15.2 TYPES

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15.2.1 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.

15.2.2 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

15.2.3 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

15.2.4 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.

15.2.5 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.

15.2.6 Others

Other types of actuators include,

- heaters

- lights

- sirens/horns