Linear actuator

The present invention relates to a linear actuator e.g. for an article of sitting and/or lying furniture of the type as stated in the preamble of claim 1. The invention further relates to a method for determining the position of the spindle nut on the spindle.

Determination of the position of the activation element of the actuator is typically performed with a magnetic encoder (cf. e.g. WO 2007/131509 A1 Linak A/S) or a rotary potentiometer (cf. e.g. EP 0 831 250 A2 Dana Corp). However, it also occurs, that an optical encoder or a linear sliding potentiometer is employed (DE 10 2005 052 796 A1 Okin GmbH). A magnetic and an optical encoder as well as a rotary potentiometer indirectly indicate the position of the activation element calculated based on rotations of the spindle or a gear wheel in the transmission. When using the encoders, information concerning the position of the activation element will be completely lost in case of power failure. A linear sliding potentiometer is attractive, as it at all times directly provides the absolute position of the spindle nut, even when the activation element is manually adjusted for instance by activating a quick release, cf. e.g. WO 03/033946 Linak A/S. When activating the quick release, a non self-locking spindle is disengaged causing the spindle nut on its own initiative to be put in motion under the load. On the other hand, the sliding potentiometer is difficult to mount as a mechanical connection between the spindle nut and the sliding contact of the potentiometer must be established. Furthermore, the sliding potentiometer is worn as a result of the friction of the sliding contact against the resistor path.

The purpose of the invention is to provide another solution for determination of the position of the activation element, which is easier to mount and which is not as exposed to wearing as a traditional sliding potentiometer.

This is achieved according to the invention by constructing the actuator as stated in claim 1. By having a linear positioning element designed as an

elongated capacitive element, comprising a number of capacitive sensors, direct physical contact between the sliding means and the elongated positioning element is avoided, which eases the assembly. In addition, there is no wearing as a result of friction. Further, the positioning element has the advantage that it does not loose information concerning the position in case of power failure.

In an embodiment of the invention, a number of capacitive sensors are located on a strip-shaped printed circuit board, positioned in a guide track in the outer tube of the actuator, parallel to the spindle. The capacitive sensors are activated in that a sliding means in the nature of an activation block made from a conductive material, preferably metal, attached to the spindle nut is moved directly over the capacitive sensors without touching these. This means that the determination of the position of the spindle nut on the spindle during detection can be performed completely without friction and otherwise wearing of the positioning element. At the same time the system is tolerant of possible impurities, as an initiation procedure compensates for a possible fault. A printed circuit board is ideal in connection with the integration of the capacitive sensors in an electrically driven linear actuator, but the positioning system can expediently also be designed as a foil with capacitive sensors, which can be placed in the actuator in the same way as a printed circuit board. The positioning system is controlled by an integrated circuit developed for functioning as a controller for a number of capacitive sensors. Such a circuit is typically used for monitoring a keyboard, with a connection to each key or to a row or column in a matrix. By placing the sensors closely together, a touch panel can be constructed. Such a touch panel can be constructed as a matrix. A matrix, where one direction has one element, and thus appears as a one-dimensional array, is called a "slider" and can be used for determining the position in one longitudinal direction. In this way, a solution is achieved, which can be used as an alternative solution where sliding potentiometers otherwise usually are employed as positioning units for actuators. Normally, the number of inputs on the touch controller is limited, which will also limit the resolution and thereby accuracy which can be

achieved by constructing a slider as a positioning element. When it, in an actuator, is desired to be able to determine the position of the spindle nut on a spindle, which for instance has a length of 600 mm, having a resolution of 1 mm, it is a challenge both for practical and financial reasons. A touch controller with a large number of input terminals is due to size and energy consumption not immediately realizable. This disadvantage can be overcome by constructing the slider so that all the sensors are connected in a row on the printed circuit board, and then continuing the row by repetition of the same sensors, but in a different order. On the spindle nut is mounted an activation block, which not only has a length greater than the extent of one sensor, but has a length of between two and three times the length of one sensor. At the same time the capacitive sensors are typically positioned successive after each other in a saw tooth-like pattern, intending the activation block to more or less cover and thereby activate more capacitive sensors simultaneously. In this embodiment, the position of the spindle nut can be calculated based on the signal amplitude of the individual sensor. A microprocessor would typically manage this function and decode the signal from the multiplexed segments. In the upper segment of the slider, where the order of the capacitive sensors has been altered compared with their order in the lower segment, the microprocessor would further attend to calculating the specific position based on the signal level of the activated sensors.

The positioning element is also expedient in that it does not loose information concerning the position of the activation block in case of power failure. When the current after a disconnection again is connected, the positioning element can immediately detect the position of the activation block based on the signal levels of the capacitive sensors without it having to be moved.

The output process of the position from the microprocessor can be performed in the same way as when using a sliding potentiometer in that the microprocessor converts the position to a relative voltage. It will also be possible to output the position of the spindle nut on the spindle as a pulse-

width modulated voltage, which is the case in other actuator systems, using optical or magnetic positioning systems. Finally, the microprocessor can output the position directly as a counter value or a measure of length by communicating with another microprocessor based unit.

For reasons of safety, end-stop switches are built into electrically driven linear actuators, which cut off the supply to the motor before the spindle nut meets the physical end stops. An example of end stop switches can be seen in EP 1 322 876 Linak A/S. Here, the end stop switches are mounted on a longitudinal strip shaped printed circuit board, which can be extended into the outer tube in a guide track running parallel to the length of the spindle. The end stop switches are activated by the spindle nut, when it moves over the end stop switches and activates the protruding contact means hereon. That the nut can move over the end stop switch without colliding with this enables the printed circuit board with end stop switches to be mounted and demounted without exposing the spindle, which has obvious advantages in terms of production and service. According to the invention, the positioning system with the capacitive sensors can expediently be positioned on the same printed circuit board as the printed circuit board for the end stop switches. As the distance between the capacitive sensors and the activation block mounted on the spindle nut has to be small in order to ensure an accurate determination of the position, it means that the spindle nut with the activation block normally cannot move over the end stop switches unless theses are mounted at the opposite side of the printed circuit board but still in such a way that they are mechanically activated when the spindle nut moves over them. In some cases a construction of the positioning system with the capacitive sensors on a separate printed circuit board may be preferred. Such a printed circuit board can be inserted into a guide track running parallel to the guide track for the printed circuit board with end stop switches or be secured to it between the end stop switches. If the printed circuit board is inserted in a separate guide track it can be fixed by being adjusted in the length so that it fits between the two end stop switches. In this embodiment, the nut can move freely over the end stop switches and thereby the printed

circuit boards can be inserted in their guide track or be removed without it being necessary that the spindle is exposed.

An example of an actuator will be described more fully below with reference to the accompanying drawing, in which

Fig. 1 , shows a longitudinal section through an electrically driven actuator,

Fig. 2, shows a section through an actuator exposing the capacitive positioning element,

Fig. 3, shows a cross section of the outer tube with a guide track for securing a printed circuit board,

Fig. 4, shows a sketch showing the capacitive sensors constructed on a printed circuit board where the capacitive sensors are activated by means of sliding means and the corresponding signal values, and

Fig. 5, shows an arrangement of printed circuit boards.

As it appears from the drawing the main components of the actuator are a two-part housing 1, a reversible motor 2, a worm drive consisting of a worm wheel 3 driven by a worm (not shown) constructed in the drive shaft of the motor, a spindle 4, a spindle nut 5 of plastic, an activation element 6 having a front mounting 7, an outer tube 8 and a rear mounting 9.

The activation element 6 consists of a tube and is with its rear end secured to the spindle nut 5. The activation element is secured to the structure in which the actuator should be incorporated by means of an eye 10 in the front mounting 7. Likewise, the actuator is connected to a second part of the construction by means of an eye 11 constructed in the rear mounting 9.

On the inside of the outer tube 8, a strip shaped printed circuit board 12 is positioned parallel to the longitudinal direction of the spindle as it appears from fig. 2. In the outer tube 8 as shown in a cross section in Fig. 3 there are two guide tracks 13 located opposite each other for inserting the strip shaped

printed circuit board 12. On the printed circuit board 12 is mounted a linear positioning element, comprising a number of capacitive sensors 14 constructed in the longitudinal direction on the printed circuit board. The capacitive sensors are activated in that an activation block 15 secured to the spindle nut 5 is moved directly over them and thus functioning as sliding means. This means that the determination of the position of the spindle nut on the spindle 4 during detection can be performed without friction and otherwise wearing of the positioning element. At the same time the system is tolerant to possible impurities, as an initiation procedure compensates for a possible fault. It is noted that the activation block, which preferably is made of metal, is isolated from the other metal components in the actuator as the spindle nut, to which it is secured, is of plastic. As it appears on Fig. 4, the capacitive sensors 14 are not led all the way to the edge of the printed circuit board. This is deliberate to avoid that the capacitive sensors 14 are affected or short-circuited electrically when the printed circuit board is guided into the guide track 13 in the outer tube 8. The positioning system is operated by an integrated circuit as controller for the capacitive sensors. This circuit can practically be positioned on the back of the printed circuit board 12. The capacitive sensors 14 are positioned in a row so that they form a "slider" and thus function for determination of the position in one longitudinal direction.

The activation block 15 mounted on the spindle nut does not only have a length greater that the extent of a single capacitive sensor 14, but a length of between two and three times the length of one sensor. At the same time, the capacitive sensors 14 are saw-toothed and positioned successive after each other in a close pattern intending the activation block to more or less cover and thus activate more of the sensors at the same time. As the controller does not only select the most clearly indicated capacitive sensor, but also expresses the activation degree of the individual sensor, it is possible to calculate the position of the spindle nut based on the signal amplitude of the individual sensor. The activation block 15 always covers at least one of the sensors completely and further at least two other sensors are partially covered. The proportion between the signal amplitude of these partially

covered sensors expresses how much the position of the spindle nut is displaced compared with the center of the fully covered sensor. A microprocessor is used for calculating this.

The slider is constructed with more segments, so to understand that all the capacitive sensors are connected in a row on the printed circuit board (a segment) for then to continue the row (second segment) by repeating the same capacitive sensors but in a different order. The microprocessor is also used to decode the signals from the multiplexed segments, where the capacitive sensors are rearranged compared with the lower segment. As the rearranging of the capacitive sensors is carried out in a way which prevents that the same combination of capacitive sensors can be covered by the activation block in other segments, an unambiguous indication of the position of the spindle nut on the spindle is achieved. Fig. 5 shows the principle of a multiplexed slider, here shown with two segments S1 and S2 each having five capacitive sensors a - e, where the signal level L for the individual capacitive sensor is indicated above. Here, it can also be seen that the activation block 15 covers the sensor c completely and the adjacent sensors b and d partially. The microprocessor selects the sensor c having the highest signal level L1 , as the basis sensor for determining the position. To determine whether the activation with the sliding means is performed in segment S1 or S2, the microprocessor selects the two sensors b, d, having the second L2 and third L3 highest signal level. These two capacitive sensors L2, L3 must be located on the slider next to the basis sensor since the physical distribution of two to three times the length of one sensor necessarily will activate the capacitive sensor adjacent to the basis sensor. It will therefore be possible for the microprocessor to determine in which segment the sliding means are located. Here, the segment is calculated to be S1. Based on the signal levels of the two adjacent capacitive sensors, the microprocessor can likewise calculate the displacement, by means of which the sliding means are located in the longitudinal direction compared with the center of the basis sensor. This is performed by calculating a factor occurring from the proportion between the signal levels of the capacitive sensors

adjacent to the basis sensor. The specific location is thereupon calculated by first indicating the physical location of the segment and then the location of the basis sensor on the segment and finally adjusting the factor for displacement compared with the basis sensor calculated from the signal levels of the capacitive sensors adjacent to the basis sensor.

In a microprocessor the position of the spindle nut will typically be represented by a numerical value. The read-out of the position of the spindle nut on the spindle from the microprocessor can be performed as usual in that the microprocessor converts the position to a relative voltage, as a pulse-width modulated voltage or directly as a numerical value or directly as a linear measure by communicating with another microprocessor-based unit.

To ensure that the activation element does not exceed its end positions, end stop switches are used, which are activated by the spindle nut 5, when this reaches an end position, and via the control stops the motor. The printed circuit board 12, shown in Fig. 5, shows a printed circuit board on which is mounted two end stop switches 16, 17. The capacitive sensors 14 may be realized on the printed circuit board 12, but since the distance between the activation block 15 embedded in the spindle nut 5 and the capacitive sensors

14 should be small, it means that the spindle nut 5 with the activation block

15 cannot move over the end stop switches. The solution is realizable but requires a particular construction of the spindle nut so that it with the ends can activate the end stop switches 16, 17 and can maintain a short distance between the activation block 15 and the capacitive sensors 14. However, the solution is unsuitable in that the printed circuit board not only can be inserted into the guide tracks 13 after the spindle 4 and the spindle nut 5 have been mounted in the outer tube 8. Simultaneously, a possibly emerged fault in an end stop switch would mean that the spindle nut and the switch are brought to meet each other in a destructive manner.

Another embodiment (not shown) is based on an alteration of the structure in which the end stop switch is mounted on the other side of the printed circuit

board 12 compared to the side where the capacitive sensors 14 are located and the spindle nut 5 moves. There is a hole in the printed circuit board so that the switch on the end stop switch may be activated from the other side of the printed circuit board 12 and still make enough room for the activation part of the switch to be pressed so far into the hole that the spindle nut with activation block can pass over the switch. For positioning in the actuator, the printed circuit board 12 is located in the same way as shown earlier in a guide track in the actuator adapted to maintain an accurate distance between the spindle nut/activation block and the capacitive sensors on the printed circuit board.

In the embodiment shown in Fig. 5, the printed circuit board 12 is fitted with end stop switches 16, 17, and an extra printed circuit board 18 with the capacitive sensors 19 is constructed, which is inserted in an additional guide track in the outer tube 8, running parallel to the other guide track for the printed circuit board 12. As the length of the printed circuit board 18 is shorter than the printed circuit board 12 with end stop switches 16, 17, so that the length of this corresponds to the distance between the end stop switches 16, 17, the guide track for this additional printed circuit board is located compared to the guide track 13 so that the printed circuit board 12 fixes the printed circuit board 18 in its position in the outer tube 8. The sliding means in the nature of the activation block 15 for activating the capacitive sensors 19 are inserted in the spindle nut 5 in such a way that this is fixed in a position immediately above the capacitive sensors 19 and at the same time is capable of passing over the end stop switches 16, 17. It occurs that the spindle nut 5 can move freely over the end stop switches 16, 17, for which reason both printed circuit boards 12, 18 can be mounted after the mechanical assembly of the actuator, and makes a possible servicing possible without having to disassemble the actuator. The printed circuit board 18 may e.g. be mounted on top of the printed circuit board 12 as a piggyback with distance pieces and electrical connections so that it can be mounted in one operation as an assembled module in the actuator.

With the invention is thus provided an inexpensive and assembly friendly positioning element, which especially is characterized by not being exposed to wearing, as it is constructed as a capacitive arrangement determined by area, distance and dielectric material, which is here the space between the capacitive sensors and the activation block.