New Electro-Mechanical Polymer Actuator Technology for Better Interactivity
Smart material actuators for haptics may help usher in a “New-Sensory Age.&rdquo
by Christophe Ramstein and Ausra Liaukeviciute
MOBILE DEVICES such as smartphones, notebooks, and wearables are rapidly transitioning into what we are dubbing the “Neo-Sensory Age” – the next phase in the evolution of computers in which devices become more personal and portable and, most importantly, in which the interface between user and computer spans all the human senses. By fully engaging the senses, the computing devices of this new age will enable immersive, effective, and multimodal interactions. In this article, we will review new materials and enabling technologies for haptics, one of the core elements of a sensory user experience that complements touch-input interfaces with tactile output.
Although essential in real life, haptics is missing or poorly implemented in today’s mobile-computing devices. Since the first mobile phones introduced vibrational alerts as a complement to audio ringtones (the Motorola StarTAC in 1996, for example), most mobile phones have included some sort of haptic-feedback solution to provide users with tactile notifications. Most typically, mobile phones employ so-called whole-body haptics, in which the entire phone vibrates when it receives a call. More recently, the use of whole-body vibrations has expanded to confirm key presses while typing text, and also for making gaming and entertainment applications more engaging. This is an inexpensive yet bulky and limited performance solution that uses small motors to deliver vibration to the mobile devices. Eccentric rotating masses (ERMs) and linear resonant actuators (LRAs) are the most popular conventional technologies for these simple buzzes; these have not
changed very much over the last 15 years.
Such conventional actuators offer limited product-design flexibility. They are not able to support the new generation of component requirements for slim and powerful mobile devices. In addition, product designers are searching for ways to deliver a richer and more realistic and advanced haptic-user experience as a key product differentiator.
Electro-mechanical polymers (EMPs), a new type of electro-active polymers (EAPs), have emerged as a leading candidate to address the growing demand for a new actuator technology – one that is thinner, lighter, and bendable, with a large tactile and audio-response range enabling a richer user experience while providing more design flexibility to the mobile-device vendors. This article reviews this new haptic technology and highlights the important role EMP actuators will have in the emerging “Neo-Sensory Age.” It will discuss the need for these actuators, examine the properties and capabilities of EMP actuators, compare and contrast them against other technologies such as piezo-ceramic (PZT) actuators and dielectric elastomer EAP actuators (dEAPs), and describe the applications for these actuators.
Haptics for Flexible Displays and Consumer Devices
When we think of our most important senses, vision, hearing, and touch come to mind – but what we often overlook is the fundamental importance of haptics, or touch feedback during interactions with our surrounding environment. The thousands of sensors located in our skin1 are the main tools we use, not only to orient ourselves spatially and physically, but also to perceive and learn about the world around us. It is through these receptors in our body and in our skin that we gain vital information that guides our interactions with the nearby environment and enables us to differentiate textures, sense pressure, feel vibrations, and experience temperature.
The human hand is richly endowed with these tactile receptors.1 Each hand has approximately 150,000 mechanoreceptors. Their density is highest on the fingertips (2500/cm2). These receptors provide tactual acuity, enabling humans to discriminate fine surface texture, instantaneously identify object contours and shapes, recognize material types, and control hand motion.
Most existing touch-screen devices, such as smartphones and tablets, support an increasing array of multi-finger gestures. Users can scroll, pinch, swipe, zoom, and rotate with multiple fingers. This makes the user interaction rich, natural, and intuitive.
From a tactile feedback standpoint, we should expect these multi-touch devices to provide a similarly rich tactile response, one that localizes the tactile response to each finger. Unfortunately, whole-body vibration solutions integrated into current multi-touch touch-screen devices are limited to one vibration at any given time
for the entire device. This makes the user’s experience incomplete. To leverage the full potential of our bodies, hands, and fingers, future flexible displays should have localized haptics to enable a true multi-touch multi-haptic experience.
To create localized haptics on multi-touch devices (Fig. 1), designers and engineers will have to re-think mobile devices and integrate multiple actuators that can create haptic responses in multiple areas of the device in the form of localized vibrations, sounds, and deformations. These will be simultaneously and fully synchronized with the multi-finger gestures being performed. Each finger should receive its own tactile response. Such actuator technology cannot be realized with existing bulky motors. These haptic devices will need to be ultra-thin, ultra-light, and easy to integrate with the cover or the touch screen of flexible devices.
Fig. 1: Whole-body haptics is insufficient to fulfill the potential of multi-finger actions. Future displays will incorporate localized haptics.
Review of Smart Material Actuator Technologies for Haptics
New actuator materials are required to enable the devices described above with multi-touch input and localized haptic, sound, and touch capabilities. The actuator material will need to allow high electro-actuation strains, meaning that it will be pliant enough to deform or stretch, yet strong enough to handle high stresses and move structures that transmit vibrations. This elastic energy density, the product of strain and stress, is one of the key metrics for quantifying the overall performance of actuator materials.
A new generation of haptic devices under development focuses on solid-state actuator technologies based on thin, smart materials with significant mechanical response to electrical stimulation (Fig. 2). Relevant material choices include piezoceramic material, such as lead zirconate titanate (PZT); dielectric electro-active polymers (dEAPs), such as soft elastomers that allow compression by electrodes under electrostatic Coulomb attraction; and electro-mechanical polymers (EMPs), such as relaxor ferroelectric electrostictive polymers [e.g., poly(vinylidene fluoride – trifluoroethylene – 1,1-chlorofluoroethylene), or P(VDF-TrFE-CFE), and poly(vinylidene fluoride – trifluoroethylene – chlorotrifluoro-ethylene), or P(VDF-TrFE- CTFE)].
Fig. 2: Electro-mechanical polymers (EMPs) are a new type of device capable of providing haptic responses in a small form factor.
Piezoelectric ceramic technology, widely used in the consumer-electronic space, is used for speakers, sensors, and vibrational motors. PZT materials, which actuate by bending and changing shape through an electrical polarization process, are capable of producing a considerable amount of force (see Table 1). These ceramic-based materials, though strong, are brittle and susceptible to breakage, which is a challenge for flexible device configurations. Moreover, they can only produce small strain levels. In bending actuations, the maximum strain of typical piezoceramics is only 0.05%. Thus, for a 100-mm-long device, the length change in electro-actuation would only reach 0.05 mm (Fig. 3).
Fig. 3: The strain and modulus of PZT, dEAP, and EMP materials are compared.
In fact, the elastic energy density of these materials is less than 0.01 J/cm3. Owing to their rigid nature and low strain capabilities, piezoceramics have a limited potential for use in developing devices with advanced localized haptic feedback and compatibility with flexible architectures.
Among the polymer-based actuators, dielectric EAPs use electrostatic forces created by two electrodes to compress a soft, elastomeric polymer film. Dielectric EAPs are capable of undergoing high strains, but they provide small forces and typically require high voltages (e.g., over 900 V, low current). When an electric charge is applied, the electrostatic forces acting upon the electrodes can result in either attractive or repulsive forces. The high voltage required is due to the thickness and modulus of the soft elastomeric interlayer. Vibration is achieved by varying the voltage signal. While the movement and potential frequency created with this movement can be high, the stress level is very low, which requires a large device area for generating a reasonable local force for vibrations or deformations. For dEAP, the elastic energy density is less than 0.03 J/cm3, higher than PZT.6
In conclusion, PZT and dEAP material are both good candidates for creating inertial resonant actuators that can shake a mass to create whole-body vibrations. PZT benders are suitable for high-definition resonant actuators (HD LRAs) with a large frequency response (100–300 Hz). They can shake a mass quickly on a short displacement. However, owing to their very low strain, their brittle nature, and their lack of design flexibility, they will not be able to accommodate the need for localized haptics. Similarly, dEAP material will be useful for creating resonant actuators (LRAs) with low-frequency responses and quiet vibrations if the mobile devices can support the high-voltage requirement. However, dEAP material cannot support localized haptic implementations such as localized vibrations and deformations; its low modulus requires very large device areas to produce the required force.
Electro-Mechanical Polymers (EMPs)
EMPs, in contrast, are a new class of EAP materials that offer both large strain and comparatively high modulus. EMP is ideally positioned for creating localized haptics – including vibrations, deformation, and sounds. EMP can also be used as a local pressure sensor. It differs from dEAP and PZT in that the origin of macroscopic deformation is a fast reversible solid-state phase transition in a piezoelectric phase induced by an applied electric field. The external electric field makes the polymer chains transform from one conformation to another structure with different dimensions, causing the material to elongate (Fig. 4).
Fig. 4: This electro-mechanical polymer working mechanism shows the elongation of the materials in the lower box.
In contrast to ceramic materials, EMP actuators can achieve much higher strain – more than 3%, which is 60 times higher than that of the ceramic-based material (Fig. 3).3–5 Furthermore, EMP actuators also exhibit higher elastic energy density, more than 0.2 J/cm3.3–5 Therefore, they can generate a great deal more force than dEAPs using a given space. The EMP material is ultra-thin, and EMP actuators are less than 200 µm in thickness. Given this combination of properties, EMPs are an ideal platform for localized haptic applications, sound, and pressure sensing. This class of materials holds significant possibilities for defining a new way of providing localized haptic feedback in consumer-electronics, medical, and automotive products, and other applications across various industries.
Summary of PZT, dEAP, and EMP Material Technologies
Table 1 summarizes key properties of the three smart material actuators described in the previous sections: PZT, dEAP, and EMP. These metrics are key to understanding the benefits and downsides of each technology in the context of flexible displays and mobile applications. As we can see, while PZT is strong enough, the movement it generates is quite low. This combination requires PZT material to be used in a thick (2.5 mm) LRA structure (where mass is added to be shaken), preventing the possibility of attachment to the surface of ultra-thin mobile devices for concentrated localized vibrations. dEAP actuators, on the other hand, have quite a lot of strain (25%), but very low force (1 MPa). This material also requires additional mass in order to vibrate structures in mobile devices. In addition, high voltages required for this technology are not applicable for consumer-electronics devices. On the other hand, EMP material balances strain (3%) and force (700 MPa) and is ultra-thin and flexible, which allows it to be attached directly to the device surface and provide localized vibrations for next-generation thin and flexible devices.
When comparing the inherent properties of the three material platforms discussed, EMPs stand out as the most promising solution for developing thin and light devices and enabling integration of advanced haptics into next-generation displays and consumer electronics.
Choices for Product Design
When evaluating haptic technologies for displays and consumer electronics, several aspects of the material should be considered: overall form factor, haptic performance, mechanical and environmental durability, power consumption, and operating voltage.
EMPs are ultra-thin, flexible, lightweight, and highly customizable. EMP actuators can directly bond to different surfaces to enable architectures that create localized vibrations, emit sound, and visibly deform. As we can see from Fig. 5, the EMP actuator attaches directly to the surface to create localized vibrations.
Fig. 5: In the image above, the electro-mechanical polymer is attached to the touch surface.
The physical properties of EMP actuators offer a variety of options for customization; they enable light and thin mobile product form factors integrated with realistic tactile feedback. EMP actuators can also complement new component technologies, such as flexible displays, to provide a more practical feature set, with haptics and sounds in one package.
Another factor to examine when choosing a haptic actuator technology is performance – the strength and frequency of vibration – and how the vibration feels and performs in different scenarios. All of the smart-material actuator technologies described in this article are capable of operating at a wide range of frequencies (100–300 Hz), generating over 1 g of acceleration – so-called High-Definition Haptics. EMPs provide multi-touch localized haptic vibrations and deformations that are only felt under the specific area touched by the user. This enables a much more realistic tactile interaction with devices and a wider range of actions.
When implementing haptics in mobile devices, it is also important to consider the overall system architecture (Fig. 6). This typically consists of a microcontroller, driver, and power electronics, plus one or multiple actuators and sensors.
Fig. 6: A typical haptic system architecture consists of a microcontroller, driver and power electronics, and one or multiple actuators and sensors.
For smart material actuators, a key factor is the operating voltage and, therefore, the electrical power-supply requirements. In consumer electronics, dielectric EMPs are not attractive because they require a very high voltage (see Table 1), which greatly limits the selection of and increases the cost of the electrical driver. Piezoceramic and EMP actuators, on the other hand, require much lower voltages. Miniature commercially available drivers for consumer electronics are ideal for driving these technologies.
Haptic Applications for EMP Actuators
Because conventional mechanical buttons seem to be on track for obsolescence, deformable, mechanically programmable user interfaces with dynamic haptic feedback could eclipse the static displays of today. Enabled by shape-changing materials such EMPs, touch screens someday may have physical 3-D buttons emerging from the surface of a portable device on demand.
Some intermediate solutions are being explored to morph buttons out of a transparent surface. For example, a solution developed by Tactus is made from a multi-layered stack with an optically clear polymer on top that deforms when liquid is being pumped in and out through micro-channels. This solution does not offer the ability to control buttons individually, nor to provide local vibrations, and requires liquid management in a mobile device. New technologies will help expand the physical limits of our devices by enriching the user experience in human-machine interfaces.
To get a better sense of how new EMP technology can be utilized in real products, three application examples are provided below: an ultra-thin keyboard, touch screens, and wearable devices.
Ultra-Thin Keyboards with Localized Haptics
While mobile devices get thinner and lighter, a major obstacle to making keyboards thinner is the physical packaging of the mechanisms under each key that provide the desired feel of a snappy “click.” Many OEMs are increasingly looking for methods to manufacture thinner and lighter keyboards without sacrificing that physical tactile feedback.
Figure 7 shows an ultra-thin keyboard prototype with localized vibrations and sounds enabled by EMP actuators. While typing on this keyboard prototype, users feel vibrations and sounds on each key to confirm button presses, making the typing experience similar to typing on a keyboard with physical buttons. This prototype, however, only provides vibrations and does not have integrated deformation. The next generation of this prototype will show the ability to bring the keys up when needed, adding the kinesthetic feel of the key shape as well as the snappy feel of a click, without requiring any mechanical structure underneath the keyboard substrate.
Fig. 7: The world’s thinnest flexible keyboard with localized haptics and sounds is a product concept enabled with EMP actuators.
Localized Haptics for Touch-Screens
The obvious next step for touch screens is to mimic the shapes of keyboard keys and other UI buttons and icons, directly on the touch screen, while also providing a physical behavior in response to multi-finger gesture. Imagine the keys of a keyboard morphing out of a touch screen so you can feel their shapes while browsing the touch screen with your fingers. While typing, you would get the snappy feel of the click, along with a subtle but localized sound! And once typing is done, the touch screen would go back to its original flat and smooth shape, better for swiping and scrolling gestures. To create a morphing touch screen, transparent EMP will be required, along a thin, compliant, transparent substrate to replace the glass. Transparent EMP actuators are currently under development.
Haptics for Wearable Devices
Wearable devices such as watches, goggles, shoes, and many others under development are an extension of smartphones, tablets, and notebooks. They are smaller, thinner, and lighter, with limited power. They have less and more specific functionality and very small and limited displays for interaction. Therefore, an alternate mode of feedback is required to give end users simple information such as alerts and notifications. Later on, EMP technology will go into fabrics to create smart digital clothing.
An Integrated Future
Our sense of touch and our ability to feel are integral parts of our existence in the physical world and play a fundamental role in daily life. As people begin to explore new, innovative ways to enhance user experience through devices that are increasingly in sync with our senses, new material technologies and products will become a way for manufacturers and designers to integrate haptic feedback in an effective, meaningful way.
1R. S. Johansson and A. B. Vallbo, “Tactile sensory coding in the glabrous skin of the human hand,” Trends in Neuroscience 6, 27–32 (1983).
2S. Wong, High Quality Multitouch Gesture PSD for Devices Posted Design Resource. getaddictedto.com (2011). Retrieved from http://www.getaddictedto.com/design-resource-multitouch-gesture/#sthash.CWDYg0gT.dpbs.
3Q. M. Zhang, V. Bharti, and X. Zhao, “Giant Electrostrictive Response and Ferroelectric Relaxor Behavior in Electron Irradiated P(VDF-TrFE) Polymer,” Science 280, 2101–2104 (1998).
4H. Xu, Z. Y. Cheng, D. Olson, T. Mai, Q. M. Zhang, and G. Kavarnos, “Ferroelectric and Electromechanical Properties of P(VDF-TrFE-CTFE) Terpolymer,” Appl. Phys. Lett. 78, 2360 (2001).
5F. Xia, Z.-Y. Cheng, H. Xu, Q. M. Zhang, G. Kavarnos, R. Ting, G. Abdul-Sedat, and K. D. Belfield, “High Electromechanical Responses in Terpolymer of P(VDF-TrFE-CFE),” Adv. Mater. 14, 1574 (2002).
6R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, “High-Speed Electrically Actuated Elastomers with Strain Greater than 100%,” Science 287, 836-839 (2000). •