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Adaptronic couplers for highly dynamic power transmission

Project Description

Electric motors clearly constitute the most common drive principle in robotics and mechatronics. Smart materials, however, offer considerably higher power-to-mass ratios than electric motors. If mechanical energy instead of electrical energy can be distributed through a system, e.g. a robot, highly dynamic and efficient torque transmission elements based on smart materials, e.g. piezoceramics, can be used to transmit torque from an input to an output element. Just like electric motors, these components - called adaptronic couplers - can provide position, velocity, and torque control of the output element. Therefore, they can combine functions of different conventional machine components such as friction clutches, continuously variable transmissions, and swivel units (cf. Fig. 1).

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Fig. 1: Application profile of adaptronic couplers.

In a joint project, the iRP and the Institute for Engineering Design (IK)(External) develop adaptronic couplers based on smart materials which can transmit variable torques highly dynamically from an input element to an output element employing static or dynamic friction. In the long run, systems (e.g. robots) based on these machine components are envisaged to compete with or even surpass systems based on the classic drive principle - electric motors - w.r.t. dynamics and torque-to-mass-ratio. Based on adaptronic couplers serial kinematic machines driven by only one motor can be designed. Thus, the weight of the robot structure may be descreased significantly which in turn increases achievable accelerations.

Two experimental models of revolute adaptronic couplers have been designed and evaluated successfully. Fig. 2 shows one experimental model that is currently used for the evaluation of control approaches and endurance tests. The electric motor(1) drives the input shaft via the bellows coupling(2). The guiding jacket(4) accomodates the piezoelectric actuator which exerts a normal force on a sliding bush that surrounds the input shaft. Thus a drive torque is generated that can be controlled to move the output element and the attached load(3). The input shaft angle and the angle of the output element are measured by encoders(5,6).

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Fig. 2: Prototype of the revolute adaptronic coupler.

Different linear control approaches combined with nonlinear models and various sensor setups have been evaluated for position and torque control. The control performance results presented below have been obtained by a cascaded position-velocity controller using encoder and angular velocity feedback. As can be seen, the following error is very low.

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Fig. 3: Control performance in position control mode.

Details on the concept(External) and preliminary results (External) have already been published. Moreover, in the context of adaptronic couplers an approach to virtually increasing encoder resolution(External) has been developed. Current work focuses on nonlinear modeling of piezoactuators, the friction contact as well as the geometric imperfections and material migration of the drive shaft. Regarding energy efficiency, a promising concept that exploits highly dynamic switching between minimum dynamic friction and static friction has been developed and is currently being implemented. Simulation results indicate a significant reduction of wear and energy loss in the friction contact.

Future work will focus on different active principles, nonlinear control approaches, and material issues to reduce wear and to further increase the achievable torque-to-mass ratio.
Apart from revolute adaptronic couplers, a prismatic variant of this concept is currently being developed and tested (patent pending).

For more information please contact Daniel Kubus or David Inkermann (External) .

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