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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5 V MCU Family Provides Water Tolerant Touch Integration

NXP Semiconductor has announced its 5 V KE1xZ family of MCUs. Based on the Arm Cortex-M0+ core, the MCUs are suited for embedded control systems in harsh electrical environments and provide an integrated CAN controller and capacitive touch from 32 KB flash. Designed for a wide range of industrial applications, the KE1xZ family offers mixed-signal integration across a range of compact memory variants. The 1-MS/s ADC and FlexTimer modules, combined with NXP’s Freemaster software tools library and Motor Control Application Tuning plugin (MCAT) enable designs of Brushless DC (BLDC) and other motor-control systems.

NXP’s KE1xZ MCU family offers advanced noise immunity, water-tolerant touch and low-power wake-on-touch operation—essential features for the strict electromagnetic compatibility (EMC) standards of the industrial and home appliance markets. NXP’s touch IP, combined with software and tools provide a high level of stability, accuracy and ease of use, with continued responsiveness and functionality through wet conditions. It can sustain 10 V in conducted noise, in alignment with International Electrotechnical Commission (IEC) 6100-4-6 test level 3.

Additional KE1xZ MCU features:

  • Internal 48MHz internal reference clock with 1% accuracy over full operating range
  • Boot ROM with built in bootloader and 128-bit unique device identifier (UID)
  • ADC self-calibration feature
  • Flash Access Control (FAC)
  • Cyclic Redundancy Check (CRC) generator module
  • Internal watchdog (WDOG) with independent clock source and external watchdog monitor (EWM)
  • On-chip clock loss monitoring
  • IEC 60730 Class B safety certification
  • LQFP package with 48- and 44-pin options

The KE1xZ MCU family will be available globally in March 2019 from NXP and its distribution partners with a suggested resale price from $0.79 at 10,000-unit quantities. NXP enables developers through its MCUXpresso software and tools ecosystem, along with its FRDM-KE15Z and FRDM-TOUCH development platforms (see image above), with respective suggested resale prices of $35 and $15. Third-party support is enabled from the broad ARM ecosystem.

NXP Semiconductor | www.nxp.com

 

Rugged Touch Panel Computer Targets Railway System Designs

ADLINK Technology has released its latest Driver Machine Interface (DMI) touch panel computer, the DMI-1210, designed specifically for train control and driver information display. Powered by the Intel Atom x5-E3930 processor (formerly Apollo Lake) and featuring a 12.1” (4:3) high resolution color display, 5-wire resistive touch screen and securable I/O interface, the DMI-1210 can be deployed as an HMI unit for driver’s desks, control panel for passenger information systems, surveillance system control/display unit or in railway diagnostics and communications applications.
The DMI-1210 is an EN 50155 certificated, cost-effective, commercial-off-the-shelf (COTS) driver interface that offers train radio display, electronic timetable, and diagnostic display functions and additional functionality such as train data recorder. The DMI-1210 supports full range DC power input from +16.8 V to +137.5 V DC. Optional MVB, GNSS, 3G/LTE, WLAN and Bluetooth through add-on modules give system integrators the necessary tools to expand use case possibilities.

With ADLINK’s built-in Smart Embedded Management Agent (SEMA) management and status LEDs on the front panel, the DMI-1210 provides easy and effective health monitoring and system maintenance. In addition, system robustness and reliability are provided by careful component selection for extended temperature operation, isolated I/Os, conformal coated circuit boards, securable I/O connectors and high ingress protection rating (IP65 front, IP42 rear).

ADLINK Technology | www.adlinktech.com

 

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.

Single-Chip Controllers for 20-Inch Automotive Touchscreens

A new family of single-chip maXTouch touchscreen controllers from Microchip Technology is designed to address a number of issues particular to automotive screens up to 20 inches in size. Even as touchscreen displays in the car grow larger, drivers expect screens to operate with the same touch experience as mobile phones. However, screens in automobiles need to meet stringent head impact and vibration tests, and consequently have thicker cover lenses that potentially impact the touch interface performance. As screens get larger, they are also more likely to interfere with other frequencies such as AM radio and car access systems. All of these factors become a major challenge in the design of modern automotive capacitive touch systems.
The MXT2912TD-A, with nearly 3,000 touch sensing nodes, and MXT2113TD-A, supporting more than 2,000 nodes, bring consumers the touchscreen user experience they expect in vehicles. These new devices build upon Microchip’s existing maXTouch touchscreen technology that is widely adopted by manufacturers worldwide. Microchip’s latest solutions offer superior signal-to-noise capability to address the requirements of thick lenses, even supporting multiple finger touches through thick gloves and in the presence of moisture.

As automakers use screens to replace mechanical switches on the dash for sleeker interior designs, safe and reliable operation becomes even more critical. The MXT2912TD and MXT2113TD devices incorporate self- and sensor-diagnostic functions, which constantly monitor the integrity of the touch system. These smart diagnostic features support the Automotive Safety Integrity Level (ASIL) classification index as defined by the ISO 26262 Functional Safety Specification for Passenger Vehicles.

The new devices feature technology that enables adaptive touch utilizing self-capacitance and mutual-capacitance measurements, so all touches are recognized and false touch detections are avoided. They also feature Microchip’s proprietary new signal shaping technology that significantly lowers emissions to help large touchscreens using maXTouch controllers meet CISPR-25 Level 5 requirements for electromagnetic interference in automobiles. The new touch controllers also meet automotive temperature grade 3 (-40°C to +85°C) and grade 2 (-40°C to +105°C) operating ranges and are AEC-Q100 qualified.

With the addition of the new maXTouch touchscreen controllers, Microchip provides full scalability to customers, offering the industry’s only complete and growing portfolio of automotive-qualified touchscreen controllers for the use of various screen sizes. Developers can design multiple platforms from small touchpads to large displays in the same development environment with the same host software interface and quality user experience. This ultimately shortens design time while lowering system and development costs.

Eight dedicated application and sensor design centers around the world help Microchip customers and partners accelerate the process of bringing their maXTouch technology designs to market. Microchip’s maXTouch technology specialists are working with all major sensor, display and touch module manufacturers.

An evaluation kit is available for each of the parts in the new maXTouch touchscreen controller family. The kit includes a Printed Circuit Board (PCB) with the maXTouch touchscreen controller, a touch sensor on a clear glass lens, the Flat Printed Circuit (FPC) to connect to the sensor display, a converter PCB to connect the kit to the host computer via USB, as well as cables, software and documentation. All parts are also compatible with maXTouch Studio, a full software development environment to support the evaluation of maXTouch touchscreen controllers.

The MXT2912TD-A and MXT2113TD-A devices are available now in sampling and volume quantities in LQFP176 and LQFP144 packages, respectively.

Microchip Technology | www.microchip.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

Tools for Cypress Semi’s PSoC BLE 6 MCU

Cypress Semiconductor has announced the public release of the PSoC 6 BLE Pioneer Kit and PSoC Creator Integrated Design Environment (IDE) software version 4.2 that enable designers to begin development with the PSoC 6 microcontroller (MCU) for IoT applications. PSoC BLE 6 is the industry’s lowest power, most flexible MCU with built-in Bluetooth Low Energy wireless connectivity and integrated hardware-based security in a single device.

Early adopters are already using the flexible dual-core architecture of PSoC 6, using the ARM Cortex-M4 core as a host processor and the Cortex-M0+ core to manage peripheral functions such as capacitive sensing, Bluetooth Low Energy connectivity and sensor aggregation. Early adopter applications include wearables, personal medical devices and wireless speakers. Designers are also utilizing the built-in security features in PSoC 6 to help guard against unwanted access to data.

CY8CKIT-062-BLEThe PSoC BLE Pioneer Kit features a PSoC 63 MCU with Bluetooth Low Energy (BLE) connectivity. The kit enables development of modern touch and gesture-based interfaces that are robust and reliable with a linear slider, touch buttons and proximity sensors based on the latest generation of Cypress’ industry-leading CapSense capacitive-sensing technology. Designers can also use the board to add USB Power Delivery (PD) with its Cypress EZ-PD CCG3 USB-C controller. The kit also includes a 2.7-inch E-ink Display Shield add-on board (CY8CKIT-028-EPD) with thermistor, digital mic, and 9-axes motion sensor.

Offering best-in-class flexibility and ease-of-use, the PSoC 6 MCU architecture can serve as the catalyst for differentiated, visionary IoT devices. Designers can use software-defined peripherals to create custom analog front-ends (AFEs) or digital interfaces for innovative system components such as E-ink displays. The architecture is supported by Cypress’ PSoC Creator IDE and the expansive Arm ecosystem. Designers can find more information about PSoC Creator at http://www.cypressw.com/creator.

The PSoC 6 BLE Pioneer Kit (CY8CKIT-062-BLE) is available for purchase for $75 at the Cypress Online Store and through select distribution partners. PSoC 6 devices are currently sampling. Production devices are expected by the end of 2017.

Cypress Semiconductor | www.cypress.com

Getting Started with PSoC MCUs (Part 3)

Data Conversion, Capacitive Sensing and More

In the previous parts of this series, Nishant laid the groundwork for getting up and running with the PSoC. Here he tackles the chip’s more complex features like Data Conversion and CapSense.

By Nishant Mittal
Systems Engineer, Cypress Semiconductor

In the previous two parts of this “Getting started with PSoC” series, I have hopefully provided you with a good base of knowledge about PSoC devices. Here, in this final part it’s time to get more in depth and discuss various data conversion protocols in PSoC and provide some design examples. I’ll also cover interfacing various peripherals with the Photo 1microcontroller. We’ll also get into how to transition from a bare silicon PSoC chip or PSoC development board to using the chip in your project.

Data conversion with PSoC

Data Conversion is an important block in any kind of instrumentation system or Internet of Things implementation. In fact, any application that uses sensors or interfaces to the external environment is an application in which Data Conversion is an integral part of the system. Although digital sensors are available today, the lower costs of analog sensors shouldn’t be overlooked.

 

PSoC Creator has a Data Conversion component that enables designers to code efficiently with less effort. The photo above shows the screenshot of the ADC (analog-to-digital conversion) component in PSoC Creator. The photo above also shows the configuration setting for ADC. First off, we need to set the Channel sampling rate (SPS). Second, we need to set the voltage reference which is necessary to do the comparison of analog signals. Here we use VDDA/2 or VDDA which is 5 V. You can select whether you For web Figure 1want a single-ended ADC or differential ADC by simply clicking the appropriate tab from the component configuration. Clock source needs to be chosen. If the source is chosen to be internal, the PLL from the internals of chip are used—otherwise you’d have to connect an external crystal to the controller using the development kit CY8CKIT-044. Other advanced settings are available for complex programs—but most of those aren’t needed in most intermediate applications.

Read the full article in the September 326 issue of Circuit Cellar

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