New applications of fluidic technology

CPavlin P Facon

This paper describes three types of application of fluidics presently under development at Bertin Company. 1. A gas analyzer for medical purposes. This circuit regulates the concentration of anaesthetic gas in air breathed by the patient. Two solutions are discussed: one is a pure fluidic circuitry, the other is an hybrid fluido-electronic system. 2. A fluidic strain gauge for measuring deformations of plastic materials. A special construction minimizes ambient pressure sensitivity of the gauges. 3. A prototype of a powered exoskeleton. At the time of writing arm and shoulder have been realized. The motions of the human operator are detected by fluidic no-contact sensors which act on the exoskeleton through pneumatic muscles and cause the slave to assume the desired position.

Introduction

This paper describes three very different applications which use fluidics in whole or in part. The work is now continuing, but the main results obtained so far are discussed.

Gas analyzer In hospitals, gas mixtures suited for patient breathing are made using metering devices. These units have the disadvantage of delivering a mixture, the composition of which varies with the flow. This is especially undesirable for anaesthetic?. Research now in progress, financed by the French Delegation for Scientific and Technical Research*, is seeking to build an inexpensive regulator of gas mixtures. The unit continuously analyzes the mikture and opens or closes the mixing valve to maintain a constant concentration independent of the flow. As we know, one means of measuring the concentration of a gas calls for the use of a sonic oscillator, the frequency of which varies as a function of the concentrations in the mixture. This sensor has two basic features:

Fig 1. Fluidic analyzer

1. It is simple and inexpensive. 2. It is not specific to the gas being analyzed and can thus be used for various mixtures.

Following is a description of two units we have built and the first results obtained. The first unit uses exclusively pneumatic techniques, with a fluidic circuit for metering and a pneumatic circuit for control. In the second unit, o d y the sensor uses fluidics, with electronic signal processing and control performed by an electric stepping motor.

The fluidic regulator This unit only requires a single power source, compressed air or a vacuum very frequently available in operating rooms. It comprises a metering section delivering a pressure which is proportional to the concentration of the gas and a motordriven valve controlled by the difference between the setting and the measured value. Metering circuit

The circuit diagram appears in fig 1. It comprises two * In connection with INSERM, unitL.UI3 Dr Pocidalo (H8pital Claude Bernard, Paris)

Fig 2 Fluidic analyzer

- complete circuitry

fluidic oscillators (0).One is fed with the reference gas (frequency Fl ) and the other with the gas to be/analyzed (frequency F2).Before the mixer amplifier (M- A), which delivers beat frequency Fl - F2,isolation amplifiers (I-A) are placed which prevent coupling and feedback oscillation between the two oscillators. The rest consists of a pulseshaping and integration circuit. The output pressure is approximately proportional to the concentration of the mixture.

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The fluidic oscillators are miniature proportional amplifers using negative feedback. The base frequency is near 2 000 Hz and is highly stable. Thus, a variation of 10% in the supply pressure around the nominal pressure only causes a change of 0.1%in the frequency. In practice, the oscillator has a stability of 1 X id" at \constant temperature. The frnal accuracy also depends on the reproducibility of the calibrated pulses which depends on the constancy of the switching pressures of the flip-flops. At constant temperature, the short-term drift of the fluidic analyzer is between 0.1 and 1%. Fig 2 is a photograph of the fluidic circuitry.

Counter A

IClcsklhlCounterd u U

I

ri u

/I MemoryBI 4

NI-Na

Memory D

L

Control circuit

It would be ideal to develop a pneumatically-operatedvalve built into the mixer. To simplify the design, we retained the existing mixer and added a motor to operate the valve. We used standard devices found in industry, such as a rotary air-oil actuator and pneumatically-operated distributors. A fluidic circuit (fig 3) compares the pressure delivered by the metering circuit corresponding to the actual concentration with a reference pressure corresponding to the difference and controls rotation of the valve to make the composition of the mixture equal to the setting. An on-off drive is used. However, to avoid excessively long correction times and highamplitude sustained oscillations, a high approach speed is used for large differences and a low final speed near the value of the setting.

I

Fig 4. Fluidic

I

- electronic analyzer

analyzed, G, ,under the same temperature and pressure conditions as for gas G,. Then, counter C counts down to zero by the number N of clock cycles previously counted. At the same time, from is subtracted the N , cycles delivered by the fluidic oscillator. When counter C has returned to zero, a number N = N , - N , remains on the counter. Since:

8,

N , =NZ =NI

FI E;

FI

-

N2 F2

1 I

Pb

where d is the fraction of gas G3 in G2 by volume

I

I P=P3 PI PI

DI

-

Diaphragm Isolator Pa. Analyser output

C IComparator Pv. Piloted valve

Pb=Bias pressure

Fig 3. Fluidic analyzer - control circuit

Fluidic-electronic regulator

In this version, only one fluidic oscillator is used. It is fed alternately with the reference gas and the gas being analyzed. The pulsating signal is immediately converted into an electric signal by a fast-response, sensitive pressure transducer. Fig 4 shows the circuit diagram. At first, the oscillator is fed with reference gas G,,and it ascillates at frequency F, . The pulses are fed to counter A containing a futed initial number N , ;it counts down to zero. At the same time, a quartz oscilIator feeds No pulses into counter C. After this, the fluidic oscillator is fed with the gas being

Since N I is given and is known, the content6 of the mixture is simply proportional to the number N, - N , on the counter at the end of the cycle. We will not go into detail here concerning the electronic signal-processingcircuit which uses CMOS components. The pressure sensor is a Pitran transistor manufactured by Stow Laboratories Inc. To regulate a gas mixture, the power element is a reversible electric motor operated by the difference between the set value and the measured value. Comparison between the two types of analyzers

At the present time, the fluidic analyzer is less expensive to manufacture than the electronic version, but its performance is more limited. Short-term stability is of the order of 1% for the fluidic unit, because it is limited by the calibration of the pulses, whereas the fluidic-electronic unit has a stability of 1 X lU4(that of the fluidic oscillator). The advantage is even greater over the long term, for use of the same oscillator for the two gases means concentration is measured independently of frequency F, and variations in temperature and pressure are avoided along with possible changes in the oscillators due to natural ageing and fouling.

Fluidic strain gauges Fluidic strain gauges have already been studied, primarily at

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HDL by Drzewiecki. The paper' outlines the theory of the fluidic gauges and describes applications in load-cell systems which operate in hostile environments. It gives a possible threshold of 1 microstrain (1U6). The work outlined here concentrates on the measurement of large deformations in materials with a small modulus of elasticity such as plastics and rubbers. A design is described which eliminates the effects of outside pressure. Theory

Fluidic strain-gauge measurement uses the variation in pneumatic resistance of a small-section tube. This resistance is developed in a flexible material to permit large deformations. Fig 5 shows the diagram of the fluidic circuit. The variable output of the gauge is used to control a high-gain fluidic amplifier. Gauge

High gain

output signal

Adjustable bios

I

Outside pressure bar

7 -

i-

- 0.1

Fig 7. Sensitivity to outside pressure

I

Air supply

Fig 5. Fluidic strain gauge circuitry

Choice of materials and construction

When the gauge is used to measure the deformation of a material, the presence of the gauge must interfere as little as possible with the deformation of the material itself. For this purpose, the gauge must be made of a very flexible material with as small a modulus of elasticity as possible. It must also be easily glued to plastics. After testing different products, we selected a silicone elastomer which can be polymerized when cold (marketed by Khone Poulenc under the name RTV 1 1 1). The adhesive used to glue the gauge is CAF 1.

-_ Mould schema

1

I

.

I - - ,

1

I

_.

3

Ordinary gauge

Fig 8. Fluidic gauge

RTV 111 has the following characteristics: 1. Tensile strength, 17 bars 2. Elongation, 200% 3. Young's modulus of elasticity, 8.5 bars

The gauge is obtained by coating a calibrated wire with the elastomer. After polymerization, the wire is removed, leaving a capillary tube. This method provides good reproducibility of the characteristics. If the measurement is made in a variable-pressure chamber, this gauge is not suitable, for it is very sensitive to outside pressure. To correct this, one or more coil springs are inserted in the elastomer concentric with the tube (fig 6). Fig 7 shows the influence of the pressure on an ordinary gauge and gauges equipped with springs. Thus, as we can see the presence of a spring reduces the influence of outside pressure about twentyfold. Sensitivity to outside pressure is further reduced with two springs. Fig 8 is a view of the gauge. Performance

Gauge equipped with spring Fig 6 Gauge moulding

Elongation with an ordinary gauge causes reduction of the cross section of the tube and a resulting increase in its resistance. On the other hand, the opposite result is obtained when using a gauge with a spring. Under elongation, the coils of the spring separate, and the space between them

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is filled in part by the material outside and the material inside the spring. This increases the flow section of the gas and reduces the pneumatic resistance of the gauge. The gauge factor, in other words the ratio between the relative variation in resistance and the elongation, is in the neighbourhood of -3. The range of measurement is between -1 5 and +30% of the elongation. Reproducibility is better than 1%. The influence of the ambient pressure is negligible up to 7 bars. We wish to thank Sociiti Europ'eenne de Propulsion which financed this research and permitted us to discuss these results.

Pneumatic-powered exoskeleton* Arhplification of human effort is a difficult problem, for the structure of the exoskeleton must surround the members of the individual while not interfering with the amplitude of the movements and the number of degrees of freedom. The structure, of reduced volume, must be capable of withstanding and transmitting the efforts. The General Electric Company has attempted to build a complete exoskeleton to multiply the individual's efforts by a factor of 50. Although arm movement has been amplified satisfactorily using electrohydraulic methods, the problem of walking has yet to be solved satisfactorily because of the very large number of interdependent control loops and very loose coupling with the individuap In this study, we have tried to control each degree of freedom separately by making maximum use of the individual's capacity for adaptation.

r-----l

fl

output

u Fluidic strain gauge

Air stream penetration sensor

t

Fig 9. Fluidic displacement sensors

A

2

Neutral

I

I

0.5

1 Displacement

I

I

1.5

2

+

mm Fig 10. Air stream sensor characteristic

Choice of solution

The operator is surrounded by an external structure designed to reproduce his movements. For this purpose, sensors placed between the operator and structure measure the relative movements and control the actuators to reduce the difference. A certain amount of play is permitted between the individual and outer structure, and the relative movements are measured by the appropriate sensors placed in carefully chosen locations. To improve control stability and introduce simple and natural sensitivity to the effort, a connection is made between the individual and structure having a certain degree of stiffness. Actuation could be obtained by hydraulic cylinders or electric motors. However, a pneumatic design was chosen for the following reasons: 1. Easy adaptation of power to the load using air decompression. 2. Very easy installation of flexible actuators on sometimes complex structures. 3. Detection, amplification and actuation can be performed with a single power source.

used under traction, compression and shear conditions. Since there is no leakage to the outside air, the chance of collapse is minimal. At the beginning, in the experimental model we used air-stream penetration sensors which are easier to adjust. Their output pressures are higher, thus eliminating one stage of amplification. Fig 10 gives the sensor's response. There is a stable area in the curve in which small movements cause no change in output pressure. This eliminates any chance of oscillation at rest. The airstream penetration sensors are obtained by chemical cutting of stainless steel. Amplifidr

Each sensor operates a muscle by means of an amplifier. Given that the muscle feed pressure is 7 bars, we used a two-stage relay amplifier with moving parts having a total gain of 3000. This is a proportional amplifier capable of inflating the muscle in 0.3 s and deflating it in 0.6 s. When employing a strain gauge, it may be necessary to use a fluidic preamplifier. Pneumatic muscle

Description of components

Movement sensor

The movement sensor has a fixed part joined to the structure and a moving part actuated by the individual. Several types of sensors have been tested. The most interesting are the fluidic strain gauge and air-stream penetration sensor (fig 9). The strain gauge is similar to the one described earlier. It can easily be incorporated in the flexible part joining the individual and structure and can be Developed under a contract of Direction des Recherches et Moyens d'Essais (DAME)

Examination of a structure surrounding the arm as close as possible, capable of moving with two degrees of freedom around the elbow and three degrees of freedom around the shoulder, shows that the choice and installation of actuators are difficult. In particular, it is very difficult to cross rigid actuators or superpose them. We chose flexible actuators consisting of an inflatable envelope surrounded by a deflatable jacket (Mackibben type). The advantage of this type of muscle is that the entire lateral surface is active, such that at the beginning of travel it can exert far more effort than the product of its section and the pressure. Consequently, the effort is not

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Engineering in medicine @ IMechE 1976

assisted uniformly. At the limits of travel, movement remains free but is no longer assisted. Fig 11 shows the theoretical and actual chatacteristics of different types of muscles. Fig 12 shows the number of movements which can be performed by these muscles as a function of the feed pressure. It should be noted that, in practice, most of the movements are made under low pressure, pointing to very acceptable durability.

9F'

I

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I

I l l l l

I l l l l

I

I

I

I

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~

A L/L f i g 13. Arm and bracelet

Fig 11. Characteristicsof muscles

904 105 Number o f movements

106 Fig 14. Experimental arm

Fig 12. Durability of flexible muscle

Structure and location of components

At the present time, only actuation of the arm and shoulder have been achieved. Actuation of the legs is simpler, and the solutions may be based on those used for the arm. The structure of the experimental model consists of sections of articulated plastic tubes on which the muscles are mounted directly. Rotation is obtained using concentric tubes separated by needle bearings capable of withstanding shear action. The shoulder articulations raise a problem, for we must obtain the equivalent of an internal ball joint. Approximate movement is obtained by a double

cardan joint. In future models, extremely rigid lightweight structures will use composite materials (resins reinforced with glass or carbon fibres). The connections between the structure and individual are made from moulded elastomers. Fig 13 shows the bracelet used to detect arm movement (rotation and bending) at the wrist. Fig 14 shows the experimental arm. Performance

In this study, we have not sought to increase greatly the maximum efforts normally exerted by an individual but

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rather to allow him to perform the same movements without fatigue. The increase can be relatively large. In practice, it i s limited voluntarily to a factor of between 10 and 30 to avoid spontaneous oscillation of the structure around the stationary arm, The particular characteristics of the flexible muscle, which make the speed of contraction depend on the load as well as the presence of an antagonist muscle whose deflation through its power relay acts as a dashpot, contribute to the stability of the design. Thus, there is no need for a correction system, even when a large degree of inertia is encountered, Despite everything, for a moderate effort the inflation and deflation times of the muscles are sufficiently short to permit arm movemeni at virtually normal speed. Table 1 shows the travel for the different movements, along with the range of assistance of effort and duration of movement. Movement

Travel (degrees)

Apsistance pf effort (degrees)

Duration of movement ( sec )

Bending, extension

110

0 to 90

0.2 to 0.3

Pronewpine

120

0 to 90

0.4 to 0.5

AnteRulsion

90

0 to 70

0.2 to 0.3

Abduction

90

0 to 80

0.2 to 0.3

The value of the syStem comes from the great flexibility of a structure which is not specialized but capable of providing sustained effort for a long period of time without excessive fatigue for the operator. Potential applications include hostile environments, in which unforeseeable tasks need to be performed and which require the use of external protection. Such conditions are encountered: 1. in regions with climate extremes of intense heat and cold, 2. in certain industrial settings, 3. at sea, 4. in fires for emergency procedures, 5. in the nuclat field, including work inside reactors where residual radioactivity can persist.

Conclusions Each of the applications described uses fluidic sensors which are'noteworthy for their simplicity, durability and low cost. However, they do not always use fluidic processing circuits. This illustrates the ever-growing tendency to integrate different techniques and use them conjointly, for example, the combination of fluidic and electronic or pneumatic techniques. Use is made of the advantages of each to achieve the best possible design.

Table 1

References

Development prospects

This research has demonstrated that an arm can be moved with five degrees of freedom using a system with simple, inexpensive parts. An individual could be surrounded completely by a mobile structure serving as external protection which, when worn alone, would be an unbearable impediment.

1. Drzewiecki, T Fluidic strain gauges and their applications. Paper A1 presented at the First International Fluidics Applications Conference, Bratislava, CSSR pp 61-68. November 11-15, 1973. 2. Flck, B R Final report on Hardiman I - prototype for machine augmentation of human strenzth and endurance. General Electric Co, Report AD 139 735. August, 1971.

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New applications of fluidic technology.

New applications of fluidic technology CPavlin P Facon This paper describes three types of application of fluidics presently under development at Be...
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