US3763412A

US3763412A - Open loop, linear, incremental positioning device

Info

Publication number: US3763412A
Application number: US00275456A
Authority: US
Inventors: D Detrick; Monte A Di
Original assignee: THRUST Inc
Current assignee: THRUST Inc
Priority date: 1972-07-27
Filing date: 1972-07-27
Publication date: 1973-10-02
Anticipated expiration: 1990-10-02

Abstract

An open loop linear incremental positioning device wherein a cylindrical electric coil surrounds an articulated magnetic plunger which is constrained to work against an opposing force. The articulated plunger consists of a plurality of magnetic segments having incremental magnetic working gaps therebetween. Upon energizing the coil with a given potential a contracting force is created and a predetermined number of gaps close in opposition to the retarding force, thereby moving the plunger an amount equal to the gaps closed. The applied potential uniquely determines the position of the plunger since for a selected potential the contracting force balances the opposing force after the predetermined number of gaps have closed.

Description

OLU'IQ llllrv UIIGLCD Detrick et al. 1 Oct. 2, 1973 [54] OPEN LOOP, LINEAR, INCREMENTAL 1,449,212 3/1923 Berry 335/265 x DEVICE 2 23%;; R5132? 5 5"; 51371 55 3 e1n ur Inventors: Dale Detrick, y, 3,430,120 2/1969 Kotaka et al. 310 14 x Angelo M. DiMonte, South Salem, Primary Examiner-D. F. Duggan [73] Assignee: Thrust, Incorporated, New York, Att rney-Peter L. Berger [22] Filed: July 27, 1972 [57] ABSTRACT [211 App]. No.: 275,456 An open loop linear incremental positioning device Related U S Application Data wherein a cylindrical electric coil surrounds an articulated magnetic plunger which is constrained to work [63] g z of 1971 against an opposing force. The articulated plunger cond an one sists of a plurality of magnetic segments having incre- [52] U 8 Cl 318/135 310/14 335/265 mental magnetic working gaps therebetween. Upon energizing the coil with a given potential a contracting [51] Int. Cl H02k 41/02 Field 310/l2 l9 force 1s created and a predetermmed number of gaps e close in opposition to the retarding force, thereby moving the plunger an amount equal to the gaps closed. The applied potential uniquely determines the position [56] References C'ted of the plunger since for a selected potential the con- UNITED STATES PATENTS tracting force balances the opposing force after the 3,183,410 5/1965 Fl 335/ 68 X predetermined number of gaps have closed. 2,935,663 5/l960 Pollak.. 335/259 548,601 10/1895 Black 335/259 X 16 Claims, 13 Drawing Figures J g l o lg D 2/ 6 ll! 1 ll lm 2 /0 l I? l I m Ml l ll 2 5 PAIEH IEU 2 973 SHEET 10F 4 M UM FIG! V INVENTOR. ALE W DETR/CK BYANGGO Dildo/v7! PAIEHTEUUCT 21973 SHEET 2 BF 4 Puu CHARACTfR/ST/C m I l 45 dc CLOSED J2 d g DISPLACEMENT Fl G .2

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l l l 1 d2 of) 0/5/ 1. CEMENT OPEN LOOP, LINEAR, INCREMENTAL POSITIONING DEVICE This is a continuation of US. Pat. application Ser. No. 107,152 filed Jan. 18, 1971 now abandoned.

BACKGROUND AND BRIEF SUMMARY OF THE INVENTION This invention relates to an electromagnetic discrete positioning system which is operable in open loop fashion to achieve accurate incremental positioning along a linear axis.

Most prior, high accuracy, linear positioning systems have been closed-loop servo systems. In these systems a position sensing device is used to develope a signal indicative of actual position and this signal is compared with a signal indicative of the desired position in order to derive an error signal representing the difference. The error signal is amplified and then generally applied to an electric servo motor which rotates in a direction tending to eliminate the difference between desired and actual positions. Such servo systems can be operated in a continuous analog mode or in an incremental digital mode.

High positioning accuracy can be achieved with servo systems, but these systems are relatively complex and costly and are subject to instability, overshoot and oscillation problems. In addition, the motion is initially created by an electrical motor and, hence, for linear positioning, a mechanical interface is required to transform rotary motion into linear motion, thereby giving rise to possible drift and backlash problems.

Solenoids have long been used to convert electrical energy into linear motion. However, solenoids generally have a very weak open gap initial pull force. The pull force builds up exponentially as the gap is closed, resulting in a powerful pull force when the gap is fully closed. Because of the uneven pull, solenoids cannot be controlled accurately at intermediate positions and, therefore, are not used in linear incremental positioning systems.

Improved solenoid type actuators including a cylindrical coil and an articulated plunger made up of multiple magnetic segments and multiple air gaps'have been developed. For example, see U. 8. Pat. No. 2,811,361 issued to James P. Watson and U. S. Pat. Nos. 3,376,528, 3,467,927, and 3,486,147 issued to James C. Macy and assigned to the assignor of this application. The improvement is achieved by breaking up the air gap into smaller increments and by reducing stray flux losses. When operated as solenoids, the air gaps close successively and rapidly providing a more uniform pull over a long traverse.

Surprisingly, on studying the pull characteristics of the articulated plunger actuator it was found that the device does not operate on the basis of the total air gap of the magnetic circuit as might be expected. Instead, the individual air gaps of the articulated plunger appear to act independently. This invention takes advantage of this discovery to provide a discrete positioning system capable of positioning a plunger at any desired one of a predetermined number of possible positions. This is achieved by employing a force opposing the solenoid pull, which force increases with displacement, and by energizing the electromagnetic coil in a particular manner.

BRIEF DISCRIPTION OF THE DRAWINGS Illustrative embodiments of the invention are shown in the drawings which form part of this specification wherein:

FIG. I is a cross-sectional view of an actuator according to one embodiment of the invention;

FIG. 2 is a curve showing the relationship between pull force and displacement for an actuator like that shown in FIG. 1.;

FIGS. 3a to 3f are a related set of curves comparing the spring pull force with the actuator pull force at several different energized states;

FIG. 4 is an electrical schematic diagram of a circuit for selectively energizing an actuator;

FIG. 5 is a curve showing the relationship of voltage applied to an actuator compared to actuator displacement;

FIGS. 6 and 7 are electrical schematic diagrams of other circuits for selectively energizing an actuator; and

FIG. 8 is a cross-sectional view of an actuator accord ing to another embodiment of the invention.

DETAILED DESCRIPTION The open loop incremental positioning device with the articulated plunger is illustrated in FIG. 1. A cylindrical, concentrically wound coil 2, surrounded by a layer of insulating material 3, is located within a magnetic structure comprising a cylindrical housing 5 surrounding the coil and upper and

lower end plates

6 and 7. The upper end plate 6 includes a central opening which accommodates the plunger structure 8. Coil 2 is energized via leads 9.

The plunger structure includes a number of generally disc shaped magnetic segments 10 which form the air gaps 11 between adjacent segments. Segments 10 are joined by nonmagnetic flange couplings 12 which limit the separation between adjacent magnetic segments but permit relative movement of the magnetic segments along the axis of coil 2. The lowest magnetic segment 13 (as viewed in FIG. 1) is secured to lower end plate 7 and the upper magnetic segment is coupled to a solid bar plunger extension 14 which extends up through the opening in upper end plate 6.

Ignoring the spring 15 surrounding plunger extension 14 for the moment, the pull characteristic for the open loop incremental positioning device is as illustrated in FIG. 2 where the pull force F is plotted against displacement D for a predetermined energization (E,,, e) of coil 2. Starting at the left of FIG. 2, which represents the fully open condition shown in FIG. I, energization of the coil results in a pull force f tending to move the plunger toward the closed position. As the plunger is displaced, the pull force increases exponentially until one of the air gaps has closed corresponding to a displacement d and a peak full force f,,. A plateau in the pull characteristic exists at displacement d since only a relatively low pull force f is then available for closing a second air gap but a higher force f would be required to begin the opening of the first air gap. Similar plateaus or equilibrium conditions exist at displacements d d d etc., corresponding to the closure of the other air gaps.

The exact explanation for the observed pull characteristic is not known. However it is observed that the pull characteristic does not correspond to that expected according to the total cumulative air gap in the magnetic circuit. Instead, if all the air gaps were ignored with the exception of that air gap in the process of closing, the pull characteristic would appear to correspond to the relationship: P k(F /L 1 where P is the pull force, F is the magnetomotive force or ampere-turns, k is a constant, and L is the length of the single gap in the process of closing. According to this relationship the pull force increases inversely as the square of the decreased gap length which appears to agree with the observed characteristics for the closure of an individual gap. The rise of the observed sawtooth envelope as the incremental positioning device closes is probably attributable to the reduction of flux losses as well as the decreased reluctance of the magnetic circuit as the total air gap for the magnetic circuit decreases.

The pull characteristic shown in FIG. 2 is substantially reversable, that is, the characteristic is substantially the same whether the actuator is opening or closing. Some hysteresis has been observed and therefore the curve for the linear positioning device while opening is somewhat higher than the curve for the device when closing.

A family of curves for an incremental positioning device at different energization levels is shown in FIG. 3b 3f. The higher the energization level applied to the coil, the higher the envelope of the pull characteristic curve becomes. The plateaus remain at the same places aligned with displacements d d d etc., corresponding to closure of the individual gaps.

Referring again to FIG. 1, a spring plate 21 is secured to the free end of plunger extension 14 to contain compression spring 15 between the spring plate and upper end plate 6. The spring is in compression even in the fully extended position, shown in FIG. 1, so that the spring always provides a force opposing the pull force of the actuator. The spring should be carefully selected since too small a spring would result in the successive and rapid closure of all gaps once the first gap closes and too large a spring will needlessly reduce the working displacement of the positioning device and may make it impossible to close some of the gaps.

An open loop linear incremental positioning device has been constructed and operated wherein the compression spring consisted of a stack of Belleville spring washers, i.e., 17 pieces of a 10 X 5.2 X 0.3 spring stacked in series plus 22 pieces of a 14 X 7.2 X 0.5 spring atacked in series. This spring structure provided the desirable characteristics in combination with a 1,600 turn coil of 22 wire and a plunger structure with 10 segments.

The force vs. displacement curve for a typical spring is shown in FIG. 3a. A minimum opposition force f. is present in the fully open position due to the compression of the spring in this condition. The opposition force provided by the spring increases with displacement.

The operation of the discrete positioning actuator can best be explained in connection with FIGS. 3a 3f. The plunger does not begin to move toward the closed position until the magnetic pull force exceeds the initial opposition forcef provided by the spring. If the energy E supplied to the coil is gradually increased eventually a value e will be found corresponding to the pull characteristic shown in FIG. 3b having an initial value f With a slight further increase in the energization level the magnetic pull force will exceed the spring opposition force and therefore the plunger begins to move. Since the magnetic pull force increases exponentially as the first gap closes and the spring force increases linearly with displacement, the magnetic pull force will continue to exceed the spring force causing the first gap to close rapidly once movement begins.

Once the first gap has closed (displacement d,) a plateau is reached on the magnetic pull characteristic such that the pull force drops and is not sufficient to exceed the spring force for any displacement beyond 4,.

In order to close a second gap the energization level for the coil must be increased to a value e to obtain a pull characteristic as shown in FIG. 30 where the pull for the d plateau has a value f sufficient to overcome the spring opposition force at displacement d In similar fashion further increases in the coil energization level will cause additional gaps to close. FIG. 3d shows the pull characteristic corresponding to energization level e which causes three gaps to close, FIG. 32 shows the pull characteristic for the e;, level which causes four gaps to close and FIG. 3f shows the pull characteristic which causes five gaps to close. Thus, a preselected energy level will cause a predetermined number of gaps to close.

A circuit for controlling energization of coil 2 is shown in FIG. 4. A transformer 30 is used having its primary winding 31 connected to an AC source and its multi-taped secondary connected to the stationary contacts of the multi-position switch 33. Coil 2 of the positioning device is connected between the movable contact of switch 33 and one end of secondary winding 32.

FIG. 5 is a curve showing the relationship between the energization potential and the displacement in discrete steps.

The potentials e to e shown in FIG. 5 would be selected for the tap voltages of secondary winding 32 (FIG. 4). With this arrangement the incremental position of the device is accurately controlled in accordance with the selected switch position.

Other arrangements for achieving the desired selective coil energization control are shown in FIGS. 6 and 7. FIG. 6 shows a DC circuit for energizing the coil, wherein resistors 41-45 are connected in series across a DC source to provide a voltage divider. The junctions between the resistors are connected to the stationary contacts of a rotary switch 46 and coil 2 is connected between the rotary contact of switch 46 and one end of the voltage divider. A diode 47 is connected across the coil to absorb inductive surges. The resistors are selected to provide the voltages e to e (FIG. 5) taken from a voltage displacement plot for the open loop positioning device.

In the arrangements shown in FIGS. 4 and 6 selective control is achieved by controlling the applied voltage which is then directly related to current and therefore controls the magnetomotive force of the magnetic circuit because of the relation wherein F is magnetomotive force, n is the number of coil turns and i is current flow through the coil. In the arrangement shown in FIG. 7 the effective number of coil turns is varied to control the magnetomotive force provided by the coil.

The taps of the actuator coil 2' are connected respec tively to the collectors of switching transistors 50-54. The emitters of the transistors are connected to the positive terminal of the source and the free end of coil 2' is connected to the negative terminal. Diodes 55 are connected between the taps on the coil to absorb inductive surges. By applying a suitable control signal to the base of a selected one of the transistors 50-54 the supply potential is applied across a selected number of coil turns.

With the structure shown in FIG. 1 a problem can sometimes arise where two of the gaps begin to close at the same time. It appears that the air gap located in the strongest portion of the magnetic field has a preference and closes before air gaps located in weaker portions of the magnetic field. Thus, the air gaps at the center of the coil tend to close first and those at the ends close last.

The likelihood of two gaps trying to close at the same time is substantially eliminated by locating all the air gaps between the center and one end of the coil. Under these circumstances there is a specific order or preference. The actuator shown in FIG. 8 is arranged with a solid bar 60 of magnetic material extending from the lower base plate 7 preferably more than halfway through coil 2 so that the magnetic segments 11 are all located between the mid-point of the coil and one end thereof.

While only a few illustrative embodiments have been described in detail it should be obvious that there are numerous variations within the scope of this invention. For example, while the force in opposition to the contracting pull of the plunger is shown as being generated through use of a compression spring, other means which produce a retarding force which increases as the plunger contracts may be employed so long as the rate of increase of the retarding force is less than the rate of increase of the pull generated in the closure of a single gap.

Also, the actuators shown in FIGS. 1 and 8 can be used as analog to digital converters by applying the analog input signal across the coil 2. A predetermined number of gaps will close corresponding to'the instantaneous value of the applied analog voltage such that the discrete position of the articulated plunger provides a digital representation of the applied analog signal.

The invention is more particularly defined in the appended claims.

We claim:

I. A linear incremental positioning device movable between end positions and capable of stopping at positions intermediate said end positions, said device comprising a core structure, said core structure comprising a plurality of axially aligned core segments having air gaps between adjacent ones of said segments, a plunger member connected to said core structure to be moved thereby between said end positions, a single electromagnetic coil wound around said core structure and at. least all of said core segments, said coil extending at least the length of said segments, said air gaps extending from one end of said coil to approximately the midpoint of the coil, and ampere turns control means for controlling the amount of ampere turns generated in said single coil for selectively closing a number of said air gaps in response to the voltage level applied to said coil to control the movement of said plunger to stop between said end positions and at a said intermediate position.

2. A linear incremental positioning device as set forth in claim 1, wherein said control means comprises electric circuit means for applying a current level to said coil and generating an electromagnetic force tending to close said air gaps and a balanced opposing force means for opposing said electromagnetic force for stopping the plunger at selected ones of said intermediate positions.

3. A linear incremental positioning device as set forth in claim 2, wherein said electric circuit means comprises means to adjust the voltage magnitude applied to said single coil.

4. A linear incremental positioningdevice as set forth in claim 2, wherein said opposing force means comprises a spring member.

5. A linear incremental positioning device as set forth in claim 4, wherein said spring member comprises a helical spring, said helical spring being axially aligned with and located around said plunger.

6. A linear incremental positioning device as set forth in claim 1, further comprising means to limit the width of each of said air gaps.

7. A linear incremental positioning device as set forth in claim 5, wherein said helical spring is a compression spring which is compressed as successive ones of said air gaps are closed.

8. A linear incremental positioning device as set forth in claim 2, wherein the force required to open a selected closed air gap is greater than the force to close said selected air gap.

9. A linear incremental positioning device as set forth in claim 1, wherein said control means comprises means for incrementally stopping said plunger between said end positions.

10. A linear incremental positioning device as set forth in claim 2, wherein said electrical circuit means comprises a multiple resistor voltage divider and a selector switch for operativelyconnecting said electromagnetic coil between selected taps of said voltagedivider.

11. A linear incremental positioning device as set forth in claim 2, wherein said electrical circuit means comprises a variable transformer.

12. A linear incremental positioning device as set forth in claim 2, wherein said electrical circuit means comprises an electrical switching means for energizing selected portions of said electromagnetic coil.

13. A method of operating a solenoid type structure, said structure comprising a core structure having a plurality of axially aligned core segments having air gaps between adjacent ones of said segments, a plunger member connected to said core structure to bev moved thereby, a single electromagnetic coil wound jaround said core structure and at least all of said core segments, said coil extending at least the length of said segments to close successive ones of said air gaps when energized, and a force opposing the closing of said air gaps, said method comprisingapplying a voltage to said coil, balancing said force opposing the closing. of said air gaps against the electromagnetic force generated by the energized electromagnetic coil at discrete positions in said core structure and selectively closing a number of said air gaps in response to the voltage levelapplied value by applying a predetermined electrical potential across the coil.

16. A method as set forth in claim 13, wherein the electromagnetic coil is energized with a preselected value by applying the electrical potential to a predetermined portion of the coil.

Claims

1. A linear incremental positioning device movable between end positions and capable of stopping at positions intermediate said end positions, said device comprising a core structure, said core structure comprising a plurality of axially aligned core segments having air gaps between adjacent ones of said segments, a plunger member connected to said core structure to be moved thereby between said end positions, a single electro-magnetic coil wound around said core structure and at least all of said core segments, said coil extending at least the length of said segments, said air gaps extending from one end of said coil to approximately the midpoint of the coil, and ampere turns control means for controlling the amount of ampere turns generated in said single coil for selectively closing a number of said air gaps in response to the voltage level applied to said coil to control the movement of said plunger to stop between said end positions and at a said intermediate position.

4. A linear incremental positioning device as set forth in claim 2, wherein said opposing force means comprises a spring member.

10. A linear incremental positioning device as set forth in claim 2, wherein said electrical circuit means comprises a multiple resistor voltage divider and a selector switch for operatively connecting said electromagnetic coil between selected taps of said voltage divider.

13. A method of operating a solenoid type structure, said structure comprising a core structure having a plurality of axially aligned core segments having air gaps between adjacent ones of said segments, a plunger member connected to said core structure to be moved thereby, a single electromagnetic coil wound around said core structure and at least all of said core segments, said coil extending at least the length of said segments to close successive ones of said air gaps when energized, and a force opposing the closing of said air gaps, said method comprising applying a voltage to said coil, balancing said force opposing the closing of said air gaps against the electromagnetic force generated by the energized electromagnetic coil at discrete positions in said core structure and selectively closing a number of said air gaps in response to the voltage level applied to said coil, said number of air gaps being fewer than the total number of air gaps.

14. A method as set forth in claim 13, comprising the additional step of controlling the amount of magnetic flux passing through said core structure.

15. A method as set forth in claim 13, wherein the electromagnetic coil is energized with a preselected value by applying a predetermined electrical potential across the coil.