You have probably heard about capacitive touch sense—it’s a common method of creating physical user interfaces that provide significant advantages over traditional solutions such as switches, buttons, and mechanical sliders. If you’re not familiar with capacitive touch sense, All About Circuits has plenty of material to get you started, including this article, this one, and this one.
But capacitance is not the only thing that can be used to sense a finger press. Inductive touch sense, as you might have guessed, is a technique in which touch events are detected by measuring changes in inductance.
Unlike capacitive touch sense, which requires only a PCB capacitor (and a fingertip), inductive touch sense involves two separate components: a PCB inductor and a metal plate.
Courtesy of Texas Instruments.
A controller circuit drives AC current through the inductor. This generates an AC magnetic field, which creates eddy currents in the conductive plate. Magnetic fields generated by these eddy currents interact with the inductor in such a way as to reduce the effective inductance. The strength of the interaction depends on the distance between the plate and the inductor; if a finger press causes the metal plate to deflect, the reduced distance between plate and inductor leads to a measurable change in inductance.
The circuitry required to implement inductive touch sense is comparable to what you need for capacitive touch sense: an inductance-to-digital converter (LDC) and a microcontroller. The LDC measures the inductance and converts it to binary data that can be processed by a microcontroller, which interprets changes in inductance and initiates the appropriate responses.
Courtesy of Texas Instruments.
Capacitive vs. Inductive
I’m not an expert on inductive touch sense, but it appears that the primary advantage over capacitive touch sense is environmental robustness. Various factors can influence the capacitance of a cap-touch sensor—temperature, dust, moisture. The inductive-touch-sense system seems to be more compatible with applications that require resistance to unfavorable environmental conditions.
A new evaluation kit from TI gives you just about everything you need to get started with inductive touch sense. The LDC2114-QUICKSTART-KIT comes with a development board that provides the relevant control and interface circuitry; the central component is the LDC2114 inductance-to-digital converter IC.
TI's LDC2114 4-channel inductive touch evaluation module. Image courtesy of Texas Instruments.
The kit also includes the LDCCOILEVM, which is a board with 19 inductive-touch-compatible PCB coils.
TI's LDCCOILEVM. Courtesy of Texas Instruments.
The LDC2114 appears to be a high-performance device. It includes a resonant circuit to drive the coils, inductance-measurement circuitry, and processing algorithms. According to the datasheet, when all is said and done the device can reliably detect deflections as small as 200 nm—in other words, a very light touch can be detected (I’m assuming it doesn’t take much force to cause a 200 nm deflection in a thin piece of metal).
As far as I can tell, TI’s development kit is the only one currently available. Microchip used to offer an inductive-touch development board, but it’s now listed as “obsolete” and is not officially available for purchase. Nonetheless, you can access the user guide for this board, and the information in this document is still helpful, especially since the Microchip board relied on a microcontroller instead of a dedicated inductance-to-digital converter. The “Theory of Operation” section, beginning on page 19, would be a good place to start if you are looking to implement inductive touch via custom firmware.
The DM183027 inductive touch sensor development board. Image courtesy of Mouser Electronics
If you are aware of any other ICs or boards that are dedicated specifically to inductive touch sense, feel free to mention them in the comments.