Perry Y. Li

Passive velocity field control of mechanical manipulators

Two concepts are advocated for the task specification and control of mechanical manipulators: 1) coding tasks in terms of velocity fields; 2) designing controllers so that the manipulator when under feedback control, interacts in an energetically passive manner with its physical environment. Based on these two concepts, a new passive velocity field controller is proposed which mimics the behavior of a passive energy storage element, such as a flywheel or a spring. It stores and releases energy while interacting with the manipulator, but does not generate any. The controller has the interesting property that it stabilizes any multiple (positive or negative) of the desired velocity field, and exponentially stabilizes the particular multiple of the desired velocity field which is determined by the total kinetic energy of the manipulator control system.

Passive bilateral control and tool dynamics rendering for nonlinear mechanical teleoperators

We propose a passive bilateral teleoperation control law for a pair of n-degree-of-freedom (DOF) nonlinear robotic systems. The control law ensures energetic passivity of the closed-loop teleoperator with power scaling, coordinates motions of the master and slave robots, and installs useful task-specific dynamics for inertia scaling, motion guidance, and obstacle avoidance. Consequently, the closed-loop teleoperator behaves like a common passive mechanical tool. A key innovation is the passive decomposition, which decomposes the 2n-DOF nonlinear teleoperator dynamics into two robot-like systems without violating passivity: an n-DOF shape system representing the master-slave position coordination aspect, and an n-DOF locked system representing the dynamics of the coordinated teleoperator. The master-slave position coordination is then achieved by regulating the shape system, while programmable apparent inertia of the coordinated teleoperator is achieved by scaling the inertia of the locked system. To achieve this perfect coordination and inertia scaling, the proposed control law measures and compensates for environment and human forcing. Passive velocity field control and artificial potential field control are used to implement guidance and obstacle avoidance for the coordinated teleoperator. The designed control is also implemented in an intrinsically passive negative semidefinite structure to ensure energetic passivity of the closed-loop teleoperator, even in the presence of parametric model uncertainties and inaccurate force sensing. Experiments are performed to validate the properties of the proposed control framework.

Passive bilateral feedforward control of linear dynamically similar teleoperated manipulators

Presents a passive bilateral feedforward control scheme for linear dynamically similar (LDS) teleoperated manipulators with kinematic scaling and power scaling. The proposed control law renders the teleoperator as a passive rigid mechanical tool with programmable apparent inertia to the human operator and the work environment by utilizing bilateral force feedforward and kinematic feedback control. The passivity of the closed-loop system is robust to force measurement inaccuracies and model uncertainty. Thus, interaction stability of the teleoperator with any passive environment is guaranteed. Coordination error and the overall motion aspects of teleoperation are controlled individually. The proposed control law is also applicable to general nonlinear robotic teleoperators if sufficiently high kinematic feedback gains are used. The proposed control schemes have been validated experimentally for both LDS and non-LDS systems.

AHS safe control laws for platoon leaders

The automated highway system (AHS) architecture of the California PATH program organizes traffic into platoons of closely spaced vehicles. A large relative motion between platoons can increase the risk of high relative velocity collisions. This is particularly true whenever platoons are formed or broken up by the join and split control maneuvers and by the decelerate to change lane control maneuver, which allows a platoon to create a gap before switching from one lane to another. In this paper we derive a safety region for the relative velocity between two platoons. By guaranteeing that the relative velocity between platoons remains in this region, impacts of high relative velocity can be avoided. Under normal operating conditions, there are four control laws for a platoon leader: leader law, join law, split law, and decelerate to change lane law. For each control law, a desired velocity profile for the platoon that satisfies safety and time-optimality requirements is derived. A nonlinear velocity controller is designed to track the desired velocity profile within a given error bound. When safety is not compromised, this controller keeps the acceleration and jerk of the vehicles in the platoon within comfort limits.

Traffic flow stability induced by constant time headway policy for adaptive cruise control vehicles

Abstract This paper is concerned with the traffic flow stability/instability induced by a particular adaptive cruise control (ACC) policy, known as the “constant time headway (CTH) policy”. The control policy is analyzed for a circular highway using three different traffic models, namely a microscopic model, a spatially discrete model, and a spatially continuous model. It is shown that these three different modeling paradigms can result in different traffic stability properties unless the control policy and traffic dynamics are consistently abstracted for the different paradigms. The traffic dynamics will, however, be qualitatively consistent across the three modeling paradigms if a consistent biasing strategy is used to adapt the CTH policy to the various modeling frameworks. The biasing strategy determines whether the feedback quantity for use in the control, is taken colocatedly, downstream or upstream to the vehicle/section/highway location. For ACC vehicles equipped with forward looking sensors, the downstream biasing strategy should be used. In this case, the CTH policy induces exponentially stable traffic flow on a circular highway in all three modeling frameworks. It is also shown that traffic flow stability will be preserved for an open stretch highway if the entry and exit conditions are made to observe the downstream biasing strategy.

Motion Planning and Control of a Swimming Machine

We propose a practical maneuvering control strategy for an aquatic vehicle (AV) that uses an oscillating foil as a propulsor. The challenge of this problem lies in the need to consider the hydrodynamic interaction as well as the underactuated and non-minimum phase natures of the AV system. The control task is decomposed into the off-line step of motion planning and the on-line step of feedback tracking. Optimal control techniques are used to compute a repertoire of time-scalable and concatenable motion primitives. The complete motion plan is obtained by concatenating time-scaled copies of the primitives. The computed optimal motion plans are regulated by a controller that consists of a cascade of linear quadratic regulator, input–output feedback linearization and sliding mode control. Time-varying linear quadratic controllers can also be time-scaled and concatenated. Therefore, they can be computed beforehand. The proposed strategy has been experimentally validated for both constrained longitudinal only maneuvers and unconstrained longitudinal/lateral maneuvers.

Control of smart exercise machines. I. Problem formulation and nonadaptive control

Concerns the design of intelligent controllers for a class of exercise machines. The control objective is to cause the user to exercise in a manner which optimizes a criterion related to the users mechanical power. The optimal exercise strategy is determined by a biomechanical behavior of the individual user, which is assumed to satisfy an affine force-velocity relationship dependent on the body geometric configuration. Consequently, the control scheme must simultaneously: 1) identify the users biomechanical behavior; 2) optimize the controller; and 3) stabilize the system to the estimated optimal states. Moreover, to ensure that the exercise machine is safe to operate, the control system guarantees that the interaction between the exercise machine and the user is passive. We formulate the control problem and propose a controller structure which satisfies the safety requirement and is capable of causing the user to execute an arbitrary exercise strategy if the users biomechanical behavior is known. The controller is of the form of a dynamic damper and can be implemented using only passive mechanical components.

Passive velocity field control (PVFC). Part II. Application to contour following

When the contour following task is represented by a velocity field on the configuration manifold of the system, the coordination aspect of the problem is made explicit. The PVFC scheme developed in the Part I (ibid. vol.29(9) (2001)) can then be applied to track the defined velocity field. However, for some contours, an encoding velocity held on the configuration manifold does not exist or is difficult to define and, as a consequence, the PVFC cannot be directly applied. For systems whose configuration manifolds are compact Lie groups and the desired contour is represented by a parameterized trajectory, a general methodology is developed, using a suspension technique, to define a velocity field on a manifold related to the configuration manifold of the system for which PVFC can be applied. With this strategy, timing along the contour can be naturally varied online by a self-pacing scheme so that the contour tracking performance can be improved. The experimental results for a 2-DOF robot following a Lissajous contour illustrates and verifies the convergence and robustness properties of the PVFC methodology.

Passive velocity field control (PVFC). Part I. Geometry and robustness

Passive velocity field control is a control methodology for fully actuated mechanical systems, in which the motion task is specified behaviorally in terms of a velocity field, and the closed-loop system is passive with respect to a supply rate given by the environment power input. The control law is derived geometrically and the geometric and robustness properties of the closed-loop system are analyzed. It is shown that the closed-loop unforced trajectories are geodesics of a closed-loop connection which is compatible with an inertia metric, and that the velocity of the system converges exponentially to a scaled multiple of the desired velocity field. The robustness property of the system exhibits some strong directional preference. In particular, disturbances that push in the direction of the desired momentum do not adversely affect the performance. Moreover, robustness property also improves with more energy in the system.

Using Steady Flow Force for Unstable Valve Design: Modeling and Experiments

In single stage electrohydraulic valves, solenoid actuators are usually used to stroke the main spools directly. They are cheaper and more reliable than multistage valves. Their use, however, is restricted to low bandwidth and low flow rate applications due to the limitation of the solenoid actuators. Our research focuses on alleviating the need for large and expensive solenoids in single stage valves by advantageously using fluid flow forces. For example, in a previous paper, we proposed to improve spool agility by inducing unstable transient flow forces by the use of negative damping lengths. In the present paper, how steady flow forces can be manipulated to improve spool agility is examined through fundamental momentum analysis, CFD analysis, and experimental studies. Particularly, it is found that two often ignored components-viscosity effect and non-metering momentum flux, have strong influence on steady flow forces. For positive damping lengths, viscosity increases the steady flow force, whereas for negative damping lengths, viscosity has the tendency to reduce steady flow forces. Also, by slightly modifying the non-metering port geometry, the non-metering flux can also be manipulated to reduce steady flow force. Therefore, both transient and steady flow forces can be used to improve the agility of single stage electrohydraulic valves. Experimental results confirm the contributions of both transient and steady flow force in improving spool agility.