We use a range of commercial and bespoke built electromagnetic (EM) sensors in our work. The bespoke sensors are designed for the specific research project / application being considered but all have the basic principle of being multi-frequency low magnetic field sensors. Examples of bespoke laboratory built EM sensors are given below:
U-shaped sensor 100mm
Portable EM sensor that can be used for characterisation of flat sheet or plate steel samples, figure 1. The sensor consists of one generating coil with 100 turns of 0.24mm insulated copper wire and two sensing coils with 86 turns of 0.16mm insulated copper wire, which were wound on a ferritic U-shaped core with a bridge of 100mm, leg lengths and thickness of 56mm and 25mm respectively. The EM sensor's excitation coil is driven using an impedance analyser Solartron (SL 1250 or 1260A) with an AC voltage of 3V. This sensor is ideal for strip or plate samples and the sensor can be operated at frequencies from1Hz to 65kHz with a high signal to noise ratio. The sensor has been used to investigate the effect of microstructure on EM signals, figure 2.
Figure 1. U-shaped multi-frequency EM sensor
Figure 2. Inductance versus frequency data for pearlitic steels with different pearlite lamellae spacings
Cylindrical EM sensors (air cored/ferrite cored) have been used to investigate rod and bar samples and with FE models (using COMSOL multi-physics software) to allow the magnetic property (low field relative permeability) to be determined for samples of interest, figure 3. The sensors have exciting and sensing coils wound around cylindrical formers of different sizes, figure 4. The EM sensor's excitation coil is driven by an impedance analyser (Solartron1250/1260A) at frequencies from 10 Hz to 65kHz with an AC voltage of 3V. Whereas U-shaped EM sensors are placed onto the surface of a test sample, samples are placed within the cylindrical sensor body and are typically rod or bar shaped, but strip samples are also tested (and modelled).
Figure 3. COMSOL Multi-physics model for cylindrical sensor and rod sample
Figure 4. Different geometry cylindrical multi-frequency EM sensors
High Temperature Cylindrical Sensor
For high temperature (and room temperature) measurements of inductance, which can be used to determine the materials magnetic property (low field relative permeability) from FE models, an air cored High Temperature Cylindrical Sensor is used, figure 5. Samples are placed within the cylindrical sensor body within a furnace and can be heated to above the Curie temperature and cooled with in-situ measurements of inductance with temperature (measured using a thermocouple attached to the sample) taken. These sensors are manufactured using an alumina former, with generating (outer) and receiving (inner) coils wrapped around the former. The sensors are encased in a high temperature solid silica coating, which both protects the sensing and receiving coils as well as holds them in place.
Figure 5. High Temperature Cylindrical EM sensor