Page 90 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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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.

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

3.1.2.1 Hall 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.

3.1.2.2 Current 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.

3.1.3 Wire 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