Kailash Krishnaswamy

On using unstable electrohydraulic valves for control

High bandwidth, high flow rate electrohydraulic valves typically have two or more stages. Most multi-stage valves are expensive, require meticulously clean fluid, and introduce higher order dynamics. On the other hand, single-stage spool valves are cheaper and more reliable. However, a majority of them are not suitable for high bandwidth, high flow rate applications due to limitations of the electromechanical/solenoid spool-stroking actuators. In this paper we investigate the feasibility of reducing this limitation by exploiting the transient flow forces in the valve so as to achieve spool dynamics that are intrinsically open-loop unstable. While conventional valves are designed to be open-loop stable, the unstable valve design has to be stabilized via closed-loop feedback. Simulation case studies are conducted to examine the potential dynamic and energetic advantages that an unstable valve may offer. These studies indicate that unstable valves provide faster response than the stable counterparts when stroking forces are limited. Moreover, unstable valves tend to require less positive power and energy to operate.

Bond Graph Based Approach to Passive Teleoperation of a Hydraulic Backhoe

Human operated, hydraulic actuated machines are widely used in many high-power applications. Improving productivity, safety and task quality (e.g., haptic feedback in a teleoperated scenario) has been the focus of past research. For robotic systems that interact with the physical environments, passivity is a useful property for ensuring safety and interaction stability. While passivity is a well utilized concept in electromechanical robotic systems, investigation of electrohydraulic control systems that enforce this passivity property are rare. This paper proposes and experimentally demonstrates a teleoperation control algorithm that renders a hydraulic backhoe/force feedback joystick system as a two-port, coordinated, passive machine. By fully accounting for the fluid compressibility, inertia dynamics and nonlinearity, coordination performance is much improved over a previous scheme in which the coordination control approximates the hydraulic system by its kinematic behavior. This is accomplished by a novel bond graph based three step design methodology: (1) energetically invariant transformation of the system into a pair of “shape” and “locked” subsystems; (2) inversion of the shape system bond graph to derive the coordination control law; (3) use of the locked system bond graph to derive an appropriate control law to achieve a target locked system dynamics while ensuring the passivity property of the coordinated system. The proposed passive control law has been experimentally verified for its bilateral energy transfer ability and performance enhancements.

Analysis of aircraft pitch axis stability augmentation system using sum of squares optimization

In this paper, we use SOS (sum of squares) programming approaches to analyze the stability and robustness properties of the controlled pitch axis (6 state system) of a nonlinear model of an aircraft. The controller is a LTI dynamic inversion based control law designed for the short period dynamics of the aircraft. The closed loop system is tested for its robustness to uncertainty in the location of center of gravity along the body x-axis. Results in the form of stability regions about a trim point are computed and verified using simulations.

PASSIVE BILATERAL TELEOPERATION OF A HYDRAULIC ACTUATOR USING AN ELECTROHYDRAULIC PASSIVE VALVE

Abstract A passive control scheme for the bilateral teleoperation of a electrohydraulic actuator and a motorized joystick is proposed. The overall system enables a human operating a motorized joystick to feel as if he is manipulating a rigid mechanical tool with which the work environment is also in contact. By ensuring that the closed loop system behaves like a passive two port device, safety and stability when coupled to other systems are improved. The control scheme is developed by first using previously developed active feedback to passify a four way proportional directional control valve, and then by the design of an intrinsically passive teleoperation controller. The coordination error between the joystick and the hydraulic actuator converges to zero for sufficiently low manipulation bandwidth. Experimental results verify the characteristics of the control scheme.

Inversion Based Multibody Control - Launch Vehicle with Fuel Slosh

ics under consideration were fully actuated. The dynamics we consider in this paper, are that of a critically underactuated system. The dynamic inversion-based nonlinear control algorithm presented in this paper will account for the coupled dynamics of interconnected rigid bodies. Specifically, the controller regulates engine angle of a vehicle by operating on the engine gimbal torque in the presence of fuel slosh. The controller uses an estimate of the slosh dynamics provided by a reduced-order observer. We compare the performance / stability improvements offered by high-fidelity controllers (those designed for a multi-body vehicle including slosh) over low-fidelity controllers (those designed for a multi-body vehicle but without regard for slosh dynamics). We use the proposed design procedure to simulate a controller that can actively damp slosh using additional thrusts.

Passive Teleoperation of a Multi Degree of Freedom Hydraulic Backhoe Using Dynamic Passive Valve

Passive systems possess the beneficial properties of ease of human control, and stable interaction with arbitrary passive systems. Motivated by these properties, our research has been directed towards developing passive hydraulic systems. A novel hydraulic element called the apssive valve was first developed. The passive valve was used in single degree of freedm passive bilateral teleoperation of a hydraulic actuator. The teleoperation control ensured a) passive 2-port behavior of the teleoperator and b) bilateral between the human and the work environment with a power scaling vactor. Although passivity, coordination between the master and slave systems was ensured only when the teleoperator (and hence the passive valve) was manipulated at sufficiently low frequency. In this paper, we extend the previously proposed teleoperation control to multi-degree of freedom control of a hydraulic backhoe, and rectifies the previously imposed bandwidth limitation by accounting for the dynamics of the passive valve. The passivitiy property of the hydraulic backhoe ensures interaction stability with any human opperator and the work environment that could be modelled passive. The intrinically passive control ensures two-port, rigid coordinated behavior of the passive valve driven teleoperated backhoe.Copyright

Sensor fusion for GNSS denied navigation

We present technologies that are being developed to address the need for a navigation solution in the absence of Global Navigation Satellite Systems (GNSS) measurements. The navigation system uses sensors such as vision systems, RADARS and LIDARS with feature extraction, matching and motion estimation algorithms. We present experimental results of using scale invariant feature transform, speeded up robust features, and modified Harris feature extraction algorithms and compare the performance. We also present methods to extract lines and planes that can aid in navigation. For motion estimation we present results for visual odometry as well as simultaneous localization and mapping navigation. We experimentally verify the algorithms in both a realtime Linux framework as well as offline. We also present ongoing work in vision integrated navigation in an attitude and heading reference system as well as an extended Kalman filter framework. All the methods we present in this paper are incremental navigation methods.

Structural vibration control for broadband noise attenuation in enclosures

This paper develops and evaluates several strategies for structural vibration control with the objective of attenuating broadband noise inside a rectangular enclosure. The strategies evaluated include model-independent collocated control, model-based feedback control and a new “modal-estimate” feedback strategy. Collocated control requires no knowledge of model parameters and enjoys the advantage of robustness. However, effective broadband noise attenuation with colocated control requires a large number of sensor-actuator pairs. Model-based controllers, on the other hand, can be theoretically effective even with the use of a single actuator. However, they suffer from a lack of robustness and are unsuitable from a practical point of view for broadband structural vibration applications where the dynamic models are of large order and poorly known. A new control strategy is developed based on attenuating a few structural vibration modes that have the best coupling with the enclosure acoustics. Broadband attenuation of these important modes can be achieved using a single actuator, a limited number of accelerometers and limited knowledge of a few modal functions. Simulation results are presented to demonstrate the effectiveness of the developed strategy.

Bondgraph Based Approach to Passive Teleoperation of a Hydraulic Backhoe

Human operated, hydraulic actuated machines are widely used in many high-power applications. Improving productivity, safety and task quality (eg. force feedback to the operator in a teleoperated scenario) has been the focus of past research. In addressing these issues, our research proposes and experimentally demonstrates a control technique that renders a hydraulic machine (teleoperated backhoe in this case) as a two-port, co-ordinated, passive machine. The passive teleoperated backhoe is driven by a human operator at one-port and interacts with the environment at the other. It guarantees interaction stability and safety with the human / work environment as the latter are usually passive. In previous work, a passive teleoperation algorithm was proposed for multi degree of freedom teleoperation of a hydraulic backhoe approximated by its kinematic behavior. The approximation led to severe performance deterioration under certain operating conditions. In this paper, a bondgraph based passive teleoperation architecture is proposed for the non-linear dynamic modeled backhoe. Passive control is designed in two stages. In the first stage, bondgraph based system inversion ideas are used to determine a coordination control law. In the second stage, a desired locked system (desired dynamics of the coordinated teleoperator) is defined and an appropriate control law is determined to ensure the passivity property of the locked teleoperator. The proposed passive control law is experimentally verified for its bilateral energy transfer ability and performance enhancements.Copyright

Passification of an electrohydraulic two-stage pressure control servo-valve

Passive systems have the beneficial property that their interconnections with other passive systems result in necessarily stable systems. This virtue of passive systems can be exploited with hydraulic systems to build powerful machines that are safe and human friendly. A difficulty in applying the passivity concept to electrohydraulic systems is that they are not intrinsically passive. In a previous paper, passification techniques were developed to render single-stage directional control valves as passive two-port devices with a supply rate related to the power input at the hydraulic port. In this paper, we extend the passification technique to two stage pressure control servo-valves. The passified two-stage valve avoids the flow rate and bandwidth limitations that exist for single-stage valves.