Inductive touch buttons measure the inductance shift cause by the deflection in a metal target above their sensor, which can be used in applications that require a seamless touch button that works based on a force applied to the surface.

Figure 2-5 Inductive Sensing Basic Operation

Inductive sensing works based on the force applied, therfore there is no need to GND the surface like in some capacitive touch buttons, and this setup allows inductive sensing to work even when the user is wearing gloves or if moisture gets on the touch surface. Devices like the LDC3114-Q1 process the change in the inductive sensor and provide a digital output that signifies a button press, relieving the MCU from any data processing of the button data. The algorithm that processes the button press also includes a baseline tracking that helps remove false detections due to dirt, damage, or environmental changes like temperature.

Figure 2-6 Baseline Tracking

Even though the sensor requires a metal target, the touch surface is not required to be metal. Examples like the TIDA-060039 use a plastic touch surface with metal tape underneath to act as the target.

Figure 2-7 Inductive Touch Button Stackup With Non-Conductive Touch Surface

Inductive touch can use many different materials as the target and offer a robust nature for sensing, therefore these buttons can be used on both the interior and exterior of a vehicle to sense a user touch input.

Many push buttons inside a car have an illumination component to them. An inductive touch button uses a metal target, therefore extra consideration is needed for how to handle the illumination. The easiest way to show illumination with the button is to offset the light from the touch surface of the button. Having the illumination off to the side, above, or below the button separates the illumination from the touch button stackup, but that approach is not always acceptable in the design. One way to include the light in the middle of the touch surface is to have an LED on either side of the sensor coil and use an illumination guide to allow light to disperse above the metal target like in the following stackup.

Figure 2-8 Illuminated LDC Touch Button With Illumination Guide

The desired illumination cutout can be used on the outer touch surface, and the metal target iteration with the sensor coils remains intact in this implementation. However, this design requires a thin illumination guide layer, along with a metal target that deforms enough for a robust button press detection. An alternative to this design is to move the illumination guide between the metal target and sensor coil and have a cutout in the metal target for light to shine through.

Figure 2-9 Illuminated LDC Touch Button Illumination Guide Alternative

The illumination guide is not required in this application if the LEDs can be used as spacers instead. This implementation potentially moves the metal target further away from the sensor coil and requires a hole or cutout to be added to the metal target. The shape and size of the hole has an impact on the target and sensor coupling by disrupting the eddy currents that form on the target. Simulating the eddy currents on a square target from a circular sensor coil yields the flowing.

Figure 2-10 Eddy Current Formation

The eddy currents formed on the target are in a circle, therefore a small hole in the center of the metal does not have a large impact on the button performance. The size of the hole is an important factor as a larger hole decreases the coupling between target and sensor coil.

Figure 2-11 Hole Size Comparison

As show from the graph above, a small hole does not pose a significant change in the sensor frequency when the target is at close ranges, but when the hole becomes larger, the amount of change on the sensor is decreased. Having less frequency change causes the button to require more motion or more force for a button press to be detected. Additionally, if the button is deforming in the center of the metal target, the impact of a larger hole is more significant to the target coupling because less metal deforms towards the sensor. Alternatively, the LED can be placed inside the sensor coil rather than on the outside to prevent the need for an illumination guide to disperse the light as seen in Figure 2-12.

Figure 2-12 Inductive Touch Button With Center LED Stackup

This implementation routes the LED traces down to the fourth layer and away from the sensor coils to keep their impact to a minimum. The metal added from the LED causes some slight change in the coupling between the sensor and metal target, but not enough to impact button performance when the target is kept close to the sensor coil. The designer can replace the conductive target with a spring to allow mechanical travel if the design requires that function. Using a spring as the target introduces a new set of concerns for the application that designers must consider. First, the spring must be separated from the coils as to not touch by expanding the spacer to cover part of the inductive coil. The shape and style of the spring is also an issue. A traditional compression spring does not always provide much inductance shift for a button application. To solve this, a spring where the first layer is not in contact with itself until the spring is compressed can be used. The spring has a gap and does not form a complete circle; therefore, eddy currents do not form normally on the spring. As the spring is compressed and creates a complete circle or ring target, the eddy currents form more similarly to Figure 2-10, and a large inductance shift occurs. Alternatively, if a conical spring is used, the hole in the center of the target decreases and allows for the inductance to shift as the hole size decreases. The possibilities for implementing an inductive touch button are not limited to just these methods. Using a metal as the target gives freedom in the design to take advantage of an existing mechanical structure as long as the implementation can provide a way for the metal to cause an inductance shift on the sensor coil when the user interacts with the touch surface.