Control Systems & PLC's

Proximity Switches

Developing Next-Generation Human Interfaces using Capacitive and Infrared Proximity Sensing

Introduction

More than a billion electronic products with advanced human interfaces capabilities are expected to ship in 2010. Based on technologies such as capacitive and infrared proximity sensing, these interfaces provide a dramatic improvement in the end-user experience while increasing system reliability and reducing total cost.

Advanced sensor-based interfaces are more reliable than traditional mechanical interfaces since they do not contain moving parts associated with buttons and dials, which are prone to failure over time. Sensor-based control panels and displays also become more flexible, allowing a single set of controls to be reconfigured based on application context so that users are presented with only those choices that are currently active.

Next-Generation Human Interfaces

Next-generation products require next-generation human interfaces to differentiate themselves in the marketplace.

• User detection: Proximity sensing can determine, for example, whether an end user is                       currently sitting at a PC and turn off the display when he or she walks away from the desk. Given the substantial power required for LCD backlighting, even simple user detection can result in significant power savings across an organization.

• Fingerprint-free displays: Many portable devices require users to touch buttons all over the screen, leaving oily marks that are both difficult to see through and clean.

• Automatic backlighting control: Part of the proximity sensing signal path is the use of an ambient light sensor (ALS) to reduce noise from external light sources.

• Invisible intrusion detection: Reflecting infrared light off the interior door surface of a system allows developers to deploy an "invisible" intrusion mechanism that avoids the unreliability and expense of mechanical switches used for the same purpose.

• Health and safety benefits: Kiosks, check-out stands and other public computers present health risks in terms of spreading disease via keyboards or touch screens. In parts of China, for example, laws require that every elevator panel be wiped down once an hour to prevent the spread of SARS. Touchless panels avoid and mitigate these public health issues.

Offloading Interface Control

One emerging trend in embedded design is to offload user interface management from the primary application processor to a dedicated 8-bit MCU. Human touch is a relatively slow event to an applications processor, and powering the entire system to check if a user has moved his or her finger consumes significantly more power than is required by an 8-bit MCU to accomplish the same task.

Unmatched System Responsiveness

Proximity sensing employs an infrared sensor and one or more infrared light-emitting diodes (LEDs). The basic operating principle is to illuminate an object and measure the intensity of the reflected light. The number of LEDs required depends upon the application and what spatial information is needed.

Reducing System Power Consumption

With the current emphasis on green, energy-saving electronics, all devices, and not just portable devices, are beginning to be designed with power conservation in mind. Silicon Labs has implemented several mechanisms for reducing overall system power consumption in its capacitive touch-sense MCUs:

• Background scanning: Since the CDC is implemented in hardware, capacitance measurements for channel scanning can be completed autonomously while the CPU operates in its power-saving suspend mode.

 

• Autonomous auto scanning: Rather than scan and convert all capacitive sensing channels, only active channels are scanned and converted.

• Channel bonding: Scanning several channels together using a single input consumes   less power than handling multiple conversions to check channels individually.

• Integrated LDO regulator: The F99x MCU's integrated low drop out (LDO) voltage regulator provides linear response while maintaining a constant, ultra-low active current at all voltages.

Most MCUs are designed to optimize either active or sleep power efficiency. The F99x architecture was designed from the ground up to offer the industry's lowest power in both active mode and sleep mode.

Advanced Development Environment

As embedded applications continue to become more complex, designing a robust implementation requires not only proven hardware but also production-ready software and world-class development tools.

QuickSense Studio is the only development tool on the market that supports both capacitive and proximity sensing, enabling developers to design a complete user interface using a single development environment. In addition to the Configuration Wizard, QuickSense Studio accelerates the design of the following:

• Infrared proximity sensing

• Ambient light sensing

• Capacitive buttons and sliders

• Capacitive proximity sensing

• Complex algorithms

• Gesture recognition

• MCU control and communications

• Capacitive touch screens

 

Silicon Labs – Proximity sensor IC and  SLIC

About the Author

Silicon Laboratories Inc. is a leading designer of high-performance, analog-intensive, mixed-signal integrated circuits (ICs) for a broad range of applications. For more information about Silicon Laboratories, please visit www.silabs.com.

How does an Inductive Proximity switch work ?

Proximity switches open or close an electrical circuit when they make contact with or come within a certain distance of an object. Proximity switches are most commonly used in manufacturing equipment, robotics, and security systems. There are four basic types of proximity switches: infrared, acoustic, capacitive, and inductive.

Infrared proximity switches work by sending out beams of invisible infrared light. A photodetector on the proximity switch detects any reflections of this light. These reflections allow infrared proximity switches to determine whether there is an object nearby. As proximity switches with just a light source and photodiode are susceptible to false readings due to background light, more complex switches modulate the transmitted light at a specific frequency and have receivers which only respond to that frequency. Even more complex proximity sensors are able to use the light reflected from an object to compute its distance from the sensor.

Acoustic proximity sensors are similar in principle to infrared models, but use sound instead of light. They use a transducer to transmit inaudible sound waves at various frequencies in a preset sequence, then measure the length of time the sound takes to hit a nearby object and return to a second transducer on the switch. Essentially, acoustic proximity sensors measure the time it takes for sound pulses to "echo" and use this measurement to calculate distance, just like sonar.

Capacitive proximity switches sense distance to objects by detecting changes in capacitance around it. A radio-frequency oscillator is connected to a metal plate. When the plate nears an object, the radio frequency changes, and the frequency detector sends a signal telling the switch to open or close. These proximity switches have the disadvantage of being more sensitive to objects that conduct electricity than to objects that do not.

Inductive proximity switches sense distance to objects by generating magnetic fields. They are similar in principle to metal detectors. A coil of wire is charged with electrical current, and an electronic circuit measures this current. If a metallic part gets close enough to the coil, the current will increase and the proximity switch will open or close accordingly. The chief disadvantage of inductive proximity switches is that they can only detect metallic objects.

Proximity switches are used in manufacturing processes -- for example, to measure the position of machine components. They are also used in security systems, in applications such as detecting the opening of a door, and in robotics, where they can monitor a robot or its components' nearness to objects and steer it accordingly.

Design of Optical Proximity Switch