BJT, FET, Darlington, etc.
• 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
• rotary motions with gearing are most common
• 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,
1. Hydraulic Fluid
2. An Oil Reservoir
3. A Pump to Move Oil, and Apply Pressure
4. Pressure Lines
5. Control Valves: to regulate fluid flow
6. Piston and Cylinder: to actuate external mechanisms
• Oil Reservoir
• DC servo motors
well suited to feedback control systems
• Stepper motors
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.
• Other types of actuators include,
Problem 12.1 A piston is to be designed to exert an actuation force of 120 lbs on its extension stroke. The inside diameter of the piston is 2.0” and the ram diameter is 0.375”. What shop air pressure will be required to provide this actuation force? Use a safety factor of 1.3.