Power Hydraulics - Switched Mode Control of Hydraulic Actuation
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Proc. 2010 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2010), October 18-22, 2010, Taipei, Taiwan
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.
In robotic applications where high power density, ruggedness
and reliability are key requirements hydraulic actuation can be a sensible choice. However, the low efficiency of proportional control can be a limitation and it is necessary to go beyond the paradigm of proportional flow/pressure control. One response to this challenge is to revisit traditional on-off
hydraulic technology and develop ?power hydraulic? devices
that behave in analogous manner to their power electronic
counterparts. ?Power hydraulics? is a challenging and little
explored technology due to the markedly non-linear behaviour
of hydraulic systems and the need of components with dynamic
specifications that are not readily available off-the-shelf.
After an analysis of the real on-off characteristics of a valve, a prototype hydraulic switching converter, inspired by the electric DC-DC Buck converter, is presented and its
performance in pressure control mode, relative to a classical proportional valve-controlled system, are assessed. An energy saving of 75% is achieved. Merits and limitations of the current design are identified.