Underactuation is a core challenge associated with controlling soft and continuum robots, which possess theoreti-cally infinite degrees of freedom, but few actuators. However,$m$actuators may still be used to control a dynamic soft robot in an m-dimensional output task space. In this paper we develop a task-space control approach for planar continuum robots that is robust to modeling error and requires very little sensor information. The controller is based on a highly underactuated discrete rod mechanics model in maximal coordinates and does not require conversion to a classical robot dynamics model form. This promotes straightforward control design, implementation and efficiency. We perform input-output feedback linearization on this model, apply sliding mode control to increase robustness, and formulate an observer to estimate the full state from sparse output measurements. Simulation results show exact task-space reference tracking behavior can be achieved even in the presence of significant modeling error, inaccurate initial conditions, and output-only sensing.

This paper reports the design, modeling, and control of an MR-compatible actuation unit comprising pneumatic stepper mechanisms. One helix-shaped bellows and one toroid-shaped bellows were designed to actuate in pure rotation and pure translation, respectively. The actuation unit is a two degree- of-freedom needle driver that translates and rotates the base of one tube of a steerable needle like a concentric tube robot. For safety, mechanical stops limit needle motion to maximum unplanned step sizes of 0.5 mm and 0.5 degrees. Additively manufactured by selective laser sintering, the flexible fluidic actuating (FFA) mechanism achieves two degree-of-freedom motion as a monolithic, compact, and hermetically-sealed device. A second novel contribution is sub-step control for precise translations and rotations less than full step increments; steady- state errors of 0.013 mm and 0.018 degrees were achieved. The linear FFA produced peak forces of 33 N and -26.5 N for needle insertion and retraction, respectively. The rotary FFA produced bidirectional peak torques of 68 N-mm. With the FFA's in full motion in a 3T scanner, no loss in signal-to-noise ratio of MR images observed.

The design of a free piston compressor (FPC) intended as a pneumatic power supply for pneumatically actuated autonomous robots is presented in this paper. The FPC is a proposed device that utilizes combustion to compress air into a high-pressure supply tank by using the kinetic energy of a free piston. The device is configured such that the transduction from thermal energy to stored energy, in the form of compressed gas, is efficient relative to other small-scale portable power supply systems. This efficiency is achieved by matching the dynamic load of the compressor to the ideal adiabatic expansion of the hot gas combustion products. The device proposed exploits this fact by first converting thermal energy into kinetic energy of the free piston, and then compressing air during a separate compression phase. The proposed technology is intended to provide a compact pneumatic power supply source appropriate for human-scale robots. The design and implementation of the FPC is shown, and preliminary experimental results are presented and discussed with regard to efficiency and energetic characteristics of the device. Most significantly, the device is shown to operate nearly adiabatically.

This paper presents a method for the impedance control of a pneumatic linear actuator for tasks involving contact interaction. The method presented takes advantage of the natural compliance of pneumatic actuators such that a load cell, typically used in impedance control, is not required. The central notion of the method is that by departing from a stiff actuation system, low-bandwidth acceleration measurements can be used in lieu of high-bandwidth force measurements. The control methodology presented contains an inner loop to control the pressure on two sides of a pneumatic cylinder, while an outer loop enforces an impedance relationship between external forces and motion and commands desired pressures to the inner loop. The inner loop enforces the natural compliance of the pneumatic actuator by controlling both the sum and difference of the pressures on both sides of the pneumatic actuator. This is accomplished by utilizing two three-way proportional spool valves instead of a four-way valve typically used in fluid power control. Experimental results are shown demonstrating the pressure tracking control of the inner loop. Experimental results are also shown that demonstrate the impedance tracking of the outer loop for free motion and the transition from free motion to contact.

This work describes the modeling and control of a proposed actuation system that is capable of pressurizing a chamber volume via the catalytic decomposition of a liquid monopropellant controlled by a binary on/off propellant valve. Two design configurations of the actuation system are presented and common portions of both are energetically modeled. The parameters of the resulting dynamic model are meaningful physical properties of either the propellant or the system. A model-based switching controller is then applied to the task of pressure tracking. Model validation and controller performance are shown experimentally.

This paper describes the design of a liquid-propellant-powered hot-gas actuator appropriate for human-scale power-autonomous robots. A prototype of the actuation system is described, and closed-loop tracking data is shown that demonstrates good motion control. Experiments to characterize the energetic performance of the actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than a battery-powered by undiluted propellant would offer an energetic figure of merit an order of magnitude greater than battery-powered DC motor actuated systems.

This paper presents a liquid-fuel powered pneumatic actuator appropriate for human-scale autonomous robotics. The motivation for this work is the development of a lightweight actuation system with system energy and power densities significantly greater than a DC motor and battery combination. For the scale of interest, the design tradeoffs between complexity of an energy conversion system and fuel specific energy density make many conventional approaches inappropriate. Conventional actuation, such as a battery powered DC motor system, does not possess adequate energy storage to perform significant amounts of mechanical work for significant periods of time autonomously. A system design comparison for a six-degree of freedom walking robot is presented which compares the expected performance of a monopropellant powered actuation system to a battery powered DC motor system. Results from this comparison demonstrate the viability of such an approach and indicate significant increases in energy and power densities. A single-degree of freedom manipulator was constructed to demonstrate this approach. Experimental results corroborate the expected energetic advantages.