Actuator Design and Testing for Dynamic Applications
For researchers interested in the design and fabrication of actuators, performance assessment is a primary concern. How does the newest design stack up to the previous version, or one that’s commercially available? For many applications, applying or measuring a very small or precise amount of force is required, in which case you need an accurate and sensitive force controller/transducer. Several great examples of this are presented below, in which our 300C or 400A-series transducers were used to measure the aerodynamic lift of a flapping-wing micro-aerial vehicle, force generation from a miniature, self-contained electroosmotic actuator, and responsiveness of a rolling diaphragm actuator designed for MR-guided biopsy procedures.
Electrostatic flapping-wing actuator with improved lift force by the pivot-spar bracket design
One of the central challenges to make an efficient flapping-wing micro-aerial vehicles (FMAV) is to effectively emulate the rotational movements. Research has shown that a complex combination of movements at the right times at the right angles can significantly contribute to aerodynamic lift force.
In this paper, the authors introduce the use of “pivot-spar” brackets, which show considerable similarity to the the rotational patterns found in a honey bee. The design uses electrostatic actuated flapping wings, which are appealing in FMAVs due to their simple structure and high energy efficiency. In addition, to minimize the weight of their design, the pivot spar brackets were made of light-weight carbon fiber, using laser cutting methods for the fabrication process.
To test the force, their experimental setup used a pivot, balancing their FMAV prototype on one end an a counterweight on the other. On the side of the FMAV, they attached the pivot to an Aurora Scientific Force Transducer to measure the lift force. They report that the pivot-spar design was highly effective, generating 79.2 µN of lift force on average.
Figure 1. Experimental setup to measure lift force using the FMAV and counterweight on a pivot attached to the ASI force sensor
Compared to the team’s previous “oval-hole” brackets, this new design demonstrated remarkable improvement. The use of carbon fiber materials and laser-cutting methods reduced weight by a factor of 20, from 1000 mg to 50 g. In addition, the improved rotational movement produced by their new “pivot-spar” brackets generated on average 2.5 times greater aerodynamic lift force, from 32 µN to 79.2 µN. They conclude,
“Future work aims at enhancing the lift force/weight efficiency by improved designs and effective fabrication procedures for the purpose of demonstrating the takeoff of the integrated electrostatic FMAVs.”
Fabrication of a Miniature Paper-Based Electroosmotic Actuator
Soft actuators are appealing for their ability to deform. In the authors’ words,
“Soft actuators are able to stretch, twist, squeeze through tight spaces, and gently grip fragile objects. Fluidic actuators work by pumping fluid into an elastic chamber to change its shape and are generally capable of exerting a large amount of force over a distance.”
Complex motions that have been very difficult to achieve historically—shaking somebody’s hand, for example—have be realized recently with “soft robots” using fluidic actuators. In this paper, the authors describe the development, fabrication and testing of novel, miniature, self-contained and untethered soft actuators. The authors note that historically there have been few soft miniature hydraulic actuators because designing a leak-proof device is difficult at a cm-scale.
Fabrication involved layer-by-layer assembly, stacking fluid-filled paper within a thin, elastic silicone membrane, with two electrodes. Upon application of an electric field through the paper, electroosmotic flow within the pores of the paper cause the membrane to bulge outwards, pushing up a force distribution plate.
Figure 2. Schematic showing the adjacent and stacked configurations of their design: an electric field generated by the electrodes causes electroosmotic fluid flow through the pores in the paper to generate hydraulic force which causes the elastic membrane and force distribution plate upwards, measured by an Aurora Scientific 300C force transducer
Performance of their actuator was assessed based on 3 metrics: displacement, speed and force, with which they could calculate work, power and energy efficiency. They used the Aurora Scientific 300C Dual-Mode lever in isotonic mode to measure all three. To assess displacement and speed, they applied a constant force of 1 g using the force transducer, cycled the voltage on/off and measured the displacement of the plate over time. Inflation of the membrane was quick, rising up to 400 µm in 0.1 seconds at 600V. To assess force, they applied an increasing force to the distribution plate until inflation dropped to zero, which occurred around 30g. In terms of pressure, that equates to approximately 15 kPa across the total membrane area.
The authors note that their device design represents a marked improvement over previous designs based on soft lithography. In terms of real-world relevance, there’s a good reason why most living things are ‘soft’ rather than rigid – it allows for a great deal of flexibility and the ability to adapt to their environment. Soft robots, like the living things they are inspired by, offer many benefits over their rigid cousins, and applications are currently being explored in areas as diverse as medicine, manufacturing and even space exploration.
Long-stroke rolling diaphragm actuators for haptic display of forces in teleoperation
Telerobotics and remote surgery is a highly useful tool in modern medicine, allowing highly trained surgeons to lend their expertise to patients in a rural community with limited access to qualified physicians, for example, or even to patients inside an MRI machine.
In this paper, the authors present a novel design and manufacturing method for a rolling diaphragm actuator designed for teleoperation, specifically MR-guided biopsy procedures. MRI-guided biopsy presents a challenge, due of course to the limited access to the patient while MRI imaging is underway. While possible using a telerobotic system, a light touch is required in some cases, like to insert a needle. Another consideration is the range of motion – the authors note that
“skin-to-target distance varies widely between cases.”
To address these challenges, the authors present a design using two identical actuators (one for the input and one for the output), moved by a rolling diaphragm. Ripstop nylon was used to reduce the risk of rupture, while silicone was used to minimize hysteretic loss and to minimize elastomer thickness.
To assess the device’s performance, they conducted various tests. To assess friction, hysteresis and viscous drag, they measured resistance to motion with output in free air. They found that while friction remained very low, some viscous drag and hysteresis were observed. Next, they measured output/input force, using a 309C Dual-Mode lever to provide 1N of force for the input, with the output pressed against a soft spring (1.26M/mm) to simulate the stiffness of a human body. Up to a frequency of 25Hz, the input/output ratio was nearly constant.
Comparing their design’s performance to two other actuators with transparent force transmission, they noted their design’s superior combination of selective stiffness, long stroke, low hysteresis and highly accurate transmission of fingertip-level forces.