Comparing capacitive and inductive sensing for embedded applications
Touch sensing has become the user interface of choice across a wide range of embedded systems. First implemented using resistive sensing technology, touch sensing had to overcome many challenges, including low sensitivity, false triggering, and short operating life. This created an opportunity for touch sensing using capacitive sensing.
Capacitive sensing has proven to be robust in many environmental use cases. However, as with any technology, capacitive sensing comes with its own challenges. As these challenges tend to application-specific, inductive sensing technology has risen to fill in the gaps.
In this article, we will be discussing 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 will compare the advantages of inductive sensing over capacitive sensing in these applications.
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 (including PCB traces and PCB materials like FR4, which has more capacitance compared to Flex one) is additive. 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 utilizes two different lines: TX (Transmitter) and RX (Receiver). TX sends a PWM signal with respect to GND the digital system supply, VDDD. RX detects the amount of charge received on the RX electrode.
One of the drawbacks of capacitive sensing is that it cannot operate 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 also require different designs. Implementing capacitive sensing with thicker glass material (display glass) and meeting capacitive sensor sensitivity with these types of materials are additional implementation challenges in Industrial applications.
Inductive sensing enables 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. 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 in the form of an eddy current that causes a circular magnetic field. This eddy current induces a reverse magnetic field, in turn leading to a reduction in inductance. A capacitor is added in parallel to the coil to create an LC tank circuit. As the inductance drops, the frequency shifts upward, changing the amplitude throughout.
With the removal of a dielectric from the sensor, compared to a capacitive sensor, inductive sensing is able to operate reliably in the presence of water. Thus, inductive sensing brings 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, 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 below.
Currently, the primary design challenge for implementing inductive sensing is designing coils with 100% production yield where inductive trace spacing is very narrow, such as using 4 mils spacing. There is also the consideration of meeting inductive values with variations in PCB laminate materials.
Use Cases for Inductive Sensing and Capacitive Sensing
Capacitive sensing is undeniably useful in a great many applications. However, for certain use cases, inductive sensing offers greater reliability. Consider the use case of a Bluetooth speaker that needs to be water resistant and is intended for use in up to two feet of water for half an hour. This use case requires more than just 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.
With capacitive sensing, such operation is partially possible using mutual capacitive sensing employing complex shielding techniques. However, the device would offer a less than ideal use experience. For example, there would be inconsistent responsiveness from the touch interface and it would operate differently than it does when it is used out of water due to changes in the dielectric introduced by the presence of water.
For this application, metal-over-touch using inductive sensing would provide a consistent and reliable user performance (see Figure 1). 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 1: The architecture of a water-resistant Bluetooth speaker using inductive sensing. (Source: Cypress Semiconductor)