GB2150282A - Measurement apparatus - Google Patents

Abstract

A measurement apparatus comprises a probe 11 pivotally mounted on a housing 10. The probe 11 has a spherical element 14 at one end and a mirror 13 at the other end. A beam from a light source 22 is reflected by the mirror 13 onto a linear array of photo-diodes 25. The position of the probe 11 can be adjusted by an electric motor which is driven by a control circuit which is responsive to the photo-diodes. The control circuit also drives a display which indicates the position of the probe. In operation, the probe 11 is driven into a demanded position and subsequent movement of the probe is indicated on the display. The probe 11 may be pivotable about two orthogonal axes, the linear array of photodiodes being replaced by a two-dimensional array. <IMAGE>

Description

SPECIFICATION Measurement apparatus This invention relates to an apparatus for use in measuring of the type in which a probe is brought into contact with an object in order to make measurements.

In a well known measuring technique, a simple probe is mounted on a measuring machine and measurements are made by bringing the probe into contact with an object. As point of contact is determined visually, the accuracy of this technique is limited. In order to improve the accuracy of measurement, an apparatus has been proposed in which a probe is movably mounted in a housing.

The probe is biassed into a rest position and displacement of the probe from this position is detected by the breaking of an electrical contact.

Although this method provides greater accuracy than the conventional technique, this accuracy is limited by the essentially mechanical way in which displacement of the probe is detected.

Accordingly an object of this invention is to provide a new or improved measurement apparatus capable of making measurements with a high degree of accuracy.

According to this invention there is provided an apparatus for use in measuring comprising a housing, a probe movably mounted on the housing and having a reflecting face, a source of radiation positioned on the housing and arranged to transmit a beam of radiation onto the reflecting face of the probe, an array of radiation sensitive elements positioned on the housing and arranged to receive a beam transmitted by the radiation source and reflected from the reflecting face of the probe, means for adjusting the position of the probe, and a control circuit responsive to an output signal of said array for providing a drive signal to said adjusting means.

In the apparatus according to the present invention, the position of the probe is detected with the array of radiation sensitive elements and therefore high accuracy may be achieved.

This invention will now be described in more detail by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional view of an apparatus for use in measuring embodying this invention; Figure 2 is a fragmentary sectional view on the line 2-2 of Figure 1; Figure 3 is a block diagram of the electrical components of the apparatus shown in Figure 1; Figure 4 is a graph of the characteristics of an amplifier used in the apparatus of Figure 1; Figure 5 is a sketch of an array forming part of the apparatus of Figure 1; Figure 6 is a drawing of the mainly mechanical components of another apparatus for use in measuring embodying this invention; and Figure 7 is a block diagram of the electrical components of the apparatus shown in Figure 6.

Referring now to Figure 1, the apparatus includes a housing 10 and a probe 11 pivotally mounted in the housing. The probe 11 comprises a conical arm 12 having a circular mirror 13 mounted at one end and a spherical probe element mounted at the other. The mirror 13 has a reflecting face on its side which is opposite to the arm 12. The pivotal mounting for the probe 11 is shown in more detail in Figure 2 and in this Figure the arm 11 has been omitted for reasons of clarity. The mirror 13 is attached to a pair of spindles 15 and 16, the spindle 15 being mounted in a bearing 17 attached to the housing 10 and the spindle 16 being driven by an electric torque motor 18 and mounted in a bearing 19 which is attached to the case of motor 18.The bearings 17 and 19 comprise pre-loaded thrust ball bearing assemblies and the parts of shafts 15 and 16 which are mounted inside the bearings are of conical configuration.

Referring again to Figure 1, the apparatus also includes a light source 21 which in the present example comprises a lamp 22, a spherical lens 23 and a slit 24 which together project a slit shaped beam of light onto the mirror 13. The beam is reflected from the mirror 13 onto a linear array of photo diodes 25, the long dimension of the beam being orientated at 90" to the long dimension of array 25.

Turning next to Figure 3, the array 25 is driven by a clock pulse generator 26 and an output signal from the array 25 is supplied to a readout and error detector circuit 27. The circuit 27 receives a demand signal from a position setting circuit 28 which in turn is provided with a set of operating keys. The demand signal from the circuit 28 indicates a desired position for the beam of light on array 25 and, in consequence, a required position for the probe 11. The circuit 27 reads the actual position of the beam on array 25 making any necessary interpolation where the beam falls onto adjacent photo diodes. An output signal from circuit 27 is supplied to a display 29 which may either display the actual position of the beam on the array 25 or its position relative to the required position.The circuit 27 also outputs an error signal indicative of the difference between the required position and the actual position and this signal is supplied to an amplifier 30. The amplifier 30 adds the error signal to an a.c. signal which it receives from an a.c. generator 31 and the resulting output signal is supplied as a drive signal to the motor 18.

In operation, the motor 18 drives the probe 11 into the position demanded by the position setting circuit 28. The a.c. signal from the generator 31 ensures that the probe is accurately positioned despite friction in the bearings 17 and 19.

The amplifier 30 will normally have linear characteristics as shown by curve 32 in Figure 4. However, if greater accuracy is required a non-linear amplifier may be used in which the gain is enhanced about zero as shown by curve 33 in Figure 4.

The required position for the beam on the array 25 will normally be a central position as indicated at 34 on the sketch of the array shown in Figure 5.

Alternatively, under some circumstances a position towards the end of the array as indicated at 35 may be chosen. Clearly, if the probe 11 moves to a position in which the beam falls beyond one of the ends of array 25, the error signal from circuit 27 will be lost. In order to prevent this, some mechanical means is necessary to limit the range of movement of probe 11 and such means may take the form of a lug mounted on 5 probe 11 which acts against a pair of springs mounted on housing 10 on opposite sides of probe 11.

The apparatus shown in Figures 1 to 3 may be used either in conjunction with a measuring machine or with a machine tool. When it is used with a measuring machine, contact between the spherical probe element 14 and the object being measured is detected by displacement of the light beam from a preset position on array 25 and a measurement is made at each point of contact. If desired, fine measurements may be read directly on the display 29. When the apparatus is used with a machine tool, it is either mounted in a free position on the spindle of the machine tool or, if there is no free position, in place of one of the tools each time it is required to make a measurement. The apparatus is arranged so that the beam occupies a known position on the array when the probe element 14 is brought into contact with a master.Measurement are then made on a workpiece which is being machined at regular intervals until the desired dimension is achieved.

The apparatus shown in Figures 1 to 3 is capable of making measurements to a high degree of accuracy. There is presently available a linear array of photo diodes comprising 1000 photo diodes spaced at 10 um intervals. The length of this array is Icm and the error in this length is typically less than 2 um. Consequently, the position of the light beam on the array 25 can be determined with an error of less than 1 um. The example shown in Figure 1, the spacing from the mirror 13 to the array 25 is equal to the spacing between mirror 13 and the probe element 14 and so the error in determining the position of probe element 14 will be half that in determining the position of the light spot on array 25. Even greater accuracy may be achieved by shortening the length of arm 11 or by lengthening the optical path from mirror 13 to array 25.The optical path may be increased by the use of additional mirrors mounted in housing 10 so as to achieve a folded path. If desired, the arm 11 together with the probe element 14 may be demountable from the mirror 13 and a set of such arms with probe elements may be supplied so as to meet various applications.

The light source 21 may take forms other than that shown in Figure 1. For example, it may be replaced with a semi-conductor laser such as a gallium arsenide laser. Moreover, the invention is not limited to the use of radiation in the visible waveband and light source 21 in the array 25 may be replaced by a radiation source and detector which operate above or below the visible waveband.

Referring now to Figure 6, there is shown an example of the invention which may be used where it is desired to make measurements in two dimensions. As shown in Figure 6, a circular mirror 50 is attached to pivot shafts 51 and 52 which are respectively mounted in bearings 53 and 54 located in a first gimbal ring 55. The shafts 51 and 52 are driven by a pair of motors 56 and 57 secured to ring 55. The ring 55 is also connected to a pair of pivotal shafts 58 and 59 which are mounted in a pair of bearings 60 and 61 located in a second gimbal ring 62. The shafts 58 and 59 are driven by a pair of motors 63 and 64 secured to ring 62.

The ring 62 is mounted to the housing of the apparatus and, although not shown in Figure 6, the mirrcr 50 is attached to a conical arm which in turn carries a spherical probe element. In the example of Figure 6, 7 the light source 21 is replaced with a light source which produces a small circular beam of light and the linear array 25 is replaced with a two dimensional array of photodiodes.

The electrical components of the apparatus of Figure 6 are shown in Figure 7. These components include a clock pulse generator 70 which drives the two dimensional, the two dimensional array being indicated by reference numeral 71. An output signal from the array 71 is supplied to a readout and error detector circuit 72, the circuit 72 receiving a demand signal from a position setting circuit 73.

The circuit 72 provides a position signal to a display circuit 74. The circuit 72, 73 and 74 are generally similar to the circuits 27, 28 and 29 shown in Figure 3 and operate with two dimensions instead of with just one. The circuit 72 provides a pair of error signals which are supplied respectively to a pair of amplifiers 75 and 76, these amplifiers also receiving an a.c. signal from an a.c. generator 77.

The amplifiers 75.and 76 respectively provide drive signals to the motors 56, 57 and 63, 64. In operation, a desired position is keyed into the circuit 73 and the motors 56, 57, 63 and 64 then drive the mirror 50 so that the probe element achieves the desired position. The actual position is then displayed by display circuit 74.

Claims (3)

1. An apparatus for use in measuring comprising a housing, a probe movably mounted on the housing and having a reflecting face, a source of radiation positioned on the housing and arranged to transmit a beam of radiation onto the reflecting face of the probe, an array of radiation sensitive elements positioned on the housing and arranged to receive a beam transmitted by the radiation source and reflected from the reflecting face of the probe, means for adjusting the position of the probe, and a control circuit responsive to an output siognal of said array for providing a drive signal to said adjusting means.

2. An apparatus for use in measuring substantially as hereinbefore described with reference to and as shown in Figures 1 to 5 of the accompanying drawings.

3. An apparatus for use in measuring substantially as hereinbefore described with reference to and as shown in Figures 6 and 7 of the accompanying drawings.