Rethink the button: An inductive-sensing approach
Inductive-sensing technology has been consistently gaining ground with touch-on-metal (ToM) buttons because it facilitates cost-effective and a highly reliable approach that is immune to moisture and dirt, and continues to work even after minor structural damage. The completely sealed case allows for modern-looking and aesthetically pleasing designs without the need for moving mechanical parts.
This article covers the fundamentals of ToM technology using an inductance-to-digital converter (LDC) and provides guidance for constructing ToM buttons using metal panels commonly found in applications such as consumer electronics and appliances.
Inductive ToM buttons vs. conventional buttons
The two most common approaches for button implementations are mechanical and capacitive:
– Mechanical buttons on appliances and consumer electronics use a resistive contact solution. Mechanical solutions rely on the proper functioning of a mechanical button and its moving parts, which are susceptible to long-term reliability issues or sticking due to [ingress of] liquids or other contaminants. This approach does not inherently seal the case completely, making the button a potential area for moisture leaks that could damage sensitive electronic components contained inside.
– Capacitive buttons, such as those for home appliances, have higher long-term reliability than mechanical buttons and have a modern, sleek look. However, capacitive solutions do not work well with ungrounded metals that can cause the output to drift or falsely indicate the strength of a button press. Additionally, capacitive solutions are affected by foreign substances and may not respond if the button surface gets wet. Capacitive buttons may be susceptible to false triggering from foreign objects and cannot detect hands encased in most types of gloves. Depending on the architecture, strong electromagnetic interference (EMI) sources such as fluorescent lighting can interfere with capacitive button operation.
Inductive-sensing-based designs overcome the challenges of mechanical and capacitive buttons by offering a completely sealed and contactless solution with a greatly simplified assembly process. They offer superior reliability with immunity to moisture and external contaminants such as dirt or oil.
Unlike mechanical buttons, inductive-sensing-based buttons can detect the amount of pressure applied, allowing for adjustable sensitivity or the flexibility to program the button for different functions depending on the amount of pressure. In addition to working with grounded and ungrounded button panels, inductive sensing also provides excellent immunity from EMI sources due to a narrowband resonant-sensing approach. LDCs have been rapidly improving low-power operation such that ToM interfaces are now a viable solution for battery-powered applications like smartphones and tablets. A simple periodically sampled approach can put the average current consumption at less than 100 µA for 10 samples per second.
These advances have [led to] many manufacturers in the consumer electronics, automotive and home-appliance industries switching from plastic buttons to LDC ToM buttons. Panels on many modern appliances and consumer electronics use stainless steel or brushed aluminium, which can be turned into a user interface with minimal or no additional material. Table 1 compares common button technologies.
Table 1: Common button technologies
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Implementing inductive ToM buttons
Figure 1 shows a block diagram of a ToM solution with two buttons. When applying light force to a button, the inner surface of the metal sheet will be pushed toward the printed circuit board (PCB) sensors. The LDC detects the deflection and a microcontroller in the system then interprets this deflection as a button press. Haptics such as vibrations, audible beeps or visual indicators may give users an acknowledgement of an accepted button press.
Figure 1. System block diagram of a ToM implementation
Inductive-sensing technology can measure proximity to metal from changes in alternating current (AC) magnetic fields. An AC current flowing in an inductor produces an AC magnetic field perpendicular to the current flow. Bringing a conductive target into the vicinity of the inductor’s AC magnetic field induces a small circulating current, or an eddy current, on the surface of the conductive target (Figure 2).
Figure 2. Eddy currents induced on a metal surface
The magnitude of the eddy current is a function of the distance, size and composition of the conductor. There is a higher density of eddy currents at the greatest concentration of magnetic field lines, as shown in Figure 3. This current produces its own magnetic field – opposing the one created by the inductor – and reduces the equivalent inductance of the coil. An LDC such as the Texas Instruments LDC1612 measures this inductance shift to provide information about the position of the target over a sensor coil. The measured inductance shift enables precise positioning of the target as it moves closer to the sensor coil.
Figure 3. Eddy-current density induced by an AC magnetic field
A sensor coil is positioned below each button location. A light press on the metal surface causes micrometers of temporary deflection. While this change can’t be seen or felt (other than by intentionally provided haptic feedback), it is easily detectable by a high-resolution LDC. The LDC can distinguish nanoHenries (nH) of inductance shift from the system noise and detect sub-micrometers of metal deflection. Incorporating this principle facilitates button designs without moving parts – designs that instead add etched contours in the metal to indicate the location of a button.
While mechanical buttons provide built-in tactile feedback for a fixed amount of force, inductive solutions offer the ability to change both the type and level of user feedback. Inductive solutions also can be used to adjust the amount of force required to trigger the event. Inductive-sensing-based buttons are either connected to a haptic driver, an audio subsystem or an optical/display subsystem to provide user feedback and a more intuitive feel.
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System example
Figure 4 shows a suitable button panel produced from a 0.8-mm thick sheet of Al6061-T6 aluminium. The outer button diameter is 12 mm. Grooves on the top and bottom of the metal sheet are etched into the metal to indicate the button’s location to the user and to ensure that sufficient metal deflection occurs at the location of the sensor.
Figure 4. Manufactured button panel
The second part of a ToM system is a sensor. Commonly, the sensor coil is drawn as an ordinary copper trace on a PCB, such as the one shown in Figure 5.
Figure 5. Sensor PCB
For maximum sensitivity, the PCB should be mounted as close as possible to the metal surface; we recommend a coil-to-metal distance of less than 20% of coil diameter to detect micrometer metal deflections. To ensure that the metal does not touch the sensor portion of the PCB, we also recommend a minimum separation of 0.2 mm, depending on the application and manufacturing tolerances. The metal panel can be flush to the PCB, but a small, >0.2-mm, cutout above the sensor will provide enough room for deflection. Alternatively, consider placing a small spacer with a cutout to allow deflection between the sensing coil and the metal panel.
Once assembled, the LDC is ready to start detecting button presses. Figure 6 shows the real-time output response when pressing the button. You can see that the LDC output is very repeatable. A configurable threshold added in software detects button-press events. More sophisticated software algorithms can also add functionality such as multibutton support and multilevel force sensitivity and remove unwanted effects caused by drastic changes in temperature or humidity, or permanent deformation of the metal. An ultra-low-power such as the MSP430 microcontroller can run these algorithms.
Figure 6. LDC real-time button-press response
Using the approach outlined in this article, designers can take advantage of LDC devices as a new human-machine interface when it comes to highly reliable ToM buttons. The large resolution and dynamic range of this device offers much flexibility when it comes to mechanical design and sensor arrangement. In battery-operated applications, the low average current consumption enables very long lifetimes on a single charge and provides minimal battery drain to the overall power budget. The algorithms and interface are simple enough to be processed by the MSP430 microcontroller with minimum central processing unit (CPU) cycle time and programming effort.
Ben Kasemsadeh and Luke LaPointe are applications engineers with Texas Instruments.
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