Control Of Parallel Mechanism Research Articles

Construction method of parallel mechanisms with a partially constant Jacobian matrix

The Jacobian matrix plays an important role in the performance analysis, configuration design, planning and control of parallel mechanisms (PMs). PMs with a constant Jacobian have simple kinematics and are easy to analyse and control. However, since the criteria for detecting the invariance of a Jacobian are rather strict, the application of most rotational and mixed-motion PMs is limited. If most elements of the Jacobian remain constant, the analysis, design and trajectory planning will be simplified, and the control accuracy and efficiency will be improved. Moreover, the application of rotational and mixed-motion PMs with simple kinematics will be expanded. In this study, a method for structuring PMs with a partially constant Jacobian based on screw theory is proposed. The scenarios of realizing a partially constant Jacobian and the construction process are expounded. PMs with a partially constant Jacobian are constructed in two steps: limb transformation and limb combination. Five approaches that can be used to achieve a partially constant Jacobian are obtained. Then, the limb combination possibilities of translational, rotational and mixed-motion PMs are discussed. Some quintessential PMs are listed, and four PMs with different mobility property as used as examples to verify the characteristics.

Type Synthesis of 1T2R Parallel Mechanisms Using Structure Coupling-Reducing Method

Direct kinematics with analytic solutions is critical to the real-time control of parallel mechanisms. Therefore, the type synthesis of a mechanism having explicit form of forward kinematics has become a topic of interest. Based on this purpose, this paper deals with the type synthesis of 1T2R parallel mechanisms by investigating the topological structure coupling-reducing of the 3UPS&UP parallel mechanism. With the aid of the theory of mechanism topology, the analysis of the topological characteristics of the 3UPS&UP parallel mechanism is presented, which shows that there are highly coupled motions and constraints amongst the limbs of the mechanism. Three methods for structure coupling-reducing of the 3UPS&UP parallel mechanism are proposed, resulting in eight new types of 1T2R parallel mechanisms with one or zero coupling degree. One obtained parallel mechanism is taken as an example to demonstrate that a mechanism with zero coupling degree has an explicit form for forward kinematics. The process of type synthesis is in the order of permutation and combination; therefore, there are no omissions. This method is also applicable to other configurations, and novel topological structures having simple forward kinematics can be obtained from an original mechanism via this method.

Research on Fast Forward Position Solution of Parallel Mechanisms

It is difficult to obtain analytical solution to forward kinematics for parallel mechanism, and the speed of the solution based on conventional numerical solution method is lower, and hard to achieve real-time performance in motion control. In this paper, in order to avoid the two problems, a forward position solution method was proposed to approach the forward kinematics analytical solution exponentially. The Stewart platform was taken as example, its kinematics model was built, and then according a complex motion trajectory of moving platform given, the driving displacement of actuated joints was solved by inverse kinematics method, and then based on the driving displacement trajectory of actuated joints, using the forward position solution method, the forward kinematics solution was obtained, the simulation results showed that the forward position solution method can not only completely ensure real-time performance, but also attain very high accuracy, and so the forward position solution method for forward kinematics can be applied in motion control of parallel mechanisms.

Projection-Based Control of Parallel Mechanisms

The unifying idea for most model-based control approaches for parallel mechanism is to derive a minimal-order dynamics model of the system and then design the corresponding controller. The problem with such a control approach is that the controller needs to change its structure whenever the mechanical system changes its number of degrees-of-freedom. This paper presents a projection-based control scheme for parallel mechanism that works whether the system is overactuated or not; it does not require derivation of the minimal-order dynamics model. Since the dimension of the projection matrix is fixed, the projection-based controller does not need to change its structure whenever the mechanical system changes its number of degrees-of-freedom. The controller also allows to specify lower and upper bounds on the actuator forces/torques, making it suitable not only for the control of parallel manipulators with limited force/torque capability of the actuators but also for backlash-free control of parallel manipulators as well as for control of tendon driven parallel manipulators. The stability of the projection-based controllers is rigourously proved, while the condition for the controllability of parallel manipulators is also derived in detail. Finally, experimental results obtained from a simple parallel mechanism, which changes its degrees-of-freedom, are appended. The results also demonstrate that the maximum actuator torque can be reduced by 20% if the actuator saturation is taken into account by the controller.

Control of parallel mechanisms

There is a great deal of research on control of parallel mechanisms that considers their characteristics or how they may be made best use of. In this review, a dynamic control method by Uecker is introduced. As to force control, it explains effectiveness of parallel mechanisms and a hybrid control scheme by Nguyen. Merits of redundant actuation are discussed. Minimization of actuation torque by Ropponen, singularity avoidance by Kumar, direct compliance control by Yokoi and relation of internal force to stiffness by Hanafusa are introduced.

パラレルメカニズムの制御

There is a great deal of research on control of parallel mechanisms that considers their characteristics or how they may be made best use of. In this review, a dynamic control method by Uecker is introduced. As to force control, it explains effectiveness of parallel mechanisms and a hybrid control scheme by Nguyen. Merits of redundant actuation are discussed. Minimization of actuation torque by Ropponen, singularity avoidance by Kumar, direct compliance control by Yokoi and relation of internal force to stiffness by Hanafusa are introduced.