Working principle and composition of hydraulic drive
Hydraulic drive is mostly used in applications requiring large output force, and its speed is lower than that of pneumatic drive under low-pressure drive conditions.
Hydraulic drives have high output force and power, and can form servo mechanisms, often used to drive the joints of large robots.
Hydraulic drive systems mainly consist of hydraulic cylinders and hydraulic valves. A hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy, performing linear reciprocating or oscillating motion. It has a simple structure and reliable operation. When using a hydraulic cylinder to achieve reciprocating motion, a speed reduction device is unnecessary, and there is no transmission backlash, resulting in smooth movement. Therefore, it is widely used in various hydraulic systems.
Reciprocating linear hydraulic cylinders controlled by solenoid valves (referred to as linear hydraulic cylinders) are the simplest and cheapest open-loop hydraulic drive devices. Linear hydraulic cylinders regulate flow through a controlled throttling orifice, allowing for deceleration at the end of the motion and control of the stopping process. Large-diameter hydraulic cylinders are inherently more expensive and require costly electro-hydraulic servo valves, but they can achieve greater output force, typically reaching 14 MPa.
Whether it's a linear hydraulic cylinder or a vane hydraulic motor (hereinafter referred to as a rotary hydraulic motor), their working principle is based on the action of high-pressure oil on the piston or vanes. Hydraulic oil is delivered to one end of the hydraulic cylinder via a control valve. In an open-loop system, this is controlled by a solenoid valve; while in a closed-loop system, it is controlled by an electro-hydraulic servo valve or a manual valve.
Hydraulic valves are further divided into check valves and directional valves. A check valve allows oil to flow only in one direction and blocks it from flowing in the opposite direction; this type of valve is also called a check valve. Directional valves are divided into spool-type directional valves, manual directional valves, mechanically operated directional valves, and solenoid directional valves. A spool-type directional valve uses the axial movement of a valve core within the valve body to connect or disconnect the corresponding oil circuit. Manual directional valves are used for manual reversing. Mechanically operated directional valves are used in mechanical movements as limit devices to restrict reversing. Solenoid directional valves are used to change the direction of fluid flow, thereby changing the state of mechanical movement, when a reversing command is issued by an electrical or control device.
Compared to mechanically driven robots, hydraulically driven robots have a much greater lifting capacity, reaching up to hundreds of kilograms. However, hydraulic drive systems have higher requirements for sealing, are not suitable for operation in high or low temperature environments, require higher manufacturing precision, and are therefore more expensive.
