Wiping devices that exploit fingerlike bending motions produced by electroactive polymeric (ionomeric) actuators are undergoing development. These wiping devices function similarly to conventional windshield wipers. However, unlike conventional windshield wipers, these devices contain no motors, gears, or drive linkages; as a result, these devices are relatively simple, compact, and lightweight. Conceived for use in wiping dust off solar cells and windows of scientific instruments to be sent to explore Mars, these wiping devices might be useful for similar purposes on Earth.
A device of this type is denoted by the acronym"SWEP" (for surface wiper actuated by electroactive polymers). The only moving part in a SWEP is the wiper arm/actuator. This part is made from electroactive polymers; namely, (a) a membrane made of an ion-exchange polymer sandwiched between (b) surface polymeric layers that contain or are coated with platinum and that serve as electrodes. When a small electric potential (typically a few volts) is applied to the electrodes, the sandwich bends. Depending on the magnitude of the applied voltage and the dimensions of the arm, the angle of bending could exceed 180°. The direction of bending depends on the polarity of the applied potential (see figure). Thus, one could make a wiper go back and forth across a surface, in the manner of a conventional windshield wiper, by applying an alternating voltage.
Electroactive polymers (EAPs) exhibit several characteristics that lend themselves well to SWEPs:
- EAPs can be mass-produced at costs much lower than those of piezoelectric materials, in large part because unlike piezoelectric materials, EAPs need not be poled.
- EAPs can readily be formed to desired sizes and shapes.
- Physical characteristics of EAPs that are particularly well suited to actuation in SWEPs include high toughness, large electrostrictive strain, and inherent damping of vibrations.
Another advantage of SWEPs is low power consumption. For example, the prototype unit shown in the figure operates with a drive power of 20 to 30 mW. In a typical application, the frequency of the alternating driving voltage would be a fraction of a hertz; however, the frequency could be made higher if necessary, because the characteristic response time of a SWEP is of the order of milliseconds.
Like a conventional windshield wiper, a SWEP for a typical practical application would preferably be constructed as a wiper/actuator arm with a wiper blade or perhaps a brush attached. The shape, size, and material of the blade or brush could be chosen by design to minimize friction and ensure effectiveness in cleaning. As in the case of a conventional windshield wiper, the wiping should be done at the minimum frequency that provides effective cleaning, in order to minimize scratching.
This work was done by Yoseph Bar-Cohen and Tianji Xue of Caltech and Mohsen Shahinpoor of the University of New Mexico for NASA's Jet Propulsion Laboratory.