HK1119763B - Tool rack for coordinate measuring machine and corresponding tool - Google Patents

Description

Tool holder for coordinate measuring machines and corresponding tools

This application claims priority from european patent application 2006EP-125417 filed on 5.12.2006, the disclosure of which is hereby incorporated by reference.

Technical Field

The present invention relates to a tool system that can be connected to a coordinate measuring machine, such as a probe, a video camera or any other mechanical or optical measuring tool that preferably transmits electrical signals, and to a tool holder that allows the tool to be automatically replaced during a measuring operation using the machine and the corresponding tool. The invention is not limited to the field of metrology or dimensional measurement and may be applied to robots having interchangeable tools or any other system that includes automated tool interchange.

Background

Several examples of coordinate measuring machines with automatic interchangeable tools are known. They are basically measuring systems in which the moving head and the tool of the measuring machine have mutual engagement means which are able to be locked and unlocked automatically and which are able to establish a very precise and stable mechanical connection between the measuring head of the measuring machine and the selected tool.

The known system also has a tool holder located in the working space of the measuring machine, in which the measuring machine can place the tool at the end of the measuring operation and retrieve it, as required by the next task. The measuring machine is thus able to perform a complex series of measurements according to the sequence in which the program runs or according to manual instructions of the operator.

The precision of the connection between the measuring head and the different tools is obviously very important for the accuracy of the measurement. It is also important that the connection has a high reproducibility, so that calibration operations can be avoided at each tool change. For this purpose, a balanced connection with exactly six connection points is usually provided between the measuring head and the tool, in order to be able to determine the position of the tool relative to the measuring head accurately. Patent application EP1577050 describes a connection with three pins whose axes are 120 ° apart on the measuring head and three ball joints which are correspondingly positioned on the tool, which produces a balanced connection, that is to say in which the tool is held in its position by exactly six linearly independent mechanical constraints. In such a system, the position and restraining force of the tool are accurately determined.

In this type of connection, the accuracy depends on the characteristics of the contact surfaces. It is therefore important to avoid or limit vibrations that may alter or alter these surfaces during the joining and separating operations.

Document US4651405 describes a measuring tool arranged to be attached to the arm of a coordinate measuring machine by means of a mechanical connection which engages with a cam of a measuring head using an appendage on the tool. The tool is locked to the measuring machine when the cam is rotated 90 deg., which rotation of the cam is accomplished by means otherwise provided for this purpose. If this operation is automatic, the tool holder must contain one or several actuators to actuate the cams and switches from the locked position to the unlocked position as required and vice versa as required.

These devices have actuators for actuating the cams and locking and unlocking the tools, respectively, for each position of the tool holder. These actuators, and associated controls, add complexity and cost to the tool holder and require the described cam system.

The patent application EP416524 describes a probe and a tool holder with a magnetic connection between the measuring head and the probe, the tool holder having a permanent magnet and an electromagnet. When the tool is switched, the magnetic force can be counteracted by an electromagnet in the measuring head, allowing the measuring head to be separated from the tool. Patent US5028901 describes a similar system in which an electromagnet is located outside and separate from the measuring head.

One limitation of these known systems is that additional elements are required, which increase the cost, complexity and size of the measuring head.

Patent EP626492 describes an interchangeable probe connected to a needle body by a magnet. The needle has no elements designed to connect the needle with an automatic tool switching system.

Patent US4604787 describes an industrial robot arm with interchangeable tools, which is able to place the tools in the storage devices of the tool holders and to pick new tools from the same tool holders. The tool holder has a storage device for tools that can be slid in a direction transverse to the storage device axis and held axially. The mechanical connection between the robot arm and the tool opens automatically under the effect of the axial separation force exceeding a predetermined threshold.

Patent EP566719 describes a tool system and a tool holder in which the tools have permanent magnets and are held magnetically on the corresponding measuring heads. Separation between the tool on the tool holder and the measuring head can occur in a similar manner to that described in document US4604787, in the event that the axial separation force exceeds the magnetic attraction.

In these systems, the axial force required to separate the tools is considerable, which leads to the problem of obtaining high precision. It is important for the accuracy of the measuring system that the tool holder, the tool and the arm of the measuring machine are not subjected to too high mechanical stresses, which would result in loss of adjustment and measurement inaccuracies.

Disclosure of Invention

One object of the present invention is to propose an interchangeable tool and a corresponding tool holder that depart from the limitations of the known devices.

The present invention also aims to improve the accuracy and repeatability of tool switching operations.

These objects are achieved by a device which is the subject of the independent claims, optional features being introduced in the dependent claims and the description.

Drawings

Examples of embodiments of the invention are presented in the specification by way of explanation of the appended drawings, in which:

FIG. 1 illustrates a tool rack having a storage device for a plurality of tools according to one embodiment of the present invention;

FIG. 2 shows the tool rack of FIG. 1 with the storage device occupied by a tool, in this embodiment a probe for a CMM;

FIG. 3 illustrates an innovative tool rack in a folded position according to one aspect of the present invention;

fig. 4 shows a detail of two storage devices of the tool rack of the invention, seen from above in a perspective view;

fig. 5 shows a detail of two storage devices of the tool rack of the invention, seen from below in a perspective view;

FIG. 6 shows the position of the tool on the tool holder after the tool has been disengaged from the measuring head; and

FIG. 7 graphically illustrates a magnetic force upon disengagement of a tool in an apparatus according to an aspect of the present invention.

Examples of the invention

Fig. 1 shows a tool holder 20 designed specifically for receiving an interchangeable tool 200 for a Coordinate Measuring Machine (CMM), as will be described hereinafter with reference to this specific application. The invention is not limited to this application, however, but it will be appreciated that the invention is also applicable to robotic tools, industrial robots and any other tool holder having interchangeable tools.

The tool holder 20 has a base 85 for securing the tool holder within the operating space of the CMM. Generally, the base 85 is stably fastened to a reference surface (not visible) of the machine, on which the measuring elements are also arranged. Other arrangements are also possible. The position of the tool holder 20 in the reference numerals of the measuring machine is determined before the measurement and at the time of tool change, for example by the coordinates of a series of predetermined reference points marked in a calibration operation, which is known to the person skilled in the art.

In the following description, the vertical direction refers to a direction perpendicular to a reference plane of the measuring machine, which is usually a horizontal reference plane. These references to conventional orientations have the sole purpose of improving the readability of the specification and not limiting the invention, and are also applicable to tool holders oriented in any direction in space. The terms "upward" and "downward", "upper" and "lower" with respect to these conventional directions are to be considered as illustrative and not restrictive words.

The tool rack 20 preferably has a plurality of storage devices for storing a plurality of tools 200. In this way, complex measurements can be performed using different tools, and the CMM is set up to place a tool in an available storage means of the tool rack 20 each time it is required, and then to retrieve another tool when required for playback on the storage means. Conveniently, with reference to fig. 2, the storage unit is supported by the upper body 650, raised with respect to the base 85 by the arm 83, so as to be able to accommodate tools of various heights without fear of contact with the reference surface.

Referring to fig. 3, the arms 83 are preferably hinged along axes 81 and 82 to enable folding in the event of a crash. In this way, the risk of damage, for example after incorrect operations on the measuring machine, which could lead to vibrations between the measuring head, or even between the measuring head and the tool holder, can be reduced. In general, the tool holder 20 is held in the unfolded position in fig. 2 by precise elastic means of known type, not shown. The shock absorber, which is also not visible in the figures, dampens the folding of the tool holder in case of a shock.

Each storage unit preferably has a protective cover 50, best seen in fig. 4, which is slidable in a horizontal direction between an open position, in which the respective storage unit is accessible, and a closed position, in which the storage unit and any tools therein are protected. According to one embodiment, the protective cover 50 may also be locked in the open position.

Each storage unit has a guide rail 60 to allow tools to be inserted into the storage unit in a horizontal direction and removed from the storage unit in the same direction. According to the embodiment of the invention shown in fig. 1 and, in more detail, in fig. 5, the tool holder 20 has a body 650 resting on the arm 83, the protective cover 50 sliding on the body 650. The guide 60 consists of two straight and parallel edges cut out in a metal plate 630 screwed or fastened to the body 650.

The tool 200 has an engagement surface or region 210 that works with the rail 60. As shown in the embodiment (fig. 1), the engagement surface is a circumferential groove surface having a square or rectangular cross-section 210 that symmetrically surrounds the circular body of the tool 200. However other arrangements are possible.

The connection between the tool and the measuring machine is made by means of an adapter 300, which is fastened to the moving head of the measuring machine. As in the embodiment shown (fig. 1), the adapter is fastened to the moving head (not visible) by a threaded rod 330. However other fastening systems are possible and are included in the present invention. The tool 200 and the adapter 300 are preferably connected in an accurate and reproducible manner by means of a magnetic element 260, for example a permanent magnet, and a system of precision contact elements, for example three pins whose axes are spaced 120 ° apart on the measuring head and three ball engagements correspondingly positioned on the tool, so that a balanced connection is created which determines precisely the positioning of the tool relative to the measuring head.

In the illustrated case of a screw coupling, an annular or at least axially symmetrical coupling surface 210 is particularly suitable. Ensuring a certain angular position of the adapter on the measuring head is very difficult, since it depends on the holding force and the friction between the surfaces. The pockets 210 do not have a preferred orientation and, as explained further below, do not rely on the relative angle or orientation of the tool with respect to a vertical axis to perform its function.

When the tool is placed on the tool holder, the measuring machine, together with the adapter 300 and the tool 200, moves its measuring head, opposite the storage means of the tool holder 20, with the slot 210 aligned with the rail 60. The measuring head is then moved horizontally in a direction parallel to the guide rail 60 until the guide rail 60 engages the slot 210. The inner diameter of the slot 210 corresponds to the distance between the rails 60 with sufficient space, e.g. a few tenths of a millimeter, to allow easy insertion without lateral restraint.

The shield 50, if only one, is normally held in the closed position by a spring or other suitable resilient member. During the above-mentioned horizontal movement, the adapter 300 contacts the protective cover and slides the protective cover so as to be able to enter the storage device of the tool rack.

Referring to fig. 4, each storage unit has teeth 100 between the rails 60. Insertion of the tool 200 between the rails 60 is terminated when the tooth 100 is at least partially engaged into the slot 210. In accordance with one aspect of the present invention, the lower surfaces 120 of the teeth 100 are not aligned in the horizontal plane of the lower surface 110 of the rail 60. For example, the lower surface 120 of the tooth 100 is moved downward 0.10-0.50mm relative to the horizontal plane of the lower surface 110 of the rail 60.

The tool 200 is disengaged from the adapter 300 in the following manner: the measuring machine is actuated, thereby causing an upward vertical movement of the measuring head. During movement, the lower surface 120 (see FIG. 5) of the tool 100 contacts the lower inner surface of the slot 210. From this point on, the tool no longer follows the vertical movement of the adapter, since the tool is stopped by the retaining surface 120 of the tooth 100. Two effects then occur:

1. because the magnetic coupling force and the force exerted by the teeth 100 are misaligned, separation between the adapter 300 and the tool 200 occurs. The latter is inclined at an angle alpha (see fig. 6) until the groove 210 of the tool contacts the lower surface 110 of the guide rail 60. Due to the lever effect, the force exerted on the tooth 100 is much smaller than the overall magnetic attraction force.

2. This tilting increases the air gap between the magnetic element on the tool 200 and the adapter 300. The magnetic force is strongly dependent on the air gap, so the inclination causes a significant drop in the attractive force and tends to untie the tool.

The tilt is limited by the lower retaining surface 110 (see fig. 5) of the guide rail 60 contacting the lower surface of the slot 210. From this point on, the vertical force applied by the tool rack 20 to the tool 200 is distributed to two surfaces, namely the lower retaining surface 110 of the rail 60 and the lower retaining surface 120 of the tooth 100. When this force exceeds the magnetic attraction force, the tool 200 is completely disengaged from the adapter 300 as the tilt is reduced.

The arrangement of the present invention provides a significant reduction in the mechanical stresses applied to the guide rails 60 during unwinding. This reduction is due to the fact that the tool is tilted and that a residual holding force is distributed between the lower holding surface 110 of the rail 60 and the lower holding surface 120 of the tooth 100, which residual holding force results from the magnetic effect. Distributing the force to three contact points or areas is advantageous for improving the accuracy of the joining.

For example, a 0.25mm offset between the lower surface of the tooth 100 and the lower retaining surface of the rail 60 is sufficient to produce a tilt angle of about 2, which is very effective.

Fig. 7 shows the vertical component of the magnetic force during tool disengagement. Curve 402 represents a tool holder with a holding surface that is arranged to tilt the tool in disengagement, whereas curve 401 is associated with a known situation in which the tool is not tilted. It should be noted that at the initial contact point 405, the breaking force in case of tilting is slightly reduced, due to the lever effect mentioned above. Point 410 corresponds to the maximum tilt of the tool, i.e. when the tool is in contact with the holding surface and the tool holder rail at three points and can no longer tilt. From this point onwards, the lever decrease ends, the magnetic force being slightly lower than the maximum force in the straight case 401.

The leaf spring 150 (see fig. 1, 4) rests above the upper surface of the slot 120, causing some friction in order to avoid any unwanted movement of the tool in the storage device, for example with vibrations, without however requiring a complete locking of the tool 200 or causing considerable stress on the measuring head when the tool is switched. In this case, when the tool is retrieved again, the measuring head is able to find the tool in exactly the same position and direction from which the tool left after disconnection. According to this embodiment, the leaf springs 150 can be replaced by permanent magnets or magnetic elements in one or more of the tool holders 20.

To load additional tools, if there is a shroud 50 by moving the shroud, the machine is actuated to center it over the known position of the tool in the selected storage device. The measuring head is then moved vertically downward until the tool 200 is connected to the adapter 300 and the slot 210 is no longer in contact with the lower retaining surface of the tooth 100 or the surface of the guide 60. Finally, the measuring head is moved horizontally in the direction defined by the guide 60 until the tool 200 is completely detached from the tool holder 20.

Accurate and reproducible engagement of the tool 200 is largely dependent on the position of the tool in the tool holder 20. In particular, it is important that the tool rests in the position of its storage means and in the same direction as it would be when the machine being measured is left on said storage means, with minimum mechanical constraints. In this way, the connection can take place accurately. The leaf springs 150 preferably allow the tools to move slightly for the joining operation, allowing the equalizing means to position themselves accurately. Improper positioning of the tool 200 or tool holder 20 on the CMM can cause vibrations during reconnection that can compromise calibration and measurement accuracy and irreversibly alter the contact surface.

To ensure stability of the tool 200 on the adapter 300, the magnetic attraction between these two elements is generally large enough, on the order of 10N or more, to prevent the tool from being lost while the machine is working, even when dynamically moved. It is difficult to disengage the tool 200 from the adapter 300 during disconnection at one time without causing unwanted displacement and vibration. The device of the invention allows this separation to be performed step by step and without the above drawbacks, the invention avoids the random separation that occurs when the tool is not tilted in a suitable way, by distributing the mechanical stress to the three holding surfaces, reducing the magnetic attraction force by tilting the tool and adapting the tool orientation for separation.

Advantageously, the inventive device ensures contact and separation of the contacts between the tool 200 and the adapter 300, and the adapter 300 always follows the same sequence during connection and separation. This significantly improves the accuracy and repeatability of the connection.

The tool holder 20 of the present invention also has the following advantages: the separation and connection of the tool is accomplished by the motion of the passive member and the measuring machine only, without the need to use additional actuators to lock or unlock the tool.

Claims (16)

1. A tool rack for storing one or more tools of a coordinate measuring machine, each tool being movably connectable to a movable arm of the coordinate measuring machine, the tool rack having at least one storage device for storing the tools, the at least one storage device comprising:

two substantially straight rails parallel to the first direction and separated by a free space to allow insertion of a tool into the free space of the storage device between the rails in the first direction and removable from the storage device in the first direction,

a retaining surface for limiting movement of the tool in a second direction,

teeth between the rails, in a plane other than the holding surface and the rails, limit movement of the tool in the second direction to enable tilting of the tool when a force is applied to the tool in the second direction.

2. A tool holder according to claim 1, wherein the retaining surfaces lie in at least two different planes perpendicular to the second direction.

3. A tool holder according to claim 1 having retaining means to inhibit movement of the tools rather than completely blocking them.

4. A tool holder according to claim 1 having retaining means for retaining the tools in the position and orientation in which they were placed.

5. A tool holder according to claim 3 wherein the retaining means provides mobility to the tool when the tool is attached to the moveable arm of the coordinate measuring machine.

6. A tool holder according to claim 3 wherein the retaining means causes friction between the tool and the rail when the tool is moved in the first direction.

7. A combination of a tool holder according to claim 1 and a tool removably attachable to a moveable arm of a coordinate measuring machine, wherein the tool has an engagement region capable of working with the guide rail and retaining surface of the at least one storage device.

8. The combination of claim 7, wherein the engagement region of the tool is symmetrical with respect to an axis of symmetry, and the axis of symmetry is parallel to the second direction when the tool is inserted into the storage device of the tool holder.

9. The assembly of claim 7, wherein the connection between the tool and the moveable arm of the measuring machine is magnetic.

10. The combination of claim 7 wherein the connection between the tool and the moveable arm of the measuring machine is made by an adapter capable of working with the tool secured to the moveable arm of the measuring machine.

11. The assembly of claim 10, wherein the coupling force between the tool and the moveable arm or adapter is reduced by tilting the tool.

12. A method for actuating a coordinate measuring machine having a movable arm with a coupling device for converting an interchangeable tool, a tool holder within the reach of the movable arm, the tool holder having at least one storage device for registering tools, the method comprising the steps of:

moving the movable arm in a first direction such that the tool is engaged to the movable arm within the storage device of the tool rack;

moving the movable arm of the tool rack in a second direction to disengage the tool from the movable arm;

wherein the at least one storage device comprises:

two substantially straight rails parallel to the first direction and separated by a free space to allow insertion of a tool into the free space of the storage device between the rails in the first direction and removable from the storage device in the first direction,

a retaining surface for limiting movement of the tool in a second direction,

teeth between the rails that limit movement of the tool in a second direction in a plane other than the holding surface and the rails to enable tilting of the tool when a force is applied to the tool in the second direction;

wherein when the movable arm is moved away from the tool holder, the retention surface of the storage device limits the movement of the tool in the second direction and exerts a force on the tool that opposes the force exerted by the interface, and exerts a tilting torque to tilt the tool and reduce the force exerted by the interface on the tool.

13. The method of claim 12, wherein the connection between the tool and the moveable arm of the measuring machine is magnetic.

14. The method of claim 12, wherein the tool and the moveable arm are coupled together in a precise and repeatable manner by a balanced coupling having a plurality of contact elements, the positioning of the tool relative to the moveable arm is precisely determined, and the contact and separation of the balanced coupled contact elements is always in the same order during assembly and disassembly.

15. The method of claim 12, wherein the tool is tilted in a preferred direction due to the teeth, thereby avoiding asymmetric sliding between the rails during separation.

16. The method of claim 12, wherein the tool is positioned in the storage device such that it is not in full contact with the guide rail, thereby preserving mobility.