EP2226514B1 - Device for measuring the axial position of a piston rod relatively to a cylinder housing - Google Patents

Abstract

The device has circular structures of bulges and recesses formed in a mantle surface of a piston rod (18), and a sensor device (28) for contactlessly scanning the structures using sensors e.g. magneto-resistive sensors, inductive sensors, eddy current sensors and ultrasound sensors, for determining an axial position of the rod. A wear-resistant cover guides the structures into a cylinder housing (10), where the structures concentrically surround the rod. The cover is formed as a pipe (26) that is coaxially pushed into the rod, and the structures form an absolutely coded solid unit. The piston rod is made of soft magnetic material or ultrasound reflecting material, and the pipe is made of magnetically transparent material or non-magnetic stainless steel e.g. austenitic steel, or ultrasound permeable material.

Description

The invention relates to a device for measuring the axial position of a piston rod with respect to a cylinder housing of a fluid pressure actuated cylinder-piston unit according to the preamble of patent claim 1.

For many applications of fluid pressure actuated cylinder-piston assemblies, i. H. of pneumatic or hydraulic units, it is necessary to measure the position of the piston rod with respect to the usually firmly mounted cylinder housing. The determination of the position of the piston rod and, where appropriate, variables derived from this position, such as speed and acceleration, are required, for example, if such units are used as a servo or actuator in automation.

For measuring the position of the pliers, it is known to form the piston rod with an axially extending material measure, which is scanned by a arranged on the cylinder housing sensor device. From the DE 100 20 764 A1 and the DE 196 48 335 C2 For this purpose, it is known to use a permanent magnetic material measure in the jacket of the piston rod or in the interior of the tubular piston rod. The onset of such a permanent magnetic material measure is expensive. Since the material measure is designed as a strip extending in the axial direction, the piston rod must be guided secured against rotation.

From the DE 198 01 091 A1 a device for measuring the axial position of a piston rod with respect to a cylinder housing of a fluid pressure actuated cylinder-piston unit is known. In the lateral surface of the piston rod an axially extending strip-shaped material measure is incorporated, the consists of radially over the lateral surface protruding elevations or in this lateral surface dipping depressions. Two magnetic sensors spaced apart in the axial direction scan this material measure. In order to guide the piston rod in the cylinder housing displaceable and sealed, the material measure is coated with a chrome coating. The incorporation of the material measure in the piston rod is expensive. A further increased manufacturing expense results from the fact that the material measure must be provided with a chrome facing, whereupon the lateral surface of the piston rod must be turned off again for a sealed guide in the cylinder housing. The strip-shaped dimensional standard also makes a torsion-proof guidance of the piston rod necessary here. The material measure is scanned incrementally, so that at the beginning of the movement of the piston rod in each case first a reference position must be approached.

As a further prior art, the EP 1 721 087 B1 called.

The invention is based on the object to provide a device for measuring the axial position of a piston rod with respect to a cylinder housing, which has a robust and simple to manufacture construction and allows versatile use.

This object is achieved by a device with the features of claim 1.

Advantageous embodiments of the invention are specified in the subclaims.

In the device according to the invention, the piston rod is formed with a material measure, which is formed from the piston rods horizontally enclosing ring structures, eg from surveys or depressions. Since these ring structures surround the piston rod concentrically, the piston rod can be produced with these ring structures in a simple manner, in particular for example by turning or grinding. Due to the rotationally symmetrical design of the ring structures, the material measure can be scanned in any rotational position of the piston rod, so that a non-rotatable guidance of the piston rod is not necessary and the unit can be used more versatile. Thus, the piston rod can be performed axially displaceable sealed, a tube is pushed onto the piston rod coaxially as a cover of the material measure. The tube is made of a wear-resistant material to minimize friction wear during axial movement. The tube is supported radially on the outer circumference of the ring structures. The tube therefore does not have to absorb any radial forces and can therefore be thin-walled. The production of the tube is simple and the sliding of the tube on the piston rod is a simple assembly process. The ring structures form an absolutely coded dimensional measure of the position, so that the position of the piston rod is immediately available even when starting after a business interruption.

According to the invention, the material measure of the piston rod is divided by equidistant dimensional ring structures into periodically successive sections. Each of these subsections is scanned absolutely by the sensor device, so that the piston position within the respective subsection can be determined absolutely. Within each subsection, an assignment ring structure is further arranged. The axial position of this allocation ring structure within the respective subsection is different for each subsection. The axial position of the assignment ring structure within the respective subsection is thus an unambiguous characteristic of the respective subsection and can unambiguously define its position within the entire axial material measure. From this unique assignment of the subsection and from the absolute position measurement within the subsection, an absolute position measurement over the entire stroke of the piston rod can thus be obtained. The accuracy of the position determination depends only on the accuracy of the absolute measurement within the sections, while the total length of the measurable stroke of this can be independently selected by the number of sections.

The sensor device has a plurality of sensors which are arranged in a line running parallel to the axis of the piston rod. When the scale is divided into sections, the sensor means has an axial length (i.e., the axial distance of the outermost sensors) at least equal to the axial length of the sections. As a result, the sensor device can respectively detect the dimensional ring structures forming the current partial section in order to interpolate the position of the piston rod within this partial section. Likewise, this ensures that the position of the assignment ring structure can be detected within the measurement ring structures.

The sensors can be designed in a manner known per se. In particular, magnetoresistive sensors, inductive sensors or eddy-current sensors can be used. The magnetic field of these sensors is influenced by the ring structures of the piston rod and their axial position relative to the respective sensors. Accordingly, the individual sensors of the sensor device provide different signals whose Amplitude of the respective different axial position of the ring structure with respect to this sensor depends. From the ratio of the signal amplitudes of the various sensors of the sensor device, the position of the ring structures with respect to the sensor device can be determined in a subsequent evaluation. In order to influence the magnetic field for these sensors by the ring structures of the piston rod, the piston rod with the ring structures of a soft magnetic material, eg. B. from a suitable iron alloy. So that the tube pushed over the piston rod does not shield the magnetic field, this tube consists of a "magnetically transparent" material, ie of a diamagnetic or paramagnetic material. This can be for example a plastic. In order to ensure the necessary wear resistance, a metallic material is preferred, this may in particular a non-magnetic steel, for. B. be an austenitic steel.

It is also possible to scan the dimensional standard of the piston rod by a sensor device with ultrasonic sensors. In order to make the material measure covered by the deferred tube ultrasonically visible, it is necessary here for the materials of the piston rod and the pushed tube to have different sound wave resistance, so that a reflection of the sound waves occurs on the surface of the piston rod with the ring structures and through the deferred Tube is measurable.

Fig. 1

im Axialschnitt schematisch ein Zylinder-Kolben- Aggregat mit einer Vorrichtung zur Messung der axia- len Position der Kolbenstange und

Fig. 2

die absolute Codierung der Kolbenstange.

In the following the invention will be explained in more detail with reference to a schematic embodiment shown in the drawing. Show it

Fig. 1

in axial section schematically a cylinder-piston unit with a device for measuring the axia- len position of the piston rod and

Fig. 2

the absolute coding of the piston rod.

A hydraulic or pneumatic cylinder-piston unit has a cylinder housing 10. In the cylinder chamber of the cylinder housing 10, a piston 12 is mounted axially displaceable and sealed by a sealing ring 14 against the cylinder wall. Via connections 16, the cylinder chambers can be acted on both sides of the piston 12 with fluid pressure. The piston 12 is connected to a coaxially arranged piston rod 18, which emerges axially guided out of the cylinder housing 10, wherein a sealing ring 20 seals the piston rod 18 on its outer periphery. The cylinder space, the piston 12 and the piston rod 18 have a coaxial circular cross section, so that the piston rod 12 is rotatable about its axis.

The piston rod 18 has on its outer circumference ring structures which surround the piston rod 18 concentrically and in the illustrated embodiment are formed by grooves inserted into the lateral surface turned grooves 22 and 24. Axially on the piston rod 18, a tube 26 is pushed, which rests radially close to the outer periphery of the piston rod 18. The tube 26 thus forms the outer peripheral surface of the piston rod 18, on which the sealing ring 20 rests and with which the piston rod 18 is guided axially displaceably in the cylinder housing 10.

At the end of the cylinder housing 10, on which the piston rod 18 emerges, on the cylinder housing 10 is a sensor device 28 disposed adjacent to the outer circumference of the tube 26 at a small distance and the piston rod 18 scans with the ring structures.

The sensor device 28 consists of a plurality of sensors, which are arranged in a row parallel to the axis of the piston rod 18 in succession. The sensors of the sensor device 28 may be formed in different ways known per se.

The sensors of the sensor device may be magnetoresistive sensors. The magnetic field of these sensors, which is axially parallel to the piston rod 18, depends on the magnetic flux in the surface of the piston rod 18, so that a high magnetic flux and thus a large signal amplitude of the sensors results when an area between the recessed grooves 22 and 24 is in an axial position located at the respective sensor. The recessed grooves 22 and 24, however, interrupt the magnetic flux in the surface of the piston rod 18, so that the signal amplitude of the respective sensor decreases when one of the grooves 22 and 24 is located axially in the region of the respective sensor. In order to guide the magnetic field in the surface of the piston rod 18, the piston rod 18, at least in its outer peripheral region of a soft magnetic material. So that the magnetic flux is not shielded by the deferred tube 26, this consists of a "magnetically transparent" material, ie of a diamagnetic or paramagnetic material. In view of the abrasion resistance and these magnetic properties, the pushed-on tube 26 is preferably made of a non-magnetic stainless steel z. B. from an austenitic steel alloy.

In another embodiment, the sensors of the sensor device 28 are inductive sensors that operate on the principle of a transformer. The magnetic alternating field of these sensors is thereby magnetically short-circuited via the mantle regions of the piston rod 18 between the grooves 22 and 24, so that here too a large signal amplitude results when the piston rod 18 with an axial region between the grooves 22 and 24 in the position of the respective sensor, while the grooves 22 and 24 lead to a reduction of the sensor signal. Also in this embodiment, the piston rod 18 is made of a soft magnetic material and the tube 26 made of a magnetically transparent material.

Furthermore, the sensors of the sensor device 28 may be eddy current sensors in which an alternating magnetic field of the sensors in the outer jacket layer of the piston rod 18 generates eddy currents when the axial regions of the piston rod 18 between the grooves 22 and 24 in the axial position of the respective sensor are located. The recessed grooves 22 and 24 interrupt the magnetic field and thus the formation of eddy currents. In this embodiment, the piston rod 18 must have good electrical conductivity at least in its outer jacket layer. The deferred tube 26 must also consist in this embodiment of a magnetically transparent material that does not shield the alternating magnetic field of the sensors.

Finally, the sensors of the sensor device 28 may also be ultrasonic sensors. In this embodiment, the mantle surface of the piston rod 18 is scanned by the sensor device 28 with ultrasound in order to determine the ring structures in their axial position. To the through the deferred pipe 26 covered surface of the piston rod 18 can be scanned by means of the ultrasonic sensors, the material of the pushed tube 26 must have a good ultrasonic permeability, while the material of the piston rod 18 must reflect the ultrasound.

FIG. 2 shows in one embodiment, the coding of the piston rod 18 through the grooves 22 and 24 for the absolute determination of the position of the piston rod 18 with respect to the cylinder housing 10 arranged on the sensor device 28th

The piston rod 18 has annular structures formed on its outer circumference by concentric grooves that are screwed in. In this case, grooves 22 are provided which are arranged equidistantly over the entire axial length of the stroke to be measured of the piston rod 18 and limit Maß-ring structures each having a constant length a. In order for the Maß-ring structures formed by the grooves 22 define in each case in the axial direction periodically adjoining sections a ₁ , a ₂ ...... a _n .

Within each of these sections a ₁ , a ₂ ... A _n there is an assignment ring structure formed by a groove 24. The axial position of this assignment ring structure within the respective subsection a ₁ , a ₂ ... A _n differs for each subsection a ₁ , a ₂ ... A _n from all other subsections. The respective axial position of the assignment ring structure within the associated subsection a ₁ , a ₂ ... A _n thus represents a clear identification of the respective subsection.

In the in FIG. 2 the example shown shifts the axial position of the groove 24 of the allocation ring structure within the respective subsection in each case by an axial distance d from a subsection to the subsequent subsection. In FIG. 2 this is shown for an axial section of the piston rod 18. In the 9th subsection a ₉ , the groove 24 of the allocation ring structure is displaced axially by the distance 9d from the end of the subsection a ₉ formed by the groove 22. In the following subsection a ₁₀ , the groove 24 is displaced by the axial distance 10d against the end of this section a ₁₀ determined by the groove 22, etc.

The sensor device 28 determines the axial position of the limited by the grooves 22 Maß-ring structures, which can be determined by the arranged in a line sensors of the sensor device 28, the axial position of the piston rod 18 within the respective section a _n absolutely. Furthermore, the axial position of the association ring structure formed by the groove 24 within the respective subsection a _{n is} determined by the sensor device 28, so that the assignment of the absolute position value within the subsection a _n can be unambiguously assigned to the respective subsection. In an evaluation connected downstream of the sensor device, the identification of the subsection a _n obtained by means of the association ring structure is combined with the position determined absolutely within this subsection a _n to form an absolute position determination over the entire length of the piston rod 18.

The constant distance of the grooves 22 and thus the axial length of the sections is selected according to the required resolution of the axial position measurement and the formation of the sensor device 28. This axial length a of the sections can be, for example, of the order of 50 mm.

LIST OF REFERENCE NUMBERS

10

cylinder housing

12

piston

14

sealing ring

16

connections

18

piston rod

20

sealing ring

22

Creasing the Dimensional Ring Structures

24

Grooving the mapping ring structures

26

pipe

28

sensor device

Claims (10)

1. Cylinder/piston unit actuated by fluid pressure, with a cylinder housing (10) and a piston rod (18), with structures constituting elevations or depressions which are formed in the outer surface of the piston rod (18) and form an axially extending material measure, with a sensor device (28) which is disposed on the cylinder housing (10) and scans the structure without contact using sensors spaced apart in the axial direction for determining the position, and with a wear-resistant cover for the structure for the purpose of guiding it in the cylinder housing (10), wherein the structures are annular structures which surround the piston rod (18) concentrically, wherein the cover is a tube (26) fitted coaxially onto the piston rod (19), and wherein the annular structures form an absolutely coded material measure, characterised in that annular measuring structures disposed equidistantly in the axial direction of the piston rod (18) divide the material measure into segments (a_n) which are absolutely scanned and follow one another periodically, and an annular assigning structure is disposed within each segment (a_n), wherein the axial position of the annular assigning structure within one segment (a_n) is different from the axial position of the annular assigning structure in any other segment and serves to unambiguously assign the given segment (a_n).
2. Cylinder/piston unit as claimed in Claim 1, characterised in that in the axial direction the sensor device (28) has a length that is at least equal to the length of one segment (a_n).
3. Cylinder/piston unit as claimed in Claim 2, characterised in that the sensor device (28) has a plurality of sensors which succeed one another in the axial direction and of which the signals are dependent upon their respective distance from the annular structures, that the signals are evaluated on the one hand for absolute determination of the position of the piston rod (18) within a segment (a_n) and on the other hand for ascertaining the position of the annular assigning structure within this segment (a_n), and that the absolute position of the piston rod (18) is formed from these two values.
4. Cylinder/piston unit as claimed in any one of the preceding claims, characterised in that the annular structures are formed by grooves (22, 24) machined into the outer surface of the piston rod (18).
5. Cylinder/piston unit as claimed in Claim 4, characterised in that the annular measuring structures are delimited by grooves (22) and the annular assigning structures are formed by grooves (24).
6. Cylinder/piston unit as claimed in Claim 4, characterised in that the tube (26) rests against the circumference of the piston rod (18) so as to be sealed radially.
7. Cylinder/piston unit as claimed in any one of Claims 1 to 6, characterised in that the piston rod (18) is made from a soft magnetic material and the tube (26) is made from a magnetically transparent material.
8. Cylinder/piston unit as claimed in Claim 7, characterised in that the tube (26) is made from a non-magnetic high-grade steel, particularly an austenitic steel.
9. Cylinder/piston unit as claimed in Claim 7 or Claim 8, characterised in that the sensor device (28) has magnetoresistive sensors, inductive sensors, or eddy current sensors.
10. Cylinder/piston unit as claimed in any one of Claims 1 to 6, characterised in that the sensor device (28) has ultrasound sensors, and that the piston rod (18) is made from a material which reflects ultrasound, and the tube (26) is made from a material which is permeable to ultrasound.

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