• These use a coil of wire to attract a ferrous core when actuated. When the coil is deenergized a spring will pull the core back out of the coil.
• These are particularly bad electrically. There is a large actuation current and when deenergized there will be a large inductive spike.
• A very popular application for solenoids is valves. These effectively use the solenoid to drive pistons that open/close ports on a cylinder.
• Very powerful and well suited to slower speeds with higher forces.
• Use of fluids, and high pressures can make this method awkward, messy, and noisy when improperly applied.
• Becoming less common in smaller force applications, as electrical systems mature
• Typical hydraulic systems use a simple solenoid valve to actuate devices, some newer developments use more sensitive devices
• A hydraulic actuation system is shown below
• Higher maximum accelerations that D.C. motors
• small time constants giving smooth operation
• time constant of hydraulic servo valve is about 5ms
• cylinders have small motions
• Incompressible fluids are used to transmit volume and pressure changes throughout a system.
• Pascal’s law basically describes these systems,
• Hydrostatic force/motion multiplier,
• The Hydrodynamic Effect - when fluid is moving quickly, it has high levels of kinetic energy. If the fluid impacts a surface, it transmits a high quantity of energy in a short period of time.
• Hydraulic Circuits typically contain,
3. A Pump to Move Oil, and Apply Pressure
5. Control Valves - to regulate fluid flow
6. Piston and Cylinder - to actuate external mechanisms
- well suited to feedback control systems
- good for low torque applications
- moves to exact positions, but all accuracy can be lost if the slip-torque is exceeded.
• good for limited position robots, it is not suited to partial actuation, either on or off.
• Some basic characteristics are,
- stroke from a few millimeters to meters in length (longer strokes have more springiness
- the actuators will give a bit
- pressures are typically up to 85psi above normal atmosphere
- the cylinder weight can be quite low
- additional equipment is required for a pressurized air supply- linear and rotatory actuators are available.
- dampers can be used to cushion impact at ends of cylinder travel.