The report on the Harbin robotic arm was published in the September 28th issue of Space: Science and Technology. Wenfu Xu is a professor in the Harbin Institute of Technology's School of Mechanical Engineering and Automation and State Key Laboratory of Robotics and Systems. He said, “In order to solve the problems of strict restrictions on the mass and size of the manipulator, as well as the high requirements for reliability and safety of the control method in space operation, we developed a light space manipulator and proposed a new control method.”
Such a robotic manipulator has to be able to exert constant force control when in operation. Xu went on to say that “For the constant force control of a plane, the direction of control force is constant, but for a curved surface in an unknown environment, its normal vector is often changed in real time, so the traditional method would fail. In order to overcome this difficulty, we propose integral adaptive admittance control that can realize real-time correction of the desired position of the end of the manipulator so that it is in full contact and realizes constant force control.”
Consider drawing a line on a piece of paper. If the paper is on a flat desktop, it is much easier to maintain even pressure across the whole line. Drawing an identical line on a sheet of paper that has been wrapped around a bouncing ball is much more difficult. It will require specific calculations to understand the movement of the ball and how much pressure to exert based on the position of both the pen and the ball.
In order to keep the force control of the space manipulator constant, the research team imposed a control method that removes the need for steady-state correction. This is a key component of control systems in well-known environments. Steady state correction applies the potential error to the full movement. This eases problems when the environment is entirely predictable. If the manipulator knows that the desk’s surface is rough and strong pressure would result in the paper being ripped, the manipulator can lighten the pressure of the pen to maintain a constant line. However, when the surface is changing and unpredictable, maintaining a constant corrective state results in more errors because not all corrections apply to all errors.
The Harbin researchers tested their control method for the lightweight manipulator and found that the mechanical arm could adjust quicker than a traditionally controlled manipulator even on an unknown surface. This results in a tracking effect that is steady enough for practical applications. Xu said, “Using the proposed light space manipulator and the integral adaptive admittance control method can solve practical problems on on-orbit servicing, such as space target capturing, on-orbital assemble, orbital repairing and so on.” According to Xu, this work can serve as a reference for the design of light manipulators in the future. The control approach developed by the Harbin team can be applied to the machining process of robotic grinders and polishers.