Jonathan E. Luntz

Distributed Manipulation Using Discrete Actuator Arrays

Distributed manipulation systems induce motions on objects through the application of many external forces. An actuator array performs distributed manipulation using a planar array of many small stationary elements (which are called cells) that cooperate to manipulate larger objects. Typically, highly dense actuator arrays are modeled as spatially continuous, programmable forcefields, althoughinmany implementations a relatively small number of actuators supports an object and continuous assumptions break down. This paper serves two purposes: to present a methodology for modeling and analyzing the dynamics of manipulation on a highly discrete actuator array and to present a methodology for designing manipulation strategies on discrete actuator arrays. This is done in the context of a particular macro-scale actuator array comprising a fixed planar array of motorized wheels. Modeling of the dynamics takes into account several models of the interaction between the actuators and the object, the distribution of the weight of the object among the supports, and the discrete nature of the system. Under certain modeling assumptions, the manipulation dynamics of an object are extremely simple for a given set of supporting cells. An inversion of these piecewise-continuous dynamics generates a fully continuous open-loop manipulation strategy, effectively smoothing out the discontinuities. The authors show that although the resulting manipulation field may stably position and orient any object in the continuous field case, discreteness causes many objects to experience unstable rotational equilibria. Thus, poor orientation precision is a limitation of open-loop manipulation using discrete actuator arrays and motivates the use of feedback. The authors also derive closed-loop manipulation strategies through an inversion of the discrete dynamics that reduce the many-input, three-output distributed control problem to a standard three-input, three-output control problem that operates under distributed control. In effect, the array of actuators is reduced to a single virtual actuator capable of applying a desired net force and moment on an object. It is proven that even in the presence of dynamic coupling and nonlinearities introduced due to discreteness, these closed-loop strategies are asymptotically stable. Multimedia extensions include a complete simulator and videos of the experimental prototype.

Parcel manipulation and dynamics with a distributed actuator array: the virtual vehicle

We are developing a materials handling system where many small simple actuators cooperate to transport and to manipulate large objects in the plane. A discrete set of cells, each comprising two actuators, are fixed in a planar array. By coordinating the actuators in the cells on which an object rests, an object can be transported and manipulated. In essence, this system is an improvement over traditional conveyor systems in that objects can be re-oriented, as well as conveyed. Such an array provides flexible materials handling in which many objects independently can be manipulated and transported at the same time. The array is coordinated in a distributed manner where each cell has its own controller and each controller communicates with its neighbors. Towards the goal of motion planning, in this paper we consider the dynamics of parcel transport and manipulation. The parcel dynamics are based on an exact discrete representation of the system, unlike other methods where a continuity assumption is made. Two types of contact models are considered.

Distributed manipulation with passive air flow

Distributed manipulation systems induce motions on objects through the application of forces at many points of contact. Current forms of distributed manipulation include multiple mobile robots, vibrating plates, actively controlled arrays of air jets, and planar micro and macro-mechanical arrays of actuators. The paper presents a form of distributed manipulation using passive air flow fields, which has been experimentally demonstrated. Rather than directly actuating the flow at each point, potential flow assumptions allow a small number of simple point-generated fields to combine to form complex manipulation fields. The paper presents a methodology for efficiently computing equilibria of objects manipulated by arbitrary combinations of radially symmetric fields (such as potential flow fields).

A distributed control system for flexible materials handling

In this research, we are developing a materials handling system where many small, simple actuators cooperate to convey large objects with three degrees of freedom in a plane. The actuators, or cells, are arranged in a regular array which is fixed to a planar surface, and objects are passed over the array. Such an array provides very flexible materials handling in which many objects can be conveyed simultaneously in different directions. The array is coordinated in a distributed manner, rather than by a central controller. Each manipulator has its own controller, and each controller communicates with its neighbours. In this article, modeling and control methods, and a real-time communication network and language are developed. Simple tests on hardware and in simulation are presented.

Mechanical extension implants for short bowel syndrome

Short-bowel syndrome (SBS) is a rare, potentially lethal medical condition where the small intestine is far shorter than required for proper nutrient absorption. Current treatment, including nutritional, hormone-based, and surgical modification, have limited success resulting in 30% to 50% mortality rates. Recent advances in mechanotransduction, stressing the bowel to induce growth, show great promise; but for successful clinical use, more sophisticated devices that can be implanted are required. This paper presents two novel devices that are capable of the long-term gentle stressing. A prototype of each device was designed to fit inside a short section of bowel and slowly extend, allowing the bowel section to grow approximately double its initial length. The first device achieves this through a dual concentric hydraulic piston that generated almost 2-fold growth of a pig small intestine. For a fully implantable extender, a second device was developed based upon a shape memory alloy actuated linear ratchet. The proof-of-concept prototype demonstrated significant force generation and almost double extension when tested on the benchtop and inside an ex-vivo section of pig bowel. This work provides the first steps in the development of an implantable extender for treatment of SBS.

Closed-loop operation of actuator arrays

An actuator array performs distributed manipulation where an object being transported and manipulated rests on a large number of stationary supporting actuators. The authors have developed a macroscopic actuator array consisting of many motorized wheels. As opposed to a MEMS array, the analysis requires the explicit modeling of the discreteness in the system, including the set of supports, distribution of weight, and generation of traction forces. Using an open-loop wheel velocity field, discreteness causes undesirable behavior such as unstable rotational equilibria, suggesting the use of object feedback. Discrete distributed control algorithms are derived by inverting the dynamics of manipulation. These algorithms reduce the many-input-three-output control problem to a three-input-three-output control problem.

Generation of quadratic potential force fields from flow fields for distributed manipulation

Distributed manipulation systems induce motions on objects through the application of many external forces. Many of these systems are abstracted as planar programmable force fields. Quadratic potential fields form a class of such fields that lend themselves to analytical study and exhibit useful stability properties. This paper introduces a new methodology to build quadratic potential fields with simple devices using the naturally existing phenomena of airflow, which is an improvement to the traditional use of the complicated programmable actuator arrays. It also provides a basis for the exploitation, in distributed manipulation, of natural phenomena like airflow, which require rigorous analysis and display stability difficulties. A demonstration and verification of the theoretical results for the special case of the elliptic field with airflows is also presented.

Distributed Manipulation of Flat Objects With Two Airflow Sinks

Distributed manipulation systems induce motions on objects through the application of forces at many points of contact. Current forms of distributed manipulation include multiple mobile robots, vibrating plates, actively controlled arrays of air jets, and planar micro- and macro-mechanical arrays of actuators. The authors have presented a new form of distributed manipulation using passive airflow fields. This paper lays out infrastructure for manipulation algorithms using logarithmic potential fields applicable to passive airflow distributed manipulators. It uses a line-integral form of the lifted force equations, and provides a numerical approach to check the uniqueness of the robust pivot point for given objects in a logarithmic potential field. The numerical method is proved analytically to require a finite resolution to find all robust pivot points. It also proposes a squeeze-like sequential manipulation algorithm to bring an object with a unique robust pivot point to a unique final pose using airflow fields without sensors. The algorithm has been verified by experiments which are conducted for three different starting orientations, and end up with a unique final pose at the end of the manipulation sequence

Application of distractive forces to the small intestine: defining safe limits.

BACKGROUND Distraction enterogenesis is a novel method for increasing small bowel length by the application of linearly directed forces. However, the magnitude of distractive forces that human and animal small bowel can safely withstand is unknown. METHODS Acute ex vivo force-displacement curves for human (n = 5) and pig (n = 6) small intestine (with and without mesentery) were made by applying increasing amounts of distractive forces to bowel immersed in normal saline (39°C). Progressive load was applied until gross disruption of the tissue was detected, or the applied force reached 1000 gram-force (gf). Histology was used to detect evidence of load-induced damage. In vivo blood flow to pig bowel with distractive loads (30-200 gf) was measured by laser Doppler. RESULTS The relationship between the level of force and degree of displacement was linear. The presence of a mesentery increased stiffness of pig bowel, but did not affect human bowel. Gross tissue disruption in pig and human tissue was seen at forces between 235 and 295 gf, respectively. However, in grossly undamaged areas, histology was unchanged even after application of higher loads. With in vivo testing, mesenteric blood flow was present up to 200 gf; however, blood flow to the bowel wall was reduced to undetectable levels at loads exceeding 100 gf. CONCLUSIONS While whole bowel tissue may tolerate greater applied loads, blood flow to the bowel wall was compromised at loads over 100 gf, suggesting that any higher forces place the bowel at risk for ischemia. These measurements will help guide the clinical application of distraction enterogenesis.

Superposition Methods for Distributed Manipulation Using Quadratic Potential Force Fields

Planar quadratic potential fields are useful for distributed manipulation because they are readily analyzable and naturally produce predictable equilibria. This generally simplifies implementation, since feedback and control may not be necessary. Traditionally, to dynamically produce moving fields for complex manipulation tasks, these fields had to be realized by highly capable, but redundant, actuator arrays. This paper suggests a new method: using simpler devices to generate basic component fields, and superposing these fields to produce desired fields with similar degrees of freedom. This approach is particularly useful for naturally produced force fields which do not allow the dynamic moving and changing of a field, but allow for superposition, and thus, can be spatially combined to produce the desired net behaviors. A vector representation of these fields is developed and applied to two problems: how to place the fields in space to span the maximum possible configuration space; and how to generate an optimal solution to generate a desired field by a superposition of a fixed field arrangement with minimal effort from each field. We experimentally validated these methods using airflow fields based on phenomenological and time superposition. Finally, we use superposition to simplify trajectory following, by blending proportions of two fields over time and by superimposing a set of component fields, each responsible for compensating a particular dynamic term