High sensitivity fluidic proximity detector

FIELD OF THE INVENTION ^όυ

This invention relates generally to fluidic controls, and particularly concerns a proximity detector of the fluidic type.

THE PRIOR ART ₃₅

The development of fluidic devices to perform sensing and control functions began quite a number of years ago, but has accelerated in recent years. As is generally known, fluidic techniques are applicable to a wide variety of applications, and have advantages over their electrical coun- ⁴⁰terparts in certain environments. For example, where it is necessary to avoid the possibility of sparking in an explosive atmosphere, or to sense the proximity of objects which do not have detectable electrical or magnetic properties, the use of electrical devices is contra-indicated. For ⁴⁵such applications, and many others as well, the fluidic approach is often chosen.

With particular reference to the problem of proximity detection, it has been known for some time that a conical jet device is particularly well adapted for this applica- ^ύtion. As seen for example in German Pat. No. 879,466 dated June 15, 1953, this type of device ejects an initially cylindrical stream of fluid which entrains air from the interior of the cylinder to create a low pressure zone _ggtherein. External air pressure causes the cylindrical stream to converge about the low pressure zone to form a generally conical configuration. When the conical jet is disturbed by the proximity of an object to be sensed, the pressure in the interior of the cone rises somewhat above ₆„ its previously reduced level. This pressure rise can be detected, by an appropriate means, to indicate the proximity of the object in question.

However, a difficulty arises with this type of device when it is used in an all-fluidic system. This difficulty is due _ggspecifically to the fact that fluid amplifier devices which are preferred for connection to a proximity detector of this type will not respond to a negative pressure (i.e., below ambient) signal. They require a positive (above ambient) pressure signal for their operation.

Attempts have been made in the design of such proximity devices to provide pressure outputs. But these at2 tempts have incurred a number of disadvantages, such as poor sensitivity and an inability to achieve sharp bistable operation. In addition, such devices are subject to inaccuracies and lack of repeatability introduced by hysteresis effects.

SUMMARY AND OBJECTS OF THE INVENTION

In terms, the object of this invention is to provide an improved fluidic proximity detector. More specifically, the objective is a proximity detector which provides a positive pressure signal, but which displays greater sensitivity, more positive response, and superior accuracy and repeatability.

The invention is carried out by providing a detector comprising a body having an annular orifice for ejecting an annular jet of fluid. Means of the body define an output port for connection to a response device, and further means connect the output port to the interior volume of the conical jet. Finally, means are provided which supply sufficient fluid to the interior volume of the conical jet to prevent drawing fluid away from the output port, and thus to avoid reducing the output pressure below ambient. Consequently, when the jet is disturbed by the proximity of an object, the output pressure rises above ambient and operates the response device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, with parts broken away and sectioned for clarity of illustratiton, of the proximity detector of this invention showing the fluid flow pattefn produced by the device during stand-by operation.

FIG. 2 is a longitudinal section of the same device showing the fluid flow pattern during the detection phase of its operation.

FIG. 3 is a front elevational view of the same detector, showing the annular orifice and pressure sensing opening thereof, and

FIG. 4 comprises graphs of output pressure versus proximity of the detected object for the device of this invention and a prior art device.

The same reference characters refer to the same elements throughout the several views of the drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An exemplary fluidic proximity detector 100 embodying this invention includes a generally cylindrical housing 102 having a generally cylindrical interior cavity. The cavty opens through the front face of the housing 102 to define an exterior orifice 104, and within the housing the cavity defines a supply chamber 106 through which a fluid under pressure is supplied to the orifice. A passage 108 extends radially through the wall of the housing 102 and receives a hose fitting 110 for the purpose of introducing the fluid into the supply conduit 106. Normally the fitting 110 is connected by suitable hoses to a pump supplying air under pressure, although there is no limitation on the nature of the fluid which can be used with this device.

The orifice 104 and supply conduit 106 both have an annular geometery by virtue of a connecting tube 112 which extends axially through the interior of the supply conduit 106 and emerges at the center of the orifice 104. The tube 112 is supported in this position by means of a plug body 114 which is received within, and seals off, the rear of the cavity in the housing 102. In the manufacture of the device 100, the tube 112 is most conveniently made integral with the plug body 114.

Plug body 114 is formed with an output port 116 within which is received a hose fitting 118 so that the port

3,545,256 can be connected to a suitable response device. In most cases the response device chosen for this purpose would be one of the various types of fluid amplifiers. The tube 112 and plug body 114 have a hollow interior 120 which provides a passage for connecting the output port 116 to a region 122 which is just outside the housing 102 and circularly bounded by the annular orifice 104.

In the operation of the detector 100, fluid is supplied under pressure through the hose fitting 110 and radial passage 108 to the supply conduit 106 and orifice 104, both of which annularly surround the connecting tube 112. As a result, an annularly shaped stream of air 130 issues from the entire circumference of the orifice 104. This stream 130 should preferably be turbulent in nature, i.e., have a uniform velocity cross-section. In order to promote such turbulence, the edges 132 and 134 of the annular orifice 104 are rounded.

As the jet emerges, it entrains nearby air molecules, an effect which creates a reduced pressure in the region 122 surrounded by the jet 130. Ambient air pressure externally surrounding the stream 130 causes it to deflect inwardly, and therefore to converge in the neighborhood of point 136 to form the generally conical configuration seen in FIG. 1. Consequently the jet 130 encloses a volume in the region 122, and effectively seals that volume off from the surrounding air. As the entrainment process continues, therefore, volume 122 becomes an isolated region of reduced air pressure.

The situation illustrated in FIG. 1 is the stand-by condition of the proximity detector 100, which occurs when there is no object within detection range. In contrast, FIG. 2 shows the situation which exists when an object 140 approaches within detectable proximity to the device 100. Under such circumstances the conical stream 130 is disrupted, to a degree dependent upon the distance between object 140 and the orifice 104. Such disruption causes some of the molecules of the conical jet 130 to be deflected into the enclosed volume 122, as indicated by the arrows at 142. The evacuating effect of the entrainment process is partly counterbalanced by the infusion of deflected molecules, and as a result the presence of the object 140 is reflected by a rise in pressure in the enclosed volume 122. However the higher pressure level which results under the circumstances depicted in FIG. 2 is still sub-ambient, unless the object 140 comes into actual contact with the orifice 104, blocking the escape of the jet 130 and terminating the entrainment process entirely.

The pressure rise in the enclosed volume 122 is communicated over the hollow interior 120 of connecting tube 112 and plug body 114 to the output port 116, and thus constitutes the output signal to the fluid amplifier or other responsive device. But since the pressure in volume 122 does not rise above ambient even when object 140 is in proximity, the output is not sufficient to switch any of the various positive pressure responsive types of fluid amplifiers which would be preferred for this type of application.

There are various reasons why a positive pressure responsive fluidic amplifier would be preferred. One of the popular types of fluid amplifiers at present is the flow mode device, in which the power stream is initially laminar but then is driven turbulent by a positive pressure signal pulse. This type of device responds only to a positive pressure signal.

Another frequently used fluid amplifier is the wall attachment device, which depends upon the Coanda effect. In this type the power stream can be pulled aside by a negtive pressure signal pulse as well as pushed by a positive pulse. But the negative pressure operating mode requires an opposing negative pressure pulse to be applied subsequently from the other side of the power stream to switch it back again, and that negative pressure switchback pulse causes problems because of its undesirable effect on the next fluidic stage. For these reasons it is highly desirable that a positive pressure responsive fluid ampli tier be connected to respond to the proximity detector 100. Accordingly, it is necessary to derive a positive pressure signal output from that device.

The prior art has experimented with a number of approaches to the problem of obtaining a positive pressure output from conical jet proximity detectors, but none of these has proved satisfactory. One solution is to isolate the fluid amplifier within a chamber where a local environment of reduced pressure is maintained. Then the output signal coming from the proximity detector, although subatmospheric, is positive relative to the level prevailing inside the isolation chamber. This approach is expensive because of the need for an isolation chamber and the additional apparatus necessary to maintain a reduced pressure therein. It also entails a high level of fluid power consumption.

Another approach is the provision of a small Pitot-type tube just outside the external boundary of the conical jet 130, which senses ambient pressure when the jet is undisturbed as in FIG. 1, but picks up fluid molecules scattered from the jet when the latter is disturbed and distorted as in FIG. 2, resulting in a positive pressure pulse which travels through the Pitot-type tube. The unsatisfactory results achieved with this approach are graphically illustrated in FIG. 4, where curve 150 shows the results of actual tests with this type of device. In this graph the ordinate Po is output pressure measured in inches of water, the zero level being ambient (local atmospheric) pressure. The abscissa is the sensing distance d in inches between the object 140 and the orifice 104 as depicted in FIG. 2.

Part 150.1 of curve 150 indicates that as the object 140 approaches the orifice 104, the output pressure Po rises above ambient at a distance d of about one inch. After a short rise, the curve assumes a rather gradual rise angle, then tops out at a pressure of about 3 inches, and subsequently tails off to a lower value. The rise from zero to maximum pressure level takes place over a fairly long approach range from one inch away to about %₀ of an inch away. Thus the device does not achieve a sharp digital switching characteristic from fully off to fully on within a narrow approach range. In addition, the sensitivity of the device is low since the maximum output pressure is only three inches of water.

Moreover, as indicated by the arrowheads 152, as the object 140 is withdrawn the output pressure declines along a different path 150.2. In other words, the prior art device in question has a hysteresis characteristic represented by the volume between the curves 150.1 and 150.2. This is disadvantageous because if the prior art device is retriggered after starting to retrace down the curve 150.2, . then the detection process begins from a point which is not on the normal rise curve 150.1. The result is an atypical triggering operation, which means that the device is not repeatable under all operating conditions.

To solve this problem in accordance with the present invention, a bleed hole 160 extends through the wall of the connecting tube 112 so that the annular supply conduit 106 communicates with thhe connecting tube interior 120. The dimensions of the bleed hole 160 are chosen in relation to the supply pressure existing in the conduit 106 so. that when the object 140 is out of detectable range the fluid flow equals and thus balances the rate of evacuation of air molecules from the enclosed volume 122 due to entrainment. On the other hand, the bleed hole 160 must provide enough Of a fluid restriction so that the pressure, in the enclosed volume 122 does not quite reach ambient, because that would reopen the converged conical jet 130 and interfere with the proximity detection function.

If the fluid flow rate through the bleed hole 160 is just sufficient to balance the entrainment losses under stand-by conditions, a dynamic equilibrium is maintained and the pressure inside the enclosed volume 122 remains

3.545.256

S near ambient. The enclosed volume 122 must draw fluid molecules from some source to replace those lost by entrainment. If it is allowed to draw those molecules from the opposite end of the connecting passageway 120, then it pulls a partial vacuum from the output port 116 and imparts a negative pressure to the fluid amplifier response device. According to the present invention it draws those molecules instead from the bleed hole 160 through the forward end of the connecting passage 120. As a result, a near ambient pressure level exists within the enclosed volume 122.

Ambient pressure prevails at the intersection of the bleed hole 160 and passage 120, the slight pressure difference being just sufficient to produce a fluid flow from the bleed hole 160 through the forward portion of passage 120 to volume 122, which numerically equals the entrainment rate. The pressure in the rear portion of the passage 120 and at the output port 116 is ambient, and this lack of a pressure differential means there is no fluid flow in this region under stand-by conditions.

Under detection conditions as illustrated in FIG. 2, once again the object 140 raises the pressure in the enclosed volume 122. But with the device of this invention, instead of increasing from a relatively low sub-ambient level to a somewhat higher sub-ambient level, the pressure in volume 122 increases from near ambient to above ambient. This pressure increase is transmitted along the connecting passageway 120 to the output port 116 to provide a positive (above ambient) pressure pulse for switching the responsive fluid amplifier device.

Looking at the detection situation from a fluid flow point of view rather than a pressure point of view, the flow through the bleed hole 160 is balanced by entrainment of molecules from the enclosed volume 122 only under stand-by conditions as illustrated in FIG, 1. Under proximity conditions as illustrated in FIG. 2, the flow through the bleed hole 160 is excessive, causing an aboveambient pressure condition at the intersection' of bleed hole 160 and passage 120, which diverts the excess flow along the rear portion of passage 120 and through the output port 116 to switch the responsive fluid amplifier device.

In FIG. 4 curve 170 provides an instructive comparison between the results achieved with the device of this invention and those of the prior art. Note that there is no response at all until the sensed object 140 comes within a distance d of about of an inch. Then, within a narrow approach range of between %o of an inch and fa of an inch the output pressure rises very suddenly and steeply to an output level exceeding the maximum of the prior art curve 150. At distances of less than^o of an inch, the output continues to rise steeply to levels approaching twice the maximum of the prior art curve 150. Accordingly, there is a much more distinct switch from off to on, and the device is also far more sensitive in that it supplies a much higher output for a given approach distance once the sensed object 140 comes within detection range. The return curve traced as the object 140 is withdrawn is identical with the triggering curve, i.e., there is no hysteresis, and no possibility of atypical operation upon sudden return of the sensed object.

It will therefore be appreciated that the present device provides a positive pressure output, which is preferable for operating various types of fluid amplifier devices, and does so with the additional advantages of greater detection sensitivity and more positive response. In addition, the device has no hysteresis effect, resulting in repeatability of operation under all conditions.

Since the foregoing description and drawings are merely illustrative, the scope of protection of the invention has been more stated in the following claims; and these should be liberally interpreted so as to obtain the benefit of all equivalents to which the invention is fairly entitled.