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.

FIGURE 1
Capacitive sensing technique

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.

FIGURE 2
Inductive sensing technique [1]

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..

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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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.

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.)

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!

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.

PSoC MCU Variant is Purpose-Built for IoT Edge Processing

Cypress Semiconductor is expanding its Internet of Things (IoT) solutions portfolio with a new member of its ultra-low-power PSoC 6 microcontroller (MCU) family. The new PSoC 6 MCU is purpose-built to address the growing needs for computing, connectivity and storage in IoT edge devices. The new MCUs include expanded embedded memory with 2 MB Flash and 1 MB SRAM to support compute-intensive algorithms, connectivity stacks and data logging.

At the same time, Cypress has announced two new development kits for the PSoC 6 family, enabling developers to immediately leverage the industry’s lowest power, most flexible dual-core MCU with hardware-based security—to prolong battery life, deliver efficient processing and sensing, and protect sensitive user data. PSoC 6 is empowering millions of IoT products today, providing the most secure and low-power processing available.
Developers can evaluate the new PSoC 6 MCUs with expanded embedded memory using Cypress’ new PSoC 6 Wi-Fi BT Prototyping Kit (CY8CPROTO-062-4343W) (shown). This $30 kit features peripheral modules including Cypress’ industry-leading CapSense capacitive-sensing technology, PDM-PCM microphones, and memory expansion modules, enabling quick evaluation and easy development. The kit is supported by Cypress’ ModusToolbox software suite that provides easy-to-use tools for application development in a familiar MCU integrated development environment (IDE).

To streamline development of products with Bluetooth Low Energy (BLE) 5.0 connectivity, Cypress has introduced the PSoC 6 BLE Prototyping Kit (CY8CPROTO-063-BLE). This $20 kit features a fully-certified CYBLE-416045-02 BLE module—a turnkey solution that includes a PSoC 63 MCU, onboard crystal oscillators, trace antenna and passive components.

Cypress’ PSoC 6 MCUs are production qualified today and are in-stock at authorized distributors. The new PSoC 6 MCUs with expanded embedded memory are currently sampling and are expected to be in production in the first quarter of 2019. The PSoC 6 Wi-Fi BT Prototyping Kit (CY8CPROTO-062-4343W) is available for $30 and the PSoC 6 BLE Prototyping Kit (CY8CPROTO-063-BLE) is available for $20.

Cypress Semiconductor | www.cypress.com

MCUs Provide Inductive Sensing Solution

Cypress Semiconductor has announced production availability of the PSoC 4700S series of microcontrollers that use MagSense inductive sensing technology for contactless metal sensing. The series also incorporates Cypress’ industry-leading CapSense capacitive-sensing technology, empowering consumer, industrial, and automotive product developers to create sleek, state-of-the-art designs using metals and other materials. The highly-integrated MCUs enable cost-efficient system designs by reducing bill-of-material costs and provide superior noise immunity for reliable operation, even in extreme environmental conditions.
Cypress also announced availability of the new CY8CKIT-148 PSoC 4700S Inductive Sensing Evaluation Kit, a low-cost hardware platform that enables design and debug of the MCUs. The kit includes MagSense inductive-sensing buttons and a proximity sensor, as well as an FPC connector to evaluate various coils, such as a rotary encoder. The PSoC 4700S series is supported in Cypress’ PSoC Creator Integrated Design Environment (IDE), which allows users to drag and drop production-ready hardware blocks, including the MagSense inductive sensing capability, into a design and configure them easily via a simple graphical user interface.

The PSoC 4700S MCUs integrate:

  • A 32-bit Arm Cortex-M0+ core
  • Up to 32 KB Flash and 4 KB SRAM
  • 36 GPIOs
  • 7 programmable analog blocks
  • 7 programmable digital blocks

Support for up to 16 sensors, enabling implementation of buttons, linear and rotary encoders, and proximity sensing.

The CY8CKIT-148 PSoC 4700S Inductive Sensing Evaluation Kit is available for $49 at the Cypress online store and from select distributors.

Cypress Semiconductor | www.cypress.com