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I don’t think I ever finished these notes.
Actuation Systems
Actuation systems are the elements of control systems which are responsible for transforming the output of a control system (such as a microcontroller or microprocessor) into a controlling action on a machine or device.
Typical Hydraulic Power System
The pump pumps oil from a sump through a non return valve and an accumulator to the system, from which it returns to the sump.
Advantages and Disadvantages of Hydraulic Power Systems
Advantages:
- Able to generate extremely large forces from compact actuators
- Easy to control speed
- Easy to implement linear motion
Disadvantages:
- Large infrastructure (high pressure pump, tank, distribution lines)
- Potential fluid leaks
- Noisy operation
- Vibration
- Maintenance
- Characteristics of fluids change with temperature and moisture
Components of the System
Hydraulic Pump
This provides the pressure and flow of the liquid in the system.
Pressure Relief Valve
This is a safety system that safely let’s out the fluid of a pressurised system if the pressure exceeds a specified safe pressure.
Non-return Valve
Accumulator
The accumulator is a container in which the oil is held under pressure against an external force. This smoothes out any short term fluctuations in the output oil pressure of the pump.
- oil pressure rises → gas bladder contracts → more volume for oil to occupy → pressure reduces
- oil pressure reduces → gas bladder expands → less volume for oil to occupy → pressure increases
Types of Pumps
Gear Pump
A gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications.
They are also widely used in chemical installations to pump high viscosity fluids.
Vane Pump
A rotary vane pump is a positive displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity.
In some cases these vanes have have variable length and/or be tensioned to maintain contact with the walls as the pump rotates.
Radial Piston Pump
The working pistons extend in a radial direction symmetrically around the drive shaft to take in fluid and output it at the outlet port.
Axial Piston Pump with Wash Plate
An axial piston pump is a positive displacement pump that has a number of pistons in a circular array within a cylinder block. It can be used as a stand-alone pump, a hydraulic motor or an automotive air conditioning compressor.
Typical Pneumatic Power System
- (Filter, Silencer) The air inlet to the compressor is likely to be filtered silenced to reduce the noise level.
- An electric motor drives the compressor
- The pressure relief valve protects the system against rising above a safe level
- (Cooler, Water Trap) Since the compressor increases the temperature of the air, there may be a cooling system and filter/water trap to cool the air and remove contaminants from the system.
- An air receiver increases the volume of air in the system, to smooth out short term pressure fluctuations
Advantages and Disadvantages of a Pneumatic System
Advantages:
- Cheaper than electrical or hydraulic actuation
- Non-flammable so can use in harsh environments
- Simple to implement
- Controllable
Disadvantages:
- Does not produce a “stiff” system
- Requires a compressor, air conditioning, and control valves
- Pressurised air can be dangerous
Actuators
Actuators generate a Force or Moment. Both hydraulic and pneumatic actuators have the same principles, but differ in size. Cylinders are the principal actuators for pneumatics:
Terminology
Supply to compressed air to one chamber requires exhaust from the other chamber.
Positive Stroke
Extends and pushes to + position.
Negative Stroke
Retracts to - position.
Single Acting Linear Actuator
The control pressure is applied to one side of the piston.
When pressure is applied the piston moves along the cylinder.
When pressure is no long applied the piston reverts back to its initial position and the air is vented from the cylinder.
Double Acting Linear Actuator
Double Acting Linear Actuators are used by applying pressure to one of two sides to move a rod in one of two directions.
Rotary Actuator
These produce a rotary motion.
A linear cylinder can be used to produce rotary motion with angles less than 360 degrees with the correct linkages:
You can also have vane type semi rotary, where the difference in pressure causes rotation:
Control Valves
Pneumatic and hydraulic systems use directional control valves to direct the flow of fluid through a system.
They may be activated to switch the fluid flow direction by means of mechanical, electrical, or fluid signal pressure.
Valve Flow Symbols
Valve Actuation Symbols
Solenoid Operated Spool Valve
Spool Directional Control Valve
Move horizontally within the valve body to control flow:
Poppet Valve
This valve is normally closed.
Pressure Control Valve
Servo Valve
An electrohydraulic servo valve is an electrically operated valve that controls how hydraulic fluid is sent to an actuator.
Servo valves are often used to control powerful hydraulic cylinders with very small electrical signal.
Servo valves can provide precise control of position, velocity, pressure, and force with good post movement characteristics.
Process Control Valve
Used to control the fluid flow rate.
A common form of pneuma
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