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01.What is a servo hydraulic cylinder?
The electro-hydraulic servo system refers to a hydraulic control system with a servo element servo valve or servo pump as the control core. It is usually composed of a command device, a controller, an amplifier, a hydraulic source, a servo element, an actuator, a feedback sensor and a load.
The servo hydraulic cylinder is the actuator in the (electro-hydraulic servo system). It can freely control the direction, position, speed or deformation of an object by applying (controllable) push, pull, pressure, twist and other forces to an object.
In the servo hydraulic system, the hydraulic cylinder and servo valve need to cooperate with each other to achieve the normal operation of the servo system. There is a combination relationship and a direct feedback relationship between them.
In order to simplify the mechanism, shorten the pipeline connection distance, and improve the control sensitivity, the system’s signal detection device and feedback device, servo valve or proportional valve and other components are generally installed directly on the servo hydraulic cylinder
02. Servo hydraulic cylinder structure and classification
The servo hydraulic cylinder is generally composed of end covers, cylinder barrel tubes, piston rods, piston assemblies, servo modules, sensor assemblies, bases and other main parts.
1. Support seat, 2. Position sensor, 3. Electro-hydraulic servo valve, 4. Cylinder, 5. Piston rod, 6. Load sensor 7. Locking ring, 8. Spherical bearing, 9. Upper end cover, 10. Valve block, 11. Lower end cover, 12. Sleeve, 13. Ear seat
The performance indicators of the servo hydraulic cylinder will directly affect the accuracy and quality of the electro-hydraulic servo system. Its performance parameters, appearance structure, installation and connection form, etc. need to be determined according to the performance indicators and functions of the servo system and the models of the selected servo valve and feedback sensor.
1. Servo hydraulic cylinder with built-in displacement sensor, 2. Servo hydraulic cylinder with buffer, 3. Servo hydraulic cylinder with integrated external displacement sensor
Compared with ordinary hydraulic cylinders, the focus of servo hydraulic cylinders is “control“, and the key point to be solved is to ensure a low damping coefficient, that is, to minimize friction. According to the different friction reduction methods, it is divided into ordinary servo hydraulic cylinders and static pressure support servo hydraulic cylinders.
Ordinary servo hydraulic cylinders often reduce friction by designing the cylinder body, improving the precision of processing and manufacturing, treating the surface process of the piston rod, and selecting seals with low friction coefficients.
Static pressure support servo hydraulic cylinders form a closed chamber through the special mechanism design inside the front cover of the piston rod, and inject a certain pressure of hydraulic oil into the closed chamber to form an oil film between the piston rod and the cylinder, which plays a certain supporting role. Due to the realization of liquid-oil lubrication, the friction resistance of the hydraulic cylinder is extremely small. Its dynamic response and position accuracy are better than ordinary servo hydraulic cylinders, but the cost is higher.
03.Comparison with traditional hydraulic cylinders
The servo hydraulic cylinder is one of the key components in the servo hydraulic system. Its structure and dynamic characteristics directly affect the performance and service life of the system. Therefore, compared with traditional hydraulic cylinders, the requirements for servo hydraulic cylinders are higher.
1. High stability
Servo hydraulic cylinders are used to drive negative workloads at high frequencies to achieve high-precision, high-response servo control, and must meet the high stability requirements of the pressure rod.
2. High strength and high stiffness
In order to meet the requirements of working pressure and high-frequency impact pressure, the thickness of the servo hydraulic cylinder is generally exceeding the calculated value of the normal strength design for safety reasons.
At the same time, the rigidity requirement of the servo hydraulic cylinder is high, that is, the slenderness ratio of the piston rod (the ratio of the piston rod length to the cross-sectional area) is smaller, otherwise the frequency of the actuator will drop significantly, and the base of the cylinder must not only meet the force requirements of the supporting cylinder, but also have a solid foundation.
3. Low friction
Servo hydraulic cylinders require the friction starting resistance and the friction resistance throughout the entire process to be reduced as much as possible, otherwise it is easy to generate limit cycle vibration and generate static dead zone and dynamic dead zone. Therefore, measures such as selecting seals with low friction coefficients must be taken to minimize static and dynamic friction resistance
4. Low leakage
Servo hydraulic cylinders are not allowed to leak externally, and require very small internal leakage.
5. High cleanliness
Ordinary hydraulic cylinders require higher cleanliness, while servo hydraulic cylinders require very high cleanliness.
