Helmut Kogler

Energy Efficient Fluid Power in Autonomous Legged Robotics

This paper is concerned with the application of fluid power in autonomous robotics where high power density and energy efficiency are key requirements. A hydraulic drive for a bioinspired quadruped robot leg is studied. The performance of a classical valve-controlled (“resistive-type”) and of an energy saving (“switching-control mode”) hydraulic actuation system are compared. After describing the bio-inspired leg design and prototyping, models for both drives are developed and energy efficiency assessments are carried out. It is shown through simulation that the switching-control mode hydraulic actuation can meet the challenge of legged robotic locomotion in terms of energy efficiency with respect to improving robot power-autonomy. An energy saving of about 75% is achieved. Limitations of the current system are identified and suggestions for improvements are outlined.Copyright

Power hydraulics - switched mode control of hydraulic actuation

This paper is concerned with the application of switching technology to hydraulic actuation. Over the last 50 years with advances in power electronics, faster and faster static switches have been developed and applied to the control of motors. Hydraulic technology evolved in the opposite direction: switching control was not considered, and more and more accurate proportional flow/pressure control devices (servovalves etc) were developed. However despite the sophistication of such valves, from an energetic viewpoint proportional control is dissipative and inefficient. Indeed, by analogy it can be seen as the equivalent of resistive (rheostatic) motor control.

Linear motion control with a low-power hydraulic switching converter - Part II: Flatness-based control

Hydraulic switching control is an opportunity for economic, robust and energy efficient hydraulic motion control. The hydraulic buck converter represents a promising concept, which meets these requirements. Part I of this publication has shown that for a convenient control performance a model-based control should be applied. In this paper a flatness-based controller for linear hydraulic motion control with a sub-kilowatt hydraulic buck converter is presented. A simple model-based on an averaging of the switching converter is employed to derive a flatness-based controller combined with a nonlinear observer to estimate all system states only from a position signal. Its performance is studied by simulations and experiments for large ramp type and sinusoidal motions of a heavy mass. Comparison of motion quality and energy consumption are made between the converter and a standard hydraulic proportional drive using a servo-valve instead of the switching converter. With this flatness-based controller a tracking performance as good as of a proportional drive can be achieved, yet at much better efficiency. Proportional control for the switching converter, however, leads to inferior results for this type of intended motions.

The use of a hydraulic DC-DC converter in the actuation of a robotic leg

This paper presents the application of a hydraulic DC-DC converter, namely a step down Buck Converter to the actuation of a robot leg that is part of the quadruped robot HyQ. The use of a Hydraulic Buck Converter (HBC) offers significant advantages in terms of improved efficiency of hydraulic actuation systems analogously to an electric switching DC-DC converter as opposed to a rheostatic-type system. In this paper, a HBC consisting of two digital valves and two check valves is introduced to improve the efficiency performance of a singl leg of a hydraulic quadruped robot (HyQ). This type of hydraulic buck converter is able to support the locomotion in two directions. The HBC operates at a switching frequency of 100 Hz in pulse-width-modulation. The better energy performance compared to proportional control is achieved by the use of fast check valves. The performance of the system with a 3-way-4-position proportional valve is compared with the HBC drive. A test rig is set up to investigate the performance of HBC with two different controllers and a Hydraulic Proportional Drive (HPD) system, based on proportional valves which control flow, by throttling it, in a dissipative manner. The performance of position tracking and energy consumption is evaluated. The experimental results indicate that HBC systems can achieve similar position tracking with relatively less consumed energy.

A Simulation Model of a Hydraulic Buck Converter Based on a Mixed Time Frequency Domain Iteration

Digital hydraulics is an opportunity to realize simple, robust, cheap and energy efficient hydraulic drives. In such systems digital on/off valves are used instead of proportional valves. Moreover, in hydraulic switching converters the valves are actuated within a few milliseconds, which create sharp pressure changes and, in turn, significant wave propagation effects in the pipe system. For a proper design of digital hydraulic systems a sound understanding of these effects is required to achieve the desired behavior of the switching drive system. In such converters, like the buck-, boost or boost-buck-converter, the inductance is one crucial component. It is realized by a simple pipe mainly for cost reasons. Furthermore, switching converters contain some components with nonlinear characteristics, like valves or accumulators, which prevent a comprehensive analysis in frequency domain. For a convenient analysis a qualified model of a hydraulic buck converter based on a mixed time frequency domain iteration is presented. Main parameters of this model are identified and wave propagation effects in the inductance pipe of the converter are investigated by simulation.Copyright

Analysis of Wave Propagation Effects in Transmission Lines due to Digital Valve Switching

In digital hydraulic systems, switching valves have opening and closing times in the range of a few milliseconds. Due to this fast switching, high bandwidth pressure pulsation is excited, which is the stimulus for airborne noise up to some kilohertz. Since the human ear is very sensitive to audible noise in this frequency range, an analysis of the influence of the valve’s opening curve on the pressure surge in the pipe system is intended. The study is based on simulations employing dynamic pipe models for linear wave propagation and laminar fluid flow. In particular, a simple pipe system with a valve at one end and a pressure boundary at the other end of the pipe is investigated. It is shown, how the valve opening characteristics of spool and seat type switching valves influences the pipe responses. Also the role of parasitic inductances due to the valve block bores is discussed and it is shown how the switching characteristics influences the dynamical effects on the pressure pulsations in the pipe system.Copyright

Investigation of a Switch-Off Time Variation Problem of a Fast Switching Valve

In a series of experiments the peak current during switch on of a fast switching valve, which was found to be out of tolerances with respect to some armature dimensions, was varied to realize different switch on times. Despite the fact that the holding current was identical for all cases and the time between switch on and off was very long, the valve’s switch off time showed an unexpected dependency on the switch on peak current value. This paper presents an explanation of this phenomenon by coarse mathematical models, demonstrating that the manufacturing error in combination with friction, skewness, and fluid stiction are responsible for this behavior.Copyright

Linear motion control with a low-power hydraulic switching converter - Part I: Concept, test rig, simulations

Switching control can be employed for hydraulic motion control. Among the many hydraulic switching methods investigated so far, the so-called hydraulic buck converter convinces by its simplicity. It consists of two switching valves, an inertance pipe, and a hydraulic accumulator to flatten the pulsation resulting from switching. This accumulator entails significant softness and nonlinearity which, in combination with sealing friction of the hydraulic cylinder, may lead to inferior performance with simple control concepts. In this paper a sub-kilowatt hydraulic buck converter, its components, design, dimensioning, and steady-state performance characteristics are presented first. Measurements on a testrig for linear motion employing a simple P-controller show the necessity of a model-based control to achieve the desired control performance. Therefore, a dynamic model of the converter and the linear drive is derived and studied by simulations. It is shown that the model incorporates the relevant physical effects and that it is qualified for a flatness-based control, which is derived in Part II of the publication.