Materials choices for product designers are vast, but often when it comes to touch control the fall back is traditionally glass or plastic, but new acoustic signal processing is expanding those options.

Acoustic signal processing has been used extensively in many fields such as oceanography and telecommunications. One of the most recent uses for acoustics is to register a touch. For touch screen and touch control applications using acoustics, there are currently several technologies on the market including 3M’s Dispersive Signal Technology (DST) that determines a touch point by measuring mechanical energy waves within a substrate that were created by a finger or stylus touching a surface and how the waves are dispersed over distance. Algorithms anticipate the “dispersion” effect and interpret the precise touch location.

Another example is Tyco Electronics’ EloTouchSystems, which uses acoustic pulse recognition technology. This concept is based on the idea that sound generated by a touch has unique characteristics that can be compared against premeasured characteristics identified for that location. Tyco recently entered into a cross-licensing agreement with Sensitive Object, a Paris-based company, that has developed an acoustic technology dubbed ReverSys. The two companies have agreed to cross-license their intellectual property in the acoustic touch field and to enter into a mutual technology development program.

Sensitive Objects technology can be used as a control panel to control lighting, as in this image, or be used in a variety of household applications such as controlling appliances and security systems.

ReverSys is a haptic technology, which means it pertains to touch and the sound waves generated from the touch. In a nutshell, the touch on a control panel creates an acoustical signature that is unique to that exact point in space. A similar touch just a few centimeters away will emit sound waves with a different acoustic signature. ReverSys uses acoustic sensors to capture the “sound,” and determine that sound’s signature, which is the key to determining the exact point of the touch. Each signature relates to a specific touch point and each touch point relates to a specific command, says Marc Vasseur, vice-president of marketing and business development for Sensitive Object.

The technology is based on the time-reversal mirror theory. Much of the development work in this arena was done in the 1990s at the Laboratoire des Ondes Acoustiques, the Paris-based acoustic laboratory of the Centre National de la Rescherche Scientifique (CNRS). Here, Professor Mathias Fink invented the “time reversal mirror,” a technique that identifies specific acoustic signatures and determines the location of the touch point using the propagation of the waves, their rate of decay and how the waves echo after they contact the edge boundaries. The technology was spun-off in 2003 and the company, Sensitive Object, was formed.

The theory states that a sound wave captured by an acoustic sensor can return that wave along its entire spatial area back to the location of the original source. The wave that is transmitted by the touch is captured by acoustic sensors, which sends the wave back to the source, where it is reflected back. As the sound wave is reversed and retransmited, it becomes more focused on the target.

A user controls consumer electronics from several feet away.

Traditionally,  the interior surface of the unit being tested had to be covered with many sensors and each had to be connected to an individual electronic circuit that stores the signals. Fink’s research found that even a single sensor could be used. According to research by company founder Ros Kiri Ing, using a single sensor and a “chaotic cavity,” the same physical process was realized that allowed the sound waves detected by the single sensor sent back to its source. According to the research, the system can blend the sound waves that are emitted in all directions spatially and relay the waves back to that single sensor.

Then, by comparing this signature to a database of pre-established signatures, the exact location of the source can be determined. In addition, because the location can be found with a single sensor, component costs can be dramatically reduced, says Vasseur.

The touch-control acoustic technology allows the company to develop “virtual” buttons that can be placed several meters away from the controllers. The buttons are called virtual in that they are not tied to a specific sensor component.

The S16 embedded system enables the development of HMI applications in any system environment. Sensors feed acoustic information to the board, which is digitized and enhanced via a proprietary process before analysis.

The receivers that capture the sound waves are typically acoustic transducers. Properly locating them is key to their optimal performance. Vasseur says that to get high volume and good performance, designers from the OEM and Sensitive Object need to work together to determine the optimal location.

ReverSys’ software engine, which can be implemented in a PC or embedded in a microprocessor, discriminates touches based on the acoustic signature and triggers any set of actions that are programmed to occur when a button at that precise location is touched. For instance, the company’s S16 embedded system features built-in sensors that feed the acoustic information to a printed circuit board, which digitizes the signal, enhances it via a proprietary process and then compares it to a library of pre-loaded acoustic signatures. The unit can store up to 64 acoustic signatures in a standard configuration, with expansion being possible. In other technologies, having 64 touch points might require 64 sensors. This acoustic approach allows designers to reduce the number of components needed and the subsequent cost for developing a complex interface.

This technology can work with the touch of a stylus, a pen, or corner of a credit card, which can be advantageous when using a point-of-sale, or point-of-service system. However, unlike other technologies, this acoustic sensing technology can’t follow a moving touch, so therefore cannot be used to create a slide-type actuator, which would require multiple touches.

The ReverSys technology can be designed to work independently of the object’s shape, meaning that the touch interface can be a traditional flat or three-dimensional object. The touch control surface can utilize virtually any type of material, including glass, plastic, metal, wood, and even soft materials such as leather. The system does not require there to be a line of sight between the object touching the control and the sensor. This is an advantage over other technologies such as infrared that requires a line of sight. In fact, when using acoustic signal processing, the substrate material can be damaged, dirty and impossible to see through, and it will still work as a touch control interface, adds Vasseur.

The system’s robustness is linked to the substrate material that the product designer chooses. The material and the thickness doesn’t matter, Vasseur says, whether that material is a 1 mm thick glass panel or a 5 mm glass panel, because once the system is programmed to recognize a signature through a particular material it will do so every time.

As a general rule, the technology can employ any material possessing a homogenous composition. “The technology is based on the way the object is resonating, if the noise is propagating through a material that is not reasonably homogenous, the level of performance could suffer,” Vasseur says. Given that the acoustic properties of most materials are well known, materials don’t need to be customized for any particular acoustic control application.

One critical challenge that arose during the development of the technology was dealing with ambient environmental noise, an issue that Vasseur says has been resolved. The problem was that any external or internal noise impacting the control surface or sensors product could “pollute” the acquisition of the acoustic signal. The solution was to model the touch signatures so that the system could be programmed to recognize the difference between the acoustic signatures of a deliberate touch and the vibrational signatures induced into the surface by ambient sound.

On the electronics side, the company put in a lot of work developing protection for the different platforms. For instance, a touch screen that the company currently sells can withstand 15 kV of electrostatic discharge.

The nature of acoustic touch control also makes EMC issues less of a concern. “By not using electricity, we are much less sensitive to electromagnetic interference issues than other technologies such as capacitive technology.”

The first application based on the technology was a virtual keyboard (VBK), an ultra thin acoustic keyboard that can be used to create a fully functional 108 key computer keyboard on any surface. Two sensors mounted underneath the surface enable the entire keyboard face to be made tactile sensitive.

Today, they are currently in use in a variety of applications such as household appliances, vending machines, medical equipment and industrial automation. The technology has evolved to include uses in touch screens, control panels or in embedded platforms — some applications may require multiple platforms to be used. For instance, a vending machine might have a keypad as well as a touch control interface.

The systems can be easily reprogrammed so that a touch might mean something different. For instance, if the price of a soft drink goes up, the unit can be reprogrammed so that a touch system will reflect the new cost.

Vasseur says that another opportunity for OEMs is in terms of releasing new appliance models without having to develop new tooling. The appliance maker can use the same physical piece of glass or polycarbonate for the touch interface on two or three different models by changing the graphic overlay on each touch panel and then programming each microcontroller to recognize the different set of touch points.

For more information, email: contact@sensitive-object.com