Cylinder with a piston rod and an optical position sensor

Abstract

Cylinder (1) with a piston rod (3) and an optical position sensor which is directed towards a marking (5) arranged on the piston rod (3), which marking has at least two code elements which can be distinguished by the position sensor and from which two code symbols ("0", "1"), each consisting of a plurality of code elements, and a separator symbol ("T") are formed, wherein a plurality of code symbols ("0", "1") form a code word, characterized in that the marking (5) has a plurality of code word pairs, wherein each code word pair has the same predetermined total length.

Description

The present invention relates to a cylinder having a piston rod and an optical position measuring device which is directed at a marking arranged on the piston rod.For a position measurement on the piston rod of a cylinder, it is known to incrementally detect the position of the piston rod, starting from a reference mark. Such a marking is used by first detecting the reference mark, whereby a starting position is known and, starting from the known starting position, the current position of the piston rod is determined incrementally via the further marking. Such a marking has the disadvantage that depending on the position of the piston rod at the start of the measurement, a long reference travel is sometimes required in order to record the reference mark as the starting point for the incremental measurement.Absolutely measuring systems are also known in which the marking on the piston rod carries a code with which the absolute position is indicated. It has proven disadvantageous in the coding of the absolute position that the coding of the position and the construction of the sensor necessary for this are complicated.WO 95 / 01510 discloses a hydraulic, pneumatic or similar cylinder having a piston rod provided with a marking, to which an optical sensor is directed. The marking provided on the piston rod allows both incremental and absolute coding of the position. To determine the absolute position, three reading heads are provided, two of which are arranged one behind the other in the direction of the marking and thus permit an evaluation of the speed. The third sensor measures independently of the first two sensors and allows an absolute position on the piston rod to be read.EP 1 426 737 B1 discloses a device for determining the position of a movable mechanical element as well as a method for producing such a coding. In the known coding, code elements are provided which form a distinguishable part of an optically readable marking. The encoding consisting of several code elements forms a numerical value in binary digits (bits). The thus encoded numerical value indicates the position on the piston rod. A code element has a portion having a predetermined width T on the surface of the piston rod. Two portions are provided for coding individual bits, a distance of the width of 1 T or 2 T indicating binary number 0 or 1, respectively. Further, end marks of length 4T are provided separating a pair of adjacent areas. For encoding, it is provided to use pseudo-random binary sequences. The position is determined by means of the random coding with the aid of a stored field.The invention is based on the object of providing a marking for a cylinder with a piston rod, which marking contains reliable and easily readable position information.According to the invention, the object is achieved by a cylinder having the features of claim 1. Advantageous embodiments form the subject matter of the dependent claims.The cylinder according to the invention has a piston rod and an optical position measuring device which is directed at a marking arranged on the piston rod. If the piston rod of the cylinder is moved, the marking moves relative to the position meter. The marking has at least two code elements that can be distinguished for the optical position sensor. The code elements form the basic components for reliable optical position detection. Two code characters are formed from the code elements. Furthermore, a separator may be formed from a plurality of code elements. Two or more different code elements may be provided to form the code characters. For hardware reasons, the two code symbols and the separator preferably each have a different physical length. In the logically following level of coding, several code characters form one code word. The invention is now particularly characterized in that the marking extending over the piston rod has a plurality of code word pairs which have a predetermined total length in pairs. The particular advantage of code word pairs of the same length is that when reading out the position from the marking, the read signals can be checked by code word pairs of the same length. If an error occurs during the reading, it is known when the read operation to form a code word pair would have to be completed and when a new read operation can be initiated.The codewords in a codeword pair are adjacent to each other.In a preferred development, the two code words in a code word pair together have two delimiters. It has been found to be practical if each code word is followed or introduced by a separator. In this assignment, it is then self-evident that two delimiters are contained in a code word pair.In a predetermined embodiment, each codeword has a predetermined number of code characters forming the codeword. The different length of the code words occurs despite the defined number of code characters if the code elements or the code characters formed therefrom have a different length.In a particularly preferred embodiment, two code words separated from one another by separation symbols, each having n code symbols, have a spacing (in the sense of a Hamming spacing) of n or (n-1) code symbols. Distance here means the number of code characters which have to be converted into a code word in order to arrive from a first code word to a second code word. Such a large distance between two words of a code word pair ensures that, if it is known that a code word pair is present, reading errors can be quickly detected and possibly also corrected.In a preferred development, the two code characters have different lengths.A preferred embodiment will be described in more detail below. The following are shown: FIG. 1 shows a hydraulic cylinder with a piston rod, to the marking of which an optical position sensor is directed, FIG. 2 shows a detail of the piston rod, FIG. 3 shows an exemplary enlarged code, and FIG. 4 shows an exemplary coding with three code elements and one separator after each code word pair.The invention is explained in more detail below with reference to an example. The particular aspect of the invention lies in the use of code word pairs, so that for an explanation of the invention, reference can be made in part to figures from the prior art, for example WO 95 / 01510. FIG. 1 shows a schematic view of a cylinder, wherein it can be a hydraulic or pneumatic cylinder, for example. The cylinder 1 has a cylinder sleeve 2 and a piston rod 3, and the piston rod 3 passes through a cover 6 disposed on a cylinder head 4. The piston rod 3 is sealed from the interior of the cylinder. A measuring device (not shown in detail) is integrated into the cover and optically detects a marking 5 on the piston rod 3.FIG. 2 shows the piston rod 3 with its marking in the detail, wherein FIG. 3 allows an enlarged view of the marking 5. The enlarged representation of FIG. 3 shows a series of code elements which consist of individual lines or lines applied to the piston rod. Each of the applied lines has a different reflection property for incident light. For example, it can be provided that no light is reflected in the darkly illustrated strips of the marking, while the light is reflected in the regions lying therebetween. This may also be the case elsewhere. It is important for the evaluation of the marking 5 that the optical sensor reads a different signal in position X than in position Y.The coding provided according to the invention has a logic structure which facilitates the reading out of the signals. At the logic lower level there are code elements which are represented by the lines shown in FIG. 3 on the piston rod, for example by the absence on the dashed line. Depending on the coding selected, a plurality of code elements can collectively represent a code character. This can be, for example, a binary "0" or a binary "1.". Furthermore, a separator can be defined from a plurality of code elements, which separator separates the code characters of one code word from the code characters of another code word. A pair of code words can be combined to form a code word pair if the added length of the two code words yields a constant predetermined value.Figure 4 shows a particular embodiment which uses four code elements. As shown, the code character "0" consists of the sequence "black-white-black-white-black". These code elements "black" and "white" have the same width. This is followed by a code element "white", which has a smaller width, and a code element "black", which has a larger width. A possible representation for the code character "0" thus points to four code elements of different widths. Alternatively, it would be possible to rely on two elementary code elements each having the width D / 2. In this illustration, the code character "0" would have the illustration "E black, E black, E white, E white, E black, E black, E white, E white, E black, E black, E black, E white, E black, E black, E black and E black", where E white and E black represent a half width code element. For practical use, it may be clearer to distinguish here four code elements, of which a white narrower and a black wider is implemented. The code character "1" differs from the code character "0" in that a black and white transition is introduced once again. The separator is represented by a further black-and-white transition.In the coding shown in FIG. 4, 2-bit coding is now present. Here, coding is performedThe code words "00" and "11" are separated from one another here by a separator character "T". The code words "00" and "11" form a code word pair having a predetermined total length. Similarly, the code word "01" and "10" yields a code word pair having the same total length.For a better understanding of the invention, we shall first consider a marker 5 which does not know any code word pairs. Assuming, for example, a coding in which a code word consists of three bits in length, the following marking can be applied to the piston rod: the separator between two code words being represented here by "T". If, in this marking, the optical sensor reads, for example, the marking 100T, it was known that the sensor is just between the fifth and sixth markings, so that an evaluation results in the position of the piston rod. In this example, all codewords appear to be of the same length since they are always 3 bits. However, it is quite conceivable for practical considerations that the code character "0" is coded, for example, as a bar-gap-bar, while the code character "1" has a representation in code elements with a different length. Assuming such a situation, the code characters "0" and "1" do not have the same length as the code words "100" and "011" have the same length.According to the invention, it is therefore provided that a plurality of code word pairs having the same length are provided in the marking 5. An example is, for example:Looking at the above marker 5, it is found that the successive codewords "000T" and "111T" are of the same length as the following pairs:The four code word pairs P 1- P 4 each have the same length, since they always have the same number of code characters "0" and "1". A further advantage of the above code word pairs is that within a code word pair the Hamming distance between two code words is "3" with the delimiter "T" left out. A further property is that the last code word of a code word pair and the first code word of a subsequent code word pair have a distance of "2". Depending on the required length and accuracy of the division of the piston rod, a different number of code characters can be selected for the representation of a code word.Since the marking 5 does not take place sequentially when using the code word pairs, a table is provided for evaluating the position of the piston rod, which table assigns a position to an individual code word.List of reference characters1 Cylinder 2 Cylinder sleeve 3 Piston rod 4 Cylinder head 5 Marking 6 Cover 7 Cable

Claims

Cylinder (1) having a piston rod (3) and an optical position meter which is directed at a marking (5) arranged on the piston rod (3) and has at least two code elements which can be distinguished by the position sensor and from which two code symbols ("0", "1") and a separator ("T") are formed, in each case consisting of a plurality of code elements, a plurality of code symbols ("0", "1") forming a code word, characterized in that the marking (5) has a plurality of code word pairs, each code word pair having the same predetermined total length.Cylinder (1) according to claim 1, characterized in that the two code words in a code word pair together comprise two separation characters.Cylinder (1) according to claim 1 or 2, characterized in that each code word has a predetermined number of code characters.Cylinder (1) according to one of Claims 1 to 3, characterized in that two code words which are separated from one another by a separator and each have n code characters differ from one another by n or (n-1) code characters.Cylinder (1) according to Claim 4, characterized in that the code words of a code word pair differ from one another in all their code characters.Cylinder (1) according to one of Claims 1 to 5, characterized in that the two code symbols have different lengths.

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