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PHD DEFENCE DOUWE DRESSCHER

CONTROLLED PASSIVE ACTUATION: CONCEPTS FOR ENERGY EFFICIENT ACTUATION USING MECHANICAL STORAGE ELEMENTS AND CONTINUOUSLY VARIABLE TRANSMISSIONS

Energy autonomous robots need to take care of their own energy supply. In the absence of a charging station, the robot will have to replenish its energy in other ways. Alternative energy sources available to a robot in an outdoor environment may include unrefined biomass, sun and wind. As these energy sources are limited, energy efficiency of mobile robots is of key importance.

With this research, we want to make a step toward energy autonomous legged robots. For this, we have two key objectives: increase insight in the mechanisms of energy loss and contribute toward the improvement of energy efficiency.  In the first part of this thesis, the energy losses in legged robots are discussed. The active supporting of the system mass and the additional movement of the legs contribute to the high energy consumption in legged vehicles. However, how significant the contributions of the various causes of energy loss are, is not well understood. We investigate the mechanisms of energy loss in electric actuators to understand their contribution to energy (in)efficiency.

As a part of this effort, a method to select a motor-gearbox combination for energy efficiency is presented and evaluated to show the possible impact. We conclude that for most robotic applications, electric actuators are the dominant cause of energy losses. Selecting a motor-gearbox combination for energy efficiency can provide a significant improvement in the efficiency. However, the energy losses in the motor-gearbox combination remain significant. As an alternative to using only electric actuators for actuation, an actuation principle that incorporates a mechanical storage element and a Continuously Variable Transmission (CVT) in the drive-train is discussed. This actuation principle is named Controlled Passive Actuation (CPA). The envisioned benefits are twofold: first, separation of the “energy supply” function and the “servoing” function enables using the electric actuator in its most efficient range of operation; and, second, by incorporating the capability to recycle energy within the mechanical domain, the energy loss in the electric actuator can be avoided.  

An important component in this actuation principle is the CVT and in the second part of this thesis, two CVT's that were designed for application in Controlled Passive Actuation are discussed. The first design in a lever with a pivot that moves along the lever. It is shown that the design is conceptually suitable for application in Controlled Passive Actuation and that the mechanical design needs improvement. The second design is based on two spheres of which the relative orientation can be changed to adapt the transmission ratio. This concept is named Dual-Hemi CVT. A prototype was built and experimental results show that it is suitable for application in CPA.  In the third part of this thesis, two versions of CPA are discussed. In the first version of CPA, a flywheel is used as mechanical storage element. By changing the rate of the transmission ratio of the Continuously Variable Transmission, the energy transfer between the source and the load is controlled. A test setup was built and experimental results show that the system can be used to achieve servoing behaviour at a limited bandwidth. In the second version of CPA, a spring is used as mechanical storage element. By changing the transmission ratio of the CVT the energy flow between the source and the load is controlled. A test setup that includes the aforementioned Dual-Hemi CVT was built and experimental results show that the system can be used to achieve servoing behaviour with limited performance. When applying CPA, achieving good performance and high efficiency has proven to be difficult. By adding a CVT, a complex device with non-idealities that have to be dealt with is added. We have shown that we can separate the energy source function from the servoing behaviour function.

We were not (yet) able to obtain good servo performance nor achieve the envisioned efficiency gains. To conclude: to achieve robots that can move with high energy-efficiency, we need to look beyond currently available electric actuators. Either the motor steepness of electric motors needs to be increased such that they offer an energy efficient way of actuating robots when used in the traditional sense, or additional facilities need to be included in the drive to let the motor operate in its current high-efficiency region and/or reduce the energy flow trough the motor. The latter can for example be achieved with CPA. A step has been made on this path but the search should continue. CVTs can help in achieving lower energy losses in the actuators, but the requirements on the CVT are high.