Taking a Fluid Approach

One of the most familiar techniques for teaching elementary electricity is to compare the flow of electrons through wires to the passage of fluids through pipes. The analogy is so valid that scientists are now changing it from a textbook explanation to practical application. They are building fluid circuits that supplement and even replace some electronic devices. By controlling and amplifying the flow of fluids (either gases or liquids), just as electron flow is controlled and amplified in electronic circuits, they have conjured up a variety of odd new fluidic devices that offer valuable improvements on their electronic counterparts.

Because they are unaffected by temperature extremes, radiation, vibration or shock--conditions that often damage or knock out electronic circuits--fluidic controls show their greatest promise in aerospace and defense work. The Army has already successfully tested a fluidic roll-rate control on its TIM (test instrumentation) missile; it is evaluating a fluidic navigational device developed by Martin Marietta Corp. for use by foot soldiers. Honeywell Inc. has developed and flown a fluidic autopilot. In less esoteric applications, the new technology is being used on New York Central locomotives, General Electric turbines and the machinery that manufactures Speidel watchbands.

Switches & Amplifiers. In place of the battery or generator that energizes an electronic circuit, fluidic devices use a continuous stream of fluid, usually air. The supply can come from a pump, from the hot gases of a jet engine, from air forced through nose vents in" missiles or airplanes, or even from a tank of compressed air. In a simple fluidic circuit, the power stream is fed into the base leg of a Ylike arrangement of tubes or channels. As the stream flows through the Y toward outlets at the end of two diverging arms, a fluid-flow phenomenon, called "the Coanda effect," causes the stream to attach itself to one side of the circuit and to flow out through only one of the arms.

A tir>y "control jet" of air, blown perpendicular to the power stream as it passes through the base leg, can force the stream to attach itself to the opposite side of the circuit. When that happens, the power stream flows out entirely through the other arm of the Y. A puff of the other control jet reverses the process, just as a small voltage change on the grid of a vacuum tube can control a relatively heavy flow of current through the tube's plate circuit. Like a vacuum tube, the fluidic circuit can thus be used as a switch to turn on or shut off a supply of power.

By etching or carving out a cavity at the point where the arms and base leg of a fluidic circuit meet (see diagram), fluidics engineers can prevent the power stream from clinging to either wall. Instead, it flows down the center of the Y and divides equally between the two outlets. In this "anti-Coanda" configuration, the application of a control jet merely deflects the power stream by an amount proportional to the intensity of the jet. As the output of the two legs varies with the strength of the control jet, the fluidic circuit is once more something of a vacuum tube. In effect, it is an amplifier, exaggerating with its power stream the fluctuations of its tiny control jet.

Computers & Rockets. Scientists have devised countless ways to make use of the controlled output of fluidic circuits. A fluidic guidance system can control the course of a torpedo by shooting out jets of gas or sucking in water. This distorts the surrounding boundary layer of water, changes its frictional effects and causes the torpedo to turn. In a rocket flying through the atmosphere, the control jets of a fluidic stabilization system are attached to vents in the rocket's nose cone. As the attitude of the rocket be gins to change, the nose vents gulp in air at different pressures, and those changing pressures control a small jet of hot gases shot at right angles into the rocket's exhaust. As the exhaust gases are deflected, they correct the rocket's attitude.

On an assembly-line conveyor belt, moving parts momentarily interrupt strategically placed jets of air shooting across the belt. The interrupted air jets, connected to the control jets of a fluidic circuit, cause power streams to flow and stop, opening and closing valves. The valves in turn activate automated pneumatic machines that process the passing parts.

Toys & Toothbrushes. Though the first promising fluidic circuits were developed only seven years ago, at what is now the Army's Harry Diamond Laboratories in Washington, scientists are fast catching up with electronic technology. They have already produced fluidic oscillators, memory and logic circuits, and have devised fluid versions of resistors and capacitors. They have also learned to etch fluid channels into small blocks of metal and plastic, producing fluidic versions of electronic integrated circuits [TIME, Sept. 2]. Though they are still no match in size for the microscopically small electronic I.C.s, several compact fluidic circuits can now be interconnected and fitted into durable and compact inch-square wafers.

For all the work "that has already been done, many fluidics problems remain. Scientists still do not fully understand some fluid-flow phenomena; fluidic circuits are still relatively cumbersome and are generally more expensive than their electronic counterparts. In addition, the speed of fluidic devices is limited by the maximum velocity of a pressure wave through the fluid--which is the relatively slow speed of sound. This places them at a distinct disadvantage in competition with electronic computers, which are limited in speed only by their size and the velocity of an electrical impulse--the speed of light.

Despite these shortcomings, many U.S. companies have established fluidics divisions. They are spending millions of dollars annually on fluidics research and development and will sell an estimated $30 million worth of fluidic products this year. One pioneering firm, Maryland's Bowles Engineering Corp., works entirely on the development of fluidic technology and systems. Appliance makers are developing washing machines and dishwashers with fluidic controls. Detroit auto manufacturers are considering a number of fluidic devices such as fluid amplifiers for gas turbine engines. Mattel Inc. is developing fluidic-controlled toys that will respond to sound, and General Time Corp. has been granted a patent on the first fluidic automatic toothbrush.

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