FIELD OF THE INVENTION

The present invention is directed to the field of mechanical actuators and arrays thereof, more specifically actuators of the field of haptics and haptic feedback, tactile displays, and actuators for touch feedback systems, for example for virtual and augmented reality applications.

BACKGROUND

Virtual and augmented reality (VR and AR) aims to provide an immersive environment where the user can interact naturally with a virtual objects for training, gaming, remote surgery or teleoperation. While humans are visually dominated, our other senses are essential for true immersion, with the sense of touch being particularly underexploited. The underusing of the sense of touch for Virtual Reality system can be explained because we lack wearable, compliant and mm-scale transducers to generate localized mechanical stimulus over large areas such as a hand or an arm. For example, wearable devices stimulating the sense of touch lag far behind the advanced development of VR headsets. The engineering challenges for developing advanced haptic interface are many, because the tactile sensors of the human body have not only high spatial resolution (as low as 1 mm) and high bandwidth, for example from DC up to 800 Hz, but also can sense a very large dynamic range of displacements, able to feel features from few micrometers up to the millimeter range, see reference [1], and sense both normal and shear forces. To provide a convincing sense of touch for a handling task requires hundreds to individual actuators to stimulating the skin on fingers and palm, yet these thin actuators must all be comfortably integrated into a glove.

The use of shear force in portable applications has been very limited, as making arrays of pins to provide normal force, such as Braille display, is already very challenging, even in a rigid configuration. Yet a great deal of how we make use of our sense of touch is based on shear force: identifying a surface merely by brushing one's finger over it, or preventing a glass of water from slipping out of one's grip exploits human's well developed shear force sensors. Braille displays, while not wearable and not flexible, are one of the most mature application of haptic interfaces. They illustrate the challenges of integrating dense arrays of actuators on a 2.5 mm pitch that must meet simultaneously force, displacement and speed requirements. The main commercial products are based on piezoelectric bender actuators that are used as levers to push pins, such as the HyperBraille™ from Medtech. Recent developments have shown portable displays using electromagnetic technologies but with reduced force, see reference [2]. Research prototypes based on electromagnetic technologies, see reference [3], shape-memory alloys, see reference [4], or active fluids, see reference [5], have also been proposed.

To make tactile display wearable, polymer-based materials have been used. For example, elastomers can easily integrated in a wearables such as a glove, and their mechanical properties are close to human's skin providing a more natural feel and interaction. Pneumatic devices such as the commercial HaptX glove, see reference [6], or examples in literature, see references [7,8] are based on an inflatable actuator membrane, limited to normal forces. The used external compressor and controller exclude untethered operation but enable generate high forces, for example 90 mN to 500 mN on a small pitch of 1.5 mm, see references [8], [9]. Controlling many actuators requires complex tubing and large number of pneumatic valves. To reduce complexity, devices have been developed based on shape memory polymers, see references [9,10] requiring a single pressure source while each taxel is addressed electrically by local heating. The control system is therefore simplified but the response time around 1s not suitable for VR applications, see references [9,10].

For wearables, direct electrical actuation rather than remote hydraulic actuation is preferable. Dielectric elastomer actuators (DEA) use an electric field to deform elastomers. They can be processed into arrays of addressable taxels, see reference [11] and soft wearable interfaces, see reference [12]. Although simple mm-scale devices generate forces too low for robust haptics (<50 mN), it is possible to increase force output using hydraulic amplification (120 mN, see reference [11]), rolled structures (450 mN, see reference [13]) or hybrid systems using a heat activated shape memory polymer as dielectric (600 mN latched force, see reference [14]).

Hydraulically amplified self-healing electrostatic actuator (HASEL) devices are a type of dielectric actuator using liquid dielectric instead of elastomers, leading to built-in hydraulic amplification, see reference [15]. The HASEL concept has been adapted to a structure (peano-HASEL) made with inextensible shells that do not require stretchable electrodes and allows using a wider range of dielectric material, potentially with high permittivity such as PVDF, see references [16] and [17]. Their concept is based on a zipping electrode that linearly contracts a fluidic cavity as a function of voltage. Peano-HASEL devices have strains of 25%, specific energy (up to 1.12 J/kg) and power (up to 160 W/kg).

Despite all these advancements in the field of actuators that can be used for touch feedback, strongly improved solutions in the field of actuators is still desired, to provide for a more compact design, high force density, and fast response times.

SUMMARY

According to one aspect of the present invention, a dielectric actuator is provided. The dielectric actuator preferably includes an enclosure forming a cavity having a dielectric fluid therein, the enclosure having a base, a flexible membrane and a reversibly-stretchable membrane, and a pair of electrodes, one electrode of the pair of electrodes arranged at the flexible membrane, the other electrode arranged at the base, the pair of electrodes arranged to generate an electric field to an area of the cavity that is filled with the dielectric fluid, wherein when the electric field is generated between the pair of electrodes, an electrostatic pressure in the dielectric fluid causes the flexible membrane to move towards the base, reducing an initial volume of the area of the cavity, increases a pressure of the dielectric fluid inside the cavity, and causes the reversibly-stretchable membrane to bump out to an expanded state.

According to another aspect of the present invention, another dielectric actuator is provided. The dielectric actuator preferably further includes a second pair of electrodes, one electrode of the second pair of electrodes arranged at a different location of the flexible membrane as compared to the one electrode of the first pair, the other electrode arranged at the base, the second pair of electrodes arranged to generate an second electric field to a second area of the cavity that is filled with the dielectric fluid.

According to still another aspect of the present invention, a matrix of a plurality of dielectric actuators are provided. Preferably, the matrix of a plurality of dielectric actuators is mounted to a base that is formed by a flexible substrate, more preferably a flexible printed circuit board.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

