US3027703A

US3027703A - Servo-controlled drive for machine tools and the like

Info

Publication number: US3027703A
Application number: US738059A
Authority: US
Inventors: Royle Joseph Kenneth
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1957-05-29
Filing date: 1958-05-27
Publication date: 1962-04-03
Anticipated expiration: 1979-04-03
Also published as: BE568130A; FR1207032A; US2968144A

Description

April 3, 1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 1958 5 Sheets-Sheet 1 A5 A 5 SW5 1 ROTARY-TO-L/NEAR V cogyveersz o p S52v0 SIGNAL M Mr, STAT! NAEY F IGJ.

5mm :IZLC d M SIGNAL Q 4 RM ACTIVE \eauzv-ro-Lwme CONVEETEQ PASS/V5, MOV/NG FIG.4.

ELC ACT/V5 52v0 P P. M Q SIGNAL QOTAQY-TD-L/NEAE cowseree M A PASSIVE, NOV/N6 1 W 2 12;. c p j seevo J M ASS/V6 SIGNAL Q EOTAQY-TO-L/NEARCONVEETf ACTIVE, MOVING FlG.6.

INVENIOR I JOSEPH KENNETH ROYLE ATTORNEYS April 3, 1962 J. K. ROYLE 3,0273%:

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 1958 5 Sheets-Sheet 2 v I Q V SEEVO SIM r- H FIG 3 A .H 1 I 1 M U pl 5' j m v f Aer/vs 552 v0 eomev-m-u/vsu CONVEQTEQ SIGNAL PASSIVE, smnamev F IG 2 S50v0 M $/GNAL Vm) Aer/v5, EL 6 Q T? I P15 :1

%77777 7 2072I/2Y-7'0-L/NE4e CONVEETEP,

PASS'IVE, STAT/OM42) INVENTOR JOSEPH KENNETH ROYLE ATTORNEYS April 3, 1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 1958 5 Sheets-Sheet 3 7777i TABLE DIGITAL r0 VALVE F (5. 4, i ANALOGUE mum/2,4102 5 I I nan/47012 CONVERTER 4 FIG.9.

l v INVEN'IIDR JOSEPH gggNNETH Rom r ATTORNEYS April 3, 1962 I J. K. ROYLE SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE 5 Sheets-Sheet 4 Filed May 27, 1958 FIG. l5.

3-PHA5E 4.6.

PHASE I PHASE 2 PHASE INVENTOR JOSEPH KENNETH ROYLE FIG. l6.

Mfwonumrs April 3, 1962 J. K. ROYLE 3,027,703

SERVO-CONTROLLED DRIVE FOR MACHINE TOOLS AND THE LIKE Filed May 27, 1958 q K 5 Sheets-Sheet 5 INVENTOR- JOSEPH KENNETH BYLE ATTORNEYS United States Patent fi 3,027,703 Patented Apr. 3, 1962 3,027,763 SERVO-CONTROLLED DRIVE FUR MACHINE TOOLS AND THE LIKE Joseph Kenneth Royle, Heat-on Moor, Stoclrport, England,

assignor to National Research Development Corporation, London, England, a British corporation Filed May 27, 1958, Ser. No. raaasa Claims priority, application Great Britain May 29, 1957 8 Claims. (Cl. 60-6) This invention relates to apparatus for driving a first object along a predetermined path in relation to a second object and is particularly applicable to the propulsion of the slides of machine tools along their slideways though the invention is not confined to such applications.

According to the invention there is provided apparatus for driving a first object in relation to a second object along a predetermined path, comprising two driving mechanisms coupled together and to the first object and the second object in such a manner that movements of the first object relative to the second object along the said path are dependent upon the algebraic sum of the individual driving actions of the two driving mechanisms, the first driving mechanism being capable of a relatively high driving acceleration over only a relatively short range of driving action and the second driving mechanism being capable of a relatively long range of driving action but being arranged to provide only a relatively low driving acceleration, means, responsive to a signal representing the motion required of the first object in relation to the second object along the said path, for actuating the first driving mechanism and means, operable before the first driving mechanism, in moving from the middle of its range of driving action, has reached an end of its range of driving action in responding to a signal as aforesaid, for actuating the second driving mechanism in such a sense as to supplement the driving action of the first driving mechanism initiated by the said signal, whereby the first driving mechanism is prevented from reaching the end of its range of driving action.

According to the invention there is further provided apparatus for driving a first object in relation to a second object along a predetermined path, having a first driving mechanism comprising a hydraulic piston and cylinder arrangement and a second driving mechanism comprising a rotary driving motor coupled to a device for transforming rotary motion derived from the said motor, into motion in the direction of the said path, the two driving mechanisms being coupled together and to the first object and the second object so that relative movement between the two objects along the said path is dependent upon the algebraic sum of the driving actions of the two driving mechanisms, the hydraulic piston and cylinder arrangement having a short stroke and being controlled by a high performance hydraulic valve, the second driving mechanism having a range of driving action which is long in relation to the said stroke but being arranged to provide a driving acceleration which is low in relation to that of the hydraulic piston and cylinder arrangement, a closed servo loop for controlling the relative motion between the two objects in the direction of the said path comprising means for receiving signals characteristic of desired relative motion between the said objects along the said path, means for receiving signals characteristic of relative motions taking place between the said objects along the said path, means for comparing the said signals so received and continuously generating an error signal representing the difference between the said signals, an actuator for the hydraulic valve, means for continuously operating the actuator in accordance with the instantaneous value of the error signal,

cooperating members coupled respectively to the piston and cylinder of the said hydraulic arrangement engaging when the piston reaches a predetermined position between the middle and an end of its stroke, means for energising the motor of the second driving mechanism in such a sense as to supplement the driving action of the hydraulic arrangement when the said members engage as aforesaid and de-energising the motor when the members have moved out of engagement.

The invention will be more readily understood from the following description of certain embodiments thereof illustrated in the accompanying drawings in which:

FIGS. 1 to 8 inclusive are diagrammatic representations of certain variants of the invention, I

FIG. 9 is an elevation of a first embodiment of the invention,

FIG. 10 is a sectional plan of the said first embodiment,

FIG. 11 is a cross-sectional side elevation of the said first embodiment,

FIG. 12 is a cross-section to an enlarged scale of a part of the said first embodiment,

FIG. 13 is a detailed underside view to an enlarged scale of a part of the said first embodiment,

:FIG. 14 is a schematic diagram of a servo control system for use with the invention,

FIG. 15 is a diagram of a motor control circuit for use with the invention,

FIG. 16 is a diagram of a second motor control circuit for use with the invention.

For convenience of description the first driving mechanism will hereinafter be referred to as the quick drive and the second driving mechanism will be hereinafter referred to as the slow drive but these terms should not be considered as in any way defining or limiting the properties of the two said mechanisms.

The invention finds its principal uses in systems where the relative positions, velocity or acceleration (or any combination of the three), for between two objects, along a predetermined path, is required to be under control of a command signal of which a characteristic is variable accordingto the relative position, velocity or acceleration (or combination of the three) required of the two objects.

Systems of this type, commonly called servo systems, present the difficulty that a driving mechanism having a quick response to a change of the command signal will tend to have a small range of driving action and conversely a driving mechanism having a large range of driving action will tend to have a slow response to a change of the command signal.

This may be illustrated by the following examples:

A hydraulic piston and cylinder under the control of a well designed valve can respond extremely quickly to a sudden change in a command signal applied to an actuator controlling the valve but if the cylinder is long this advantage is lost due to the compliance or compression of the hydraulic fluid contained in the cylinder. The compressibility of this volume of fluid reduces the stiffness of the system so that the resonant frequency of the inertia load is lowered.

The stiffness of a long hydraulic cylinder can be increased by increasing the diameter of the bore but this in turn increases the volume of the fluid to be moved which requires a larger valve with a slower response.

The quick response of a hydraulic piston and cylinder arrangement is therefore only obtainable when the stroke is short.

To provide a relatively large movement between the two objects various mechanisms, many of which involve rotating elements, are available. An example of the latter is the well known lead screw and nut, one element being rotated by a rotary motor. Another example is a rack and worm. These mechanisms can be designed to provide great stiffness combined with a long stroke but it is correspondingly difiicult to procure quick response without complication and expense. For instance, if the rotating element is driven by an electric motor, the armature cannot be rapidly accelerated without the application of considerable power. This difficulty becomes more acute the heavier the loads encountered.

A long hydraulic piston and cylinder combination is another instance of a driving mechanism suitable for providing relatively large movement between the two objects but it has already been explained that such a drive lacks stilfness unless it has a large diameter.

The invention combines the virtues of both types of driving mechanism and avoids their limitations.

FIGURES l to 7 inclusive, show various kinematically equivalent combinations of a quick drive and a slow drive for providing linear relative motion between a first object and a second object, these objects being illustrated as a machine tool work table riding on slideways anchored to the fixed parts of the machine tool, and the said fixed parts respectively. The quick drive is represented as a short stroke hydraulic ram Q, controlled by a valve V operated by an actuator A responsive to command signals and the slow drive is represented as a bar working in combination with a rotary-to-linear converter RLC (such as a lead screw and nut respectively or a rack and worm respectively) driven by a motor M.

In FIG. 1 the quick drive is carried by the work table and has a moveable member attached to the bar, the latter cooperating with a rotary-to-linear converter RLC anchored to the slideways and rotated by a motor similarly anchored.

In FIG. 2 the quick drive is again carried by the work table and its moveable member is attached to the bar. The bar rotates so that the moveable member of the quick drive must also rotate unless a rotary joint is interposed between them. If the moveable member is the piston of a hydraulic piston and cylinder arrangement, the piston can be given sufiicient clearance to enable it to rotate in the cylinder without undue friction. The bar cooperates with a non-rotating or passive rotary-tolinear converter RLC which is anchored to the slideways and the bar is rotated by a motor M carried by the work table, via a splined joint which permits endwise movement of the bar to the extent of the stroke of the quick drive.

In FIG. 3 the quick drive is floating, with its moveable member coupled to the work table. A non-rotating or passive rotary-to-linear converter RLC is anchored to the quick drive and cooperates with a bar rotated by a motor M anchored to the slideways. In practice the quick drive would ride on slideways parallel to the slideways for the work table.

In FIG. 4 the quick drive Q is anchored to the slideways and its moveable member is coupled to a non-rotating or passive rotary-to-linear converter RLC cooperating with a bar rotated by a motor M anchored to the worktable. This arrangement has the advantage, where a hydraulic device is used for the quick drive, that it can be supplied by rigid piping.

In FIG. 5 both the quick drive Q and the motor M of the slow drive are anchored to the slideways and the quick drive moveable member is coupled to the bar of the slow drive either directly or through a rotary coupling and the bar co-operates with a non-rotating or passive rotary-to-linear converter RLC anchored to the work table. Where there is no rotary coupling between the bar and the moveable member of the quick drive the latter must rotate. The motor M drives the bar through gearing and a splined joint which permits endwise movement of the bar to the extent of the stroke of the quick drive. With this arrangement all power supplies are coupled without the need for flexible leads.

In FIG. 6, the quick drive Q is anchored to the slideways, the motor M of the slow drive and a rotating or active rotary-to-linear converter RLC are connected to the moveable member of the quick drive and this assembly would in practice be mounted on additional slideways parallel to the main slideways supporting the work table and providing free movement along the additional slideways to the extent of the stroke of the quick drive. The rotary-to-linear converter cooperates with a nonrotating or passive bar anchored to the work table.

In FIG. 7 the quick drive Q the motor M of the slow drive and a sliding coupling for rotating the bar of the slow drive, are all anchored to the work table. The bar of the slow drive is coupled to the moveable member of the quick drive so that the latter must rotate unless a rotary coupling is introduced between them. The bar of the slow drive cooperates with a non-rotating or passive rotary-to-linear converter RLC anchored to the slideways.

in the form where the quick drive is capable of relative linear movements between its driving members over a limited length of stroke and where the slow drive incorporates a rotary motor and a rotary-to-linear converter.

FIG. 8 illustrates a rotary version of the invention in which a work table journalled for rotation in bearings (not shown) is driven by a slow drive in the form of a worm wheel coupled to the work table (shown as machined from the under side of the rim of the work table). This worm wheel cooperates with a Worm driven by the motor M of the slow drive which are mounted on a subtable which is also journalled for rotation coaxial with the work table in bearings (not shown). The quick drive Q is anchored to fixed parts of the machine and can rotate the subtable carrying motor M to and fro over a limited angle by means of a connecting rod. This arrangement is the rotary equivalent of FIG. 6.

A practical embodiment of the invention will now be described in relation to FIGURES 9 to 15 of the accompanying drawings.

FIG. 9 is an elevation of a milling machine incorporating the invention.

The machine has a main casting 1 providing a horizontal platform 2 supporting slideways 3 upon which a work table 4 may ride from right to left and vice versa, as shown in the figure. Behind the slideways 3, a vertical pillar 5 fixed to main casting 1 supports a beam 6 overhanging the work table 4 and beam 6 has on its underside slideways not visible in the drawing, upon which rides a vertical tool spindle mounting 7 which is capable of horizontal movement on the slideways of the beam 6 in a direction normal to the plane of the paper. The slideways 3 and the slideways on the beam 6 permit relative movement between a tool carried on the spindle 8 and a work piece fixed to work table 4, along two axes at right angles to one another.

The work table 4 is moved along slideways 3 by means of a motor 9 driving a worm 10 (shown in dotted lines in FIGURE 9) which cooperates with a rack 11 (shown mainly in dotted lines in FIGURE 9). Motor 9 is coupled to worm 10 via a worm and worm wheel gear train 12 and a shaft 13 (shown in dotted lines in FIG- URE 9). Shaft 13 is coupled to worm It) by gearing 14 not shown in FIGURE 9 but visible in FIGURE 10.

Rack 11 rides in guides 15 in the underside of work table 4 (visible in FIGURE 11) and is coupled to the work table via the piston rod 16 of a hydraulic piston and cylinder arrangement 17 the cylinder of which is bolted to Work table 4.

FIGURE 10 shows the milling machine in plan, sectioned in a horizontal plane running through the centre of worm 10. FIGURE 11 is a vertical section of the milling machine along line 18, looking in the direction of the arrows.

Shaft 13 is supported at one end by bearings in a hous- FlGS l to 7 illustrate principal varients of the invention ing 19, fixed rigidly to slideways 3, which accommodates the gear train 12.

At its other end shaft 13 is supported in bearings secured to the floor of the structure of slideways 3. Worm 10 is journalled for rotation in a cradle 21 and this cradle is capable of rotating bodily about the axis of shaft 13 which passes through bearing bushes 22 in cradle 21 which provide a pivot for the latter. Special provisions are taken to prevent endwise movement of shaft 13, worm 10 and cradle 21. These provisions, which are omitted to simplify the drawing, may take the form of conventional thrust bearings or may include a thrust bearing loaded by a hydraulic thrust arrangement. Cradle 21 has a lug 23 on the side remote from shaft 13 which rests on a hydraulic jack unit 24. Jack unit 24 is supplied with hydraulic fluid at a suitable pressure and forces the ing 23 upwards so that cradle 21 is urged to rotate about shaft 13 clockwise in relation to FIGURE 11 whereby Worm 10 is forced upwards into engagement with rack 11 for the elimination of back-lash.

FIGURE 12 shows the hydraulic piston and cylinder combination 17 in section to an enlarged scale.

A cylinder body 25 is secured to the end of work table 4 by means of bolts such as bolt 26. A piston 27 slides in the bore linear 28 and is coupled to rack 11 by piston rod 16, which passes through a glanded cylinder end 29, by means of the screw-threaded extension 30. The other end of the cylinder is closed by a cylinder end 31, both cylinder ends being secured to the cylinder body 25 by means of bolts such as 32. The side of the cylinder to the right of piston 27, as seen in FIGURE 12, commun cates with a hydraulic valve 33 via a port 34 in cylinder end 31.

Valve 33 has a bore 35 in which rides a spool 36 having two controlling lands 37 and 38 and two sealing lands 39 and 40. The lands 37 and 38 co-operate with annular ports 41 and 42 grooved out of the surface of bore 35 and communicating via supply ports 43 and 44 with unions (not shown) for the attachment of pipes leading to the high pressure and low pressure sides respectively of a hydraulic pressure supply system. The controlling lands 37 and 38 are dimensioned so that, when the spool is in the central position they do not completely isolate ports 41 and 42 from one another whereby there is a small flow of hydraulic fluid between supply ports 43 and 44. This arrangement is commonly referred to as underlap and makes for stable operation of the valve at small openings.

The side of the cylinder to the left of piston 27, as seen in FIGURE 12 communicates via a port 45 in cylinder end 29, with a union (not shown) for attachment of a pipe leading to the high pressure side of the hydraulic pressure supply system. The full pressure of the said system is therefore constantly applied to the left hand face of piston 27 but over an effective area which is reduced by the cross sectional area of piston rod 16. The pressure on the other side of piston 27 operates upon its full area however and is thus able to overcome the pressure on the left of piston 27 and force the latter to the left when the valve 33 is operated in the appropriate sense (Le. upwards as seen in FIGURE 12). Conversely, when the valve 33 is operated in the opposite sense the right hand side of piston 27 is acted upon by the low pressure of the hydraulic system, which is overcome by the high pressure to the left of the piston even though it works upon a smaller elfective area of piston surface.

A groove 46 surrounds the outer end of piston rod 16 and is in communication via port 47 with a union (not shown) for attachment to the low pressure side of the hydraulic supply system, whereby oil leaking along the piston rod is scavenged.

Spool 36 is operated axially by an actuator which, for the sake of simplicity, is omitted from the drawing. This actuator is of a type responsive to command signals characteristic of movements required to be made by work table 4 along slideways 3. Such actuators are well known in the art, a suitable type being of the type commonly known as a torque motor. This type of actuator works on the same principle as a centre zero moving coil electric meter movement but is of robust construction and capable of exerting a substantial force via a finger corresponding to the pointer of a meter.

In operation, the command signals produce movements of piston 27 which cause corresponding movements of work table 4, along slideways 3, in relation to rack 11,

which, when motor 9 is at rest, is locked against axial movement by worm 10, the slant angle of which is such as to make the rack and worm combination virtually irreversible in the sense that loads applied to the rack 11 cannot rotate the worm 10.

Rack 11 carries along one side, cams such as 48 in FIGURE 11 and shown in detail in FIGURE 13 which gives an enlarged underside view of part of the work table 4 and rack 11. Fast with the work table 4 are switches such as 49 each having a trigger member 50 cooperating with a cam 48.

At least two switch-and-cam sets 48/49 are provided at convenient sites along rack 11 and they are so placed that one switch engages its cam when the piston 27 moves off-centre in one direction and the other switch engages its cam when piston 27 moves off-centre in the other direction. It is arranged that the operation of the switches 49 control the motor 9 in such a way that the action of the hydraulic piston/cylinder combination 17 is supplemented by movement of rack 11 before piston 27 reaches the end of its stroke.

For this to be effective it is in general necessary for the signals applied to the actuator for valve 33 to be subject to a feed-back servo system. An example of such a system is schematically illustrated in FIGURE 14.

FIGURE 14 shows a record tape 51, for instance a magnetic record tape, carrying signals characteristic of the motions required to be executed by work table 4. Tape 51 is carried on spools 52 and passed by conventional magnetic record play-back methods over a read ing head 53. The reproduced command signals pass to a comparator 54 which has another input from apparatus which originates signals characteristic of motions made good from time to time by Work table 4 along slideways 3. FIGURE 14 illustrates one arrangement of this type in which an elongated optical diffraction grating 55 is attached to and moves with the work table and a smaller grating 56 is anchored to the slideways, the two gratings being so placed and aligned that the small grating is narrowly separated from and overlies some part of the large grating in all positions of the work table along its slideways. The directions of the rulings of the two gratings are relatively inclined at a small angle so that alternate dark and light bands (so-called moir fringes) are seen when looking through the two superimposed gratlngs at a light source. These bands are approximately normal to the direction of the lines of the two gratings and move in the direction of their breadth when the small grating moves along the large grating on movement of the work table along its slideways. When an optical slit, parallel to the longitudinal axes of the bands, is introduced into the light path, a photo electric cell such as that indicated at 57 in FIGURE 14, trained on the light emerging from the gratings and the slit, has an output (ideally sinusoidal in wave-form) which fluctuates according to the movement of the bands.

The pitch of the rulings on the

diffraction gratings

55 and 56 may be of the same order of magnitude as the limits of accuracy to which the machine is to be controlled. A movement of the work table equal to the distance between two adjacent rulings produces amovement of the bands (or moir fringes) such that one band occupies the place previously held by its immediate neighbour. As the bands and their spacing is many times greater than those of the grating lines the device provides a sensitive measure of work table movements, and the output of the light cell passes through one minimum and one maximum for each said movement of the work table.

Various proposals have been made for distinguishing between diiferent directions of movement of the work table, which results in different directions of movement of the bands or moir fringes, but it is not considered necessary to describe such methods. It is sufiicient to say that light cell 57 gives an output which fluctuates in time with small increments ofmovement of the work table of predetermined length and that the direction of movement can be distinguished. If the command signals from pickup 53 are in the form of signal elements each representing one of the said small increments of movement of the work table and having one form for one direction of movement and another form for the opposite direction of movement, the two inputs into comparator 54 can be compared and any difference can be arranged to represent the discrepancy at any instant between the movements required of the work table and the movement actually made good. This difierence or error signal, derived from fluctuating indications, will be in incremental or digital form. To apply such an error signal to valve 33 a digitalto-analogue converter 58 is interposed between the comparator 54 and the valve actuator 59'. The analogue signal applied to actuator 59 may be a voltage corresponding to the number of units of error held in comparator 54 at any instant.

Such a voltage applied to valve actuator 59 will operate valve 33 to cause relative movement between piston 27 and cylinder 17 and as the piston is held by the rack, the cylinder will move and with it the work table. If the command signals from pick-up 53 call for continuous movement of work table 4 there will be a continuous error signal in 54 since, as soon as a cancelling signal is received from light cell 57 to cancel one command signal, the error is registered again on receipt of a new command signal. When the piston moves a predetermined distance from its central position in the cylinder one of the switches 49 is operated and motor 9 starts up. The worm and

rack

10, 11 then drive the work table in the same direction as it is already being driven by piston 27 and the two superimposed motions cause a momentary excess of signals from light cell 57 which result in signals of reversed sense being passed from comparator 54. The result is that the valve 33 is operated in the reverse sense to move piston 27 back towards its central position. If the table is moving at the correct speed under control of motor 9 and the worm and

rack

10, 11, the piston will not move far in this reverse direction and may not move far enough to disengage the switch 49, in which case the motor will remain in operation until the required work table movement is exceeded. Since however, the motor, worm,

rack combination

9, 10, 11 must be capable of centering piston 27 even in the presence of continued command signals in the same sense, the system must be arranged so that the maximum speed of the motor, worm, rack combination exceeds the maximum speed required of the work table. Therefore, if the motor 9 remains switched on, the work table will soon be moving faster than it should which will result in reverse signals from comparator 54 and compensatory movement of the piston 27 towards its central position. The motor 9 will be switched oil before this position is reached by piston 27 but will continue to revolve by inertia for some little time so that the piston may, in compensating for any continued excess speed of the work table, overshoot its central position and operate the other switch 49 so as to reverse the motor. In practice there will be some hunting as between the piston and the motor. This will not matter in many cases since the sensitivity of the hydraulic piston and cylinder assembly 17 to signals received via valve actuator 59 will be sufficient to correct momentary discrepancies between the actual and the required movements of the work table, within acceptable limits of accuracy.

An arrangement to minimise this hunting will be described later however.

FIGURE 15 shows a method of actuating motor 9 under control of the two switches 49. These switches are represented by their respective (normally open) contacts SB and SF one of which is connected in series between a battery supply and a relay CB and the other of which is connected in series between the battery supply and a relay CF. These relays may be heavy duty contactors of the type used in conventional motor starter units. In FIG- URE 16, motor 9 is shown as a three phase alternating current motor. Such a motor may be reversed by transposing the first and third phase connections from the supply mains to the motor. The three phase wires from the supply mains are connected to the motor, first via three normally open contacts cfll, cfZ, and cf3 of relay CF and secondly via normally open contacts cbl, CM and cb3 of relay Cii. As the two switches 49 cannot be simultaneously operated only one of the relays CB and CF at a time, can be operated. When CF is operated the phase I, phase 2 and phase 3 phase wires from the supply mains are connected to the upper, middle and lower terminals of the motor respectively. With CB operated the phase 1, phase 2 and phase 3 wires are connected to the lower, middle and upper terminals of the motor respectively.

The cams 48 are so shaped and located that when piston 27 moves oil-centre sufiiciently to bring one of the switch triggers 5% into engagement with the cooperating cam 48, the trigger remains operated by the cam during further movements of the piston in the same direction, to the end of its stroke.

The points in the stroke of piston 27, when switches 49 operate, must be set in from the ends of the piston stroke by an amount which will be dictated by the acceleration capabilities of motor 9 and also by its slowing down time. To minimise the risk of piston 27 reaching the end of its stroke before motor, worm,

rack combination

9, 10, 11 of the slow drive can come to its aid, the switching points should be fairly close to the central position. When the slow drive is moving the work table too fast, or in the wrong direction (for instance when the command signals call for an abrupt reversal of direction on the part of the work table), it is desirable that motor 9 should be switched off at an early point in the recovery stroke of the piston 27 to avoid a prolonged over-run of the slow drive. This calls for a wide separation of the switching points from the central position. A suitable compromise must be made between these two conflicting requirements.

To minimise hunting, additional cam and switch combinations such as 48, 49 may be provided. These will be located so as to operate on movements of the piston nearer to the ends of its stroke than the operating points of switches SF and SB. These additional switches have contacts each of which can operate a single relay and this relay has three pairs of normally open contacts each pair being connected in parallel with a resistance, and one of these resistances being connected in series with one of the three phase wires from the supply mains on the left of FIG. 16. Normally, when only switch SP or SE is operated, motor 9 will be energised at reduced power. When one of the additional switches is operated, however, the three series resistances will be short circuited and full power will be applied to motor 9.

In all embodiments of the invention it is preferable that the complete chain of linkages from one to the other of the two objects between which relative movement is required, should be free from back-lash and elasticity since any lost motion due to these factors constitutes an absolute limit to the accuracy with which the driving mechanisms can control the relative movements between the two objects. It is nevertheless permissible to have a certain amount of back-lash and elasticity present in such linkages (indeed it is scarcely possible to eliminate them entirely in practice), so long as the total lost motion remains substantially below the limits of accuracy with 9 which the relative motion between the two objects is to be controlled.

The slow drive should preferably be irreversible in the sense that it cannot yield under loads imposed upon it by the driving action of the quick drive.

A certain amount of reversibility is permissible however so long as such reversibility operates at an extremely low mechanical advantage and so long as it involves the acceleration of some element of substantial mass. This may be illustrated in connection with the embodiment of the invention shown in FIGURES 9, 10, ll, and 12. Whereas the worm It can be rotated by motor 9 to propel rack 11, an endwise thrust on the rack 11 cannot cause the worm 10 to revolve to any significant extent because the angles of the mating teeth are such as to provide an overwhelming mechanical disadvantage to such reversed movement. If the mating teeth were cut at a more oblique angle, however, the worm 10' might be capable of turning in the presence of high endwise thrusts imparted to rack 11 by the quick drive 17. This would not seriously impair the operation of the machine, however, so long as the worm or elements rigidly coupled to it had high inertia so as to retard any reversed motion of the rack 11. Suppose, for instance, that the quick drive is acting under command signals calling for movement of the work table 4 to the right as seen in FIGURE 9. If worm 10 were slowly to start turning in response to the end thrust on rack 11, then the rightwards movement of work table 4 would slowly fall behind the rate of movement called for by the command signals. This would produce (in a monitoring feed-back system such as that illustrated in FIGURE 14) a slowly rising deficit of signals from the monitoring equipment (i.e. from light cell 57) as against the incoming command signals (i.e. from reading head 53). In response to this deficit, quick drive 17 would increase the speed of its driving action to the right and would soon operate one of the switches 49 to energise motor 9. Even if motor 9 took some time to reach full speed, it Would develop enough torque to arrest the rotation of worm 10 earlier than this and in any event the acceleration of worm 10 due to relatively high inertia would be slow enough to give motor 9 time to start and reverse the movement of piston 27 before it reached the end of its stroke. Without this inertia worm 10 might yield so quickly that the quick drive could exhaust its stroke before motor 9 had picked up sufficiently to come to the assistance of the quick drive.

The invention has been illustrated by embodiments in which the quick drive is in the form of a hydraulic piston and cylinder arrangement and the slow drive is in the form of a device such as a lead screw and nut or a rack and worm, driven by a rotary motor. The quick drive and the slow drive can of course take other forms within the scope of the invention. For instance, in an extremely high performance equipment the quick drive may take the form of an electro-mechanical transducer such as a ma-gneto-strictive transducer and a suitable slow drive for use with this quick drive might take the form of a short stroke hydraulic piston and cylinder arrangement such as is used for the quick drive in the embodiment of the invention illustrated in FIGS. 9 to 11. This arrangement would have an extremely rapid response and a correspondingly short range of movement, and if this range of movement proved to be inadequate it could form part of a triple drive with a still slower drive such as the slow drive illustrated in FIGS. 9 to 11 operating in tandem with the magneto-strictive transducer and the hydraulic piston and cylinder; each of the relatively slower drives being energised so as to prevent the next quicker drive from exhausting its stroke.

I claim:

1. Apparatus for driving a first object in relation to a second object along a predetermined path, comprising two driving mechanisms coupled together and to the first object and the second object in such a manner that movements of the first object relative to the second object along the said path are dependent upon the algebraic sum of the individual driving actions of the two driving mechanisms, the first driving mechanism being capable of a relatively high driving acceleration over only a relatively short range of driving action and the second driving mechanism being capable of a relatively long range of driving action but being arranged to provide only a relatively low driving acceleration, means, responsive to a signal representing the motion required of the first object in relation to the second object along the said path, for actuating the first driving mechanism and means, operable before the first driving mechanism, in moving from the middle of its range of driving action, has reached an end of its range of driving action in responding to a signal as aforesaid, for actuating the second driving mechanism in such a sense as to supplement the driving action of the first driving mechanism initiated by the said signal, whereby the first driving mechanism is prevented from reaching the end of its range of driving action.

2. Apparatus as claimed in claim 1 in which the means for actuating the second driving mechanism is operated when two parts of the first driving mechanism, which execute relative movement to provide the driving action of the first driving mechanism, have moved to the relative positions they occupy when the first driving mechanism is at a predetermined distance from an end of its range of driving action.

3. Apparatus as claimed in claim 2 in which the means for actuating the second driving mechanism comprise two mutually cooperating members one coupled to one of the said parts of the first driving mechanism and the other to the other of the said parts, the second driving mechanism being actuated under control of the said two members.

4. Apparatus as claimed in claim 3 in which the second driving mechanism comprises an electric motor and the said two members cooperate to connect the said motor to a source of electric current.

5. Apparatus as claimed in claim 3 in which the second driving mechanism comprises a rotary motor coupled to a device for transforming rotary motion derived from the motor into motion in the direction of the said path.

6. Apparatus as claimed in claim 3 in which the said two members come into engagement to actuate the second driving mechanism when the said parts of the first driving mechanism have moved to the relative positions they occupy when the first driving means is at a predetermined distance from an end of its range of driving action and in which the said two members disengage to terminate the actuation of the second driving mechanism when the said parts of the first driving mechanism, having moved to the relative positions they occupy when the first driving means is at the said predetermined distance from an end of its driving range, leave those relative positions on the return of the first driving means towards the middle of its range of driving action.

7. Apparatus for driving a first object in relation to a second object along a predetermined path, having a first driving mechanism comprising a hydraulic piston and cylinder arrangement and a second driving mechanism comprising a rotary driving motor coupled to a device for transforming rotary motion derived from the said motor, into motion in the direction of the said path, the two driving mechanisms being coupled together and to the first object and the second object so that relative movement between the two objects along the said path is dependent upon the algebraic sum of the driving actions of the two driving mechanisms, the hydraulic piston and cylinder arrangement having a short stroke and being controlled by a high performance hydraulic valve, the second driving mechanism having a range of driving action which is long in relation to the said stroke but being arranged to provide a driving acceleration which is low in relation to that of the hyddraulic piston and cylinder arrangement, a closed 1 1. servo loop for controlling the relative motion between the two objects in the direction of the said path comprising means for receiving signals characteristic of desired relative motion between the said objects along the said path, means for receiving signals characteristic of relative motions taking place between the said objects along the said path, means for comparing the said signals so received and continuously generating an error signal representing the diiference between the said signals, an actuator for the hydraulic valve, means for continuously operating the actuator in accordance with the instantaneous value of the error signal, cooperating members coupled respectively to the piston and cylinder of the said hydraulic arrangement engaging when the piston reaches a predetermined position between the middle and an end of its stroke, means for energising the motor of the second driving mechanism in such a sense as to supplement the driving action of the hydraulic arrangement when the said members engage as aforesaid and de-energising the motor when the members have moved out of engagement.

8. Apparatus as claimed in claim 7 in which the said transforming device comprises a rack and Worm combination.

References Cited in the file of this patent UNITED STATES PATENTS 1,961,090 Smith May 29, 1939 2,367,492 Pickett et a1 I an. 16, 1945 2,426,910 Wilson 2 Sept. 2, 1947 2,535,909 Ernst Dec. 26, 1950 2,684,443 Tiball i July 20, 1954 2,785,353 Fenmore v Mar. 12, 1957 2,835,142 Foster a- May 20, 1958 2,867,759 Comstock Jan. 6, 1959 2,907,937 Apgar Oct. 6, 1959 FOREIGN PATENTS 760,321 Great Britain Oct. 31, 1956 OTHER REFERENCES Publication: Aircraft Production, July 19-55, pages 267- 272, Computer-Controlled Machine-Tools, by-D. T. N. Williamson.