What is a Hydraulic Actuator ?

What is a Hydraulic Actuator?

Hydraulic actuators are components that transform working fluid energy into mechanical energy for reciprocating motion. The most often used materials in power hydraulics are defined, and various surface changes of the abovementioned elements are offered with the goal of increasing actuator operating parameters. Iron alloys are the most commonly utilized materials for actuator elements.

However, as environmental concerns grow, there is a trend toward looking for new substitutes that provide the same or better mechanical or tribological performance. Sealing systems are generally made of thermoplastic or elastomeric polymers, which have a low friction coefficient and guarantee the greatest possible seal-to-cooperating-element interaction. In the realm of surface modification, the topic of chromium plating of piston rods has been explored, which should be replaced by other means of increasing surface qualities due to the toxicity of hexavalent chromium.

Hydraulic Actuator

The hydraulic supply and return line is linked to the lower chamber of the actuator, allowing hydraulic fluid to flow to and from it. The piston’s motion is transmitted to a valve via the stem.

The spring force retains the valve in the closed position when there is no hydraulic fluid pressure. As more fluid enters the bottom chamber, the pressure inside it rises.

This pressure causes a force on the bottom of the piston that is the polar opposite of the spring’s force. The piston begins to move upward when the hydraulic force exceeds the spring force, the spring compresses, and the valve opens.

The valve continues to open as the hydraulic pressure rises. When hydraulic oil is evacuated from a cylinder, the hydraulic force is reduced.

The operation of a hydraulic actuator is similar to that of a pneumatic actuator. To move the valve, each uses some motive power to overcome spring force. Hydraulic actuators can also be configured to fail-open or fail-close as a fail-safe feature.

Hydraulic Actuator

Hydraulic Actuator is the type of actuator that utilizes the hydraulic pressure as input in order to provide excitation to the plant (controlled process) of the control system. The controller provides the control signal to the actuator that hydraulically applies the equivalent signal to the plant in order to have the desired output in controlled operations.

The hydraulic systems possess an operating pressure range between 1 to 35 Mega Pascals.

These are widely used in closed-loop control systems whenever there is a need for high speed as well as large force operations.

What is an Actuator in Control System?

A device used in a control system that uses the output of the controller to provide a control signal to the plant to get the desired output is known as an actuator. An actuator is a crucial unit of a control system because the control signal from the controller signifies the change needed for converting the achieved output into the desired one.

We have already discussed this while discussing the operation of the closed-loop control system that the feedback unit provides a part of the achieved signal back at the input side of the system. Here the reference input is compared with the signal achieved at the output in order to check the existing difference between the two.

The difference in the value of reference input and achieved output signifies the error signal. This error signal is provided to the controller that generates the control signal that corresponds to the error signal.

This control signal is provided to the actuator and the actuator further changes it into the type of signal which can drive the plant(motor) involved in the operation.

This implies that the control signal from the controller drives the actuator of the control system. Further, the actuator unit excites the plant of the system. And this way, the controlled output is generated.

Actuators are generally classified into two categories:

  • Hydraulic Actuators
  • Pneumatic Actuators

Here in this article, we will discuss hydraulic actuators. So, let us proceed.

Operating Principle

The operating principle of a hydraulic actuator system is based on a concept proposed by Pascal and known as Pascal’s Law or Pascal’s Principle.

Pascal’s Law asserts that pressure applied to a confined fluid in a container at a precise location is transmitted equally in all directions within the fluid as well as the container’s walls, with no loss.

Assume that pressure P is supplied to an area A, and the resultant force owing to the applied pressure is:

F= P * A

When a force F is applied in a smaller region to create pressure P in a confined fluid, the force produced on a greater area as a result can be proportionally larger than the force provided by the pressure.

This approach is employed by numerous hydraulic systems to generate very large forces using the applied pressure at a specific spot.

Fluid Hydraulics

The hydraulic fluid used in the system must be incompressible by definition. This is because the fluid’s incompressible nature allows power to be sent to all parts of the fluid at the same time. Petroleum-based oils or non-flammable synthetic oils are used in these systems.

The use of such oils also offers lubrication to the hydraulic system’s moving parts, decreasing friction losses and facilitating component cooling, which decreases the risk of fire threats in the event of excessive heat generation during operation.

These systems mostly use petroleum oils since they have low compressibility. This oil also works as a seal, preventing any of the components from leaking.

Advantages of Hydraulic Actuators

  1. Hydraulic actuators are tough and well-suited to situations requiring a lot of force. They can generate 25 times the force of comparable pneumatic cylinders.
  2. They can also withstand pressures of up to 4,000 pounds per square inch.
  3. Due to the incompressibility of fluids, a hydraulic actuator can maintain constant force and torque without the pump adding more fluid or pressure.
  4. Pumps and motors for hydraulic actuators are available.

Disadvantages of Hydraulic Actuators

  1. Fluid will leak from hydraulics. Loss of fluid, like pneumatic actuators, causes less efficiency and cleanliness issues, which can lead to damage to adjacent components and surroundings.
  2. A fluid reservoir, motor, pump, release valves, and heat exchangers, as well as noise reduction devices, are all required for hydraulic actuators.

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