Introduction

An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which an agent acts upon an environment. The agent can be either an artificial intelligence agent or any other autonomous being (human, other animal, etc.).

Perhaps the most common type of actuator is powered by air and is called a pneumatic cylinder or air cylinder. This type of actuator is an air-tight cylinder, typically made from metal, that uses the stored energy of compressed air to move a piston when the air is released or uncompressed. These actuators are most commonly used in manufacturing and assembly processes. Grippers, which are used in robotics, use actuators that are driven by compressed air to work much like human fingers.

Many actuators have more than one type of power source. Solenoid valves, for example, can be powered by both air and electricity. Alternatively, a solenoid can be powered by both hydraulics and electricity.

What is an actuator?

An actuator is something that converts energy into motion. It also can be used to apply a force. An actuator typically is a mechanical device that takes energy — usually energy that is created by air, electricity or liquid — and converts it into some kind of motion. That motion can be in virtually any form, such as blocking, clamping or ejecting. Actuators typically are used in manufacturing or industrial applications and might be used in devices such as motors, pumps, switches and valves.

Electrical signal can be low power or high power. In case of low power signal, additional circuitry is required to drive the actuator otherwise it can be driven directly.

Actuators output can be position or rate i. e.linear displacement or velocity. Actuation can be from few microns to few meters.

Hydraulic actuator:

Hydraulic systems are used to control & transmit power.A pump driven by prime mover (electric motor) creates flow of fluid.

An actuator is used to convert the energy of the fluid back into mechanical power Amount of output power developed depends upon the flow rate, pressure drop across the actuator & its overall efficiency.

TYPES OF ACTUATORS

Linear actuator (hydraulic cylinder)-Provides motion in straight line-Linear displacement depends on stroke length (length of actuator)-Usually referred to as cylinders, rams (single acting cylinders) or jacks (cylinder used for lifting)

Rotary actuators (Hydraulic motors)-Produces continuous rotational motion-Pump shaft is rotated to generate flow, a motor shaft is causedto rotate by fluid being forced into the driving chambers

Semi rotary actuators

-Produces non-continuous rotational motion-Limited to less than one revolution (<360°)-Used to produce oscillatory motions in mechanisms

LINEAR HYDRAULIC ACTUATORS (CYLINDERS)Common Types#Single acting cylinder#Double acting cylinder#Displacement cylinder

Special Types1. Plunger or ram2. Telescoping.3. Cable4. Diaphragm5. Bellow6. Tandem7. Duplex8. Rotary actuators

Pneumatic actuator

It convert energy formed by compressed air at high pressure into ether linear or rotary motion.

The devices intended for transformation of potential and kinetic energy of the stream of compressed gas in mechanical energy of the output link that can be, for example, a rod of the piston, a shaft of the turbine or the case of the jet device is called pneumatic engines of the automated actuator.

All pneumatic actuators can be subdivided into the following types:

• diaphragm pneumatic actuators.

• pneumatic power cylinders.

• gas-engine pneumatic actuators.

• turbine pneumatic actuators.

• jet-stream pneumatic actuators.

• pneumomuscles.

• combined pneumatic actuators.

Advantages and disadvantages of Pneumatic Actuators.

Advantages:• Simplicity of realization relatively to small back and forth motions;• Sophisticated transfer mechanisms are not required;• Low cost;• High speed of moving;• ease at reversion movements;• Tolerance to overloads, up to a full stop;• High reliability of work;• Explosion and fire safety;• Ecological purity;• Ability to accumulation and transportation.

Disadvantages:• Compressibility of the air.• Impossibility to receive uniform and constant speed of the working bodies movement.• Difficulties in performance at slow speed.• Limited conditions - use of compressed air is beneficial up to the definite values of pressure.• Compressed air requires good preparation.

SINGLE ACTING CYLINDER

OPERATION OF SINGLE ACTING CYLINDER

• Produces linear motion in one direction.

• Consists of cylinder (barrel), piston, piston-rod (ram) & inlet port at piston end or blank end (other end is known as rod end).

• Cylinder is machined to high surface finish (honing).

• Fluid enters through inlet port into piston end or blank end –pressure build up-force generation on piston-movement of piston –EXTENSION or FORWARD STROKE.

• RETRACTION or RETURN by compression spring or under the influence of gravity (only in case of vertical mounting).

• Usage of seals –Piston seal (imparts clearance between piston & cylinder for clear movement), Rod seal or End seal (prevention of leakage from cylinder), Bearing (supports piston rod) & Wiper (prevention of dirt/dust entry into the cylinder).

•Drain hole helps in removing the leaked oil across piston seal.

•Widely used in hydraulic systems operations.

DOUBLE ACTING CYLINDER

• Produces linear motion in two directions.

• May be single rod ended or double rod ended.

• Piston is connected to smaller diameter piston rod.

• Fluid pressure acts on either side of piston alternatively.• Both sides of piston has oil ports.

• Parts of double acting cylinder.• Fluid enters through left port causing extension stroke while when it enters through right port causes retraction stroke, for present case.

• For a given pressure double acting cylinder (single rod type) exerts greater force when extending than when retracting.

CONSTRUCTIONAL FEATURES OF CYLINDER

• Five basic parts –Base cap & Bearing cap with port connections, a cylinder barrel, piston & piston rod.

• End caps are secured to barrel through welding or threaded connection.

• Smooth inner surface of barrel (seamless drawn tube) to prevent wear & leakage.

• Pistons (separates high & low pressure zones) are usually made of cast iron or steel.

• Function of End seal, bearing & wiper seal.

DISPLACEMENT CYLINDER

• Produces motion in one direction.

• No piston. Need no precision machining.

• Consist of cylinder, rod & end cap with bearing.

• Rod end inside cylinder is provided with collar while other end is connected to load.

• Collar prevents ejection of rod under fluid pressure.

• Amount of fluid entering in cylinder equals rod displacement from cylinder hence the name displacement cylinder.

• Maximum thrust exerted by the displacement cylinder is given by Maximum thrust = Pressure X rod area.

• Extend speed of the rod is given by Rod speed =Flow rate of fluid entering cylinder /Area of cylinder rod displacement.

TANDEM CYLINDER

• Also known as combination cylinder.

• Two separate pistons are mounted on same rod.

• Two double acting cylinders are connected in series.

• Suitable for higher force (2 cylinders –twice force) generation with smaller cylinders.

• High volume of oil is required to drive the cylinders.

TELESCOPIC CYLINDER

• Multiple cylinders mounted concentrically within one another.

• Suitable for longer strokes with shorter retraction.

• Operates on displacement principle.

• Stop rings limit the movement of each section.

• When the cylinder extends, all the sections move together until the outer section is prevented from further extension by its stop ring.

• Remaining sections continue out-stroking until the second outermost section reaches the limit of its stroke and so on until all sections are extended, the innermost one being last of all.

• For a given input flow rate, the speed of operation will increase in steps as each successive section reaches the end of its stroke.

• For a specific pressure the load lifting capacity reduces for each successive section.

• Example : high lift fork truck, tilting of truck dump bodies telescopic.

COUSHIONED CYLINDER

• End caps (base cap) have to withstand shock loads (fluid pressure or from kinetic energy of the moving parts) at extremes of piston travel

• Reduction of shock loads with the help of cushion valves build in end caps

• Exhaust fluid flow is unrestricted until plunger enters the cap.

• As plunger enters end cap port fluid experiences blockage, passes through deceleration valve (adjustable needle valve) which in turn reduces speed & the end of travel impact.

• Deceleration valve is adjustable to allow the deceleration rate to be set.

• A check valve is included in the end cap to bypass the deceleration valve & give near full flow as the cylinder extends.

Screw: lead screw, screw jack, ball screw and roller screw actuators all operate on the principle

of the simple machine known as the screw. By rotating the actuator's nut, the screw shaft moves in a line.

Wheel and axle:

Hoist, winch, rack and pinion, chain drive, belt drive, rigid chain and rigid belt actuators operate on the principle of the wheel and axle. A rotating wheel moves a cable, rack, chain or belt to produce linear motion. [1]

Cam: Cam actuators function on a principle similar to that of the wedge, but provide relatively

limited travel. As a wheel-like cam rotates, its eccentric shape provides thrust at the base of a shaft.

Electrical actuator

Electrically actuated system are very widely used in control system because easy interface with control system which are also electric and because electricity is easily available unlike fluid power which require pump and compressor .

The disadvantage of electric actuator’s Electric equipment is more hazard than other system unless made intrinsically safe, in

which case it become expansive.

Electric actuator have a poor torque-speed characteristic at low speed.

Electric actuator are all basically rotary motion and complicated mechanism needed to convert rotation into other form of motion.

There are three types of motor used in control system D.C. motor

A.C. motor

Stepper motor

Working Principle of motor

Every motor works on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.

Classification of Motors

AC Induction motor Components:

Stator: It carries three phase winding and is fed from 3-phase supply.Rotor: It carries winding which works as a secondary as in transformer.

AC Synchronous motor

Stator: It carries three phase winding and is fed from 3-phase supply.

Rotor: It carries permanent magnet poles that rotate exactly with same speed as that of the stator magnetic field, hence the name synchronous motor.

It is not self starting.

Stepper motorsA stepper motor is an electromechanical device which converts electrical pulses into discrete

mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.

Advantage:

The rotation angle of the motor is proportional to the input pulse. The motor has full torque at standstill (if the windings are energized) Precise positioning and repeatability of movement since good stepper motors have an

accuracy of 3 – 5% of a step and this error is non-cumulative from one step to the next. Excellent response to starting/ stopping/reversing. A wide range of rotational speeds can be realized as the speed is proportional to the

frequency of the input pulses.

Disadvantage:

Not easy to operate at extremely high speeds.

Types of stepper motor

Permanent magnet type Variable reluctance type Hybrid type

SOLENOIDS (Electric actuator)

Solenoids are the most common actuator components. The basic principle of operationis there is a moving ferrous core (a piston) that will move inside wire coil as shown in Figure. Normally the piston is held outside the coil by a spring. When a voltage is applied to the coil and current flows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of the coil. The piston can be used to supply a linear force

Applications

Pneumatic valve.

Car door openers.

Solenoids controlled valves

The flow of fluids and air can be controlled with solenoid controlled valves. Anexample of a solenoid controlled valve is shown in Figure. The solenoid is mounted on the side. When actuated it will drive the central spool left. The top of the valve body has two ports that will be connected to a device such as a hydraulic cylinder. The bottom of the valve body has a single pressure line in the center with two exhausts to the side. In the top drawing the power flows in through the center to the right hand cylinder port. The left hand cylinder port is allowed to exit through an exhaust port. In the bottom drawing the solenoid is in a new position and the pressure is now applied to the left hand port on the top, and the right hand port can exhaust. The symbols to the left of the figure show the schematic equivalent of the actual valve positions. Valves are also available that allow the valves to be blocked when unused.

Piezoelectric actuators (Electric actuator)

Pierre and Jacques Curie discovered the piezoelectric effect in 1880.

The application of an electric field to a piezoelectric crystal leads to a physical deformation of the crystal.

Piezoelectric materials are: Quartz, Ceramics, PZT(lead zirconate titanade).

Advantages

An ability to create high forces.

A high efficiency and a high mechanical durability.

short response time.

Disadvantage

Have small strains. (0.1-.2%)

High supply voltage needed.(60-1000V)

Large hysteresis. (actuator doesn’t go back to exactly where it started).

Types of piezoelectric actuators

1.Piezoelectric Stack Actuator

Perhaps the easiest way to produce a linear motion by the piezoelectric effect is to use astack actuator, which is a multilayer construction: each stack is composed of severalpiezoelectric layers, as depicted in Figure.

The required dimensions of the stack can be easily determined from the requirements of the application in question. The height is determined in respect to the desired movement and the cross–sectional area in respect to the desired force.

The main problem of the stack actuators is the relatively small strain (0,1 – 0,2 %) obtained. The movement can be increased by using for example levers or hydraulic amplifiers. It is noticeable, that in addition to the desired longitudinal movement some lateral movement typically also occurs. Therefore, a guiding has to be used if only longitudinal motion is desired.

Figure illustrates the deviations from the straight-line accuracy.

2. Linear Motors

Since the strain of the piezoelectric ceramics is relatively small, displacement amplifiers or hybrid structures are needed. There are many amplification techniques such as levers and hydraulic systems, and piezoelectric motors.

In the lever systems, the amplification is achieved with lever arms which magnify the displacement. The output force of the lever system is significantly smaller than the actuator force. Hydraulic systems use generally a piston for the amplification. The Micro- and Nanosystems Research Group has developed a hydraulic amplifier based on the use of bellows.

Figure: A schematic of the Figure: A 3D-CAD model of the Piezo hydraulic actuator Piezo hydraulic actuator

The linear steppers include an inchworm motor, a stick and slip actuator, and an impact drive motor. The ultrasonic motors can be divided into standing wave and traveling wave ultrasonic motors. In this paper, the operating principles of the inchworm motor, the stick & slip actuator and the traveling wave ultrasonic motor are described.

In inchworm motors, the linear movement is achieved by using three piezo elements. The operation principle is illustrated in above Figure.

3. Piezoelectric BendersPiezoelectric bending actuators (or piezoelectric cantilevers, or piezoelectric bimorphs)

bear a close resemblance to bimetallic benders. The application of an electric field across the two layers of the bender result in one layer to expand, while the other contracts. The net result is a curvature much greater than the length or thickness deformation of the individual layers. With a piezoelectric bender, relatively high displacements can be achieved, but at the cost of force and speed.

There are some benders that have only one piezoelectric layer on top of a metal layer(unimorph), but generally there are two piezoelectric layers and no metal (bimorph).

This way, the displacement is doubled in comparison to a single layer version. If the number of piezoelectric layers exceeds two, the bender is referred as a multilayer. With thinner piezo layers, a smaller voltage is required to produce the same electric field strength, and therefore, the benefit of the multilayer benders is their lower operating voltage.

Bimorph and multilayer benders can be built into one of the two types: a serial or parallelbender. In a serial bender, there are two piezoelectric layers with an anti-parallel polarization connected to each other, and two surface electrodes.

Other types of actuator:

Resistance heater Cartridge heater and Band heater are used with temperature sensors and temperature

controller to control the temperature in automated molding equipment and in soldering operation.

The resistive heating element 1 comprises a substrate 2 which serves as carrier material and which is substantially plate-shaped, e.g. of

graphite, and an electrically conducting layer 3 in the form of a, in the present embodiment, meandering conductor path

4 (shown with dash-dotted lines) e.g. of pyrolytic carbon. An insulating layer 5, e.g. of pyrolytic boron nitride is disposed between the substrate 2 and the conducting

layer 3 forming the conductor path 6 for protecting the conductor path 4 from external effects which also consists e.g. of

pyrolytic boron nitride. The ends of the conductor path 4 connecting to an outer side of the resistive heating element 1 serve as terminals.

7, 8 for applying an electric voltage.

Light

Lights are used on almost all machines to indicate the machine state and provide feedback to the operator.

Display devices are also actuator.

LEDWhenever we needed bright display we use LED an LED array for displaying number’s.

Gas plasma display

CRT

LCDLCD is low power consuming. It don’t generate light.It control the transmission of light.

Sirens/Horns –

Sirens or horns can be useful for unattended or dangerous machines to make conditions well known.