A Novel Spider-Inspired Rotary-Rolling Diaphragm Actuator with Linear Torque Characteristic and High Mechanical Efficiency

Abstract

We present a novel, fluid-driven rotary-rolling diaphragm actuator with direct rotary output. Its working principle is inspired by the spider leg's hydraulically operated joints and the diaphragm design of rolling diaphragm actuators. The new actuator is fully sealed, shows minimal output torque losses, and minimum friction during operation. Stiction and Coulomb friction are avoided by design. Our proposed mechanism can be used as a compliant actuator in soft robots, or as a stiff transmission device, depending on the fluid and working pressure. The rotary-rolling diaphragm is the defining component of the actuator. The diaphragm is based on silicone rubber, reinforced by a fabric with anisotropic tensile strength characteristics. The diaphragm is custom-designed to follow the actuator's toroidal shape and to ensure the smooth unrolling behavior throughout the stroke. Our actuator outputs a constant torque throughout its stroke compared with monolithic, rotary soft robot actuators with a change in torque. Our design offers a high mechanical efficiency of 95%, compactness, a wide working range of 100°, and a low mechanical complexity from a single chamber.

Keywords: MRI compatible; actuator; diaphragm; efficient; fluidic; rolling; rotary; spider-inspired.

FIG. 1.

Design methodology leading to the rotary-rolling diaphragm actuator requirements. (a) Conventional fluidic actuators. They can achieve high force efficiency. The O-ring as the seal between the cylinder and the piston leads to Coulomb friction and stiction. (b) Rolling diaphragm actuators avoid Coulomb and stick friction by a diaphragm rolling between walls. They output telescopic movement. (c) Vane actuators directly output torque but require two seals with high mounting and manufacturing precision, and a complex shape. (d) We propose a rotary-rolling diaphragm mechanism that uses the rolling diaphragm concept, but for direct torque output. The actuator can be manufactured with the mere help of a 3D printer and manual tools. It is important to note that the pairing of the pinion/rack and direct linkage system with the O-ring or rolling diaphragm cylinder is arbitrary and could be switched. In this case, the points “nonlinear output” and “mechanically complex” would switch. 3D, three-dimensional. Color images are available online.

FIG. 2.

Simplified view (a) of a common house spider leg (genus Tegenaria), with a close look at the membrane of the tibia–metatarsus joint (b, b1–b3). Several arc-shaped (c), stacked membrane segments unfold successively at joint opening. (1) Pressurized fluid inflow, (2) coxa and trochanter, (3) femur, (4) patella, (5) tibia, (6) metatarsus, (7) tarsus. (d) A linear fluidic actuator based on a rolling diaphragm. In our work, we combine the idea of a rolling diaphragm with the stacked membrane segments of the spider's joint, into (e) a rotary-rolling diaphragm actuator. (8) Housing, (9) rolling diaphragm, (10) piston, (11) and (12) membrane segments, (13) housing, (14) rotating piston. (f) Microscope images of the tibia–metatarsus joint, with its schematic presentation in (b). Color images are available online.

FIG. 4.

Uniaxial stress–strain characteristics of the fabric core and the resulting diaphragm material. Embedded is a microscope image of the diaphragm. Stress is shown in [N/mm] instead of [Pa]. The thickness of the diaphragm (0.5 mm) is largely determined by its fabric core thickness (0.35 mm). The rotary-rolling diaphragm here is stressed to maximum values barely visible in this plot, around 0.11 N/mm, indicated by the small rectangle at the plot's origin. Hence, by replacing the 3D printed plastic with a stiffer material, the actuator's maximum working pressure can be increased. Color images are available online.

FIG. 3.

Diaphragm fabrication process. The diaphragm's core is sewn together from seven fabric patches and coated with silicone rubber. Seams run in parallel to the rolling direction. (a) The patch is shaped by dividing the piston's surface into seven segments, unrolling the segments into 2D, and adding margins for sewing and clamping. (b) Patches are cut from a microfiber cloth with a laser cutter. (c) Patches are sewn together with a sewing machine. (d) The diaphragm core fabric is placed over a positive form for coating, defining its final shape, and then manually soaked with liquid silicone. Excessive rubber is brushed off, and the upper rim (mold included) is cast immediately. The rim will clamp down the diaphragm in the actuator. (e) The cured and coated diaphragm is placed into a second mold to cast the lower rim. (f) Assembly: the diaphragm is clamped to the piston and then between the two shell halves. Color images are available online.

FIG. 5.

(a–d) Opening sequence, cut view of the rotary-rolling diaphragm actuator. (1) Shell (gray), (2) piston (green), (3) diaphragm (red), (4) bridge (green), (5) axis, mounted outside the shell, (6) torque is applied to the moving leg segment (green). The diaphragm is shown in red, the pressurized fluid volume in blue. When the piston moves, the outer diaphragm portion covers the largest distance and thus unrolls rapidly. Here, a smooth and low-friction behavior is essential. Shell and piston are split into two parts connected by screws clamping the diaphragm. Shell and piston are 3D-printed from ABS; the base of the shell is closed by a transparent polymethylmethacrylate plate. The blue hose is the supply hose for pressurized air and connects to a valve. The smaller transparent hose connects to the pressure sensor. ABS, acrylonitrile butadiene styrene. Color images are available online.

FIG. 6.

(a) Side view and (b) Top view of the test-stand. The test-stand is composed of two arms connected to the piston and the shell, respectively. The shell connects to the left arm, fixed to the ground via a load cell to measure the torque. A position sensor records the opening angle. A pressure sensor measures chamber pressure and is connected by a second, short hose. The pressure source and regulator are wall-mounted and not shown here. Color images are available online.

FIG. 7.

Isometric experiments: both actuator arms are immobile, and joint torque is measured. The actuator's output torque is shown, for chamber pressures from 0 to 50 kPa and joint angles from 0° to 100°. (a) Eleven curves are overlaid, each smoothed with a fourth-order polynomial fit. The maximum isometric actuator torque measured is 7.3 Nm. (b) For each joint angle position from 0° to 100°, the chamber pressure was slowly increased to 50 kPa and then slowly decreased again. Color images are available online.

FIG. 8.

Experimental results from work-loop experiments. (a) Torque and pressure curves for an average pressure of 10.25 kPa (target pressure of 10 kPa) and joint speed of 8°/s. The graph shows five consecutive measurements, merged. The upper curves represent values at joint closing, the lower at joint opening. The pressure at joint closing is higher than at opening since additional air has to flow out of the actuator. The torque shown is corrected for the difference caused by the deviation of the instantaneous pressure from average pressure [M_equiv, Eq. (5)] using the value established in the static experiments (Ar = 1.46 × 10 − 4 m³). The equivalent (“corrected”) torque at joint closing is higher than that at the opening, due to friction. The area enclosed by the two torque curves represents the energy lost due to friction. The bump at 30° is caused by a small spot of additional silicone rubber in that area of the diaphragm. The uncorrected torque curve shows larger fluctuations (a gray area), which follows the pressure fluctuations (blue area). Here, the instantaneous pressure is not identical between experiments. However, after correcting for pressure fluctuation, the torque curve's uncertainty is small, which confirms that pressure compensation works as intended and indicates that the actuator's torque–friction relationship is repeatable. (b) The mean friction and the ratio of friction to nominal torque dependent on the operating pressure. The error bars in Y-direction represent the 95% confidence interval of the mean value computed from five measurement cycles. The error bars in X-direction represent the mean deviation of the pressure from the mean pressure during the individual measurements. For low pressures up to 25 kPa, the friction slowly and almost linearly increases; for higher pressures, the increase is steeper. The maximum torque efficiency of 97% is at 30 kPa. (c) Work-loop experiments, at a joint speed of 8°/s and recalculated torque curves, for pressure levels between 1 and 50 kPa. The upper curve of a pair represents the value while closing the joint, the lower while opening. The effect of the piston bending outward at high pressures (Fig. 9) can be seen in the increase of torque difference, for high-pressure scenarios (i.e., 50 kPa), and opening angles above 40°. (d) Like (a), but at 100°/s and 10 kPa. Color images are available online.

FIG. 9.

Deformation of the bridge (9) caused by high chamber pressures (dark blue fluid) and large opening angles (c). (1) and (10) are stoppers. (a) The chamber pressure (2) applies force (blue arrows) to the piston area (4), measured by a force sensor at (1). The piston (4) and its bridge (9) are ABS printed. They are curved, and the force pushes the piston radially outward, deforming them elastically (c). While the piston is inserted in the shell (5), the gap's fluid pressure produces a force pushing the piston/bridge toward the center. The effective (net) force (6) depends on the chamber pressure (1), and the pressurized perimeter area (7), that is, the actuator's opening angle. At small actuator angles, the fluid force along the perimeter compensates for the unwanted piston deflection. At combinations of large actuator angle (b) and high chamber pressure (c), the piston's bridge deflects eventually outward, closing the outer rotary-rolling diaphragm gap. Even before both diaphragm surfaces contact each other, friction increases: the diaphragm unrolls with a less than optimal gap size. Color images are available online.