The present invention relates to an actuator of the type referred to in the preamble of claim 1. An actuator of this kind is known from WO 2014/079593.

Linear actuators are used in machine tools and industrial machinery, in computer equipment such as disk drives and printers, for valves and mufflers - and many other places where linear movement is desired. As actuators of this type are often part of a construction in which one or more parts of the construction are movable, there are of- ten exact requirements for power, velocity and the length of the movement. The problem with known actuators is that their size and especially length are big compared to the movement, power and velocity they are able to provide. This invention overcomes these problems by being extremely compact compared to its capacity. It will therefore be considerably easier to dimension the entire construction. Generally, the most im- portant mechanical, functional parameters characterizing an actuator are:

Stroke:

Many actuators are available with standard strokes, e.g. 50 mm, 100 mm, 150 mm, etc., but they can most often be ordered with a specific, special stroke. Here, it is im- portant to have a large margin of delivery options.

Built-in dimensions:

The built-in dimensions are most often a combination of a minimum length + the stroke. Here, it is important that the minimum length is as small as possible as this is regarded as a "dead" length.

Push and pull forces:

The push and pull forces describe the force that the actuator is able to exercise. Here, it is of course important to be able to obtain as great a force as possible relative to the size.

Efficiency:

The efficiency characterized by the relation between the supplied energy relative to the energy that the actuator can release. As previously mentioned, the purpose of the actuator is to transform a rapidly rotating movement into a slow, linear movement. The known principles for this transformation of movement are:

Threaded spindle and nut. This principle is by far the most widespread and is most often organized in such a manner that the motor drives the threaded spindle - most often with some kind of reduction gear inbetween which reduces the rotation speed. On the threaded spindle, there is mounted a nut which is fixedly connected to the piston. When the motor rotates the threaded spindle, the nut is displaced and with it the piston. The advantage of this principle is that it is simple, inexpensive and robust. The disadvantage is that there is a relatively large loss in the mechanism due to the friction between the spindle and the nut.

Ball circuit. This principle is a variant of the threaded spindle and the nut. The spindle is constructed with a special thread with circular threaded recesses, and the nut is provided with a large number of balls rolling in a threaded spiral pattern. The advantage of this principle is that it is almost friction-free and thus lossless. The disadvantage is that it is more complex, vulnerable and expensive.

Roller screw actuator. Reminds somewhat of both the threaded spindle-nut and the ball circuit types. However, a roller is inserted with either external thread or simple smooth grooves between an interior threaded spindle and an external pipe thread. In principle, this principle works as a threaded spindle and a nut, but the friction between the two elements has been eliminated as an almost clean roll is established by the threaded rollers instead of sliding. The advantage of this system is that there are almost no losses in the system, and that is it robust and able to transmit large forces. The disadvantage is that it is relatively complex and expensive.

Toothed wheel and rack. Especially used for more stationary installations. Is not suited for closed, compact all-round actuators for general use.

Cam belt or chain-driven actuators. Like toothed wheel and rack, this type is best suited for stationary installations such as production and process equipment.

The purpose of the invention is to improve one or more of these parameters for tuator as described above. The invention consists in a linear actuator comprising a linear tubular movable piston, a stationary part with a smaller diameter than the piston and constituting a motor housing in which an electric motor is arranged, as well as a rotor part forming the connection between the piston and the motor housing in which the piston is influenced axially by a number of smaller cylindrical planetary wheels which are provided with external grooves or threads attached to a rotor housing which, via the rotor part, is driven by the electric motor, the peripheral geometry of said planetary wheels with either radial grooves or threads engaging with the interior thread of the piston, and in which these planetary wheels are attached to the rotor part so that they can rotate about their own rotational symmetry axis.

As regards all the parameters mentioned above, the actuator provided in accordance with the invention has the same or better values than the known actuators. The stroke may be adapted to any desirable value, the built-in dimensions are undisputedly the shortest and thus the best on the market, the push/pull forces like the best competitor, the efficiency is better than the best on the market. In the actuator according to the present invention, no reduction gear is required. It is the same threaded planetary gear that takes care of the transformation of rotation into linear movement and the reduction of the velocity.

According to the invention, a system is used that reminds most of category c (see above), but is different in many ways. Most often, actuators use electric motors to supply the energy to the desired linear power and displacement. Here, the problem is that electric motors of the desired size and price rotate very rapidly, e.g. 4 - 8,000 RPM, whereas the desired linear movement is often slow, e.g. 5-50 mm/second. Thus, a large reduction of the velocity must take place at the same time as the movement must be transformed from rotating to linear. In almost all the known actuators, this is solved by inserting a reduction gear after the motor. The reduction gear reduces the rotation speed of the motor typically by a factor of 5-100, but the movement is still rotation.

The number of threads on the interior thread of the piston is preferably equal to the product of the number of rollers distributed on a round of the rotor part and the number of threads on each planetary wheel, in which the following formula describes the axial velocity of the piston V = (Z x p - D x p / d), wherein v is the axial, linear velocity of the piston, Z is the number of planetary rollers, p is the thread pitch of the planetary rollers, D is the pitch circle diameter of the interior thread of the piston, and d is the pitch circle diameter of the exterior thread of the planetary wheel.

In the preferred embodiment, the rotation of the planetary wheels is guided so that they rotate with the same speed. Preferably, the rotation of the planetary wheels is guided so that they rotate with the same mutual speed, and the rotation speed is guided so that there is preferably rolling between the planetary wheels and the interior thread of the piston in the engagement region. Preferably, the rotation of the planetary wheels is guided so that they rotate with the same mutual speed and the motor is fixedly connected with the sun gear. Thus, the rotation speed of the planetary wheels is reduced as the synchronizing gear wheels function as a classical planetary gear. The invention will be further described with reference to the working examples of the accompanying drawings in which:

Fig. 1 a shows a schematic diagram of a known actuator,

Fig. 1 b shows a schematic diagram of an actuator according to the invention, Fig. 2 shows the position of the planetary wheels while the planetary thread drive carries out a rotation,

Fig. 3 shows the actuator according to the invention in its shortest and longest position, respectively,

Fig. 4 shows an exploded view of an embodiment of the invention in which the

planetary wheels are rotating freely,

Fig. 5 shows a variant of the actuator of fig. 4,

Fig. 6 shows a detailed view of the synchronizing mechanism in an actuator according to the invention, and

Fig. 7 shows a vertical section of the assembled actuator. As will be apparent from fig. 1 which shows an actuator of the prior art (fig. 1 a) and an actuator according to the invention (fig. 1 b), respectively, an actuator of the prior art is much longer than the actuator according to the invention as motor 1 , reduction gear 1 a and the piston as well as the thread spindle 3a are positioned in continuation of one another. On the actuator according to the invention, the piston 3 is positioned on the outside of the motor 1 and the gear in the rotor part 2 and thus does not give rise to an additional length.

Fig. 2 shows the position of the planetary wheels while the planetary thread drive car- ries out a rotation. The function of this planetary thread drive may be described as follows: Fig. 2 shows a cross section of the actuator, and in fig. 2A the planetary wheel (4) marked with a black dot is facing upwards. On the planetary wheel there is drawn a circle pointing away from the centre line of the actuator. In fig. 2B, the planetary thread drive (2) has rotated until this latter circle marking again points directly away from the centre line of the actuator. The threaded periphery of the planetary wheel has now completed one revolution relative to the interior thread of the piston and thus displaced the piston by the length p corresponding to the thread pitch of the planetary wheel - but as it has simultaneously followed the threaded track of the piston, it has been displaced by approximately 1/3 revolution. However, the threaded track of the piston has 3 pipe threads and thus an actual thread pitch of 3 times p. This means that the planetary wheel has worked its way forward by the length p, but has simultaneously rolled back by about p. The resulting linear movement of the piston is close to 0 (zero). Fig. 2C shows the position of the planetary wheel when it has completed another revolution, and fig. 2D shows the position of the planetary wheel when it has completed exactly 3 revolutions. It can be seen that the planetary thread drive has now turned one revolution + the angle e. The resulting displacement of the piston is (3 x p) - 3 x p + (3e/360 x p). In other words, it is the thread diameter of the planetary wheel relative to the thread diameter of the piston that determines the gearing and thus the piston speed. If the thread diameter of the planetary wheel is exactly 1/3 of the thread diameter of the pis- ton, the piston is standing still no matter how fast the motor is spinning and no matter how big the thread pitch is as long as the piston has the same number of pipe threads as the number of rollers (here 3).

Fig. 3 shows the actuator in its shortest and longest position, respectively. Normally, the built-in length L is defined as the sum of the constants END1 , END2, DL and the variable S. The smallest value of L appears by setting DL+S to be equal to MD. Then, the associated value of S represents the minimum stroke. In practice, a longer stroke than this minimum stroke is always used, thus leaving a free unused space behind the motor. This free space may e.g. be used for a longer and thus more powerful motor which would give the actuator a larger push/pull force and this without an extra built-in length. In the known actuators, an extra long motor will entail an extra long built-in length.

Fig. 4 shows the simplest embodiment in which the planetary wheels are rotating freely. This requires a good contact between planetary wheels and the thread of the piston.

Fig. 5 shows the same construction as fig. 4, but with synchronization of the planetary wheels by means of the planet gears (10) which are fixedly connected with the plane- tary rollers (4) via the shafts (5). The sun gear (11 ) sees to it that all planet gears (10) and thus the planetary rollers (4) will follow the same rotation speed.

Fig. 6 shows a close-up of the synchronizing mechanism described above. Fig. 7 shows a vertical section of the assembled actuator as described above.