GB2260821A

GB2260821A - LVDT for propeller pitch change system

Info

Publication number: GB2260821A
Application number: GB9222011A
Authority: GB
Inventors: William J Bagge
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1991-10-25
Filing date: 1992-10-20
Publication date: 1993-04-28
Anticipated expiration: 2012-10-20
Also published as: FR2689853B1; FR2689853A1; GB2260821B; GB9222011D0

Abstract

A linear variable differential transformer assembly 60 for generating a feedback signal indicative of the pitch setting of blades 20 comprises a target armature 62 of magnetically permeable material mounted to the translating (and rotating) fluid delivery tube 50 and a stationary coil assembly 70 disposed coaxially about the fluid delivery tube 50 over the axial length thereof traversed by the magnetically permeable target 62. The stationary coil assembly 70 comprises a pair of identical secondary coil windings 72 and 76 disposed in axially spaced relationship and a primary coil winding 74 disposed centrally therebetween. The output signal 61 from the LVDT 62 is the difference in voltages induced in the two secondary coil windings 72 and 76. A second embodiment uses a pair of axially spaced target armatures and respective sets of coils (Fig. 3 not shown) to provide redundancy. <IMAGE>

Description

LVDT FOR PROPELLER PITCH CHANGE SYSTEM Technical Field This invention relates generally to pitch change systems for adjusting the pitch of a variable pitch propeller and, more particularly, to an improved linear variable differential transformer responsive to the linear displacement of a pitch change torque tube for providing a feedback signal indicative of the pitch angle at which the propeller blades are set.

Background Art Conventional aircraft propeller systems typically incorporate a plurality of variable pitch propeller blades mounted to a rotary hub driven by the aircraft's engine, with each propeller blade extending radially outwardly from the hub along the longitudinal axis of the blade. In order to permit pitch adjustment, each blade is mounted to the hub for pivotable movement about its longitudinal axis.

The hub typically encloses a chamber within its interior wherein a pitch change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch of the blades thereby altering air resistance to the rotation of the blades to thereby control engine speed.

Typically, the actuation system includes a pitch change actuator of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of the hydraulic fluid which drives the actuator. The adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by way of collateral apparatus, a change in pitch change fluid pressure whenever the monitored engine speed departs from the desired engine speed setting.

To control blade pitch, the net pressure force exerted by the pitch change fluids selectively directed in response to a departure from desired engine speed against the opposite faces of the piston, that is the difference between the pressure force exerted by the fine pitch change fluid on one face of the piston and the pressure force exerted by the coarse pitch change fluid on the opposite face of the piston, is varied thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston.

Typically, the fine and coarse pitch change fluids are delivered through independent conduits in an axially elongated tube assembly to opposite sides of the pitch change piston. For example, the fine pitch change fluid is delivered to a fine pitch fluid chamber adjacent the forward face of the pitch change piston and the coarse pitch change fluid to a coarse pitch fluid chamber adjacent the rearward face of the pitch change piston. The fluid delivery tube assembly, typically referred to as a torque tube, commonly comprises a pair of co-axially disposed tubes forming an annular fluid delivery conduit therebetween opening to one of the fluid chambers and an inner conduit within the interior of the inner tube opening to the other fluid chamber, the fine pitch change fluid being delivered through one of these conduits and the coarse pitch change fluid through the other conduit.The inner tube is mounted at its forward end to the propeller hub and the outer tube is mounted at its forward end to the pitch change piston whereby the outer tube not only rotates with the propeller hub but also translates axially with the pitch change piston, while the inner tube rotates with the propeller hub but does not translate with the pitch change piston.

In order to provide a feedback signal indicative of blade pitch setting to the controller that selectively meters the pitch change fluids, it is common practice to monitor the axial movement of the rotating and translating fluid delivery tube since this tube is attached to the pitch change actuator piston and moves therewith. It is well known in the art to utilize a linear variable differential transformer (LVDT) of conventional sliding armature/surrounding coil construction as a means of generating such a feedback signal indicative of the axial position of the translating tube of the fluid delivery assembly.Customarily, a pair of independent LVDT's are used to provide redundant feedback signals, each LVDT disposed axially parallel to the translating tube with its armature mounted to the distal end a spring loaded shaft which in turn is operatively attached through bearing means at its other end to the rotating and translating outer fluid delivery tube so as to translate but not rotate therewith whereby the armature reciprocates within a stationary cylinder housing the LVDT coils. The stationary cylinder is typically mounted to the non-translating tube of the fluid delivery assembly and houses a pair of axially spaced secondary coils and a primary coil disposed centrally therebetween.As the pitch change piston moves axially in response to a change in blade pitch, the rotating and translating outer fluid delivery tube will correspondingly move axially and the armature of the LVDT will slide within the stationary cylinder, thereby causing the voltages induced in the secondary coils to change responsively. The difference between the voltages induced in the axially spaced secondary coils is indicative of the displacement of the armature from its null position, i.e. a central position between the two axially spaced secondary coils. Thus, the LVDT measures the stroke of the pitch change piston and provides a feedback signal which is indicative of blade pitch setting.

Although such conventional LVDT feedback systems function satisfactorily, they are cumbersome to install and properly calibrate to a reference blade pitch. Furthermore, as both the translating tube and the non-translating tube of the fluid delivery assembly rotate with the propeller hub, the attachments for operatively mounting the LVDT between the translating tube and the non-translating tube must include bearings to separate rotation from translation. The armature of the LVDT must be mechanically grounded to the outer race of the bearing and means must be provided to prevent rotation of the armature.

Disclosure of Invention It is an object of the present invention to provide a mechanically simpler blade pitch feedback system utilizing an LVDT.

It is also an object of the present invention to provide such an LVDT feedback system wherein target means mounted to the rotating and translating fluid delivery tube serves as the sliding armature for a pair of redundant stationary LVDT coil assemblies disposed circumferentially about the rotating and translating fluid delivery tube.

According to the present invention, linear variable differential transducer means for generating a feedback signal indicative of blade pitch in a pitch change system utilizing a translating piston to affect blade pitch positioning comprises target means of magnetically permeable material, such as for example a nickel iron alloy, mounted to a pitch change fluid delivery tube operatively connected to the pitch change piston to translate therewith, and at least one stationary coil assembly disposed coaxially about the translating fluid delivery tube and including a pair of axially spaced secondary coil windings and a primary coil winding disposed centrally between the secondary coil windings. The coil assembly extends over the extent of travel of the magnetically permeable target such that as the target translates with the translating fluid delivery tube, the voltages induced in the spaced secondary coils upon energization of the central primary coil will vary in response to the position of the target means relative thereto. The difference between the voltages induced in the secondary coils, one being located forward of the central primary coil and the other being located rearward of the central primary coil, constitutes the desired feedback signal which is indicative of the relative position of the pitch change piston and therefore the pitch setting of the blades.

Advantageously, a pair of the LVDT's of the present invention are employed simultaneously to generate independent feedback signals to provide a desired redundancy. The paired LVDT's may comprise independent first and second coil assemblies disposed axially one behind the other along the translating fluid delivery tube and coaxially about separate first and second magnetically permeable targets, respectively, mounted to the fluid delivery tube in axially spaced relationship. Alternatively, the paired LVDT's may comprise independent first and second coil assemblies disposed coaxially, one about the other, about a single magnetically permeable target mounted to the translating fluid delivery tube.In either arrangement, each coil assembly produces an output voltage signal which varies with the position of its respective target and is therefore indicative of the axial position of the pitch change piston and the pitch setting of the blades operatively connected therewith.

Brief Description of Drawing The aforementioned objects, as well as other objects, features and advantages of the present invention will become more apparent in light of the detailed description of the embodiments thereof illustrated in the accompanying drawing, wherein: Figure 1 is a partially sectioned, partly schematic, elevational view of a pitch change system incorporating an embodiment of the linear variable differential transformer of the present invention; Figure 2 is a sectioned, elevational view of the embodiment of the linear variable differential transformer of the present invention incorporated in the pitch change system of Figure 1; and Figure 3 is a sectioned, elevational view of an alternate embodiment of the linear variable differential transformer of the present invention.

Best Mode for Carrying Out the Invention The present invention is directed to a linear variable differential transformer assembly for use in a blade pitch change system for changing the pitch of the blades of a variable pitch propeller of the type used on propeller driven aircraft. As depicted in the Figure 1, the propeller system includes a hub 10 into which a plurality of propeller blades 20, of which only one is shown, are mounted. Each blade is mounted at its root end on bearings 12 in an socket 14 in the disc end of the hub 10 so as to be pivotally adjustable for pitch change about its longitudinal axis.

An eccentric roller 16 extends outwardly from the lower end of the root portion of the blade 20 and is received within a cam slot 18 of a desired shape provided in a hydraulically actuated blade pitch change actuator 30 having an axially translatable actuation piston 40 which includes an axially elongated shaft extending rearwardly and threadably mounted on a pitchlock screw 46, and a pitchlock piston 44 mounted on a ballscrew 48. The blades 20 are operatively connected to the shaft of the actuation piston 40 of the pitch change actuator 30 such that an axial translation of the actuation piston 40 in one direction, in this case rearwardly (i.e. left to right in Figure 1), accomplishes a change to a coarser, i.e. higher, blade pitch and in the other direction, in this case forwardly, accomplishes a change to a finer, i.e. lower, blade pitch.

During normal operation, to effect a pitch change or maintain a desired pitch setting despite changing blade load, the axially translatable actuation piston 40 of the pitch change actuator 30 is selectively pressurized with a fluid such as hydraulic oil pumped from a fluid reservoir (not shown) via a main pump 110, or in the event of a malfunction of the main pump 110 via an auxiliary pump 112 as in conventional practice, through a control valve 100 to and through a transfer tube 50 to be applied against the appropriate face of the actuator piston 40. The axial position of the actuator piston, which indicates the actual blade pitch, is continuously sensed and fed back to the electronic controller 120 by position feedback means 60 operatively associated with the transfer tube 50 comprising the linear variable differential transformer (LVDT) assembly 60 of the present invention.

The axial position of the actuation piston 40, and therefore the pitch of the blades 20, is maintained at equilibrium at any desired pitch setting by balancing the net pressure force on the actuation piston 40 with the blade loading transmitted to the pitch change actuator 30, the net pressure force being the difference between the pressure force exerted by the coarse pitch change fluid supplied to the coarse pitch fluid chamber 32 and acting upon the forward face 42 of the actuation piston 40 and the counter acting pressure force exerted by the fine pitch change fluid supplied to the fine pitch change fluid chamber 34 and acting upon the pitchlock piston 44 of the pitch change actuator 30 to drive the pitchlock piston 44 against the rearward face of the actuation piston 40.The effective area of the forward face 42 of the actuation piston 40 is advantageously substantially greater than, for example approximately twice, the effective area of the face of the pitchlock piston 44. Thus, by adjusting the pressure of the coarse pitch fluid, the net pressure force on the actuation piston 40 may be readily balanced against any change in the blade loading force on the actuation piston, which acts to drive the blades 20 to a finer pitch setting, thereby counteracting movement under the influence of an increase in blade loading to a finer pitch, thereby maintaining the blades at a desired pitch setting.Additionally, the magnitude of the net pressure can be readily varied by modulating the pressure of the coarse pitch fluid relative to the fine pitch fluid to increase or decrease the magnitude of the rearwardly acting net pressure force relative to the blade loading force so as to effect a desired translation of the actuation piston 40 and the pitchlock piston 44, together as a unit in back to back relation during normal operation, rearwardly or forwardly so as to drive the blades 20 to a different pitch setting.

As the LVDT assembly of the present invention may be utilized with any configuration of the control valve 100, the particular configuration of the control valve 100 is not germane to the present invention. However, the blade pitch control system 100 may comprise a primary electrohydraulic valve (EHV) 130 controlled by the electronic controller 120, a protection EHV 160, and a protection solenoid 150 for transferring authority to the protection EHV 160 away from the primary EHV 130 in the event of a malfunction of the electronic controller 120, such as disclosed in commonly assigned copending application serial number 07/743,943, filed August 12, 1991, the entire disclosure of which is hereby incorporated by reference. However, the particular configuration of the control system 100 is not germane to the present invention and it is to be understood that the linear variable differential transformer of the present invention may be utilized in any pitch change system wherein the pitch setting of the blades may be correlated with the position of a translating fluid delivery tube. The protection electrohydraulic valve 160 functions independently of the primary electrohydraulic valve 130 to assume control authority for feathering and for low pitch stop protection and overspeed protection in the event of an electrical failure.During normal operation, the protection EHV 160 functions solely as a conduit through which the pitch change fluid passes and does not in anyway interfere with the authority of the primary EHV 130 which is controlled by the electronic controller 120 to modulate the pressure of the coarse pitch fluid between a lower drain pressure, i.e. the pressure of a supply fluid reservoir (not shown), and a higher supply pressure, i.e. the pressure at which the fluid is received from the pump, while maintaining the pressure of the fine pitch fluid at higher supply pressure.To effect a pitch change to a coarser pitch setting, the electronic controller 120 modulates the primary EHV 130 to increase the pressure of the coarse pitch fluid and passes the coarse pitch change fluid from supply through a first pitch change fluid conduit 53 in the axially elongated transfer tube 50 to the coarse pitch fluid chamber 32 thereby increasing the pressure acting on the forward face 42 of the actuation piston 40 such that the net pressure force acting on the actuation piston 40 exceeds the blade loading, thus resulting in the pitch change actuator 30 being translated rearwardly to drive the blades 20 to a new coarser pitch.To effect a pitch change to a finer pitch setting, the electronic controller 120 modulates the primary EHV 130 to decrease the pressure of the coarse pitch fluid, while passing fine pitch change fluid at supply pressure through a second pitch fluid conduit 57 in the axially elongated transfer tube 50 to the fine pitch fluid chamber 34 thereby decreasing the pressure acting on the rearward face of the pitchlock piston 44 such that the net pressure force acting on the actuation piston 40 is now exceeded by the blade loading, thus resulting in the pitch change actuator 30 being translated forwardly as the blades 20 are driven under the blade loading to a new finer pitch setting and the coarse pitch change fluid drains back to the fluid reservoir.

Except in the event of a failure, normal pitch change logic and scheduling is provided by the electronic controller 120 over the entire blade pitch envelope including in flight pitch control, ground pitch control including low pitch stop override and blade reversal, and emergency feathering. The electronic controller 120 comprises a microprocessor programmed with all the pitch change logic necessary to control and schedule blade pitch over the entire operating range. On multi-engine aircraft, the electronic controller 120 may also be programmed to synchrophase the propellers in a conventional manner by biasing blade pitch between the propeller shipsets.

Advantageously, the electronic controller comprises a full authority digital electronic controller (FADEC), a type of electronic controller that has been used for some time as an electronic engine controller on aircraft turbine engines to automatically operate and modulate hydromechanical fuel flow controllers and are not per se new in the art. Such full authority controllers are provided with redundant control channels and redundant inputs and outputs to provide added safety.

The primary EHV 130, which in the depicted embodiment comprises a conventional jet pipe EHV of the type well known in the art, is operatively connected to a torque motor 122 which is controlled by the electronic controller 120 in a conventional manner well known in the art to selectively port pressurized fluid from a supply conduit to the coarse pitch fluid conduit 53 and the fine pitch fluid conduit 57 of the fluid delivery tube 50.The protection EHV 60 comprises a hydraulically actuated spool valve incorporating a spool having smaller area end face which is continuously exposed to supply pressure fluid and a the larger area end face exposed either to a regulated pressure fluid passed from an emergency overspeed governor 160 which has authority only in the event of failure of the electronic controller 120 or to supply pressure fluid from conduit 151 in the event that the protection solenoid 50 opens to allow supply pressure fluid to flow to the right end of the valve chamber to either effect emergency feathering of the blades or in response to a low pitch warning.

In the event that the electronic controller 120 receives a warning signal from a low pitch stop sensor 170 operative to provide such a warning signal whenever the pitch change actuator has moved beyond a preselected limit in the fine pitch direction, the protection solenoid 50 is also energized by the electronic controller 20. The low pitch stop sensor 170 senses the axial movement of the fluid delivery tube 50 and when the tube 50 has moved beyond a preselected limit in the fine pitch direction, the sensor 170 sends a pressure signal to the switch 172 which transmits a warning signal proportional to that pressure signal to the electronic controller 120.The particular type of the low pitch stop sensor 170 utilized is not germane to the present invention and although the low pitch stop sensor 170 may comprise any of a number of known low pitch stop sensors conventionally used on propeller pitch control systems, the low pitch stop sensor 170 may advantageously comprise the low pitch alarm assembly disclosed in commonly assigned co-pending application docket number H2118-PP of the applicant filed on even date herewith, the entire disclosure of which is hereby incorporated by reference.

Emergency overspeed protection is also provided in the event of failure of the electronic controller 120, such as would occur in the event of complete electrical failure, by modulating the protection valve 160 through the regulated pressure signal produced by the emergency overspeed governor 190 and applied to the right end face of the spool thereof.

Although depicted as a conventional mechanical overspeed governor of the type well known in the art and commonly employed in pitch change control systems, the emergency overspeed governor 190 may alternatively be an electrohydraulic overspeed governor of conventional type known in the art. The mechanical overspeed governor 190 operates in a manner well known to those skilled in the art to modulate the regulated pressure relative to the supply pressure. The overspeed governor 190 functions in conjunction with the protection valve 160, to position the blades to maintain engine speed within preselected limits, for example from 100% to 107% of normal rpm, during emergency overspeed conditions when the electronic controller 120 has failed to avoid an overspeed condition.

In accordance with the present invention, the linear variable differential transformer assembly 60, which generates for transmittal to the electronic controller 120 a feedback signal 101 indicative of blade pitch setting, comprises an target armature 62 of magnetically permeable material, such as for example a nickel iron alloy, mounted to outer surface of the rotating and translating fluid delivery tube 50 and a stationary coil assembly 70 disposed coaxially about the fluid delivery tube 50 over the axial length thereof traversed by the magnetically permeable target 62 as the fluid delivery tube translates with the blade actuator 130.The stationary coil assembly 70 comprises a pair of identical secondary coil windings 72 and 76 disposed in axially spaced relationship and a primary coil winding 74 disposed centrally therebetween, each of these coil windings circumscribing the fluid delivery tube 50 and being mounted on the wall of a support housing 78 disposed about the fluid delivery tube 50 over the length traversed by the target 62. As in conventional LVDT's, the secondary coil windings 72 and 76 are connected externally in a series-opposing circuit such that the output signal 61 from the LVDT 62 is the difference in voltages induced from the primary coil winding 74 in each secondary coil winding 72 and 76.

As the fluid delivery tube 50 rotates and translates axially with the blade actuator 30, the magnetically permeable target 62 mounted thereto moves reciprocally within the housing 78. When the magnetically permeable target 62 is positioned centrally between the spaced secondary coil windings 72 and 76, the voltages induced in the secondary coils 72 and 76 from the primary coil winding 74 are identical and there is no net voltage output signal 61. However, when the magnetically permeable target 62 is positioned off center, that is a greater portion of the target 62 lies within one of the secondary coil windings 72, 76 and a lesser portion of the target 62 lies within the other secondary coil winding, the voltage induced in one secondary coil winding will be greater than the voltage induced in the other secondary coil winding.

Consequently, there will be a voltage output signal 61 equal to the net voltage difference between the voltages induced in the secondary coil windings 72 and 76 from the primary coil winding 74. Since the magnitude of the output voltage difference signal 61 varies in response to the relative position of the magnetically permeable target 62 within the coil assembly 70, the output signal 61 from the LVDT 60 is indicative of the relative position of the translating fluid delivery tube within its range of travel from a forward most position at reverse blade pitch setting and rearward most position at the coarsest blade pitch setting, i.e. blade feather.

Therefore, the output signal 61 is also indicative of blade pitch setting, each position of the magnetically permeable target 62 relative to the two secondary coil windings 72 and 76 circumscribing the fluid delivery tube 50 being indicative of a specific blade pitch setting between full reverse pitch and feather.

Advantageously, the magnetically permeable armature target 62 comprises a cylindrical member which completely encompasses the outer surface of the rotating and translating fluid delivery tube 50 so that the coil windings, which also completely encompass the fluid delivery tube 50, will sense a uniform magnetically permeable target even when the fluid delivery tube is rotating, as well as translating, as is typically the case in hydromechanical pitch change systems. It is to be understood, however, that the LVDT assembly of the present invention is suitable for use on both rotating and non-rotating fluid delivery tubes.

Further, as in conventional practice, it is desirable to provide a redundant feedback signal from a second LVDT for transmittal to the electronic controller 120 as a margin of safety. This redundancy may be readily provided using the LVDT assembly of the present invention by disposing a pair of independent coil assemblies about the translating fluid delivery tube 50 with the coil assemblies either disposed in axially aligned relationship or disposed in concentric relationship.

Referring now to Figure 2, the LVDT assembly 60 depicted therein comprises a pair of identical, independent, concentrically disposed stationary coil assemblies 70a and 70b disposed coaxially about the fluid delivery tube 50 and a single magnetically permeable armature target 62 mounted to the outer surface of a portion of the rotating and translating fluid delivery tube encompasssed by the concentric stationary assemblies 70a and 70b. Both the radially inner coil assembly 70a and the radially outer coil assembly 70b comprise a pair of axially spaced secondary coil windings 72 and 76 and a primary coil winding 74 disposed centrally therebetween, each coil winding 72,74,76 circumscribing the fluid delivery tube 50 and being mounted on the wall of its respective support housing 78.As the fluid delivery tube 50 rotates and translates axially with the blade actuator 30, the magnetically permeable target 62 mounted thereto moves reciprocally within the region encompassed by the concentric stationary coil assemblies 70a and 70b. As discussed hereinbefore, when the magnetically permeable target 62 is positioned off center relative to the secondary coil windings 72 and 76 of the coil assemblies 70a and 70b, different voltages are induced in the secondary coils 72 and 76 from the primary coil winding 74 and pair of equal voltage output signals 61a and 61b will be generated by the LVDT coil windings 70a and 70b, respectively, equal to the net voltage difference between the voltages induced in their respective secondary coil windings 72 and 76.One of these voltage output signals is transmitted as a feedback signal to a first channel of the electronic controller 120 and the other is transmitted as a feedback signal to a second channel of the electronic controller 120 thereby providing the desired redundancy.

Referring now to Figure 3, the LVDT assembly 60 depicted therein comprises a pair of identical, independent, stationary coil assemblies 70a and 70b disposed in axially aligned relationship coaxially about the fluid delivery tube 50 and a pair of magnetically permeable targets 62a and 62b mounted to the outer surface of the fluid delivery tube 50 in axially spaced relationship, the first target 62a being encompasssed by the first coil assembly 70a and the second target 62b being encompassed by the second coil 70b. Both the first coil assembly 70a and the second coil assembly 70b comprise a pair of axially spaced secondary coil windings 72 and 76 and a primary coil winding 74 disposed centrally therebetween, each coil winding 72,74,76 circumscribing the rotating and translating fluid delivery tube 50 and being mounted on the wall of its respective support housing 78.As the fluid delivery tube 50 translates axially with the blade actuator 30, the first and second magnetically permeable targets 62a and 62b mounted thereto move reciprocally within the region encompassed by their associated stationary coil assembly 70a and 70b, respectively. As discussed hereinbefore, when the magnetically permeable target 62 is positioned off center relative to the secondary coil windings 72 and 76 of its associated coil assembly, different voltages are induced in the secondary coils 72 and 76 from the primary coil winding 74. Thus, in response to the relative axial position of the magnetically permeable target, a pair of equal voltage output signals 61a and 61b will be generated by the axially aligned LVDT coil windings 70a and 70b, each signal equal to the net voltage difference between the voltages induced in their respective secondary coil windings 72 and 76. One of these voltage output signals is transmitted as a feedback signal to a first channel of the electronic controller 120 and the other is transmitted as a feedback signal to a second channel of the electronic controller 120 thereby providing the desired redundancy.

Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.

We claim:

Claims

1. A linear variable differential transformer assembly for generating a feedback signal indicative of blade pitch in a pitch change system utilizing a hydraulically actuated translating blade actuation piston to affect a change in pitch of at least one blade and having an axially disposed fluid delivery tube operatively associated with said blade actuation piston so as to translate therewith, said transducer assembly comprising: a. target means mounted to the translating fluid delivery tube so as to translate therewith, said target means formed of a magnetically permeable material; and b. at least one stationary coil assembly disposed coaxially about the translating fluid delivery tube and extending axially over the extent of travel of said target means as said target means translates with the fluid delivery tube, said coil assembly having a pair of axially spaced secondary coil windings and a primary coil winding disposed centrally between the spaced secondary coil windings, the secondary coils being connected in a series-opposing circuit whereby a feedback signal is generated proportional to the difference between the voltages induced in the secondary coils through the magnetically permeable target means from the primary coil.

2. A transformer assembly as recited in claim 1 wherein said target magnetically permeable target means comprises a cylindrical member of a magnetically permeable material which completely encompasses a band-like portion of the fluid delivery tube.

3. A transformer assembly as recited in claim 2 wherein said magnetically permeable target means comprises a cylindrical member made of a nickel iron alloy.

4. A transformer assembly as recited in claim 1 wherein said at least one coil assembly comprises a first radially inner coil assembly disposed coaxially about the fluid delivery tube and a second radially outer coil assembly disposed coaxially about the first coil assembly, each of the first and second coil assemblies producing an independent, redundant feedback signal.

5. A transformer assembly as recited in claim 1 wherein said magnetically permeable target means comprises a first target mounted to the fluid delivery tube and a second target mounted to the fluid delivery tube in axially spaced relationship from the first target, and said at least one coil assembly comprises a first coil assembly disposed coaxially about the fluid delivery tube and extending over the extent of travel of the first target and a second coil assembly disposed coaxially about the fluid delivery tube and extending over the extent of travel óf the second target.