The human arm has six degrees of freedom in operational space and most industrial robots also have the same degrees of freedom. However general robotic systems that are used in industry just imitate only the appearance of the human. The human arm shows seven joint space freedoms and has 29 muscles to operate the joints. This is the point that the human is wholly different from the robotic systems. It is called kinematic redundancy that the joints of the system are larger than the operational space freedoms. The similar concept, redundant actuation refers to the use of more actuators than the kinematic degrees of freedom of a system. Human arm shows several unique functions that general robotic systems cannot generate. The representative examples of them are obstacle avoidance by the kinematic redundancy and active stiffness modulation via redundant actuation. In this thesis, redundant actuation is studied and the focus of research concentrates into the utilization of actuation redundancy.
Redundantly actuated system shows several characteristics that cannot be found in non-redundant system. First of all, redundant actuation generally is incorporated with closed-chain mechanism rather than serial-chain mechanism. Utilization of redundant actuation can eliminate the singularities in the closed-chain structure. A redundantly actuated system has a larger load handling capacity as well as improved operational velocity and acceleration, compared to those of a non-redundant system. Moreover, redundant actuation admits fault tolerance capability when some of the actuators fail, and thus, a system having redundant actuators can be beneficially used in specific environments such as space or nuclear power plants where high performance and reliability are vital. Furthermore, redundant actuation allows the system to employ several load distribution schemes for enhancement of its performance or beneficial internal load generation in the development of several additional subtasks. The redundantly actuated system can produce proper internal load by distributing the loads of the actuating input. Then, spring like effect can be produced by antagonistic internal load with conjunction with complex nonlinear geometry of the mechanism. The antagonistic stiffness, which is called by the name of active stiffness, can be modulated arbitrarily by controlling actuation inputs without feedback control. The active stiffness is distinguished from conventional feedback stiffness control by its feedforward control scheme.
The load distribution of the actuation inputs is treated in this thesis. The realization of active stiffness modulation and the optimal load distribution algorithm to reject impulsive disturbance are studied in detail. Active stiffness modulation is applied to five-bar mechanism with redundant actuation. In the first place, the kinematics and dynamics of five-bar mechanism are evaluated and performance analysis is carried out based on the kinematic and dynamic model. The four operational performances, which are load handling capacity, operational velocity, operational acceleration and Jacobian isotropy, are compared by varying the number of actuators and the positions of actuating joints. The number of actuators is set to four and the optimal positions of the actuating joints are determined from the analysis results. Moreover, kinematic optimization is performed to maximize the efficiency of active stiffness modulation. To do this, new index is developed and used for optimization. The index, named by active stiffness index, is defined as isotropy in modulation of active stiffness. The optimized five-bar mechanism with four actuators is designed and fabricated based on the results of the performance analysis and the kinematic optimization. The experiment on the modulation of active stiffness is carried out. It is verified that the wanted stiffness is successfully achieved by comparing the frequency of the end-effector’s free motion with the given frequency. The application of the active stiffness modulation is expected to be a programmable spring such as an active remote compliance center system and a force-reflecting controller for remote control or virtual reality. It also can be applicable to a programmable dynamic modulator for an exact experiment or a test bed.
The study on the application of redundant actuation to the impact phenomenon is followed. Before the study on the impulsive disturbance rejection, the dynamics of the colliding multi-body system is investigated carefully. When a robot system collides with environment, not only the contact point but also the joints of the robot system experience impulsive forces or moments. However, analysis on the so-called internal impulses acting on the joints has not attracted much attention although excessive impact may cause damage to the joint. A closed-form internal impact model applicable to general classes of robotic mechanisms is proposed. The characteristics of the internal impulse are analyzed through simulation for serial-chain, closed-chain, and hybrid-chain systems. The proposed internal impact model is verified by the experiment on the five-bar mechanism. It is shown that the external impulse is distributed to the internal impulses and the configurations best for external impulses might not be best for internal impulses. It is also found that the internal impulse has strong inter-relationship with momentum change of linkage systems. Thus, in order to minimize the internal impulse the motion planning of the given linkage system becomes crucial. It is expected that the model and analysis about the internal impulse will be useful in the analysis and design of human body motion or in the health-care area of sport science.
Unexpected impact may cause very large position errors. Large actuation effort will grow up to compensate for these errors. The sudden increases of actuator torque can violate the torque limit. The torque limit violation may lead the control performance unsatisfactory. Utilization of redundant actuation makes it possible that the position errors caused by impact decrease and the speed of error convergence becomes fast. An optimal load distribution algorithm for redundantly actuated system is proposed to reduce the effect of shocks. This scheme distributes the compensating effort to the system actuators in consideration of the torque limit. The disturbance rejection capability of the proposed scheme is compared with those of other two load distribution methods by simulation and experiment. The solution reflecting torque limit shows the best performance of disturbance rejection due to external impact. From the results, we can conclude that application of the redundant actuation and the load distribution algorithm provides several merits when the system is exposed to impact and the impact is crucial to the system`s performance.
