Daniel Xu

Self-sensing dielectric elastomer actuators in closed-loop operation

Because of their large output strain, dielectric elastomer actuators (DEAs) have been proposed for tunable optics applications such as tunable gratings. However, the inherent viscoelastic drift of these actuators is an important drawback and closed-loop operation of DEAs is a prerequisite for any accurate real-world application. In this paper, we show how capacitive self-sensing can be used to drive a DEA in closed-loop without the need for any external sensor. The method has been demonstrated on a DEA tunable grating based on a VHB acrylic and silicone membrane. The results show that the widely used VHB presents a time-dependent drift between the capacitance of the electrodes and their strain. The silicone-based grating does not exhibit such a drift, and its strain can be stabilized by regulating the capacitance of the device to a constant value. We also report on an new fabrication method for thin deformable gratings based on replication on a water-soluble master and a 27% change in the grating period has been obtained on a VHB-based device.

Stretch not flex: programmable rubber keyboard

Stretchability is a property that brings versatility and design freedom to human interface devices. We present a soft, flexible and stretchable keyboard made from a dielectric elastomer sensor sheet. Using a multi-frequency capacitance sensing technique based on a transmission line model, we demonstrate how this keyboard can detect touch in two dimensions, programmable to increase the number of keys and into different layouts, all without adding any new wires, connections or modifying the hardware. The method is efficient and scalable for large sensing systems with multiple degrees of freedom.

Enabling large scale capacitive sensing for dielectric elastomers

Hand motion is one of our most expressive abilities. By measuring our interactions with everyday objects, we can create smarter artificial intelligence that can learn and adapt from our behaviours and patterns. One way to achieve this is to apply wearable dielectric elastomer strain sensors directly onto the hand. Applications such as this require fast, efficient and scalable sensing electronics. Most capacitive sensing methods use an analogue sensing signal and a backend processor to calculate capacitance. This not only reduces scalability and speed of feedback but also increases the complexity of the sensing circuitry. A capacitive sensing method that uses a DC sensing signal and continuous tracking of charge is presented. The method is simple and efficient, allowing large numbers of dielectric elastomer sensors to be measured simulatenously.

Artificial muscle actuators for a robotic fish

Biology is a source of inspiration for many functional aspects of engineered systems. Fish can provide guidance for the design of animal-like robots, which have soft elastic bodies that are a continuum of actuator, sensor, and information processor. Fish respond to minute pressure changes in water, generating thrust and gaining lift from obstacles in the current, altering the shape of body and fins and using sensory nerves in their muscles to control them. Dielectric Elastomer (DE) artificial muscles offer a mechanism for a fish muscle actuator. DE devices have already been shown to outperform natural muscle in terms of active stress, strain, and speed[1-3]. DEs also have multi-functional capabilities that include actuation, sensing, logic and even energy harvesting, all achievable through appropriate control of charge[4, 5]. But DE actuators must be designed so that they provide enough torque to drive the tail and develop useful forward thrust.

Scalable sensing electronics towards a motion capture suit

Being able to accurately record body motion allows complex movements to be characterised and studied. This is especially important in the film or sport coaching industry. Unfortunately, the human body has over 600 skeletal muscles, giving rise to multiple degrees of freedom. In order to accurately capture motion such as hand gestures, elbow or knee flexion and extension, vast numbers of sensors are required. Dielectric elastomer (DE) sensors are an emerging class of electroactive polymer (EAP) that is soft, lightweight and compliant. These characteristics are ideal for a motion capture suit. One challenge is to design sensing electronics that can simultaneously measure multiple sensors. This paper describes a scalable capacitive sensing device that can measure up to 8 different sensors with an update rate of 20Hz.

Localised strain sensing of dielectric elastomers in a stretchable soft-touch musical keyboard

We present a new sensing method that can measure the strain at different locations in a dielectric elastomer. The method uses multiple sensing frequencies to target different regions of the same dielectric elastomer to simultaneously detect position and pressure using only a single pair of connections. The dielectric elastomer is modelled as an RC transmission line and its internal voltage and current distribution used to determine localised capacitance changes resulting from contact and pressure. This sensing method greatly simplifies high degree of freedom systems and does not require any modifications to the dielectric elastomer or sensing hardware. It is demonstrated on a multi-touch musical keyboard made from a single low cost carbon-based dielectric elastomer with 4 distinct musical tones mapped along a length of 0.1m. Loudness was controlled by the amount of pressure applied to each of these 4 positions.

Tunable grating with active feedback

We report on the use of capacitive self-sensing to operate a DEA-based tunable grating in closed-loop mode. Due to their large strain capabilities, DEAs are key candidates for tunable optics applications. However, the viscoelasticity of elastomers is detrimental for applications that require long-term stability, such as tunable gratings and lenses. We show that capacitive sensing of the electrode strain can be used to suppress the strain drift and increase the response speed of silicone-based actuators. On the other hand, VHB actuators exhibit a time-dependent permittivity, which causes a drift between the device capacitance and its strain.

Non-verbal communication through sensor fusion

When we communicate face to face, we subconsciously engage our whole body to convey our message. In telecommunication, e.g. during phone calls, this powerful information channel cannot be used. Capturing nonverbal information from body motion and transmitting it to the receiver parallel to speech would make these conversations feel much more natural. This requires a sensing device that is capable of capturing different types of movements, such as the flexion and extension of joints, and the rotation of limbs. In a first embodiment, we developed a sensing glove that is used to control a computer game. Capacitive dielectric elastomer (DE) sensors measure finger positions, and an inertial measurement unit (IMU) detects hand roll. These two sensor technologies complement each other, with the IMU allowing the player to move an avatar through a three-dimensional maze, and the DE sensors detecting finger flexion to fire weapons or open doors. After demonstrating the potential of sensor fusion in human-computer interaction, we take this concept to the next level and apply it in nonverbal communication between humans. The current fingerspelling glove prototype uses capacitive DE sensors to detect finger gestures performed by the sending person. These gestures are mapped to corresponding messages and transmitted wirelessly to another person. A concept for integrating an IMU into this system is presented. The fusion of the DE sensor and the IMU combines the strengths of both sensor types, and therefore enables very comprehensive body motion sensing, which makes a large repertoire of gestures available to nonverbal communication over distances.