Touch-type actuation devices have become increasingly popular in a wide variety of products because they are easy to use, easy to clean, and they provide environmental sealing and a broader range of aesthetic options. Resistive touch screens and glass-front capacitance controls have become more extensively used in recent years, and are well-suited for many applications. But there are many situations where rough handling, extreme environments, or the potential for vandalism demand something more rugged and resistant to abuse. Piezoelectric switches are often used in these cases, but they, too, have some limitations.

To overcome the drawbacks of existing touch-actuation devices, ITW ActiveTouch, Buffalo Grove, Ill., developed a novel actuation/sensing technology that is sensitive enough to register a light touch, but tough enough to resist blows from a hammer. The division of Illinois Tool Works spent seven years researching and developing the technology, which is based on the principle of trapped acoustic resonance.

Given the name ActiveTouch, the touch-sensitive switch can be made from a variety of rigid materials, but when made from metal it becomes an extremely robust and durable actuator. ITW says the switch can be made from stainless steel up to 1/2-in. thick, making such a switch not only vandal-proof, but also waterproof, shockproof and even explosion-proof, as well as unaffected by high-temperature sterilization. Such switches can, therefore, be designed to operate in harsh or hazardous environments, including underwater. With no moving parts, and no ingression of water or chemicals possible, the switches offer virtually unlimited lifespan.

How it works

Contoured regions on the switch plate create resonant cavities that trap the ultrasonic  energy.

The principle of trapped acoustic resonance rests upon the fact that a material capable of supporting shear and torsional mechanical waves at ultrasonic frequencies can have those waves trapped, or localized, by creating specific features or geometries in the material. The cylindrical regions designated to trap the ultrasonic energy serve as resonant cavities that are set into motion by ultrasonic transducers emitting in the area of 1 MHz.

The ultrasonic waves induce a twisting motion, which is called the trapped torsional mode, and confines the motion to the cylindrical cavity. In doing so, the ultrasonic vibrations initiate a resonance in the cavity, which is analogous to ringing a bell. As with a bell, the resonance from the cavity will decay, or ring down, at a predictable rate, depending upon its material and geometry. Touching the outside surface of a resonant cavity has a damping effect that accelerates the rate at which the resonance decays, just as someone touching a struck bell damps the vibrations and hastens the end of the ringing sound. The damping occurs when the surface is touched by a finger, gloved finger, or any acoustic-absorbing object.

The time-to-decay rates for both states of the resonant cavity — touched and untouched — are stored within a microcontroller, which can then determine whether the cavity is in a touched or untouched state by comparing the measured decay rate with the decay-rate profiles stored in memory.

In a novel application of the piezoelectric effect, the resonance from a cavity is detected by the same piezoelectric transducer that initially generated the ultrasonic energy. The transducer, which is bonded to a switch surface, serves two roles, enabled by the two-way characteristics of piezoelectric materials. Using the indirect piezoelectric effect, the transducer converts electrical pulses into ultrasonic pulses when energized. When de-energized, the transducer acts as a sensor, using the direct piezoelectric effect to convert the reflected ultrasonic vibrations into electrical pulses.

In other words, the piezo element “rings the bell,” then pauses to listen to it, determining whether anyone has touched it. This cycle of ringing and listening occurs nearly every microsecond. The microcontroller manages the whole communication and can be easily multiplexed to monitor multiple switch positions. It can ring many such bells and listen to them simultaneously. This constant cycling makes it an active system, hence its name, ActiveTouch. Though operating in an “always on” mode, the system’s power draw is minimal, with 25 microamps being typical of most designs.

Micro managing

Enlarge this picture When the ultrasonic transducer operating at 1 MHz vibrates the resonator, a twisting motion is induced. It is known as “trapped torsional mode” because the motion is confined to the shape of the cylinder.

Despite the use of acoustic resonance as a principle, the performance of the sensors is not affected by external sources of vibration. This is partly due to the high frequency used by the system, but due in larger part to the microcontroller’s program that recognizes only those specific waveforms pertinent to the system’s design.

Management by microcontroller offers a variety of advanced feature options that can be specified by the design engineer. Switch activation can be designated as normally open (NO), normally closed (NC), normally high (NH), normally low (NL), momentary, latched (maintained), or proportional. Outputs can be serial, USB, RS232, CAN bus, in series or in parallel, or any number of customer-specified outputs.

The design engineer can specify the sensitivity threshold from near zero to much higher levels, and can select the length of damping time needed to output a signal.

The microcontroller can also be programmed to self-calibrate the system to adapt to changing conditions such as temperature swings or deformations in surface material. Because it is an active system, it can learn to identify a new waveform as the nominal, untouched state, and still distinguish it from the waveform of the touched state. It can also apply that learning capability to self-diagnostic functions. For example, upon identifying a changed nominal state, a new baseline, the system can be programmed to communicate an error signal, generate an alarm, call for service, or simply deactivate the abnormal switch area and transfer its function to a backup location. When necessary, these features permit faster troubleshooting and repair to reduce or eliminate system downtime.

Defining distinctions

Enlarge this picture The two waveforms represent the normal resonance decay, or ring-down, without being touched. The two waveforms at right represent the accelerated decay when the resonator has been damped by a touch.

Because an ActiveTouch switch employs the piezoelectric effect, both direct and indirect, it is important to distinguish its differences from a standard, commonly used piezoelectric switch, which is also a solid-state component.

A piezo switch uses the direct piezoelectric effect, where a mechanical force exerted upon the piezo element causes it to generate an electrical pulse. This signal is then read by a controller that interprets the signal as a touch. The piezo switch can also be placed behind a front panel and still register the touch. The behind-the-panel approach provides design advantages such as sleek styling and protection of the elements.

Piezo switches, however, have some limitations compared to ActiveTouch switches. The elements in piezo switches require a small amount of mechanical deflection, which creates two issues involving material thickness and spacing. Typically, the thickness of the material over the piezo element must be 0.030-in. or less in order to produce the required deflection. Thicker materials require a larger force to actuate, making it more difficult for the user to operate. Limiting the top material thickness can be a drawback in applications where sharp impacts or potential vandalization are important issues.

The required mechanical deflection also limits how closely the piezo elements can be spaced in a grouping, as on a keypad, for example. When placed too closely together, applying a force to one switch point might inadvertently activate an adjacent switch point due to the strain spreading across the surface material.

And because piezo switches are actuated by strain in the material, they can be subject to false actuation when objects inadvertently come into contact with the switch or by pressure effects.

Another disadvantage of piezo is that, unless advanced circuitry is employed, the output is typically only a brief, discrete pulse regardless how long the user presses the surface. By contrast, the ActiveSwitch can detect continuous touching by the user and utilize that awareness if desired in the application.

Design freedom

Enlarge this picture Graphic depiction of resonant decay when touched and untouched states. The rapid decay is recognized as a touch and trigger actuation.

ActiveTouch switches also provide a product designer with a high degree of design flexibility. The surface material for an ActiveTouch switch can be virtually anything capable of exhibiting acoustic resonant properties, including glass, ceramic, hard plastics, metals, and even stone materials, such as granite. In some cases, the material can be specified up to 1/2-in. thick to create an extremely durable device.

The touch surface can be contoured for finger positioning or for other locating aids, and can incorporate Braille legends. Identifying legends can also be applied by engraving, etching, printing, or use of graphic overlays bonded to the surface. Illumination from the back of the touch surface can be provided by clear, epoxy-filled channels, or by bonding a graphic overlay with light guides on top of the surface.

In addition, the active switch area that defines a touch can range from small to large, and can be curved or irregularly shaped. The technology can be employed as discrete, modular switch elements, or grouped together to form keypads and keyboards. Switch locations can be placed close together — up to six within a linear inch — making the technology feasible for even small control panels or portable devices.

The combination of design flexibility, durability, and environmental sealing makes the new actuation technology suitable for a diverse range of applications, including outdoor power equipment, industrial equipment, commercial appliances, vending machines, ATMs, kiosks, security control panels, medical equipment, fitness equipment, and portable electronic devices subject to rough handling.

For more information, email: info@itwactivetouch.com