The present invention relates to a movement restraining device that has particular application for restraining relative movement between the parts of a linear motor, preferably a tubular linear motor.

GB 2,235,783-A discloses a basic linear motor which can be used in combination with the brake of the present invention. The disclosure of this document is hereby incorporated by reference.

Linear electric motors are now in widespread use in industrial applications requiring the rapid and accurate positioning of one component relative to another. An example of such an application is the rapid and precise positioning of a test probe over a printed circuit board to check the circuit pathways thereof, prior to loading it with expensive integrated circuits. A further application is one in which very smooth motion is required, with imperceptible variation in the velocity of the component being moved. Linear motors are able to achieve tight and precise servo control over the whole range of motion of the motor's rotor relative to its stator. Further, a high resolution of detected movement is obtainable. The extreme accuracy and high speeds obtainable with a linear motor have served increase their popularity.

One drawback of linear motors is that they contain, in and of themselves, no means for stopping relative movement, especially in the event of a power outage. In the event that current is interrupted to the linear motor, the load carried by the linear motor will continue to move under gravitational and/or inertia forces. The use of high performance linear bearings which minimise sliding friction exacerbates this problem. The problem is particularly acute when linear motors are mounted with their axis in a vertical direction, as shown in Figure 1 of the accompanying drawings. In this illustration, the linear motor comprises a stator 10 mounted to a vertical wall 12. The stator 10 has a vertically disposed passageway 14 and a rotor 16 moves vertically therein. The rotor 16 has a plurality of permanent magnets disposed along its length, as disclosed in GB 2,235,783-A. Please note that GB 2,235,783-A refers to the stator 10 as an "armature" and to the rotor 16 as a "stator".

The rotor 16 has end stops 18 and carries a load 20 at its bottom end. If the power suddenly fails the relative force that serves to move and position the rotor 16 relative to the stator 10 disappears and the rotor 16 falls under gravity until the top end stop 18 hits the stator 10. If the power fails when the rotox 16 is in its uppermost position, then the end stop 18 can impact on the stator 10 with, substantial force and can cause damage to the rotor or stator. Furthermore, persons working underneath the linear motor can be struck by the falling load 20.

One conventional method for mitigating against this drawback is to use a counter-balancing arrangement, as shown in Figure 2 of the accompanying drawings. In this arrangement, the top end of rotor 16 is connected to a rope or chain 22 and is passed around pulleys 24 such that the weight of the rotor 16 and load 20 is counter¬ balanced by a counter-balancing load 26. In this arrangement, if power is suddenly lost, the counter-balancing load 26 balances against the weight of the rotor 16 and the load 20 such that the rotor 16 and load 20 do not suddenly drop in the stator 10. Although this arrangement mitigates against the above-mentioned problem, it introduces new problems in that the pulley configuration takes up space, requires maintenance and requires the load 26 to be changed in accordance with the size of the load 20.

For robot applications, an example of which is shown in Figure 3, the counter-balancing solution may not be possible. A robot that is designed to pick up a load, move it and put it down again cannot be effectively counter-balanced because the value of the load 20 changes with time. Taking the example of Figure 3, the linear motor stator 100 is connected to a rotational joint 110. In the positions shown, the rotor 160 moves vertically with respect to the stator 100. However, for other positions of the rotational joint 110 the rotor 160 will move in. different directions with respect to the stator, sometimes horizontally. The range of movements possible with the robot of Figure 3 precludes the use of a counter-balancing arrangement. There is therefore a desire for a means to restrain movement of the linear motor rotor 16 with respect to its stator 10, especially in the event of a power outage. Preferably, the means should be suitable for use in any orientation of the linear

motor.

Thus, a need exists for a brake for stopping linearly moving loads and particularly for use in stopping linearly moving loads in the event of a power outage, emergency stop, parking, or similar situations. WO 02/38977 discloses a linear motor brake. As will be appreciated from

Figure 6 of this document, a great many parts are required and the brake is non- compact and heavy. Furthermore, the brake relies on having a supply of compressed fluid (such as air) and requires a secondary braking track to be set up alongside the linear movement track (see Figure 1 of the document). The use of compressed air increases complexity and is also a burden in terms of power supply.

It would be beneficial to provide a linear motion brake that does not use a pneumatic or hydraulic fluid and which is "passive" in its braking action. Furthermore, there is a desire for this brake to be lightweight, easy to assemble, compact in size and for it to not unduly affect the performance of the linear motor. Preferably, any such brake is retro-fϊttable to any existing linear motor, particularly a tubular linear motor.

The present invention addresses these and other problems by providing, in a preferred form, a brake for restraining linear movement between two parts that otherwise move linearly relative to one another in the axial direction. Such a brake is preferably rigidly connectable to a first of said two parts and preferably comprises a gripping member that is arranged, in use, to grip a second of said two parts. In tin ^' s manner, the gripping member can restrain movement of the second part relative to the gripping member in at least a first axial direction of movement.

Preferably, the gripping member itself is axially movable so that, in use, the gripping member can move axially relative to the first part.

The capability for axial movement of the gripping member allows the gripping member to respond to tiny relative movements between the two parts. The axial movement capability of the gripping member is preferably very small suck that it is capable of moving axially over a very small distance, preferably less than 5nim ₅ more preferably less than 3mm.

A-

It has been found to be convenient for the gripping member to be arranged to grip the second part with a radial gripping force. Such a radial gripping force is achieved in a preferred embodiment by providing the gripping member with a plurality of fingers movable in the radial direction so as to grip or ungrip the second part as the fingers move towards or away from the second part respectively.

The brake is preferably arranged such that axial movement of the gripping member in a first axial direction causes the gripping member to grip the second part more strongly and/or axial movement of the gripping member in a second axial direction (opposite to the first axial direction) causes the gripping member to grip the second part less strongly. This allows a brake having a uni-directional application to be provided. In the context of Figure 1, the brake can be arranged such that the "first axial direction" is the downwards direction. In this instance, as the rotor 16 attempts to move downwards under gravity relative to the stator 10, the gripping member grips the rotor 16 more strongly to prevent such movement. However, movement of the rotor 16 in an upward direction causes the gripping member to grip the rotor 16 less strongly and thus allows relative movement between the rotor 16 and the stator 10. This in a preferred embodiment allows the brake to be reset using solely the power of the linear motor.

Preferably, the gripping member is arranged, in use, to grip the second part lightly such that movement of the second part in an axial direction causes an initial corresponding movement of the gripping member in this axial direction. The gripping member is thereafter preferably urged to grip the second part more or less strongly in accordance with the direction of movement of the second part.

It is preferable to provide a limit member for limiting the axial movement of the gripping member. Such a limit member may additionally or alternatively provide a limit on the amount of gripping force that the gripping member applies to the second part.

The brake preferably comprises a disengagement member arranged, in use, to disengage the gripping member from the second part so as to allow free movement of the second part relative to the gripping member in at least the first axial direction of

movement, preferably also in the second axial direction of movement.

The disengagement member thus allows the brake to be completely disengaged such that normal motion of the second part relative to the first part is not impeded. The disengagement member is preferably arranged to release the gripping member such that it re-engages with the second part as the gripping member moves in the first axial direction.

When the gripping member comprises a plurality of fingers movable in the radial direction, the disengagement member is conveniently configured to open up the fingers of the gripping member as the gripping member moves in the second axial direction.

A preferred embodiment of the invention uses an electromagnet and a ferrous material (preferably disc or ring-shaped) that are arranged to be held together when the electromagnet is activated. The holding together of the electromagnet and the ferrous material can be arranged to ensure contact between the disengagement member and the gripping member such that the gripping member is substantially prevented from gripping the second part. Preferably, the electromagnet is connected to a brake housing and the ferrous material is connected to the gripping member. The brake housing is preferably that part of the brake that is rigidly connectable to the first of the two parts.

The gripping member is preferably positioned such that it may be acted upon by an external resetting member such as an end stop arranged at the end of the second part, so as to cause the disengagement member to disengage the gripping member from the second part. In this case, the electromagnet is preferably positioned such that it holds the gripping member in a fixed position relative to the first part when the disengagement member causes the gripping member to no longer grip the second part. In this manner, the electromagnet and corresponding ferrous material can be used as a holding means which hold the brake in the disengaged position. Then, when power is cut from the electromagnet, the gripping member is able to be released from the disengagement member such that it can grip the second part.

Movement of the second part in the first direction thereafter causes the gripping member to restrain movement of the second part, hi this way, the brake can be activated simply by cutting power to an electromagnet.

When the resetting member is an end stop on the linear motor rotor, the brake can be fully disengaged from the second part by driving the rotor in the second direction until the end stop bears upon the gripping member and pushes it onto the disengagement member. This same force also serves to bring the electromagnet and ferrous material together which are then activated to hold the brake in the disabled position. The brake preferably has a housing, which housing has a tapered part that, directly or indirectly, acts on a tapered part of the gripping member so as to cause the gripping member to grip the second part more strongly. The tapered part of the housing may indirectly act on the tapered part of the gripping member via the intermediary of a bearing. Such a bearing serves to reduce the effective friction between the two tapered parts and ensures that the friction between the second part and the gripping member is greater than the friction between the housing and the gripping member. This helps to prevent the second part slipping against the grip of the gripping member. The bearing is preferably radially compliant and may be embodied by, for example, a helical tension spring that is formed in a loop to encircle the gripping member or by a rubber annulus (donut shaped) around the gripping member.

The housing preferably surrounds the gripping member. The gripping member preferably surrounds the second part in use and is substantially rotationally symmetric about the axis of the second part. This allows the gripping member to act on the whole circumference of the second part and provides for a stronger grip. Means are preferably provided to bias the gripping member against the second part. A biasing member, which can be a spring, is particularly convenient. In an especially convenient embodiment, the biasing member and the bearing member are embodied by the same part. In the best mode for performing the invention, the function of both the biasing member and the bearing member is carried out by a

helical tension spring that is pre-tensioned so as to squeeze the gripping member against the second part.

Movement of the gripping member in the first axial direction preferably acts against the biasing member and so transforms axial movement of the gripping member into a radial grip force acting on the second part. This effect is most preferably achieved by utilising a taper on the inside of a housing which acts, via said biasing member, on a taper on the outside of the gripping member.

Movement of the gripping member in the first axial direction preferably compresses the biasing member in the radial direction and such radial compression provides an equal and opposite reaction force that pushes the gripping member radially against the second part.

When a tension spring is used as the biasing member or the bearing member, it is preferable that all of its coils are separated from one another in all of the positions it can adopt during use. This prevents a buckling mode collapse of the tension spring as it is compressed radially.

In the most preferred embodiment, the brake is for particular application to a linear motor and the first and second parts are the two relatively moving parts of a linear motor, namely the rotor and the stator thereof. The invention is of particular applicability to tubular linear motors in which the second part is a tubular rotor. Preferably, such a rotor is cylindrical and/or is smooth. The present invention can be embodied in a compact configuration.

The brake can be adapted to be connectable directly to the stator of the linear motor. In this way, the brake can be retro-fitted to already existing linear motors and can be arranged to act on the rotor as it moves relative to the stator. The various components of the brake, such as the gripping member, the disengagement member and/or the housing are preferably formed of plastic. This is beneficial because if metallic elements are used in the vicinity of the linear motor rotor, a phenomenon known as "cogging" occurs in which the movement of the rotor in the stator is no longer smooth and judders. Ths occurs when ferrous materials interfere with the magnetic field and also when non-ferrous, but conductive,

materials (such as aluminium) have eddy currents induced in them as the rotor (containing permanent magnets) moves relative to them. It is thus an advantage of the present invention that the main components that come into the vicinity of the rotor can be made from plastic. If an electromagnet and ferrous member, such as a disc, are used to hold the brake in the disabled position, these components can be situated far enough away from the rotor to avoid the undesirable cogging phenomenon. This is achieved by radially extending outwards the housing and apart of the gripping member so as to mount the electromagnet and the ferrous material on the extension away from the rotor.

In one embodiment of the invention a circuit can be provided which monitors a current flow between the electromagnet and its associated ferrous member. When the current flows, it indicates that the electromagnet and ferrous member are in contact and so indicates that the brake is disabled. When no current flows, it is an indication that the two components have been separated and so the brake is activated. This allows feedback to a controller or the like of the status of the brake.

The brake is particularly well suited to act as a fail-safe brake because it can be arranged to restrain movement of the second part in at least the first axial direction when the power to the brake is cut. The present invention also comprises a linear motor including the brake having any, some or all of the preferable features mentioned above. The invention is particularly useful when the linear motor is mounted in such a way that there is at least a component of vertical movement.

The present invention also provides a novel use of a helical tension spring in a configuration where the ends of the spring are connected together to form a loop, the spring acting as both a linear bearing and as a means for providing a radially inward force.

The present invention also provides a novel method of disengaging a brake in which the end stop of a moving member of a linear motor is caused to bear against the brake so as to disengage a gripping member of the brake from the moving part.

An electromagnet is preferably energised to hold the gripping member in this disengaged position. The brake can be constructed in accordance with the above mentioned preferable aspects of the invention.

The invention also provides a novel method of applying a brake. In this method the removal of electrical power to an electromagnet causes a gripping member to be biased against a moving part, with any further attempted movement of the moving part in a particular direction causing said gripping member to grip the moving part with a stronger grip. Again, this method can be carried out using the above mentioned brake design. Another preferred form of the invention provides a brake for a linear motor, said brake comprising a gripping member for gripping a moving part, said gripping member comprising a plurality of fingers movable in the radial direction so as to grip or ungrip said moving part; a disengagement member arranged, in use, to prise open said plurality of fingers so as to allow free movement of said gripping member relative to said moving part.

The active prising open the fingers of the gripping member with the disengagement member prevents any accidental or incidental contact between the gripping member and the moving part of the linear motor when the brake is in the disengaged position. There is preferably provided a biasing member which, in use, biases the fingers of the gripping member against the moving part. In this case, the action of the disengagement member to prise open the fingers works against the biasing force of the biasing member. When the disengagement member is moved so as to no longer prise open the fingers, the biasing member causes the fingers to once again act against the moving part to retard movement of the part.

Preferably, the biasing member is donut shaped and encircles the fingers of the gripping member. A helical tension spring looped back on itself has been found to be an advantageous biasing member. The biasing member can also act as a linear bearing between the fingers of the gripping member and an internal surface of a provided housing.

The disengagement member is preferably annular and preferably has a tapered leading edge to facilitate the prising open of the fingers of the gripping member.

The disengagement member is preferably arranged to be held in position holding open the fingers of the gripping member by an electromagnet. In this way, if power suddenly fails, the electromagnet is released and the disengagement member is pushed out from underneath the fingers of the gripping member by the action of the biasing member, which tends to close the fingers of the gripping member against the moving part.

The present invention will now be further described by way of non- limitative example only, with reference to the accompanying drawings in which:-

Figure 1 schematically shows a vertically arranged linear motor comprising a stator and a rotor;

Figure 2 schematically shows the linear motor of Figure 1 which has been counter-balanced to prevent sudden dropping of the rotor in the event of a power failure;

Figure 3 schematically shows a robot comprising two linear motors and one rotational joint;

Figure 4 is a perspective view of a brake according to the present invention;

Figure 5 is a schematic cross-sectional view showing the main components of the brake according to the present invention;

Figure 6 is an exploded view showing the cover member, gripping member, housing and disengagement member of the brake according to the present invention;

Figure 7 is a cross-sectional view drawn to scale of the brake of the present invention in the disengaged position; and Figure 8 is a cross-sectional view, similar to Figure 7, but with the brake in the engaged position.

Figure 4 shows a perspective view of a brake according to the present invention. The visible components are a housing 200, cover member 21O ₃ gripping member 220, mounting bolt 230, grub pin 240 and retaining screw 250. There is also

provided an electrical power cord 260 for supplying electrical power to an electromagnet (not shown). The housing 200 is designed to be rigidly mounted to the bottom of the stator 10 of the linear motor shown schematically in Figure 1. The rotor 16 passes through the central open hole in the brake where it is directly gripped by the gripping member 220 when the brake is activated. The gripping member 220 is axially movable -with respect to the housing 200 and is rigidly connected to the cover 210 by means of two grub pins 240. As is shown in Figure 6, the cover member 210 is capable of axial sliding movement with respect to the housing and is guided in such movement by the provision of webbing 212 on the inner part of cover member 210 and slot 202 in housing 200 that is shaped and configured to closely receive the webbiag 212.

The internal components of the brake are shown schematically in Figure 5. Please note that Figure 5 shows the brake of Figure 4 vertically inverted, i.e. with the housing 200 at the top of the page and with the cover member 210 nearer the bottom of the page. For trie sake of simplicity, the gripping member 220 is shown as being integrated with the cover member 210. For reasons of manufacturing ease, these members are separate in the preferred embodiment and are connected together by grub pins 240.

The brake is shown in Figure 5 in the engaged position, although a clearance is represented between the gripping member 220 and the rotor 16 for the purposes of clarity.

As can be better seen from Figure 6, the gripping member 220 comprises a plurality of fingers extending in the axial direction. The fingers flare radially outwardly so as to provide an outer taper 290 on the outer surface of the gripping member 220. The part of the gripping member 220 that grips the rotor 16 is surrounded by an annular part 270 of housing 200. The internal contour of the annular part 270 has a taper 280. This taper has a relatively small angle, for example 4°. Limit members 330, 340 are provided on the gripping member 220 and the housing 200 to limit movement of the gripping member in the axial direction relative to the housing.

A disengagement member 350 is provided on said housing in the vicinity of the fingers of the gripping member 220. As will be appreciated from Figure 5, the disengagement member 350 also serves to limit axial movement of the gripping member 220 in the upward direction. The disengagement 350 is shown integral with the housing 200 in Figure 5 but, as is shown in Figure 6, is can be a separate member that is screwed to the housing and this is preferably the case to aid in assembling the brake.

As shown in Figure 6, the disengagement member 350 is an axially extending annular member that has a tapered leading edge on its outer surface. This tapered leading edge facilitates the opening up of the fingers of the gripping member 220, as will be apparent from Figure 5.

An electromagnet 300 is provided in the housing radially spaced away from the rotor 16 and gripping member 220. A disc 310 of ferrous material is disposed on the cover 210, also radially spaced from the rotor 16 and the gripping member 220. A linear bearing member 320 is disposed in the annular space between the gripping member 220 and the annular part 270 of the housing 200. This linear bearing is donut shaped and encircles the gripping member 220 such that it can roll up or down its circumference. This rolling action of the bearing member reduces any frictional forces which may serve to impede relative movement of the gripping member 220 with respect to the annular part 270 of the housing 200. The bearing member can be made from a donut shaped rubber ring or from a tension spring looped back on itself so as to encircle the gripping member 220. The bearing member is preferably radially compliant. Thus, as the gripping member 220 moves downwards in Figure 5 the bearing member 320 gets compressed between the taper 280 ^' of the annular part 270 and the taper 290 of the gripping member 220. This radial compression creates an opposite and equal reaction force that serves to push the fingers of the gripping member 220 against the rotor 16. This is the basis for the braking action of the present invention. It will therefore be appreciated that movement of the gripping member 220 in a first axial direction (downwards in Figure 5) causes the gripping merαber 220 to grip the rotor 16 more strongly.

Correspondingly movement of the gripping member in a second axial direction, opposite to the first axial direction, (upwards in Figure 5) causes the gripping member 220 to grip the rotor 16 less strongly.

The bearing member 320 can also act as a ^" biasing member which serves to cause the gripping member 220 to lightly grip the rotor 16. This light gripping of the rotor 16 is present for all axial positions of the gripping member 220, except when the brake is disengaged by the disengagement member 350 (as will be explained later). This light gripping means that external movement of the rotor 16 (for example under gravity) causes a corresponding movement of the gripping member 220. For example, if the rotor 16 should move downwards as shown in Figure 5, the light gripping of the gripping member 220 against the rotor will cause the gripping member 220 to also move downwards. This will serve to radially compress bearing/biasing member 320 such that the gripping member 220 grips the rotor 16 more strongly. Further downward force on the rotor 16 will thus be transmitted by the gripping member and the bearing/biasing member 320 into a radial force against the rotor 16. As a result, gripping member 220 grips the rotor 16 more strongly as the rotor 16 attempts to move downwards. The brake therefore passively applies a gripping force to the rotor 16 commensurate with the amount of force that is acting on the rotor 16 to move it. The maximum gripping force is applied when the limit member 330 on the annular part 270 comes into contact with the limit member 340 on the gripping member 220. This gripping force can be quite strong, and it is possible for the brake of the present invention to hold a rotor 16 having a 50kg load on it, for example.

A cross-sectional view of the brake in the engaged position is shown in Figure 8. The rotor 16 is not shown for clarity. The orientation in Figure 8 is vertically reversed compared to that in Figure 5. Thus, Figure 8 shows a brake that brakes against movement of the rotor 16 as it tries to move upwardly.

Referring back to Figure 5, the brake can ^" be disengaged by actuating the linear motor such that the rotor 16 moves in the second direction (upwardly in Figure 5). The gripping member 220 moves upwardly also because it grips the rotor 16.

This serves to reduce the compression of the bearing/biasing member 320 and so serves to reduce the grip of the gripping member 220 against the rotor 16. As the gripping member 220 moves upwards, its fingers bear upon the disengagement member 350 and the disengagement member 350 serves to open ixp the fingers of the gripping member 220. This prising open of the fingers occurs against the biasing . force of the biasing member 320 which naturally serves to close the fingers such that the gripping member grips the rotor 16.

As the rotor 16 moves up, the grip provided by the gripping member 220 diminishes to a point at which it is possible for the rotor 16 to mo ^" ve upwards relative to the gripping member. If the rotor 16 is propelled upwards by the linear motor the end stop 18 of the rotor will eventually bear against the cover member 210. Continued actuation of the linear motor causes the end stop 18 to push the cover member 210 upwardly which in turn causes the gripping member 220 to be pushed upwardly such that its fingers are spread out by the disengagement member 350. At the same time, the ferrous material 310 comes into contact with th.e electromagnet 300. The electromagnet can then be energised with the result that the gripping member 220 is held open by the disengagement member 350. In this position the brake is disabled and provides no gripping force at all against the xotor 16. Figure 7 shows a cross-section of the brake in the disabled position, with trie ferrous material 310 hard up against the electromagnet 300 and with the fingers of the gripping member spread out around the disengagement member 350.

The electromagnet 300 need only be strong enough to overcome the biasing force created by the elasticity of the fingers of the gripping member 220 and by the biasing member 320 which would otherwise tend to move the gripping member 220 away from the disengagement member 350. A strong electromagnet is therefore not required, allowing a cheap construction that uses little power.

When the electromagnet is deactivated, the resilience of tb_e fingers of the gripping member 220 together with the resilience provided by the Hasing member 320 together with the external shape of the disengagement member 350 and the internal profile of the fingers of the gripping member 220 cause the gripping member

220 to move away from the disengagement member 350 in the first axial direction. The biasing member 320 thereafter causes the gripping member 220 to grip the rotoi 16 and any further movement of the rotor 16 in the first axial direction serves to strengthen the grip of the gripping member 220 thereon, as described above. It will be appreciated that the bearing/biasing member 320 is optional and trie invention can be carried out in substantially the same manner by having the taper 2SO on the annular part 270 of the housing 200 act directly on the taper 290 on the outer surface of the gripping member 220. In this case, provision must be made for ensuring that the friction between the rotor 16 and the gripping member 220 is greater than the friction between the taper 280 and the taper 290. Otherwise, the rotor 16 will slip in the gripping member 220. Friction reducing coatings (e.g Teflon) may be applied to the tapered parts to assist in preventing slipping of the rotor. Similarly, the electromagnet and accompanying ferrous material is optional and can be replaced by other means for holding the gripping member 220 against the disengagement member 350, for example a ratchet mechanism or suction pad.

As can be seem from Figure 6, the brake of the present invention has relatively few parts as will be appreciated by comparing Figure 6 of this application with Figure 6 of WO 02/38977. Furthermore, the brake uses a simple electrical actuator to hold it in the disengaged position and fails-safe into the engaged position should electrical power be lost. There is no requirement for complicated or expensive pneumatic or hydraulic arrangements and the brake can be brought into th.e disengaged position using the power of the linear motor itself. Furthermore, the brake can act directly on the rotor 16 of the linear motor and can be connected directly to the stator of the linear motor. The brake is compact in size and can be used on linear motors in any orientation. If braking is required in both directions, two brakes can be positioned in opposite orientations end to end. In this configuration it is not possible to reset the brake using the movement of the linear motor and so the above-described brake may require modification to use external resetting means or to replace the electromagnet and ferrous material with means that actively pull the housing 200 and cover member 210 together, for example a piston.

The above mentioned description relates to a preferable embodiment and features of the embodiment may be omitted or replaced by equivalent features in accordance with the scope of the appended claims.