LIBRARY ı6ıul 0) - Cranfield University

c) Ultrasonic sensors;
d) Through-the-arc sensing;
e) Optical sensors.
2.6.1.3.1 Electromechanical sensors [refs. 114,124]
These sensors often appear in the form of a contact probe which outputs
distance changes based on an electric signal with a tracer applied to a grove. The
tracer is classified into a one or two degree-of-freedom sensor (see Figure 2.19). It
can also be categorised by the output signal, which can be analog, proportional to a
distance (potentiometer, LDT") or digital (ON/OFF), based on the application of a
limit switch. They have been widely used for a long time but they are slowly being
substituted by more sophisticated methods such as through-the-arc sensing and
optical sensing.
One major disadvantage of this method is that the sensor must be mounted
close to the torch, which can cause problems in joints with difficult access.
2.6.1.3.2 Inductive sensors
These are non-contact sensors normally used for measuring distance from a
conducting material. They are based on the principle that when a magnetic flux which
interlinks a conductor is subject to change, an eddy current is generated in the
conductor in order to compensate for the former change. In the case of a sensor, the
magnetic field is generated by a coil, which has its impedance increased when a
distance to a conductor increases [ref. 114].
When these sensors are required to be used in a welding environment, they
must have an enhanced design to protect against the generation of welding-induced
disturbances due to the arc electromagnetic field and its intense heat. Such enhanced
design is accomplished by using different coils for excitation and detection [refs. 114,
125]. A common approach is to use a magnetic field generation coil and two
detection coils [ref. 114] which are designed based on the same winding number and
differential connection so as to inhibit the effect induced by the other magnetic field.
In a single coil type, a weak DC current is imposed on a high frequency current and
the drop in the DC resistance is used to detect the temperature of the coil and
compensate its effect. Practical magnetic sensors present sensitivities of 0.1 min and
the scope of measurement distance is 10mm. Depending on the type of joint, different
geometrical arrangement of the coils may be used [refs. 125,127].
Seam tracking techniques based on magnetic sensors are reported by Nomura
et al. [ref. 114] and Goldberg [ref. 127].
2.6.1.3.3 Ultrasonic sensors [ref. 114,126]
An ultrasonic sensor can be used to measure the distance between the sensor
and a base material by determining the time of flight of a ultrasound pulse, which is
emitted by the sensor, echoed on the surface of the base material and received by a
" Linear differential transformer.
32