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Introduction

A robot must be able to interact physicallywith the environment in which it is operating .

Actuators are the components of a robot thatenable it to affect the environment, say, by

exerting forces upon it or moving through it.

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Types. . . .

Electrical

Hydraulic

Pneumatic

Other

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Electrical Drives

The most common actuators in mobile robots.

Used both to provide location by poweringwheels or legs, and for manipulation byactuating robot arms.

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Types. . .

DC Motor

Stepper Motor

Servo Motor

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DC Motor

The working principle is theattraction and repulsion of magnets.

They are simple and cheap.

They are easy to control

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DC Motor Control

By controller and H-bridge .

PWM control

Speed control bycontrolling motorcurrent

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Stepper Motor

A stepper motor is a brushless electric motor.

It can divide a full rotation into a large numberof steps.

The motor s position can be controlledprecisely without any feedback mechanism.

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Characteristics

They are constant power devices.

As motor speed increases ,torque decreases.

They exhibit more vibration than other motortypes.

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Types . . .

Permanent magnet Stepper motor

Hybrid Synchronous Stepper motor

Variable Reluctance Stepper motor

Lavet type Stepper motor

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Contd . . .

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Advantages

Their control is directly compatible withdigital technology

They can be operated open loop bycounting steps, with an accuracy of s 1 step.

They can be used as holding devices, sincethey exhibit a high holding torque when therotor is stationary

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Servo Motor

A servo is an automatic device that uses error-sensing negative feedback to correct theperformance of a mechanism.

It has a positionable shaft that can bearranged in a number of angled positions via acoded signal.The position of the shaft changes as it receivesdifferent signals.

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Contd . . .

It has control circuit and potentiometer thatis connected to the output shaft.

The amount of power applied to the motor isproportional to the distance it needs totravel. So, if the shaft needs to turn a largedistance, the motor will run at full speed. If itneeds to turn only a small amount, the motorwill run at a slower speed. This is calledproportional control.

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Contd . . .

The control wire is used to communicate theangle. The angle is determined by theduration of a pulse that is applied to thecontrol wire. This is called Pulse CodedModulation.

Servo motors are used where precise controlis required with size constrain.

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Hydraulic Actuators

It s a cylinder or fluid motor that convertshydraulic power into useful mechanical work.

The mechanical motion produced may belinear ,rotary or oscillatory.

Operation exhibits high force capability , highpower per unit weight and volume , goodmechanical stiffness , and high dynamicresponse.

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Contd . . .

These features lead to wide use in precisioncontrol systems and in heavy-duty machinetools , mobile, marine and aerospaceapplications.

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2005/2006 I. 19

Hydraulic actuators: cylindersCylinder types:

Single acting:work can be done only in one direction

Piston

Double acting piston:

Piston rod on both sides

PlungerWork is done in both dir ections

Telescopic Telescopic Fast moving

Tandem

Fast moving

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2005/2006 I. 20

Rotary hydraulic actuators

Swivel vane rotary actuator:

Limited angle in both directions

Maximum angle always smaller than 3 60°

The same torque in both directions

Piston rotary actuator:

With rack and gear couplingHere maximum angle may be larger than 3 60°

Limited angle rotaryactuator

Limited angle rotaryactuator

Parallel

pistonrotaryactuator

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Disadvantages Efficiency of a volumetric hydraulic actuator is a little bit lower, than

efficiency of mechanical and electric transfers, and duringregulation it is reduced.

conditions of operation of a hydraulic actuator (temperature)influence its characteristics.

Efficiency of a hydraulic actuator is a little reduced in the process of exhaustion of its resource owing to the increase in backlashes andthe increase of outflow of liquid (falling of volumetric efficiency).

sensitivity to pollution of working liquid and necessity of highculture service.

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Pneumatic Actuator

A pneumatic actuator converts energy in the form of compressed air into motion.The motion can be rotary or linear, depending on thetype of actuator.Types

± Tie rod cylinders ± Rotary actuators ± Grippers ± Rodless actuators with magnetic linkage or rotary cylinders ±

Rodless actuators with mechanical linkage ± Pneumatic artificial muscles ± Speciality actuators that combine rotary and linear

motion frequently used for clamping operations ± Vacuum generators

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Contd . . .

A Pneumatic actuator mainlyconsists of a piston, a cylinder,and valves or ports. The pistonis covered by a diaphragm, orseal, which keeps the air in theupper portion of the cylinder,allowing air pressure to forcethe diaphragm downward,

moving the piston underneath,which in turn moves the valvestem, which is linked to theinternal parts of the actuator.

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Contd . . .

Pneumatic actuators may only have one spotfor a signal input, top or bottom, dependingon action required.

Valves require little pressure to operate andusually double or triple the input force. Thelarger the size of the piston, the larger theoutput pressure can be.

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Contd . . .

On 100 kPa input, you could lift a small car(upwards 1 ,000 lbs) easily.

Valves input pressure is the "control signal."This can come from a variety of measuringdevices, and each different pressure is adifferent set point for a valve. A typicalstandard signal is 20 100 kPa.

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Other Actuators

Piezoelectric

Magnetic

Ultrasonic

SMA(Shaped Memory Alloy)

Inertial

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Examples

Surgical Robot(pneumatic)

Military Robot(Hydraulic)

Military Robot(Electric)

Moira(pneumatic)

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