New touch sensor uses trapped acoustic resonance technology to monitor contacts

Touch-sensitive devices have become common in cell phones, industrial touch panels, and appliances. These devices typically use capacitive sensors to create “touch” sensitivity. In reality, capacitive actuation does not require touch, but merely close proximity of an object capable of changing the capacitance of the system. Capacitive systems are typically limited to nonmetallic materials. They may not work well with gloves and can be susceptible to false activations caused by water. Even with these limitations, capacitive touch sensors work well for many consumer applications.

However, some situations involve more stringent requirements. A definitive touch, wet or underwater applications, and areas with high vandalism potential can benefit from another technique: Trapped acoustic resonance technology expands touch-sensing capabilities onto metallic substrates. The technology, referred to simply as ActiveTouch, was developed by ITW (Illinois Tools Works) over the past seven years. It can turn a solid steel plate up to a half-inch-thick into a touch-sensitive surface with multiple switch points. The technology even works with ballistic steel, creating the potential for placing a touch switch in a bulletproof steel plate with no seams.

Trapped acoustic resonance
Trapped acoustic resonance is most simply described with an analogy: the ringing of a bell. When a metal bell is struck, it produces a sound wave through vibration. As the metal’s vibrating energy dissipates, the sound slowly decays. If one touches the bell shortly after striking it, the sound dies quickly as the touch damps vibrating energy. This change in vibration decay rate can be detected and used to indicate when an individual is touching a surface that’s ringing.

In practice, realizing this phenomenon requires a substrate material capable of supporting shear and torsional mechanical waves at ultrasonic frequencies. This property is described as a high quality factor or, more simply, a high Q. When the surface of the high-Q material is contoured to create small islands of vibration, the material can trap and localize ultrasonic energy. These trapped energy regions are referred to as resonant cavities.

A wave motion is induced when a resonator is set into vibration in the megahertz range by a properly positioned piezo transducer. The motion is confined to the shape of a cylinder and extends through the thickness of the metal.

The transducer serves double duty: It creates the mechanical resonance and monitors its decay. A microprocessor controls the frequency and duration of the electrical input into the transducer. To use the bell analogy, it is the hammer striking the bell. When the transducer is no longer electrically driven by the microprocessor, the resonant vibrating energy in the substrate drives the transducer to create an electrical output signal. The piezoelectric hammer has turned into a microphone that now listens to the ring of the bell. The transducer signal defines the characteristic decay of the resonance in the substrate.

The microprocessor develops an understanding of the normal decay rate for the system. Much like the bell, when the surface of the resonant cavity is touched, the finger absorbs the energy in the substrate creating a much faster energy decay rate. This continuous process of pinging, listening, and evaluating happens hundreds of times each second.

When the time to reach a threshold value in the decay curve is shorter than the predetermined time of the normal decay rate, the micro-controller registers a touch. The microprocessor is programmed to make the touch sensor function like a switch with normally open or normally closed, momentary or latching contacts. It can even incorporate a time delay before or after activation.

The decay rate is a function of the contact material and the normal force driving the contact. Some rubber materials dampen the ringing better than a finger while some fabrics, such as a fleece or silk, produce smaller dampening effects. Leather work gloves dampen well. Likewise, the harder a user pushes on the switch surface, the quicker the vibrations in the resonant cavity decay. This rate of decay can be used to determine relative pressure sensitivity. For example, pressure sensitivity could control the scroll speed of a menu. The harder the user presses, the faster the menu scrolls.

It was noted earlier that many capacitive switches do not work well with water. Water also transmits acoustic energy — compression or sound waves travel well in water. However, the torsional and shear vibration created in the ActiveTouch substrate does not couple well with liquids. This greatly reduces the water sensitivity of an ActiveTouch sensor. Algorithms employed in the microprocessor filter any residual water effect to eliminate any water sensitivity.

Benefits beyond a typical switch
As the sensor goes through its pinging, listening, and evaluating hundreds of times a second, it can be thought of as having a heartbeat. The microprocessor is constantly monitoring the switch for a decay signal. If that decay signal is not evident, in either a touched or untouched state, the microprocessor can register an error and communicate this appropriately. In this manner the sensor can self-monitor performance and report abnormalities, an aid in trouble shooting or preventive maintenance.

Because the shear and torsional energy is trapped in resonant cavities, putting multiple switch points into one substrate is possible. With microprocessor advances, a single processor can monitor multiple resonant cavities simultaneously. These two factors make multiple switch points economical. Multiple cavities in close proximity to one another, much like capacitive sensors, can create a scroll pad or slider — only this time it has a metal surface.

Traditional piezo switches
While the ActiveTouch technology uses a piezo-electric transducer, the similarities to traditional piezo switches end there. A traditional piezoelectric switch must create stress in the piezoelectric crystal to produce an electric pulse output. The force an individual applies to the metal surface of a piezo switch creates a stress in both the metal substrate and the piezo crystal. To reduce the force needed to generate stress on the crystal, the metal substrate often becomes quite thin.

This can create several types of problems. A thin substrate can be susceptible to external shocks and vibrations. These external shocks may couple with the metal substrate to create erroneous switch activations. ActiveTouch is not susceptible to external shocks and vibrations as the ultrasonic energy must be absorbed to trigger a switch. The ultrasonic frequency and narrow frequency band monitored prevent vibrations from affecting the ActiveTouch technology in most applications.

The thin metal substrate may also impact the spacing of piezo switch points. The switch points must be spaced so a deflection on one location does not create a false trigger on an adjacent switch point. This leads to sense points spaced sufficiently apart that a scroll pad or slider is no longer practical. Resonant cavities in the thicker ActiveTouch substrate eliminate this problem.

Finally, ActiveTouch’s thicker substrate also makes the switches less prone to vandalism. A typical piezo switch’s surface is less than 0.03 in. thick. Keypads using ActiveTouch technology are over 0.1 in.

Limitations to ActiveTouch
While the ActiveTouch technology has many advantages over conventional switches, it does have its limitations. In reality, it is a three-wire sensor programmed to act like a switch. The output can be modified via secondary circuits to meet virtually any application. But unlike mechanical switches, the ActiveTouch switch does need a relay to switch high current or voltage.

The sensor needs a high-Q material. This eliminates many popular plastics such as ABS or polycarbonate. However, the technology does work well with a high-glass-filled PPS.

A material capable of dampening the ultrasonic vibrations is required to trigger the switch. Fingers and leather gloves work well. Very thick synthetic fleece mittens require more force. Hard surfaces generally will not trigger a switch at all.

There are many benefits to the ActiveTouch technology which previously were absent in input devices. While there are numerous switch and interface technologies to choose from for benign environment applications, ActiveTouch technology provides the interface for those extremely rugged, mission critical applications.