1.2.1 Solenoids

 

• 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.

 

 

 

1.2.2 Hydraulic

 

• 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

 

1.2.3 Hydraulics

 

• 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

 

 

 

1.2.4 Electric

 

• DC servo motors

- very common

- 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.

 

1.2.5 Pneumatic

 

• 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.

 

 

1.2.6 Others

 

• Other types of actuators include,

- heaters

- lights

- sirens/horns