Capacitive vs. Inductive Sensing

Touch Trade-Offs

Touch sensing has become an indispensable technology across a wide range of embedded systems. In this article, Nishant discusses capacitive sensing and inductive sensing, each in the context of their use in embedded applications. He then explores the trade-offs between the two technologies, and why inductive sensing is preferred over capacitive sensing in some use cases.

By Nishant Mittal

Touch sensing was first implemented using resistive sensing technology. But resistive sensing had a number of disadvantages, including low sensitivity, false triggering and shorter operating life. All of that discouraged its use and led to its eventual downfall in the market.

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

In this article, we’ll discuss capacitive sensing for embedded applications and how it can be used in various applications. We will then explore the use of inductive sensing in embedded products and why inductive sensing is preferred over capacitive sensing in some use cases. Finally, we’ll compare the advantages of inductive sensing over capacitive sensing in these applications.

Capacitive Sensing for Embedded

Capacitive sensing operates on the principle of monitoring the change in parasitic capacitance due to a finger touch (Figure 1). Capacitive sensing has been used primarily in two different forms: self-capacitance and mutual-capacitance. In self-capacitance mode, the net capacitance due to a finger touch and board capacitance is additive. This capacitance includes PCB traces and PCB materials like FR4, which has more capacitance compared to Flex materials and many similar dielectrics. Self-capacitance mode is useful in general touch application like buttons for touch-and-respond applications. In contrast, mutual capacitance is well-suited for applications involving more complex sensing such as gestures, multi-touch and sliders.

Mutual capacitance sensing uses two different lines: TX(Transmitter) and RX(Receiver). The Transmitter sends a PWM signal with respect to the system VDD and GND. The Receiver detects the amount of charge received on the RX electrode.

One of the difficult use cases of capacitive sensing is that it cannot operate perfectly underwater. It also requires relatively strict design guidelines to be followed for error-free operation. Capacitive sensing performance is also impacted by nearby LEDs and power lines on PCBs. Implementing auto-tuning with variation in trace capacitance, variation in capacitive sensing buttons and different slider sizes and shapes all require different designs. Implementation challenges in industrial applications include using capacitive sensing with thicker glass material (display glass) and meeting capacitive sensor sensitivity requirements with those types of materials.

Inductive Sensing for Embedded

Inductive sensing enables the next-generation of touch technology in applications involving metal-over-touch use cases such as in automotive, industrial and many embedded and IoT applications. Inductive sensing is based on the principle of electromagnetic coupling, between a coil and the target (Figure 2). 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. This phenomenon causes some energy to get transferred to the metal target—referred to as eddy current—that causes a circular magnetic field. Eddy current induces a reverse magnetic field, in turn leading to a reduction in inductance.

To cause the resonant frequency to occur, a capacitor is added in parallel to the coil to cause the LC tank circuit. As the inductance starts reducing, the frequency shifts upward changing the amplitude throughout. In contrast to a capacitive sensor, inductive sensing is able to operate reliably in the presence of water thanks to the removal of a dielectric from the sensor. This advantage brings inductive sensing touch sensing to a wide range of applications that involve liquids such as underwater equipment, flow meters, RPM detection, medical instruments and many others. Inductive sensing also supports biomedical applications. In general applications, inductive sensing enables replacement of mechanical switches and proximity sensing of metal objects. For example, in automotive applications, inductive sensing can be used to replace mechanical handles as well as detect car proximity. Some of these examples will be discussed in detail later..
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Read the full article in the May 346 issue of Circuit Cellar
(Full article word count: 1842 words; Figure count: 6 Figures.)

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