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07-01-2011, 11:17 PM
When one stream of fluid is permitted to impinge on another, direction of flow changes and the tendency of a fluid to strike the wall also changes. This concept gives rise to a new engineering system known as ‘fluidics. The term fluidics is the contraction of the words fluid and logic. Tremendous progress has been made in last twenty years in design and application of fluidic devices.
The current interest in fluidics for logic and control function was launched by the U.S Army’sHarry Diamond Laboratories. In March 1960 this laboratories invented the first fluid amplifier. This work was later expanded through a series of research and development contracts and the work reported in this section was sponsored by the U.S Airforce.The environmental capability of fluidic devices permits direct measurement of required control parameters within the engine.
These devices are more economical, faster and smaller than hydraulic control elements employing moving parts such as valves etc. Fluid devices have no moving parts hence they are more reliable and have long life. Fluidics is now offering an alternative to some other devices being operated with the help of electronics. It can operate where electronic devices are unsatisfactory, such as high temperature, humidity, in presence of severe vibrations, in high fire risk or where ionizing radiations are presents.
FLUID AMPLIFIERS.DOC (Size: 1.4 MB / Downloads: 216)
BASIC PRINCIPLE OF FLUID AMPLIFIER
Most popular application of fluidics is amplifiers. A fluid amplifier is basically a flat piece of metal or plastic in which shallow passages are engraved for the flow of fluid.
The arrangement of these passages differs for each type of fluid amplifiers, but in principle all the amplifiers have a main power stream, which changes its direction of control jet.
The tendency of fluid to chose one side of a symmetrically diverging channel and flow in an asymmetric way is called “Conda effect” in honor of the first man (in 1930’s) to observe and utilize the phenomenon. Subsequent work has shown that, if symmetrically placed splitter is very close to the entrance part of the main power jet, then this jet is divided equally in both channels.
(a) ORIGINAL FLOW (b) CONTROL JET
© CONTROL JET TURNED OFF
Fig 1. Flow in a diverging channel when the splitter is very close to the entrance in a diverging portion
If a control jet is activated on one side, this power jet is deflected to flow entirely to the opposite channel. After the control jet is turned off, the original divided flow occurs again.
(a) ORIGINAL FLOW (b) CONTROL JET ACTIVATED
© CONTROL JET TURNED OFF
Fig 2. Flow in a diverging channel when splitter is at a large distance
When splitter is moved to a distance more than five diameter from the power jet opening, the power jet prefers one of the two channels, if the control jet on this side is activated. This power jet will be deflected to another channel and remain flowing there even after the control jet is turned off.
The basic stability mechanism of the fluid amplifier can be explained by examining the stability of a submerged fluid jet (air jet into air or liquid into liquid) which is flowing between two adjacent symmetrical walls. Consider the first case where the two walls are located far from the jet and have practically no effect. The fluid which is accelerated and carried away by the jet is replaced by other fluid from the surrounding environment.
If the walls are brought somewhat close to the jet, the entrained fluid cannot be so easily replaced because the jet flow and the entrainment flow must now share a more limited flow area. As a result, more fluid is carried by the jet than the entrainment flow can supply, and then pressure in the vicinity of the jet exist becomes less than that of the surroundings until equilibrium is established. The difference between the pressure forces on either side of the jet tends to deflect the jet towards the reduced pressure region, further restricting the entrainment flow. The resulting secondary reduction in pressure results in continuing jet deflection until the jet becomes attached to the adjacent wall where stability is finally achieved.
Amplification function can be achieved by employing fluid amplifier. The amplifiers may be electronic or fluidic. Amplifier is a device which gives a large change in output of either pressure or both as a result of small change in control input. In other words we can say that amplifies its input signals.
Generally amplifiers are of following types.
1. Digital amplifiers
2. Analogue amplifiers
It is like an ON-OFF switch. In these amplifiers there are two outputs and the flow takes place either from one or another output depending upon where the control signal is present or not. In this flow is either completely from one output or completely from other .Digital amplifiers are more commonly used.
In analogue amplifier output varies in proportion to the control signals.
These are stable in any one of its two output states in presence or absence of an applied signal. The bistable amplifier operates on turbulent flow. According to the splitter position, these can be divided into three categories.
Fig 4. Effect of load and Splitter Position
(a) Splitter very close to the nozzle:
With the splitter very close to the nozzle, the main or power stream is divided and flow in equal amount of output. A control jet drives it to one wall or another. But on removal of this jet, the power stream is reversed back to its original position and exit through both the outlets.
(b) Splitter more than 5 nozzle width:
When the splitter is at a distance of more than 5 nozzle widths from the nozzle, the stream flows in the direction in which it once starts flowing due to external reasons even when this cause is turned off.
© Splitter at more than 10 nozzle width:
In this case the device is more stable. A blocked load diverted the jet from the output. It prefers to the opposite output, but it returns to its preferred output when the block is removed.
These amplifiers are commonly used in fluid logic circuits. In the turbulent
amplifier, a laminar jet from supply tube is direct at a collector or output tube.
A control jet is provided at right angle to the main power jet which changes the laminar flow into the turbulent flow and pressure recovery in the pressure tube is reduced. The turbulent amplifiers are operated at very low pressure and are generally uses air and require only about 0.1 m3 of air per hour and require very small power. It produces very less output pressure. For its use it is necessary to step the pressure with the help of step up relay or boosters.
Fig 5. Turbulent Amplifier
Fig 6. Turbulence Amplifier
Logic is the thought used to evolve the solution of a problem. Critical flow devices are also capable of giving a large change in output pressure or flow with a small change in input controle flow, and thus work as amplifiers. (Eg: Turbulent amplifiers) Digital devices are widely used and in performance potential are directly capable with electronic gates. The main logic gates are
Functions Input signal Output
1. NOR Neither input A nor B nor C nor D An output
2. NOT One input No output
3. OR Any input A or B or C or D An output
4. AND All inputs A and B and C and D An output
The main difference is that the speed of response is lower, but fluidic devices have the advantage that they do fit into environments where electronic devices are unsuitable. Electronics or fluidics offers minimiaturizataion which becomes increasingly favorable when more sophisticated control is required. Thus the choice of control depends primarily on the particular application concerned and any special requirements of the system. By connecting the fluid amplifiers in different manners, we can obtain various basic logic function and these can be used in timing, counting and sensing purposes.
1. Logical function NOR:
(a) Working Principle (b) Circuit
Fig 8. Logic State NOR
This logic state can be achieved by using only one turbulent amplifier. If input signal from neither A nor B nor C nor D is present, then output will be available.
2. Logical function NOT:
This logic function can also be achieved by employing one turbulent amplifier. If one input signal is used, the result is NOT i.e. No output will be available. This is also known as ‘signal inversion’.
3. Logical function OR:
This logic function can be achieved by two turbulent amplifiers. If an input signal either A or B or C or D is applied to turbulent amplifier X, then output from this amplifier will be off. As shown in figure this will cause the input signal E to turbulent amplifier T to be off. This will result in output from the amplifier.
Fig 10. Logic State OR
4. Logical function AND:
This function can be achieved by with the help of three turbulent amplifiers, where amplifiers X and Y both are connected to the amplifier Z. When either input signal A or input signal B is not applied, then output from either X or Y respectively will be available. (i.e. Either X or Y will be “on’’) which will sent either an input signal D to amplifier Z. This input signal C or D causes the turbulent amplifier Z to be off. There fore to get an output from this amplifier Z, both inputs A and B must be present so that both input C and D remain off.
APPLICATION OF FLUIDIC CIRCUITS:
Fluidic circuits are now being applied in several fields. For sensing, counting and timing functions in industrial field, turbulent amplifiers are used.
This technique is used in machine process and gauging and is based on logic state NOT. When a component is brought near to the sensing head, the output pressure from the sensing head is increased as shown in the pressure gauge located in the left of the circuit. This output is fed as an input signal to the turbulent amplifier (shown by NOT) which causes no output , as also indicated by the pressure gauge on the right hand side. Thus we can say that when the components (or moving probs for measuring the flatness of the surface) are closed to the sensing head, the pressure rise will “switch off” the output of the turbulent amplifier. When the component is away from the sensing head, pressure in the left gauge will be nil. (i.e. No input signal) and this will result in output from the turbulent amplifier and the pressure of which will be indicated by the right hand side pressure gauge. Thus the rise in pressure in sensing head due to the very near position of the component, ‘turn off’ the step up relay and end the machine operation.
To count the signals with fast power output NOT-NOR logical functions are used. Initial input signals are obtained from 2 ways (one from sensing head and another is from interruptible jet) When signal A is not applied to the sensing head, the NOT element (Turbulent amplifier) continuously. As a signal is applied to the NOR element, there will be no output from it and no signal can be supplied to the step up relay.
If signal A is applied to the sensing head (or some times also know as a pressure build-up head) the NOT element will be switching off. While with the vane revolving, signal B from the interruptible jet will on and off the input signal to the NOR element. This results in switching the NOR element output on and off. This output controls the step up relay which operates the counter to count the interruption in jet B from the vane revolutions.
This method is used to blow the whistle at pre adjusted time and employs NOT-AND logic functions.
Supply tubes are counted to the clock from the rear at 12 points (at 5 minute intervals). The output tubes are fixed in the clock glass exactly opposite to the supply tubes. The minute hand moved in the gap between these supply tubes and output tubes. These minute hands interrupt the passage of the jet.
Air is supplied to all the 12 supply tubes and to any one of the output tubes. The output tube is connected to the NOT element through a flexible pipe. In the normal position (when minute hand is not interrupting the jet) input signal remains on’ which results switching off the output. Thus the NOT element normally remains ‘off’ and it starts when the minute hand interrupts the air system.
When the lever valve is on position and supply from it is on, AND element will not operates until NOT elements also supplies the output as shown in the figure. Thus when both signals are supplied (i.e. at the point when minute hand interrupts the air system) the AND unit will operate the step up relay which then blow the whistle.
Thus the whistle can be blow at any preset time, by providing the interruptible jet at that particular position on the dial. Timing –with the minute hand interrupting the jet, output tube at 3 (at 15 minute position) and lower valve is in on position, AND function is complete, whistle is blowing.
1. These have long life, since there are no moving parts, hence no wear.
2. These do not have the effect of shock and vibrations.
3. It can operate at high temperature and humidity.
4. These amplifiers are unaffected by radiation and can be operated even in the nuclear reactor.
5. These are small and compact.
6. Fluidic circuits used for sensing are extremely simple and robust, where as electronic systems require complex and expensive transducers.
7. These are immune to corrosion.
8. The environmental condition required by electronic systems is expensive packing and the protection for the system is also expensive. Fluidic systems are more economical, since they do not requires protective devices.
1. High power consumption.
2. Slowness of response.
3. High pressure air or oil is to be made available.
1. Automobile engineering: - Use of fluidics in carburetion. It controls fluid flow and mixing. In modern automobiles wind shield wiper is controlled by fluidic air valves.
2 Aerospace applications: - Missile control, aircraft control and jet engine control.
3. Steel industry: - Severe environments, high temperature and dirty atmosphere, together with the low cost and robustness of air jet sensing systems make fluid control as a natural choice.
4. Nuclear application: - Neutron flux detector.
5. Industrial application:-Sewing machine control.
6. Marine application:-Turbine speed sensors.
7. Medical application: - Artificial heart and lung ventilators.
8. Domestic application: - Mining control for hot and cold water, vacuum cleaners.
According to fluid power experts there will be more use of fluidics, better material and product evolution in this century. Silicon chips can serve as microminature pipelines for processing fluids. Channels of this IC sensor chips are 40 micrometers in diameter. This will lead to more efficient system component and will be smaller, thus reduces machine size and weight. For example U.S armed force are studying ways of developing subminiature computers that used fluids such as mercury instead of electric current. These devices would be immune to the electron magnetic pulse and radiation effects that can destroy solid state electronic devices.
1. INTRODUCTION TO FLUID AMPLIFIER – Rossell. W. Henks
2. FLUID AMPLIFIER – Joseph. M. Krishner
3. A GUID TO FLUIDICS – Arthus Conway
4. FLUID AMPLIFIER OFFERS NEW INDUSTRIAL TOOL – Reilly
5. TURBULANT AMPLIFIER DESIGN & APPLICATION - Auger
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22-10-2011, 10:29 PM
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24-10-2011, 11:02 AM
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