The sense of touch is a critical element into an immersive user experience. Haptics is, essentially, the science of touch. According to IDTechEx, the haptics market will be worth nearly $5bn by 2025. How can an electronic device enhance the user experience through interacting with the sense of touch? Haptic technologies have been present in gaming and cell phone for a long time. More recent developments are enabling far more sophisticated user experiences.
KEMET Electro-Mechanical Polymer-based actuators are thin, light, flexible, and provide a wide range of haptic feedback that are mild, pleasing and distinguishable from one another thereby providing a wide range of localized feedback.
The paper was presented by Marina Innocenti, KEMET Electronics, Bologna, Italy at the 3rd PCNS 7-10th September 2021, Milano, Italy as paper No.4.2.
KEMET Film Flex Assembled Actuator
Inspired by the idea to make the world a better, safer and more connected place to live, inventors are the ones who push the boundaries of innovation and create new devices towards these goals. KEMET’s latest piezoelectric polymer haptic actuator is one such invention. This new class of haptic actuator uses Electro-Active Polymer (EAP) technology, which is paper-thin at 150 μm thickness, featherweight and can easily mimic real-world stimuli for inducing haptic effect sensations. The working principle of these devices relies on implementation of a unique electro-active polymer, which when activated by means of electric field, realigns its molecules in a direction that elongates the film, creating in this way a piezoelectric effect that can effortlessly convey specific material textures and familiar feelings, like the clicks and clacks of buttons, and more. The technology and the applications for this kind of product, with a more nuanced, localized, natural-feeling experience than previous haptic devices, are endless. Bonding or framing the actuator to a rigid substrate transforms actuator elongation into an out-of-plane vibration, creating the haptic effect.
The wide bandwidth of the devices coupled with some physiology of touch and sensation allows for a very innovative haptic response rather than the simple on/off, buzz, or no buzz notifications associated with ERMs.
KEMET haptic actuator is built through the same manufacturing process as the film capacitors and this allow to get the best from the polymer film properties described above. The dimensions of the first launched KEMET haptic actuator are L=10mm, W=11mm, T=110um. This is expected to be the first haptic actuator of a family whose dimensions can vary depending on the market needs either in terms of performances and dimensions. The easiest way to take advantage of the haptic effect, produced by the KEMET haptic actuator when the electric field is applied, is to constrain it within a flexible surface: in this way, the alignment of the molecular dipoles within the film may be perceived as an out of plane displacement, depending on the frequency and amplitude of the applied electrical field. A flexible printed circuit board (flex PCB) has been used for that purpose, with a layout aimed at allowing wiring and fixing of the final Film Flex Assembled Actuator underneath the target surfaces (Fig. 2).
Integration of KEMET Film Flex Assembled Actuator in a finished product
KEMET actuators, unlike LRAs and ERMs, do not shake the entire device, can be embedded directly into a product’s surface and act as a haptic skin for devices. They are capable of providing localized and meaningful haptic feedback and enhancing the user experience, increase immersion and satisfaction. Because the integrated haptic devices vibrate over a wide range of frequencies, the user experience is enhanced. Rich, low frequencies provide pleasant sensations, and then higher frequencies impart the detail and overtones, creating effects with unusually natural sensations [37].
Mechanical integration of KEMET Film Flex Assembled Actuator is possible within both rigid shapes and flexible surfaces or volumes. The performances of the actuator are transferred to the user depending of its installation, positioning and material interface (also referred as “cover” in the following), so it is important to develop the structure of the final geometry around the FFAA depending of the needs of the users and application. Positioning the FFAA near the user facing surface, on the flex PCB side the stronger haptic effect will be achieved. Also the cover plays an important role in the modulation of perceived sensation. There are a variety of textiles and polymer blend sheets suitable and available in the market to be used as cover to guarantee good performances, safety and reliability of the final assembled product. Below are reported some of the most important characteristics (electrical insulation, resistance to bending, wear rate, waterproofness, conformability) that has to be considered for a cover material:
- Electrical insulation: surface cover material and cabling must fulfil electrical insulation requirements for an operating voltage ≥ 250Vac.
- Resistance to bending: the stiffness and thickness of the material used influence the capability to bend of the final product, the actuator’s energy transfer into a perceivable haptic sensation and the visibility of the FFAA on the cover surface. If the sheet is too flexible, a stiffener layer could be added as support on the PCB side where the actuator is assembled.
- Wear rate: scratch and wear resistance are important design factors for the user facing surfaces. Aesthetic characteristics must have the same requirements of standard products.
- Waterproofness: the whole integration of the haptic module should be waterproof. The FFAA is an electric device, so it has to be protected by water.
- Conformability: the user facing surface can also wrap around the whole integrated product part for seamless construction.
Considering some possible application of KEMET FFAA in different devices and technologies, in the next parts of the paper will be analysed the integration of the actuator with both hard and flexible shapes.
Hard shape integration
Hard shape integration is when the actuator has to be included within a rigid structure, various examples can be easily found in several human machine interfaces (buttons, joysticks, touch pad…). To allow proper haptic effect, the integration structure can be realized as the per next description. Considering the mechanical structure of the FFAA, with the active part in the centre of the flex PCB, to keep the best performances and to allow the right positioning of the soldering areas, the Film Flex Assembled Actuator should have a mechanical supporting structure working in the perimeter around the active part of the actuator (free margin of the flex PCB of the FFAA). Any rigid plastic, constituting the mechanical support of the FFAA, should be designed with the aim to avoid interference with the actuator, (active part of the FFAA) to prevent damages or stresses. On the contrary, any soft material such as foam can be used as substrate for the active part of the actuator to have light support, when working. When designing the FFAA mechanical support, the depth of the seat where the FFAA is placed, can be considered within the tolerance range of the FFAA itself (Fig. 3 and Table 1).
A high depth in the seat of the actuator may lead to an undesirable bending of the PCB surface with a modification or dampening of the haptic feedback. Another important aspect to be considered is the wiring. The overall design should allow the wires to easily pass thorough without applying any mechanical stress to the flex PCB and the overall FFAA. Finally, our suggestion from an aesthetical point of view, is that the external cover of mechanical support should be designed with the aim to create the best compromise between thickness and hardness to guarantee that the outline of the FFAA is not visible from the user side and that best haptic performances are preserved. Below will be discussed different examples of integration developed to integrate the FFAA in rigid mechanical support.
