US3159168A

US3159168A - Pneumatic clock

Info

Publication number: US3159168A
Authority: US
Inventors: Trevor D Reader
Original assignee: Sperry Rand Corp
Current assignee: Sperry Corp
Priority date: 1962-02-16
Filing date: 1962-02-16
Publication date: 1964-12-01
Anticipated expiration: 1981-12-01
Also published as: GB1027817A; NL289079A; CH410484A

Description

Dec. 1, 1964 T. D. READER 3,159,163 I PNEUMATIC cLocx med Feb. 16, 1962 l 2 sheets-sheet 1 I mvENToR. Tnevon o. nemen BY Mam/d ATTORNEYS T. D- READER PNEUIIATIC CLOCK Dec. l, 1964 2 Sheets-Shoot 2 Filed Feb. 16. 1962 Tl 1 1 l um. @t

United States Patent O 3,159,168 PNEUMATIC CLOCK Trevor D. Reader, Wayne, Pa., assignor to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware t Filed Feb. 16, 1962, Ser. No. 173,694 15 Claims. (Cl. IS7-81.5)

This invention relates to a pure fluid device which is vcapable of generating pulses at predetermined intervals,

and more particularly, to a clock pulse generator whose mode of operation may be either symmetrical or asymmetrical without requiring any change in the physical dimensions of the device.

Pure fluid devices have become of increasing interest in those fields which require the characteristics of ruggedness and reliability, together with relatively small physical size and intermediate response times. These devices generally employ no moving parts other than a power stream of fluid which may be selectively switched into different output channels by other control fluid streams which interact with the power stream to be switched. Pure fluid amplifiers have been developed in which a control stream of relatively low energy is used to deflect a power stream of relatively high energy. By providing one or more channels to which the power stream may be directed by said control stream, the proportion of the power stream within an output channel may be varied in accordance with the value of the control stream so that a utilization device connected with the output channel may receive power far greater than that of a control stream, but proportional thereto.

Among other pure fluid devices which have been recently developed are monostable and bistable flip-flops, and free running multi-vibrators. In the bistable flip-flop, the power stream is deflected into one of -two output channels by a temporarily applied control stream pulse, after which said power stream is maintained in this output channel until a different control stream pulse subsequently switches it into the other output channel where its flow is also maintained.- This basic device may be converted into a free running multibrator by providing a negative feedback passageway from each of the output channels to a respective control stream orifice, so that the power stream flow through an output channel eventually results in a control stream issuing from an associated control orifice in such a fashion as to switch the power stream to the other output channel. Thus, the power stream switches back and forth between the output channels in cyclic fashion, where this frequency normally depends upon the finite length of the negative feedback passageways. Therefore, in order to change the frequency of the prior art free running multi-vibrator, there normally has to be a change in the physical dimension of the device such as changing the finite length of the feedback passageway.

The present invention provides a pure fluid oscillator for generating a train of pulses whose frequency may be varied without change in any of the physical dimensions of the device. In general, the invention provides means for maintaining each of the output channels at a quiescent pressure level which may or may not be the same for both channels. Upon deflecting the power stream into an output channel, the pressure therein begins to build up from the quiescent value to a particular higher threshold value. A negative feedback passageway is provided between the output channel and a control stream orifice to transmit the pressure in the output channel back to the orifice to eventually switch the power stream from this output channel to the other output channel. However, the sizeof the passageway and control orifice are such that a minimum threshold pressure must be attained in the 3,159,168 Patented Dec. 1, 1964 output channel before switching of the power stream is possible. The time required for this minimum threshold value to be reached after the power stream has begun to flow through the channel is generally dictated by the value of the quiescent pressure of the channel and the pressure of the power stream itself. Therefore, by merely changing the quiescent pressure maintainedin the output channel to a different value, the length of time that the power stream remains flowing in a channel can be varied.

It is therefore an object of the present invention. to provide a pure fluid device which operates in symmetric cyclic fashion and whose repetition rate can be changed by means other than changing the physical dimension of the device.

Another object of the present invention is to provide a pure fluid oscillator in which the value of a predetermined quiescent pressure in the output channels will generally determine the repetition rate of the device, there being means for selectively changing said quiescent pressure.

It is further within the province of this invention to provide means for allowing the pure fluid oscillator to have an asymmetrical mode of operation as opposed to the above described symmetrical mode. In the asymmetrical mode of operation, the quiescent pressure and/or the threshold pressure in each output channel may be adjusted to values different from those existing in the other channel such that a power stream remains longer in one output channel than the other. In this event, theintervals between adjacent output pulses from the device will not be equal.

It is yet another object of the present invention to provide a pure fluid oscillator having an asymmetrical mode of operation such that the intervals between the output pulses are not equal.

Yet another object of the present invention is to provide a pure fluid clock pulse generator which can be selectively operated to produceoutput pulses separated by equal or unequal intervals.

If it is possible to vary the pressure of the power stream itself according to the output channel through which it flows, then the device may be made to produce a train of output pulses alternate ones of which differ in magnitude from the others.

Therefore, another object of the present invention is to provide a pure fluid clock pulse generator for producing a train of pulses of unequal magnitudes.

These and other objects of the present invention will become apparent during the course of the following description which to be read in conjunction with the drawings, in which;

FIGURE la is a sectional plan view of one embodiment of the invention;

FIGURE lb is an elevation view of the device of lFIGURE la;

FIGURE 2a is a sectional plan view of another embodiment of the invention;

FIGURE 2b is an elevation view of the embodiment in FIGURE 2a; and

FIGURESa-Bg show various pressure wave form diagrams illustrating the various modes of operation of the present invention.

Referring first to FIGURES la and 1b, there is shown one embodiment of the present invention which utilizes the well known boundary layer effect for maintaining a power stream in an output channel until the threshold pressure in that channel is attained. Reference numeral 10 generally refers to the body of the device which contains an interconnected system of fluid ducts in the manner shown. Fluid may be introduced to the device via input channel 11 by means of a pump or compressor not shown in any of the figures. The entering fluid in channel 11 issues into a chamber 12 via a nozzle orifice 13. Chamber 12 is formed by the two converging outer walls 14 and 15 of the respective output channels generally indicated by 16 and 17. In the preferred embodiment each outer w'all 14 or 15 may have an acute change of slope in the region 18 or 19 such that the output channel cross section becomes larger until it is terminated by

end walls

20 and 21. However, it is not essential that such acute slope changes be provided, nor need the

output channels

16 and 17 be terminated within body 10.

Associated with each of

lthe output channels

16 or 17 is a respective feedback passageway 22 or 23 which connects its associated output channel with a respective one of control orifice nozzles 24 or 25 which in turn is placed in the side walls 14 or 15 of chamber l2. It will be noted that each wall 14 and 15 is offset with respect to the power stream orifice 13. This configuration, coupled with the presence of regions 18 and 19, aids the power stream to be locked on toa wall 14 or 15 of the output channel to which it is directed until the threshold pressure in said output channel is reached. This operation will be subsequently described in fuller detail.

Connected with each of the

output channels

16 or 17 is a

respective pressure source

26 or 27. These connections are effected by means of respective conduits 28 or `29 which terminate in -

respective ports

30 or 31 of output to cause switching of the power stream. Thus,

pressure sources

26 and 27 regulate the pulse cycle by either decelerating or accelerating the pressure build up in their associated output channels during the time that the power stream is directed therein.

Intermediate output channels

16 and 17 is a channel 32 lwhich exits from chamber 12 at the intersection of the inner walls of said output channels. As the power stream from orifice 13 switches from one output channel to the other, it temporarily iiows through channel 32. The output of channel 32 may be connected to any utilization device not shown which employs a train of clock pulses.

As mentioned before, the physical geometry of the device `shown in FIGURE '1 is such as to cause the power stream to maintain its flow within the output channel to which it is diverted even after the initial diverting signal has been terminated. For example, if the power stream from orifice 13 is diverted so as to flow into output channel 16, its velocity is such as to create a low pressure boundary layer region between it and the outside wall 14 of channel 16. The boundary layer phenomenon is well layer is destroyed, thus diverting the power stream back into channel 16. Each time that the power stream switches from

channel

16 or 17, or vice versa, it `temporarily flows into channel 32 from whence it may be directed to the utilization circuitry as a pulse output.

The condition under which the power stream boundary layer is disrupted is the following. Assume that the power stream has been kdiverted into output channel 16 .having a quiescent pressure determined by the pressure of source 26. As soon as the power stream commences to flow into channel 16, the pressure therein begins to rise due to the increasing volumeof fiuid trapped therein. The magnitude of the pressure in channel 16 determines the pressure in feedback passageway 22. Passageway 22 and control orifice 24 have physical dimensions such that upon a cer- 4 ,v v tain threshold pressure being reached in channel 16, the pressure transmitted via passageway 22 to orifice 24 is sufficient to disrupt the boundary layer region along wall 14. When this occurs, the power stream is diverted and changes its direction to now flow into output channel 17.l

Of course, as soon as the power stream is diverted away from output channel 16, the pressure therein begins to decrease toward its quiescent value so that control orifice 24 thereafter has no further effect upon the power stream. However, once the powerstream has been deflected into output channel 17, a bound-ary layer condition is created adjacent wall 15. Thus, the power stream remains locked in output channel 17 where it subsequently increases the pressure therein from the quiescent value toa threshold value suflicient to destroy the boundary layer region on wall 15. When this occurs, the power stream again begins to flow into output channel 16 to repeat the cycle.

The oper-ation of the clock ypulse 4generator in FIGURE l is graphically illustrated in FIGURE 3. The device may be symmetrically designed so that the same threshold pressure in each output channel is required to disrupt the boundary layer region along the respective side walls. Furthermore,

pressure sources

26 and 27 may be adjusted so that the quiescent pressures in the output channels are equal. This mode of operation is illustrated by FIGURE 3a where it is seen that the pressures in

channels

16 and 17, respectively, represented by t-he dotted and the dot-dash lines, vary between the same quiescent pressure value Q and the same threshold value T. For example, under the assumption that the .pressure of the power stream remains relatively constant no matter through which output channel it flows, it is seen that power stream flow into channel 16 raises the pressure from the quiescent value Q to the threshold value T in a finite llength of time. Upon the channel 16 pressure reaching this threshold value, the pressure now existing at control orifice 24 is sufficient to disrupt t-he boundary layer region adjacent wall 14. The power stream is thereforediverted away from output channel 16 and into channel 17, with a temporary flow of the power stream occurring in channel 32 du-ring this switching time. Thus, a clock pulse (shown by the solid line wave form in FIGURE 3a) is generated in channel 32.

Upon the power stream switching from channel 16 to channel 17, the pressure in channel 16 begins to rapidly drop toward its quiescent value Q, while the pressure in channel 17 begins to ascend from the same quiescent value Q to the same threshold value T. Upon the threshold value T being attained in channel 17, the pressure in passageway 23 and at control orifice 25 is such as to disperse the boundary layer region along wall 15 and so force the power stream back into channel 16. This switching from channel r17 to channel 16 also causes a -temporary flow of fluid in channel 32 to thereby generate another clock pulse. Thus, the times l1 and t2 lbetween adjacent clock pulses are equal, and the clock pulses themselves are of equal magnitude. This mode of operation 1s termed symmetrical" 'because of the equal time inter- /rals between the generated equal length clock pulses in the By varying the quiescent pressures maintained in the

output channels

16 and 17, the time interval t1 and t3 will also be changed. As an example, in FIGURE 3b equal threshold pressures and equal quiescent pressures are maintained in the output channels, but the quiescent pressure Q is of greater magnitude than the quiescent pressure Q in FIGURE 3a. Therefore, when the power stream is switched into one of the output channels, less time is required for the threshold pressure to be attained than was the case in FIGURE 3a. This modeof operation is also symmetrical in that there are equal time intervals between adjacent output equal length clock pulses; however, the frequency of the generated clock pulse train is greater in FIGURE 3b than in FIGURE 3a. This invention thereby provides means to vary the fre-

V channel

16 and 17.

quency of the oscillator by merely changing the quiescent pressure which is maintained in the output channels. This novel concept consequently permits the establishment of any frequency within a band of frequencies without changes in frequency being limited to discrete steps. No change in the physical dimension ofthe device need be effected. Therefore, this invention leads itself for use in a completely pure fluid system wherever a variable frequency oscillator need be included whose repetition rate must be changed in accordance with the pressure of some fluid signal. Such a fluid signal may be utilized to establish the quiescent pressure in the output channels. In FIGURE 1, therefore, it is obvious that a single quiescent pressure source may be provided for both

output channels

16 and 17 in the event that the quiescent pressurein each must be equal.

The geometry of output channels l16 and 17, lfeedback passageways 22 and 23, and control orifices 24 and 25 may also be designed so that a different thershold pressure is required in each output channel in order -to destroy the boundary layer regions. If this is the case, FIGURE 3c illustrates the situation where it is assumed that the URES 3a through 3f, the pressure of the power stream itself has been assumed to remain constant throughout the entire cycle. If the pressure of the power stream is assumed to vary, however, according to the output channel in which it is tiowing and/or upon the instantaneous value ofpressure in the output channel, then adjacent generated clock pulses may be of unequal magnitude. This is illustrated in FIGURE 3g. However, the quiescent pressures Q1 and Q1, as well as the threshold pressures t1 and t1, may still be designed so .that there may or may not be equal intervals of time between adjacent clock pulses, notwithstanding the fact that the pressure of the power stream varies.

FIGURE 2 is a slightly different embodiment of the present invention wherein a positive feedback channel is provided from each output channel in order to maintain the power stream in the output channel to which it is diverted until the threshold pressure is attained. In this configuration, the principle of momentum exchange besame quiescent pressure is maintained in each output However, a thershold pressure of magnitude t1 must -be attained in channel 16 as compared to a different higher threshold 'pressure t2 in channel 17. A longer time is now required for the pressure in channel 16 to reach its threshold value t1 than is required for the pressure in channel 17 to increase from the common quiescent value Q to its -threshold value t1. Thus, the intervals t1 and t2 between adjacent clock pulses are not equal because the .power stream spends unequal periods of time in the different output channels. This mode of operation may therefore be termed asymmetrical.

FIGURE 3d illustrates a case where .the threshold pressures are equal, but the quiescent pressure main-tained in one channel differs from that in the other. As an example, assume that the quiescent pressure maintained in channel 16 is of value Q1 while that maintained in channel 17 is of value Q2 where Q1 Q2. As shown in FIGURE 3d, a longer time is required for .the threshold 1 pressure to be attained inchamber 16 than is required for chamber 17. Therefore, intervals t1 and t2 between adjacent output clock pulses in channel 32 are unequal, thus defining the asymmetrical mode of operation.

FIGURE 3e illustrates a symmetrical mode of operation even though both threshold and quiescent pressures differ for each of the output channels. Assume that the quiescent pressure in chamber v16 is of value Q1 while that in channel 17 is of value Q2. Each of the output channels -and associated feedback passageways and control orifices are also assumed to bevconstructed so that a different threshold pressure t1 and t, is required to cause switching of the power stream from one channel to the other. However, threshold pressures t1 and t1 are designed, relative to quiescent pressures Q1 and Q1, so that the power stream is maintained for the same lengthrof .time in each of the

output channels

16 or 17. Thus, FIGURE 3e illustrates the situation where equal time intervals t1 and t2 `appear between adjacent generated clock pulses. On the other hand, threshold pressures i1 and t2 may alternatively be designed so that the asymmetrical mode of operation occurs, as illustrated by FIGURE 3f. In this case, and for the same pair of quiescent pressures Q1 and Q1 used in FIGURE 3e, the time intervals t1 and t1 between adjacent clock pulses are unequal. Since threshold pressures are normally determined by the physical geometry of the device and thus cannot be readily changed, the modes of operation illustrated in FIGURES 3e and 3f may also be obtained by selectively varying the quiescent pressures Q1 and Q1 so that the power stream does or does not spend an equal amount of time in each output channel.

In all of the previous examples as illustrated byFIG- tween a control stream and the power stream is utilized. Reference numeral 35 generally refers to a body of material having a connected system of fluid ducts therein, with an input power channel 36 being connected to a source of fluid for generating a power stream issuing forth from power nozzle orifice 37into chamber 38. Output channels 39 and 40 diverge from chamber 38, with a channel 41 being intermediate therebetween in order to detect the switching of the power stream from one channel 39 to the other 40, or vice versa.

Associated with each of the output channels 39 or 40 is a positive feedback channel 42 or 43, respectively. As shown -in FIGURE 2a, positive feedback passageway 42 opens into a side wall of output channel 39 and is connected back to a control orifice 44 on `the side of the power stream opposite to that facing output channel 39. In similar fashion, a positive feedback passageway 43 opens into output channel 40 and is connected to control orifice 45 on the side of the power stream opposite to that facing channel 40. When the power stream is diverted into channel 39, for example, a portion of its energy is fed back via passageway 42 and emerges from orifice 44 as a control stream in a direction tending to maintain the power stream in output channel 39. If the power stream is diverted into output channel 40, a portion of its energy is fed back via passage 43 to impinge upon the power stream in a direction to maintain the latter flowing into channel 40. This principle of momentum exchange is well known in the fluid amplifier art, and is merely another way of achieving the bistable characteristic.

As with FIGURE 1, a quiescent pressure is maintained in output channels 39 and 40 -by pressure sources connected to `these channels via

conduits

46 and 47 and

ports

48 and 49, respectively. When a power stream is diverted to flow into one of the output channels the pressure in that channel increases from its quiescent value and is fed back v-ia feedback passageway 50 or 51. These negative feedback passageways 50 or 51 terminate in control orifices 52 and 53, respectively, so that control streams issuing therefrom have a direction to impinge upon the power stream and defiect it into an output channel other that that from which the control stream originates. 52 therefore oppose e-ach other, as do the control streams from orifices 45 and 53. However, the threshold pressures in the output channels are such that a control stream from the orifice 52 or 53 will override the simultaneously appearing control stream from orifice 45 or 44, respec-v tively, so that the power stream is diverted from one output channel to the other. For example, assume that the power stream flows in output channel 39 so that the pressure therein increases from the quiescent value to the designed threshold value. As soon as the power stream is diverted in-to channel 39, a control stream issues from orifice 44 which is of sufficient energy to The control streams from orifices 44 and' maintain Ithe power stream in channel 39. As the pressure in channel 39 increases, however, the control stream from orifice 52 -also increases until eventually a threshold pressure in channel 39 is reached whereby the energy of the control stream from orifice 52 is greater lthan the energy of the control stream from orifice 44. In this case, the n et force applied tothe power stream in chamber 38 is in a direction to switch 'the power stream to channel 40. Upon being diverted to channel 40, the positive feedback control stream from orifice 45 maintains the power stream in this output channel until the design threshold pressure is reached, whereby the control stream from orifice 53 becomes greater in energy magnitude to force .the lpower stream back into `channel 39. Therefore, the embodiment of FIGURE 2 differs from that of FIGURE 1 only in the maner in which the power stream is maintained in an output channel until the threshold value is attained. Otherwise, the novel princples are the same, and the modes of operation illustrated by FIGURES 3a through 3g are as applicable to FIGURE 2 as they are to FIGURE l.

Although several preferred embodiments of the invention have been shown and described, it is believed tha-t modifications thereto may be apparent to persons skilled in the art which lie within the scope of the appended claims.

I claim:

1. In a pure fluid device of the type wherein a fluid power jet stream under pressure maint-ains its fiow in an output channel to which it is diverted, the combination comprising:

(a) first means for selectively maintaining the quiescent pressure in said output channel, which exists during the absence of said power stream therein, within a range of values below a certain threshold;

(b) second means temporarily acting to divert said power stream into said output channel where it thereafter remains to increase the pressure therein up to said threshold value within a finite time depending upon the values of said channel quiescent pressure and said power stream pressure; and

(c) third means responsive to said threshold value of pressure is said output channel for diverting said power stream away from entering sa-id output channel so that the pressure therein subsequently returns to its quiescent value.

2. The combination according to claim l wherein the fluid power jet streamis enabled by the physical geometry of the device yto create a boundary layer condition so as to lock on to a side wall of said -output channel upon being diverted thereto.

3. The combina-tion according to claim l wherein said third means comprises a control orifice located adjacent said power stream, and a feedback passageway connecting said control orifice with said output channel for permitting the threshold pressure in the latter to apply a force against said power stream and said first means comprises a source of pressure connected to said output channel.

4. The combination according to claim 3 wherein the fiu-id power jet stream is enabled by the physical geometry of the device to create a boundary layer so as to lock on to a side wallof said output channel upon being diverted exists during the absence of said power stream therein,

within a range of values below a first threshold; (b) second means for selectively maintaining a `second quiescent pressure in said second output channel,

which exists during the absence of said power stream therein, within a range of values below a. second threshold, where the diversion of said powcr stream into either one of said first or second output channels thereby increases the pressure in said output channel up to said first or second threshold value, respectively, within a finite time depending upon the values of said power stream pressure and said first or second quiescent pressure, respectively;

(c) third means responsive to said first threshold value of pressure in said first outputchannel for diverting said power stream into said second output channel; and

(d) fourth means responsive to said second threshold value of pressure in said second output channel for diverting said power stream into said first output channel.

6. The combination according to claim 5 wherein said first means comprises a first source of pressure connected to said first output channel, and said second means comprises a second source of pressure connected to said second output channel.

7. The combination according to claim 6 wherein said third means comprises a first control orifice located adjacent said power stream and a first feedback passageway connecting said first control orifice with said first output channel for permitting the first threshold of pressure in the latter to apply a force against said power stream, and said fourth means comprises a second control orifice located adjacent said power stream and a second feedback passageway connecting said second control orifice with said second output channel for permitting the second threshold pressure in the latter to apply a force against said power stream.

8. The combination according to claim 7 wherein said first and second threshold values are unequal.

9. The combination according to claim 7 wherein said first and second quiescent pressures are unequal.

10. In a pure fluid device of the type wherein a fiuid power jet stream under pressure maintains its flow in either one of first or second output channels to which it is diverted, the combination comprising:

(a) first means for selectively maintaining a first quiescent pressure in said first output channel, which exists during theabsence of said power stream therein, within a range of values below a first threshold;

(b) second means for selectively maintaining a second quiescent pressure in said second output channel, which exists during the absence of said power stream therein, within a range of values below a second threshold, where the diversion of said power stream into either one of'said first or second output channels thereby increasesthe pressure in said output channel up to said first or second threshold value, respectively, within a finite time depending upon the values of said power stream pressure and said first or second quiescent pressure, respectively;

(c) third means responsive to said first threshold value of pressure in saidv first output channel for diverting saidpower stream into said second output channel;

(d) fourth means responsive to said second threshold value of pressure in said vsecond output channel for diverting said power stream into said first output channel; and

(e) fifth means for receiving said power stream at the times at which the diverting of said power stream occurs.

ll. The combination according to claim l() wherein said first and second threshold values are unequal.

l2. The combination according to claim l0 wherein said first and second quiescent pressures are unequal.

13. The combination according to claim 10 wherein lsaid first means comprises a first source of pressure connected to said first output channel, said second means comprises a second source of pressure connected to said second output channel, said third means comprises a first control orifice located adjacent said power stream and a first feedback passageway connecting said first control orifice with said first output channel for permitting the first threshold pressure in the latter to apply a force against said power stream, said fourth means comprises a second control orifice located adjacent said power stream and a second feedback passageway connecting said second control orifice with said second output channel for permitting the second threshold pressure in the latter to apply a force against said control stream, and

said iifth means comprises a channel located intermediate 1 said first and second output channels into which said power stream temporarily flows upon said power stream being diverted from said first output channel to said second output channel, or vice versa.

14. The combination according to claim 13 wherein said first and second threshold values are unequal.

15. The combination according to claim 13 wherein said first and second quiescent pressures are unequal.

Hurvitz Sept. 26, 1961 Horton Mar. 13, 1962

Claims

1. IN A PURE FLUID DEVICE OF THE TYPE WHEREIN A FLUID POWER JET STREAM UNDER PRESSURE MAINTAINS ITS FLOW IN AN OUTPUT CHANNEL TO WHICH IT IS DIVERTED, THE COMBINATION COMPRISING: (A) FIRST MEANS FOR SELECTIVELY MAINTAINING THE QUIESCENT PRESSURE IN SAID OUTPUT CHANNEL, WHICH EXISTS DURING THE ABSENCE OF SAID POWER STREAM THEREIN, WITHIN A RANGE OF VALUES BELOW A CERTAIN THRESHOLD; (B) SECOND MEANS TEMPORARILY ACTING TO DIVERT SAID POWER STREAM INTO SAID OUTPUT CHANNEL WHERE IT THEREAFTER REMAINS TO INCREASE THE PRESSURE THEREIN UP TO SAID THRESHOLD VALUE WITHIN A FINITE TIME DEPENDING UPON THE VALUES OF SAID CHANNEL QUIESCENT PRESSURE AND SAID POWER STREAM PRESSURE; AND (C) THIRD MEANS RESPONSIVE TO SAID THERESHOLD VALUE OF PRESSURE IS SAID OUTPUT CHANNEL FOR DIVERTING SAID POWER STREAM AWAY FROM ENTERING SAID OUTPUT CHANNEL SO THAT THE PRESSURE THEREIN SUBSEQUENTLY RETURNS TO ITS QUIESCENT VALUE.