Preliminary investigation of the design space of geared magnetorheological actuators for safer robotic manipulators

Abstract

Geared magnetorheological (MR) actuators have the potential to provide safe and fast physical interactions between human and machine due to their low inertia and high bandwidth. The use of MR actuators in collaborative robotics serial manipulators is only emerging and the design space of this approach is unknown. This paper provides a preliminary understanding of this design space by studying how much gearing can be used between the MR actuators and the joint outputs while maintaining adequate safety levels for collaborative tasks. An analytical collision model is derived for a 6 degrees-of-freedom serial manipulator based on the geometry of the well-known UR5e robot. Model validity is confirmed by comparing predictions to experimental collision data from two robots, a UR5e and a MR5 equivalent. The model is then used to study the impact of gearing level on safety during eventual collisions with human. Results show that for both technologies, robot safety is governed by the balance between the reflected mass due to structural mass and actuator rotational inertia. Results show that, for the UR5e geometry studied in this paper, MR actuators have the potential to reduce the reflected mass in collisions by a factor ranging from 2 to 6 while keeping gearing ratios above 100:1. The paper also briefly studies the influence of robot shape on optimal gearing ratios showing that smaller robots with shorter range have lower structural mass and, thus, proportionally benefit even more of MR actuators. Delocalizing wrist actuators to the elbow has a similar impact since it also reduces structural mass. In all, this work suggests that MR actuators have a strong potential to improve the "hapticness" of collaborative robots while maintaining high gearing ratios.

Keywords: actuator; collaborative robots; gearing ratio; magnetorheological clutch; physical human-robot interaction; robot architecture; robot safety.

FIGURE 1

Proposed architecture of MR actuator using two power chains. (a) Schematic view of the two power chains, depicting the motors, pre-clutch gearing, MR clutches, actuator gearing and output. (b) Detailed view of a MR actuator, showing the motors, MR clutches and gearing. (c) Schematic comparison of a geared motor with a high reflected mass at the output and a MR clutch driven actuator with a low reflected mass at the output from its low inertia system (Véronneau et al., 2023).

FIGURE 2

1-D collision model when masses are: (a) pre-impact, (b) at the instant of impact, and (c) post-impact where maximal force is reached. Adapted from St-Jean et al. (2024).

FIGURE 3

Power flow of (a) conventional gearmotor and (b) geared MR actuator with 2 actuation chains. Adapted from Plante et al. (2025).

FIGURE 4

Motor torque-to-mass vs. motor torque data extracted from various commercially available motors. Adapted from Plante et al. (2025).

FIGURE 5

MR clutch mass vs. clutch torque data extracted from various MR clutch designs built over the years. Adapted from Plante et al. (2025).

FIGURE 6

Experimental setup of the collision tests carried out on the MR actuated cobot MR5.

FIGURE 7

Measured maximum impact force from a collision test compared to the model prediction, as a function of end-effector impact speed, for both UR5 and MR5 cobots.

FIGURE 8

Results from the analytical model for the global maximal reflected mass $(M_{gm})$ based on the global gearing ratio on a UR5e for the 4 studied actuators considering: (a) Structural inertia matrix $A_s$ and Actuator’s inertia matrix $A_a$ , (b) Structural inertia matrix $A_s$ only, and (c) Actuator’s inertia matrix $A_a$ only. Dotted lines on (a) are positionned at the minimum $(M_{gm})$ for each curve.

FIGURE 9

Results from the analytical model for the reduction ratio of the reflected mass $(R_{\mathit{100:1,HD}})$ based on the global gearing ratio on a UR5e from the use of UR Harmonic-drive actuators with a 100:1 gearing ratio compared with the use of MR actuators (Gen1, Gen2 and Gen3) and UR Harmonic-drive actuators with a different gearing ratio.

FIGURE 10

Results from the analytical model for the reflected mass reduction from the use of Gen1 actuators instead of 100:1 Harmonic-Drive actuators $(R_{\mathit{100:1,HD}})$ based on the robot reach.

FIGURE 11

Results from the analytical model for the (a) Global maximal reflected mass $(M_{gm})$ and (b) Reflected mass reduction from 100:1 HD actuators $(R_{\mathit{100:1,HD}})$ , on an UR5e with wrist actuators delocalized to the elbow.