Multilayer microhydraulic actuators with speed and force configurations

Abstract

Electrostatic motors have traditionally required high voltage and provided low torque, leaving them with a vanishingly small portion of the motor application space. The lack of robust electrostatic motors is of particular concern in microsystems because inductive motors do not scale well to small dimensions. Often, microsystem designers have to choose from a host of imperfect actuation solutions, leading to high voltage requirements or low efficiency and thus straining the power budget of the entire system. In this work, we describe a scalable three-dimensional actuator technology that is based on the stacking of thin microhydraulic layers. This technology offers an actuation solution at 50 volts, with high force, high efficiency, fine stepping precision, layering, low abrasion, and resistance to pull-in instability. Actuator layers can also be stacked in different configurations trading off speed for force, and the actuator improves quadratically in power density when its internal dimensions are scaled-down.

Keywords: Electrical and electronic engineering; Electronic properties and materials.

Fig. 1. A detailed view of a single microhydraulic rotational layer.

a A low magnification picture taken from the droplet side, showing the outside rails, inside rails, the droplet array, and the electrode array. b A magnified view of the droplet side with the drive droplets and outside rails. Features on the electrode side are also visible since polyimide and water are transparent. c Electrode side with the four-phase Al drive electrodes, and the Pt brush electrodes. d Confocal microscope height map of the droplet side, showing the surface curvature of the drive and rail droplets as well as the edge of the droplet wall. e A height map of the fluidic via from the electrode side. Liquid profiles on both sides have a radius curvature of ~18 µm, which corresponds to a Laplace pressure of 3.8 kPa or 31 cm of 8 M LiCl with the air/water surface tension. Curvature is similar to that caused by the 24 cm of 8 M LiCl used in the fabrication of the actuator with the oil/water surface tension. A cross-section diagram of a cycle section of the actuator is shown in f. Rotational microhydraulic layers were 10-µm thick and had a 6-mm outer diameter.

Fig. 2. A cross-sectional view (left) and a top-down micrograph (right), of a stacked five-layer microhydraulic rotational actuator.

The first layer sits in a base and does not move, subsequent layers move by having the drive droplets glide on the fluoropolymer coating of the drive electrodes in the layers below. The layer tag numbers for layers 1–5 are 2R, 3R, 4R, 5R, and T, respectively, from the bottom to the top. Layers 2–5R have inverted electrode order (indicated by the R); with the final layer T displaying the MIT Logo text. Dodecane oil surrounds the entire actuator and forms the insulating ambient fluid between layers.

Fig. 3. Images of five-layer stack actuation in the speed (top) and force (bottom) configurations, left column also shows the cycle cross-section profile for the corresponding configuration.

Absolute velocity vectors are indicated for each layer with an arrow next to the layer tab. For the speed configuration (with layer order 2R, 3R, 4R, 5R, and T), each subsequent layer moves with a fixed velocity (F_cycD_pitch) relative to the layer below it. As a result, the fifth layer (T) moves four times as fast as the second layer (3R). In the force configuration (with layer order 2R, 7, 3R, 8, T), each layer moves in the opposite direction relative to the one below it. Relative to the base, the odd layers (2R, 3R, T) remain stationary, while the even layers (7, 8) move at a uniform velocity (F_cycD_pitch). Movies of these actuations are available in the Supplementary Materials.

Fig. 4. An image of a five-layer linear microhydraulic actuator in the speed configuration.

In this example, the actuator translates 19 steps per layer from initial position, one step short of the brush electrode pitch of five cycles. A movie of this actuation is available in the Supplementary Materials.

Fig. 5. A diagram of two different stacking configurations.

The high force M = 1 configuration is shown in (a), and the M = 2 configuration is shown in (b). Absolute velocity vectors for each layer are shown with stacked arrows. For any value of M, the stacking order will alternate, with M forward going layers stacked on top of M backward going layers. In general, any M configuration will have M maximum velocity and 1/M force density of the M = 1 or force configuration.

Fig. 6. A plot of the maximum unloaded rotational velocity and blocked torque density for various rotational actuators.

Inductive motors tend to have a high speed at a low torque density, while microhydraulic motors and biological joints tend to have a low velocity and a high-torque density. Different M configurations can exchange speed for torque. The 40 µm droplet pitch devices are shown in this work, while the 15 µm droplet pitch devices are projected from scaling trends. Metrics for the Microdrive 303–102 motor were measured in our laboratory, Maxon motor metrics were taken from online datasheets. Elbow and ankle measurements were obtained from the first author using a load cell and a gyroscope and are typical of biological muscle performance. Normalization masses for the muscle torque calculations were the arm mass below the shoulder, and the leg mass below the knee.

Fig. 7. A diagram showing the electrical power distribution network for a linear multilayer microhydraulic actuator.

Alternating current flows from the base up through the fluidic rails and vias to the brushes, then to the drive electrodes. It then couples to the drive droplets in the layer above and returns through the fluidic rails and vias back to the base. Driving (Al) electrodes are shown in black, brush (Pt) electrodes are shown in gray, and liquid interconnect components (water 8 M LiCl, rails, droplets, and vias) are shown in blue. The inset shows the cross-section through the fluidic via and the brush electrode double layer. The table shows the measured network parameters with the resistance and capacitance normalized to a subunit of the actuator containing a single brush electrode.

Fig. 8. Images of the multilayer microhydraulic actuator at various stages of fabrication.

Top row shows the wafer-based processing for all major lithography steps: Al metal (a), metal via (b), Pt metal (c), droplet wall (d), outside and die etch (e), and hydrophilic patterning (f). Bottom row shows the major custom microhydraulics steps: the peel and laydown for the electrowetting fluoropolymer coating and the addition of release wax (g), droplet wetting and pressurization (h), layer release (i), actuator assembly (j), and the finished actuator in a testing dodecane bath (k). Peel and wetting videos are available in the Supplementary Materials.