Piezo switch technology enables a metal touch-control panel.

Touch controls on appliances present a sleek and pleasing appearance and are easy to clean. They also provide the ability to create a more intuitive user-interface, a growing imperative, given the increase of available features on a modern, high-tech, upscale appliance. (Bosch Series 800 electric and induction cooktops use SteelTouch control panel. )

Most touch controls utilize glass overlays because they typically use a capacitance-based technology requiring a dielectric material for the touch surface. Several years ago, Dyna Systems, Scheidig, Germany, and Baran Advanced Technologies, Omer, Israel, partnered together on a project to provide appliance designers the option of using a metal touch control. Using aluminum or stainless steel as the overlay material, such a control panel enabled appliance makers to differentiate their products from competitors.

The metal control panel incorporated Baran’s Everswitch Versatile Piezo Module, which combined all the necessary aspects of the piezo switch technology into a single component that was easy to design into an appliance application and easy to install. (See Fig. 1.) Unlike other common piezo electric technologies, the VPM is a mechanically assembled, stable structure that is unaffected by extreme temperature fluctuations and not prone to deterioration of components. There is no adhesive used in the assembly process and there is no need for soldering wires directly to the sensitive piezo ceramic.

Using piezo technology is a safe and durable way to combine switches with compact metal surfaces. However, until recently, the integration of this sensor system was neither simple or inexpensive. To address these issues, Dyna Systems and Baran began a new approach and developed a method to manufacture piezo sensors in a more cost-effective way. The resulting system is called CrimpEx. (Fig. 1. The Everswitch Versatile Piezo Module was a piezo touch technology that came before CrimpEx. The VPM (assembled at left, disassembled at right) combined all the necessary aspects of the piezo switch technology into a single component. )

CrimpEx is a piezo switch component that has all the advantages of the Everswitch VPM technology, but differs in that it is automatically assembled on a printed circuit board by a newly developed crimping process that produces a slim and small component. With a sensor height of 3.0 mm and width of 7.5 mm, a key raster with 9 mm can be realized.

CrimpEx allows the cost-effective production of thinner user-interfaces with a smaller key size. The technology delivers benefits to both appliance designers and appliance end-users. For example, CrimpEx needs only a very low actuation force compared to other piezo technologies. The placement of the sensor directly behind the overlay material makes the system comfortable and sensitive and creates a short response time.

Like other piezo technologies, CrimpEx relies upon the direct piezoelectric effect. When a mechanical force is applied to a piezoelectric material, surface charges are induced by the displacement of the dielectric material, building up an electric field. But the similarities end there.

At the heart of the technology is a unique design for incorporating the piezo ceramic material. The CrimpEx sensor consists of three elements: a rectangular brass sheet, a printed circuit board, and a piece of conductive silicone elastomer between them. (See Fig. 2.) The underside of the brass sheet is printed with the piezo ceramic material, essentially creating a lamination. (Fig. 2. Illustration of a CrimpEx module. The brass sheet has piezo ceramic material printed on the underside of it. When pressed, the piezoelectric material generates a voltage signal that travels down through a layer of conductive silicone elastomer to CMOS transistors on the circuit board that translate the signal into a switching action. )

The components are assembled in a completely automated operation. First, electronic components are installed onto the circuit board using surface mount technology. After the circuit board is completed, a thin piece of conductive silicone elastomer is positioned on top of the board. Then the rectangular brass-ceramic sheet is put in place and, using a specially designed forming operation, has its edges bent then crimped around the back of the circuit board. The crimping holds the entire assembly tightly, elminating the need for any fastening operation. The stability of the assembly eliminates the need for a separate housing, which makes the unit very economical to produce..

In another automated process, the CrimpEx module is then glued to the backside of the metal overlay. (Note: while the piezo technology makes it possible to use a metal overlay, the use of metal is not a necessity. The technology works just as well with a decorative plastic overlay when desired.) The optimal overlay thickness ranges between 0.3 mm and 0.5 mm, depending on the material used. The backside of the control panel is potted with a polyurethane compound to provide environmental sealing. This produces a control panel component that is simple for the appliance manufacturer to install. The total thickness of the panel is less than 4 mm.

In operation, the appliance user presses a graphics-designated spot on the front side of the metal overlay. The required actuation force is between 1.5 N to 2.5 N, depending on the material used. The mechanical force causes the piezo ceramic material to produce a charge, which passes through the conductive silicone down the circuit board. There, the voltage signal is applied to CMOS transistors that transform it into a switching action. The transistors are positioned on the circuit board to be as close as possible to the piezo material to minimize the effect of the piezo ceramic’s impedance. (In addition to serving as an electrical link, the silicone elastomer functions as a shock-absorbing physical interface between the brass-ceramic layer and the circuit board.) The switching signal from the transistors goes to microcontrollers that contain the application software, as well as interface protocols.

Appliance makers can be assured that the technology meets their industry criteria. The VDE association has certified CrimpEx to all essential harmonized standards specified by the household appliance industry and the product is licensed for use for mains switches. The product was also successfully tested to UL and CSA standards. The technology also meets environmental standards, being sufficiently sealed to achieve an IP-68 rating. And it operates in temperatures up 105 DegC.

A number of optional features are available to enhance the user interface. Illumination is one example. By placing LEDs on the printed circuit board, the CrimpEx modules can be designed to display simple illuminated dots or bars, or even symbols, in the center of the key area, without the need for light guides or other optics. More complex illumination designs are made possible by using injection molding to fill transparent plastic into the cutout areas of the overlay.

Tactile feedback is another option that can be incorporated into the CrimpEx system simply by adding haptic modules into the component and then controlling the haptic effects through embedded software code.

The CrimpEx technology is optimized for a linear key configuration, and an individual CrimpEx module can hold one or two working switches. So the system can provide an appliance with a double row of switches. Given the linear configuration, there is no inherent limit as to how many CrimpEx modules can be placed in a row. Circuit boards with a length of 450 mm holding 32 switches have already been successfully realized.

The combined possibilities of close spacing and multiple switches in row permits the creation of a touch slider control with this technology. Slider controls are particularly useful for applications requiring the adjustment of variable parameters such as temperature, time, fan speed, etc.

The linear configuration makes the technology a good fit for many home appliance applications, which already tend toward linear layout of control keys. The CrimpEx technology was first deployed on a Bosch cooktop introduced in Europe in 2009. (The appliance received the reddot award for product design.) In the U.S., CrimpEx is used in the recently launched Bosch Series 800 electric and induction cooktops where the technology is promoted as SteelTouch.

While the CrimpEx technology is well suited for appliances, it can also be used to create durable, environmentally sealed user-interfaces in other applications such as medical equipment, commercial foodservice appliances, vending machines, kiosks, and security sytem keypads. Dyna Systems and Baran AT are already working together on piezo technology enhancements to meet the specific needs of these product segments.

For more information, visit:
Baran AT: www.baran-at.com
Dyna Systems: www.dynasystems.de