silicon plate with two pairs of connecting cables。 The first pair feeds the device with a current and the device then responds by delivering a signal on a second pair of cables which is proportional to the magnetic field and thereby the current。 The benefit of the Hall Effect device is that it is a non-contact sensor and does not interfere with the current of the welding power source。 The sensor is limited in bandwidth which is usually in the order of 100 kHz or more and a typical slew rate is 50 A/μs This is usually sufficient and will, provided the data read I/O channels are designed in accordance with the specification of the sensor, result in a rise time of less than 1% of the peak pulse time during pulsed GMA welding。