20 Matching Annotations
  1. Apr 2020
    1. during contraction of the two-unit actuator was 614 W/kg; specific work during contraction was 70 J/kg

      Power and work of the actuators were measured for their ability to lift a hanging mass.

    2. However, the thick elastomer shells (>1 mm) used in this work required high voltages to reach electric fields large enough for actuation.

      HASEL actuators are made from inexpensive, readily available materials that only require basic fabrication techniques. However, because the elastomer shells are quite thick, they require high voltages to reach the electric fields required for actuation. As a result, authors hope to make new versions of the HASEL actuators with thinner elastomeric shells and using dielectric layers with higher permittivity, so that less voltage is required for actuation.

    3. The use of hydraulic principles in HASEL actuators results in the capability to scale actuation force and strain

      HASEL actuators combine the strengths of soft fluidic actuators and electrostatic actuators. They rely on soft matter hydraulic architectures and local hydraulic pressure generated by electrostatic forces. As a result, actuation force and strain can be scaled up.

    4. capacitive self-sensing has been used for closed-loop control of DE actuators (31).

      Capacitive self-sensing is an sophisticated approach to drive actuators in closed loop because it does not require any additional or external sensing elements. A simple proportional integral (PI) controller was used in this work to successfully demonstrate the application of capacitive self-sensing for closed-loop operations, and a tunable grating actuator was used to test the actuation scheme.

    5. To demonstrate self-sensing actuation, we powered a robotic arm with two planar HASEL actuators combined in parallel and simultaneously measured capacitance (Fig. 4, fig. S16, and movies S7 and S8).

      HASEL actuators are able to sense strain through capacitance, which is a direct relationship: when strain is low, capacitance is low; when strain is high, capacitance is high. In this experiment, two planar HASEL actuators are combined in parallel to simultaneously measure capacitance during various movements of a robotic arm. Capacitance is low when the arm is flexed, while capacitance is high when the arm is extended.

    6. analyzing the phase and amplitude of voltage and current signals (fig. S15)

      The actuation voltage signal and sensing voltage signal were combined in LabVIEW and output from a data acquisition system to a high voltage amplifier. The amplified voltage signal was then applied to the HASEL actuator. Current and voltage were monitored from the high voltage amplifier and input to the data acquisition system, allowing self-sensing of HASEL actuators.

    7. capacitance can be measured transiently by applying a low-amplitude AC voltage

      Previous work using an electromechanical oscillator consisting of three artificial muscles was used to measure capacitance by applying low-amplitude AC voltage. Dielectric Elastomer Actuators (DEAs) were used as flexible capacitors. The oscillator supported a set of rails on which a ball was placed. Upon actuation, the rails tilted, rolling the ball.

    8. Nonetheless, the ability of planar HASEL actuators to tolerate high electric fields applied over large areas enabled us to scale up actuation force by combining six planar HASEL actuators in parallel to lift a gallon of water (~4 kg, which corresponds to ~120 kPa) at 69% linear actuation strain (Fig. 3D and movie S6).

      The force of actuation can be significantly increased by combining multiple planar HASEL actuators. In this experiment, authors combined six planar actuators in parallel to test the work and strain. A gallon of water weighing 8.82 lbs was lifted, and a linear actuation strain of 69% was achieved.

    9. relative to donut HASEL actuators, gas bubbles were more easily trapped between the electrodes (fig. S14)

      While performing experiments with planar and donut HASEL actuators, authors observed that gas bubbles formed after dielectric breakdown occurred. Gas bubbles negatively impact the performance of the actuators. The bubbles reduce the dielectric breakdown voltages and lead to subsequent breakdowns at the same locations.

      Gas bubbles were found to be more easily trapped between electrodes in the planar HASEL actuators compared to the donut HASEL actuators, indicating that shape plays a role in the overall success of the actuator.

    10. Planar HASEL actuators were also able to self-heal from dielectric breakdown for at least 50 cycles

      In this experiment, the authors tested the ability of planar HASEL actuators to self-heal using a six-unit actuator. Voltage was steadily applied at 0.5 kV/s until dielectric breakdown occurred. After a minute, voltage was reapplied until dielectric breakdown occurred again. This cycle was repeated 50 times to evaluate the self-healing performance of the actuator.

    11. A single unit actuator was able to operate under a large applied load of 1.5 kg [corresponding to a stress of 0.3 MPa, near the maximum value for mammalian skeletal muscle (26)] and still achieved 16% strain (fig. S13)

      The operation of a planar HASEL actuator was tested under a large load and high stress to assess if it is comparable to mammalian skeletal muscle. The load and stress applied were near the maximum values of mammalian skeletal muscle, and a 16% strain was achieved. This result depicts that the single-unit actuator is very successful and shows promise to be used in a variety of applications because multiple actuators can be combined to achieve greater results.

    12. until mechanical rupture occurred at 158,061 cycles (fig. S12D)

      To determine the cycle life of the actuator, contractions were repeated until failure. 158,060 mechanical contractions were successfully completed before the actuator ripped, showing that the actuator is able to operate efficiently under large loads for an extended period of time.

    13. Cycle life at high mechanical output power was demonstrated with a single-unit HASEL actuator, which provided 358 W/kg average (586 W/kg peak) specific power during contraction

      Power of the single-unit actuator was measured in the same way as the two-unit actuator. Similar to the two-unit actuator, the single-unit demonstrated 586 W/kg of peak power and an average power of 358 W/kg while lifting a 1 kg mass. The actuator is able to operate efficiently under large loads for an extended period of time.

    14. Peak specific power

      The peak specific power is the maximum power that the power supply can sustain for a short time.

    15. a single-unit actuator achieved 107% linear strain under a load of 250 g (actuation stress ~32 kPa) and a two-unit actuator achieved 124% linear strain under a load of 700 g (actuation stress ~114 kPa)

      The planar HASEL actuators were more effective when they are operated near their resonant frequencies. A single actuator stretched 107% of its original length when a 250 gram weight is attached. A double-unit actuator stretched 124% of its original length when a 700 gram weight is attached.

    16. (C to G) A soft gripper fabricated from two modified stacks of donut HASEL actuators handled fragile objects such as a raspberry [(C) to (E)] and a raw egg [(F) and (G)].

      Because soft gripping is a common application for soft robotics, two stacks of donut HASEL actuators were modified to operate as a soft gripper. Actuators within the stacks were constrained on one side to produce a tilting motion. When a DC voltage was applied to the stacked HASEL actuators, the device grasped delicate objects, such as a raspberry and a raw egg, without causing any damage to the objects.

    17. (A) Schematic depicting a stack of five donut HASEL actuators oriented such that adjacent electrodes are at the same electrical potential (cross-section view).

      Because the HASEL actuators can self-heal, a greater actuation can be achieved by combining multiple actuators. Here, five donut HASEL actuators are stacked to achieve a larger actuation stroke.

  2. Mar 2020
    1. (B) Demonstration of linear actuation with stacked-donut HASEL actuators.

      The stack of five donut HASEL actuators achieved 37% linear strain, which is comparable to linear strain achieved by biological muscle and corresponds to a movement of 7 mm.

    2. resonant frequency

      Simply put, resonant frequency is the natural frequency at which it is easiest to get an object to vibrate. Most objects have several resonance frequencies, such as stringed musical instruments that vibrate at their resonance frequencies when plucked or struck, and their vibrations against the surrounding air produce sound. This type of resonance is found when an object is in equilibrium with acting forces and could keep vibrating for a long time under perfect conditions.

    3. linear actuation

      An actuator creating movement in a straight line, rather than a circular motion.<br> In this study, a fixed pre-stretch is applied in one direction (downward pull).