Increasing Energy Efficiency of High-Speed Parallel Robots by Using Variable Stiffness Springs and Optimal Motion Generation

Abstract

The classical approach to decrease the energy consumption of high-speed robots is by lowering the moving elements mass in order to have a lightweight structure. Even if this allows reducing the energy consumed, the lightweight architecture affects the robot stiffness, worsening the accuracy of the mechanism. Recently, variable stiffness actuators (VSAs) have been used in order to reduce the energy consumption of high-speed pick-and-place robots. The idea is to smartly tune online the stiffness of VSA springs so that the robot is put in near a resonance mode, thus considerably decreasing the energy consumption during fast pseudo-periodic pick-and-place motions. However, the serial configuration of springs and motors in the VSA leads to uncontrolled robot deflections at high-speeds and, thus, to a poor positioning accuracy of its end-effector. In order to avoid these drawbacks and to increase the energy efficiency while ensuring the accuracy, this paper proposes the use of parallel arrangement of variable stiffness springs (VSS) and motors, combined with an energy-based optimal trajectory planner. The VSS are used as energy storage for carrying out the reduction of the energy consumption and their parallel configuration with the motors ensure the load balancing at high-speed without losing the accuracy of the robot. Simulations of the suggested approach on a five-bar mechanism are performed and show the increase on energy efficiency.

Performing Energy-Efficient Pick-and-Place Motions for High-Speed Robots by Using Variable Stiffness Springs

“…In order to validate the theoretical formulations for performing energy-efficient motions, here, we define two sets of desired pick-and-place conditions for testing the energyefficient motion generator developed in Section 3. It is worth mentioning that, in what follows, for computing the energetic losses for the two experimental scenarios, we estimate the losses thanks to the model presented in [28] that is not recalled here for reasons of brevity. This is because, due to technological limitations, we cannot have a direct measure of the motor energy consumption.…”

“…Recently, the authors of the present paper have shown in [28,29] the effectiveness of using VSS in parallel for noticeably increasing the energy efficiency of pick-and-place robots operating at high speeds. Additionally, it has been shown that the energy required to adjust the spring equilibrium position of the VSS (associated to the VSS stiffness) is not negligible when analyzing the energetic losses in the full actuation chain.…”

“…For this, let us exploit the results shown in [28] in which it is shown that more that 90 % of loss for the actuation chain in a typical high-speed pick-and-place operation are due to the resistive and conduction losses which are quadratic functions of the motor electric currents, and thus, of the motor input efforts τ τ τ and τ τ τ vss , during the deceleration phase. That is why, based on this previous hypothesis, by imposing τ τ τ and τ τ τ vss to be null during the deceleration phase, we may considerably reduce the energy losses.…”

“…Additionally, it has been shown that the energy required to adjust the spring equilibrium position of the VSS (associated to the VSS stiffness) is not negligible when analyzing the energetic losses in the full actuation chain. Regarding the strategies for energy savings in [28,29], firstly, the work in [28] seeks to exploit an energetic model that groups the losses in the full actuation chain of the parallel robot with VSS, by generating trajectories through an optimization formulation. Even if it is shown that the energy efficiency can be increased by 48 %, the motions used in the optimization are polynomial-based trajectories that do not fully exploit the robot natural dynamics leading to the convergence towards a local minima of the energetic criteria.…”