Exploring the touch-on-metal button technology

With the onset of inductive-sensing technology, we are seeing a revolution in the traditional approach to designing human-machine interfaces (HMIs). System designers are rethinking fundamental building blocks such as simple on/off buttons and attempting to solve many of the problems that have existed in button designs for decades.

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.

How do inductive ToM buttons compare to conventional buttons?
The two most common approaches for button implementations are mechanical and capacitive:

1. 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 liquids or other contaminants. This approach does not naturally seal the case completely, making the button a potential area for moisture leaks that could damage sensitive electronic components contained inside.

2. 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 100µA for 10 samples per second.