12. Actuators

• Actuators cause motion on command in engineered systems.

• There are a wide range of actuators. Some require simple on/off control while others require more sophisticated control.

• Some common actuators are,

Stepper motors

Servo motors

Hydraulic valves (to drive hydraulic systems)

Pneumatic valves (to drive Pneumatic systems)

Solenoids

Synchronous motors

Magnetic coils for attraction, or repulsion

etc.

• Most suppliers of actuators also sell drivers for their devices. These will amplify small signals that computers put out, to drive the devices, and these devices occasionally come with position (or equivalent) feedback abilities.

12.1 Discrete Actuators

• Outputs are sensitive to power spikes and might inadvertently turn on when there are transient voltage spikes. A resistor may need to be put in parallel with a load to ensure enough current is drawn to turn on the triac. The resistor size can be determined by

 

Relays

Transistors

BJT, FET, Darlington, etc.

Triacs

12.2 Types

12.2.1 Solenoids

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• 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.

 

12.2.2 Hydraulic

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• 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

12.2.3 Hydraulics

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• 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

 

12.2.4 Electric

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• 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.

12.2.5 Pneumatic

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• 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.

12.2.6 Others

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• Other types of actuators include,

heaters

lights

sirens/horns

12.3 Problems

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.

 

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