Large Internal Forces Research Articles

The bacteriophage straight phi29 portal motor can package DNA against a large internal force.

As part of the viral infection cycle, viruses must package their newly replicated genomes for delivery to other host cells. Bacteriophage straight phi29 packages its 6.6-microm long, double-stranded DNA into a 42 x 54 nm capsid by means of a portal complex that hydrolyses ATP. This process is remarkable because entropic, electrostatic and bending energies of the DNA must be overcome to package the DNA to near-crystalline density. Here we use optical tweezers to pull on single DNA molecules as they are packaged, thus demonstrating that the portal complex is a force-generating motor. This motor can work against loads of up to 57 pN on average, making it one of the strongest molecular motors reported to date. Movements of over 5 microm are observed, indicating high processivity. Pauses and slips also occur, particularly at higher forces. We establish the force-velocity relationship of the motor and find that the rate-limiting step of the motor's cycle is force dependent even at low loads. Notably, the packaging rate decreases as the prohead is filled, indicating that an internal force builds up to approximately 50 pN owing to DNA confinement. Our data suggest that this force may be available for initiating the ejection of the DNA from the capsid during infection.

Payload maximization for open chained manipulators: finding weightlifting motions for a Puma 762 robot

Although the dynamic equations of motion of open-chained robot systems are well known, they are seldom taken into account during the planning of motions. In this work, we show that the dynamics of a robot can be used to produce motions that extend the payload capability beyond the limit set by traditional methods. In particular, we develop a point-to-point weightlifting motion planner for open-chained robots. The governing optimal control problem is converted into a direct, SQP parameter optimization in which the gradient is determined analytically. The joint trajectories are defined by B-spline polynomials along with a time-scale factor. The algorithm is applied to a Puma 762 robot, with its physical limitations incorporated into the formulation. The torque limits are formulated as soft constraints added into the objective function while the position and velocity limits are formulated as hard, linear inequality constraints, on the parameters. The solutions obtained with our algorithm extend the robot's payload capability while reducing the joint torques. Interestingly, nearly all the trajectories found pass through singular configurations, where large internal forces from the robot are applied to the payload and little torque is needed from the motors.

Super Bar Theory and BP System in Super Span Flat Space Frames

In this paper, possible solutions to the constraints of flat space frames in large spans are summarized, including the application of multilayer forms, the introduction of novel simplexes, and the improvement in structural components. Then the base-reinforced pyramid (BP) system, which is formed by reinforcing the base of a rectangular pyramid by crossing bars, is introduced as a successful attempt to reduce maximum bar forces in double-layer space frames without densifying grids. Properties of the BP system are reviewed. Super bar theory is introduced to cope with the buckling and large internal forces of long bars. Therefore, large grids can be realized in super span flat space frames with lightweight and large clear space. Under this concept, either a reciprocal prism or, generally, its derivatives, which are composed of much shorter bars, replace long bars. Factors affecting the efficiency of super bars are analyzed in detail, and their joint design is also recommended. Details of designing super bars are clarified. The efficiency of a 200-m-span flat double-layer BP grid employing super bars as long components is proven by case studies. Finally, a new concept with reduced cross-section is developed for a super bar system to achieve a further reduction in weights.

A theoretical study of super-span latticed shells

In this paper, solutions to the constraints of space structures in large span are summarized firstly. Super span double-layer and triple-layer cylindrical shells based on conventional design are studied and compared. Studies show that it might be insufficient to exceed force limit and thus to increase structure's span by adopting multi-layer forms in latticed shells. Then, super bar system, as a novel concept in space structures, is introduced to cope with bar-buckling and large internal forces of long bars. Under this concept, long bars are replaced by a reciprocal prism or rather its improved derivatures. Its joint design is also recommended. The efficiency of 500 m span cylindrical shell employing super bar concept is also presented.

PD-Type Control Schemes for Cooperative Manipulator Systems

ABSTRACTKinematic control of cooperative manipulator systems is based on the solution of an inverse kinematics problem providing the single-arm reference joint trajectories corresponding to the assigned cooperation task. If an independent joint control scheme is used to realize the desired joint trajectories, tracking error components causing violation of closed-chain constraints lead to building of large internal forces. A simple PD-type control law is presented in this article which is based on kineto-static filtering of the joint error components causing internal forces. The control law retains the simplicity of independent joint control schemes and achieves null internal force at steady state. The addition of internal-force feedback is also analyzed. A case study for a planar cooperative system is developed which shows the effectiveness of the proposed kineto-static filtering and makes a comparison with classical PD control.

Äußere Stabilisatoren im Fußbereich

The stability in the lateral direction is particularly important in indoor sports as well as tennis, where the frequency of injury at the lateral aspect of the ankle is considerably high (according to various sources between 20-30%). The goal of the present work was to discuss the reasons of instability at the ankle and the effect of external stabilizers. In a number of investigations the stability was measured via film analysis during sports activities in barefoot and shoe conditions. It was concluded that stability and blocking of a movement cannot be regarded as equal. The latter restricts necessary movements of the foot at the ankle and may also provoke large internal forces elsewhere. Negative in respect to stability are stiff and thick shoe soles, positive are: the torsional ability of the sole, softness of the shoe sole edge, the link between the shaft and the sole, shaft height, taping and bracing.

Evolution of the power ("squeeze") grip and its morphological correlates in hominids.

The "squeeze" form of power grip is investigated for the purposes of clarifying the hand posture and activities associated with the grip, assessing the potential in chimpanzees for using the grip, and identifying morphological correlates of an effective power grip that may be recognized in fossil hominid species. Our approaches include: (1) the analysis of the human grip, focusing on both the hand posture involved and hand movements associated with use of the grip in hammering; (2) the analysis of similar chimpanzee grips and associated movements; (3) comparative functional analysis of regions in the hand exploited and stressed by the grip and its associated movements in humans; and (4) a review of the literature on the power grip and its morphological correlates. Results of the study indicate that humans use a squeeze form of power grip effectively to wield cylindrical tools forcefully as extensions of the forearm. Several morphological features occur in high frequency among humans which facilitate the grip and are consistent with the large internal and external forces associated with it in hammering and in other tool-using activities. Chimpanzee hand postures resembling this form of human power grip are not fully comparable and lack some of these morphological correlates that facilitate its use. The hand of Australopithecus afarensis does not appear to have been stressed by use of the grip, but there is some evidence for this type of stress in the metacarpals from Sterkfontein Member 4. Hands from Olduvai and Swartkrans do not provide sufficient evidence for assessment of power grip capabilities.

Parametric Investigation of Cable‐Stayed Bridges

A parametric study on three‐span cable‐stayed bridges with a radiating system of cables is presented. It is intended to investigate the effect of the stiffnessses of cables, pylons, and girders, as well as the effect of the central unsupported length of the main span, on the behavior of the various elements of the bridge superstructure. The study provides a better understanding of the bridge performance involving the cable forces, the girder and pylon displacements, as well as the pylon moments and axial forces due to live load. This contributes to the design of a real bridge, a laboratory model, or even a numerical example. It is shown that there is an optimum value for the central unsupported length of the main span, depending on the number of stay cables in the main span. The study shows also that the reduction in the bending stiffness of the girder results in excessive girder and pylon deflections, as well as large internal forces in the pylons and cables.