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Stiffness actuator with 6 degree of freedom


Hexa platform is a mechanism with a closed-loop kinematic structure. The beginning of parallel robots dates back to 1947 when the first concept of a manipulator with a closed kinematic chain appeared. This was the hexagonal Gough platform improperly called the Stewart platform, the prototype of which was constructed in 1955. However, in robotics prismatic drives has given way to rotary drives because of constructional requirements and final costs of lot production. So, in 1983 a concept of the delta architecture with 6 degrees of freedom and rotary drives appeared, the modification of which brought the well-known parallel structure Hexa into being. It is just this kinematic structure has been chosen to construct the active environment with 6 degrees of freedom. The principal consideration in designing the Hexa platform has been adhering to specifications set by the particular application. The most important of them are: small workspace (only a few centimeters for translational motions; a few dozens degrees for rotational motions); six-DOFs; high dynamics; high accuracy (not very critical for the application of force control) and low-cost.

Fig 1: Real system.


A quite limited workspace and lack of stringent requirements on accuracy and repeatability of positioning has made it possible to employ cheap Hitec-manufactured servos HSR-5995TG to drive the Hexa platform. This is a robot-dedicated solution, which is based on a high-dynamic coreless motor, a titanium gear and a precision potentiometer set true with the rotational axis of the servo. The servo provides a rotational speed of 60deg/0.12s, a driving torque of 30kgcm and a holding torque of 39kgcm. It should be stressed here that the electronics delivered by the manufacturer of the servo has been removed and replaced by a tailor-made solution (see Fig. 2).


The motion control hardware (Fig. 2) consists of the drives, its power electronics, and a microcontroller-based position and current supervision system. For rapid control prototyping purposes, a PC-based real-time environment with QNX running as real-time operating system and MiRPA (Middleware for Robotic and Process Control Applications) as communication layer is directly connected to the low-level control unit. To control the process two ADDI DATA cards have been employed.

Fig 2: Hexa control architecture.

MiRPA is a message driven middleware designed for meeting special demands in robotic and automation applications. With MiRPA it is possible to design very modular and open control systems. Here, we have used the MATLAB/Simulink environment to realize the investigated control structure and to observe the control results. All connections and communication steps to the robot control system are established and managed by MiRPA. The unique programming interface, the so called Manipulation Primitive, of the control system allows the logical integration of these additional modules into robot programs on the programming level. At the initial stage of tests the classic independent axes control based on PD controller was implemented. Utilizing the full motion potential offered by the platform calls for employing a feedforward control structure, which is based on the inverse process model. Tests carried out have shown that the maximum acceleration attainable by the platform is at the level of 3g.

Building the parallel platform (Fig. 1) was not the major aim of this work, but since the platform itself is the crucial component in the system synthesis regarding high-performance force control with industrial manipulators. Find out more on: Active environment concept for efficient force control systems. If you are interested on this project, please contact Rafal Osypiuk.

Demonstration of Hexa motion (trajectory generator).
Video download: Hexa_demo1.wmv (Windows Media, 1.2MB)

Demonstration of Hexa motion (space mouse).
Video download: Hexa_demo2.wmv (Windows Media, 4.0MB)

Demonstration of Hexa motion (JR3 force/torque sensor).
Video download: Hexa_demo3.wmv (Windows Media, 3.9MB)

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