Electric rotary motors are typically built from components made of rigid, dense materials such as steel, copper, aluminum, and piezoceramics. These types of motors are found widely in products with moving parts, such as electric toothbrushes, automobile windscreen wipers, electric trains, computer cooling fans, and toys. Alternative motors fabricated from soft, low density materials would open up new design opportunities to produce motors that are simpler, cheaper, moldable, and flexible. In particular, such soft motors can be used in manipulator devices for micro-surgery and soft robotic devices. In principle, soft motors are actuated using dielectric elastomer (DE) artificial muscles, which are essentially soft polymer membrane dielectrics with stretchable electrodes on their free surfaces. After a voltage is applied, the charge accumulated on the electrodes produces electrostatic forces that deform the DE. Charges of opposite polarity draw the positive and negative electrodes together to compress the elastomer (see Figure 1). In contrast, charges of the same polarity expand the area of the electrode. When the charge is removed, the elastic energy of the elastomer returns it to its original shape. Here, we describe our progress toward DE soft muscle motors with an expanded range of motion.
Figure 1. Actuation process of dielectric elastomers (DEs). When a voltage is applied, charge accumulates on the free surfaces. Resultant electrostatic forces cause the DE to become thinner (vertical arrows) and stretch in-plane (horizontal arrows).
The first reported DE-based rotary motor mechanically converted vertical motion to rotary motion using a rocker arm that transmitted torque to a wheel through a one-way roller clutch.1 We have taken a different approach by radially arranging the DEs in a membrane around a shaft. Phasic actuation of separate segments of the membrane can turn the shaft using a crank or an orbiting gear mechanism.2 For the orbiting gear, stepping the muscle actuation around the membrane results in slow counter-rotation of the shaft (see Figure 2). For a geared system, this motor is fairly simple and is composed of only six parts. The shaft is supported on two roller bearings with a membrane stator, orbiting gear, and stator frame, and all parts but the bearings and shaft are fabricated from polymers. Unfortunately, such a geared motor can only be rotated, which limits the range of motion that can be achieved.
Figure 2. A schematic of a multi-phase DE rotary motor. The trapezoid areas (A-F) represent sectors with electrodes. The orbiter moves away from the active sector. Contact between the translating orbiter and shaft results in counter-rotation of the shaft.
By redesigning the motor to work without hard bearings, we have given the motor the ability to do more than just turn a shaft. Inspired by the dexterity of the human hand, we made the central gear deformable and used membrane muscles to control it.3 We did this by first stretching the polymeric membrane and then placing the gear onto it. With further development, it would be possible to mold the gear into the membrane as a single part using a moldable material such as silicone.
Our soft motor with deformable central gear manipulated a shaft in a number of different ways. For example, the shaft can be gripped—see Figure 3(a)—or rotated by sequential actuation of opposing sectors of the membrane, similar to moving, for example, a spoon between one's thumb and forefinger: see Figure 3(b). Additionally, the shaft can be shifted to the side: see Figure 3(c). We can also change the speed of the shaft—relative to the frequency of membrane actuation—by altering the circumference of the rubber gear electro-actively.4 Rigid bearings are no longer required because the soft motor can tilt, reposition, and turn the shaft (see Figure 4 and video5).
Figure 3. A schematic of the actuation modes for our soft motor. The trapezoid areas (A-F) represent sectors with electrodes. (a) Gripping through simultaneous actuation of all sectors, (b) turning the central rotor, and (c) repositioning the rotor sideways by differential actuation of sectors.
Figure 4. Our five-freedom soft gear motor is capable of tipping a shaft, repositioning, and rotating it without the need for additional mechanisms.
In summary, we have developed a bearing-less soft muscle motor with different actuation modes for improved range of movement. Dexterous operations for our soft gear motor will only be possible if the shaft position can be precisely controlled. To that end, our lab is currently developing techniques to simultaneously actuate a DE and sense its strain-dependent electrical parameters in real-time. We are also exploring how these parameters can be used for the motor. Such artificial muscle strain-sensing will open new vistas in mechanical engineering for soft machines.
Iain A. Anderson
Auckland Bioengineering Institute
University of Auckland
Auckland, New Zealand
Iain Anderson is group leader of the Biomimetics Laboratory and senior lecturer at the Department of Engineering Science, University of Auckland. More information about the biomimetics group can be found online at http://www.abi.auckland.ac.nz/biomimetics.
1. R. Kornbluh, R. Pelrine, J. Eckerle, J. Joseph, Electrostrictive polymer artificial muscle actuators, IEEE Int'l Conf. Rob. Autom
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2. I. A. Anderson, T. Hale, T. Gisby, B. O'Brien, T. Inamura, T. McKay, S. Walbran, E. Calius, A thin membrane artificial muscle rotary motor, Appl. Phys. Mater. Sci. Process
. 98, pp. 75-83, 2010. doi:10.1007/s00339-009-5434-5
3. I. A. Anderson, T. C. H. Tse, T. Inamura, B. M. O'Brien, T. McKay, T. Gisby, A soft and dexterous motor, Appl. Phys. Lett
. 98, pp. 123704, 2011. doi:10.1063/1.3565195
4. I. A. Anderson, T. C. H. Tse, T. Inamura, B. O'Brien, T. McKay, T. Gisby, Flexidrive: a soft artificial muscle motor, Proc. SPIE
7976, pp. 79761T, 2011. doi:10.1117/12.880714