Electrostatic Latch Design Notes
In building an ensemble - or meta-module - of claytronic devices, latch design demands accurate and fast engagement, facile release and firm, strong adhesion so that the modules can execute thousands of changes in location within the ensemble at computational speeds.
A starburst latch design addresses these performance criteria. Its geometry enables reliable coupling of modules and creates a strong electrostatic force to bind modules in motion. By its close alignment, the latch also creates a more compacted space between modules and thus creates structural stability within an ensemble.
This geometry also supports a robust electronic system, which centers upon a parallel plate capacitor using flexible electrodes made of aluminum foil (20 nm thick) and dielectric Mylar film (6 µm thick).
As shown in the picture and schematic to the right, the capacitor design places 1/2 of a complete plate on each face. Thus, when two faces come together, a complete capacitor forms a full latch. This closing of a latch also opens circuits for communication and the exchange of power between modules.
In earlier designs, flexible foil electrodes, which bend to fit the contour of each mate, relied upon a normal electrostatic force. Formed across the space between the electrodes (in a direction perpendicular to the foil sheets), this initial design created a face-to-face adhesion across the foil sheets of the latch. In operation, it proved to be weak where the sheets did not come close enough to create a strong bond. When external forces pulled against the sheets, the latches separated and peeled apart from these weaker points.
Capturing Friction to Strengthen Latching Force
Besides the normal electrostatic force that binds in a direction perpendicular to the electrodes, the capacitor also harnesses a shear force in a direction that is parallel to the contact of the faces. Thus, the mechanical reinforcement from the fitting of the two faces of the capacitor captures friction from the electrostatic field. This shear force might be compared to the way that adhesive tape holds fast when pulled along its length against a surface to which it adheres. As with the tape pulled on the horizontal plane, the shearing force makes the electrostatic latch more resistant to the peeling that occured in designs that laid the aluminum faces of the latch atop each other without mechanical support. In those earlier designs, the separation pf the foil faces was comparable to the ease of lifting tape from a surface by pulling -- or peeling -- it from above.
The design of the "genderless" faces - neither male nor female - to hold the electrodes improves upon earlier designs for mating such devices. The star-shaped plastic frames in which the electrodes sit incorporate degrees of angle - a 45 degree blade at the top of each comb - to support passive self-alignment as the devices engage and to support ease of retraction during the disengagement - with a five-degree release angle along the vertical faces.
The placement of electrodes in the faces can be seen in the photograph to the left. The use of light, flexible aluminum film for the electrostatic latch rather than heavier mechanical devices or magnets also incorporates an important economy with respect to other latch designs. These latches weigh little with respect to the rest of the robot.
The use of multiple electrodes to construct the faces, each containing a half capacitor, also permits alternative ways to manage voltage and activate the latch. Use of multiple electrodes confers an important advantage. While some electrodes operate as electrostatic latches, others create a circuit for power transfer or data transfer between modules.
As a binding force, the effectiveness of tens of Newtons over hundreds of cm2 depends upon the extremely close spacing (measured in microns) of the electrodes that form the plates of the parallel capacitor. To maintain the closest approach of the two plates, flexible electrodes offer a way to achieve mechanical compliance between the adjacent plates, enabling closer spacing and larger electrostatic force. Rigid electrodes do not achieve that result. Rough surfaces and dirt on rigid electrodes too often interfere with the required closeness of the plates. Mechanical flexibility in the material of the plates allows the electrodes to bend and more closely approximate the mating plate's contour when electrostatic force is applied.
The quest for a strong, lightweight, robust device that can be manufactured guides the design criteria for an electrostatic latch that can be used to connect robotic modules in claytronic ensembles. An ideal latch will provide structural stability, power distribution and a communication channel between adjacent robots. It will be self-aligning, genderless, and quick to engage and disengage with minimal force for insertion and removal. It will weigh little with respect to the rest of the robot and consume no power to maintain the latch connection.
While this MEMS system is built at macro-scale, testing of the device confirms that its design and performance will scale to a micro-scale module with similar characteristics.