EP4720542A1

EP4720542A1 - Actuator with push force control

Info

Publication number: EP4720542A1
Application number: EP24736067.0A
Authority: EP
Inventors: Valerio Farolfi
Original assignee: Maunoa Srl
Current assignee: Maunoa Srl
Priority date: 2023-06-01
Filing date: 2024-05-29
Publication date: 2026-04-08
Also published as: WO2024246760A1; IT202300011193A1

Abstract

The actuator (1) comprises a nut (7) and a screw (1 ) which translates along axis Z by rotation of the nut (7), which is fixed in the floating case (10). The carrier (6) is mounted in the floating case (10). The electric motor (5) makes the nut (7) rotate. The chamber (19) is supplied with compressed air, through the conduit (8). The spring (3) pushes the floating case (10) downwards to produce a preload. The pressure of the compressed air acts on the floating case (10) to position it along axis Z. The floating bush (20) arranged inside the floating case (10) is kept in position by a second spring (21 ), and the floating bush (20) can interrupt the light beam (16) generated by the photocell (11).

Description

TRANSLATION (Rule 12.3) 26 JUNE 2024

TITLE: “Actuator with push force control”

Field of the invention

The present invention relates to an actuator having push force control.

Background art

Actuators of the recirculating ball screw type are known with a recirculating sleeve which allow combined linear motion and rotary motion to be performed on the same axis.

Being subjected to a combined action of two motors, these actuators are not very suitable for carrying out some types of operations that require force control such as those called pick&place which, specifically, work in the following way: picking of a screw by the shank by inserting it into a “cannula” and keeping it in position by the creation of a vacuum, with the shank facing upwards, inserted into the cannula, arranged with the head downwards with the screw drive exposed, subsequent placing of the screw onto a screwdriver of the appropriate shape and consequent distancing of the cannula from the screw which remains inserted on the screwdriver, again with the shank upwards and the head downwards.

When using a recirculating ball screw device with a recirculating sleeve it is difficult to control the correct insertion of the screw head into the screwdriver as the placing on the screwdriver takes place by interpolation of two axes, the first axis for the aligning of the key (specifically a screwdriver) on the screw drive and the second axis for carrying out the insertion after key and screw aligning. If the second axis reaches the contact height between the screw head and the screwdriver, without the key being aligned, the first axis rotates while the second axis continues its stroke applying an increasing pressure which, combined with the rotation, can lead to negative consequences, e.g.

- the screwdriver is inserted, but the screw head is damaged due to the excessive pressure exerted by the second axis while the first axis continues to rotate to align the screwdriver and the screw head with the result of damaging the screw;

- when the second axis comes into contact with the screw head it stops mechanically, having “collided” with the screw head, while the first axis can continue to rotate, if friction allows it or it can be blocked in turn with the result of damaging the screw and losing information on the position of the axes, if the system does not have encoder feedback, or generating tracking errors, if the system has position feedback.

If a torque control system is inserted on the motors, the feedback would be difficult to manage as the torque generated by one axis affects the torque generated by the other axis, e.g. for performing a stroke only along the second axis a reaction is required with torque on the first axis to prevent rotation of the entire mandrel and generate pure translation. The “resistant” torque of the first axis is in turn a function of other variables, e.g. the amount of grease on the ball tracks.

For the same reason it is not easy to precisely regulate the pressure that the actuator exerts on the screw being processed. For controlling the push force of such an actuator, intervention normally takes place through the application of external sensors to the actuator itself, e.g. of the load cell or internal type, mounted on the fastening flange of the nut.

Both in the case of an external sensor of the mechanical type and with an integrated sensor, feedback must be implemented between an analog force transducer and a servo drive already feedback-controlled on the motor encoder.

Therefore, these devices have such disadvantages that the need is felt to find a new solution and to arrange an actuator with push force control that is compact and versatile so that it can be used in various applications.

Summary of the invention

The aforementioned problem is solved, in accordance with a first aspect of the present invention, by means of an actuator defining a longitudinal axis Z, comprising a screw, a nut, wherein the screw translates along the axis Z by rotation of the nut , a conduit, a photocell capable of generating a light beam, a floating case, inside of which the nut is fixed, a carrier inside of which the floating case is mounted, to which a torque can be transmitted coaxially to the axis Z, enabling the floating case (10) to slide along the direction of the axis Z, a motor arranged coaxially to the axis Z, capable of making the nut rotate, a load fixing element integrally sustained to the lower end of the screw, a chamber arranged in the lower part of the case which can be supplied under pressure with compressed air, supplied via the conduit, a first spring which pushes the upper part of the floating case downwards to produce a preload, wherein the pressure of the compressed air acts on the surface of the floating case so that the floating case is positioned along the axis Z based on a resultant one of the forces that are generated in the chamber, a floating bush arranged inside the floating case is kept in position by a second spring a second spring, so that the floating bush can interrupt the light beam.

An important advantage offered by the solution of the invention is that it allows the adjustment of the action force of the on the air pressure in a conduit and the feedback is performed digitally by a photocell.

In the case of an application of the actuator of the invention actuator by acting on machines for testing screws, or on machines for automatic screwing, a “pick&place” type function can be carried out on small elements without the risk of damage due to the application of excessive pressure. Furthermore, the solution of the invention allows the compensation of the weight of the recirculating ball screw with grooves of a particular pick&place type used for controlling “micro screws” for dental implants. The “micro screws” can be picked and moved, gripping them by the shank with the head directed downwards, so as to insert a particular measurement caliper into their screw drive which is stationary with respect to the screw. The insertion can be carried out delicately with a roto-translation movement.

With a state-of-the-art system, torque control and force control would have to be inserted as the reading of the supply current absorption of two motors (with different actions) acting on the same shaft would not provide sufficiently reliable values for managing the required precision in placing the “micro screws”.

With the actuator of the invention, thanks to the floating nut sliding inside a guide that allows the transmission of the rotation of the electric motor, it is possible to prevent impacts or contacts with excessive forces. The contact force of the tool is thus adjustable by varying the pressure of the air downstream of the device.

With suitable choice of geometries, it is possible to compensate the weight of any tool or apparatus fixed onto the end part of the actuator.

Therefore, the main advantages of the actuator are the contact force it can produce, which is pneumatically adjustable, as we act directly on physical quantities such as the supply air pressure and the action surface is defined without software intermediation. This offers the actuator of the invention greater strength and safety. Preferably, it is applicable with screws, recirculation screws or ball recirculation screws having ball recirculation grooves. Thus, a simple adjustment and feedback are carried out even when compound movements are performed.

The actuator of the invention is easy to implement as the feedback is carried out digitally by a photocell.

The dependent claims describe preferred embodiments of the invention, forming an integral part of the present description.

Brief description of the figures

Further features and advantages of the invention will become more apparent in light of the detailed description of preferred, but not exclusive, embodiments of an actuator shown by way of non-limiting example, with the aid of the accompanying drawings, in which:

Fig. 1 shows a longitudinal section on a longitudinal axial plane of the actuator of the invention,

Fig. 2 shows an axonometric view with a partial longitudinal section of the actuator in Fig. 1 ;

Fig. 3 shows an axial section of the actuator in Fig. 1 in a first operating position;

Fig. 4 shows an axial section of the actuator in Fig. 1 in a second operating position; Fig. 5 shows an axial section of the actuator in Fig. 1 in a third operating position.

The same reference numerals and letters in the drawings identify the same elements or components.

Detailed description of a preferred embodiment of the invention

With particular reference to Figures 1 and 2, the actuator of the invention, indicated globally with reference numeral 100, defines a longitudinal axis of symmetry Z, and comprises a screw 1 screwed into a nut 7. The screw 1 is free to translate along axis Z by the rotation of the nut 7, but its angular movement about axis Z with any method is prevented.

The nut 7 is fixed inside a floating case 10, which is in turn mounted inside a carrier 6 which transmits torque thereto for rotation about axis Z, leaving it free to run along the axis Z direction.

The electric motor 5, coaxial to axis Z, is able with its rotation to rotate the nut 7, but it could also be a hydraulic or pneumatic or other type of motor, or any transmission member, such as a pulley for a toothed belt, a pinion, a gear wheel, etc., connected to a motor arranged outside the working area. A motor support 14 is also provided, fastened to the upper part of the motor 5.

At the lower end of the screw 1 an element is solidly fastened for the fastening of a load, such as a tool or apparatus, not shown in the figures, e.g. a keying or locking set 12.

The load, being fastened to the keying set 12 integral with the nut 1 , its weight, added to the weight of the screw itself, bears on the nut 7.

The nut 7 can remain in a certain angular position around axis Z by virtue of the torque imposed by the motor 5.

The lower part of the case 10 faces a chamber 19 supplied under pressure with compressed air which is supplied through the conduit 8 while the upper part of the case 10 is pushed downwards by a preload spring 3 which has the upper part of the box 2 as a reaction support. The air pressure in the chamber 19 acts on the surface of the case 10 which is floating and is positioned along axis Z according to the resultant of the forces generated by the difference between sections D1 and D2, indicated in Fig. 3. The sealing elements 15, e.g. the rings, guarantee the maintenance of a certain air pressure in the chamber 19. Inside the floating case 10, there is also a floating bush 20 held in position by a spring 21 , both coaxial to the case 10 itself. The end part of the floating bush 20, during its movement, is able to interrupt the light beam 16 generated by the photocell 11 .

An encoder ring 4 and an encoder head 13 may be advantageously provided.

A description of the operation of the actuator 100 follows. With particular reference to Figure 3, during the vertical downward or upward movement without obstacles, the nut 7 is placed in rotation by the electric motor 5 and produces the vertical translation of the screw 1. The floating bush 20 interrupts the beam 16 of the photocell 11 which sends a signal of normal operation, in the application logic.

With particular reference to Figure 4, during the descent movement of the screw 1 , when the apparatus or tool, not illustrated in the figures, connected to the locking set 12 comes into contact with an obstacle, the nut 7 is still free to rotate under the action of the electric motor 5, but causing translation along axis Z of the floating case 10 in which the nut 7 is inserted. The lifting of the floating case 10 and simultaneously of the floating bush 20 uncovers the beam 16 of the photocell 1 1 which at this point returns a signal which can be used in the application logic. The maximum translation along axis Z of the floating case 10 in which the nut 7 is inserted is delimited by the presence of the two surfaces 30 and 31 provided in the box 2 against which said two surfaces of the floating case 10 abut. The surface 30 stops the translation of the floating case 10 downwards and the surface 31 stops the translation of the floating case 10 upwards.

Similarly, with reference to Figure 5, during the upward movement of the screw 1 , when the keying set 12 comes into contact with the floating bush 20, the nut 7 is still free to rotate under the action of the motor 5 generating the upward translation of the floating bush 20 only. The lifting of the floating bush 20 uncovers the beam 16 of the photocell which then returns a reference signal or indicating the stroke end. The device of the invention described herein is applicable to screws, recirculating ball screws or recirculating balls with recirculating ball grooves, such as those of the axes for commercial robots known as SCARA.

An important advantage of the actuator of the invention is that it can be used when precise adjustment of the force applied to the actuator is required.

Non-limiting examples of particularly advantageous applications of the actuator of the invention are:

- those in which tools are applied for placing small mechanical parts and

- those for direct integration on plasma cutting heads to detect the part being processed

- for gluing heads,

- for surface finishing systems that require roto-translation movements;

- for anti-crushing safety devices arranged on linear axes, i.e. for the transformation into collaborative linear axis;

- for movement devices of safety guards for machines.

The application of the actuator for operations in the field of screw tightening is particularly advantageous for the reason that the floating nut 7, being an integrated stroke compensation system, allows the actuator to be transformed into a completely electric tightener without a pneumatic stroke of the screwdriver, as normally operated in tighteners of the prior art. Furthermore, the totally electric management of a screwdriver fastened to the actuator for screwing operations, implies other advantages, including full control of the movement of the screwdriver, being able to produce different advancement speeds in different advancement positions along axis Z, and having the possibility to implement other additional functions along the stroke of the screwdriver itself. The actuator of the invention can be transformed into a system for the safety of machine elements by simply replacing the photocell with a “safe” sensor such as, for example, an inductive safety sensor or photoelectric sensors connected to an appropriate safety control unit.

Claims

1. An actuator (100), defining a longitudinal axis Z, comprises a screw (1 ), a nut (7), wherein the screw (1 ) translates along the axis Z by rotation of the nut (7), a conduit (8), a photocell (1 1 ) capable of generating a light beam (16), a floating case (10), inside of which the nut (7) is fixed, a carrier (6) inside of which the floating case (10) is mounted, to which a torque can be transmitted coaxially to the axis Z, enabling the floating case (10) to slide along the direction of the axis Z, a motor (5) arranged coaxially to the axis Z, capable of making the nut (7) rotate, a load fixing element (12) integrally sustained to the lower end of the screw (1 ), a chamber (19) arranged in the lower part of the case (10) which can be supplied under pressure with compressed air, supplied via the conduit (8), a first spring (3) which pushes the upper part of the floating case (10) downwards to produce a preload, wherein the pressure of the compressed air acts on the surface of the floating case (10) so that the floating case (10) is positioned along the axis Z based on a resultant one of the forces that are generated in the chamber (19), a floating bush (20) arranged inside the floating case (10) is kept in position by a second spring (21 ), so that the floating bush (20) can interrupt the light beam (16).

2. The actuator according to claim 1 , wherein an encoder ring (4) and an encoder head (13) are provided in the upper part.

3. The actuator according to one of the preceding claims, wherein the motor device (5) is an electric motor or a pulley for a toothed belt or toothed wheel.

4. The actuator according to one of the preceding claims, wherein the load fixing element is a locking set (12) to which a screwdriver is fixed.

5. A pick&place device, comprising the actuator according to one of claims 1 to 4.

6. An automatic screwdriver comprising the actuator according to one of claims 1 to 4.

7. A plasma cutting device comprising the actuator according to one of claims 1 to 4.

8. A gluing head comprising the actuator according to one of claims 1 to 4.

9. An anti-crushing safety device arranged on linear axes comprising the actuator according to one of claims 1 to 4.

10. A surface finishing system requiring roto-translation movements comprising the actuator according to one of claims 1 to 4.