Modelling and Control of a 2-DOF Robot Arm with Elastic Joints for Safe Human-Robot Interaction

Abstract

Collaborative robots (or cobots) are robots that can safely work together or interact with humans in a common space. They gradually become noticeable nowadays. Compliant actuators are very relevant for the design of cobots. This type of actuation scheme mitigates the damage caused by unexpected collision. Therefore, elastic joints are considered to outperform rigid joints when operating in a dynamic environment. However, most of the available elastic robots are relatively costly or difficult to construct. To give researchers a solution that is inexpensive, easily customisable, and fast to fabricate, a newly-designed low-cost, and open-source design of an elastic joint is presented in this work. Based on the newly design elastic joint, a highly-compliant multi-purpose 2-DOF robot arm for safe human-robot interaction is also introduced. The mechanical design of the robot and a position control algorithm are presented. The mechanical prototype is 3D-printed. The control algorithm is a two loops control scheme. In particular, the inner control loop is designed as a model reference adaptive controller (MRAC) to deal with uncertainties in the system parameters, while the outer control loop utilises a fuzzy proportional-integral controller to reduce the effect of external disturbances on the load. The control algorithm is first validated in simulation. Then the effectiveness of the controller is also proven by experiments on the mechanical prototype.

Mechanical Design and Analysis of a 3D-printed Planar Compliant-Rigid Hybrid Robot Prototype

“…Fig. 13 shows the posture of the robot when following a rectangular trajectory with two diagonal points (4,10) and (8,15), though due to the maximum length of the robot arm made it impossible to reach the point (8,15), by adding the length of either rigid arm or compliant arm could solve the problem (Fig, 14).…”

“…The simulation result of the control algorithm is given below, notice that the coordinates are relative, therefore does not link to the real-world scale of the robot. 10 shows the iteration process of the robot when the target point is set as (8,8), the red arc as well as the straight line connected by the green dot represent the movement of the robot arm as well as the joint J_1, the black arc and the straight line represent the initial position manually set, the blue ones show the ending position of the robot arm when it reaches the target point.…”

“…Combination of them to overcome their own individual drawbacks will be good in the field of robotics. There is very little work reported on this area, except in [8], Hua Minh Tuan et.al has designed a robot arm with elastic joints, which would provide safe robot interaction. However, it is different from the robot structure this paper is going to introduce, due to the structure they introduced is rigid, and the structure this paper is about to present is compliant and rigid hybrid.…”

A Novel Adaptive Sliding Mode Controller for a 2-DOF Elastic Robotic Arm

“…Then, in Reference [28], we compared this controller and a reinforcement learning algorithm. Then, in Reference [29], we implemented the controller on a two DOFs robot arm with elastic actuators. However, to the best of our knowledge, an adaptive control aiming to both stabilize the system and deal with uncertainties to eliminate the influence of external force/torque has not been released.…”

“…In the inner loop, there is one MRAC for each joint to stabilise the motor side. The MRAC was presented in our previous papers [27,29]. Firstly, the conventional sliding mode controller is established.…”