Xiangrong Shen

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

Simultaneous Force and Stiffness Control of a Pneumatic Actuator

This paper proposes a new approach to the design of a robot actuator with physically variable stiffness. The proposed approach leverages the dynamic characteristics inherent in a pneumatic actuator, which behaves in essence as a series elastic actuator. By replacing the four-way servovalve used to control a typical pneumatic actuator with a pair of three-way valves, the stiffness of the series elastic component can be modulated independently of the actuator output force. Based on this notion, the authors propose a control approach for the simultaneous control of actuator output force and stiffness. Since the achievable output force and stiffness are coupled and configuration-dependent, the authors also present a control law that provides either maximum or minimum actuator output stiffness for a given displacement and desired force output. The general control and maximum/minimum stiffness approaches are experimentally demonstrated and shown to provide high fidelity control of force and stiffness, and additionally shown to provide a factor of 6 dynamic range in stiffness.

A Gas-Actuated Anthropomorphic Prosthesis for Transhumeral Amputees

This paper presents the design of a gas-actuated anthropomorphic arm prosthesis with 21 degrees of freedom and nine independent actuators. The prosthesis utilizes the monopropellant hydrogen peroxide as a gas generator to power nine pneumatic type actuators. Of the nine independent actuators, one provides direct- drive actuation of the elbow, three provide direct-drive actuation for the wrist, and the remaining five actuate an underactuated 17 degree of freedom hand. This paper describes the design of the prosthesis, including the design of small-scale high-performance servovalves, which enable the implementation of the monopropellant concept in a transhumeral prosthesis. Experimental results are given characterizing both the servovalve performance and the force and/or motion control of various joints under closed-loop control.

Energy Saving in Pneumatic Servo Control Utilizing Interchamber Cross-Flow

This paper proposes a structure and control approach for the energy saving servo control of a pneumatic servo system. The energy saving approach is enabled by supplementing a standard four-way spool valve controlled pneumatic actuator with an additional two-way valve that enables flow between the cylinder chambers. The “crossflow” valve enables recirculation of pressurized air, and thus enables the extraction of stored energy that would otherwise be exhausted to the atmosphere. A control approach is formulated that supplements, to the extent possible, the mass flow required by a sliding mode controller with the recirculated mass flow provided by the crossflow valve. Following the control formulation, experimental results are presented that indicate energy savings of 25‐52%, with essentially no compromise in tracking performance relative to the standard sliding mode control approach (i.e., relative to control via a standard four-way spool valve, without the supplemental flow provided by the crossflow valve). DOI: 10.1115/1.2718244

On the enhanced passivity of pneumatically actuated impedance-type haptic interfaces

The stable simulation of high-stiffness surfaces remains a challenge in impedance-type haptic simulations of mechanical environments. In this paper, the authors propose an approach to achieving a stable, high-stiffness surface in a haptic interface by leveraging the open-loop properties of pneumatic actuators. By using the open-loop component of the actuator stiffness as a primary component of stiffness simulation in a haptic interface, the system requires a comparatively small component of simulated stiffness from the closed-loop control of the actuator. A passivity analysis is presented describing how the presence of an open-loop stiffness enhances the range of passivity of haptically simulated high-stiffness surfaces. Experimental results both with and without a human operator are presented that demonstrate the effectiveness of the approach and its enhanced passivity relative to motor-actuated devices.

Independent Stiffness and Force Control of Pneumatic Actuators for Contact Stability during Robot Manipulation

This paper proposes a control approach that controls the stiffness and force of pneumatic actuator independently. This independent control of stiffness and force removes a primary cause of contact instability when utilizing stiffness or impedance based force control during interaction with stiff environments. Specifically, in typical stiffness or impedance control, since the force is defined in terms of motion variables, the desired force term appears as high gain position feedback when a manipulator is in contact with a stiff environment. In the proposed approach, since stiffness and force are controlled independently, the force appears as an exogenous input rather than as high gain position feedback, and therefore this source of instability is removed. The paper describes the control approach for independent stiffness and force control, which is based on an MIMO sliding mode controller design. Experimental results are presented that demonstrate the effectiveness of the control approach.

A forearm actuation unit for an upper extremity prosthesis

This paper presents the design of a 14 degree-of-motion forearm actuation unit for an upper extremity prosthesis. The forearm utilizes pneumatic type actuators which use the reaction products of a monopropellant gas generator as a working fluid. The use of pneumatic type actuators provides a near-human power density, such that the fourteen actuator forearm unit can deliver approximately one half of the force and power capability of a human arm, with a total package size that fits within the volumetric constraints of a 50th percentile female forearm (excluding the cartridge of liquid propellant). This paper describes the design of the forearm. The design has been fabricated, and experimental results are presented that demonstrate the closed-loop force tracking capability. An accompanying video further demonstrates the forearm performance.

Progress Towards the Development of a Highly Functional Anthropomorphic Transhumeral Prosthesis

This paper presents progress towards the development of a gas-actuated anthropomorphic arm prosthesis with 21 degrees of freedom and nine independent actuators. The system is designed to utilize the monopropellant hydrogen peroxide as a gas generator in order to power the nine pneumatic-type actuators. The design incorporates four actuators to provide direct-drive actuation of the elbow joint and three wrist degrees-of-freedom, while the remaining five actuate an underactuated 17 degree-of-freedom hand. This paper describes the prosthesis design, including the design of small-scale high-performance servovalves that enable implementation of the monopropellant concept in a transhumeral prosthesis. Video frame sequences of the prosthesis under closed loop control demonstrate its functionality in performing tasks representative of activities of daily living.

Leveraging the Open-Loop Stiffness of Pneumatic Actuators to Enable High-Stiffness Surface Simulation in Impedance-Type Haptic Interfaces

The stable simulation of high-stiffness surfaces remains a challenge in impedance-type haptic simulation of mechanical environments. In this paper, the authors propose an approach to achieving a stable, high-stiffness surface in a haptic interface by leveraging the open-loop properties of pneumatic actuators. By utilizing the open-loop component of actuator stiffness as the primary component of stiffness simulation in a haptic interface, the system requires a comparatively small component of simulated stiffness from closed-loop control of the actuator. A stability analysis is presented indicating that this approach enhances significantly the range of passivity of haptically simulated high-stiffness surfaces. Experimental results both with and without a human operator are presented that demonstrate the effectiveness of the approach.Copyright