Gerrit Adriaan Folkertsma

Parallel stiffness in a bounding quadruped with flexible spine

Legged locomotion involves periodic negative and positive work, which usually results in high power consumption. Improvement of the energy efficiency is possible by using energy storage elements to reversibly store the negative work performed during a walking or running cycle. While series elastics with high impedance (high gear ratio) actuators are widely used, we investigate the application of parallel stiffness with highly backdriveable actuators. We specifically show that the use of parallel springs in a bounding quadruped with a flexible spine can lower power consumption by over 50%.

Design and analysis of an optimal hopper for use in resonance-based locomotion

Quadrupedal running is an efficient form of locomotion found in nature, which serves as an inspiration for robotics. We believe that a resonance-based approach is the path towards energy-efficient legged locomotion and running robots. The first step in working towards this goal is creating an energy-efficient one-legged hopper. Such a one-legged hopper was designed and constructed. The impact efficiency of the mechanism is calculated analytically, determined in simulation and measured with the prototype. The impact efficiency as calculated from the experiments is found to be in agreement with the analytical expectation and simulation results. Finally, using an electric motor to inject energy by creating a virtual spring with reverse hysteresis, hopping is achieved.

Application of substantial and sustained force to vertical surfaces using a quadrotor

In the field of aerial robotics, one of the key challenges is to enable aerial manipulators to exert substantial forces on the environment. Enabling this will allow the technology to perform meaningful tasks airborne, such as cleaning or grinding surfaces. While in contact and applying a large, continuous force, control of the UAVs attitude is a challenge. In this work, we show that a regular (PID-based) attitude controller is incapable of stabilizing aerial manipulators that apply physical contact forces on the environment that are comparable to the UAVs weight. We present a novel control algorithm that uses an LQR-optimized state feedback on the roll and yaw angle while in contact. Experiments on a UAV of 1.5 kg show that the proposed controller is capable of applying a contact force of over 15 N — equal to the UAVs weight — sustained for several minutes.

Power-continuous Synchronisation of Oscillators: a novel, energy-free way to synchronise dynamical systems

Synchronisation is an essential part of many controlled dynamical systems, in particular in the limb motion of legged robots. In this paper we introduce a novel control strategy that allows synchronisation of two oscillators without using any external power, but by modulating the power flow between the two oscillators. We then derive a separate energy-level controller that regulates the oscillation amplitude, again without changing the systems total energy. Finally, we show that the strategy works on a realistic mechanical system, by synchronising the phase difference and apex height of two bouncing masses.

Robird: A Robotic Bird of Prey

Ever since the start of aviation, birds and airplanes have posed a mutual risk: Birds are killed when struck by aircraft, but, in return, bird strikes cause billions in damage to the aviation industry. Airports employ bird-control methods such as audiovisual deterrents (like scarecrows, lasers, and noise), weapons, and chemicals to relocate, suffocate, or otherwise terminate the birds [2]. While the latter methods work, they are ethically questionable. The problem of audiovisual deterrents is that they quickly lose effectiveness due to habituation. The approach that works consistently is the use of predator birds to scare off the prey birds and permanently relocate them away from runways. However, the predators themselves cannot be precisely controlled and, in turn, also pose a threat to airplanes.

A General Approach to Achieving Stability and Safe Behavior in Distributed Robotic Architectures

This paper proposes a unified energy-based modeling and energy-aware control paradigm for robotic systems. The paradigm is inspired by the layered and distributed control system of organisms, and uses the fundamental notion of energy in a system and the energy exchange between systems during interaction. A universal framework that models actuated and interacting robotic systems is proposed, which is used as the basis for energy-based and energy-limited control. The proposed controllers act on certain energy budgets to accomplish a desired task, and decrease performance if a budget has been depleted. These budgets ensure that a maximum amount of energy can be used, to ensure passivity and stability of the system. Experiments show the validity of the approach.

Energy-based and biomimetic robotics

All physical systems interact by exchanging power, or energy. This energy can be explicitly taken into account when designing robotic systems, in dynamic models of systems and controllers, leading to more insight in energy-related effects. In this thesis, a biomimetic cheetah robot is developed, by first identifying core dynamic (energy-based) principles of the real cheetah and then translating them into a mechanical design. Theoretical aspects required for this analysis and design are amongst others synchronisation of limit cycles, energy-efficiency of hopping robots, passivity and energy-aware controllers, morphological computation. Finally, we introduce a biomimetic robotic bird on which we aim to apply the methods and tools used and developed for the cheetah.

Energy in Robotics

Energy and energy exchange govern interactions in the physical world. By explicitly considering the energy and power in a robotic system, many control and design problems become easier or more insightful than in a purely signal-based view. We show the application of these energy considerations to robotics; starting from the fundamental aspects, but, most importantly, continuing to the practical application to robotic systems. Using the theory of Port-Hamiltonian Systems as a fundamental basis, we show examples concerning energy measurement, passivity and safety. Control by interconnection covers the shaping and directing of energy inside the controller algorithms, to achieve desired behaviour in a power-consistent manner. This idea of control over the energy flows is extended to the physical domain. In their mathematical description and analysis, the boundary between controller and robot disappears and everything is an interconnected system, driven by energy exchange between its parts.

Reduction of time-varying nanotesla magnetic fields from electric power lines by twisting

Time-varying magnetic fields generated by electrical power lines in the laboratory can disturb electron microscope imaging. Modern microscopes require these fields to be below 10 nT [2]. We calculated and measured magnetic fields from straight and twisted current-carrying wires, and show that without twisting, this value cannot be reached.