A Novel Metal Object Detection System using Asymmetric Triangular Gradiometers for High-power Inductive Power Transfer Applications

A new metal object detection (MOD) system appropriate for various high-power inductive power transfer (IPT) applications such as EV/robot chargers and cordless home appliances is proposed. An asymmetric triangular gradiometer structure based on two vertically stacked triangular spiral coils is newly introduced to overcome a chronic low signal-to-noise (SNR) issue in conventional MOD detection coils. The proposed gradiometers are arrayed to form a detection coil on the surface of a transmitting (Tx) coil so that the detection coil output can represent the magnetic field distribution on the Tx coil surface. A signal processing circuit is newly designed based on an analog divider circuit to distinguish small variances in the detection coil output, which come from an existence of metal objects (MOs), under the high noise environment. The proposed MOD system was fabricated and experimentally verified with a 1kW-level IPT system with reference MOs in Ki standard, which is an international standard for cordless kitchen platform and published by the Wireless Power Consortium. To eliminate blind spots in a target MOD area, two detection coils were overlapped, where the each detection coil consists of 60 triangular gradiometers having a side of 40mm. The results showed that the output voltage of the proposed MOD system varies more than 200 mV for a steel disk having a diameter of 15 mm, which is the smallest MO in Ki standard, apart 5mm from the Tx coil surface.