METHOD AND DEVICE FOR REGULATING A FLUID FLOW TO HYDRAULIC OR PNEUMATIC DRIVE MEANS

Abstract: With a method and device for regulating a fluid flow to hydraulic or pneumatic means a twin-stage servovalve device is used. This valve device comprises a primary stage (P) which is actuatable by a control signal to open and close a passage for a controlling fluid stream towards a spring biased valve poppet (L) in the secondary stage of the device in such a way that said valve poppet is positioned to set a flow aperture (O) for the regulated fluid flow in agreement with the control signal. An adjustable pulse generator (PG) is connected with the primary stage (P) and made to supply this stage with a control signal in the form of a sequence of pulses, the pulse frequency and/or the pulse width of which is adjustable in agreement with the fluid flow to be regulated, in such a manner that the primary stage (P) is actuated to open and close in pace with the supplied pulses and thereby set the flow aperture (O) of the secondary stage in accordance with the frequency and width of these pulses.

Description: Method and device for regulating a fluid flow to hydraulic or pneumatic drive means. The present invention is related to a method for regulating a fluid flow through a twin-stage servovalve device to hydrau¬ lic or pneumatic drive means in accordance with a control signal, and a corresponding valve device for this purpose. Prior art twin-stage servovalves of this type are e.g. described in US patent specifications Nos. 2.884.906 and 2.884.907 and comprise a primary stage having a directionally controlled nozzle, the angular position of which relative to the inlet ends of two identical control conduits is adjusted by an electromagnetic control element in agreement with intensity and direction of electric current supplied to the element. These control conduits terminate in individually associated ends of a cylinder, in which an elongated valve member is disposed with tight fit against the cylinder walls, but having a certain freedom of movement back and forth in the cylinder. The cylinder and the valve member constitute the second stage of the servovalve, as the movement of the valve member in the cylinder to a varying degree opens and closes the fluid passage through the valve, possibly along several alternative flow paths. The movement and position of the valve member in the cylinder is determined by the fluid flow through the two fluid con¬ duits to the respective extreme ends of the cylinder. In absence of electrical control signal to the primary stage the directionally controlled nozzle occupies a symmetrical position with respect to the inlet ends of the control conduits and equally large fluid streams are then supplied from the nozzle to the two control conduits. The valve member is then actuated by the same drive force at both ends and maintains an unchanged position. With a supplied control signal of any polarity the nozzle is correspondingly angularly displaced, and the fluid flow through one of the control conduits would then dominate and displace the valve member in its cylinder. The movement of the valve member is, however, fed back via a feedback link to the inlet ends of the control conduits, which thereby are displaced in the same direction as the directionally control¬ led nozzle. The displacement of the valve member as well as the inlet ends of the control conduits is then maintained until the control conduits again have reached a symmetric position with respect to the nozzle, and the valve member consequently again is actuated by the same drive force from both sides. Thus, in this position the displacement of the valve member is proportional to the displacement of the nozzle and then also to the supplied electric control current to the electro¬ magnetic control element in the primary stage. Known twin-stage servovalves of the above type may vary as to practical design, but are always based on the servotechnical principles mentioned above for the purpose of obtaining continuous agreement between control signal and valve position. Apart from being used in aircrafts such valve devices are utilized essentially in hydraulic and pneumatic robotsystems, for the regulation of fluid flows to and from the drive means of the robot in accordance with its programmed movements. As the servovalves in the latter application is subject to continuously changing adjustment, because not only the position of the valve member, but also its positional varia¬ tions at all times must be strictly in accordance with the values and variations of the control signal, the mechanical embodiments of such valve have to meet very high demands. Thus, it is required that the servovalves should operate in such a way that the regulated fluid flow always is propor- tional to the momentary value of the electric control signal. Furthermore, zero signal current to the valve should provide zero fluid flow through the same, whereas maximum signal current should result in maximum fluid flow through the open valve. Also, the polarity of the control signal should at all times correspond to the flow direction through the valve. Signal values between zero and maximum should further provide a regulated fluid flow which constitute the corresponding relative portion of the maximum fluid flow. The operational characteristics of the servovalve must be maintained approximately independent of oil temperature and wear. The above requirements are not satisfactorily met by the servovalves which now are used for robot operation, and in addition these valves are expensive in production and sensi¬ tive to wear due to the complicated mechanical design mentioned above. With these valves it is a common error that zero input signal does not provide zero fluid flow through the valve. This means that the servovalve must be driven out in one direction or the other to make it close completely. The servo system react to this and compensates automatically, but at the expense of the accuracy of the system as to positional adjustment. These relations also vary with the temperature of the working fluid and the ageing of the valve. Also, friction and re anent magnet fields in the primary stage constitute error sources. These have the same effect as the zero point displacement mentioned above, but are dependent of prior current applied to the electromagnetic control system. This error varies with time and is difficult to eliminate. Consequently, it is a main object of the invention to arrive at a method for regulation of a fluid flow through servovalve devices of the type mentioned above in such a way that the indicated deficiencies are overcome. Thus, the invention concerns a method for regulating a fluid flow through a twin-stage servovalve device to hydraulic or pneumatic drive means in accordance with a control signal in the form of a sequence of pulses, the pulse frequency- and/or pulse width of which are adjustable in agreement with the intended flow volume of the fluid flow to be regulated, the servovalve device comprising a primary stage actuable by the pulse sequence of the control signal to open and close in pace with the supplied pulses a passage for a controlling fluid stream toward a spring biased valve poppet in the secondary stage of the device, in such a manner that said valve poppet is positioned to set a flow aperture for the regulated fluid flow in agreement with the pulse adjustment of the control signal. In principle such a method is known from the Swedish Publi¬ cations Prints Nos. 359.621 and 409.250 and on this back¬ ground of prior art the unique features of the method according to the invention is that the controlling fluid stream is made to pass from a narrow inlet channel on the upstream side of the primary stage- through a pressure chamber located between the primary stage and the valve poppet of the secondary stage and further through a narrow outlet channel on the downstream side of the chamber. It is also an object of the invention to provide a twin-stage servovalve which is substantially easier and less cost- demanding to manufacture than prior art servovalve designs of the present type. Thus, the invention also realtes to a twin-stage servovalve device for regulating a fluid flow to hydraulic or pneumatic drive means in accordance with a control signal, said valve device comprising a primary stage operatively adapted to be actuated by the signal and a secondary stage disposed to be set by the primary stage, as well as an adjustable pulse generator which is connected with the primary stage and operatively adapted to supply this stage with a control signal in the form of a sequence of pulses having adjustable pulse frequency and/or pulse width, for opening and closing a passage for a controlling fluid stream towards a spring biased valve poppet in the secondary stage of the device in order thereby to set a flow aperture for the regulated fluid flow through the secondary stage in agreement with the pulse adjustment of the control signal. The unique feature of this device then resides in the fact that a pulse integrating pressure chamber is disposed between the primary stage and the valve poppet of the secondary stage for receiving the controlling fluid stream from a narrow inlet conduit on the upstream side of the primary stage, a narrow outlet conduit being disposed on the downstream side of the chamber for the discharge of the fluid stream from the pressure chamber. The primary stage preferably comprises an electromagnetically controlled nozzle operatively disposed to be opened and closed in pace with the supplied electric current pulses forming said control signal. The flow conduit for the controlling fluid stream is in such case conveniently provided from the upstream side to- the downstream side of the secondary stage. Obviously, in this case the controlling fluid stream is extremely small as compared with the fluid stream to be regulated. The invention is now described in further detail by means of an exemplified embodiment and with reference to the accompanying drawing, whereon. Figure 1 shows diagrammatically a plain embodiment of a twin- stage servovale device according to the invention. Figure 2 shows an ordinary connection of servovalves with drive means . Figure 1 shows a twin-stage servovalve device according to the invention and comprising a primary stage P and a secondary stage which is generally designated by S. The fluid flow to be regulated flows in throught an inlet Bl and out through an outlet B2. From the inlet Bl a small branch conduit Cl is tapped for conveying a controlling fluid stream to the primary stage P. The primary stage includes an electromagnetically controlled nozzle M, which is closed in currentless state, but is opened by supplying an electric current of appropriate intensity. For the control of the nozzle current pulses are applied from the pulse generator PG. The nozzle of the primary stage is then opened and closed in pace with these pulses, and during the open inter¬ vals control fluid flows from the branch conduit into a chamber K, which through a further narrow branch conduit C2 is connected with the outlet B2 of the servovalve device. The fluid volume in the chamber exerts a pressure against a valve poppet L, one side of which constitutes a movable wall T of the chamber. This wall is through a steam E connected with a valve disk V forming the movable part of a poppet valve having a valve seat N. This poppet valve V, N consti- tues the secondary stage of the valve device. The valve poppet L is biased by means of a helical spring F, which is disposed in an interspace between the insides of the piston wall T and the valve seat N, respectively. Thus, the fluid pressure in the chamber K in proportion to the spring force of the spring F determines to which extent the secondary stage of the valve device is open and conse¬ quently also the fluid volume which per time unit flows through the inlet Bl, the valve aperture O and the outlet B2. The pressure in the chamber K is in turn defined by the ratio of open intervals to closed intervals for the pulse control¬ led nozzle M of the primary stage. Consequently, this press¬ ure may be varied and regulated by adjusting and changing the pulse frequency and or the pulse width of the pulses supplied to the primary stage from the pulse generator PG. With low pulse frequency and small volume of the chamber K in propor¬ tion to the flow cross-section of the branch conduits Cl and C2, the pressure in the chamber K would fall approximately to zero between the pulses, and the valve poppet L would tend to move in pace with the flow pulses through the nozzle M. With higher pulse frequency and larger volume of the chamber K compared with the cross-section of the branch conduit, however, the chamber would tend to function as an integrator, in which the chamber pressure would not decrease abrubtly towards zero between the pulses, but continuously maintain a value corresponding to the mean value of the ratio of pulse width to interpulse interval for a certain number of pulses. Thus, in agreement with the operational conditions the work¬ ing parameters may be adjusted in such a manner that either a secondary stage to be opened or closed in pace with the sup¬ plied pulses to the primary stage, or a secondary stage being open to an extent which varies continously with the presented pulse frequency or pulse width of the pulse sequence supplied from the pulse generator PG, is achieved. In both cases the fluid flow through this stage from the inlet Bl through the valve aperture 0 to the outlet B2 would be essentially prop- portional to the pulse frequency or pulse width. A twin-stage servovalve device as indicated in Figure 1 has a far simpler mechanical design and conseπpaently higher opera¬ tional reliability than the prior art servovalve embodiments discussed above. Furthermore, servovalves of the present kind are not subject to zero-point errors of the type pre¬ vailing with the prior art designs, as the valve device according to the invention in all essential operates digitally with the advantages this implies as against the analogue control of the previously known devices. Figure 2 shows an ordinary connection of servovalves for regulated fluid supply to pneumatic or hydraulic drive means D. The shown drive means is constituted by a cylinder U having a piston R disposed for sliding movements in the cylinder and provided with a piston rod RS. The fluid conduits DB and DA open into the cylinder U at respective sides of the valve R. The drive cylinder is together with its supply conduits DB and DA arranged in bridge connection with four servovalves, as indicated in Figure 2. The bridge connection is on mutually opposite sides connected with a pressure source TK and a return conduit RL, respectively. For the movement of the piston R in the drive cylinder drive fluid must be supplied to the one side of the piston and discharged from the other side. For this purpose the servo¬ valves are opened in pairs, i.e. valves 1 and 3 at the same time for piston movement in the one direction and valves 2 and 4 for piston movement in the other direction. In both cases a drive fluid may then flow from the pressure source through one of the open valves towards one side of the piston, while a corresponding amount of fluid is discharged on the other side of the piston and transferred through the other open valve to the return conduit RL. The velocity of the piston is determined by the magnitude of the flow aperture of the open valves and consequently by the pulse frequency or pulse width when servovalves according to the invention are used. For operation ' of the servovalves in pairs according to Figure 2, two interconnected secondary stages may be associated with one and the same primary stage.

Claims: PATENT CLAIMS 1. Method for regulating a fluid flow through a twin-stage servovalve device to hydraulic or pneumatic driVe means in accordance with a control signal in the form of a sequence of pulses, the pulse frequency and/or pulse width of which are adjustable in agreement with the intended flow volume of the fluid flow to be regulated, the servovalve device comprising a primary stage (P) actuable by the pulse sequence of the control signal to open and close in pace with the supplied pulses a passage for a controlling fluid stream toward a spring biased valve poppet (L) in the secondary stage (S) of the device, in such a manner that said valve poppet is positioned to set a flow aperture (O) for the regulated fluid flow in agreement with the pulse adjustment of the control signal, c h a r a c t e r i z e d i n that the controlling fluid stream is made to pass from a narrow inlet channel (Cl) on the upstream side of the primary stage through a pressure chamber (K) located between the primary stage (P) and the valve poppet (L) of the secondary stage (S) and further through a narrow outlet channel (C2) on the downstream side of the chamber. 2. Method as claimed in claim 1, ' c h a r a c t e r i z e d i n that the controlling fluid stream is tapped as a branch stream (Cl, K, C2) from the fluid flow to be regulated and made to pass from the upstream (Bl) to the downstream (B2) side of the secondary stage. 3. A twin-stage servovalve device for regulating a fluid flow to hydraulic or pneumatic drive means in accordance with a control signal, said valve device comprising a primary stage (P) operatively adapted to be actuated by the signal and a secondary stage (S) disposed to be set by the primary stage, was well as an adjustable pulse generator (PG) which is connected with the primary stage (P) and operatively adapted to supply this stage with a control signal in the form of a sequence of pulses having adjustable pulse frequency and/or pulse width, for opening and closing a pass¬ age (M) for a controlling fluid stream towards a spring biased valve poppet (L) in the secondary stage (S) of the device in order thereby to set a flow aperture (O) for the regulated fluid flow through the secondary stage (S) in agreement with the pulse adjustment of the control signal, c h a r a c t e r i z e d i n that a pulse integrating pressure chamber (K) is disposed betweeen the primary stage (P) and the valve poppet (L) of the secondary stage for receiving the controlling fluid stream from a narrow inlet conduit (Cl) on the upstream side of the primary stage, a narrow outlet conduit (C2) being disposed on the downstream side of the chamber for the discharge of the fluid stream from the pressure chamber (K). 4. Servovalve device as claimed in claim 3, c h a r a c t e r i z e d i n that a flow conduit (Cl, K, C2) for the controlling fluid stream is provided from the upstream (Bl) to the downstream (B2) side of the secondary stage. 5. Servovalve device as claimed in claims 3 or 4, c h a r a c t e r i z e d i n that the primary stage (P) comprises an electromagnetically controlled nozzle (M) opera¬ tively disposed to open and close said passage in pace with supplied electric current pulses forming said control signal. 6. Servovalve device as claimed in claims 3-5, c h a r a c t e r i z e d i n that several secondary stages are mutually interconnected and controlled through one and the same common primary stage (P).

Inventors: ANFINDSEN OLE ARNT, ANFINDSEN, OLE, ARNT
Applicants: TRALLFA ROBOT AS, TRALLFA ROBOT A/S
Application year: 1986
Main classification: F15B9
IPC codes: F15B13, F16K31, G05D7