Optical position measuring device

Details Optical position measuring device Optical position measuring device

1999-11-18

2001-01-16

Pham, Hoa Q. (Department: 2877)

Optics: measuring and testing

By polarized light examination

With light attenuation

C356S370000, C250S231130, C250S23700G

Reexamination Certificate

active

06175414

ABSTRACT:

The present invention relates to an optical position measuring device which is designed for determining the relative position of two movable objects and which supplies a reference pulse signal for at least one defined relative position of the two objects.
DESCRIPTION OF RELATED ART
Known optical incremental position measuring devices for determining the relative positions of two objects that are movable with respect to each other include as a rule an incremental graduation track used as part of a scale. This track is scanned using a scanning unit for generating incremental signals. In addition, provision is also made for the possibility of generating so-called reference pulse signals at one or more clearly defined relative positions of the two objects. For this purpose, on the scale, adjacent to the incremental graduation track, a further reference track is arranged which includes one or more scale reference mark structures placed at defined positions. For generating the reference pulse signals, the structures are also scanned using a scanning unit. The scanning unit, for example, can include a scanning reference mark structure as well as one or more optoelectronic detector elements. As a rule, a reference pulse signal is generated and the reference mark structures on the scale-side and scanning-side are so configured, that at any reference position a signal is generated having a maximum or minimum value at those positions. In order to assure reliable detection of the reference pulse signal, the signal that is generated in this manner is placed in a differential connection with a reference signal and, for this purpose, is connected to the reference signal across the two inputs of a signal subtraction element. At the output of the subtraction element, a signal identifying the respective reference position is then available for further processing.
As a reference or trigger signal for the subtraction element used in optical position measuring systems, a so-called constant light signal having a constant amplitude is used, the constant light signal resulting from the optical scanning of a constant light track.
In addition, it is also known from U.S. Pat. No. 4,691,101 to generate a reference signal as a so-called push-pull reference pulse signal, which has a characteristic curve that is phase-shifted by 180° with respect to the phase of the actual clock reference pulse signal. According to
FIGS. 12A and 12B
in the above-mentioned document, this is achieved by selecting a correspondingly aperiodical arrangement of the individual immediately adjoining detector elements on the scanning side as a function of an aperiodical sequence of transparent and non-transparent partial areas on the scale. Furthermore, individual scanning-side detector elements are directly assigned to transparent or non-transparent partial areas on the scale. For generating the clock reference pulse signal, those detector elements assigned to the transparent partial areas are connected to each other, while for generating the push-pull reference pulse signal, those detector elements that are assigned to the non-transparent partial areas of the reference pulse scale structure are connected to each other. The two signals are then fed to a subtraction element, at whose output the desired reference pulse signal C is available for further processing.
It is a particular disadvantage of generating output-side reference pulse signals in this manner that the proposed position measuring device having a scale- and scanning-side detector arrangement requires a relatively small distance between the scale and a scanning unit. In many applications, this cannot be guaranteed. Furthermore, due to the irregular incidence of scattered light onto the detector elements, it is possible that the generated reference pulse signals are triggered incorrectly or in a fluctuating manner.
A similar variant for generating a reference pulse signal is also known from German Patent 195 12 258. There provision is made for the direct assignment of planar detector elements to certain areas of a scale-side reference mark structure. The disadvantages of this arrangement are identical to those of the document discussed above.
SUMMARY OF THE INVENTION
The present invention is therefore an optical position measuring device which, in contrast to the related art, makes possible the dependable generation of a reference pulse signal. This is to be assured particularly in the case of larger scanning distances as well as in the case of the incidence of scattered light.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention is an optical position measuring device for determining a relative position of two objects movable with respect to each other in a measuring direction, by generating a reference pulse signal corresponding to at least one defined relative reference position. The position measuring device includes a scale having a scale reference mark structure, the scale being connected to one of the two objects and the scale reference mark structure being composed of n (n=2,3,4 . . . ) blocks disposed in the measuring direction, the scale reference mark structure blocks having a plurality of sequentially arranged partial areas of varying optical characteristics disposed in the measuring direction. The device also includes a scanning unit connected to the other of the objects and including a scanning plate with a plurality of detector elements and a scanning reference mark structure composed of n blocks arranged sequentially in the measuring direction, the scanning reference mark structure blocks having assigned to them corresponding detector elements. At least a first group of one of the scale reference mark structure blocks and scanning reference mark structure blocks is configured, at the reference position in the plane of a respective other one of the scanning reference and scale reference mark structures, to supply an intensity distribution that is identical to the structure of the associated first group of blocks, and at least a second group of another of the scale reference mark structure blocks and the scanning reference mark structure blocks, in a plane of a respective other one of the scanning reference and scale reference mark structures, supplies an intensity distribution that is complementary to the structure of the associated second group of blocks. The detector elements are interconnected such that output signals of the detector elements of each group of blocks are connected to each other to generate a reference pulse output signal resulting from the output signals, said reference pulse output signal having a maximum or a minimum at the defined relative position of the two objects.
According to the present invention, scanning of one or more scale reference mark structures is accomplished using the scanning reference mark structure in connection with detector elements arranged in a defined manner. In this way, a larger potential scanning distance can be realized.
Due to the larger scanning distance, it is now also possible without difficulty to electrically connect the detector elements employed on their light-sensitive side, using bonding wires, without allowing the latter to be damaged by the scale due to the small scanning distance.
Because the surfaces of the two groups of blocks having the scale and scanning reference mark structures can be selected in the invention so as to be identical, insensitivity with respect to scattered light influences is assured. This is because scattered light influences all of the signal components equally, and the signals that are phase shifted by 180° and that are discharged from both groups of blocks are influenced identically by the scattered light.
In contrast to the related art already discussed, it is possible according to the invention to use relatively insensitive detector elements, not requiring great expense to manufacture, to generate the required high-resolution reference pulse signal.
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