Fluid power and control
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Advantages of hydraulic systems
- low weight/power ratio
- small sizes
- good controlability of performance requirement (pressure and flowrate)
- good dynamic response time
- power can be transmitted over reasonably long distances
- simplicity of linear motion (and long strokes)
- heat conduction directly from components and cooling of the fluids is easily achieved
- simplicity - relatively low cost of electrical interfacing and computer control
Disadvantages of hydraulic systems
- High energy consumption (although more thought given to component efficiency and energy saving techniques)
- Fluid maintenance - particle contamination for mineral oils, degradation of water-based fluids
- Noise radiation
- Fluid losses
Measure of the fluid's resistance to compressibility
Pressure relief valve (general)
- Must be placed at the output line of a postive displacement pump
- If the output line of the pump is shut off, the pump flowrate can only rapidly compress the oil in the line -> rapid pressure increase -> weakest mechanical point fractures -> severe damage
- Pressure controlled by balancing forces using spring retained elements which move under spring compression to provide a small clearance creating flowpath for fluid to escape
- Proportional to cost and power to be controlled
What direction is the fluid reaction force in?
The direction closing the orifice (because the pressure is smaller)
The over-centre valve allows flow passing through it freely via check valve built in, T -> P. This function is used for lifting the press head. In lowering, flow can only pass the valve P -> T when the pressure at P port can overcome the counterbalance set. However, if pilot control is used the counterbalance function will be removed, allowing longer press force and saving energy.
Describe the main causes of flow losses and torque losses in an axial piston pump
- Flow loss is cause by pressure difference at flowing areas, clearances between port plate and piston block, piston seal and cylinder bore, slipper and swashplate.
- Torque loss is cause by friction at above areas and the pump bearings.
Frequency response method
Define gain margin and phase margin
Describe the factors which affect the fluid pressure drop along a pipe and the factors which affect the pressure drop across a restrictor
Another way of stabilising the close-loop behaviour of a lightly damped servoactuator
A pressure difference feedback plus a high pass filter can be commisioned to stabilise the system
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