GB2158971A - Digital servovalve structure and method - Google Patents

Abstract

A digital servovalve structure includes a spool valve 26, solenoid valves 16 or 18 for varying the pressure at the opposite ends of the spool valve to produce desired movement of the valve spool, position sensing structure 78 for sensing the position of the valve spool and digital electronic means including a central processing unit 82 and current driver amplifiers 84,86 for developing pulse width modulated signals for actuating the solenoids to position the valve spool in accordance with the difference between the sensed position of the valve spool and a desired position thereof. The method of the invention includes the steps of sensing the position of the valve spool of a spool valve, comparing the sensed position of the valve spool with a desired position thereof and pulsing solenoid(s) in accordance with a command signal representative of the difference between the sensed position and desired position of the spool valve to move the valve spool to the desired position thereof. <IMAGE>

Description

SPECIFICATION Digital servovalve structure and method The invention relates to an electrohydraulic servovalve and method and refers more specifically to an electrohydraulic servovalve including a valve spool, solenoid actuated valve means for positioning the valve spool in accordance with pulse width modulated electric command signals fed thereto, and means for sensing the actual position of the valve spool and developing the command signals for the solenoid actuated valve means in accordance with the difference between electric signals representing the sensed position of the valve spool and a desired position thereof and the corresponding method of controlling the position of the valve spool.

Most present electrohydraulic servovalves include a torque motor into which an analog electrical control signal is fed. In such servovalves, the electrical control signal is converted to a main spool valve position by a force motor and pilot valve. The force motor and pilot valve in most cases include what is known in the trade as a flapper and nozzle valve. An example of such valves is found in United States Patent No. 3,777,784.

Such servovalves require precision manufacturing and assembling of many small parts. Thus, such servovalves are difficult to manufacture, expensive and must be utilized carefully to prevent contamination thereof and/or damage thereto.

In accordance with the present invention, the torque motor of a previous electrohydraulic servovalve is replaced by one or more solenoids. The position of the valve spool of the servovalve is sensed by an electrical position transducer and a pulse width modulated command signal is produced in accordance with the difference between a desired position of the valve spool and the sensed position thereof and is used to actuate the solenoids and position the spool valve as desired.

The use of solenoids in place of a force motor offers a significant cost reduction and is better suited to accept digital pulse inputs from a microprocessor based controller or computer.

The following is a specific description intended to illustrate the invention, by way of example only, reference being made to the accompanying drawings in which: Figure 1 tis a partly schematic partly block diagram of digital servovalve structure constructed in accordance with the invention for effecting the digital servovalve method of the invention.

Figure 2 is a schematic representation of actuating fluid flow through the digital servovalve structure illustrated in Fig. 1.

Figure 3 is a graph plotting error volts and pulse width parameters of the digital servovalve structure illustrated in Fig. 1 which is useful in explaining the invention.

Figure 4 is a graph showing time and error signal relationship in digital servovale structure such as shown in Fig. 1, again useful in explaining the invention.

Figure 5 is a graph of time and solenoid current parameters of the digital servovalve structure shown in Fig. 1 also useful in explaining the invention and in particular showing the dual value current pulse width modulated command signals utilized in the digital servovalve structure shown in Fig. 1.

Figures 6-9 are similar to Fig. 2 and illustrate various possible modifications of the digital servovalve structure illustrated in Fig. 1.

The digital servovalve structure illustrated in Fig. 1 includes a spool valve 12, control electronics 14 and solenoid valves 1 6 and 1 8.

Spool valves such as spool valve 1 2 are well known and will therefore be considered only briefly. Spool valve 1 2 includes the outer cylindrical case 20 having closed ends 22 and 24.

Spool valve 1 2 further includes the cylindrical valve spool 26 having the two end surfaces 28 and 30. The valve spool 26 further includes the annular metering recesses 32,34 and 36 therearound spaced apart axially as shown in Fig. 1.

A valve actuating media such as hydraulic fluid is provided from a hydraulic fluid supply 38.

Hydraulic fluid is supplied to the spool valve 1 2 through conduit 40 which is as shown connected through the cylindrical outer case 20 into the metering recesses 32 and 36. A hydraulic fluid return conduit 42 is connected at one end through the cylindrical case 20 to the metering recess 34 and at the other end to the hydraulic fluid supply 38 again as shown in Fig.

1.

The hydraulic fluid is also supplied to a device controlled by the digital servovalve 10 through conduits 44 and 46 connected through the cylindrical case 20 between the metering recesses 32 and 34 and 34 and 36 respectively with the spool valve 1 2 in a centered or closed position as shown in Fig. 1.

As will be readily understood by those in the art, control fluid is metered to a controlle device through control conduits 44 and 46 either from the hydraulic fluid supply or retuned to the hydraulic fluid supply in accordance with the axial movement of the valve spool 26 within the cylindrical case 1 2. The position of the valve spool 26 in the cylindrical case 1 2 is determined by the pressure in the chambers 48 and 50 at the opposite ends of the valve spool 26.

In the digital servovalve structure 10 illustrated in Fig. 1, the pressure is varied in the chamber 50 through a hydraulic bridge circuit 52 including a branch 54 from the hydraulic fluid supply 38 having the parts 56 and 58 including restrictions 60 and 62 therein and with the chamber 50 of the spool valve 1 2 connected therebetween. The other branch 64 of the hydraulic bridge circuit 52 includes the parts 66 and 68 having the solenoid valves 1 6 and 18 therein as shown. The solenoid valve 1 6 is connected at one side to the hydraulic fluid supply 38 through conduit 40 while solenoid valve 1 8 is connected at one side to the hydraulic fluid return conduit 42.The other sides of the solenoid valves 1 6 and 18 are connected together and to the chamber 48 at the other end of the spool valve 1 2 as shown.

Over all actuation of the spool valve 1 2 on energizing of the solenoids 1 6 and 18 will be considered subsequently in conjunction with the operation of the control electronics 14.

Control electronics 14 include an electromechanical transducer 74 including as a sensing element the linear voltage differential transformer 76 which provides an electrical signal representative of the axial position of the valve spool 26.

As shown, the linear voltage differential transformer 76 includes a core 78 mechanically attached to the spool 26 for axial movement therewith and a coil 80 which is mounted in a stationary position with respect to the movable coil 78. The electromechanical transducer 74 is again a known device operable to provide an output electrical signal which is representative of the position of the valve spool 26.

The control electronics 14 further includes the central processing unit 82 and a pair of driver amplifiers 84 and 86.

The central processing unit 86 is an off the shelf purchased item which is programmable to accept command input signals which may be digital signals over conductor 88 representing desired positions of the valve spool 26 and to compare the electrical signals representing the actual position of the valve spool 26 which may also be digital signals with the command input signals and to provide to one or the other of the driver amplifiers 84 or 86 pulse width modulated command signals in accordance with the difference between the desired and actual positions of the valve spool 26 which pulse width modulated command signals are operable through the driver amplifiers 84 and 86 to actuate the solenoid valves 1 6 and 1 8 to produce movement of the valve spool 26 to a desired position thereof whereby a controlled metering of hydraulic fluid through the spool valve 1 2 is accomplished.

More specifically, the solenoid valves 1 6 and 1 8 are high speed solenoids and may include standard E laminates and coil forms. The armature of the solenoid operation, the solenoid valves are normally closed by spring force. Energizing the coil pulls the armature against a stop and opens, a ball and cone type valve.

The solenoid structure is a compromise between cost and performance. Alternate solenoid designs may include a laminated armature and stator which will provide the bet dynamic response, a single piece armature and stator which will provide the lowest cost, flapper and nozzle solenoid valve which will also lower the cost and a spool type valve which will permit higher operating pressure but is more complex. Other solenoid designs may be employed in the digital servovalve concept of the invention.

Similarly, while a linear voltage differential transformer 76 is shown as the valve spool position sensing element of the electromechanical transducer 74 the function of the electromechanical transducer 74 is to provide an indication of the spool position needed for closed loop flow control in the digital servovalve 10 and other sensing devices are optionally available.

Thus, a linear potentiometer may be substituted for the linear voltage differential transformer at a lower cost but will probably have a more limited life. A light sensitive potentiometer may be substituted for the linear voltage differential transformer and is a newer development which may have lower cost potential and possible direct digital output. Another possible substitution for the linear voltage differential transformer is a non-contacting radio frequency sensor which is a lower cost item and has a higher frequency response and will add no additional mass to the valve spool to inhibit valve response.

The central processing unit 82 may be any of a plurality of known units including a programmable microprocessor such as a Z80 microprocessor chip. This digital logic circuit compares a command input digital signal with the actual valve position feed back signal which may also be a digital signal and produces an error signal. Based on the error signal, the logic programmed into the central processing unit generates a command signal to driver amplifier 84 or 86 to open either solenoid valve 1 6 or 1 8 and thus produces a change in pressure in one of the chambers 48 and 50 to cause movement of the valve spool 26 to eliminate the error signal.

The driver amplifiers 84 and 86 convert the error signals from the central processing unit to solenoid actuating current. Driver amplifiers 84 and 86 may be simple on/off transistors amplifying central processing unit input power. However, better system performance is obtained by using driver stages having for example two current level operation as shown in Fig. 5.

Such driver amplifiers provide a higher current during the initial or pull-in time of the solenoids 1 6 and 18. The higher pull-in current speeds up the solenoid response and permits the use of smaller minimum valve open time valves thereby permitting improvement of both the static resolution and dynamic response of the servovalve 10 through increased pulse repetition rates. Other driver amplifiers such as two voltage level driver amplifiers may also be used, some of which provide slight reduction of performance at a lower cost.

With particular reference to the embodiment of the digital servovalve illustrated in Figs. 1 and 2, the operation is as follows. A pressure divider consisting of orifices 60 and 62 sets a fixed pressure in chamber 50 on the right side of the spool 26. When the valve spool 26 is not moving the pressure in chamber 50 is approximately one half the pressure from the hydraulic fluid supply 38 through conduit 40 minus the pressure of the return hydraulic fluid in the conduit 42 providing the orifices 60 and 62 are the same size.

With the solenoids valves 1 6 and 1 8 closed, the pressure in the chamber 48 at the left of the valve spool in Fig. 1 will become equal to the pressure in the chamber 50.

Pulsing solenoid 1 6 for a short time establishes a pressure increase in chamber 48 and a quantity of fluid entering the chamber 48 will move the valve spool 26 to the right.

Pulsing solenoid 1 8 for a short time establishes a pressure reduction in chamber 48 and the quantity of fluid moving out of 'chamber 48 will cause movement of the valve spool 26 to the left.

Accordingly, the motion of the valve spool 26 will not be continuous in the digital servovalve 10 with time but will take place in small increments. For a well performing digital servovalve these increments need to be small and the repetition rate at which the pulses may be applied should be high. Such operation may be ahieved by the use of the high speed solenoids 1 6 and 1 8 and the two current level driver amplifiers 84 and 86 in accordance with the above disclosure.

Spool motion will take place in accordance with the following basic equations:

= = A spool. A x AQ = t qdt o thus

in3 Fluid quantity displaced A spool in Area of spool driving surface Ax x in -Incremental motion of spool t sec Time solenoid is open (for average flow rate) q in sac Flow rate through solenoid Static performance, or the resolution of the digital servovalve 10 is defined by the smallest repeatable value of Ax, It may be seen that the incremental displacement Ax, relates to the minimum opening time of the valve combined with the flow rate through the solenoid and is inversely proportional to the spool driving area.High pulse repetition rate is also very desirable, and is essential for the dynamic response of the valve.

Referring more specifically to Fig. 3, the pulse width is described as a function of the error signal. Thus, at zero error signal neither driver amplifier 84 or 86 is pulsed. As the error signal increases to plus e0 a minimum pulse width of to is generated. The use of a minimum pulse width assures that the current is applied long enough to a solenoid valve to produce a reliable and repeatable minimum response from the solenoid valve.

The minimum pulse width may be less than the pulse width required to fully open the solenoid valve. For best system operation, it is found that in fact the minimum pulse width should be smaller than that required to produce full solenoid valve opening.

Reliable short pulses may open the solenoid valve only five to twenty percent. During this time, the flow through the valve does not reach its maximum and the resulting valve Ax becomes significantly smaller and is a nearly linear function of the error signal.

As the error signal becomes larger, the corrective pulse width becomes proportionally greater as shown in Fig. 5, until the pulse width t, as shown in Fig. 3 becomes the same as the pulse repetition interval 90 as shown in Fig. 4. At this point, the pulse stays on continuously.

In the current implementation, the length of pulse is established in the logic circuit before the pulse command is issued for processing. This is a somewhat simple but acceptable implementation. It is of course possible to terminate the pulse by a variety of different digital logic. Thus, as shown in Fig. 4, for example, in the central processing unit the pulse width is determined for the error signal recognized immediately proceeding each command signal to the solenoid valves through the driver amplifiers at the pulse repetition interval.

In overall operation of the digital servovalve 10 in accordance with the invention, with the solenoid valves 1 6 and 1 8 closed and witch the valve spool 26 of the spool valve 1 2 in a stable position whereby conduits 44 and 46 are closed, an input command conductor 88 is provided to the central processing unit 82 which input command may be in digital form from a computer or the like, and which may indicate a desired position for the spool 26 of the servovalve 1 2 to effect a flow through the conduits 44 and 46 to perform a desired operation.

The current position of the valve spool 26 is sensed through the electromechanical transducer 74 including the linear voltage differential transformer 76 and a signal which may be a digital signal may be provided to the central processing unit 82 from the electromechanical transducer 74 representing the current position of the valve spool 26.

The command input signal and the position indicating signal are then compared in the central processing unit and an error signal which may be positive or negative is developed by the central processing unit and a pulse width for the drive pulses selected. The pulse width modulated command signal is then provided to the dual current driver amplifier 84 and 86 in accordance with the polarity of the error signal to pulse the solenoid valve 1 6 or 1 8 to effect the desired movement of the spool valve 26 right or left as shown in Fig. 1 in a direction so as to ultimately reduce the error signal as determined by the central processing unit to zero.

As shown in Fig. 4, in the preferred embodiment illustrated, a pulse width modulation decision is made in the central processing unit immediately prior to each command signal at the operating pulse repetition rate of the digital control valve 10.

While one embodiment of the digital servovalve structure and method of the invention and modifications thereof has been disclosed in detail it will be understood that other embodiments and modifications of the digital servovalve structure and method of the invention are contemplated by the inventor.

Thus, in the preferred embodiment of the digital servovalve disclosed above, the two solenoids are placed on the same end of the spool valve. The digital servovalve of the invention may also be implemented by placing one solenoid valve on each end of the servovalve. Also, the digital servovalve of the invention may be implemented using only a single solenoid.

Thus, for example in Fig. 6, wherein the same elements have been given the same reference numerals as in Figs. 1 and 2, the solenoid valves are placed between the ends of the spool valve and the hydraulic fluid return while in Fig. 7, the solenoids are placed between the hydraulic fluid supply and the opposite ends of the valve spool. In Fig. 8, only one solenoid is utilized in the conduit between one end of the spool valve and the hydraulic fluid return while in Fig. 9 only one solenoid is used between the hydraulic fluid supply and one end of the spool valve. In both of the embodiments of the invention, of Figs. 8 and 9, an additional fixed orifice 92 is substituted for the second solenoid.

Further, it will be understood that other modifications may include other central processing unit control logic and possible feed back from other sources for example from the process output in place of spool valve position.

Thus, for example, it would be possible to feed hyudraulic fluid under pressure to the spool valve 1 2 through conduit 42 and receive returning hydraulic fluid through conduit 40.

It is the intention to include all such modifications and embodiments of the invention as are defined by the appended claims within the scope of the invention.

Claims (17)

1. Digita! servovalve structure comprising a spool valve including a valve spool, a hydraulic fluid supply and return for supplying operating hydraulic fluid to and exhausting operating fluid from the spool valve connected to the spool valve and digital electronic control means connected to the means for supplying operating hydraulic fluid to and exhausting operating fluid from the spool valve for varying at least one of the hydraulic fluid supply to and exhaust from the spool valve to vary the position of the valve spool.

2. Structure as set forth in claim 1, wherein the means for supplying operating hydraulic fluid to and exhausting operating fluid from the spool valve includes passages forming a two branch, four part hydraulic bridge circuit between a hudraulic fluid supply and a hydraulic fluid return with opposite ends of the valve spool connected to the bridge circuit at points between the hydraulic fluid supply and return.

3. Structure as set forth in claim 2, including fixed orifices in one branch of the bridge circuit between the hydraulic fluid supply and one end of the valve spool and between the one end of the valve spool and the hydraulic fluid return and solenoid valves between the hydraulic fluid supply and the other end of the valve spool and between the other end of the valve spool and the hydraulic fluid return.

4. Structure as set forth in claim 2, including a fixed orifice between the hydraulic fluid supply and one end of the valve spool and a solenoid valve between the one end of the spool valve and the hydraulic fluid return and a fixed orifice between the hydraulic fluid supply and the other end of the spool valve and a solenoid valve between the other end of the spool valve and the hydraulic fluid return.

5. Structure as set forth in claim 2, including a solenoid valve between the hydraulic fluid supply and one end of the valve spool and a fixed orifice between the one end of the valve spool and the hydraulic fluid return and a solenoid valve between the hydraulic fluid supply and the other end of the spool valve and a fixed orifice between the other end of the spool valve and the hydraulic fluid return.

6. Structure as set forth in clim 2, including a fixed orifice between the hydraulic fluid supply and one end of the spool valve and a fixed orifice between the one end of the spool valve and the hydraulic fluid return and a fixed orifice between the hydraulic fluid supply and other end of the spool valve and a solenoid valve between the other end of the spool valve and the hydraulic fluid return.

7. Structure as s set forth in claim 2, including a fixed orifice between the hydraulic fluid supply and one end of the valve spool and a fixed orifice between the one end of the valve spool and the hydraulic fluid return and a solenoid valve between the hydraulic fluid supply and the other end of the spool valve and a fixed orifice between the other end of the spool valve and the hydraulic fluid return.

8. Structure as set forth in claim 1, wherein the digital electronic control means includes an electromechanical transducer connected to the valve spool of the spool valve for providing an electrical signal representing the position of the valve spool, a central processing unit for receiving the signal representative of the position of the valve spool and comparing the valve spool position representing signal with a command input signal and providing output command error signals representative of desired movement of the spool valve, driver amplifying means for receiving the command error signals and amplifying them at least one solenoid valve for varying the hydraulic pressure of the opposite ends of the valve spool connected in the means for supplying operating hydraulic fluid to and exhausting operating fluid from the spool valve in accordance with the error signals to produce desired movement of the valve spool.

9. Structure as set forth in claim 8, wherein the elctromechanical transducer is one of a linear voltage differential transformer, a linear potentiometer, a light sensitive potentiometer, and a non-contacting radio frequency sensor.

10. Structure as set forth in claim 8, wherein the driver amplifying means is one of a dual current and dual voltage amplifier providing higher initial actuating energy for solenoid valve operation.

11. A digital electro-hydraulic servovalve comprising a spool valve including an outer cylindrical case, an inner valve spool reciprocably mounted in the case for movement axially thereof in accordance with hydraulic fluid pressure at the opposite ends thereof including three annular metering recesses spaced apart axially therealong, a source of hydraulic actuating fluid under pressure, conduit for passing hydraulic actuating fluid under pressure to one of the end annular metering recesses and the control annular metering recess on the valve spool, a conduit for returning hydraulic actuating fluid to the source from the other of the end annular metering recesses and the like central annular metering recess on the valve spool, means for connecting one end of the spool valve between one end of the cylindrical case and one end of the valve spool to the conduit supplying hydraulic actuating fluid under pressure through a first fixed orifice and to the conduit for return of hydraulic actuating fluid through a second fixed orifice, a first and second high speed solenoid valve, means for connecting the first high speed solenoid valve between the supply of hydraulic actuating fluid under pressure and the other end of the spool valve between the other end of the valve spool and the other end of the cylindrical case, means for connecting the second high speed solenoid valve to the spool valve between the other end of the valve spool and the other end of the cylindrical case and to the hydraulic actuating fluid, return conduit, an electromechanical transducer including a linear voltage differential transformer connected between the cylindrical case and valve spool for providing an electrical output signal representative of the position of the valve spool within the cylindrical case, a digital logic circuit connected to the electromechanical transducer for receiving the electrical signal representative of the position of the valve spool and comparing the signal representative of the position of the valve spool with input command signals representative of a desired position of the valve spool and to provide error, output command signals to drive the valve spool toward the desired position thereof, dual current driver amplifiers connected between the digital logic circuit and each of the solenoid valves for providing pulse width modulated dual current level signals to the solenoid valves for opening the solenoid valves in accordance with the error output command signals to drive the valve spool to the desired position and reduce the error, output command signals to zero.

1 2. Structure as set forth in claim 11, wherein the hydraulic fluid under pressure is passed to the end annular metering recesses of the valve spool and returned from the central annular metering recess.

13. Structure as set forth in claim 11, wherein the hydraulic fluid under pressure is passed to the central annular metering recess of the valve spool and returned from the end annular metering recesses.

14. The method of digitally controlling a spool valve including a valve spool comprising sensing the position of the valve spool, comparing the position of the valve spool with a desired position thereof and providing pulsed error signals repreentative of the difference between the actual and desired position of the valve spool and controlling the pressure at the opposite ends of the valve spool in accordance with the error signals to move the valve spool to the desired position thereof.

1 5. The method as set forth in calim 12, and further including the step of pulse width modulation of the error signals.

1 6. The method as set forth in claim 12, and further including controlling the pressure at the opposite ends of the valve spool by pulsing at least one solenoid valve connected between the spool valve and a source of hydraulic fluid under pressure with the error, output command signals.

17. The method as set forth in claim 13, and further including the step of two stage amplification of the error, output command signals before pulsing of the solenoid therewith.

1 8. A digital servovalve substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

1 9. A method of digitally controlling a spool valve substantially as hereinbefore described with reference to the accompanying drawings.