HK1146113B - Instrument for measuring dimensions and height gauge - Google Patents

Description

Instrument for measuring dimensions and height gauge

Technical Field

The present invention relates to an instrument for measuring dimensions along a linear axis, and in particular, but not exclusively, to an instrument for measuring linear dimensions along a vertical measuring axis and a horizontal measuring axis.

Background

Measuring instruments such as altimeters and machines for measuring coordinates are used to measure and control the dimensions of high precision mechanical parts. They provide measurements with accuracy and reproducibility on the order of micrometers.

These instruments allow relative and absolute measurements to be performed. They are usually associated with a horizontal surface table or measuring table on which the component to be measured or controlled is placed in a selected position so that this common element constitutes a reference plane between the device and the component to be measured.

Typically, the dimensional measurement device includes three guides oriented parallel to three orthogonal coordinate axes. In the case of altimeters, two of these axes are ignored, since they are inscribed in the measuring table plane and the guides oriented perpendicularly to the measuring table constitute vertical measuring axes. It is also contemplated that other configurations may be implemented that allow measurements to be taken from one of the other axes, the other two axes being considered to define a reference plane.

These devices are designed for measuring parts made of rigid material whose deformation under force is considered to be zero. This limits the field of application of these devices.

On the measuring column, the guide is fastened vertically on a rigid base comprising a bottom plate that can be moved by sliding, either on an air cushion or on a measuring table, so as to be able to access all parts of the lateral sides of the component to be measured.

The measuring instrument usually has a fixed position reference on each measuring axis, a high precision position encoder and an electronic control and display device, possibly included in a console visible to the operator, for displaying the measured dimensions of one or more measuring modes.

European patent EP 0579961 in the name of the applicant describes an altimeter with a measuring carriage sliding on vertical guides. The measuring carriage comprises a "floating" drive carriage that slides relative to the measuring carriage and which transmits the vertical forces exerted on the measuring carriage. In this instrument, the connection between the measuring carriage and the driving carriage is elastic: the adjusting spring system connecting the two carriers is balanced and at the same time guarantees a minimum contact force acting on the measuring carrier to determine the measuring direction and to ensure good contact with the component to be measured.

Other measuring columns with similar force-applying means are known, for example from the documents CH667726 and EP 0223736. The devices of these documents comprise an elastic transmission element between the drive motor and the measurement probe and are arranged to perform dimensional measurements when the deformation of the elastic element reaches a predetermined value and therefore the pressing force of the probe on the component to be measured approaches this predetermined value.

Devices are also known, for example from patent US 4924598, in which the deflection (deflections) of the elastic element transmitting the measured force to the sensor probe is not measured continuously, but is detected by an electrical contact when a deformation threshold is exceeded. However, these simpler devices do not allow measuring the force overshot during the measurement. Moreover, such a device does not allow the measurement conditions and in particular the pressing force of the probe to be changed or corrected during the measurement.

In all known devices based on the measurement of the dimensional deformation of an elastic element, the dimensional deformation of the elastic element will be substantially determined by a displacement sensor, such as a potentiometer or a touch probe of the very easy type, which gives information on the relative position between the two carriages. However, these low cost sensors have certain drawbacks because they do not provide accurately referenced information and do not provide a perfect correspondence to contact forces, their calibration may change over time and they require dust protection as well as water and/or oil projection (projection).

Furthermore, known floating carriage systems have additional degrees of freedom that may affect the accuracy of the measurements. The travel of the floating carriage is considerable and the time required to stabilize the system does not allow fast measuring operations to be performed.

It is also known that for certain measurement or scanning operations (such as to search for a minimum), the elastic connection between the measurement system and the drive slows down the measurement. The floating carriage arrangement may also constitute a weak element during fine adjustment of the probe position, since the system does not provide sufficient flexibility to perform measurements when the measurement force varies only slightly. The system then has to continuously seek a constant force balance point, which continuously requires a stabilization period before providing dimensional information.

In a dual-carriage measuring column, the elastic elements need to be specifically adjusted for each orientation of the system, i.e. the weight of the measuring carriage is kept balanced by this elastic system and therefore changes in the orientation of the system require modifications to these elastic elements.

Disclosure of Invention

It is an object of the present invention to propose an instrument for measuring dimensions which does not have the limitations of the known devices. According to the invention, these aims are achieved in particular by an instrument for measuring dimensions, comprising: a linear guide supporting a position reference defining a measuring axis (measuring axis); a moving carriage movable parallel to the guide; drive means connected to the carriage by a transmission in order to determine the linear displacement of the mobile carriage; a feeler (feeler) attached to the mobile carriage and designed to come into contact with the component to be measured; a position transducer (transducer) disposed on the carrier to provide a measurement of the position of the stylus relative to the measurement axis. Wherein the carriage is connected to the drive means by at least one force sensor capable of measuring a contact force exerted by the probe on the component being measured.

This solution has in particular the advantage over the prior art of directly converting the contact force into an electrical signal without having to base the measurement on an indirect, inaccurate measurement made of the relative displacement between the two carriages. This allows measuring the actual contact force applied along the axis under observation and advantageously reduces the number of components that may cause errors. A more compact, more rigid structure, better measurement accuracy, faster response to position changes and good immunity to projections can thus be obtained.

The instrument of the present invention includes a force sensor for measuring along the instrument displacement and measurement axis. Preferably, the sensor is pre-stressed, as it must allow measuring the force variation in both displacement directions. This configuration is not limiting as other sensors may be added to determine the orientation of the contact force along different orthogonal axes.

Furthermore, the inventive measuring instrument preferably comprises one or more additional elastic connections or means for limiting the driving force by means of friction elements to protect the force sensor. One of the first additional elastic elements, located at a height controlled by the drive system, makes it possible to limit the force exerted by the operator or by the motor when the probe is in contact with the part to be measured. Other resilient elements may be added continuously to the force sensor to increase the resilient displacement limit of the measurement system and thus protect the sensor. The addition of these protection means is not restricted, since the elastic deformation does not affect the measurement of the position. Stops and/or elements parallel to the force sensor can also be added in order not to exceed the limit of deformation tolerable for the sensor and thus avoid (shoot) mechanical overloads.

A direct reading of the contact force enables a better control of this basic measurement parameter. Indeed, the contact force may have a dimensional effect on the particular material. Thus, a finer measurement of the applied contact force enables the elasticity of the measurement material to be interpolated (interpolated) from several measurements of different contact forces, and thus also extrapolated (extraolated) from the actual dimensions of the measurement component achieved with a zero contact force corresponding to zero elastic deformation. Moreover, in the case of measuring surfaces that are not orthogonal to the measuring axis of the instrument, the dimensions of the component to be measured are no longer affected by parasitic contact forces for zero contact force.

Advantageously, the present invention provides an improved search for optimal conditions, which can be performed by following changes in force and position. The measured dimensional values are therefore no longer limited to a predefined contact force. The device of the invention makes it possible to correct the dimensional measurement according to the variations in contact force, which is not possible with the prior art instruments. The device also takes into account each material and optimizes the value correction according to the elasticity of the material without necessarily requiring a positional mechanical correction.

The invention is not limited to vertical measurement systems but can also be used for absolute or relative measurements along a horizontal axis or even for measurements along any axis in a volume, for example at the end of an articulated arm. Due to the greater fineness of the measurement with respect to the contact force, dimensional measurements in various orientations can be guaranteed without any loss of accuracy. Due to its rigid and simple structure, the linear measurement is no longer limited to certain orientations of the system, and the orientation of the measurement surface is no longer limited to surfaces parallel to the reference plane. In different orientations, the effect of the weight of the carriage only affects the operating point of the sensor without any modification to the elastic element. The choice of pre-stressing the force sensor can be made to cope with different possible orientations without adapting the balance point.

Known measurement columns are generally limited to surface measurements parallel to a reference plane. When such a device measures a surface that does not belong to this category or is no longer associated with a reference plane, it must be able to distinguish the orientation of the contact force. Otherwise, the contact force may exceed the force measured by the instrument along its measurement axis. Furthermore, the dimensions of the measuring element (e.g. a spherical measuring probe) must take into account the orientation of the surface to correctly account for the dimensions (projection) of the measuring probe relative to the instrument displacement axis.

In a not shown embodiment of the invention, the system enables to distinguish the orientation of the contact force perpendicular to the measuring surface with respect to the measuring axis of the device. Thus, an optimum measuring force corresponding to a surface orthogonal to the measuring axis can be found.

In another embodiment using other sensors, the contact force in each direction can be measured in order to adapt the force applied during the measurement to the orientation of the surface.

Drawings

Examples of embodiments of the invention are indicated in the description illustrated by the accompanying drawings, in which:

fig. 1 schematically shows an overall view of a measuring column according to an embodiment of the invention.

Fig. 2 and 3 show details of the carriage of the measuring column of fig. 1.

Fig. 4 shows the column of fig. 1 in a side view.

Reference numerals:

25 baseboard seat

30 frame

40 bracket

50 cable

55 upper pulley

60 handle

56 balance weight

59 Probe

100 force sensor

120 elastic element

150 measurement console

Detailed Description

The height gauge of fig. 1 comprises a baseplate seat 25 and a vertical frame 30, the baseplate seat 25 being designed to rest on a reference plane, the vertical frame 30 comprising a position reference defining a vertical measuring axis. The carriage 40 slides along the frame 30 and its position is read by a position encoder, which is not visible in the figure. The position encoder is preferably an optical encoder that can read the carriage position with an accuracy on the order of or greater than microns. Other position measurement systems are possible.

Movement of the carriage 40 is ensured by the cable 50 by means of the force sensor 100, the cable 50 forming a closed loop around the upper pulley 55 and a lower pulley (not visible). The lower pulley is driven by a handle 60, the handle 60 being actuable by the operator and allowing movement of the carriage 40. Counterweight 56 (visible in fig. 4) balances the carriage weight. A probe 59, preferably interchangeable, is brought into contact with the surface to be measured.

According to an embodiment of the invention, not shown, the displacement of the carriage 40 is ensured by a motor, which drives the lower or upper pulley with a partially elastic mechanism. Optionally, the cable may be replaced with a strip or band of metal or other suitable flexible transmission element.

The force sensor 100 is, for example, a strain gauge, a system of piezoresistive elements, a charge cell or any other suitable sensor, and at least one resilient element 120 is interposed on each side between the cable 50 and the carrier 40. The elastic element 120 determines the prestress of the sensor 100. Such a system allows measuring the contact force in two directions along the measuring axis. The measurement force applied to the probe 59 is read directly by the sensor. The sensor generates an electrical signal representative of the contact force and transmits this information to the measurement console 150. The elastic element also has the function of protecting the force sensor and increasing the elastic displacement limit of the system in a controlled manner.

The measurement console 150 preferably includes a display device for automatically displaying the dimension measured along the measurement axis and the contact force measured by the sensor, and a command and data input device (e.g., a keyboard). The measurement console 150 preferably includes an acoustic warning device to signal to the operator that the measured force reaches a desired target value and that remembers the height measurement corresponding to this desired contact force. More preferably, the measurement console comprises a computing means (e.g. a digital processor) arranged to read data from the position encoder of the carriage 40 and from the force sensor 100 according to a program stored in its memory and for providing measurement results, such as diameter, center, position of the translation point, parallelism, elasticity measurements, finding the orientation of the measurement surface, etc.

In the illustrated embodiment, the measurement console 150 has a housing in which all of its components are housed: display, keyboard, processor, etc. However, this is not a limiting feature of the invention, which also includes the following measuring instruments: where these components are placed differently, for example in the base or in the frame or in an external device connected to the measuring instrument, for example a computer.

In a motorized version of the invention, the measuring console also comprises a control unit driving the motors, which controls the motors according to a program (for example according to electrical signals of the forces from the sensors 100). The control unit is for example programmed for stabilizing the contact force measured by the force sensor at a predetermined value. The manual version of the invention preferably comprises a handle with an elastic coupling, which gives the operator tactile feedback on the contact force.

According to an embodiment of the invention, the control unit is programmed so as to allow the position to be measured according to different measurement modes, characterized by different methods for managing the contact force. The following main three modes of operation can be distinguished: a predefined value pattern, a pattern between intervals and an instantaneous value pattern, or any combination of these patterns. The predefined value mode can be measured with a force value fixed or set by the operator. The position is measured when the force is reached and stable. The mode between intervals allows for rapid measurements without seeking too accurate a measurement of the point of contact. This mode is comparable to the previous mode and requires a shorter settling time. The instantaneous value mode allows the operator to immediately perform a quick measurement when in contact with the component. The force and position measurement is achieved as soon as the measurement is triggered. Hybrid mode may be implemented, for example, by combining trigger thresholds between intervals with instantaneous measurements, which enables knowledge of force/position pairs (calls) while still remaining within the predetermined contact force value interval.

According to a preferred embodiment of the invention, the control unit is programmed for determining several measurements of the same contact point for different contact force values, and this will enable extrapolation of the corresponding measurements when the contact force equals zero. This also allows determining the material type and measuring the elasticity of the measured material. It is also possible to start from a predetermined elasticity value and to seek the normal of the measuring surface by comparing the change in the elasticity value measured at different contact points with the measurement at different contact forces. This allows to take into account the size of the probe in the contact direction projected on the measuring axis.

According to a variant embodiment of the invention, the probe fastening system comprises at least a second force sensor for measuring a force orthogonal to the measuring axis of the instrument. The orientation of the normal axis of the measuring surface can thus be determined relative to the measuring axis. This thus enables the contact force to be maintained at a known value as it is no longer combined with the force measured along the instrument displacement axis. Such a system allows to take into account the correct dimensions of the probe viewed on the measuring axis, regardless of the orientation of the measuring surface.

According to a variant embodiment of the invention, the measurement program performs a series of successive measurements along each orthogonal axis, in order then to obtain the coordinates of the points in the two-dimensional representation or in the three-dimensional representation.

According to an embodiment of the invention, the measurement procedure performs measurements with respect to known reference points of the component to be measured. Selecting the instrument orientation so that the point to be measured is located on the instrument displacement axis enables the control time to be optimised while taking into account the orientation of each measurement surface.

Claims (18)

1. An instrument for measuring dimensions, comprising:

a linear guide supporting a position reference defining a measuring axis;

a moving carriage movable parallel to the guide;

a drive device connected to the carriage by a transmission member for determining a linear displacement of the moving carriage;

a feeler attached to the mobile carriage and designed to come into contact with the component to be measured;

a position transducer arranged on the carrier so as to provide a measurement of the position of the contact relative to the measurement axis;

the method is characterized in that:

the carriage is connected to the drive means by at least one force sensor capable of measuring the contact force exerted by the probe on the component being measured.

2. The instrument of claim 1, wherein the force sensor comprises a piezoresistive element or a strain gauge.

3. The instrument according to claim 1, characterized by comprising a cable ensuring the movement of said carriage and at least one elastic element interposed on each side between said cable and said carriage.

4. An apparatus according to claim 1, characterized by comprising a motor acting on or comprised in the drive means, and a control unit arranged to control the motor in dependence of the measurement of the contact force.

5. An instrument according to claim 4, wherein the control unit is arranged to stabilize the measured contact force at a predetermined value.

6. The apparatus of claim 1, wherein the drive means is manually actuable by an operator, including display means capable of indicating to the operator the applied contact force.

7. An instrument according to claim 1, characterized by being arranged for determining several height measurements of the part by several contact force values, and for extrapolating the corresponding measurements for which the contact force is equal to zero.

8. The instrument according to claim 4, characterized in that said control unit is programmed so as to be able to measure the position according to different measurement modes, characterized by different methods for managing said contact force.

9. The apparatus according to claim 8, wherein the force is a predetermined value or within a predetermined interval or instantaneous value.

10. The apparatus of claim 1, wherein the force sensor is protected from mechanical overload by a stopper and/or a resilient element that increases the limit of elastic displacement of the system.

11. The instrument of claim 10, wherein the force sensor is pre-stressed to enable measurement in each direction along the measurement axis.

12. An instrument according to claim 1, characterized in that the elastic deformation of the component to be measured can be determined and minimized.

13. The instrument of claim 1, comprising at least a second force sensor arranged along one of the axes orthogonal to the instrument measurement axis for determining the orientation of the contact force.

14. The apparatus of claim 1, wherein the orientation of the surface of the component to be measured can be determined.

15. The instrument of claim 1, wherein the dimension can be measured in any orientation regardless of the instrument.

16. The apparatus according to claim 1, characterized in that it allows measuring elements that are not orthogonal to the measuring axis of the device.

17. The apparatus of claim 1, wherein the position transducer has an optical sensor.

18. The instrument of claim 1 including a vertical dimension height gauge, said guide being a frame connected vertically to the floor base.