Linear actuator

The invention relates to an actuator as stated in the preamble of claim 1.

The explanation of the invention departs from a linear actuator of the type comprising a linear movable, tubular activation element. A linear actuator comprises an electric motor, which through a transmission drives a spindle. On the spindle is riding a spindle nut, to which the tubular activation element is secured with its one end. The other end of the tubular activation element is secured with a front fixture to the movable element in the structure, in which the actuator is incorporated, while the actuator with a rear fixture is secured in the stationary part of the structure or vice versa. By rotating the spindle nut, this will either move outwards or inwards on the spindle depending on the direction of rotation, and the tube section thus moves axially outwards or retracts in a manner corresponding to the piston rod on a telescopic cylinder.

Linear actuators are widely used within the area of adjustable furniture, such as beds, chairs or tables, where they are used for adjusting e.g. the height of a table or the mattress surface of a bed.

In relation to adjustable furniture personal safety is an area of priority. A linear actuator exerts large forces during the adjustment of an article of furniture, for which reason there is a risk for personal injury if a part of the body is caught in the furniture. Even though the consideration for personal safety is the main objective, a further consideration should also be shown for the actuator. In case of an adjustable table, an

encounter with a windowsill/computer shelf or another table could e.g. block the movement and result in the actuator being overloaded and damaged.

A device for squeeze protection in connection with a table is known from US 5,495,811 to Ergoflex Systems, where a trapped object will activate a switch in the form of a ribbon switch, which subsequently interrupts the power supply to the actuator. The device requires that the contact element is spread out over all surfaces, where there might be a risk of squeezing. In case of a table where squeezing may occur between an object and the upper side and underside of the tabletop respectively, the outlined solution is difficult to realize and not practical.

Another device is known from US 2003/0052626 Al to Dewert Antriebs- und Systemtechnik, where a measurement of the motor current is carried out as an expression for the load on the motor. A blocking of the mechanism would cause the motor current to rise momentarily, as a result of which the current to the motor may be interrupted to avoid squeezing of an object and overload of the appliance and the motor. The arrangement is as far as it goes fine by being a simple solution, which is easy to realize. The challenge is that the load on an adjustable table varies with the amount of equipment, which is placed on the table and the position of said equipment. The same applies to a bed, where the load depends on the weight of the person using the bed and whether the person is fully or partly positioned in the bed. As the actuator must be able to exert forces which counteract the load, a threshold for the limitation of the motor current will have to be so high that no gentle protection from injuries due to squeezing is given for the person.

A squeeze protection for use in connection with an actuator is further known from WO 03/056976 to Linak A/S. The squeeze protection is arranged in a housing, which with a special fixture can form a connection between the actuator and the table top. The device may be supplied with new systems but is also suitable for retrofitting. The solution is as far as it goes fine but complicated as the electrical connection for the squeeze protection separately will have to be led to the operation unit. Further, it is beset with great difficulties to install the device. Another factor is that the constructional height of the column is extended by the height of the device, for which reason the lowest adjustment height for the column is undesirably increased. Since the device consists of many parts, it is also relatively expensive to realize.

A solution is thus needed, which safely but in a simpler manner solves the problem of injuries due to squeezing during the adjustment of the above-mentioned adjustable articles of furniture.

The purpose of the invention is to provide a solution to the outlined problem, i.e. to achieve a safe squeeze protection, which besides from being simple and compact also functions independently from the load on the appliance. Further, a solution is desired, which if possible is completely integrated with the actuator, so that unnecessary wiring and assembling is avoided. A solution is sought, which as far as possible cooperates with the existing parts in the actuator and thus in an integration does not take up more space and is inexpensive .

This is achieved according to the invention by designing the linear actuator as stated in claim 1, where the squeeze protection device is arranged in direct or indirect connection with an end of the electric motor.

In that the squeeze protection device is built in a compact and simple manner, it is thus possible to place the device in the housing in connection with an end of the motor, thus avoiding difficult assembly work during the assembly of the actuator and the squeeze protection on the appliance, which is otherwise the case in the prior art. Further, extra wiring between a device for squeeze protection, located outside the actuator, and the actuator or to a control box for operating the actuator is avoided, as the required cable connections may be run internally in the housing of the actuator. As the squeeze protection device is compact, it may by means of small modifications of the actuator be arranged in already existing hollows in the actuator. The actuator with the device for squeeze protection may likewise in a more inexpensive manner be manufactured as a joint unit in a production. As it appears, the invention provides a solution, which in a highly satisfactory manner meets the requirements for the invention stated in the introductory part.

The device for squeeze protection comprises a piezo element, which is particularly expedient, as a piezo element does not express a static state, but only a dynamic movement when the piezo element is activated. The activation is caused by the piezo element getting squeezed or bent, at which a brief voltage pulse is generated. In case the piezo element is of the type, constituting the membrane in an acoustic sound generator, this is moved, when being used as an acoustic sound

generator, in that an electric voltage is applied between two points on the disc, causing the piezo crystals to expand or retract . A corresponding effect is achieved when a piezo element is bent at which a voltage is generated and may be used as an expression for the deflection of the piezo element.

In that the device for squeeze protection is located in connection with an end of the motor, it is achieved that the squeeze protection with an end is embedded to an unyielding part in the housing. A second part of the device for squeeze protection is connected to another unyielding part, which is connected to a fixture. Between the two unyielding parts is located a compressible part, which originally is inserted to obtain a mechanical noise dampening in the actuator. The compressible part will, since the forces exerted by the actuator run through this part, be compressed or expanded respectively, depending on whether the actuator exerts pressure or pull between the two unyielding parts.

The compressible part is equipped with a hollow in which the device for squeeze protection may be arranged. The device for squeeze protection thus follows the movement in the compressible part by being placed between the two unyielding parts .

The device for squeeze protection may expediently be designed as a module, consisting of a housing in which a piezo element suspended in a spring is mounted.

To retain the device for squeeze protection in connection with the first unyielding part, the housing is equipped with snap- locking means, which with co-operative snap- locking means, designed in connection with the motor,

retains the device in connection with the motor. The first unyielding part should further be designed so that the piezo element does not rest on this with its entire surface. This is practically done in that the circular membrane-like piezo element with its outer rim rests on an edge, which forms a guide on the first unyielding part . When the second unyielding part is placed on top of the compressible part, having the squeeze protection device mounted, the second unyielding part will press the housing with the squeeze protection further into the compressible part. The spring, which with one end is secured to the housing, will at a point, preferably the center point, on the membrane- like piezo element exerts a pressure on the piezo element, which thus in the normal state, where the actuator does not exert forces, will be supplied with a pretension. This pretension ensures that the piezo element is capable of indicating a squeezing in both directions of the movement. The design with the compressible part and the spring for transferring the forces between the two unyielding parts is particularly expedient in that the piezo element only tolerates a minimal deflection. The spring is thus dimensioned to, in relation to the compressible part, only to transfer a small portion of the forces, exercised between the two unyielding parts.

The piezo element may be equipped with a cable connection, which practically may be designed with a plug connection. The actuator is already furnished with a printed circuit board having a design, which comprises a micro processor to which the piezo element may be connected. However, this does not preclude the piezo element from being equipped with its own separate circuit on the printed circuit board, which independently from other circuits in the actuator may momentarily interrupt

the current to the motor, alternatively briefly reverses the rotation of the motor in order to release a trapped object. Signal cables, used for communicating with a control unit from which the adjustment of the actuator may be performed, are led from the printed circuit board out of the actuator. The design with the micro processor results in that functions, such as determination of the position of the spindle nut during its travel on the spindle, may be performed, but also that data may be communicated from the actuator to the control unit via a cable. The squeeze protection device thus benefits from this design in that it may easily be built-in and incorporated in the actuator, not only mechanically but also electrically.

A linear actuator according to the invention will be explained more fully below with reference to the accompanying drawing, in which:

Fig. 1, shows a height-adjustable table, where the table top is shown as transparent and the actuators are shown separately and in their retracted state,

Fig. 2, shows a cross section through a column for a table with a detailed view of the construction of the squeeze protection device,

Fig. 3, shows and exploded view of the squeeze protection device, and

Fig. 4, shows a schematic diagram, showing the squeeze protection device mounted in connection with a column.

The table, shown in Fig. 1, comprises a lifting column 1 at each side. The lifting column 1 consists of three cooperative telescopic members 2a, 2b, 2c, where the lower member 2a is fixedly connected to a foot 3. Further, it can be seen that the two columns 1 are interconnected with a cross beam 4 , attached to the lower telescopic member 2a of the first or the second column 1 respectively. The table top 5 is mounted on the topmost telescopic member 2c, which is not shown in the drawing to avoid unnecessary details. The mounting of the table top 5 varies from one table to the other, but generally it applies that the table manufacturer uses brackets, which may be connected to the fixtures on the column 1, where after the table top 5 is mounted on the lower frame 1, 3, 4 with the bracket. The movement of the column 1 and thus the adjustment of the height of the table 5 is performed by a motor drive 8 which is a linear actuator. To the actuator is connected a control device 6 and an operation device 7 for adjusting the actuator 8.

In the specific construction of the actuator 8, shown in Fig. 2, the actuator 8 is designed in such a way that it constitutes an assembled column 1, where the motor 10, over a transmission 11, drives a drive tube 12 with a spindle nut 13, in which a hollow spindle 14 is driven on an external thread. The hollow spindle 14 is likewise equipped with a spindle nut 15 so that the hollow spindle 14 may be driven in relation to a solid spindle 16. The solid spindle 16 is secured to the bottom of the column and fixed against rotation. When the motor 10 is activated, then the members will be telescopically displaced in relation to each other in one direction or the other. When inserted in the lifting column 1, the actuator 8, will by means of a control and operation unit

6, 7 be capable of performing an adjustment of the height of the table.

The detailed drawing in Fig. 2 shows a section of the actuator 8 where the squeeze protection device is located. In Fig. 3 the same squeeze protection device is shown in an exploded view for the sake of clarity. An unyielding first part in the form of a motor rear fixture 17 for the motor 10 is with a fixture in the form of screws 18 secured to a second unyielding part in the form of a top flange 20 through rubber bushings 19. Between the two unyielding parts 17, 20 is located a flexible spacer ring 21, typically made from a rubber- like material. The spacer ring 21 is originally inserted in the construction for mechanically to dampen noise of the lifting column by avoiding that vibrations from the motor 10 is transmitted acoustically into the parts of the column 1 which constitute the telescopic leg. In order to have a noise dampening effect, the compressible spacer ring 21 must be capable of yield when the parts are moved. This further means that the forces exerted by the linear actuator 8, when they run through the distance from the motor rear fixture 17 to the top flange 20, will cause the compressible spacer ring 21 to be compressed or expanded. Between the two unyielding parts 17, 20 the squeeze protection device 22 is located. The device is based on a membrane-like circular piezo element 23, positioned on a ring-shaped projection 24 in connection with an end of the motor 10 on its motor rear fixture 17. The piezo element 23 is positioned so that it is not supported in the middle and thus may be moved moderately both in the downwards and upwards direction. In the center of the piezo element 23 is mounted a spring 25 which protrudes vertically upwards. At the other end, the spring 25 is connected to a housing 26, which in a

preferred material is made from plastic - although it does not limit the selection of other materials - in that the spring 25 fits over a pin 27, constructed on the inside of the plastic housing 26. In that the top of the plastic housing 26 rests against the top flange 20, pressure on the top flange 20 would simultaneously exert pressure on the plastic housing 26 and result in pressure on the spring 25 in connection with the piezo element 23, thus causing the piezo element 23 to be subjected to load. The plastic housing 26 forms a protective housing for the piezo element 23 and is equipped with flaps 28, which together with counterparts 29, on the motor rear fixture 17, forms co-operative snap-locking means 28, 29 for retaining the plastic housing 26 in connection with the motor. The flaps 28 allows the plastic housing 26 to be pushed further down towards the motor rear fixture 17, but forms due to the unciform design a stop for the movement of the plastic housing 26 away from the motor rear fixture 17 when the top flange 20 is not mounted. Particularly expediently, the piezo element 23 is somewhat pre-stressed near the spring 25, so that the piezo element 23 is moved both in case of squeezing above and below the table top 5. For understanding of the invention will be appreciated that it with the described construction it is achieved that the travel of the piezo element 23 is proportional to the deformation of the compressible spacer ring 21. The forces exerted by the actuator 8 are thus divided between the compressible spacer ring 21 and the device for squeeze protection 22. The forces, which may be absorbed in the squeeze protection device 22, are intelligible minimal compared to the forces received in the compressible spacer ring 21, for which reason the spring element in the compressible spacer ring 21 in relation to the spring element in the coil spring 25 in connection with the

piezo element 23 forms a gear, with the purpose of protecting the piezo element 23 against overload. The principle of this is shown in Fig. 4 where a lifting column 8 is shown having an incorporated device for squeeze protection. It is noted that the piezo element 23 is embedded in a bowl-shaped holder 24 in connection with one end of the motor 10 and thus without hindrance may be moved upwards and downwards in the center point . Between a center point on the piezo element 23 and a flange 20, fixedly mounted on the column, is mounted a spring 25. It can be seen that the compressible spacer ring 21 also is shown as two springs, as a squeezing will cause a compression of the spacer ring 21. The load transferred to the piezo element 23 during a squeezing is thus minimal as the spacer ring 21 absorbs most of the load. As stated above, the two spring elements 21, 25 forms a gear, which reduces the mechanical load on the piezo element 23.

The signal, generated by the piezo element 23, is a dynamic signal, which reflects a change in the load of the actuator 8. A control unit 6 , as shown in Fig . 1 , which receives and processes the signals, must thus be fitted to respond to sudden changes, since they indicate a potential squeezing. On the other hand scenarios must be foreseen, where an indication error may occur, like e.g. during start-up of the motor 10 in the actuator 8.

The realization of the invention is particularly simple in that a standard actuator 8, as requested by the customer, during the production is fitted with a module

22 comprising the entire squeeze protection device 22 in that the piezo element 23, the spring 25 and the plastic housing 26 constitutes an integral unit 22. In that the unit 22 as shown in Fig. 3 is also fitted with a cable

30, preferably having a plug connected, the signal from the piezo element 23 is transferred in a convenient manner to a printed circuit board 31 mounted in the actuator 8, and forms a connection with the control unit 6 in that a cabled connection 32 is led externally out of the actuator 8. A control unit 6 can in this way react to a signal from the squeeze protection device 22 and in case of a squeezing both stop the motor 10 and reverse the motor 10 so that a trapped object is released. When desired, this function may be constructed separately on the printed circuit board 31 in the actuator 8 or as mentioned above be implemented in the control unit 6.

Even though the invention here is explained in connection with an actuator constructed as a lifting column for adjustable tables it is noticed that the invention may also be realized in connection with lifting columns for beds cf. WO2009/033486 Al Linak A/S. The invention may also be realized in connection with single actuators cf. WO 02/29284 Al Linak A/S.