A measuring apparatus for determining the position of a movable element with respect to a reference

Abstract: A measuring apparatus for determining the position of a movable element with respect to a pair of spaced reference means, e.g. mirrors (9,10), rotatable around parallel shafts (11,12). The movable element (4) emits a light beam from a position to both mirrors. As a result of the rotation, the two mirrors convert the reflected light beam into a light pulse series. Each of these light pulse series is detected by a detector (15,16), which detected light pulse series are supplied to a processing unit (6) for calculating from the light pulse series and from a starting pulse the position of the movable element with respect to both mirrors.

Description: This invention relates to a measuring apparatus for determining the position of a movable element according to the preamble of claim 1. Such a measuring apparatus for determining the position of the movable element with respect to the reflecting surface is known from EP-A- 0 202 468. For calculating the position of the movable element by means of the reflecting surfaces preferably in the form of rotary mirrors, use is made, according to this known measuring apparatus, of the fact that the distance from this element to each of the reflecting surfaces with respect to the initial moment at which this element emits, e.g., a light beam, is a function of the phase shift with respect to the light wave pulses of a series of light wave pulses supplied by one mirror and a series of light wave pulses supplied by the other mirror, which series of light pulses result from the rotation of the mirrors, and is also a function of the amount of the rotational speed. In order to calculate as reliably as possible, it is desirable that the rotational speed of both mirrors be the same, i.e. that in each angular position the mirrors rotate in mirror-symmetrical relationship with each other. Such an accurate rotation, however, considerably increases the constructional requirements of the measuring apparatus. Yet also in case of some small difference in rotational speed a rather accurate measurement can be obtained by means of the pulse series over a great many revolutions. This rotation problem, however, can be solved according to the invention by providing a starting pulse generator for emitting also a propagation energy wave of, e.g., the same type as that of the movable element, and which starting pulse generator is located such that the propagation energy wave emitted by the starting pulse generator is reflected by the reflecting surfaces into a rotational position, which reflected propagation energy wave can be detected by the detectors cooperating with the reflecting surfaces, the position of the movable element to each of the reflecting surfaces being calculable by the processing unit from the traversed angle between a pair of angular positions of the reflecting surfaces, the propagation energy wave of the starting pulse generator being detected in one angular position and the propagation energy wave of the movable element in the other angular position, the traversed angle being a measure for the distance from the movable element to the reflecting surfaces. The advantage of this embodiment is that the measurement has become independent of the rotational speed of the reflecting surfaces and that it is further unessential that the rotational speeds of both reflecting surfaces must be equal, and also that the reflecting surfaces always rotate in mirror-symmetrical relationship with each other. A further advantage is that the reliability of the measurement has considerably increased. Moreover, the measuring distance from the movable element to the reflecting surfaces may considerably increase, and when a laser beam is used, the measuring distance can in principle be measured as far as the horizon. The invention will be described in more detail hereinafter with reference to the accompanying drawings in which: Fig. 1 is a practical example of the measuring apparatus ; Fig. 2 is a diagram for further explanation of Fig. 1. The embodiment shown in Fig. 1 comprises a base plate 1 and a housing 2 mounted thereon on a longitudinal side of the base plate. The part of base plate 1 projecting from housing 2 may serve as information surface, e.g. by providing it with information relevant for the measurement, such as a function division, system of axes, etc. (not shown) for an object 3 to be measured, which is suitably placed under base plate 1. This object may be a printed circuit board, a document or otherwise. By means of a movable object, e.g. in the form of a pen-shaped holder 4, the object 3 can be scanned. For this purpose, holder 4 comprises a pen 5 depressible with respect to the longitudinal direction thereof. Provided in pen 5 is, e.g. a laser source (not shown), which comes into action after depressing pen 5. The voltage required for the laser source can be obtained by building a holder (not shown) into the holder 4, in which holder an on-and-off circuit (not shown) is disposed for switching on the laser source, e.g. at the moment when pen 5 is pressed on object 3. The light energy emitted is received by a pair of mirrors 9 and 10 rotating in opposite directions. These mirrors are accommodated in housing 2, in which they are spaced, each rotating about shafts 11,12, which are parallel to each other. The light beams a and b received by mirrors 9 and 10 are reflected by the mirrors and are each converged to points F 1 , F 2 through correction lenses 13 and 14. Disposed at these points F 1 and F 2 are photosensitive detectors 15,16, by which light beams a and b can be detected which, by means of a known per se logic circuit 17, are supplied to processing unit 6 for calculating position P of pen 5 on the object with respect to both mirrors serving as reference means. The rotation of the mirrors will result in that the light beams reflected by the mirrors in the form of light pulses are supplied to detectors 15 and 16. Each of the mirrors therefore supplies a series of light pulses. Depending on the position of the pen, these series of light pulses will be more or less phase-shifted with respect to each other. From these phase-shifted light pulse series and from the starting signal the coordinates of point P can be determined in the known manner by processing unit 6 on the movable element with respect to each of the mirrors. For mirror 9 this would be, as shown in Fig. 1, P a (x,y) and for mirror 10 P b (c-x,y), c being the distance between the two mirrors, and the dashed lines 19 and 20 being taken as boundary lines, within which the position of the movable elements 4,5 can be determined. It will be clear, however, that the position of the movable element can also be determined outside these boundary lines 19 and 20, i.e. within those boundaries within which the mirrors are still capable of reflecting the light to the detectors. For positions to the left of line 19, however, a negative value should then be selected for x. The coordinates calculated by processing unit 6 may be shown, e.g. on a display 18, or may be stored in a memory for further use. By constantly displacing the movable element 4,5 the object 3 can be scanned and thus all scanned points can be determined. The calculation is based on the angular displacement of mirrors 9 and 10 from a selected first angular position to a second angular position, which depends on the position of movable element 4 on object 3. To achieve this, there is accommodated in housing 2 a starting pulse generator 21 for generating and emitting a propagation energy wave of the same type as that of pen 5. The operation of the starting pulse generator can be controlled from processing unit 6 through line 22. The starting pulse generator preferably lies near the common plane of the rotary shafts 11 and 12, e.g. at the midpoint between these shafts and in sucn a manner that the propagation energy wave emitted by the starting pulse generator, e.g. in the form of a laser beam, reflected by mirrors 9 and 10 in preselected angular positions A and B, respectively, can be detected by detectors 15 and 16. The rotation of the mirrors will result in that at each revolution of the mirrors in the preselected angular positions A,B the deteccors receive a light pulse from the starting pulse generator. Therefore, a series of pulses is formed used as starting pulses t l ......t n (see Fig. 3a,b) for the calculation of the coordinates of point P. When mirrors 9 and 10 reach the angular positions A' and B', respectively, the light beam emitted by pen 5 from point P is detected by detectors 15 and 16 in the form of pulses ta and tb, respectively. From the difference t a1 -t 1 and t b1 -t 1 , which is a measure for the traversed angle a and β, respectively, between the preselected angular positions A, B and the angular positions A' and B', respectively, the coordinates of P can be calculated by the processing unit. It will be clear that when the mirrors used are reflective on both sides, the detection of the light beams takes place at each half-revolution of the mirrors. The reference means used in the selected embodiment are plane mirrors which are reflective on one or both sides. It will be clear that other forms may also be used, e.g. cylindrical means, of which the planes of cross-section perpendicular to the rotary shafts 11,12 are squares and of which each of the vertical sides of the cylinder surface is a reflective surface, or in general forms with planes of cross-section which are polygons and of which the number of sides is n > 2. If desired, the correction lenses 13,14 used in the given embodiment may be left out. In that case, however, the amount of light received by the detectors will be considerably reduced. This need not be a drawback if the laser source and the photosensitive detector are properly selected. In order to avoid that false light is received by the detectors, a guard tube (not shown) may be provided around the beam of reflected light between the mirrors and the associate detectors. Instead of a laser source, an infrared source may be disposed both in pen 5 and in starting pulse generator 21, but then the detectors must be suitable for detecting the infrared portion of the light energy wave. An advantage of infrared light is that ambient light does not affect the measurement, although a properly selected intensity of the laser source may also rule out any disturbing influence of ambient light.

Claims: An apparatus for determining the position of a propagation energy wave emitting movable element (4, 5) said apparatus comprising at least two spaced rotatable reflecting surfaces (9; 10), a detector (15,16), a processing unit (6) each of the reflecting surfaces (9; 10) cooperating with said detector (15; 16) to detect the propagation wave energy received by the reflecting surfaces (9; 10) and supply it to said processing unit (6) for calculating the position (P) of the movable element (4, 5) with respect to the reflecting surfaces (9; 10), characterized in that a starting pulse generator (21) is provided for emitting also a propagation energy wave, which starting pulse generator is located such that the propagation energy wave emitted by the starting pulse generator is reflected by the reflecting surfaces (9; 10) in a particular rotational position (A,B) thereof, which reflected propagation energy wave can be detected by the detectors (15, 16) cooperating with the reflecting surfaces, the position (P) of the movable element (4, 5) to each of the reflecting surfaces being calculable by the processing unit (6) from the traversed angle (α, β) between a pair of angular positions of the reflecting surfaces, the propagation energy wave of the starting pulse generator being detected in one angular position (A,B) of the reflecting surfaces and the propagation energy wave of the movable element in the other angular position (A',B') of the reflecting surfaces, the traversed angle (a, β) being a measure for the position of the movable element to the reflecting surfaces. An apparatus as claimed in claim 1, characterized in that the propagation energy wave is a light energy wave. An apparatus as claimed in claim 2, characterized in that the light energy wave may be, e.g., a laser beam or an infrared wave. An apparatus as claimed in anyone of claims 1-3, characterized in that a correction lens (13; 14) is provided between each reflecting surface (9; 10) and associate detector (15; 16) in such a manner that at least part of the energy wave beam reflected by the reflecting surfaces is converged to the detector.

Inventors: PROPER ROBERT JAN, PROPER, ROBERT JAN
Applicants: PROPER ROBERT JAN, PROPER, ROBERT JAN
Application year: 1992
Main classification: G01S5
IPC codes: G01S5