GB1571398A - Arrangement for ascertaining deviations from straightness or planeness - Google Patents

Description

(54) ARRANGEMENT FOR ASCERTAINING DEVIATIONS FROM STRAIGHTNESS OR PLANENESS (71) We, DR JOHANNES HEIDENHAIN GmbH, of Postfach 1260 D-8225 Traunreut, Federal Republic of Germany, a jomt stock company organised under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be described in and by the following statement:- The invention concerns an arrangement for ascertaining deviations from straightness or planeness of test pieces by means of a measuring sensor, movable along a guide, transversely of a measuring direction.

A straightness measuring device is already known from German Offenlegungsschrift 2,208,004. In this device, a sensor pin or bolt is moved over the test piece surface in the direction of a laser beam. The sensor pin moves up and down transversely of the light-beam direction in response to unevenness of the test piece.

These movements are transmitted to a shutter in the path of the laser beam. The up and down movements of the sensor pin vary the height of a shadow strip in the laser beam, resulting in corresponding variations of the residual laser beam, which variations are converted into electrical measurements in a photo-electric detector. Owing to its small cross-section, small aperture angle, approx- imately Gaussian distribution of brightness over the cross-section, as well as high energy density, the laser beam is well suited for this purpose.

Laser beams have already been used sucessfully in the building industry for levelling work and direction measurements. In this case, the requirements as to accuracy are slight. A much higher accuracy (e.g. +1 ,um in 1 m measuring length) is demanded, for example in the measurement of machine guides.

The straightness of a test piece is defined by laying a straight line through two points connected to the test piece and measuring the deviations of the test pieces from this ideal reference straight line.

According to the present invention there is provided an arrangement for ascertaining deviations from straightness or planeness of a surface of a test piece, the arrangement including a measuring sensor having a sensor member extending therefrom for measuring variation in displacement of the said surface from a reference line, means for establishing a laser beam to define the reference straight line, a guide at which the measuring sensor is mounted and relative to which the measuring sensor is movable transversely of the said displacement, the guide being so arranged that the movement of measuring sensor relative thereto is substantially parallel to the reference straight line, and a servo-system provided for ensuring that, in operation, light from the laser beam impinges on a fixed reference point which is remote from the means for establishing the laser beam.

Embodiment examples of the invention are represented in the drawings, in which: Figure 1 is a diagrammatic longitudinal section through arrangement embodying the invention, Figure 2 is a detail of Figure 1 on a larger scale, Figure 3 is a section on the line III-III of Figure 2, Figure 4 is a diagrammatic longitudinal section through another embodiment of the invention, Figure 4a is a cross-section through the arrangement of Figure 5, Figure 5 is a diagram of the path of light rays in another embodiment, Figure Sa is a side view of Figure 5, Figure 6 is a diagram of the path of light rays in a further embodiment, Figure 6a is a plan view of Figure 6, Figure 7 is a block circuit diagram of a circuit for photoelectric cells in a possible embodiment, Figure 8 is a diagram of the path of light rays an embodiment having dynamic evaluation.

Figure 8a is a plan view of Figure 8, partly in section, Figures 9, 9a, 9b are graphical representations of output signals of photoelectric cells of the embodiment shown in Figure 8, Figure 9c is a graphical representation of shaped output signals of Figures 9a and 9b, and Figure 10 is a diagram of an electric circuit for evaluating the signals shown in Figures 9a and 9b.

In Figure 1, the beam 201 emitted by a laser 101 falls onto a partly transparent mirror 301 and is split up into two partial rays 401 and 501. The laser 101 is held in two diaphragms 601 fixed to a base plate 801 pivotal about the axis 701. The base plate 801 can be rotated about the axis 701 by a spindle 100 driven by a motor 901. The axis 701, spindle 100 and motor 901 are fixed to a heavy foot 110 standing on the measured object 120.

The partial beam 501 split off by the mirror 301 is deflected by a mirror 130 through 90 , so that it runs parallel to the partial beam 401. The two mirrors 301 and 130 are fixed to the base plate 801. Fixed to the foot 110 is one end of a hollow body 140. The other end of the hollow body 140 is fixed to a foot 150 resting on the measured object 120.

Mounted on the foot 150 are two differentially connected photoelectric cells 160, illuminated by the partial beam 501. The photoelectric cells 160, the motor 901 and the spindle 100, together with an amplifier (not shown), form a servo-system which varies the attitude of the base plate 801 and therefore also of the partial beam 501 until the two photo-electric cells 160 are illuminated equally brightly and the electric output signal resulting from the difference between the outputs of the cells 160 amounts to zero.

On the hollow body 140, which on one side has a slot covered by flexible sealing lips 240, a carriage 170 is movable in the direction of the partial laser beams. Fixed on the carriage 170 by means of spring elements 230 is an enclosure 180 which carries an electronic measuring sensor 190. Connected to the enclosure t80 by a rod 210 are also differentially connected photo-electric cells 200 on which the partial beam 401 impinges. Deviations of the carriage 170, on which the measuring sensor 190 is mounted, from the reference straight line defined by the laser beam 401 are detected by means of the photo-electric cells 200. The photoelectric cells 200 and a motor 220, together with an amplifier (not shown) form a servosystem effecting compensation of guiding errors.This takes place by the enclosure 180 being adjusted in the direction of the arrow X until the photo-electric cells 200 are equally brightly illuminated and their elec tricel output signal difference is zero.

In Figure 4, w hich on a larger scale shows a section through the measuring sensor 190, 500 denotes a sensor pin feeling the measured object 120. The sensor pin 500 is a component of a measured value pick-up which in this example is an electrical digital length-measuring sensor with incremental distance pick-up. Digital measuring sensors are known, for example, from German Utility Model Specification 7,506,036. In the case of incremental measuring sensors, the measured values are indicated in digital form on an electronic up-down counter (not shown). A counter particularly suitable for the length-measuring sensor of the embodiment example is shown in Patent Specification No. 1 466 483. The use of a digital measuring sensor results in a number of advantages.An essential advantage of digital measuring sensors is the wide measuring range, coupled with high accuracy. In addition, they permit convenient reading of the measured values and/or further processing or storage of the measured values.

The sensor pin 500 is mounted so as to be movable in the axial direction in a precision guide 510. The precision guide 510 also has a resistance to twisting of the sensor pin 500. Fixed to the sensor pin 500 is a grating scale 520, whose grating graduation forms the rectilinear continuation of the sensor pin axis. A spring 530, supported by a sleeve 540, produces a definite pressure of sensor pin 500 on the measured object 120. The grating scale 520 is disposed adjacent a grating scanning plate 550. The grating scanning plate 550, a lamp 570, a condenser 580, and a carrier 590 for photo-electronic components 600 and 61d are fixed by holders to the housing wall 560.

The hollow body 140 (Figures 1 and 3) can be evacuated by way of a tube connection 250. All movable parts are connected by bellows vacuum tight to the hollow body 140. The two mirrors 301 and 130 together with the end of the laser tube, from which the primary laser beam 201 exits, are located together in a vacuum-tight housing 270, which is connected to the hollow body 140 by way of a movable bellows 260.

Where measurement is to be carried out over a long distance, the hollow body is long, and it is expedient to provide supports at a number of points of the hollow body 140. One such support 320 is shown in Figure 3. The hollow body 140 is fixed to a slide 330 movable in the support 320. By means of a screw 350 (Figure 3), the slide 330 can be adjusted in a direction which is at right-angles to the measuring direction and at right-angles to the travelling direction of the carriage 170. In this embodiment, the photo-electric detector mounted on the rod 210 is a four-quadrant diode 340 incorporated in a suitable circuit shown in Figure 7.

By operating the screw 350, the hollow body 140 is shifted laterally until the fourquadrant diode 340 (Figure 7) is illuminated symmetrically. thereby ensuring that the laser beam thus lies exactly on the prolongation of the measuring axis of the sensor 190.

In the embodiment shown in Figures 1 to 3. due to the servo-control of the laser beam by the photo-electric cells 160 and motor 901. the control of the electronic measuring sensor 190 by the photo-electric cells 200 and the servo-motor 220 and the provision and mounting of the carrier body 140, the following important advantages are obtained.

1. High accuracy even over a considerable length, since the position of the laser beam 201 with respect to the test piece 120 is definitely fixed relative to fixed points defined by the photo-electric cells 160 and the rotary joint 701, angular variations of the laser tube 101 or the heavy foot 110 having no influence on the measurement, 2. High security from disturbance since both laser beams 401 and 501 lie in quiet air or preferably in a vacuum in the hollow body 140, 3. Simple adjustment since the compensation system formed by the photo-electric cells 200 and the servo-motor 220 permits of wide deviations between the test piece 120 and the hollow body 140 which serves as the guide for the measuring sensor 190.

Figures 4 and 4a show a further advantageous embodiment of the arrangement according to the invention.

In a first hollow body 1401 with a longitudinal slot closed by sealing lips 240, there is a second hollow body 1402 which also has a longitudinal slot closed by sealing lip 2401 shown in Figure 6a. The hollow body 1402 is fixed to the first hollow body 140, with the interposition of good heatinsulating spacing bushes 280.The two partial laser beams 401 and 501 are within the second hollow body 1402. The space between the hollow bodies 140, and 1402 may be evacuated through a tube 250, communicating with the interior of the first hollow body 1401. A vacuum is produced in the hollow body 1402 when the hollow body 140 is evacuated. since the sealing lips 24 1 of tile second hollow body 1402 are only effective against external excess pressure.

The construction of the measuring sensor carriage and of the servo-system corresponds to that shown in Figures 1 to 3.

The addition of the second hollow body 1402 achieves further calming of the enclosed air, or, with evacuation of the hollow bodies, improved airtightness is achieved.

Figures 5 and 5a show a further sossi- bility of splitting the laser beam 201 into two partial beams.

The beam 201 emitted by the laser 101 impinges on a partly transparent mirror 290.

The reflected partial beam of the laser beam 201 impinges on two differentially connected photo-electric cells 200. The photoelectric cells 200 and the partly transparent mirror 290 are connected by a base plate and by the rod 210 to the enclosure 180 shown in Figure 1.

The partial beann which is passed by the dividing mirror 290, and which serves for recontrol of the laser 101, impinges on the photo-electric cell 160 mounted on the foot 150 at the end of the hollow body.

The aforesaid arrangement avoids disadvantages which may possibly occur in Figure 1 owing to the partial beams 401 and 501.

Figures 6 and 6a show a further advantaeous way of splitting the laser beam.

in this case, the laser beam 201 impinges on a diffraction grating vaporised onto a thin glass plate 300, and whose grating strips lie parallel to the measuring direction of the electronic measuring sensor 190 (Figure 1).

The diffraction grating 300 divides the laser beam into two partial beams which impinge on differentially connected photo-electric cells 200. The photo-electric cells 200 and the diffraction grating 300 are connected by the rod 210 to the enclosure 180 of the carriages 170. As already shown in Figures 1 to 3, the electronic measuring sensor 190 is controlled by means of the photo-electric cells 200.

The non-deflected principal beam 201 of the laser beam again impinges on the photo-electric cells 160 at the end of the hollow body and serves for recontrolling the laser 101.

The arrangement of Figures 6 and 6a results in the principal beam 201 being almost undisturbed when incident on the photo-electric cells 160 at the end of the hollow body. Rotations and displacements of the diffraction grating 300 have almost no effect on the path of the principal beam 201.

Instead of a diffraction grating vaporised onto glass, a self-supporting grating may be used, for example a grating ot thin gold foil.

The direction of the principal beam 201 is then no longer influenced at all by the grating.

The arrangements of Figures 1 to 6 use static methods for trapping and controlling the laser beam 201 or the measuring sensor 190.

Figures 8 and 8a show an arrangement with dynamic evaluation. The light beam leaving the stationary laser 101 is incident on a stationary mirror 390 and is directed by the latter so as to be incident on a mirror 400 oscillating about an axis 380 and driven by a motor 440. The beam deflected by the mirror 400 sweeps across a photo-electric cell 410 fixed by means of a rod 210 to the carriage 170 (figure 1), and also across the photo-electric cells 420 and 430 at the end of the measuring distance.

Figure 9 shows the output signals of the photo-electric cell 410. By means of the adjusting motor 220 (figure 1), the photoelectric cell 410 is moved until the times T1 and T2 are equal, ie.e. zero balance is established.

Figure 9a shows the output signals of the photo-electric cell 420, and Figure 9b the output signals of the photo-electric cell 430.

Figure 10 shows a circuit for further evaluation of the output signals of the photo-electric cells 420 and 430. Both output signals are amplified in amplifiers Vl and V2 and are supplied to respective pulse-shapers P, and P2. The amplifier V2 is an inverting amplifier. The sum of the shaped signals is then integrated in an integrator J. The sum of the signals applied to the integrator J is shown in Figure 9c. In the case of symmetrical sweeping of the laser beam relative to the centre of the separation of the photo-electric cells 420 and 430, the integral over the voltages is equal to zero. In the case of departure from symmetry relative to this centre, an error signal is supplied to the amplifier V3 feeding the adjusting motor 440.At the same time, there is supplied to the amplifier V3 an A.C. signal produced by an oscillator 0 which excites the motor 440 to make its oscillating movements. In the case of displacement of the laser beam, the plan of symmetry of the oscillation made by the mirror 400 is adjusted by the motor 440 until the error signal is zero.

Figure 7 shows the four-quadrant diode 340 referred to hereinbefore in connection with Figures 3 and 5, and the circuitry therefor. At the output of the amplifier 260 appears the difference of the sum of the quadrants I and II and the sum of the quadrants III and IV. The output signal of amplifier 360 therefore provides information regarding displacement of the laser beam upward or downward, i.e. in the measuring direction of the sensor 190 (figure 1). At the output of the amplifier 370 appears the difference between the sum of the quadrants I and III and the sum of the quadrants II and IV. The output signal of the amplifier 370 therefore provides information regarding the displacement of the laser beam to the left or to the right. The diode 340 is connected to the enclosure 180 of the carriage 170 (Figure 1) by the rod 210.

If the line of separation of the quadrants I/II and III/IV is not exactly perpendicular to the axis of the digital measuring sensor 190, then when there is a deflection of the measuring sensor 190 perpendicular to the measuring direction, the laser beam generates a voltage difference signal, which cannot be distinguished from the signal produced in the event of a real displacement of the measuring sensor 190 in the measuring direction. The above-mentioned error may be compensated by adding to the difference signal which is formed at the output of the amplifier 360 in the event of a displacement in the measuring direction with respect to the laser beam, a part of the difference signal which is formed at the output of the amplifier 370 in the case of a lateral displacement with respect to the laser beam, this addition being effected by means of an electronic adding circuit (not shown).The oblique position of the line of separation of the diode 340 with respect to the measuring axis must be known and the proportionality factor will be selected in accordance with the oblique position.

Matter described hereinbefore is described and claimed in our co-pending application no. 44073/76. (Serial No.

1571397).

WHAT WE CLAIM IS: 1. An arrangement for ascertaining deviation from straightness or planeness of a surface of a test piece, the arrangement including a measuring sensor having a sensor member extending therefrom for measuring variations in displacement of said surface from a reference straight line, means for establishing a laser beam to define the reference straight line, a guide at which the measuring sensor is mounted and relative to which the measuring sensor is movable transversely of the said displacement, the guide being so arranged that the movement of measuring sensor relative thereto is substantially parallel to the reference straight line, and a servo-system provided for ensuring that, in operation, light from the laser beam impinges on a fixed reference point which is remote from the means for establ ishing the laser beam.

2. An arrangement according to claim 1, wherein the measuring sensor is carried by a carrier mounted directly on the said guide and a detector for detecting deviations of the carrier from the reference straight line defined by the laser beam and an adjusting system which effects a compensating adjustment of the measuring sensor relative to the said carrier are provided.

3. An arrangement according to claim 2, wherein the detector is mounted on the said carrier.

4. An arrangement according to claim 2

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   with dynamic evaluation. The light beam leaving the stationary laser 101 is incident on a stationary mirror 390 and is directed by the latter so as to be incident on a mirror 400 oscillating about an axis 380 and driven by a motor 440. The beam deflected by the mirror 400 sweeps across a photo-electric cell 410 fixed by means of a rod 210 to the carriage 170 (figure 1), and also across the photo-electric cells 420 and 430 at the end of the measuring distance.

   Figure 9 shows the output signals of the photo-electric cell 410. By means of the adjusting motor 220 (figure 1), the photoelectric cell 410 is moved until the times T1 and T2 are equal, ie.e. zero balance is established.

   Figure 9a shows the output signals of the photo-electric cell 420, and Figure 9b the output signals of the photo-electric cell 430.

   Figure 10 shows a circuit for further evaluation of the output signals of the photo-electric cells 420 and 430. Both output signals are amplified in amplifiers Vl and V2 and are supplied to respective pulse-shapers P, and P2. The amplifier V2 is an inverting amplifier. The sum of the shaped signals is then integrated in an integrator J. The sum of the signals applied to the integrator J is shown in Figure 9c. In the case of symmetrical sweeping of the laser beam relative to the centre of the separation of the photo-electric cells 420 and 430, the integral over the voltages is equal to zero. In the case of departure from symmetry relative to this centre, an error signal is supplied to the amplifier V3 feeding the adjusting motor 440.At the same time, there is supplied to the amplifier V3 an A.C. signal produced by an oscillator 0 which excites the motor 440 to make its oscillating movements. In the case of displacement of the laser beam, the plan of symmetry of the oscillation made by the mirror 400 is adjusted by the motor 440 until the error signal is zero.

   Figure 7 shows the four-quadrant diode 340 referred to hereinbefore in connection with Figures 3 and 5, and the circuitry therefor. At the output of the amplifier 260 appears the difference of the sum of the quadrants I and II and the sum of the quadrants III and IV. The output signal of amplifier 360 therefore provides information regarding displacement of the laser beam upward or downward, i.e. in the measuring direction of the sensor 190 (figure 1). At the output of the amplifier 370 appears the difference between the sum of the quadrants I and III and the sum of the quadrants II and IV. The output signal of the amplifier 370 therefore provides information regarding the displacement of the laser beam to the left or to the right. The diode 340 is connected to the enclosure 180 of the carriage 170 (Figure 1) by the rod 210.

   If the line of separation of the quadrants I/II and III/IV is not exactly perpendicular to the axis of the digital measuring sensor 190, then when there is a deflection of the measuring sensor 190 perpendicular to the measuring direction, the laser beam generates a voltage difference signal, which cannot be distinguished from the signal produced in the event of a real displacement of the measuring sensor 190 in the measuring direction. The above-mentioned error may be compensated by adding to the difference signal which is formed at the output of the amplifier 360 in the event of a displacement in the measuring direction with respect to the laser beam, a part of the difference signal which is formed at the output of the amplifier 370 in the case of a lateral displacement with respect to the laser beam, this addition being effected by means of an electronic adding circuit (not shown).The oblique position of the line of separation of the diode 340 with respect to the measuring axis must be known and the proportionality factor will be selected in accordance with the oblique position.

   Matter described hereinbefore is described and claimed in our co-pending application no. 44073/76. (Serial No.

   1571397).

   WHAT WE CLAIM IS: 1. An arrangement for ascertaining deviation from straightness or planeness of a surface of a test piece, the arrangement including a measuring sensor having a sensor member extending therefrom for measuring variations in displacement of said surface from a reference straight line, means for establishing a laser beam to define the reference straight line, a guide at which the measuring sensor is mounted and relative to which the measuring sensor is movable transversely of the said displacement, the guide being so arranged that the movement of measuring sensor relative thereto is substantially parallel to the reference straight line, and a servo-system provided for ensuring that, in operation, light from the laser beam impinges on a fixed reference point which is remote from the means for establ ishing the laser beam.
2. 2. An arrangement according to claim 1, wherein the measuring sensor is carried by a carrier mounted directly on the said guide and a detector for detecting deviations of the carrier from the reference straight line defined by the laser beam and an adjusting system which effects a compensating adjustment of the measuring sensor relative to the said carrier are provided.
3. 3. An arrangement according to claim 2, wherein the detector is mounted on the said carrier.
4. 4. An arrangement according to claim 2

   or 3, wherein the detector comprises two differentially connected photo-electronic components, whose electrical output signal controls the adjusting system, the adjusting system being such as to adjust the said measuring sensor until an equal amount of laser light is incident on the two photoelectronic components.
5. 5. An arrangement according to any preceding claim, wherein the said reference point is defined by a photo-electric detector mounted fixedly at one end of the guide.
6. 6. An arrangement according to claim 5, wherein the photo-electric detector at the end of the guide has two differentially connected photo-electric cells.
7. 7. An arrangement according to any preceding claim, wherein the means for establishing the laser beam is mounted on a base plate which is tiltable at a hinge and the said servo-system is such as to control the attitude of the base plate.
8. 8. An arrangement according to claim 7, wherein the means for establishing the laser beam is held in two diaphragms fixed to the base plate.
9. 9. An arrangement according to any preceding claim, wherein a diffraction grating is provided for splitting laser beams into two partial beams which impinge on photoelectric cells connected to the measuring sensor while the principal beam passing through the diffraction grating impinges on the said reference point and defines the reference straight line.
10. 10. An arrangement according to any one of claims 2 to 8, wherein the means for establishing a laser beam includes a mirror system which splits a primary laser beam into two mutually parallel partial beams, one of the partial beams defining the reference straight line and being directed to the said detector and the other partial beam being directed to impinge on the said reference point.
11. 11. An arrangement according to claim 9, wherein the diffraction grating consists of a self-supporting thin foil.'according to foil.
12. 12. An arrangement according to claim 1, wherein means are provided for setting the laser beam in oscillatory motion including a deflecting system, there being a photo-electric cell securely connected to the measuring sensor, two photo-electric elements defining the said reference point, and circuitry which from the time difference of the beam passages produces electrical signals for use in controlling the measuring sensor and the laser beam.
13. 13. An arrangement according to claim 2, wherein the said detector is a fourquadrant diode, whose elements are so connected that deviations of the laser beam in a first direction and at right angles thereto appear as an electrical signal, and furthermore the guide is laterally shiftable by a plurality of adjusting elements distributed over the length of the guide so that the four-quadrant diode can be moved in the lateral direction until the output signal therefrom is zero.
14. 14. An arrangement according to claim 2, wherein a digital distance measuring system is provided which indicates displacement of the said carriage along the guide.
15. 15. An arrangement according to any preceding claim, wherein the measuring sensor is a digital electric measuring sensor.

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