EP2769104B1 - Position-measuring device for fluid cylinder - Google Patents

Description

The invention relates to a position measuring device according to the preamble of claim 1.

In many applications of fluid cylinders, it is advantageous if the position of a piston or the piston rod of a fluid cylinder is known in order to move movement members driven by the fluid cylinder exactly to desired positions or to determine their exact position.

A large number of position measuring devices is known from the prior art, which can detect optically detectable markings on the piston rod by means of a sensor arrangement fixed to the cylinder with a light source and a light sensor. Such a position measuring device for absolute measurement of a piston rod position is for example off DE 100 14 194 A1 , Another generic position measuring device is off WO 2009/112895 A1 whose sensor arrangement is based on a technology used in computer mice. A disadvantage of such a position measuring device is that, in spite of the generally high measuring resolution of such a system, slight measurement inaccuracies increase in the course of use and according to FIG WO 2009/112895 A1 again and again calibration positions must be approached.

A disadvantage of the first-mentioned embodiment is that in order to determine the absolute position of the piston rod in each case an entire barcode of the code grinder must be detected, but this is done sequentially in this case. In a sequence of many short movements that are shorter than the respective bar codes, resulting measurement inaccuracies, which are canceled only when a movement of the piston rod takes place, which is greater than the length of a bar code and thus an exact absolute position can be detected ,

All previously known such position measuring devices have in common that they have only an insufficient combination of robustness and measurement reliability and universally applicable compact dimensions, which is why those on optical measurement So far, only a small number of measuring systems have been successful in practice and in the marketplace.

Other position measuring systems are off US 6,327,791 B1 and WO93 / 20403 A1 known.

The object of the invention is to provide a position measuring device for a piston rod of a fluid cylinder, which has a high reliability in mechanical and metrological terms despite harsh operating conditions and is still economical to produce.

The object of the invention solves a generic position measuring device with the characterizing features of claim 1.

By virtue of the fact that, as is known from the prior art, the illumination source and the image sensor are fastened to a common printed circuit board of the sensor arrangement and a light-guiding element for transmitting the luminous flux from the illumination source to the measuring cut-out is arranged between the illumination source and the measuring section, the measuring section is also included Using a simple, compact and cost-effective illumination source sufficiently lit and reliable measurements can also be done with a simple measuring optics. The light guide is based on total reflection of a majority of the output from the light source luminous flux at its boundary walls and it can be optimally distributed the luminous flux with very low losses on the measurement section. The light-guiding element uses the principle of a light guide, similar to that which is also used in signal transmission technology, and the light-guiding element is formed by a coherent, in particular one-piece, translucent body of light-transmitting material.

By virtue of the fact that the light-conducting element according to the invention has a cross-section increasing from the light-entry surface to its light-emitting surface, even when using very small illumination sources with a small radiating surface, a measurement section sufficiently large for measurement purposes can be uniformly illuminated.

The light entry surface and the light exit surface on the light guide are translucent, while the lateral boundary walls may be opaque and, for example, may also have an internally mirrored coating that further improves the reflection of the luminous flux at the boundary walls.

The invention is applicable to all types of fluid cylinders as in single-acting cylinders, plunger cylinders, in which the piston rod is formed by the piston itself, and double-acting cylinders with a piston rod or double-sided or continuous piston rod. The position measuring device according to the invention can be doubly provided on a single fluid cylinder because of its favorable manufacturing costs and its compact dimensions, whereby a redundant position measuring system is provided, as e.g. in steering cylinders advantageous or prescribed. In the case of a fluid cylinder with a piston rod on both sides, the position measuring devices can be arranged at both ends or else both at one end.

Optimum utilization of the luminous flux emitted by the illumination source is effected when the light-guiding element directly adjoins the illumination source or identifies a light-entry surface which is positioned at a distance of less than 2 mm from the illumination source. The light-guiding element can thus contact the illumination source directly or is arranged at least in its immediate vicinity, whereby the majority of the luminous flux enters the light-guiding element and is available for illuminating the measuring section.

A further advantageous embodiment variant is that the light-guiding element extends over at least 50%, preferably over at least 75%, of the smallest distance between the illumination source and the piston rod surface. This also ensures that the smallest possible portion of the luminous flux between the illumination source and the measurement section is lost. Ideally, the light guide extends from the illumination source to just before the piston rod surface, but still ends at such a distance that the measurement section can be detected by the measuring optics without being obstructed by the light guide.

If an LED element is used as the illumination source, the sensor arrangement has a high durability and low power consumption. LED elements are available in many embodiments and can be obtained with different frequencies of light, wherein the light frequency of the LED element or the illumination source can be generally adapted to the optimum sensitivity of the image sensor.

Image sensor and / or illumination source can be designed in particular as applied to the circuit board SMD components and thereby contribute to the reduction of the sensor arrangement.

The illumination source can advantageously have an approximately square emission surface which, with a sufficiently high luminance, also enables a very small component size for the illumination source. The shape of the radiating surface can also differ substantially from the shape and size of the measuring section due to the use of the light-guiding element, since the light-guiding element can be optimally used for guiding, shaping and steering the luminous flux.

If the light-guiding element has a matted light entry surface and / or a matted light exit surface, the luminous flux is distributed relatively uniformly inside the light-guiding element and / or the light exit surface itself acts like a radiating surface of an illumination source, which, however, lies substantially closer to the measuring section than the actual one illumination source. As a result of the matting of the light entry surface or light exit surface, the luminous flux emitted by the illumination source is converted into an approximately diffuse luminous flux, which is also largely guided in the interior of the light guide element by total reflection to the light exit surface and effects a very uniform illumination of the measurement detail.

A compact embodiment of the sensor arrangement is particularly possible if the edge length of the radiating surface is less than 2 mm and the dimensions of the measuring section are at least 10 mm by 2 mm. By the light guide, the emitted from a small radiating surface intense light can be transferred to a relatively large Meßausschnitt, whereby even with thicker bar marks a Coding of many piston rod positions is possible because the measuring section detects a sufficiently large section of the code pattern.

This is also facilitated if the light-guiding element has a rectangular cross-section, with a longer side of the rectangular cross-section running parallel to the piston rod axis. As a result, a measuring section extending in the longitudinal direction of the piston rod axis can be illuminated in the best possible uniform manner.

A compact embodiment of the sensor arrangement is possible in particular when the light exit surface of the light guide element corresponds at least to three times the light entry surface. The luminous flux of the illumination source can thereby be spread or expanded starting from a small, approximately punctiform radiating surface, whereby an illumination of the measuring excerpt which is advantageous for the optical measurement is effected.

Light entry surface and / or light exit surface can be formed simply by the use of the light guide in contrast to pure lens systems, in particular substantially planar surfaces, whereby the production of the light guide is particularly inexpensive and still the lighting for reliable measurements of the piston rod position is sufficiently strong and uniform.

The light exit surface may be concave and / or sectionally convex at least in sections, whereby the light guide can be locally influenced by the light guide or the luminous flux distribution on the measuring section compared to purely flat surfaces. The concave light exit surfaces thereby cause a scattering of the exiting luminous flux, while convex sections of the light exit surface have a concentrating effect.

Another possibility for advantageously influencing the illumination of the measurement section may be that the light-conducting element breaks a local disturbance in the center of the beam path in the form of a transverse bore, a cover panel or an optical system has the elements. As a result, an excessively large illumination difference between the central region of the measurement section and its edge regions can be substantially reduced.

A further advantageous embodiment of the position-measuring device may consist in that the light-guiding element and / or the measuring optics is mounted pivotably in the housing with respect to a pivot axis parallel to the piston rod axis. This allows the light stream or the beam path of the measuring optics are adapted to different geometric conditions that can occur, for example, when the sensor assembly is to be used for piston rods of different diameters or the distance between the piston rod and circuit board of the sensor arrangement varies for structural reasons.

In order to facilitate or ensure the mutual positioning of light-guiding element and measuring optics, it is possible for the light-guiding element to be mounted pivotably on the measuring optics or for the measuring optics to be pivotably mounted on the light-guiding element, whereby the two components are, so to speak, coupled together in one direction and in one to perpendicular direction to each other are adjustable.

An embodiment with easy-to-produce code pattern on the piston rod is given when the code pattern, the width of the bar marks and the unmarked code sections each about one millimeter or an integral multiple of a millimeter. The requirements for the accuracy in the production of the code pattern are relatively low in this case, whereby the associated manufacturing costs are relatively low. A width of the line markings of one millimeter or a multiple thereof can be reliably scanned with simple embodiments of an image sensor in combination with simple measuring optics, whereby the measurement reliability is very high and a resolution of the position measurement sufficient for many applications of a fluid cylinder is given.

An advantageously usable for many applications measuring system is given when the code pattern is formed by a binary pseudo-random code with interlaced, mutually different code words, the length of the measurement section at least the length of the codewords corresponds and the codewords a length of at least 8th Bit, in particular 15 bits. Such a code is for example off WO 86/00478 A1 known, and it is a way to clearly identify a plurality of optically detectable absolute positions of the piston rod with simple stroke marks. With a code word length of 15 bits contained in a code pattern corresponding to a 1000-digit binary number, which corresponds to a length of one meter when the width of the bar marks of 1 mm is one meter, using a codeword length of 15 bits more than 1000 different codewords, giving a measurement resolution of 1mm.

For the practice of fluid cylinder applications, it is advantageous if the measurement section on the piston rod surface covers a length of at least 15 bits of the code pattern. With the length of the codewords used, the number of possible different codewords also increases, as a result of which even very long measuring lengths can be assigned to unique absolute positions of the piston rod.

An alternative embodiment of a position measuring device which also employs a code system advantageous for the practical cases is when the code pattern is constituted by a binary block code having an alternating sequence of constant number and width information bits and guard bits of constant number and width, and the Piston Rod Position Measurement uses a measurement technique that detects the position or shift of the guard bits in the span and the information bits each define unique absolute references for the piston rod position.

A sufficient and cost-effective embodiment for position measurements is when the image sensor is formed by a parallel to the piston rod axis line camera element or linear sensor array. However, the image sensor can of course also be formed by a two-dimensional CCD element, wherein a one-dimensional sensor element is also sufficient for the one-dimensional piston rod position measurement.

The measurement accuracy requirements that occur in practice can easily be met if the line scan camera element has between 32 and 2048 sensor points, in particular 128 sensor points or pixels. Such image sensors are available in many embodiments and inexpensively. At the same time, such an image sensor has very compact dimensions, which allows the compactness of the entire measuring device and the universal applicability.

Sufficient measurement accuracy of the position measuring device for the applications occurring in practice is given when the line scan camera element resolution between 200 and 1200 dpi, in particular 400 dpi. Again, a variety of embodiments is available at low cost on the market.

A compact embodiment of the sensor arrangement is possible by selecting the distance of the circuit board from the piston rod surface in a range between 15 mm and 25 mm. In conjunction with the light-guiding element and the measuring optics of the position-measuring device according to the invention, such can be provided for many different sizes of fluid cylinders without requiring extensive structural changes to them.

For the reliability of the measuring method, it may be advantageous if the main axis of the measuring optics is oriented eccentrically or skewed with respect to the piston rod axis. The image sensor is thereby relatively independent of the reflectivity of the piston rod surface and thus insensitive to occurring in the operation of such a fluid cylinder changes the piston rod surface due to wear or other environmental influences. Alternatively or in addition, the main axis of the measuring optics may alternatively also have an oblique position relative to the main propagation direction of the light beams of the illumination source, which are reflected by the piston rod surface, that is to say directed approximately eccentrically to the illumination maximum on the code pattern. As a result, reflections affecting the measurement on the piston rod surface can also be prevented.

A very compact size of the sensor arrangement is given if the center distance of the illumination source and the image sensor on the circuit board is selected from a range between 5 mm and 12 mm. By using a light-guiding element, the illumination source can be approached very close to the light sensor without the measuring section being shaded by the measuring optics.

Another measure for miniaturizing the sensor arrangement is to arrange the evaluation unit on the circuit board. In the previously described embodiments of the image sensor and of the code pattern, the computing power required for the evaluation can also be achieved with evaluation units and microprocessors of very small size whereby a first image data evaluation can already be integrated in the sensor arrangement.

If the measurement results are not output directly to the sensor arrangement but the position measurement data are further processed by an external evaluation unit, for example a control and regulating device of the device comprising the fluid cylinder, it is advantageous if an interface for the power supply and data transmission is arranged on the circuit board is.

The compactness of the sensor arrangement can also be increased by connecting a second printed circuit board parallel or at an angle to the printed circuit board, to which an evaluation unit and / or a power supply component and / or a data interface is arranged. The components of the sensor arrangement are divided by this embodiment into several circuit boards, whereby the individual circuit boards require a smaller area and the housing of the sensor arrangement can be kept relatively narrow and it does not protrude over the outer diameter of the fluid cylinder.

A structurally advantageous embodiment of the measuring device is that the housing is made in two parts and the circuit board is attached to an outer removable housing portion or housing cover. The sensor arrangement is thereby easily accessible, since the sensor arrangement is separated from the fluid cylinder when the outer housing section is removed and so an exchange of the sensor arrangement is substantially facilitated.

Furthermore, it is possible for the printed circuit board to be adjustably fastened in its position within the removable housing section from the outside, in particular by means of adjusting screws. On the one hand, this can be offset by adjusting the circuit board of the measuring section on the piston rod in the circumferential direction, for example, if faulty position measurements occur due to local damage to the code pattern. Furthermore, it is possible to attach the outer removable housing portion in a constant size with different housing dimensions, which may be required for different piston rod diameters, structurally unchanged. Due to the adjustability of the PCB inside the removable housing portion can be made in this case an optimal adaptation to the respective piston rod diameter.

One possibility of reducing the production costs of such a position measuring device is further that the measuring optics and / or the light-guiding element substantially comprise optical elements made of PMMA (plexiglass) or polycarbonate. Since the requirements of the optical quality are also met by such components, thereby expensive optical components made of glass can be avoided.

So that the sensor arrangement or the housing does not protrude with respect to its outer diameter, even with small sizes of fluid cylinders, it is advantageous if the printed circuit board has a rectangular basic shape with a maximum edge length of 40 mm. Thereby, the housing or the removable housing portion can be kept accordingly small.

To permanently protect the sensor assembly against harmful influences and to ensure a long service life with high measurement reliability, it is advantageous if the piston rod completely comprehensive sealing rings or stripping elements are arranged on both sides of the measuring section.

Due to the cost-effective design of the position measuring device according to the invention and its compact dimensions, it is further possible to equip a fluid cylinder with at least two such position measuring devices, whereby a redundant position measurement is easily possible, which is often prescribed in safety-related systems. The individual sensor arrangements can each be assigned a separate code pattern on the piston rod, but it is also possible for both sensor arrangements to access the same code pattern, in particular with staggered measurement sections.

In fluid cylinders with double-sided piston rod, such as. in steering cylinders, the position measuring devices can be arranged at the two opposite ends.

For a better understanding of the invention, this will be explained in more detail with reference to the following figures.

Fig. 1

eine Ansicht eines Fluidzylinders mit einem möglichen Ausführungsbeispiel einer Positionsmessvorrichtung;

Fig. 2

einen Ansicht einer in einem abnehmbaren Gehäuseabschnitt angeordneten Sensoranordnung;

Fig. 3

einen Querschnitt durch eine mögliche Ausführungsform einer Positionsmessvorrichtung;

Fig. 4

einen Radialschnitt durch eine mögliche Ausführungsform einer Positionsmessvorrichtung im Bereich des Lichtleitelements;

Fig. 5

einen Radialschnitt durch eine mögliche Ausführungsform einer Positionsmessvorrichtung im Bereich der Messoptik.

Each shows in a highly schematically simplified representation:

Fig. 1

a view of a fluid cylinder with a possible embodiment of a position measuring device;

Fig. 2

a view of a arranged in a removable housing portion sensor assembly;

Fig. 3

a cross-section through a possible embodiment of a position measuring device;

Fig. 4

a radial section through a possible embodiment of a position measuring device in the region of the light guide;

Fig. 5

a radial section through a possible embodiment of a position measuring device in the region of the measuring optics.

By way of introduction, it should be noted that in the differently described embodiments, the same parts are provided with the same reference numerals or the same component names, wherein the disclosures contained in the entire description can be mutatis mutandis to the same parts with the same reference numerals or component names. Also, the location information chosen in the description, such as top, bottom, side, etc. related to the immediately described and illustrated figure and are to be transferred to the new situation mutatis mutandis when a change in position. Furthermore, individual features or combinations of features from the different exemplary embodiments shown and described can also represent independent, inventive or inventive solutions.

All statements on ranges of values in objective description are to be understood as including any and all sub-ranges thereof, eg the indication 1 to 10 should be understood to include all sub-ranges, starting from the lower limit 1 and the upper limit 10 ie all subareas begin with a lower one Limit of 1 or greater and end at an upper limit of 10 or less, eg 1 to 1.7, or 3.2 to 8.1 or 5.5 to 10.

Fig. 1 shows a view of a fluid cylinder 1 with a position measuring device according to the invention 2. The fluid cylinder 1 comprises substantially a cylinder tube 3 by a non-illustrated piston is guided which is displaced by supply or removal of fluid in the cylinder tube 3. The cylinder tube 3 is closed at one end with a cylinder bottom 4 and closed at the opposite end with a cylinder head 5 through which a piston rod 6 connected to the piston is led out. The fluid cylinder 1 can be designed as a hydraulic cylinder or pneumatic cylinder, the embodiment of the position measuring device 2 according to the invention is not relevant. Furthermore, in the case of a single acting plunger cylinder, the piston rod may be formed by the piston itself, and the invention is not limited to the fluid cylinder 1 having a separate piston rod 6 but is applicable to plunger cylinders.

For many applications, it is advantageous if the position of the piston rod 6, with which moving members are moved or positioned in device, can be detected and thereby precisely approach such a fluid cylinder 1 by means of control and regulating devices preprogrammed positions or can comply preprogrammed motion sequences exactly.

The position measuring device 2 is based on the fact that a measuring graduation in the form of an optically detectable code pattern 8 is arranged on the outer circumference 7 of the piston rod 6, and a sensor arrangement 9 is fixedly arranged on the fluid cylinder 1 in the region of the cylinder head 5, with which a partial section of the code pattern 8 is optically detected and evaluated. As a result of the displacement of the piston rod 6 along the piston rod axis 10 during a movement of the piston, the code pattern 8 is moved past the sensor arrangement 9 and the position of the piston rod 6 is determined by means of an evaluation unit from the part section of the code pattern 8 optically detected therefrom.

The code pattern 8 is composed for example by a binary sequence of transverse to the piston rod axis 6 oriented bar marks 11, which are mounted on the outer circumference 7 of the piston rod 6. The code pattern 8 or the bar marks 11 can be applied for example by galvanic or metallurgical processes. A possible production method for the code pattern is that heat energy is introduced selectively on a chromium-plated coating of the piston rod surface 6 by means of concentrated laser radiation, whereby the boundary layer assumes tempering colors that differ in color from the thermally untreated sections. Such a code pattern applied in this way can additionally be provided with an optically transparent protective coating, for example an SiO 2 coating, as a result of which the code pattern 8 has increased wear resistance.

The optically detected by the sensor assembly 9 portion of the code pattern 8 corresponds to a codeword, which is associated with a unique piston rod position. The sensor arrangement 9 detects a specific measurement section of the code pattern 8, which extends in the direction of the piston rod axis 10 and is assigned an unambiguous piston rod position by an evaluation unit an optically detected code word. If, for example, the measuring section in the piston rod direction 10 has a length which corresponds to eight times the line width of a bar marking 11, then the respective position of the piston rod 6 can be detected by a code word with 8 bits.

An advantageous embodiment of the code pattern 8 for practical applications is that the width of the bar marks 11 and the unmarked code sections corresponds in each case to one millimeter or to an integral multiple of one millimeter. The narrowest bar mark has in this case a width of 1mm and the narrowest unmarked code section has a width of 1mm. Code patterns 8 with this width of the bar marks 11 are not very sensitive to punctate damage of the code pattern 8, e.g. due to wear, since the resolution here does not have to be so fine that even punctual damage would cause measuring errors. In this case, a certain minimum width of the captured measurement section is helpful in that not only a narrow line on the code pattern 8 is scanned optically, but an area with a width of two or more millimeters is imaged on the image sensor.

A possible embodiment of the code pattern 8 and of the evaluation method can advantageously consist in that the code pattern 8 is formed by a binary pseudo random code with interlaced, mutually different code words, wherein the length of the measurement section corresponds at least to the length of the code words and the code words have a length of at least 8 bits, in particular 15 bits.

An alternative embodiment of the code pattern 8 may also be that the code pattern 8 is constituted by a binary block code having an alternating sequence of constant number and width information bits and constant number and width guard bits, e.g. a marked or unmarked section 4 mm in length, is formed and the measurement of the piston rod position uses a measuring method in which the position or shift of the protection bits is detected in the measurement section and the information bits each define absolute references for the piston rod position. The optically evaluated measurement section is chosen so large that it contains at least one complete coherent group of information bits.

The sensor arrangement 9 is arranged in a housing 12, which is arranged stationarily on the fluid cylinder 1 and contains in its interior sufficient clearance for the sensor arrangement 9 as well as for the light guidance to or from the piston rod surface 7. The housing 12 can, as in FIG Fig. 1 be arranged outside the fluid cylinder 1 on the cylinder head 5 or be integrated in the cylinder head 5 or be formed by this and protects the working on optical base sensor assembly 9 from stray light and harmful environmental influences such as mechanical loads or dirt. To protect against contamination is provided in particular that the provided on the piston rod 6 Meßausschnitt is protected on both sides by the piston rod 6 comprehensive sealing rings or stripping.

In the illustrated embodiment, the housing 12 is made in two parts and includes a removable outer housing portion 13 or housing cover, which is removable from the rest of the housing 12, whereby the sensor assembly 9 is more accessible. Furthermore, 12 bushings for the purpose of data exchange or power supply can be provided on the housing. This can be accomplished in particular by a connection socket 14.

Based on FIGS. 2 to 5 is described in more detail the position measuring device 2 with the sensor assembly 9 and its operation.

Fig. 2 shows a view of the removed from the rest of the housing 12 housing portion 13, wherein in the illustrated embodiment, the sensor assembly 9 is fixed in this, whereby it is particularly easily accessible. The attachment of the sensor assembly 9 in a removable housing portion 13 makes it possible to mount this structurally unchanged on housing bases of different sizes, whereby only these must be adapted to the respective fluid cylinder 1.

The sensor arrangement 9 essentially comprises as main components an illumination source 15 for illuminating the measurement section to be detected on the code pattern 8, as well as an image sensor 16 for optical detection of the measurement section. Illumination source 15 and image sensor 16 are arranged on a common printed circuit board 17, which is fastened in the removable housing section 13 in the illustrated embodiment. An evaluation unit 18 in the form of a microprocessor 19 is also arranged on this printed circuit board 17, which determines the absolute position of the piston rod 6 from the image data captured by the image sensor 16 or processes the image data in such a way that it can be evaluated by an external further evaluation unit.

The image sensor 16 is formed, for example, by a line camera element 20 which is arranged parallel to the piston rod axis 10 and has 128 sensor points or pixels. The optical image of the measurement section on the code pattern 8 on the image sensor 16 by means of a measuring optics 21, which images the illuminated by the illumination source 15 Meßausschnitt optically to the image sensor 16. For example, the measurement section has an extension of 16 mm in the direction of the piston rod axis 10 and a width of 3 mm transversely thereto, while the line scan camera element 20 has an effective sensor length of approximately 8 mm and an effective width of 0.1 mm.

For this purpose, the measuring optics 21 contain optical lens elements, for example made of Plexiglas (PMMA) or polycarbonate, and comprise in particular a cylindrical lens adjoining the line camera element 20 and two piston rod-side lens elements with non-rotationally symmetrical surfaces. The measuring optics thus has an anamorphic property and is optimally suitable for the imaging of a rectangular measurement section on a line-shaped line camera element 20.

In the position measuring device 2 according to the invention, the luminous flux emitted by the illumination source 15 is transmitted to the measuring section on the outer circumference 7 of the piston rod 6 by means of a light-guiding element 22 arranged between the illumination source 15 and the measuring section, whereby the code pattern 8 is optimally illuminated for the purpose of optical detection becomes. In this case, the light-guiding element 22 has a light inlet side facing the illumination source 15 and a light outlet side facing the code pattern 8, and the incoming luminous flux is guided in the interior of the light-guiding element 22, whereby only a very small portion of the light quantity does not reach the light exit surface due to total reflection at the lateral boundary surfaces ,

The light-guiding element 22 preferably directly adjoins the illumination source 15 or the light-entry surface of the light-guiding element 22 is located at least a distance of less than 2 mm from the illumination source, whereby the majority of the emitted light quantity enters the light-guiding element 22. Furthermore, the light-guiding element preferably extends over at least 50% of the smallest distance between the illumination source 15 and the outer circumference 7 of the piston rod 6, whereby the light exit surface is relatively close to the code pattern 8 and this is optimally illuminated.

The illumination source 15 is preferably formed by an LED element 23, which is characterized by a long service life and a very high light output based on its energy consumption. Advantageous for the position measurement, the use of a surface-emitting thin-film LED has proven, with the wavelength ranges white, red and infrared provide good results. Furthermore, such LED elements 23 can be used in very small dimensions. In order to keep the dimensions of the illumination source 15 small, it may be provided, in particular, that it has a square radiating surface 24 which, for example, has an edge length of less than 2 mm. By means of the very effective light-guiding element 22, it is also possible to use such an approximately point-shaped illumination source 15 for the uniform illumination of a rectangular measuring section. The light-guiding element 22 makes it possible, for example, to uniformly illuminate a measuring section of 16 mm length and 3 mm width with an edge length of the radiating surface 24 of, for example, 1x1 mm.

In order to further improve the uniform illumination of the measurement section, the light-guiding element 22 may have a frosted light entry surface and / or a frosted light exit surface, whereby the intensity differences within the luminous flux are reduced.

As Fig. 2 shows further, the light guide 22 may have an approximately rectangular cross-section, wherein a longer side of the rectangular cross-section is parallel to the piston rod axis 10 and thereby the light exit surface has approximately the shape of the Meßausschnittes to be illuminated. In the case of an approximately point-shaped illumination source 15, it is advantageous if the cross-section of the light-guiding element 22 increases from the light entry surface to the light exit surface, whereby the light flux can also increase in its cross section. In the example shown, the light exit surface of the light-guiding element 22 corresponds at least to three times the light entry surface.

In order to be able to optimally adapt the measuring optics 21 and the light-guiding element 22 to one another and thereby also be able to use the sensor arrangement 9 for different piston rod dimensions, it is possible for the light-guiding element 22 and / or the measuring optics 21 to be pivotable with respect to a pivot axis 25 parallel to the piston rod axis 10 Housing 12 are stored. As a result, the area illuminated by the illumination source 15 on the piston rod surface can be varied and / or the area covered by the measuring optics 21 can be optimally adjusted. As a result, differences in the distance between the circuit board 17 and piston rod surface 7 can be compensated for different piston rod diameters.

In this case, it is possible for the light-guiding element 22 to be pivotably mounted on the measuring optics 21 or, conversely, for the measuring optics 21 to be mounted pivotably on the light-guiding element 22. These are thus directly coupled to each other and possible axle deviations are minimized by assembly errors.

The measuring section on the piston rod surface 7 detected by the image sensor 16 preferably covers a length of at least 15 bits of the code pattern 8. By acquiring code words with a length of 15 bits, a matching pseudo-random code can be used easily more than 1,000 different absolute positions of the piston rod 6 are clearly identified.

For the optical measurement of the code pattern 8, the line scan camera element 20 has a resolution between 200 and 1200 DPI, for example 400 DPI, which gives a slightly sufficient resolution for the detection of bar marks 11 with a width of 1 mm.

Fig. 3 shows a section transverse to the cylinder axis by the position measuring device 2, Fig. 4 a radial section through a position measuring device 2 in the region of the light guide 22 and Fig. 5 a radial section through the position measuring device 2 in the region of the measuring optics.

In this case, the respective beam paths are indicated, wherein in each case only individual beams are shown.

This in Fig. 4 exemplified light guide 22 is arranged so that its light entrance surface 26 is disposed at a distance from the radiating surface 27, which is less than 2 mm, the light entrance surface 26 could even connect directly to the illumination source 15 and it is ensured by this arrangement that the largest proportion of the emitted light enters the light guide 22. As a result, the light entering at the light entry surface 26 is passed on to the light exit surface 28 in which it directly penetrates the material of the light guide element 22 or is deflected on its side walls by total reflection in the direction of the measurement section 29.

The light entry surface 26 is executed in the illustrated embodiment as a substantially flat surface, which is particularly easy to manufacture and can still meet the requirements for the light guide to the measurement section 29. Notwithstanding the concave design shown in solid lines and the light exit surface 28 may be designed as a substantially flat surface, as in Fig. 4 is indicated and whereby the production of the light-guiding element 22 is further simplified.

The illumination source 15 in the form of the LED element 23 is fastened to the printed circuit board 17, which is fastened in the removable outer housing section 13 in the illustrated embodiment. As Fig. 4 shows, it is possible to the circuit board 17 parallel or at an angle to provide a second circuit board 30 on which also components of the sensor assembly 9 can be arranged. This builds for something higher, but can be performed with a smaller width and length in the direction of the cylinder axis 10. Thus, for example, the second printed circuit board 30 may have interfaces 31 for the data transmission and / or power supply and may be led to the connection socket 14 via an interface cable.

As Fig. 4 shows, the light guide 22 extends over most of the distance 32 between the illumination source 15 and the outer circumference 7 of the piston rod 6, but advantageously over at least 50% of the distance 32. This ensures that the majority of the output from the illumination source 15 luminous flux to the measurement section 29th is directed. Further shows Fig. 4 Two other ways to achieve the most uniform illumination of the measurement section 29. The first possibility is to use a concave light exit surface 28, which, however, may also be designed to be locally planar or also partially convex. The illustrated concave design causes a further dispersion of the luminous flux and is thereby achieved a uniform illuminance. As an alternative or additional measure to achieve a uniform illumination, the light-guiding element 22 has at its center a local disturbance 33, for example in the form of a bore 34 transversely to the direction of light propagation, whereby a maximum illumination in the middle of the measuring section 29 is lowered and thus the illumination intensity is also more uniform becomes.

As an alternative to a bore 34, it is also possible to attach a local cover panel or a more concave or convex section in the center region of the light exit surface 28.

Experiments have shown that it is advantageous for the manufacturing costs and the compactness of a position measuring device 2 according to the invention if the printed circuit board 17 has a distance from the piston rod surface 7 from a range between 15 and 25 mm. Furthermore, the circuit board 17 can be dimensioned so that it is a rectangular Basic shape with a maximum edge length of 40 mm, whereby even after installation in a housing 12 protrudes little compared to the outer circumference of common fluid cylinder 1.

In order to allow a further adaptation to different dimensions of a fluid cylinder 1, even if in each case the same upper housing portion 13 is used, it may be provided that the circuit board 17 is fixed in its position within the removable housing portion 13 from the outside, for example by means of adjustable screws , Thus, the sensor assembly 9 can be optimally adapted to different mounting positions and piston rod diameter in a simple manner.

Fig. 3 shows a section through a position measuring device 2 according to line III - III in Fig. 4 ,

In this embodiment, the main axis of the measuring optics 21 is oriented in the direction of the piston rod axis 10, but it is also possible to provide these eccentrically or skewed to the piston rod axis 10, whereby the measuring optics 21 is directed to a certain extent not perpendicular to the outer periphery 7 of the piston rod 6. As can further be seen, the illumination source 15 is arranged together with the adjoining light-guiding element 22 at a relatively small center distance 35 to the image sensor 16, here in the form of a line-scan camera element 20. The center distance 35 is for example from a range between 5 mm and 12 mm, whereby the sensor assembly 9 can be made very compact and the position measuring device 2 has little restrictions in its possible application due to their small dimensions.

In Fig. 3 is further indicated by dashed lines, that the light guide 22, as already based on Fig. 2 described about a pivot axis 25 pivotally mounted in the housing 12, for example in a position shown by dashed lines 22 ', whereby the main propagation direction of the luminous flux can be adjusted and the illumination of the Meßausschnittes 29 can be further optimized in a simple manner or in dependence on the relative position of Illumination source 15, image sensor 16 and piston rod surface 7 can be adjusted.

The optical detection is performed by the image sensor 16 at a high frequency e.g. with more than 1000 images per second, which gives a quasi-continuous position detection.

The dimensions and technical specifications mentioned with reference to the exemplary embodiment are optimally suited for piston rod diameters between 20 mm and 80 mm, but can be suitably adapted for deviating dimensions of piston rods 6. The described embodiment of the sensor arrangement 9 has sufficient tolerances with regard to component tolerances or installation tolerances, which is why a very wide range of applications is given here. For the working temperatures of -40 ° C and +100 ° C, the components used are suitable and also temperature effects on the measuring accuracy negligible, for higher temperatures corresponding substitute materials should be used, such as plastics with higher softening points or replacement by metallic, ceramic or crystalline materials.

In the exemplary embodiment illustrated, the light-guiding element 22 has a light entrance surface 26 of 6 mm × 4 mm and a light exit surface 28 of 24 mm × 4 mm, whereby on the one hand the radiating surface 24 of approximately 1 mm × 1 mm at the illumination source 15 and the measuring section 29 at the Piston rod surface 7 are sufficiently covered and only a small proportion of the luminous flux is lost.

Fig. 5 shows in radial section still the optical image of the measuring section 29 on the image sensor 16 in the form of the line camera element 20, wherein the bundling takes place in a direction immediately in front of the line camera element 20 by a cylindrical lens.

The embodiments show possible embodiments of the position measuring device 2, wherein it should be noted at this point that the invention is not limited to the specifically illustrated embodiments thereof, but also various combinations of the individual embodiments are mutually possible and this possibility of variation due to the teaching of technical action representational invention in the skill of those skilled in this technical field. So there are also all conceivable variants, by combinations of individual details the illustrated and described embodiment variant are possible, encompassed by the scope.

For the sake of order, it should finally be pointed out that for a better understanding of the construction of the position measuring device 2, these or their components have been shown partially unevenly and / or enlarged and / or reduced in size.

The task underlying the independent inventive solutions can be taken from the description.

Above all, the individual in the Fig. 1; 2 ; 3; 4 and 5 shown embodiments form the subject of independent solutions according to the invention. The relevant objects and solutions according to the invention can be found in the detailed descriptions of these figures.

REFERENCE NUMBERS

1

fluid cylinder

2

Position measuring device

3

cylinder tube

4

cylinder base

5

cylinder head

6

piston rod

7

outer periphery

8th

code pattern

9

sensor arrangement

10

Rod axis

11

line marking

12

casing

13

housing section

14

socket

15

lighting source

16

image sensor

17

circuit board

18

evaluation

19

microprocessor

20

Line camera element

21

measurement optics

22

light guide

23

LED element

24

radiating surface

25

swivel axis

26

Light entry surface

27

28

Light-emitting surface

29

measurement cutout

30

circuit board

31

interface

32

distance

33

disorder

34

drilling

35

center distance

Claims (15)

1. Position-measuring device (2) for a piston rod (6) leading out of a fluid cylinder (1), comprising a material measure in the form of an optically detectable code pattern (8) arranged at the outer circumference (7) of and along the piston rod (6), in particular a binary sequence of stroke markings (11) oriented transversely to the piston rod axis (10) and a sensor arrangement (9) for optically detecting a rectangular measurement section (29) of the code pattern (8) arranged in a housing (12) in a fixed manner on the fluid cylinder (1), comprising an illumination source (15) emitting a luminous flux onto the code pattern (8), an image sensor (16), measuring optics (21) for transferring an image of the measurement section (29) onto the image sensor (16) and an evaluating unit (18) for determining the absolute position of the piston rod (6) using the image information captured by the image sensor (16), wherein the illumination source (15) and the image sensor (16) are fastened to a common printed circuit board (17) of the sensor arrangement (9) and a light guiding element (22) for transferring the luminous flux from the illumination source (15) onto the measurement section (29) is arranged between the illumination source (15) and the measurement section (29), characterized in that the light guiding element (22) has a cross-section which increases from the light inlet surface (26) to the light outlet surface (28) thereof.
2. The position-measuring device (2) according to claim 1, characterized in that the light guiding element (22) directly adjoins the illumination source (15) or has a light inlet surface (26) which is positioned at a distance of less than 2 mm from the illumination source (15).
3. The position-measuring device (2) according to claim 1 or 2, characterized in that the light guiding element (22) extends over at least 50%, preferably over at least 75%, of the smallest distance (32) between illumination source (15) and outer circumference (7) of the piston rod (6).
4. The position-measuring device (2) according to any one of claims 1 to 3, characterized in that the illumination source (15) is formed by an LED element (23).
5. The position-measuring device (2) according to any one of claims 1 to 4, characterized in that the light-guiding element (22) has an approximately rectangular cross-section, wherein a longer side of the rectangular cross-section runs parallel to the piston rod axis (10).
6. The position-measuring device (2) according to any one of claims 1 to 5, characterized in that the light outlet surface (28) of the light guiding element (22) corresponds to at least three times the light inlet surface (26).
7. The position-measuring device (2) according to any one of claims 1 to 6, characterized in that the light-guiding element (22) at the centre of its beam path has a local perturbation (33) in the form of a transverse bore (34), a cover plate or an optically refracting element.
8. The position-measuring device (2) according to any one of claims 1 to 7, characterized in that the light-guiding element (22) and/or the measuring optics (21) are mounted pivotably in the housing (12) relative to a pivot axis (25) parallel to the piston rod axis (10).
9. The position-measuring device (2) according to any one of claims 1 to 8, characterized in that the code pattern (8) is formed by a binary pseudo-random code with interlaced code words which differ from one another, wherein the length of the measurement section (29) at least corresponds to the length of the code words and the code words have a length of at least 8 bits, in particular 15 bits.
10. The position-measuring device (2) according to any one of claims 1 to 8, characterized in that the code pattern (8) is formed by a binary block code with an alternating sequence of information bits of constant number and width as well as protective bits of constant number and width and the measurement of the piston rod position uses a measurement method in which the position or displacement of the protective bits in the measurement section (29) is recorded and the information bits each define absolute references for the piston rod position.
11. The position-measuring device (2) according to any one of claims 1 to 10, characterized in that the image sensor (16) is formed by a line scan camera element (20) arranged parallel to the piston rod axis (10) or a linear sensor array.
12. The position-measuring device (2) according to any one of claims 1 to 11, characterized in that the principal axis of the measuring optics (21) is oriented eccentrically or skew relative to the piston rod axis (10) and/or is oriented eccentrically or skew relative to the principal direction of propagation of the light beams reflected from the piston rod surface (7).
13. The position-measuring device (2) according to any one of claims 1 to 12, characterized in that the centre-to-centre distance (35) from the illumination source (15) to the image sensor (16) on the printed circuit board (17) is selected from a range between 5 mm and 12 mm.
14. The position-measuring device (2) according to any one of claims 1 to 13, characterized in that the evaluation unit (18) is disposed on the printed circuit board (17).
15. Fluid cylinder (1) having at least one piston rod (6) guided out therefrom, characterized in that at least two position-measuring devices (2) according to any one of claims 1 to 14 are provided for measurement of the piston rod position.

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