Developing a new generation of human-machine interfaces using capacitive and infrared proximity sensing

Introduction

It is expected that the shipment of electronic products with advanced human-machine interfaces will exceed one billion in 2010. These human-machine interfaces use technologies such as capacitive and infrared proximity sensing to significantly improve the end-user experience while increasing system reliability and reducing overall costs. In addition to making products easier to use and more visually attractive, these human-machine interfaces shield the growing complexity of electronic products, allowing manufacturers to bring products with advanced features to market faster.

Advanced sensor human-machine interfaces are more reliable than traditional mechanical interfaces because they have no moving parts connected to buttons and dials, which are more susceptible to failure over time. Sensor-based control panels and displays have also become more flexible, allowing a single set of control components to be reconfigured according to the application environment so that customers can implement their own applications based on existing functions. When gesture recognition and "non-contact" technology are combined, developers can make device interfaces more intelligent, predict user needs, and innovate usage patterns, making products more friendly, intuitive and easy to use. Firmware can be quickly and easily adjusted according to market needs, without having to completely rebuild the system or redesign the appearance of the device.

Next-generation human-machine interface

New products call for new human-machine interfaces to differentiate themselves in the market. By making electronic devices more aware of their operating environment, new features enhance ease of use, improve power efficiency and reduce system costs. In addition, its high sensitivity, low noise and moisture resistance ensure its reliability even in the most challenging environments.

The two main technologies driving the development of the next generation of human-machine interfaces are capacitive and proximity sensing. Capacitive sensors detect the presence of a user's finger by sensing changes in the capacitance of the device. It enables advanced controls such as sliders and scroll wheels, and can better recognize physical feedback-based near-range interface operations that users are used to, such as pressing buttons. Proximity sensing uses infrared sensors (using infrared reflection technology) to measure the distance to an object, up to 1 meter away. Proximity sensors can also identify objects in the air for "non-contact" gesture tracking.

The combination of the two technologies enables better control of the user interface. Many end users are already familiar with capacitive sensing technology from the use of some consumer products, most notably the iPod and the iPhone. So far, proximity sensing has usually been used for simple tasks, such as cheek detection on mobile phones. However, its application areas are far from limited to this:

User detection: For example, proximity sensing can detect if an end user is currently in front of a computer and be able to turn off the display when the user steps away. Considering that LCD backlights are very power hungry, even simple user detection can save a lot of energy across the enterprise. User detection can also be used for devices such as USB chargers/drives so that the device can be prepared for sudden unplugging.

Fingerprint-free display: Many portable devices require users to touch buttons on the screen, leaving marks that are difficult to identify and remove. Portable multimedia players with contactless interfaces allow users to watch videos without touching the screen. Similar applications include: enabling users to easily turn pages in e-books without touching the screen; allowing doctors to interact directly with touchscreen systems during surgery without touching the electronic screen.

Automatic backlight control: Part of the proximity sensing signal path uses an ambient light sensor (ALS) to eliminate noise from external light sources. The same sensor can also be used to monitor background lighting conditions and automatically adjust the display backlight to reduce energy consumption.

Invisible Intrusion Detection: By reflecting infrared light directed toward the surface of the door within the system, developers can implement an “invisible” intrusion detection mechanism that avoids the unreliability and wear and tear of mechanical switches that perform the same function.

Health and safety considerations: Multimedia information stations (kiosks), check-in counters, and other public computers present a risk of disease transmission through keyboards and screens. For example, in some areas of China, elevator control panels are required by law to be disinfected every hour to prevent the spread of SARS. Touchless panels avoid and mitigate these public health issues.

Remove interface controls

A trend in embedded design is to remove user interface management from the main application processor and assign it to a dedicated 8-bit microcontroller (MCU). Touch is a relatively slow action for the application processor, and using the entire system to detect whether the user moves a finger consumes much more energy than using a dedicated 8-bit MCU to implement the same function.

Capacitive touch sensing MCUs, such as the F99x family from Silicon Labs, are ideal for managing the next generation of user interfaces. By providing up to 25 MHz of operating performance and optimized peripherals for the task, the F99x MCUs provide the processing and input capabilities required for intelligent and accurate sensing. Combined with the Si11xx proximity sensing family, developers can implement efficient human-machine interfaces in a single development environment.

The capacitive sensing performance of the F99x MCUs is further enhanced by a hardware-implemented capacitance-to-digital converter (CDC). Silicon Labs' CDC contains two current inputs (digital-to-analog converters or DACs). The first is a variable DAC that measures the current into the external sensing capacitor; the second is a constant current source for the internal reference capacitor (see Figure 1). Capacitance measurements are made using the successive approximation register (SAR) technique, an efficient process that eliminates the effects of direct current (DC) offsets and requires no external components.

Figure 1: Hardware-implemented CDC provides high performance, 16-bit accuracy, high reliability, and DC offset rejection—no external components required

The 16-bit CDC of the F99x MCU is highly reliable and accurate. By performing a two-stage external capacitor discharge, the CDC is able to eliminate the ambient noise introduced during the discharge process. In contrast, other methods require additional external components (such as series resistors, etc.) and more than one I/O/channel (thus increasing the MCU size and wiring difficulty).

The dynamic range of the CDC is further improved by using adjustable gain. The dynamic range is also enhanced by reducing the source current to change the charging time and more directly reflecting the capacitive sensor voltage when both the source current and the series impedance are high (such as when using a touch panel or ESD protection capacitor to press the pad). Higher sensitivity provides developers with greater signal redundancy, allowing them to use thicker plastics, smaller electrodes, and ensure operational reliability even in noisy environments. The CDC can also dynamically adjust the conversion time using pin monitoring functions to eliminate interference caused by high current switching conversions on nearby pins. In short, the CDC has an excellent signal-to-noise ratio (SNR), with an SNR of 50-100 in typical capacitive sensing implementations.

Unparalleled system responsiveness

Proximity sensing uses an infrared sensor and one or more infrared light-emitting diodes (LEDs). It basically works by illuminating an object and then measuring the intensity of the reflected light. The number of LEDs required depends on the application and whether 3D information is required. For example, a paper towel dispensing sensor only needs one LED to detect if someone is standing in front of the dispenser. To detect left/right or up/down gestures, two LEDs are required. To support 3D navigation, three LEDs are required. In each case, only one physical sensor is required. However, each additional sensor adds the processing required to identify the strength of the signal from each LED and triangulate the location of the detected object.

Processing also requires filtering noise (i.e. background light) from the received signal. The more powerful the processor or embedded controller, the more samples it can obtain and the better the filtering. Increasing the sampling rate improves the resolution of the system, while better filtering also improves accuracy. Fast sampling and high-precision filtering require a robust interface, and developers must weigh the trade-offs of each approach to optimize their application.

Typically, low-sensitivity photodiodes are associated with extended acquisition times that allow light sources (such as fluorescent lights) to flicker, reducing accuracy. Silicon Labs' highly sensitive photodiode technology—proven in the industry for more than a decade—has good immunity to electromagnetic interference (EMI) and flicker, and can reliably detect objects up to 50 cm away without the use of external lenses or filters. Based on robust photodiode technology, the Si11xx sensor family has the option of integrating an ambient light sensor.

The power consumption of the proximity sensing subsystem is mainly driven by the infrared light-emitting diode (LED). Silicon Labs' QuickSense™ development environment helps developers define configuration parameters to optimize accuracy, detection range, and power consumption. For example, advanced control capabilities allow developers to dynamically adjust the LED current for specific applications and detection ranges. For ultra-low power operation, developers can use innovative single-pulse proximity sensing to minimize the LED on time, which can improve power efficiency by up to 4000 times, as shown in Figure 2.

Figure 2: The QuickSense MCU features innovative single-pulse proximity sensing that minimizes the LED on time.

Power efficiency increased by up to 4000 times

Reduce system power consumption

Nowadays, people are paying more and more attention to green and energy-saving electrical appliances. Not only portable appliances, but all electrical devices are beginning to consider applying energy-saving and environmental protection concepts to their designs. One of the efficient and low-power strategies is to minimize the CPU running time and maximize the sleep time of as many components as possible in the system. Silicon Labs reduces the overall system power consumption of capacitive touch sensing MCUs by adopting the following mechanisms:

Background Scanning: Since CDC is implemented in hardware, the capacitance measurement channel scanning can run fully automatically even when the CPU is in power-saving suspend mode.

Autonomous Auto-Scan: Scans and converts only active channels instead of all capacitive sensing channels.

Channel bonding: The power consumption of scanning multiple channels simultaneously with a single input is lower than the power consumption required to process multiple channels separately. For example, the system can use a single input to scan the entire slider and wake up the CPU if any active channel is detected to be touched. Once the CPU wakes up, it scans each channel separately to determine which channel is touched and start to recognize gestures.

Integrated LDO regulator: The LDO voltage regulator integrated in the F99x MCU provides a linear response while maintaining a constant, ultra-low effective current at all voltages. In addition, the F99x has special circuitry to preserve RAM contents when the LDO regulator is in sleep mode.

Flexible operating voltage: For many MCUs, the CPU must run at a lower frequency when the operating voltage is reduced. This increases the operating time and power consumption. If AA/AAA batteries are used, even if the MCU can operate at a minimum of 2.2V, 20% of the battery life will be wasted. Since the operating voltage can be reduced to 1.8V under full-function operation at 25MHz, the F99x can maximize battery life in different applications.

Most MCUs are designed to optimize power efficiency when running or sleeping. The F99x architecture has the lowest power in the industry in both running and sleeping modes (see Table 1). The internal power management unit (PMU) limits leakage, making the current in both running and sleeping modes less than half of that of competing F99x products.

*When operating at 0.9 to 1.8 V, the C8051F99x MCUs achieve greater average power efficiency by using an internal boost converter.

Table 1: F99x operating and sleep mode power consumption

Quick touch wake up

An important way to reduce power consumption is to turn off device displays and control interfaces that are no longer in use and put the entire system in sleep mode. A key factor in interface design is how the system responds to the user when the system transitions between sleep and run modes, that is, how to wake up faster. In a capacitive sensing system, when the system is in sleep mode, there is no backlight to indicate the function of the capacitive button or slider to the user. Therefore, the first key press is only used to wake up the system.

With proximity sensing technology, the system can detect users up to 1 meter away. This allows the proximity sensor to wake up the system when the user approaches or reaches the device, and prepare the display when the user is ready to press a key. In practical applications, this changes the way users interact with devices, making the system more intelligent and friendly. For example, devices such as car stereos or set-top boxes can close the control panel when not in use and fully open when the user approaches.

Wake-up time is the interval between acknowledging wake-up and executing the first instruction. Wake-up time depends on many factors, including regulator stability and analog device settling time. When reading a capacitor or proximity sensor, the CPU first performs an analog measurement. If the analog peripherals are not ready, the effective wake-up time is extended. Wake-up time affects not only system responsiveness but also power efficiency. During wake-up, the MCU is not operating but still consumes power. Therefore, shortening the wake-up time reduces the power consumption of the CPU during wake-up.

Estimating wake-up time is complex, and different vendors use different standards to measure wake-up time. Some MCUs wake up and trigger an interrupt service routine (ISR), and must wait until the analog detection is completed. In this case, the wake-up time is measured from the start of the wake-up event to when MCLK is valid on the appropriate pin, or when the interrupt vector is fetched. To get the same wake-up time before the first code instruction is executed, developers must add a few µs/CPU cycle to the measurement.

The F99x MCU wake-up time has been optimized to only 2us from sleep to wake-up. In addition, its analog device settling time is only 1.7us, which is 15 times faster than competing MCUs. Therefore, the effective wake-up time from event occurrence to the first analog measurement is less than 4us, which is up to 7 times faster than the closest competitor.

In addition to fast response, the F99x MCUs have the industry's lowest power capacitive touch sensing on the market. They feature outstanding performance of 150uA/MHz within the operating voltage range of 1.8-3.6V, and the industry's lowest power touch wake-up current of less than 1uA. The 14 CDC channels have ultra-fast 40us acquisition time, 16-bit accuracy, and built-in averaging, which increases reliability; and are immune to low-frequency noise and DC offset interference. The F99x MCU's CDC is the fastest and most sensitive capacitive digital converter currently available, while other products with the same sensitivity require more than 1000 times longer sampling times. To achieve higher sensing reliability, the highly programmable F99x MCU enables developers to dynamically adjust active and inactive thresholds to adapt to changes in environmental factors (see Figure 3).

Figure 3: To achieve higher sensing reliability,

Developers can dynamically adjust the active and inactive thresholds to adapt to changes in environmental factors

Silicon Labs' QuickSense portfolio includes a variety of sensing devices. In addition to the F99x MCU, Silicon Labs' F8xx and F7xx MCU families also provide advanced capacitive sensing, optimal performance, efficient power consumption and low cost for a variety of applications. For proximity sensing, developers can choose the industry-leading Si1102 infrared proximity sensor or the Si1120 infrared proximity sensor and ambient light sensor. Both devices support energy saving, single pulse technology and contactless gesture recognition. Silicon Labs' infrared proximity sensor is the fastest sensing device on the market, providing the longest sensing distance without compromising power efficiency.

Advanced Development Environment

As embedded applications become increasingly complex, designing a robust application requires not only proven hardware, but also productized software and first-class development tools. To help developers, Silicon Labs provides the QuickSense Studio development kit, which combines hardware, software and development tools to enable developers to quickly and easily apply capacitive and proximity sensing to their projects.

From an application perspective, capacitive and proximity sensors can be viewed as simple inputs to the system. By abstracting their implementation through APIs, developers can access user interaction information regardless of their source. Touches or gestures can be easily mapped to specific functional activities, greatly simplifying application and interface development. The easy-to-use, graphical user interface (GUI)-based QuickSense Configuration Wizard accelerates the development process by generating the required application configuration code and firmware drivers, without requiring developers to understand or write low-level code for MCU peripherals used to monitor sensors. Industry-proven firmware controls different capacitive sensing interface options—including touch buttons, sliders, and wheels—and capacitive proximity sensors. Developers have full control over important sensing characteristics such as sensitivity, operating thresholds, response speed, and code size.

The QuickSense Studio development kit also automatically calibrates sensors and provides complete debugging and performance analysis capabilities to ensure that product designs respond quickly, stably and reliably. For example, even if there are switches of the same size and shape, their location on the printed circuit board (PCB) will affect their active and inactive capacitance, taking into account the proximity to other conductive elements, the impact of the ground plane, and the presence of electronic interference. During development and productization, each switch needs to be calibrated and written to the Flash memory. In addition, if the influence of environmental factors (such as temperature, humidity, voltage and pollution) is large enough, incorrect measurements can lead to erroneous sensing events. The QuickSense Studio development kit adapts to the dynamic characteristics of these environmental factors by periodically reconfiguring them.

The QuickSense Studio development kit is the only development tool on the market that supports both capacitive and proximity sensing, enabling developers to design complete user interfaces using a single development environment. In addition to the configuration wizard, the QuickSense Studio development kit also accelerates product design through the following features:

Infrared proximity sensor

Ambient light sensor

Capacitive buttons and sliders

Capacitive proximity sensing

Complex Algorithms

Gesture Recognition

MCU control and communication

Capacitive touch screen

Silicon Labs also offers a variety of complete development tool kits to help developers integrate capacitive and proximity sensing into their applications. These resources include a complete wireless development suite (WDS), battery life estimator, sample code and comprehensive application notes.

summary

Effective human-machine interfaces require aesthetics combined with innovative ways to interact with electronic devices. Manufacturers seeking to differentiate their products can turn to a new generation of capacitive and proximity sensing interfaces to provide an easier-to-use, more intuitive user experience with less impact on system cost and power consumption. By using devices with capacitive and proximity sensing capabilities, such as the C8051F99x capacitive touch sensing MCU and Si1120 proximity sensor, developers can quickly apply a new generation of gesture and contactless interfaces to any system, conveniently using industry-proven hardware and firmware.