Hydraulic servo control offers numerous advantages, leading to its wide-ranging applications. However, it also has some disadvantages, which limit its use.
1. Advantages of Hydraulic Servo Control
The power-to-weight ratio and torque-to-inertia ratio (or force-to-mass ratio) of hydraulic components is high, enabling the creation of compact, light-weight, high-acceleration servo systems. This advantage is particularly evident in medium and high-powered servo systems.
To illustrate this, a comparison is made between hydraulic and electrical components.
The minimum size of electrical components depends on the maximum effective magnetic flux density and the heat generated by power loss (related to current density). The maximum effective magnetic flux density is limited by the saturation of magnetic materials, and heat dissipation is challenging. Therefore, electrical components are larger and have smaller power-to-weight and torque-to-inertia ratios.
The heat generated by the power loss of hydraulic components can be carried to the radiator by the oil, and its size primarily depends on the maximum working pressure. Because the maximum working pressure can be high (currently up to 32MPa), hydraulic components are compact and lightweight, yet provide large output force or torque, resulting in high power-to-weight and torque-to-inertia ratios (or force-to-mass ratios).
Generally, the weight of hydraulic pumps is only 10%-20% of that of equivalent power electric motors, and their size is about 12%-13% of the latter. The power-to-weight ratio of hydraulic motors is generally 10 times that of equivalent capacity electric motors, and the torque-to-inertia ratio is 10 to 20 times that of electric motors.
Hydraulic power components have excellent speed characteristics, leading to quick system responses. Due to their high torque-to-inertia ratio (or force-to-mass ratio), these components have strong acceleration capabilities, allowing for high-speed starts, stops, and reversals. For instance, accelerating a medium-powered electric motor takes seconds, while accelerating a hydraulic motor of the same power takes only about a tenth of that time.
The high bulk modulus of the hydraulic oil in the system results in a large hydraulic spring stiffness from the compressibility of the oil. Coupled with the small inertia ratio of the hydraulic power components, this results in a high natural frequency for the hydraulic system, hence a fast response speed.
Pneumatic systems with the same pressure and load as hydraulic systems have a response speed only 1/50th of that of hydraulic systems.
The hydraulic servo system has a high stiffness against load, meaning that the output displacement is minimally affected by load variations, ensuring accurate positioning and high control precision.
Due to the high inherent frequency of hydraulics, hydraulic servo systems, especially electro-hydraulic servo systems, can have a large open-loop gain, thereby achieving high accuracy and responsiveness.
Moreover, the compressibility of the oil in the hydraulic system is very small, and leakage is also minimal, resulting in great velocity stiffness of hydraulic power components and high positional stiffness when forming a closed-loop system. The open-loop speed stiffness of an electric motor is about one-fifth that of a hydraulic motor, and the positional stiffness of the electric motor is near zero.
Therefore, electric motors can only be used to form closed-loop position control systems, while hydraulic motors (or cylinders) can be used for open-loop position control, and of course, the stiffness of a closed-loop hydraulic position control system is much higher than in open-loop. Due to the compressibility of gas, the stiffness of a pneumatic system is only 1/400 that of a hydraulic system.
2. Disadvantages of Hydraulic Servo Control
Hydraulic components, especially precision hydraulic control components (such as electro-hydraulic servo valves), have poor resistance to contamination and require high cleanliness of working hydraulic fluid. Contaminated fluid can cause valve wear, degrade its performance, or even clog it up, preventing normal operation.
This is a primary cause of failure in hydraulic servo systems, necessitating the use of fine filters in such systems.
The bulk modulus of elasticity of the hydraulic fluid varies with oil temperature and the amount of air mixed into the oil. The viscosity of the oil also changes with temperature, so changes in oil temperature can significantly impact system performance.
When the sealing design, manufacturing, and maintenance of hydraulic components are improper, it can easily lead to external leakage, causing environmental pollution. Currently, hydraulic systems still widely use flammable petroleum-based hydraulic oil, and oil leakage could potentially cause a fire, making it unsuitable for some applications.
The manufacturing precision required for hydraulic components is high, leading to high costs.
The acquisition and long-distance transmission of hydraulic energy are not as convenient as electrical systems.
3. In conclusion
Hydraulic servo systems are compact, lightweight, offer high control precision, and respond quickly. These advantages are extremely vital for servo systems.
Furthermore, there are additional benefits:
- Good lubrication and long lifespan of hydraulic components;
- Wide range of speed adjustment, with excellent low-speed stability;
- Convenient power transmission via oil pipes;
- Easy energy storage with the help of accumulators;
- Hydraulic actuating elements come in two forms – linear displacement type and rotary type, enhancing their adaptability;
- Overload protection is straightforward;
- Addressing system temperature rise issues is convenient.