DEVICE FOR A MOVABLE CARRIAGE CARRIED IN A BEARING OF AIR

Abstract: The present invention relates to an arrangement for compensating for dynamic effects of a slide mounted on a beam via an air gap. Input air pressure (Pin1 and Pin2) is delivered to the air gap. A bearing cushion arrangement (3, 4) compensates for the limited pressure rigidity of the air gap, by equalising air pressure in air gap (11, 12) between beam and bearing cushion arrangement, so as to compensate for rotation about the Y-axis.

Description: Device for a movable carriage carried in a bearing of air The present invention relates to an arrangement of the kind defined in the preamble of Claim 1. In particular, the invention relates to an arrangement for compensating for dynamic effects of a slide mounted on a beam with the aid of an air-cushion bearing. BACKGROUND OF THE INVENTION In respect of coordinate measuring machines and measuring equipment where it is essential to know the precise position of a slide that moves along a beam at each moment in time, air- cushion bearings are used beneficially between the slide and the beam as movement of the slide is then free from friction and minor variations in the surface are smoothed-out during operation. Air-cushion bearings, however, have a limited (low) rigidity in contact perpendicular to the surface. This makes it difficult to determine the position of the slide precisely in dynamic sequences, since the finger or line of application of the force applied almost always deviates from the gravitational centre of the slide when the slide is accelerated. Many technical solutions have been applied to eliminate such drawbacks. For instance, the air-cushion bearing has been allowed to drag against the underlying surface, which while significantly increasing rigidity also greatly restricts speed and increases the demands placed on the slide surfaces and on the cleanliness of the surroundings. Other variants have utilised the option of measuring against a reference zone. OBJECT OF THE INVENTION One object of the invention is to provide in respect of a slide that shall be moved on a beam an air-cushion bearing that is rigid perpendicularly to the beam. Another object of the invention is to provide with respect to a slide that shall be moved on and along a beam, a path that has the smallest possible twist or turn about an axis (the Y- axis) in the horizontal plane perpendicular to the beam. THE INVENTION The invention relates to an arrangement for compensating for dynamic effects of a slide mounted on a beam through the medium of an air gap, wherein input air pressure is delivered to the gap. The invention is characterised by a bearing cushion arrangement that compensates for the limited pressure rigidity of the air gap, by equalising air pressure in said gap between beam and bearing cushion arrangement, so as to achieve compensation against rotation about the Y-axis. The bearing cushion arrangement preferably includes a pressure measuring unit and a pressure compensation unit arranged close to both ends of the slide in its direction of movement. The two pressure compensation units may be identical to one another and each includes a diaphragm arrangement that has a chamber which faces towards a passageway that is formed through the bearing cushion and opens out into said air gap, and an element which is connected to the diaphragm and actuated by the pressure exerted by the slide. Alternatively, the pressure compensation units may be different to one another, wherewith one of said units includes a diaphragm device which is common to both of the pressure compensation units and includes a first chamber on one side of the diaphragm that is influenced by the air gap of the pressure compensation unit, and a second chamber which is influenced by the air gap of the other pressure compensation unit, and wherein the first pressure compensation unit is provided with an element which is connected to the diaphragm and influenced by slide pressure. Alternatively, each pressure equalising unit may include a passageway through the air- cushion arrangement, and an air line connected to a pressure sensor, wherein the pressure sensor is common to both common equalising units and indicates the differential pressure in the two air passageways, and wherein one compensation unit gives compensation to the bearing cushion arrangement so as to equalise the sensed differential pressure. The differential pressure may then be sensed with a differential pressure sensor for each passageway, and a calculating unit may be used to calculate the pressure difference and the absolute pressure from the signals delivered by the differential pressure sensors, and the pressure difference servocontrolled to zero and the absolute value controlled to a predetermined value, by controlling the input pressures to the bearing cushion units. The arrangement may include a force sensing unit, e.g. of a piezoelectric kind, which has a relatively high rigidity, and whose height and thickness is varied by an applied voltage based on the voltage signal or signals from the air pressure sensor or sensors. A signal based on the voltage signal of the air pressure sensor or sensors can then be delivered to the power sensing unit or power sensing units to compensate for detected variations in air pressure. The slide can rest on the air cushion arrangement through the medium of at least two setting screws, and one power sensor can be placed beneath each screw. Each force sensor can be supplied with a signal based on the signal from a separate air pressure sensor that senses the air pressure between the air cushion arrangement and the beam in a region that lies in the proximity of the region in which the force sensor is seated, as seen vertically. Alternatively, the slide can rest on the air cushion arrangement through the medium of at least two setting screws, wherein a force sensor is located beneath one of the setting screws and a differential air pressure sensor senses the differential air pressure in the air gap between the beam and the air cushion arrangement at two different positions, and a voltage signal based on the sensed differential air pressure is delivered to the force sensor in order to compensate for variations in the differential air pressure. A force sensor may optionally also be placed beneath the other setting screw, wherewith both force sensors are supplied with a biasing voltage but only one force sensor is supplied with a signal based on the differential air pressure. ADVANTAGES AFFORDED BY THE INVENTION Because the air gap against the beam is relatively large in relation to normal or standard air gaps, the arrangement provides greater tolerance with respect to dirt and irregularities on the surface of the beam. The relatively low rigidity can be compensated for by an arrangement in which the air-cushion bearing is either comprised of two parts or is elongate with a rigid coupling to the slide. In this case, the slide may be coupled to the bearing cushion arrangement through the medium of at least two setting screws, of which the outermost screws are located in the proximity of each end of the slide. The invention results in relative rigidity in the direction of rotation about the Y-axis and compensates for dynamic effects in respect of slide movement. BRHCF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 illustrates a first embodiment of an inventive arrangement; Fig. 2 illustrates a second embodiment of an inventive arrangement; Fig. 3 illustrates a third embodiment of an inventive arrangement; Fig. 4 is a part view of Fig. 3 and shows another indicating and control system; and Fig. 5 illustrates a fourth embodiment of an inventive arrangement. DETAD ED DESCRIPTION OF THE DRAWINGS Shown in Fig. 1 is a slide 1 that moves along a beam 2, i.e. in the direction of the X-axis. The slide 1 is mounted on the beam through the medium of two air cushions 3 and 4 placed close to each end of the slide. The slide rests on each air cushion through the medium of a respective setting screw 5 and 6. Each air cushion has a screw field 7 and 8 for respective setting screws. As shown in Fig. 1, an air layer is obtained between the bearing cushions and the beam with resetting between the bearing cushions 3 and 4, by virtue of each cushion having a narrow pipe 9 and 10 that opens into the respective gaps 11 and 12. Provided in the upper part of the bearing cushions is a respective recess 13 and 14, which is preferably round and has a significantly larger diameter than respective narrow pipes 9 and 10. Each recess 13 and 14 accommodates a tensioned diaphragm 15 and 16 respectively. The upper ends of respective pipes 9 and 10 open into a space of thin volume beneath the diaphragm. The pipe returns pressure air from the gap 11, 12 between the bearing and the beam to the volume beneath a diaphragm (3). The screw field 7 (8) is disposed on a lever 17 (18) whose one end rests on a screw support 19 (20) against the actual bearing cushion, and whose other end is deflected, this deflected end of the lever supporting against the upper surface of the diaphragm 15 (16) approximately in the centre thereof. The diaphragm 15 (16) is rigid centrally and as far out radially as possible, although not further than movement of the diaphragm is sufficient to allow full compensation through the medium of the lever 17 (18). The relationship between those parts of the lever located between diaphragm-support and setting screw 5 (6) and between setting screw 5 (6) and screw support 19 (20) is chosen so that when the load on the bearing cushion 3 (4) increases, and therewith the pressure in the gap 11 (12), the pressure through the pipe 9 (10) will also increase, wherewith the diaphragm 15 (16) rises and therewith compensates for the change in position of the screw attachment 7 (8). The apparent stiffness of the bearing cushion (3 (4) against the beam (2) is therewith adjustable through the medium of the relationship chosen with respect to the position of the screw attachment 7 (8) on the lever 17 (18). Thus, when two bearing cushions 3 (4) of this kind are placed one after the other, the rigidity of the slide 1 will be much greater than when it is supported by an air-cushion bearing in a conventional manner and will thus be more independent of longitudinally acting dynamic forces, i.e. less rotational error about the Y-axis. Pressure air is delivered to each bearing cushion 3 (4) in a manner typical in respect of bearing cushions for air-cushion bearings, i.e through a pipe 21 (22) where the air supply end of the pipe is placed in the side of the bearing cushion and the air exhaust end opens into a nozzle 23 (24) at the air gap 11 (12). Fig. 2 illustrates another embodiment of the mechanical resetting facility. In this case, one bearing cushion 30 has essentially the same design as the air cushion 3 of the Fig. 1 embodiment, with the exception that an upper closed "volume" 31 has been included above the diaphragm 32. The recess 33 is thus seated further down in the bearing cushion 30 than the recess 13 of the Fig. 1 embodiment. Inserted above the recess 33 is a sealing element 34 that includes an opening which accommodates the deflected part 35 of the lever 36 that supports against the diaphragm 32, with a fine fit against the part 35 so as to achieve the smallest possible leakage. A narrow gauge hose 38 extends from the volume 31 above the diaphragm 32 to a narrow passageway 39 in the other bearing cushion 37, said passageway extending from one side of the cushion and opening into the air gap 40 between the bearing cushion 37 and the beam 2. Thus, the pressure of the air-cushion bearing at the bearing cushion 37 is delivered to the volume 31 of the bearing cushion 30 via the narrow hose 38. The diaphragm 32 is therewith influenced by the differential pressure between the air bearings 41 and 40. The zero state is obtained with the aid of some suitable spring arrangement 42 that keeps the differential pressure diaphragm 32 in an approximate central position when the diaphragm is not subjected to the effect of dynamic forces. In this respect, rotational movements about the Y-axis are compensated for in the same way as with Z-bearings, apart from the fact that the bearing cushion 30 therewith also compensates for the Z-movement occurring through the resilience of the bearing cushion 37. Normally, the Z-movement that occurs is of less importance. Fig. 3 illustrates an electrically controlled embodiment. Each of the two bearing cushions 45 and 46 includes a respective narrow passageway 47 and 48 which open into a respective air gap 49, 50. A pressure line or conduit 51 is connected to the passageway 47 and a pressure line or conduit 52 is connected to the passageway 48. The pressure lines each give the pressure in the air gaps 49 and 50 respectively. Two alternative indications are described below. According to the first indication shown in Fig. 3, the pressures are transmitted from the pressure lines 40 and 50 to an electric differential pressure sensor 53, the output signal of which will then be proportional to rotation of the slide about the Y-axis. This differential signal is A/D-converted in an A/D converter 59 and the digital signal is then sent to a calculating unit Dl, e.g. a computer, for compensation of the values measured. In another embodiment of the signal processing part of the arrangement, illustrated in Fig. 4, the pressure is measured in each state by an individual pressure sensor 62 and 63 and is converted in a respective A/D converter 60 and 61. Both signals are delivered to a calculating unit D2, e.g. a computer, and utilised digitally to compensate for said movement, wherewith rotation about the Y-axis is proportional to Pι-P 2 and the translation along the Z- axis is proportional to (Pι+ P 2 )/2. As in the case of the other embodiments, pressure air is delivered to each bearing cushion in a manner typical with respect to bearing cushions for air-cushion bearings, i.e. through a pipe 54 (55) whose supply end is placed in the side of the bearing cushion and the exhaust end or outlet end opens into a nozzle 56 (57) at the air gap 49 (50). The calculating unit Dl or D2 may also be programmed to servocontrol the pressure air supply Pi„l and P m 2 to the two pipes 54 and 55, so that the pressure air difference sensed by the unit 53 will be equal to zero. D2 may also have a subprogram that calculates the translation along the Z-axis and adapts the total pressure air supply to the pipes 54 and 55 to a predetermined value. Fig. 5 illustrates an embodiment where only one bearing cushion 70 at one end of the slide is shown, since the other bearing cushion is formed in mirror image. In the case of this embodiment, setting screw 71 of the slide rests via a screw field 72 on an electric force sensor 73, e.g. of a piezoelectric type, where the pressure-related piezoelectric voltage can be compensated for by an applied voltage. The voltage from a pressure sensor 74 of, e.g., the kind shown in Fig. 4, can therewith be returned directly to the force sensor 73, by amplifying the pressure sensor signal and adapting said signal so as to compensate for the position of the screw 71, whereby the bearing is made much more rigid than in the basic design shown in Fig. 4. This arrangement can thus be provided for each bearing cushion, where the signal of an absolute pressure sensor 74 influences the force and position sensor 73 beneath the screw 71, via an amplifier 75 whose amplification is set for the purpose intended. As illustrated by a broken line in Fig. 5, an output signal may optionally be taken from the force sensor 73 and delivered to a control unit 77 that includes the amplifier 75, to which the signal from the pressure sensor 74 is also delivered, and the signal to the force sensor 73 can be controlled by the control unit 77, for instance so that its output signal will remain in the vicinity of a preset value. In the Fig. 5 embodiment, the two air-cushion units 70 are controlled separately and independently of each other. Fig. 6 illustrates an embodiment in which control is effected from a differential pressure sensor of generally the same design as that in Fig. 3. In the case of the Fig. 6 embodiment, the air cushion arrangement includes an air cushion 80 which extends beneath the slide 1 practically throughout the full length of the slide, and which has an air inlet with an air distribution along the length of the cushion, said air inlet having an internal passageway 81 which is branched from an air inlet passageway 82. However, it is only important that air is distributed beneath the cushion, and not the manner in which this distribution is achieved. A sensing passageway 83, 84 for sensing the air pressure beneath the cushion is provided at each end of the bearing cushion 80. A narrow hose 85 extends from the sensing passageway 83 to one side of a differential pressure sensor 87, while a narrow hose 86 extends from the sensing passageway 84 to the other side of said differential pressure sensor 87. An output signal from the differential pressure 87 is delivered to an amplifier 88. Each of the setting screws 89, 90 of the slide rests on a respective screw field 91, 92 adjacent a respective end of the bearing cushion. Either only one force sensor 93, e.g. a piezoelectric type sensor of the same kind shown in Fig. 1, may be placed beneath one setting screw 89 and then supplied with the signal from the amplifier 88 with the other setting screw 90 placed directly against the bearing cushion, as shown in full lines in Fig. 5, or the other setting screw 90 may also be placed on a force sensor of the same type as the sensor 93 as shown in broken lines in said Figure, and both force sensors supplied with the same biasing voltage but only one force sensor 93 supplied with the signal from the amplifier 88. Although not shown, some form of servocontrol can also be included in this embodiment. It is also possible, and suitable, to supply the other sensor with the differential air pressure signal in counterphase. Many variations of the aforedescribed arrangement are possible within the scope of the invention as defined in the accompanying Claims. For instance, although the majority of the embodiments have been described as including two separate bearing cushions for the sake of simplicity, it lies within the scope of the invention for all embodiments to have solely one elongate bearing cushion with equalisation of the air pressure between its ends and the beam. In this case, input air may either be delivered to one position or to several positions, although in a manner such as to distribute the air along the surface of the air gap in a generally uniform fashion.

Claims: CLAIMS 1. An arrangement for compensating for dynamic effects of a slide mounted on a beam through the medium of an air gap, wherein input air pressure P m \ and Pi„2) is delivered to the air gap, characterised by a bearing cushion arrangement (3, 4) which compensates for the limited pressure rigidity of the air gap by equalising air pressure in the air gap (11, 12) between beam and bearing cushion arrangement so as to obtain compensation against rotational movement about the Y-axis. 2. An arrangement according to Claim 1, characterised in that the bearing cushion arrangement (3, 4) includes a pressure supply unit (21, 22) and a pressure compensation unit (9, 15 and 10, 16) in the proximity of both ends of the slide in its direction of movement. 3. An arrangement according to Claim 2, characterised in that the two pressure compensation units (9, 15 and 10, 16) are mutually identical and each include a diaphragm device (15, 16) that includes a chamber facing towards a passageway (9, 10) which extends through the bearing cushion and which opens into said air gap, and an element (17) which is connected to the diaphragm and influenced by the pressure from the slide (1). 4. An arrangement according to Claim 2, characterised in that the pressure compensation units are mutually different, wherein one unit has a diaphragm device (32) which is common to both of said pressure compensation units and which includes a first chamber on one side of the diaphragm influenced by the air gap of the one pressure compensation unit, and a second chamber influenced by the air gap of the other pressure compensation unit; and in that the first pressure compensation unit includes an element (36) which is connected to the diaphragm and influenced by the pressure exerted by the slide (1). 5. An arrangement according to Claim 2, characterised in that each pressure equalising unit includes a passageway (47, 48) through the bearing cushion arrangement and an air line connected to a pressure sensor, wherewith said pressure sensor is common to both pressure equalising units and indicates the differential pressure in the two air lines; and in that the arrangement includes a compensation unit (59, Dl; 60, 61, D2) which gives compensation to the bearing cushion arrangement for equalising the sensed differential pressure. 6. An arrangement according to Claim 5, characterised in that the differential pressure is sensed with a differential pressure sensor for each passageway (47, 48); and in that the arrangement includes a calculating unit (D2) which calculates from the signals delivered by the differential pressure sensors both the differential pressure (P1-P2) and the absolute pressure (Pl+P2)/2) and servocontrols the differential pressure to zero and the absolute pressure to a predetermined value, by controlling the inlet pressures (P in l and P m 2) to the bearing cushion units. 7. An arrangement according to any one of Claims 5-7, characterised by a force sensing unit (73; 93), e.g. of the piezoelectric kind, that has a relatively high rigidity and whose height and thickness are varied by an applied voltage which is based on the voltage signal or signals of the air pressure sensor or sensors (74; 87) for compensating for sensed variations in air pressure, said voltage being delivered to the force sensor unit or force sensor units for compensating for sensed variations in air pressure. 8. An arrangement according to Claim 7, characterised in that the slide (1) rests on the air cushion arrangement via at least two setting screws (71; 89, 90); and in that one force sensor (73; 93) is disposed beneath each setting screw. 9. An arrangement according to Claim 8, characterised in that each force sensor (73) is supplied with a signal based on the signal from a separate air pressure sensor that senses the air pressure between the air cushion arrangement and the beam in a region in the vicinity of the region in which the force sensor (73) is located, as seen vertically. 10. An arrangement according to Claim 7, characterised in that the slide (1) rests on the air cushion arrangement via at least two setting screws (89, 90); and in that one force sensor (93) is located beneath one of the setting screws; in that a differential air pressure sensor (87) senses the differential air pressure in the air gap between the beam and the air cushion arrangement at two different positions; and in that a voltage signal based on the sensed differential air pressure is delivered to the force sensor (93) so as to compensate for variations in the differential air pressure. 11. An arrangement according to Claim 10, characterised in that a force sensor is also placed beneath the second setting screw (90); and in that the two force sensors are powered either with or without a biasing voltage, the one with a superposed signal based on the differential air pressure. 12. An arrangement according to Claim 11, characterised in that the other of said two force sensors is supplied with a signal with the differential air pressure in counterphase.

Inventors: PETTERSSON BO, WIKLUND RUDOLF, PETTERSSON, BO, WIKLUND, RUDOLF
Applicants: JOHANSSON AB C E, PETTERSSON BO, WIKLUND RUDOLF, C.E. JOHANSSON AB, PETTERSSON, BO, WIKLUND, RUDOLF
Application year: 1999
Main classification: B23Q1
IPC codes: B23Q1, B23Q11, F16C29