System Configuration and Control Using Hydraulic Transformer

Abstract

Hydraulic power transmission offers multiple benefits over competing technologies including an order of magnitude higher power density than electric systems, relatively low cost, fast response, and flexible packaging. Hydraulics are often used in high-performance mobile robots that demand power, precision, and compactness. However, typical hydraulic systems suffer from low system efficiency from the wide usage of throttle valves. The research described in this dissertation focuses on developing hydraulic transformers that transforms hydraulic power from one set of pressure and flow to the other set of pressure and flow to replace throttle valves such that a compact and efficient fluid power system can be realized. A dynamic model capable of capturing operating characteristics and losses is developed to establish a quantitative comparison between two major designs of the hydraulic transformer. A traditional design where a pump and motor are coupled together in a single package is chosen for the research. This design has three possible configurations with unique operating characteristics, and if these configuration modes can be switched, the resulting transformer is shown to be more compact and efficient. A trajectory tracking controller for a cylinder and force controller for a hydraulic human power amplifier is developed to demonstrate potential applications for the hydraulic transformer. The controller developed proves that utilizing hydraulic transformer need not sacrifice the control performance. Control methodologies ensuring efficiency of the transformer driven system are developed. Transformer operating speed is optimized to minimize the power loss through the transformer. Transformer configuration is switched actively to operate the transformer in its most optimal mode. These methods further improve the efficiency benefit of using the transformer. A hydraulic transformer system utilizing developed controllers compared against a throttle valve system tracking a trajectory with various loading conditions reveals that transformer system can achieve an efficiency of 81.2% which is more than threefold increase over the throttling system with an efficiency of 26.2%. This efficiency improvement is possible with the ability of a transformer to capture regenerative energy to reduce the net energy consumption. This dissertation successfully presents the controller development for a hydraulic transformer that captures both precision and efficiency.