Capacitive Proximity Sensing



Remote interaction with user interface offers new opportunities for product differentiation.

Capacitive touch technology is still all the rave in the user-interface space, as it rightfully should be. It is becoming rarer that someone wouldn’t have a smartphone with a touchscreen on it today. And the thought of carrying a paper planner or even a notepad around is almost archaic as your capacitive touchscreen tablet can do all of that, plus enable you to surf the web, read a book, look at pictures and chat with friends – all through one device. The interface to these products helps to make the appeal associated with them, the ease of use and the fun they are to interact with. Capacitive interfaces enable all this, with no moving components which would break down over time, no gaps or creases which can get containments in them which are really tough to clean, low power to make it so your battery life is longer and the ability to reliably detect and reject a finger press when it is supposed to, even in noisy conditions. Since they already offer so much, how could you make them better? What if there was a way you could interact with your phone, remote control, thermostat, etc. without actually touching a surface? Almost as if the electrical device knows what you are going to do before you actually do it? That remote know-how is called proximity detection.

capacitive_prox_1
Self-capacitance proximity detection shows hand coupling with the electricfield.
capacitive_prox_2
A hand over a table that has a PCB and sensor resting on it to detect proximity.

Imagine as you arrive home from a long day at work, open the door and move your hand over the area where you know your home control panel is located; this home control panel then lights up and comes alive through capacitive proximity detection. It provides you with options to turn the lights on, open the blinds and listen to music simply by touching it. There are many other potentially revolutionary applications that could be driven by proximity sensing. Although the FCC has regulations in place around specific absorption rates (SAR), which specify how much WiFi and RF signals that electronics are allowed to emit in the close presence of a human being, many tablets are implementing capacitive proximity detection within these allowable limits. For example, some mobile phones are upgrading their infrared sensor in order to avoid problems such as direct sunlight turning your screen off or on unexpectedly. A wireless mouse is yet another example, where the user can immediately move the cursor with the mouse instead of having to shake-it to wake it up. Automotive applications include car handles, center consoles, light fixtures and even the trunk so you can open it with both hands full of groceries! As you can see, proximity detection can take an already great technology based on capacitive sensing and make it even more interactive.

Not only does this interface technology add a “cool” element to your device, but this can also improve many other aspects of your product:

  1. Power – Power increases the length of time that your device can run on one set of batteries since the power consumption is actually less. Let’s say there are 16 – buttons on a panel; instead of all 16 – capacitive buttons having to continuously wake up, determine if there is a finger press and then go back to sleep, over and over, now the buttons can stay in sleep mode until after the proximity detection has been triggered. This will save upwards of 16x the power.
  2. 3D Gesturing – With proximity ranges as far as 10” (25cm), it is possible to implement 3D gestures so that users can simply wave their hand in front of the sensors and increase the volume of the TV or turn the page of their book, etc. There are no limits to what could potentially be unlocked taking this technology from two dimensions to three.
  3. Replacing IR – Today’s most common method of doing proximity detection in electronics is infrared (IR). There are many downsides to IR, though, that capacitive proximity detection can overcome. IR needs a lot of power to drive the LED and can be very expensive, unlike capacitive. IR has to have visible windows on the screen; also not needed with capacitive. IR can be fooled by changing lighting conditions like direct sunlight; this will not happen with capacitive.
capacitive_prox_3
QTouchADC sensing algorithm is based on standard self-capacitance; the SAR ADC is over-sampled.
capacitive_prox_4
An animation of a hand approaching a dead-fronted thermostat and it waking-up.

There are two primary implementations of capacitive sensing: self capacitance, where you are measuring the capacitance delta between one electrode and earth ground (your hand); and mutual capacitance, where you are measuring the capacitance change between two electrodes. There are many variations of each of these (software, hardware blocks, ADC, etc.), but the same overarching concept reigns true throughout. Today, the self-capacitance implementation seems to serve best for longer range proximity distances. The device charges a sense electrode of unknown capacitance to a known potential. The electrode is typically a copper area on a PCB (could be any conductive material, including ITO). The sensor layout can be done in a variety of ways, but the most effective and efficient is the loop sensor (or commonly called loop antenna). This is just a copper trace of approx 10mm thick around the outside area that is to be proximity enabled. For a thermostat this would likely be a loop around the outside of the entire box, completely invisible to the user, but able to drive long-range detection. The larger the loop, the longer the total range (diminishing returns at a certain size). As with all technologies, there are trade-offs associated with power, response rate and noise immunity. If you can increase the acquisition rate with shorter burst lengths, this will result in lower power consumption and increased proximity range. The total reliable range you can achieve will depend on all of these factors, plus your particular application, packaging and environmental conditions.

All capacitive proximity can be setup to do physical touch sensing (where the finger makes contact with the sensor) or tuned to do proximity sensing (where the hand only needs to couple with the sensor’s e-field). Ideally, the same IC can drive both proximity sensing and standard button, slider and wheel implementations, while minimizing the number of external components required.

There are other forms of proximity detection. As discussed earlier, the most commonly used today is IR / PIR, but it has its trade-offs. There are also magnetic, optical, ultrasonic and others. Each has their respective pros and cons associated with them. Capacitive proximity detection, in most applications and environmental conditions, offers the most advantages (product differentiation, low power, low cost, no visible sensors, etc.), from physically touching a surface to a distance away of 6” or 15cms reliably (capacitive proximity is capable of much longer ranges, just with all the outside factors and trade-offs associated with them). This revolutionary technology can make all of the above benefits happen so that you are able to differentiate your end product.

 


capacitive_prox_5

Patrick Hanley joined Atmel Corporation in August 2010 as the marketing manager for the touch technology team focusing on buttons, sliders and wheels (BSW). Mr. Hanley’s responsibility includes defining new products, pricing, channel management, and many other functions for the success of the BSW product line within the Touch Business Unit at Atmel. Mr. Hanley holds a bachelor of science degree in electrical engineering (BSEE) and a minor in business administration from Marquette University in Milwaukee, WI.

Share and Enjoy:
  • Digg
  • Sphinn
  • del.icio.us
  • Facebook
  • Mixx
  • Google
  • TwitThis