Eric J. Barth

Design and energetic characterization of a liquid-propellant-powered actuator for self-powered robots

This paper describes the design of a power supply and actuation system appropriate for position or force controlled human-scale robots. The proposed approach utilizes a liquid monopropellant to generate hot gas, which is utilized to power a pneumatic-type actuation system. A prototype of the actuation system is described, and closed-loop tracking data are shown, which demonstrate good motion control. Experiments to characterize the energetic performance of a six-degree-of-freedom actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than battery-powered DC motors. Projections based on these experiments indicate that the same system powered by undiluted propellant would offer an energetic figure of merit in an order of magnitude greater than a comparable battery-powered DC motor actuated system.

A Globally Stable, Load-Independent Pressure Observer for the Servo Control of Pneumatic Actuators

Pneumatic actuators are governed by nonlinear dynamics. Thus, robust precision motion control of pneumatic systems requires model-based control techniques such as sliding-mode and/or adaptive control. These controllers typically require full-state knowledge of the system, i.e., pressure, position, velocity, and acceleration. For measuring pressure states, pneumatic servo systems require two expensive pressure sensors per axis, and hence, it makes the system economically noncompetitive with most electromagnetic types of actuation. This paper presents the development of a Lyapunov-based pressure observer for pneumatically actuated systems. The pressure observer is energy-based and has the useful feature of not requiring a model for the load of the system, i.e., it is load-independent. This pressure observer is proven to be globally stable with the added feature of having a response bandwidth equal to that of the modeled pressure dynamics. A robust observer-based controller is developed to obtain a low-cost precision pneumatic servo system. Experimental results are presented that demonstrate and validate the effectiveness of the proposed observer.

Nonlinear Model-Based Control of Pulse Width Modulated Pneumatic Servo Systems

This paper presents a control methodology that enables nonlinear model-based control of pulse width modulated (PWM) pneumatic servo actuators. An averaging approach is developed to describe the equivalent continuous-time dynamics of a PWM controlled nonlinear system, which renders the system, originally discontinuous and possibly nonaffine in the input, into an equivalent system that is both continuous and affine in control input (i.e., transforms the system to nonlinear control canonical form). This approach is applied to a pneumatic actuator controlled by a pair of three-way solenoid actuated valves. The pneumatic actuation system is transformed into its averaged equivalent control canonical form, and a sliding mode controller is developed based on the resulting model. The controller is implemented on an experimental system, and the effectiveness of the proposed approach validated by experimental trajectory tracking. DOI: 10.1115/1.2232689

Sliding mode approach to PWM-controlled pneumatic systems

This paper presents a modeling and control design method for PWM-controlled pneumatic systems. Difficulties associated with servovalves typically used to continuously control pneumatic systems, in addition to a general lack,of analytic control design techniques available for MW controlled pneumatic systems, motivate the development, of a modeling and control design technique for PWM pneumatics. Specifically, the method presented here is to model a PWM-based pneumatic system using a state-space averaging approach. This provides the analytic machinery necessary to remove the discontinuities associated with switching and results in a model tractable to standard nonlinear control design techniques. Thereby, issues such as stability robustness and performance bandwidth may be addressed directly. The control of a single degree of freedom pneumatic positioning system illustrates this technique experimentally.

Impedance Control of a Pneumatic Actuator for Contact Tasks

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.

Dynamic Constraint-Based Energy-Saving Control of Pneumatic Servo Systems

This paper proposes a control approach that can provide significant energy savings for the control of pneumatic servo systems. The control methodology is formulated by decoupling the standard four-way spool valve used for pneumatic servo control into two three-way valves, then using the resulting two control degrees of freedom to simultaneously satisfy a performance constraint (which for this paper is based on the sliding mode sliding condition), and an energy-saving dynamic constraint that minimizes cylinder pressures. The control formulation is presented, followed by experimental results that indicate significant energy savings with essentially no compromise in tracking performance relative to control with a standard four-way spool valve.

Control Design for Relative Stability in a PWM-Controlled Pneumatic System

This paper presents a control design methodology that provides a prescribed degree of stability robustness for plants characterized by discontinuous (i.e., switching) dynamics. The proposed control methodology transforms a discontinuous switching model into a linear continuous equivalent model, so that loop-shaping methods may be utilized to provide a prescribed degree of stability robustness. The approach is specifically targeted at pneumatically actuated servo systems that are controlled by solenoid valves and do not incorporate pressure sensors. Experimental demonstration of the approach validates model equivalence and demonstrates good tracking performance.

Energy saving control for pneumatic servo systems

This paper proposes an energy saving approach to the control of pneumatic servo systems. The control methodology is presented followed by experimental results that indicate significant energetic savings and essentially no compromise in tracking performance relative to a purely active approach. Specifically, experiments indicate an energy savings of 10 to 46% (depending on the desired tracking frequency) relative to standard 4-way spool valve pneumatic servo actuator control. The savings are generally higher at lower tracking frequencies, since tracking of higher frequencies requires more control effort to minimize tracking error and therefore limits the degree of possible energy savings.

Control-based design of free-piston stirling engines

A control-based analysis and characterization of a free-piston Stirling engine is presented, and proposed as a lightweight power supply for untethered robots. Typically, such devices are designed from the point of view of a thermodynamic cycle in terms of traditional thermodynamic equations of state. Such equations of state are independent of time and therefore lend little insight when dynamic elements are incorporated into the design. The approach presented here is from a system dynamics and control perspective. Equations of state are replaced by dynamic system modeling elements. Utilizing these dynamic elements, control concepts are applied to evaluate a given configuration and ensure an unstable oscillatory response and therefore transform heat into useful work. A simulation of a commercially available free-piston engine is presented, and standard control design tools are applied to its linearized model. The results show promising potential in utilizing small-scale free-piston Stirling engines as portable power supply for robotic systems.

A unified force controller for a proportional-injector direct-injection monopropellant-powered actuator

This paper describes the modeling and control of a proportional-injector direct-injection monopropellant powered actuator for use in power-autonomous human-scale mobile robots. The development and use of proportional (as opposed to solenoid) injection valves enables a continuous and unified input/output description of the device, and therefore enables the development and implementation of a sliding-mode-type controller for the force control of the proposed actuator that provides the stability guarantees characteristic of a sliding mode control approach. Specifically, a three-input, singleoutput model of the actuation system behavior is developed, which takes a nonlinear noncontrol-canonical form. In order to implement a nonlinear controller, a constraint structure is developed that effectively renders the system single-input, single-output and control canonical, and thus of appropriate form for the implementation of a sliding mode controller. A sliding mode controller is then developed and experimentally implemented on the proposed actuator. Experimental results demonstrate closed loop force tracking with a saturation-limited bandwidth of approximately 6 Hz.