Position measuring device

Applicants claim, under 35 U.S.C. §§ 120 and 365, the benefit of priority of the filing date of Nov. 30, 2002 of a Patent Cooperation Treaty patent application, copy attached, Serial Number PCT/EP02/13547, filed on the aforementioned date, the entire contents of which are incorporated herein by reference, wherein Patent Cooperation Treaty patent application Serial Number PCT/EP02/13547 was not published under PCT Article 21(2) in English.

Applicants claim, under 35 U.S.C. § 119, the benefit of priority of 1) the filing date of Jan. 17, 2002 of a German patent application, copy attached, Serial Number 102 01 496.5, filed on the aforementioned date, and 2) the filing date of Sep. 23, 2002 of a German patent application, copy attached, Serial Number 102 44 235.5, filed on the aforementioned date, the entire contents of each of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to a position measuring device for determining the absolute position, as well as a method for absolute position measuring 9.

2. Description of the Related Art

Absolute position measuring devices are increasingly employed in many fields, wherein the absolute position information is derived from a code track with code elements arranged one behind the other in the measuring direction. In this case the code elements are provided in a pseudo-random distribution, so that a defined number of sequential code elements constitutes a bit pattern. In the course of displacing the scanning device with respect to the code track by a single code element, a new bit pattern is already formed, and a sequence of different bit patterns is available over the entire measuring range to be absolutely detected.

Such a sequential code is called a chain code or a pseudo-random code.

It is stated in the publication “Absolute Position Measurement Using Optical Detection of Coded Patterns” by J. T. M. Stevenson and J. R. Jordan in Journal of Physics E/Scientific Instruments 21 (1988), No. 12, pp. 1140 to 1145, that each code element includes a predetermined sequence of two partial areas with optical properties which are complementary to each other.

Reference is made in the publication to GB 2 126 444 A. For generating the binary information in connection with such a Manchester coding, it is proposed there to compare the analog scanning signals of the code areas with a predetermined trigger threshold and to generate a binary information 0 or 1 as a function thereof.

This comparison with a fixedly predetermined trigger threshold has the disadvantage that fluctuations in the analog scanning signals can lead to an erroneous generation of the binary information.

Thus, an object of the present invention is based on creating an absolute position measuring device of high dependability or operational reliability, by which the generation of the absolute position as error-free as possible is therefore possible.

This object is attained by a position measuring device including a code having a first code element and a second code element arranged one behind the other in a measuring direction, wherein each of the first and second code elements includes a first partial area and a second partial area, which are complementary to each other and are arranged sequentially in the measuring direction. A scanning device including a plurality of detector elements that scan the first and second code elements and for forming a first scanning signal within the first partial area of the first code element and a second scanning signal within the second partial area of the first code element. An evaluation unit including a comparison device, which compares the first scanning signal with the second scanning signal and forms binary information for the first code element as a function of the comparison.

A further object of the present invention is based on disclosing a method for determining an absolute position, by which a generation of the binary information, and therefore of the absolute position, as free as possible of errors is made possible.

This object is attained by a method for absolute position measuring that includes scanning a code having a first code element and a second code element arranged one behind the other in a measuring direction, wherein each of the first and second code elements includes a first partial area and a second partial area, which are complementary with respect to each other and are arranged following each other in the measuring direction. Generating a first scanning signal within the first partial area of the first code element and a second scanning signal within the second partial area of the first code element. Comparing the first and second scanning signals with each other and forming binary information from the comparing.

The present invention will be explained in greater detail by the drawings, wherein is shown in:

FIG. 1 shows an embodiment of a position measuring device in a schematic representation in accordance with the present invention;

FIG. 2 shows the principle of an embodiment of an error test in accordance with the present invention;

FIG. 3 shows possible signals for error testing in accordance with FIG. 2 ;

FIG. 4 shows an embodiment of a position measuring device with an incremental track for generating control signals in accordance with the present invention;

FIG. 5 a shows possible analog scanning signals from the incremental track of FIG. 4 ;

FIG. 5 b shows possible control signals from the analog scanning signals in accordance with FIG. 5 a;

FIG. 6 a shows a first scanning position of the position measuring device of FIG. 4 ;

FIG. 6 b shows a second scanning position of the position measuring device of FIG. 4 ;

FIG. 6 c shows a third scanning position of the position measuring device of FIG. 4 ; and

FIG. 6 d shows a fourth scanning position of the position measuring device of FIG. 4 .

A position measuring device equipped in accordance with the present invention is schematically represented in FIG. 1 . This position measuring device operates in accordance with the optical scanning principle, wherein a code C is scanned of the transmitted light method. A scanning device AE arranged, movable in the measuring direction X relative to the code C, is used for scanning the code C.

The code C includes a sequence of code elements C 1 , C 2 , C 3 of equal length, arranged one behind the other in the measuring direction X. In turn, each code element C 1 , C 2 , C 3 includes two partial areas A and B of equal length, sequentially arranged in the measuring direction immediately following each other, which are designed complementary to each other. In this case complementary means that they have inverse properties, i.e. in case of an optical scanning principle are transparent or not transparent, or in case of incident light scanning are reflecting or non-reflecting.

The sequential code C is scanned by the scanning device AE, which contains a light source L, whose light illuminates, via a collimator lens K, several code elements C 1 , C 2 , C 3 following each other. The light is modulated as a function of the position by the code C, so that a position-dependent light distribution is created behind the code C, which is detected by a detector unit D of the scanning device AE.

The detector element D is a line sensor with a sequence of detector elements D 1 to D 11 , which are arranged in the measuring direction X. In each relative position, at least one detector element D 1 to D 11 is unequivocally assigned to each partial area A, B of the code elements C 1 , C 2 , C 3 , so that in every relative position of the detector unit D with respect to the code C a scanning signal S 1 A to S 3 B is obtained from each partial area A, B. These scanning signals S 1 A to S 3 B are conducted to an evaluation device AW, which compares each of the two scanning signals S 1 A, S 1 B, S 2 A, S 2 B, S 3 A, S 3 B of the two partial areas C 1 A, C 1 B, C 2 A, C 2 B, C 3 A, C 3 B of a code element C 1 , C 2 , C 3 with each other and, by its comparison creates a digital value, or a bit B 1 , B 2 , B 3 for each code element C 1 , C 2 , C 3 . A sequence of several digital values B 1 , B 2 , B 3 results in a codeword CW, which defines the absolute position. In case of a displacement of the detector unit D with respect to the code C by a width or length of a code element C 1 , C 2 , C 3 , a new code word CW is generated, and a multitude of different code words CW is formed over the measuring area to be absolutely measured.

FIG. 1 shows a momentary position of the code C in relation to the scanning device AE. The detector elements D 1 to D 11 are arranged in sequence at a distance of one-half the width of a partial area C 1 A to C 3 B of the code C. By this it is assured that in each position at least one detector element D 1 to D 11 is unequivocally assigned to a partial area C 1 A to C 3 B and does not scan a transition between two partial areas C 1 A to C 3 B. In the position represented, the partial area C 1 A is scanned by the detector element D 1 , and the partial area C 1 B by the detector element D 3 . The detector elements D 1 , D 3 detect the light distribution and generate, as a function of the light intensity, an analogous scanning signal S 1 A, S 1 B. Since the two partial areas C 1 A and C 1 B are embodied complementary to each other, the intensity of the scanning signals S 1 A and S 1 B is also inverse with respect to each other, therefore the signal levels are distanced far apart from each other.

This signal distance is now utilized for generating the binary information B 1 in that a check is made which of the two scanning signals S 1 A, S 1 B of the code element C 1 is greater. This check can be made by forming a quotient or by forming a difference. Difference formation is used in the example, wherein in accordance with FIG. 1 a trigger module T 1 is used as the comparison device. The trigger module T 1 generates B 1 =0, if S 1 A is less than S 1 B, and B 1 =1, if S 1 A is greater than S 1 B. In the same way binary information B 2 and B 3 is obtained by scanning the code elements C 2 , C 3 and comparing the analog scanning signals S 2 A, S 2 B, S 3 A, S 3 B, and by comparison of the partial areas C 2 A, C 2 B, C 3 A, C 3 B of respective code elements C 2 , C 3 by trigger modules T 2 , T 3 .

Therefore a first digital value is assigned to the first sequence of the partial areas A, B, which are embodied complementary to each other, and a second digital value is assigned to a second sequence of the partial areas A, B, which are embodied complementary to each other. In the example, the value 0 is assigned to the sequence opaque→transparent, and the value 1 to the sequence transparent→opaque.

Since the two partial areas A and B of each code element C 1 , C 2 , C 3 are complementary to each other, the signal-to-noise ratio of the scanning signals S is very large. A change in the light intensity of the light source L affects the scanning signals S of both partial areas A and B equally.

With the position measuring device correctly operated, because of the complementary embodiment of each of two partial areas A, B of a code element C 1 , C 2 , C 3 , analog scanning signals S, whose difference exceeds a preselected value, must be generated by scanning these partial areas A, B. A good error check is possible by observing this difference value. The basis of this error check is that it is possible to assume that, when the difference value falls by a predetermined value, the binary information B 1 , B 2 , B 3 is questionable and therefore an error signal F 1 is generated in connection with this binary information B 1 , B 2 , B 3 .

The principle of the generation of the error signal F 1 is represented in FIG. 2 . The analog scanning signals S 1 A and S 1 B of the code element C 1 are conducted to an error check device P. The error check device P compares S 1 A and S 1 B by forming a difference (S 1 A−S 1 B) and checks whether the difference value exceeds a predetermined comparison value V or does not exceed it. If the difference value (S 1 A−S 1 B) does not exceed the predetermined comparison value V, an error signal F 1 is output. These signal relations are represented in FIG. 3 .

The arrangement of the two partial areas A and B of each code element C 1 , C 2 , C 3 sequentially directly next to each other in the measuring direction X has the advantage that the detector elements D 1 to D 11 can be arranged next to each other at a small distance in the measuring direction X, so that therefore the position measuring device is insensitive to twisting of the detector unit D with respect to the code C, i.e. to Moiré fluctuations. Moreover, the sensitivity against interference because of dirt is low, since it can be assumed that both partial areas A and B of a code element C 1 , C 2 , C 3 are equally affected.

In connection with the example of the detector elements D 1 and D 2 it is easy to see in FIG. 1 that, in the course of a displacement of the code C by the length of a partial area A, B toward the left, the detector element D 1 scans the partial area C 1 B, and the detector elements D 3 the partial area C 2 A, i.e. partial areas of two code elements C 1 , C 2 . Thus the trigger module T 1 cannot provide binary information B 1 , B 2 , B 3 assigned to a code element C 1 , C 2 , C 3 . In what follows, steps will be explained by which it can be assured that the correct detector elements D 1 to D 11 are used for creating code words, i.e. those detector elements D 1 to D 11 which respectively scan the partial areas of a single code element C 1 , C 2 , C 3 .

A preferred method for this is described by FIGS. 4 to 6 . In accordance with FIG. 4 , an incremental track R with a periodic graduation of the period length corresponding to the length of a code element C 1 , C 2 , C 3 is arranged parallel next to the code C. In a known manner, the incremental track R is scanned by at least two detector elements DR 1 , DR 2 , which are offset with respect to each other in the measuring direction X by ¼ graduation period, for generating two analog scanning signals SR 1 , SR 2 , which are phase-shifted with respect to each other by 90°. These analog scanning signals SR 1 , SR 2 are interpolated in a known manner, and the interpolated position value is combined with the code word CW, so that the rough absolute position measurement is refined by the high-resolution incremental measurement.

The length of each code element C 1 , C 2 , C 3 is interpolated by the incremental measurement. A differentiation between the right and the left partial area of a code element C 1 , C 2 , C 3 is now possible in a simple manner by the interpolation value. A quadruple interpolation, i.e. a one-time triggering of the analog scanning signals SR 1 , SR 2 is sufficient for differentiating the partial areas A and B. The bit combination from the digital signals E 1 , E 2 obtained from this defines the sequence of the partial areas A, B unequivocally, and it is used as the control signal for determining the detector element D 1 to D 11 from which a correct code word CW can be created. Thus, the digital signals E 1 , E 2 define which scanning signals S must be compared with each other, and from which scanning signals S it is possible to obtain digital values B 1 , B 2 , B 3 for the code word CW.

Four different positions P 1 , P 2 , P 3 , P 4 of the code C with respect to the detector unit D are represented in FIGS. 6 a to 6 d for the further explanation of this method. The detector elements D 1 to D 11 are arranged in the measuring direction X at distances corresponding to half the length of a partial area A, B, and respectively two of the detector elements D 1 to D 11 arranged at a mutual distance corresponding to the length of a partial area A, B are differentially connected.

The position P 1 is represented in FIG. 6 a , in which the information E 1 =0 and E 2 =0 is obtained from the incremental track R. The bit B 1 of the code element C is formed by forming the difference from the detector elements D 4 and D 6 , i.e. (D 4 −D 6 ). At the position P 2 in accordance with FIG. 6 b , E 1 =0 and E 2 =1, so that the detector elements D 3 and D 5 are selected by a control unit M. At the position P 3 in accordance with FIG. 6 c , E 1 =1 and E 2 =1, so that the detector elements D 2 and D 4 are selected by the control unit M for forming the difference. At the position P 4 in accordance with FIG. 6 d , E 1 =1 and E 2 =0, so that the detector elements D 1 and D 3 are selected.

The correct detector elements for forming further bits of the code word CW are determined in the same way. If, for example, the detector elements D 1 and D 3 were selected for forming the bit B 1 , the detector elements D 5 and D 7 are used for forming the bit B 2 , and the detector elements D 9 and D 11 for forming the bit B 3 , as represented in FIG. 1 . Wherein in FIG. 1 only the trigger modules T 1 , T 2 , T 3 are represented, which are used in this momentary position.

A further possibility for determining the correct detector elements D 1 to D 11 , or the correct analog scanning signals S, includes that all detector elements D 1 to D 11 , which are spaced apart from each other at the distance of the length of a partial area A, B, are compared with each other. At the distance of a code element C 1 , C 2 , C 3 there are the detector pairs D 1 , D 3 and D 5 , D 7 —in accordance with the example of the momentary position P 4 represented in FIG. 6 d —each of which scans in a desired manner the difference of the partial areas A, B of a code element C 1 , C 2 . The further detector pairs D 3 , D 5 scan successive partial areas A, B of two successive code elements C 1 , C 2 , and in this way create an error signal F 1 by the error check device P explained by FIG. 2 . Now, for determining the correct detector elements D 1 to D 11 , a search is made for the detector group D 1 , D 3 , D 5 , D 7 in which error signals F occur the least. In detail, for performing this second possible method the following arrangement, or the following method steps, is/are required:

detector elements D 1 to D 11 are arranged in the measuring direction X at distances corresponding to half the length of a partial area A, B, the detector elements D 1 to D 11 form a first group (the even-numbered detector elements D 2 , D 4 , D 6 , D 8 , D 10 in FIGS. 6 a to 6 d ) at a mutual distance corresponding to the length of a partial area A, B, the detector elements D 1 to D 11 form a second group (the odd-numbered detector elements D 1 , D 3 , D 5 , D 7 , D 9 ) at a mutual distance corresponding to the length of a partial area A, B, the detector elements D 2 , D 4 , D 6 , D 8 , D 10 of the first group are arranged offset by half the length of a partial area A, B with respect to the detector elements D 1 , D 3 , D 5 , D 7 , D 9 of the second group, detector elements of a group immediately following each other are respectively differentially connected, the results of the comparison of those pairs of detector elements of the two groups are now used in a pattern corresponding to the length of a code element C 1 , C 2 , C 3 for forming the code word CW, whose sequence generates the least errors F, thus, in accordance with FIG. 6 d , the sequence (D 1 −D 3 )=B 1 , (D 5 −D 7 )=B 2 , etc. the detector elements D 1 to D 11 form a second group (the odd-numbered detector elements D 1 , D 3 , D 5 , D 7 , D 9 ) at a mutual distance corresponding to the length of a partial area A, B, the detector elements D 2 , D 4 , D 6 , D 8 , D 10 of the first group are arranged offset by half the length of a partial area A, B with respect to the detector elements D 1 , D 3 , D 5 , D 7 , D 9 of the second group, detector elements of a group immediately following each other are respectively differentially connected, the results of the comparison of those pairs of detector elements of the two groups are now used in a pattern corresponding to the length of a code element C 1 , C 2 , C 3 for forming the code word CW, whose sequence generates the least errors F, thus, in accordance with FIG. 6 d , the sequence (D 1 −D 3 )=B 1 , (D 5 −D 7 )=B 2 , etc.

The two partial areas A, B of each code word C 1 , C 2 , C 3 can be embodied to be optically scannable, wherein then one partial area A is embodied transparent or reflecting the scanning light, and the other partial area B opaque or non-reflecting. However, the present invention is not limited to the optical scanning principle.

The absolute position measuring device can be employed for measuring linear or rotary movements, wherein the code C is applied to one of the movable objects, and the scanning device AE to the other of the movable objects. In this case the code C can be applied directly to the object to be measured, or on a scale which in turn is coupled with the object to be measured.

Here, the objects to be measured can be the table and the carriage of a machine tool, of a coordinate-measuring machine, or the rotor and the stator of an electric motor.

Further embodiment variations of the method in accordance with the present invention of course exist besides the explained example.