A two-speed actuator for robotics with fast seamless gear shifting

Abstract

This paper present a novel dual-speed actuator adapted to robotics. In many applications, robots have to bear large loads while moving slowly and also have to move quickly through the air with almost no load. This lead to conflicting requirements for their actuators. Multiple gear ratios address this issue by allowing an effective use of power over a wide range of torque-speed load conditions. Furthermore, very different gear ratios also lead to drastic changes of the intrinsic impedance, enabling a non-back-drivable mode for stiff position control and a back-drivable mode for force control. The proposed actuator consists of two electric motors coupled to a differential; one has a large gear ratio while the other is almost direct-drive and equipped with a brake. During the high-force mode the brake is locked, only one motor is used, and the actuator behaves like a regular highly-geared servo-motor. During the high-speed mode the brake is open, both motor are used at the same time, and the actuator behaves like a direct drive motor. A dynamic model is developed and novel controllers are proposed for synergic use of both motors. The redundancy of motors is exploited for maintaining full control of the output during mode transitions, allowing for fast and seamless switching even when interacting with unknown environments. Results are demonstrated with a proof-of-concept linear actuator.

Kinematically redundant actuators, a solution for conflicting torque–speed requirements

“…While previous works (Girard and Asada, 2015; Lee and Choi, 2012) have reported on the working range in quadrant I, we also offer a view on the negative power quadrants II and IV, which are also of importance in the field of robotics. The main difference between those quadrants is the role of friction and gearbox losses.…”

“…The results in Verstraten et al (2018), for example, indicate that a DMA design can actually be made lighter than a single-motor alternative having the same operating range, provided that its composing drivetrains are selected in a smart way. With integrated designs, where the motors and the planetary differential are combined in a single housing (Girard and Asada, 2015), the actuator’s volume and weight can be further reduced. Even greater advantages may be achieved by adding non-backdrivable elements to the design, as they allow shaping the working range of the actuator to fit the application more closely.…”

“…This makes their concept unsuitable for applications in robotics. More recently, Girard and Asada (2015) presented a similar actuator where the worm gears were replaced by controllable brakes. For this actuator, they developed several control strategies to divide the required power over the motors and to deal with the holding brakes (Girard and Asada, 2016, 2017).…”

“…By using a DMA with brakes, the unused part of the operating range can be eliminated. As a result, the DMA can be composed of small motors, reducing the overall weight and size of the actuator (Girard and Asada, 2015) and improving its energy efficiency over that of a single-motor alternative (Verstraten et al, 2018). However, non-backdrivable mechanisms also have several disadvantages.…”

“…We thus conclude that the operating range can be enlarged by creating an internal power flow between the sun motor and the ring motor. Although the operating range of kinematically redundant actuators has been analyzed previously, this interesting observation has never been reported, because back-driving of motors was prevented by design (Lee and Choi, 2012) or simply not considered (Girard and Asada, 2015). Nevertheless, we believe that this is an important feature that can potentially be exploited to reduce the need for backdrivable mechanisms or brakes.…”