Inductive Sensing Touch-On-Metal Buttons Design Guide

This application note covers the fundamentals of Touch-on-Metal (ToM) technology using an LDC1612 Inductance-to-Digital Converter (LDC) and provides guidance for constructing ToM buttons. Simple on and off buttons can be easily implemented using inductive sensing. Additionally, by using a high resolution LDC, microscopic movements in a flat metal button can be sensed and processed to determine how hard a given button was pressed. This approach allows reuse of existing metal surfaces commonly found in many applications such as consumer electronics and appliances. This report contains a design example for a multi-button brushed aluminum panel and provides guidance on the mechanical system and sensor design, as well as measured performance results of the complete system.

Trademarks

All trademarks are the property of their respective owners.

1 ToM Basics

ToM buttons refers to using a flat metal surface as a button and a high resolution inductance converter such as the LDC1612 to detect the microscopic metal deflection that occurs when the button is pressed. Figure 1 shows a block diagram of a touch-on-metal solution with two buttons. When even a light force is applied to a button, the inner surface of the metal sheet will be pushed towards the PCB sensors. The metal sheet does not contact the sensors but the small amount of deflection from the press causes a shift in the sensor inductance that can be detected by the LDC and then interpreted as a button press by a microcontroller (MCU). Haptics such as a vibration, audible beep, or visual indication may also be triggered to give the user an acknowledgement of an accepted button press.

Figure 1. System Block Diagram of a Touch-On-Metal Implementation

Inductive-sensing based designs for touch-on-metal offers a completely sealed and contactless solution with a greatly simplified assembly process. In addition to being insensitive to dirt, moisture, and other contaminants, inductive touch-on-metal buttons offer a robust solution that does not use moving mechanical parts, and offers a flat surface that is easy to clean for home appliances. Unlike mechanical buttons, inductive sensing-based buttons can detect the amount of pressure on the button, allowing for adjustable sensitivity or the ability to program the button for different functions depending on the amount of pressure applied. In addition to working with grounded and ungrounded button panels, inductive sensing also provides excellent immunity towards EMI sources due to a narrow-band resonant sensing approach.

2 How Are Inductive Touch-On-Metal Buttons Implemented?

Inductive-to-Digital Converters (LDC) are able to measure proximity to metal by detecting the subtle changes in an AC magnetic field resulting from the interaction with the metal target. The LDC generates an AC magnetic field by supplying an AC current into the parallel LC resonant circuit shown in Figure 2.

Figure 2. LC Sensor Components

If a conductive target is brought into the vicinity of the inductor’s AC magnetic field, small circulating currents known as eddy currents will be induced by the magnetic field onto the surface of the conductor shown below in Figure 3.

Figure 3. Eddy Currents Induced on Metal Surface

These eddy currents produce their own magnetic field that opposes the one created by the inductor which reduces the effective inductance of the coil. The resulting inductance shift is measured by the LDC and can be used to provide information about the position of the target over a sensor coil such as distance or equivalently the force of a button press.