In prior test rigs for imposing stress on aircraft wings to simulate flight conditions hydraulic jacks have bee used. Such jacks have been arranged in either of two ways: (1) an open loop system in which all jacks are controlled through a single valve,* or (2) a closed ^' loop system in which each jack is controlled independently through its own contro valve. The closed loop system is pi*eferred because it permits more accurate simulation of stress conditions, but it has suffered some instability. In particular, it has been found that one or more of the jacks may apply abnormal loading thereby creating an undesirable stress condition which may damage the wing(s) under test.

It is an object of the present invention to provide a fluid control circuit for controlling the pressure applied to an article by a plurality of fluid operable thrusting means having a fail safe response to application ^' of an abnormal pressure to the article.

According to the present invention there is provided a fluid control circuit for controlling the pressure applied to an article by a plurality of fluid operable thrusting means drivable from a source of pressurized driving fluid, the circuit including a fluid supply lin

for carrying driving fluid to and from each thrusting means and locking means operable in response to detection of an abnormal condition to interrupt fluid supply to or from each thrusting means by closing each fluid supply line to fluid flow and thereby locking the thrusting means in their respective conditions when the abnormal condition is detected.

Preferably, the locking means comprises a cutoff valve in each fluid supply line, each cutoff valve being operable to close the respective fluid supply line to fluid flow when the abnormal condition is detected. Each cutoff valve may be a pilot operable check valve operable to hold the respective fluid supply line open to fluid flow as long as a supply of pressurized pilot fluid is connected thereto. In this arrangement, each check valve is connecte to the supply of pressurized pilot fluid by a respective pilot supply line, the pilot supply line including a pilot control valve for selectively disconnecting the supply of pilot fluid from the associated check valve. The pilot control valve is operable, when it disconnects the supply of pilot fluid from the associated sheck valve, to open the pilot supply line between the pilot control valve and the check valve to a sump to thereby relieve the pressure of pilot fluid at the check ^" valve and cause the check valve to close the associated fluid supply line.

Each fluid supply line may be connected to a sump through a respective fluid dump line, each fluid dump line having a selectively operable dump valve therein which is operable to normally hold the dump line closed to fluid flow and to open the dump lines when pressure on the articl

is being relieved. One or more of the fluid dump lines may be provided with a bleed control valve operable to control the rate of fluid flow through the dump line when the dump valve is open. 5 The thrusting means may be drivable in one direction and be adapted to return, to their inoperative positions due to reaction of the article which is being worked upon. In this arrangement the fluid supply line would be a single line to each thrusting means. i0 However in the preferred, ^• more generally applicable arrangement, each thrusting means is positively drivable in both directions so that it can apply both compressive and tensile lo-ads to the article. In this case each thrusting means is connected to a respective input fluid supply line S and an output fluid supply line, the locking means being operable to close both input and output fluid supply lines to fluid flow when the abnormal condition is detected.

This latter arrangement using thrusting means drivable in both directions will now be further described with . 0 reference to the accompanying drawing which illustrates . an hydraulic circuit diagram for one thrusting means of a control circuit according to a preferred embodiment of the present invention.

The fluid control circuit illustrated is operable to ⁵ i control the pressure applied to an article by a plurality of fluid operable thrusting means 10 drivable from a source of pressurized driving fluid (not shown) . One thrusting means 10 is shown as an hydraulic jack 12 of generally known construction. The jack 12 is positively drivable in both directions for applying both compressive and tensile

loads to the article.

The circuit includes a fluid supply line 14 for carrying driving fluid to and from each thrusting means 10. Locking means 16 is operable in response to detection of an abnormal condition to interrupt fluid supply to or from each thrusting means 10 by closing each fluid supply line 14 to fluid flow and thereby locking the thrusting means 10 in their respective conditions when the abnormal conditio is detected. In the case of jack.12 which can be positively driven in both directions, the jack 12 is connected to an input fluid supply line 18 connected to one side of the jack piston and output fluid supply line 20 connected to the other side and the locking means 16 is operable to close both input and output fluid supply lines 18,20 to fluid- flow when the abnormal condition is detected.

Control means 22 is provided for selectively switching supply of driving fluid to the thrusting means 10. The control means 22 may comprise a control valve for simultaneously operating all the thrusting means 10. Howeve as illustrated the control means 22 preferably comprises a selectively operable control valve 23 for each thrusting means 10 and located in the respective fluid supply line 14. Each control valve 23 comprises a control servo valve having a valve member 24 with a supply position allowing supply of driving fluid to the respective thrusting means 10, a central cutoff position (as shown) in which the fluid supply line 14 is closed to fluid flow, and a drain position allowing draining of driving fluid from the thrusting means 10, the control servo valve being solenoid driven so as to allow selective changing of the position of the valve member

24 .

The valve member 24 is adapted to be positively driven by the solenoid(s) in either direction from its central cutoff position so that the valve 23 has a relatively rapid response to electrical control signals. The rapid response characteristic. ay be used to provide an oscillating load to the article. For example in the case of aircraft wings, an oscillating load at a frequency of up to 2. hertz may be applied to the wings to simulate flight conditions.

The locking means 16 comprises a cutoff valve 25,26 in each of the fluid supply lines 18,20 responsive to a predetermined fault condition to close the respective fluid supply line 18,20 to fluid flow and hold the thrusting means in its condition at the instant of detection of the fault condition. Each cutoff valve is a selectively operabl check valve. Each cutoff check valve is 'pilot operable check valve 27,28 operable to hold the associated fluid supply line 13,20 open to fluid flow as long as a supply of pressurized pilot fluid is connected to the check valve' 27,28. In this embodiment there is provided a pilot supply line 29 to each cutoff check valve 27,28 associated with the thrusting means 10. In the illustrated case where two check valves 27,28 are provided for the jack 12, the pilot supply line 29 is divided at a T-connection 30 to supply the two check valves 27,28. The pilot supply line 29 is provided with a pilot control valve 31 for selectively disconnecting the supply of pilot fluid from the cutoff check valves 27,28. The pilot control valve 31 is solenoid operated. As shown, the pilot control valve

is located in the common pilot supply line 29 and is arranged to maintain that line 29 open to the source of pressurized pilot fluid as long as the solenoid is energized and to close that line on interruption of power to the solenoid thus providing a fail safe mode of operatio Thus an electrical fault that interrupts power supply to th solenoid will result in closing the pilot supply line 29. When the pilot control valve 31 closes it is arranged to- open the pilot supply lines to the cutoff valves 27,28 to a sump 32 to relieve the pressure of pilot fluid at the cutoff valves 27,28 so that the cutoff valves 27,28 close the fluid supply lines 18,20 to and from the jack 12 and thereby hold the jack 12 in its condition at the time the pilot control valve 31 is operated.

In use the fluid control circuit may be coupled to sensing means (not shown) operative to detect-, the load applied to the article by each thrusting means 10. In the case of aircraft wing loading , the sensing means may include a plurality of transducers in the form of load cell The outputs of the load cells may be fed to an electrical monitoring circuit for monitoring the load at each point of the aircraft wing where the thrusting means 10 is operating, the monitoring circuit being operative to detect any loading above a predetermined level to activate the locking means 16, -which in the preferred embodiment includes the solenoid operated pilot control valves 31.

Each fluid supply line 18,20 is connected to a sump 35,36 through a respective fluid dump line 37,38 each fluid dump line 37,38 having a selectively operable dump valve 39,40 therein which is operable to nor allyhbld hedump line

32^ 8 closed to fluid flow and to open the dump lines 37,38 when pressure on the article is being relieved. Each fluid dump line 37,38 extends from a point between the cutoff valve

27,28 and control valve 23 to the sump 35,36. Each dump valve 39,40 is a pilot operated check dump valve 43,44 couple to a respective or, as_ illustrated, a common dump pilot suppl line 45. The pressure in the common dump supply line 45, and hence at all the dump valves 43,44 may be controlled by a central dump control valve (not shown) to selectively open and close the dump valves 43,44. During relief of pressure on the article the dump valves 39,40 are opened enabling pressure relieving^ flow from the pressurized side the jack piston and fluid take-up from the sump on the other side of the piston to prevent cavitation. in the fluid supply lines on the latter side. The central dump control valve* may be operated simultaneously with the pilot control valve

31 to relieve any pressure that may build up in the fluid supply lines 18,20 between the cutoff valves 27,23 and control valves 24. Also simultaneously ^' the control valves 24 may be operated to return to their cutoff conditions'.

Each fluid dump line 37,38 includes a bleed control valve

41,42 operable to control the rate of fluid flow through the dump line 37,38 when the dump valve 39,40 are open. The bleed control valve 41,42 loacted in the fluid dump line

37,38 are temperature and pressure compensating so that the rate of discharge of fluid through the bleed valves 41,42 may be controlled. For example, in the case of aircraft wing testing, different pressures may be exerted on the various jacks by the wings during pressure relief and it is desirable to be able to individually control or set the rate of

'discharge of each jack so that a wing is not damaged. Similarly fluid temperature can vary particularly when an oscillating load is applied and this can affect the viscosity of the fluid and hence its discharge rate.

Each fluid supply line 18,20 includes an auxiliary check valve 47,48 operable, when the associated dump line 37,38 is open, to close the fluid supply line 18,20 against leakage flow other than through the bleed control valve 41,42. In seme cases it* may occur that leakage flow through the con¬ trol valve 23 negates the setting of the bleed control valves 41,42. The auxiliary check valves 47,48 prevent such leakage flow and ensure that pressure relieving flow is controlled as desired by the bleed control valves 41,42. The auxiliary check valves 47, 43 as shown can be operated from the common dump supply line 45 so that the auxiliary check valves 47,48 close when the dump valves 43,44 open.

In operation of the illustrated fluid control circuit according to the preferred embodiment of the present invention when used for loading aircraft wings, the hydraulic jack 12 is connected to a source of pressurized■liquid throug the associated control valve 23. The control valve 23 is ope in response to an electrical signal from a central control location to commence the loading operation of the respective jacks and this may occur simultaneously for all jacks. Each jack 12 has associated therewith a sensing means (not shown) which is connected to the electrical monitoring circuit (not shown) so that the loaded condition of the jack 12 or stress on the wing can be monitored and controlled through the control valve 23. In the event of a failure in the electrical control circuit or hydraulic fault in any

jack or any other _. event to cause a jack to apply a ^■ abnormal load, the pilot control valves ^' 31 will operate to interrupt pilot fluid supply to the cutoff valves 25,26 in the fluid supply lines 18,19 of the jacks 12. The jacks 12 therefore will be held in the active or load applying condition. For example if one jack ."sticks" while all the other jacks are relieving pressure on the aircraft wing the detection of the abnormal stress at the sticking jack will initiate locki of all the jacks in the condition at the instant the fault is detected. The faulty jack may then be inspected and serv iced so that it will operate correctly. The pilot control valve associated with the faulty jack may be independently operated to ensure that that jack is then operating correctly. While the jacks are being held in their load applying condition any excess pressure in the circuit on the side of the cutoff valves 25,26 remote from the jacks 12 can be purged through the dump lines 37,38 through operation of the pilot operated dump valves 37,40.

If the operator decides that it is safe to unload a jack the pilot supply to the cutoff valves 25,26 is reconnected by energizing the solenoid of the pilot control valve 31. During relief of pressure on the aircraft wing the cutoff valves 25,26 in the supply lines 18,20 are still maintained open and the fluid dump valves 39,40 are opened so that pressure relieving flow to and from the respective sumps 35,36 can take place. The pilot control valve 31 will still be operative in response to detection of a fault condition to close the fluid supply lines 18,20 to pressure relieving flow so that the circuit can operate to protect the aircraft wing at all stages throughout the

the testing operation.