Encapsulated Photoelectric Measuring System

This invention relates to an improvement in encapsulated photoelectric measuring systems for measuring the relative position of two objects, of the type which include a measuring scale, a scanning unit for scanning the scale, means included in the scanning unit for illuminating at least a first surface of the scale, means for connecting the scanning unit with one of the two objects, a housing which encapsulates the scale and the scanning unit and defines an opening through which the connecting means passes, and at least one sealing element for sealing the opening.

Such photoelectric measuring systems are used for example in machine tools for measuring lengths or angles. These measuring systems typically must therefore be protected against environmental influences in the form of oil, cooling water, processing chips, dust and the like. One prior art approach to protection of a measuring system is shown in German Patent DE-PS No. 28 46 768. The disclosed system includes a housing for the scale and the scanning unit. This housing defines a slit running along the longitudinal direction, which slit is closed by elastomeric sealing lips. A follower extends between these sealing lips to connect the scanning unit with one of the two objects to be measured. Such sealing lips have been found to operate effectively to prevent the penetration of liquids and chips or shavings into the interior of the housing. However, such sealing lips typically cannot completely keep out liquids in the form of vapor and mist that may condense within the housing. In the event mist condenses to form liquid drops on the surfaces of the scale or the scanning plate of the scanning unit, such drops have the optical effect of collecting lenses. These drops can therefore interfere with the optical scanning of the scale and can disturb the photoelectric scanning of the graduation of the scale. Measuring inaccuracies can result from such disturbances. In addition, when a scanning unit includes printed circuits, as for example in conjunction with photosensors included in the scanning unit, condensing mist presents the danger of electrical short circuits.

The present invention is directed to an improved measuring system which operates at high measuring accuracy even when used in a harsh environment which includes high concentrations of vapor, fog, mist and the like.

According to this invention, an encapsulated photoelectric measuring system of the type described initially above is provided with means for hermetically sealing the scanning unit. This scanning unit sealing means includes a photopermeable surface positioned adjacent to the first surface of the scale and substantially parallel thereto. The photopermeable surface and the first surface of the scale define a gap therebetween having a width sufficiently small to spread any liquid drops on the first surface in the region of the photopermeable surface into a continuously homogeneous liquid layer.

The present invention provides the important advantages that with a remarkably simple structure a photoelectric measuring system is provided which is fully usable even under extremely unfavorable conditions, for example in the case of processing centers closed on all sides, without impairment of measuring accuracy. Further advantageous features of the invention are set forth in the dependent claims.

The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.

FIG. 1a is a transverse sectional view taken along line Ia--Ia of FIG. 1b of an encapsulated length measuring system which operates according to the direct illumination measuring principle and incorporates a first preferred embodiment of this invention.

FIG. 1b is a sectional view taken along line Ib--Ib of FIG. 1a.

FIG. 2a is a transverse sectional view taken alone line IIa--IIa of FIG. 2b of an encapsulated length measuring system which operates according to the transmitted light principle and incorporates a second preferred embodiment of this invention.

FIG. 2b is a sectional view taken along line IIb--IIb of FIG. 2a.

FIG. 3a is a transverse sectional view taken along line IIIa--IIIa of FIG. 3b of an encapsulated length measuring system which operates according to the transmitted light principal and incorporates a third preferred embodiment of this invention.

FIG. 3b is a sectional view taken along line IIIb--IIIb of FIG. 3a.

Turning now to the drawings, FIGS. 1a and 1b represent in cross section and longitudinal section two views of an encapsulated, incremental, length measuring system which operates according to the reflected light measuring principle and incorporates a first preferred embodiment of this invention.

This measuring system includes a housing G.sub.1 which is secured by means of screws 1.sub.1 to a bed 2.sub.1 of a machine tool. The housing G.sub.1 defines a groove U in which is mounted a scale M.sub.1. The scale M.sub.1 defines a measuring graduation T.sub.1 which is scanned by a scanning unit A.sub.1. The scanning unit A.sub.1 includes a light source L.sub.1, a condensor lens K.sub.1, a scanning plate AP.sub.1 and a photosensor P.sub.1. The scanning plate AP.sub.1 defines a graduation (not shown) which corresponds to the graduation T.sub.1 of the scale M.sub.1. The scanning unit A.sub.1 is guided along the scale M.sub.1 by means of rollers 3.sub.1 and two leaf springs BF.sub.1 on a base surface 4, and by means of rollers 5.sub.1 and a pressure spring DF.sub.1 on a side surface 6.sub.1 of the housing G.sub.1. The housing G.sub.1 defines a slits S.sub.1 which extends along the longitudinal length of the housing G.sub.1 and is closed by means of a pair of flexible sealing lips D.sub.1 sloped together in the form of a roof. A coupling member N.sub.1 which defines a sword-shaped middle section extends through the slit S.sub.1 and is in sealing engagement with the sealing lips D.sub.1. The coupling member N.sub.1 is fastened by means of screws 7.sub.1 to a slide piece 8.sub.1 of the machine tool. The slide piece 8.sub.1 is slidable relative to the bed 2.sub.1 along the direction of the scale M.sub.1. The coupling member N.sub.1 is coupled by means of a coupling KU.sub.1 in the form of a wire rigid in the measuring direction to the scanning unit A.sub.1. When the machine parts 2.sub.1, 8.sub.1 move relative to one another, the graduation T.sub.1 of the scale M.sub.1 is scanned by the scanning unit A.sub.1, which generates measuring signals in response thereto that are fed over line 9.sub.1 through the coupling member N.sub.1 to an evaluating and display unit (not shown). The outputs of the line 9.sub.1 at the scanning unit A.sub.1 and at the coupling member N.sub.1 in the interior of the housing G.sub.1 are sealed by means of seals 10.sub.1.

The sealing lips D.sub.1 prevent the penetration of liquids and processing chips into the interior of the housing G.sub.1. However, the sealing lips D.sub.1 cannot completely keep out fluids in the form of vapor, fog and mist that may condense in drop form inside the housing G.sub.1. In order to prevent liquid drops (which have the optical affect of collecting lenses) from forming on the graduation surface TM.sub.1 of the scale M.sub.1 and on the surfaces of the elements L.sub.1, K.sub.1, AP.sub.1, P.sub.1 of the scanning unit A.sub.1 (which may disturb the beam path of the photoelectric scanning), the scanning unit A.sub.1 is hermetically sealed chamber which includes the scanning plate AP.sub.1 as one of the walls of the chamber. The scanning plate AP.sub.1 is positioned adjacent and parallel to the scale M.sub.1, and a gap is defined between the graduation surface TA.sub.1 of the scanning plate AP.sub.1 and the graduation surface TM.sub.1 of the scale M.sub.1. The width of this gap is dimensioned so small that any liquid drops which may possibly be present on the graduation surface TM.sub.1 of the scale M.sub.1 in the zone of the scanning plate AP.sub.1 of the scanning unit A.sub.1 are spread or formed into a continuously homogeneous liquid layer. Preferably, the width of this gap is no more than about 0.5 mm.

FIGS. 2a and 2b present in cross section and in longitudinal section two views of an encapsulated incremental length measuring system which operates according to the transmitted light measuring principle in which includes a second preferred embodiment of this invention. The measuring system of FIGS. 2a and 2b corresponds in many respects with the measuring system shown in FIGS. 1a and 1b, and corresponding elements of the system of FIGS. 2a and 2b are provided with the same reference symbols, but with the subscript "2".

In contrast to the measuring system of FIGS. 1a and 1b, the scale M.sub.2 is fastened by means of an adhesive layer 11.sub.2 to an inner surface of the housing G.sub.2. A scanning unit A.sub.2 is guided by means of rollers 3.sub.2 and a leaf spring BF.sub.2 on the graduation surface TM.sub.2 of the scale M.sub.2. The scanning unit A.sub.2 extends around the scale M.sub.2 on both sides of the scale M.sub.2. In order to avoid distortions of the beam path of the photoelectric scanning system by means of liquid drops, the upper part of the scanning unit OA.sub.2 which includes a light source L.sub.2 and a condenser lens K.sub.2 is hermetically sealed as a chamber which includes a scanning plate AP.sub.2 as one wall of the chamber. The lower part of the scanning unit UA.sub.2 includes a photosensitive element P.sub.2, and this lower part of the scanning unit A.sub.2 is also hermetically sealed as a chamber, one wall of which is defined by a glass plate GP.sub.2. As shown in FIG. 2a, the graduation surface TA.sub.2 of the scanning AP.sub.2 is directly opposite, parallel, and spaced with respect to the graduation surface TM.sub.2 of the scale M.sub.2. In addition, the glass plate GP.sub.2 defines a surface OG.sub.2 which is situated directly opposite to the surface OM.sub.2 of the scale M.sub.2 lying opposite the graduation surface TM.sub.2. The separation between the graduation surface TM.sub.2 and the graduation surface TA.sub.2 on the one hand as well as the separation between the surface OM.sub.2 and the surface OG.sub.2 on the other hand are both made so small that any liquid drops possibly present on the graduation surface TM.sub.2 and the surface OM.sub.2 of the scale M.sub.2 in the region of the scanning plate AP.sub.2 of the scanning unit A.sub.2 are formed or spread into a continuously homogeneous liquid layer. Each of the separations should preferably be less than approximately 0.5 mm.

FIGS. 3a and 3b provide cross sectional and longitudinal sectional views of an encapsulated incremental length measuring system which likewise operates according to the transmitted light measuring principle and which incorporates a third preferred embodiment of this invention. The system of FIGS. 3a and 3b corresponds essentially with the system of FIGS. 2a and 2b and corresponding elements in the two embodiments are provided with the same reference symbol, but with a subscript "3" in the system of FIGS. 3a and 3b.

In contrast to the length measuring system of FIGS. 2a and 2b, the scanning unit A.sub.3 includes an upper scanning unit component OA.sub.3 and a lower scanning unit component UA.sub.3 which are joined together one with the other by means of a joint or a spring parallelogram FP.sub.3. The spring parallelogram FP.sub.3 can be made up of a number of leaf springs arranged parallel to one another so as to allow the lower scanning unit component UA.sub.3 to deflect left and right as shown in FIG. 3b. The upper scanning unit component OA.sub.3 is guided by means of rollers 3.sub.3 ' on a graduation surface TM.sub.3 of a scale M.sub.3 , and the lower scanning unit component UA.sub.3 is guided by means of rollers 3.sub.3 " on a surface of the scale M.sub.3 lying opposite to the graduation surface TM.sub.3. In this way even with relatively great tolerances in the thickness of the scale M.sub.3 a small, parallel spacing is assured between the graduation surface TM.sub.3 of the scale M.sub.3 and the graduation surface TA.sub.3 of the scanning plate AP.sub.3 of the upper scanning unit component OA.sub.3. Similarly, a small, parallel spacing is assured between the surface OM.sub.3 of the scale M.sub.3 lying opposite to the graduation surface TM.sub.3 and a surface OG.sub.3 of a glass plate GP.sub.3 of the lower scanning unit component UA.sub.3 of the scanning unit A.sub.3. These small parallel spacings on both sides of the scale M.sub.3 form any drops adhering to the scale M.sub.3 into a continuously homogenous liquid layer in the region of the scanning plate AP.sub.3 of the scanning unit A.sub.3. The upper scanning unit component OA.sub.3 which includes a light source L.sub.3 and a condenser lens K.sub.3 is hermetically sealed in part by the scanning plate AP.sub.3. Similarly, the lower scanning unit component UA.sub.3 of the scanning unit A.sub.3 which includes a photosensor P.sub.3 is hermetically sealed in part by the glass plate GP.sub.3.

The spacings between the surfaces TM, TA and OM, OG, respectively, of the scale M and of the scanning unit A are preferably chosen to be so small that even a dirty or cloudy liquid on the scale M has no substantial influence on the photoelectric scanning. Because the scanning unit is hermetically sealed or encapsulated, problems related to electronic short circuits in printed circuits by fluid action in the scanning unit A are entirely avoided.

The scanning unit A.sub.1, A.sub.2 as well as the upper scanning unit component OA.sub.3 and the lower scanning unit component UA.sub.3 of the scanning unit A.sub.3 include in each case a case which is closed over a cover, which case encloses at least the illumination unit L, K as well as the photoelectric sensors P.

Of course, it should be understood that a wide range of changes and modifications can be made to the preferred embodiments described above. For example, the invention can readily be adapted for use with photoelectric angle measuring systems as well as in encapsulated measuring systems in which openings in the housing are closed with any desired sealing means. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.