SNOA961A February   2017  – February 2023 LDC2112 , LDC2114 , LDC3114 , LDC3114-Q1

 

  1.   Inductive Touch System Design Guide for HMI Button Applications
  2. 1Mechanical Design
    1. 1.1 Theory of Operation
    2. 1.2 Button Construction
    3. 1.3 Mechanical Deflection
    4. 1.4 Mechanical Factors that Affect Sensitivity
      1. 1.4.1 Target Material Selection
        1. 1.4.1.1 Material Stiffness
        2. 1.4.1.2 Material Conductivity
      2. 1.4.2 Button Geometry
      3. 1.4.3 Spacing Between Target and Sensor
    5. 1.5 Layer Stacks of Touch Buttons
      1. 1.5.1 Conductive Surface
      2. 1.5.2 Non-Conductive Surface
    6. 1.6 Sensor Mounting Reference
    7. 1.7 Sensor Mounting Techniques
      1. 1.7.1 Adhesive-Based
      2. 1.7.2 Spring-Based
      3. 1.7.3 Slot-Based
    8. 1.8 Mechanical Isolation
  3. 2Sensor Design
    1. 2.1 Overview
      1. 2.1.1 Sensor Electrical Parameters
      2. 2.1.2 Sensor Frequency
      3. 2.1.3 Sensor RP and RS
      4. 2.1.4 Sensor Inductance
      5. 2.1.5 Sensor Capacitance
      6. 2.1.6 Sensor Quality Factor
    2. 2.2 Inductive Touch
    3. 2.3 LDC211x/LDC3114 Design Boundary Conditions
    4. 2.4 Sensor Physical Construction
      1. 2.4.1 Sensor Physical Size
      2. 2.4.2 Sensor Capacitor Position
      3. 2.4.3 Shielding INn traces
      4. 2.4.4 Shielding Capacitance
      5. 2.4.5 CCOM Sizing
      6. 2.4.6 Multi-Layer Design
        1. 2.4.6.1 Sensor Parasitic Capacitance
      7. 2.4.7 Sensor Spacers
      8. 2.4.8 Sensor Stiffener
      9. 2.4.9 Racetrack Inductor Shape
    5. 2.5 Example Sensor
  4. 3Summary
  5. 4Revision History

1.1 Theory of Operation

Consider a flat metal plate held at a fixed distance from an inductive coil sensor, as shown in #T4726003-1. If a force is applied onto the metal plate, the metal will deform slightly. For example, with a 1 N force, which is approximately the weight of a computer mouse, a 1-mm thick aluminum plate that is 15 mm × 15 mm will deform by about 0.2 µm. This deformation moves the opposite side of the plate closer to the LDC sensor. When the force is removed, the plate will return to its original unstressed shape.

When the conductive material is in close proximity to the inductor, the magnetic field will induce circulating eddy currents on the surface of the conductor. The eddy currents are a function of the distance, size, and composition of the conductor. If the conductor is deflected toward the inductor as shown in #T4726003-1, more eddy currents will be generated.

GUID-D8F062CB-A1C5-44DD-ABF8-CC34BF1B3FCB-low.gifFigure 1-1 Metal Deflection

The eddy currents generate their own magnetic field, which opposes the original field generated by the inductor. This effect reduces the inductance of the system, resulting in an increase in sensor frequency. As the conductive target moves closer to the sensor, the electromagnetic coupling between them becomes stronger. As a result, the change in sensor frequency is also more significant.