Technical Article

Introduction to the oil replenishment performance of direct-drive electro-hydraulic servo hydraulic

Over the past decade, against the backdrop of the widespread application of volume control systems and AC servo motors, an electro-hydraulic servo system featuring direct drive and volume control of bidirectional fixed displacement pumps using permanent magnet AC synchronous servo motors has emerged, also known as the DDVC system or valve-less electro-hydraulic servo system. In this system, the three major functions of the hydraulic actuator, namely direction change, speed regulation, and pressure adjustment, are all directly controlled by the AC servo motor, hence the term "direct drive". The change in oil flow from the pump is also directly controlled by the motor. This system offers advantages such as high efficiency and energy saving, compact integration, simple operation and control, and low cost, which have contributed to its rapid development.

Hydraulic presses are widely used in the automotive industry for parts processing, as well as in various industries for product sizing, edge trimming, straightening, shoe making, handbag manufacturing, mold and die assembly, press fitting and stamping of shaft sleeves, and processes such as bending, stamping, and shape stretching of sheet metal parts. However, due to the oil replenishment issue in their open circuits, they typically employ the addition of auxiliary oil tanks or dynamic oil replenishment, where a hydraulic pump is solely used as an oil replenishment pump. This approach increases the system's volume and weight, and gives rise to issues such as heat generation. To address this problem, a structural solution utilizing a passive oil replenishment valve can be adopted. By designing a sealed pressure oil tank, the pressure can effectively solve the oil replenishment and drainage issues of asymmetric hydraulic cylinders.

The direct-drive control system of the hydraulic press utilizes a sealed pressure oil tank for passive oil replenishment. Since the oil tank maintains a certain pressure at all times, it compensates for pressure losses in the system's valves, pipelines, etc., eliminating issues such as insufficient oil replenishment, noise, and creeping. In systems with short-duration intermittent operations or frequent starts of the hydraulic pump, the pressure oil tank can serve as an auxiliary energy source, thereby reducing the power of the hydraulic pump, conserving energy, lowering investment costs, and extending the lifespan of both the motor and the hydraulic pump.