This thesis presents the design of analogue electronics, including non-invasive magnetic field and surface potential sensors, for a new food sensing and imaging system. This system is targeted towards the dairy processing industry, to detect and characterise foreign objects flowing through food processing pipes. The food sensing and imaging system employs a new imaging technique known as electric and magnetic field detection electrical admittance tomography (EMFDEAT). This involves applying a sinusoidal excitation to a semi conductive medium and reconstructing the conductivity distribution from measurements of the surface potential and surrounding magnetic flux density. Two rings of electrodes, each containing 16 segments, are used to apply a 10 kHz sinusoidal excitation across a length of pipe. Changes in local conductivity inside the pipe due to a foreign object distort the applied current distribution, which is detected by an array of 128 magnetic flux and 128 surface potential sensors. The sensor array consists of 8 stacked rings of sensors, each containing 16 magnetic flux and 16 surface potential sensors. The sensor rings, which are located between the two electrode rings, are designed to fit around a 110mm (outer diameter) pipe.

The magnetic field sensors consist of search coils, made by winding 1000 turns of 0.1mm diameter copper wire around an air-cored coil former, connected to an amplifier. The magnetic field sensors achieve a sensitivity of 2.45 V/μT with a 1.5 pT /√ Hz noise level at 10 kHz and dynamic range of ±2 μT. The surface potential sensors use transimpedance amplifiers to amplify a displacement current between the food surface and an electrode embedded in a circuit board through a 5mm PMMA (perspex) pipe wall. The surface potential sensors achieve a sensitivity of 10V/V with a 17.5 μV/√ Hz noise level at 10 kHz and dynamic range of ±500mV. Both sensors fit within a 20 × 20mm printed circuit board (PCB), with the surface potential sensor electrode located directly underneath the search coil, making the two sensors collocated in the circumferential and axial directions. Preliminary test results using a single ring of sensors indicate that raw data from the sensors can be used to detect the presence of a small plastic or metal object within a pipe filled with a saline solution that has a conductivity similar to milk (approximately 0.5 S/m).

A Current transformer (CT) is used to produce a scaled-down replica of the excitation current at each electrode, which is measured using a transimpedance amplifier. An output filter is designed for a class-D amplifier chip, which can be used to generate the 10 kHz excitation at each electrode.