Sensor design plays a crucial role in achieving optimal inductive-sensing designs.
Sensor Resonance Frequency
The sensor requires a capacitor (C) in parallel to form a parallel resonant circuit. The inductance and capacitance of that circuit determine the resonant frequency (f0) according to the simplified expression f0= 1/(2π √LC)
The range of currently supported inductance is indicated by the graph below, where the minimum resonant frequency of operation is 45 kHz and the maximum is 3 MHz.
Range of Supported Inductance and Capacitance Values
The supported capacitance range is 0.1 nF to 470 nF. The 0.1 nF lower limit is defined to reduce the effect of coil parasitic capacitance on the resonant frequency (f0). The upper limit is based on the availability of NPO (also referred to as COG) grade capacitors, which are easy to obtain up to 470 nF.
The supported inductance range is 1 μH to 10,000 µH. The 1 μH lower limit is defined by practical inductance values that can operate with the Lx frequency up to 3 MHz. Inductances > 10 μH are preferred.
The shape of the inductive sensor is important because it determines the shape of the generated magnetic field and therefore the change in inductance in the presence of a target metal object. The following are common shapes of PCB and flex coils:
- Circular coil: Circular coils are generally used when sensing a target object that is moving orthogonal to the sensor plane.
Circular Coil with Illustration Showing Optimal Plane of Movement
- Hexagonal coil: Hexagonal coils are designed to approximate circular coils in cases where a circular oil is difficult to manufacture.
- Square coil: Square coil provides optimal performance with respect to sensitivity in both horizontal and vertical directions.
Hexagonal Coil and Square Coil Examples
- Rectangular coil: Rectangular coils can be used to detect movement along a preferred axis.
Rectangular Coil with Illustration Showing Optimal Plane Movement
For a given shape, the sensor coil is specified by the following parameters, some of which are illustrated in the figures above:
- n, the number of turns.
- s, the turn spacing.
- Din, the inner diameter.
- Dout, the outer diameter – usually either Din or Dout needs to be specified; the other can be derived from other parameters
To find out more about inductive-sensing designs, check out the Inductive-Sensing Design Guide.