H. Harry Asada

Kinematic analysis of workpart fixturing for flexible assembly with automatically reconfigurable fixtures

The basic concept of an adaptable fixturing system and its hardware implementation are described. The system employs reconfigurable fixture elements that are used to locate and hold various workparts for assembly. The fixture configuration can be changed automatically depending upon the workpart geometry and the assembly operations required. Analytic tools are developed for designing fixture layouts. Kinematic modeling, analysis, and characterization of workpart fixturing are presented. The condition for a fixture layout to locate a given workpart uniquely at a desired location is derived. Desirable fixture layout characteristics are obtained for loading and unloading the workpart successfully despite errors in workpart manipulation. The fixturing of a plastic cover of an electrical appliance with complex shape is used as an example to verify the analytic results and for demonstrating the concept.

Mobile monitoring with wearable photoplethysmographic biosensors

We address both technical and clinical issues of wearable biosensors (WBS). First, design concepts of a WBS are presented, with emphasis on the ring sensor developed by the authors group at MIT. The ring sensor is an ambulatory, telemetric, continuous health-monitoring device. This WBS combines miniaturized data acquisition features with advanced photoplethysmographic (PPG) techniques to acquire data related to the patients cardiovascular state using a method that is far superior to existing fingertip PPG sensors. In particular, the ring sensor is capable of reliably monitoring a patients heart rate, oxygen saturation, and heart rate variability. Technical issues, including motion artifact, interference with blood circulation, and battery power issues, are addressed, and effective engineering solutions to alleviate these problems are presented. Second, based on the ring sensor technology the clinical potentials of WBS monitoring are addressed.

A Geometrical Representation of Manipulator Dynamics and Its Application to Arm Design

A new approach to the geometrical representation of manipulator dynamics is presented. The inertia ellipsoid, which is used to represent dynamic characteristics of a single rigid body, is extended to a general ellipsoid for a series of rigid bodies in order to represent the manipulator dynamics. The geometrical configuration of the generalized inertia ellipsoid (GIE) represents the characteristics of the manipulator as a whole. One can understand the complicated inertial effect and nonlinearity of multi-degree-of-freedom motion by simply investigating the GIE configuration. In the latter half of the paper, this method is applied to aid the design of a mechanical arm, in which dimensions of an arm structure and its mass distribution are optimized through the evaluation and graphical representation of the arm dynamics.

Analysis and Design of a Direct-Drive Arm With a Five-Bar-Link Parallel Drive Mechanism

The direct-drive arm that has no gears between motors and their loads have several important advantages including no backlash, small friction, and high mechanical stiffness. The arm mechanism, however, becomes extremely massive, when each motor is directly attached to its joint along a serial linkage mechanism. The complicated dynamics resulting from varying inertia, interactions, and nonlinearities, is also more prominent than that of a robot with gears. This paper describes a lightweight arm mechanism with invariant and decoupled inertia characteristics. Instead of having motors at serial joints, a parallel drive mechanism with a closed-loop five bar linkage is utilized. The dynamic behavior of this mechanism is analyzed and the condition for the elimination of the interactions and nonlinearities in the mass properties is derived. The decoupled and invariant arm dynamics significantly reduces the complexity of controlling the direct-drive arm. In the latter half of the paper, a prototype robot developed on this basis is described. By using high torque brushless motors which were specially designed for the direct-drive robot, top speed and maximum acceleration were increased by an order-of-magnitude to about 10 m/s and 5 G, respectively.

Utility of the Photoplethysmogram in Circulatory Monitoring

The photoplethysmogram is a noninvasive circulatory signal related to the pulsatile volume of blood in tissue and is displayed by many pulse oximeters and bedside monitors, along with the computed arterial oxygen saturation. The photoplethysmogram is similar in appearance to an arterial blood pressure waveform. Because the former is noninvasive and nearly ubiquitous in hospitals whereas the latter requires invasive measurement, the extraction of circulatory information from the photoplethysmogram has been a popular subject of contemporary research. The photoplethysmogram is a function of the underlying circulation, but the relation is complicated by optical, biomechanical, and physiologic covariates that affect the appearance of the photoplethysmogram. Overall, the photoplethysmogram provides a wealth of circulatory information, but its complex etiology may be a limitation in some novel applications.

Multivariable Control of Vapor Compression Systems

This paper presents the results of a study of multi-input multi-output (MIMO) control of vapor compression cycles that have multiple actuators and sensors for regulating multiple outputs, e.g. superheat and evaporating temperature. The conventional single-input single-output (SISO) control was shown to have very limited performance. A low order lumped-parameter model was developed to describe the significant dynamics of vapor compression cycles. Dynamic modes were analyzed based on the low order model to provide physical insight of system dynamic behavior. To synthesize a MIMO control system, the Linear-Quadratic Gaussian (LQG) technique was applied to coordinate compressor speed and expansion valve opening with guaranteed stability robustness in the design. Furthermore, to control a vapor compression cycle over a wide range of operating conditions where system nonlinearities become evident, a gain scheduling scheme was used so that the MIMO controller could adapt to changing operating conditions. Both an...

Inter-finger coordination and postural synergies in robot hands via mechanical implementation of principal components analysis

Human hands employ characteristic patterns of actuation, or synergies, that contain much of the information required to describe an entire hand shape. In some cases, 80% or more of the total information can be described with only two scalar component values. Robotic hands, however, commonly only couple intra-finger joints, and rarely take advantage of this inter-finger coordination. In this paper, real-world data on a variety of human hand postures was collected using a data glove, and principal components analysis was used to calculate these synergies, resulting in what we call eigenpostures. A novel mechanism design is presented to combine the eigenpostures and drive a 17-degree-of-freedom 5-fingered robot hand. The hand uses only 2 DC motors to accurately recreate a wide range of hand shapes. We also present a design improvement that allows us to distinguish between high-precision and low-precision tasks, as well as greatly reduce overall error.

The ring sensor: a new ambulatory wearable sensor for twenty-four hour patient monitoring

This paper describes the development of a ring sensor for twenty-four hour patient monitoring. The ring is packed with LEDs and photodetectors where the technology of pulse oximetry is implemented for blood oxygen saturation monitoring. The measured data are transmitted to a computer through a digital wireless communication link. The ring sensor is worn by the patient at all times, hence the health status is monitored 24 hours a day. Detailed descriptions of the hardware and the software of the ring sensor will be presented. Also, the effects of motion artifact and ambient light will be investigated.

Modeling of Vapor Compression Cycles for Multivariable Feedback Control of HVAC Systems

This paper presents a new lumped-parameter model for describing the dynamics of vapor compression cycles. In particular, the dynamics associated with the two heat exchangers, i.e., the evaporator and the condenser, are modeled based on a moving-interface approach by which the position of the two-phase/single-phase interface inside the one-dimensional heat exchanger can be properly predicted. This interface information has never been included in previous lumped-parameter models developed for control design purpose, although it is essential in predicting the refrigerant superheat or subcool value. This model relates critical performance outputs, such as evaporating pressure, condensing pressure, and the refrigerant superheat, to actuating inputs including compressor speed, fan speed, and expansion valve opening. The dominating dynamic characteristics of the cycle around an operating point is studied based on the linearized model. From the resultant transfer function matrix, an interaction measure based on the Relative Gain Array reveals strong cross-couplings between various input-output pairs, and therefore indicates the inadequacy of independent SISO control techniques. In view of regulating multiple performance outputs in modern heat pumps and air-conditioning systems, this model is highly useful for design of multivariable feedback control.

Inverse Dynamics of Flexible Robot Arms: Modeling and Computation for Trajectory Control

The inverse dynamics of robot manipulators based on flexible arm models are considered. Actuator torques required for a flexible arm to track a given trajectory are formulated and computed by using special moving coordinate systems, called virtual rigid link coordinates. Dynamic deformations of the flexible arm can be represented in a simple and compact form with use of the virtual coordinate systems