A METHOD AND DEVICE FOR TOUCH-FREE DETERMINATION OF THE TRANSLATORY OR ANGULAR POSITION OF A BODY

Abstract: In a method for touch-free determination of the translatory or angular position of a body, a defined measuring path (M) on the body (K) in the direction of said translatory or angular motion of the body (1) is detected spot-wise by means of an optical sensor head (1), which emits laser light towards the measuring path and provides an output signal responsive to light reflected from said path (M), the sensor head (1) and the body (K) with the measuring path (M) being movable relative to each other. In order to provide for an exact position determination with high resolution in an inexpensive way the invention suggests that the surface profile or characteristics of the body (K) along the measuring path (M) is scanned by illumination from the sensor head (1), which consists of an optical pick-up for compact discs.

Description: A method and device for touch-free determination of the translatory or angular position of a body The present invention refers to a method and a device for touch-free determination of the translatory or angular position of a body, in which a defined measuring path on the body in the direction of the translatory or angular motion of the body is detected spot-wise by means of an optical sensor head, which emits laser light towards the measuring path and provides an output signal respon¬ sive to light reflected from said path, the sensor head and the body with the measuring path being movable relative each other. In many instances within above all the field of machinery, such as in machining or controlling, it is desi¬ rable to measure positions exactly with high resolution directly on the measuring surface without greater modifi¬ cations having to be made of portions comprised in the measuring object and without introduction of expensive measuring scales. This is the case e.g. when linear posi¬ tions of a setting means or angular movements of a shaft are to be measured. Up to now this has not been possible to achieve with only one sensor located closely adjacent the measuring surface and being of sufficiently small size. The latest technical developments have matured into a very compact design of optical sensor heads operat¬ ing with laser light which at the same time can be manu¬ factured inexpensively. This is particularly the case in playing devices for grammophone discs of compact size, usually named CD-discs. The present invention is based upon the idea to utilize such sensor heads in connection with touch-free position determination of bodies. According to the invention this is now achieved substantially in that the surface profile or characteris¬ tics of the body along the measuring path is scanned by illumination from the sensor head, which is directed substantially at right angles to the measuring path and preferably consists of an optical pick-up for compact discs and further comprises a receiver for light reflec¬ ted towards said sensor head and a sensor means providing an analog output signal responsive thereto, said output signal after transformation into a digital signal in a way known per se, by known correlation techniques is compared with reference values from an initial scanning of the sur¬ face profile or characteristics of the measuring path, which values are stored in a digital memory and calibrated in absolute positions within said measuring path, thus eliminating the need for particular preparation of the surface of the body along said measuring path. The invention also intends to provide a device for carrying out said method and the features essentially dis- tinguishing said device are that the sensor head is direc¬ ted substantially at right angles to the measuring path and adapted to scan the surface profile or characteristics of the body along said measuring path, and that the sensor head preferably is an optical pick-up for compact discs and within itself also comprises a receiver for light reflected towards the sensor head, and a detector means for determination of the character of said light and for providing an analog output signal in dependency thereof, which by correlation technique and after transformation in a way known per se into a digital signal is compared with reference values from an initial scanning of the sur¬ face profile or characteristics of the measuring path, which values are stored in a digital memory and calibrated in absolute positions within the measuring path in order to thus provide a definite value of the momentary position of the sensor head along said measuring path. To use laser light for measuring the translation etc. of a surface in an electro-optical measuring system has previously been suggested in Swedish patent applica¬ tion No. 8402159-1. According thereto a comparison is made between a so-called speckle pattern created by the laser light and a previously recorded pattern of this type. This implies that the measuring system as such operates according to an entirely different principle than the present invention and also becomes much more complicated. A measuring device for identifying the surface profile of an object in an electro-optical manner has been previous¬ ly suggested in Swedish patent No. 8404376-9 but this device can not be utilized for touch-free position deter¬ mination in accordance with the present inventive consept. Owing to the present invention position or angular movement can be measured exactly and touch-free directly on the object. It is thus possible to measure the angular movement of a shaft in a robot for industrial purposes, the position of a coordinate table, the position of a piston rod in a pneumatic-hydraulic cylinder etc. without any requirement for the surface to be pre-treated. As mentioned the sensor head is of relatively small size and contains inexpensive components in relation to prior position sensors with corresponding high resolution abi- lity, namely about 1 /xm. No bulky measuring scales or similar equipments need to be used and this implies that the mechanical part towards which the sensor operates, does not need to be modified to any extent worth mentio¬ ning. By way of example, the invention will be further described below with reference to the accompanying dra¬ wings, in which Fig 1 illustrates a block diagram of a device for carrying out the method according to the inven¬ tion and Fig 2 illustrates a similar block diagram of the reference value comparison with known correlation methods. With reference to Fig 1, for the positioned deter¬ mination according to the present invention is used a sensor head 1, which consists of a touch-free measuring means, preferably an optical pick-up of substantially the same type as is used in a compact disc player. Such a pick-up has the advantage of automatically focusing a very small illumination spot on the surface of the size of about 1 μm along a normal to a measuring path M on a body K and with a dissolution of about 20 nm. The analog output signal from said sensor head 1 is a function of the light reflected from the illumination spot, which in turn is dependent of i.a. the surface profile along the given mea¬ suring path M. Said characteristics of the sensor head 1 allow that the optical characteristic of the surface pro¬ file can be read or measured with high resolution within the measuring path M. This implies furthermore, that the levels of the sensor signal in principle are re-created for each position within the given measuring path M. The dissolution of the position will be of the same order of magnitude as the diameter of the illumination spot. The functional principle of this device and hence the method according to the present invention is diagramma- tically illustrated in Fig 2. As a preparatory step to the position determination method, a reference is formed in a phase I by storing the sensor signals of the measuring path M in a digital memory 3. In order to calibrate the reference to the absolute position within said measuring path M and to arrange the reference in equidistant parts of this path the sampling frequency can be controlled by means of e.g. a speed signal if such is available. For obtaining a measuring path of a length of 1 m at a reso- lution of 1 μm one million values thus are to be stored. The random character (dependent on i.a. the surface pro¬ file) of the signal converted into digital form in a con¬ verter 2, makes said levels more or less unique to each position of the sensor within the measuring range. This makes it possible to compare continuously in the next phase, namely phase II, by means of known correlation technique in a micro-processor 4 the measured digital values with the values of the reference buffer or memory 3 and easily and rapidly retrieve the position of the sensor head 1 within the measuring path M. One kind of correlation technique which is suitable to use in this connection is a comparison of the measured values with the reference values by means of subtraction. The absolute values of the differences then form a corre¬ lation function called "average magnitude difference func¬ tion", AMDF. An actual sample x which is subtracted over a number of reference samples containing an equivalent sample » , forms a minimum value for |x-x' | , such as illustrated in Fig 2. The position of this extreme value in the corre¬ lation function becomes a measure of the position of the sensor head 1 along the measuring path . Starting from where the sensor head 1 is located at the sampling time t=t(n) and previous sampling times an approximated position can be obtained at the sampling time t=t(n+l) . This implies that the correlation only needs to be made over a restric¬ ted number of values in the memory 3 which substantially shortens the calculation time. However, it is also obvious that other types of known correlation technique also can be used in connection with the present invention.

Claims: C l a i m s 1. A method for touch-free determination of the trans- latory or angular position of a body, in which a defined measuring path on the body in the direction of said trans¬ latory or angular motion of the body is detected spot-wise by means of an optical sensor head, which emits laser light towards the measuring path and provides an output signal responsive to light reflected from said path, the sensor head and the body with the measuring path being movable relative each other, c h a r a c t e r i z e d i n that the surface profile or characteristics of the body along the measuring path is scanned by illumination from the sensor head, which is directed substantially at right angles to the measuring path and preferably consists of an optical pick-up for compact discs and further com¬ prises a receiver for light reflected towards said sensor head and a sensor means providing an analog output signal responsive thereto, said output signal after transforma¬ tion into a digital signal in a way known per se, by known correlation techniques is compared with reference values from an initial scanning of the surface profile or charac¬ teristics of the measuring path, which values are stored in a digital memory and calibrated in absolute positions within said path, thus eliminating the need for particular preparation of the surface of the body along the measuring path. 2. A method according to claim 1, c h a r a c t e- r i z e d i n that the comparison of the digital output signal with the reference values is carried out by means of subtraction such that the absolute values of the diffe¬ rences form a correlation function called "average magni¬ tude difference function", AMDF. 3. A device for carrying out the method according to claim 1 or 2 for touch-free determination of a translatory or angular position of a body and comprising an optical sensor head which emits laser light for spot-wise illumi¬ nation of a defined measuring path (M) on the body (K) in the direction of the translatory or angular motion and adapted to provide an output signal dependent of light reflected from the measuring path, the sensor head (1) and the measuring path (M) being movable relative each other, c h a r a c t e r i z e d i n that the sensor head (1) is directed substantially at right angles to the measuring path (M) and adapted to scan the surface profile or charac- teristics of the body along said measuring path, and that the sensor head (1) preferably is an optical pick-up for compact discs and within itself also comprises a receiver for light reflected towards the sensor head (1) and a sensor means for determination of the character of said light and for providing an analog output signal in depen¬ dency thereof, which by correlation technique and after transformation in a way known per se into a digital signal is compared with reference values from an initial scanning of the surface profile or characteristics of the measuring path, which values are stored in a digital memory (3) and calibrated in absolute positions within the measuring path in order to thus provide a definite value of the momentary position of the sensor head along said measuring path.

Inventors: ANDERSSON SOEREN, DURING CARL, WIKANDER JAN, ANDERSSON, SOEREN, DURING, CARL, WIKANDER, JAN
Applicants: ANDERSSON SOEREN, DURING CARL, WIKANDER JAN, ANDERSSON, SOEREN, DURING, CARL, WIKANDER, JAN
Application year: 1992
Main classification: G01D5
IPC codes: G01B11, G01D5, G01S17, G11B7