Abstract
Primary control of linear motion by variable speed electric motors driving a hydraulic cylinder via a constant displacement pump is an established and successful concept with a frequent use in industry. One problem arises when low or zero motion speed has to be realized under high pump pressure conditions. Such load scenarios occur frequently in certain pressing processes, e.g., for sintering or veneering. Most pumps have a lower speed limit, below which critical tribological conditions occur which impair lifespan and efficiency. In addition, pump speed control and pump fluctuation suffer from the mixed lubrication conditions in such an operation range. For a circumvention of such low speed pump operation, a digital valve control concept is presented and studied in this paper. Valve control is used in load holding phases with low speed. Pressure is provided by an accumulator which is charged by the pump in short charging cycles at reasonable pump speeds. It is shown that the mean control error during load holding phase lies within the desired band and the fluctuations of the control force are lower than those of the pump control. In addition, the unfavorable pump operation conditions can be avoided via digital control.
Highlights
Hydraulic drives with primary control based on variable speed motors and constant displacement pumps are broadly used in industry
The concept of combining powerful speed variable motors with a hydrostatic transmission instead of a mechanical transmission to generate and control mechanical motion has some basic advantages: the unrivalled robustness of the hydraulic cylinder, the good energetic efficiency, the possibility to employ mature and widely available electric motor control algorithms and electronics, the ease of generating high forces, passive load holding without a motor torque, simple gear shifting by switching a double stroke cylinder from rapid feed to work feed, fast emergency stop by actuating fast bypass valves, and the higher flexibility of geometric arrangement of the motor and actuator for a favorable system design, to name just a few
This paper studies a switching type valve control
Summary
Hydraulic drives with primary control based on variable speed motors and constant displacement pumps are broadly used in industry. If asynchronous motors with frequency converters are used, the precise control of slow speeds may be a big challenge, with high demands on power electronics For these reasons, it may be advantageous to hand over such operation phases to a separate actuation concept which can fulfil the requirements more and effectively. The review paper [18] addresses the low speed problem and proposes the development of appropriate pumps, capable of withstanding such operation conditions It presents a circuitry with two 2/2 proportional valves to drain off the difference of oil needed and delivered by the pump at acceptable minimum speed.
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