Measuring system and pressure medium cylinder comprising a measuring system

Abstract

The linear measuring system includes one part of a (usually steel) rod, beam or strip that is profiled with a trapezium like (subset of a) De Bruijn sequence. Each element of the alphabet is represented by a discrete height of the profile. The profile can be coated with a material with low magnetic susceptibility to enable the application on a piston rod of a pressure medium cylinder. The second part of the measuring system reads the subsequences of the main sequence by using the signals of arrays of linear Hall effect sensors, placed in a magnetic field as input for a signal processing algorithm. With a look-up table, the position of this second part of the measuring system with respect to the profile is determined. An interpolation algorithm can be used to improve the resolution to well below the sequence pitch.

Claims

1. A measuring system for determining the position of a piston rod of a pressure medium cylinder relative to a reference point, wherein the measuring system has a code track that is applied to the piston rod, runs along the piston rod, and is formed by grooves and plateaus running in the circumferential direction in a base material of the piston rod, and wherein the measuring system has at least a first sensor system that is stationary relative to the reference point and has a plurality of sensor elements that are arranged parallel to and radially spaced apart from the piston rod, and wherein each sensor element scans a limited number of code elements, characterized in that, to form the code track, at least trinary code elements are used, which form a De Bruijn sequence and/or a De Bruijn subsequence.

2. The measuring system according to claim 1, characterized in that the sensor elements are designed as linear Hall sensors.

3. The measuring system according to claim 1, characterized in that a pitch of the code elements along the piston rod is between 3 mm and 8 mm, and preferably approximately 5 mm.

4. The measuring system according to claim 1, characterized in that at least a second sensor system is provided.

5. The measuring system according to claim 4, characterized in that the sensor elements of the sensor systems are each arranged in a row, wherein a first sensor element of the first sensor system is arranged next to a first sensor element of the second sensor system, and such arrangement continues with n sensor elements.

6. The measuring system according to claim 3, characterized in that a sensor element of the first sensor system is arranged opposite in the region of a pitch of a code element, and in that a sensor element of the second sensor system is arranged opposite between two code elements with an offset that is approximately a pitch or pitch, and/or in that a 1/n pitch offset is provided for a plurality of sensor systems.

7. The measuring system according to claim 1, characterized in that the code elements do not have 0-2 and/or 2-0 jumps.

8. The measuring system according to claim 1, characterized in that long sequences of identical code elements are avoided.

9. The measuring system according to claim 1, characterized in that 0- and 2-code elements are arranged as far as possible from one another.

10. The measuring system according to claim 1, characterized in that, upon the scanning of a code element, a plausibility check is performed based upon the previous scanned code element and/or the direction of movement along the sensor elements.

11. The measuring system according to claim 4, characterized in that each sensor system has eleven Hall sensors as sensor elements.

12. A pressure medium cylinder having a measuring system according to claim 1.

Description

(1) FIG. 1 a partial perspectival view of a pressure medium cylinder with a measuring system, and

(2) FIG. 2 in a schematic cross-section of a cylinder head, the measuring system and the allocation between sensor elements and code elements.

(3) FIG. 1 shows a pressure medium cylinder 1 with a cylinder housing 2. This housing contains a piston known from the prior art, which is not described in detail here. A piston rod 4 is fastened to this. This piston rod exits from a cylinder head 6, wherein the piston rod 4 is sealed inside the cylinder head 6. The piston rod 4 is formed from a base material 8, which can be a steel core, for example, and a thin ceramic or metallic layer 10 applied around the base materialfor example, by flame spraying or build-up welding. The layer 10 is not magnetizable, or is so only to a limited extent.

(4) A measuring system 12 with a housing 14 is inserted into the cylinder head 6, wherein the measuring system is designed to detect an absolute position of the piston rod 4. The measuring system 12 is located in a non-pressurized region of the cylinder head 6 between the seal and a scraper.

(5) The base material 8 of the piston rod 4 is provided with an at least partially radially circumferential code track running along the piston rod 4. This code track is formed from individual code elements, which are described in more detail below.

(6) FIG. 2 shows a cross-section through the cylinder head 6 of a measuring system 12 according to the invention. As already indicated above, the base material 8 of the piston rod 4 has a code track. This is formed from a plurality of code elements 16, 18, 20. Furthermore, a sensor assembly 22 is part of the measuring system 12. In the present exemplary embodiment, this assembly is formed from two sensor systems, wherein the first sensor system in this exemplary embodiment has eight sensor elements 24, and the second sensor system also has eight sensor elements 26. The individual sensor elements 24, 26 are designed as linear Hall sensors. Also shown in FIG. 2 is a pitch 28, which represents the length of a code element 16, 18, 20. According to the present exemplary embodiment, a pitch 28 is also the length of two sensor elements 24, 26.

(7) FIG. 2 shows a trinary code track that has the sequence 0-1-2-1-1-0-0-1. The code track is a subsequence and is based upon a De Bruijn sequence, which is distinguished by the fact that each arrangement of a certain number of code elements 16, 18, 20 is present only once in the entire code track. In principle, it can be stated that the smallest arrangement for which the above statement is true is equal to or greater than the number of sensor systems. Accordingly, with the measuring system 12 according to the invention, it would be possible to generate a code track, wherein a sequence of code elements 16, 18, 20 with a length equal to the number of sensor elements 24, 26 in a sensor system occurs only once. However, for manufacturing and/or measurement technology reasons, further measures must be taken into account when generating the code track.

(8) As a general rule, long series of identical code elements 16, 18, 20 should be avoided, and it should be noted that the 0-code elements 16 and the 2-code elements 20 have a comparatively small proportion of the total code track and are not arranged directly adjacent to one another. In other words, 0-2 and 2-0 jumps are to be avoided to protect the protective layer 10 and the base material 8 and the top layer 10 (not shown) in the region of the jumps from avoidable stress peaks.

(9) It should also be noted that a series of identical code elements 16, 18, 20 is maximized in sequencefor example, to four identical code elements 16, 18, 20 for an eight-digit code track. In addition, a continuous trend in one direction, i.e., from 0-code elements 16 to 2-code elements 20, or in reverse order, must be avoided. Rather, the goal should be a constant change between the individual code elements 16, 18, 20.

(10) In the present exemplary embodiment according to FIG. 2, two sensor systems are formed with eight linear Hall sensors 24, 26 each. The sensor elements 24 of the first sensor system have an identical pitch to the code elements 16, 18, 20. The sensor elements 26 of the second sensor system also have an identical pitch. The sensor elements 24 of the first sensor system are radially spaced apart, i.e., opposite one another, within a pitch of the code elements 16, 18, 20, whereas the sensor elements 26 of the second sensor system are arranged at a pitch offset from the code elements 16, 18, 20. In this arrangement, the sensor elements 24, 26 are located directly adjacent to one another. This ensures that at least one of the sensor systems can determine a sufficient signal when the piston rod is displaced. In an exemplary embodiment not shown here, the sensor elements 26 of the second sensor system are located with a pitch offset from the sensor elements 24 of the first sensor system. This creates a second measurement channel that acts as a redundant sensor system to the first sensor system. If more than two sensor systems are used, e.g., n sensor systems, it is advantageous to arrange the sensor systems with a 1/n pitch offset.

(11) When the piston rod 4 is displaced, the following steps are performed: The signals of all sixteen sensor elements 24, 26 are read out, and a filtering, offset, and gain correction are performed. The determined signals and, if available, the signals from previous measurements, are used in conjunction with the direction of movement of the piston rod 4 in order to determine the position within the code track, and thus track back to the absolute position of the piston rod 4.

(12) When using a De Bruijn code track, no start code elements are necessary, as is the case with a binary code track from the prior art. Furthermore, redundant code elements 16, 18, 20 are not necessary. The length of the code track, in the case of equal or higher information density, is reduced from 0.104 m to 0.033 m, compared to the solutions known from the prior art, which allows a maximum piston length of, for example, 30 m instead of the 26.52 m known. The length of the sensor assembly is also reduced from 0.1352 m to 0.033 m.

(13) A measuring system and a pressure medium cylinder having a measuring system are disclosed.

LIST OF REFERENCE SIGNS

(14) 1 Pressure medium cylinder 2 Cylinder housing 4 Piston rod 6 Cylinder head 8 Base material 10 Protective layer 12 Measuring system 14 Housing 16 0-code element 18 1-code element 20 2-code element 22 Sensor assembly 24 Sensor element of the first sensor system 26 Sensor element of the second sensor system 28 Pitch