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Inductive Sensing with PSoC MCUs

Tougher Touch Tech

Inductive sensing is shaping up to be the next big thing for touch technology. It’s suited for applications involving metal-over-touch situations in automotive, industrial and other similar systems. Here, Nishant explores the science and technology of inductive sensing. He then describes a complete system design, along with firmware, for an inductive sensing solution based on Cypress Semiconductor’s PSoC microcontroller.

By Nishant Mittal

Touch sensing has become an important technology across a wide range of embedded systems. Touch sensing was first implemented using resistive sensing technology. However, resistive sensing had several disadvantages, including low sensitivity, false triggering and shorter operating life that discouraged its use and led to its eventual downfall in the market.

Today whenever people talk about touch sensing, they are usually referring to capacitive sensing. Capacitive sensing has proven to be robust not only in a normal environmental use cases but, because of its water-resistant capabilities, also underwater. As with any technology, capacitive sensing comes with a new set of disadvantages. These disadvantages tend to more application-specific. And those have opened the door for the advent of inductive sensing technology.

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Figure 1
Inductive sensing technique (Source: Cypress Semiconductor application note AN219207 – Inductive Sensing Design Guide).

Inductive sensing is based on the principle of electromagnetic coupling, between a coil and the target. When a metal target comes closer to the coil, its magnetic field is obstructed and it passes through the metal target before coupling to its origin (Figure 1). This phenomenon causes some energy to get transferred to the metal target named as eddy current that causes a circular magnetic field. That eddy current induces a reverse magnetic field, and that in turn leads to a reduction in inductance.
To cause the resonant frequency to occur, a capacitor is added in parallel to the coil to create the LC tank circuit. As the inductance starts reducing, the frequency shifts upward changing the amplitude throughout.

Some Use Cases

Figure 2
Shown here is the architecture of a water-resistant Bluetooth speaker using inductive sensing.

Consider the use case of a Bluetooth speaker that needs to be water resistant and is intended for use in up to 2″ of water for half an hour. This use case requires that the product is functional underwater. It also requires that the user can adjust the speaker in these circumstances. Such operation needs to be simple, consistent and reliable—even in the presence of water. Inductive sensing provides the solution for this. That’s because it has nothing much to do with the change in dielectric, but is only concerned with the metal detection.

For this application, metal-over-touch using inductive sensing would provide a consistent and reliable user performance (Figure 2). A light defection in metal can be detected as touch. Alternatively, a mechanical button and/or dial could be used. However, a mechanical interface is costly compared to a coil printed on a PCB and connected to a few passive components. Additionally, a mechanical button can break or fail, providing a much shorter useable lifespan than an inductive button would.

Figure 3
Using inductive sensing to determine vehicle proximity in an automotive application.

Consider another use case for proximity sensing using inductive sensing technology. A vehicle detection system needs to monitor when another vehicle approaches within 2 m and signal the driver on the dashboard or navigation panel. This functionality can be implemented using inductive sensing. A hardware board containing multiple coils at different locations using a flex cable, all around the dashboard, can be designed around the four corners and center of the headlight areas (Figure 3). Data from the inductive coils is collected by an inductive sensing controller such as the PSoC 4700S from Cypress Semiconductor. The controller would then analyze the data to determine the presence or absence of other cars in a 4-m vicinity around the vehicle. …

Read the full article in the February 343 issue of Circuit Cellar
(Full article word count: 2411 words; Figure count: 13 Figures.)

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Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.


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