WO1995015478A1

WO1995015478A1 - Sensor arrangement for detecting the position of a body, such as a rod

Info

Publication number: WO1995015478A1
Application number: PCT/DK1994/000450
Authority: WO
Inventors: Allan Northeved; Bent SØRENSEN
Original assignee: Force Institutterne; Bent Sørensen Holding Aps
Priority date: 1993-12-03
Filing date: 1994-12-02
Publication date: 1995-06-08
Also published as: AU1106695A; DK135493A; DK135493D0

Abstract

A sensor arrangement for detecting the position of a rod (13) movable relative to a fixed position. The sensor arrangement comprises a plurality of magnetic sensors (31), each sensor being able to emit an electric signal proportional to the magnetic field intensity adjacent the surface of the rod (13). The signals are transmitted to a pattern recognition device comparing said signals with preregistered signal patterns representing specific positions of said rod (13). The sensor arrangement is circumferential and comprises 360 equidistantly arranged sensors (31). As a result, it is not necessary to provide a specific magnetic structure in the surface of said rod (13) as the sensor arrangement (12) utilizes the magnetic state in which the piston rod (13) happens to be.

Description

SENSOR ARRANGEMENT FOR DETECTING THE POSITION OF A BODY , SUCH AS A ROD

Technical Field

The invention relates to a sensor arrangement for detecting the position of a body, such as a rod, movable relative to a fixed position.

5 Background Art

EP Publication No. 296,808 discloses a sensor arrangement for sensing the position of a movable rod, such as a piston rod. Zones of a varying magnetic character are provided in the rod by means of exact local heat¬ ings of said rod. A fixed transducer is able to detect these zones and 10 thereby to indicate the position of the rod. It is, however, difficult to provide such locally heated zones.

EP Publication No. 446,969 discloses a method of providing a desired magnetic structure in the surface of a rod by incorporating magnetic material in the surface of the rod. As a result, the position of the rod can 15 be detected by means of a permanently placed sensor arrangement com¬ prising a plurality of magnetic field sensors. Such magnetic materials to be incorporated in the rod increase, however, the fabrication costs.

Brief Description of the Invention

The object of the invention is to provide a sensor arrangement for detect- 20 ing the position of a rod, and which does not require a predetermined specific magnetic structure in the surface of the rod.

A sensor arrangement of the above type is according to the invention characterised in that it comprises a plurality of magnetic sensors, each sensor being able to emit an electric signal proportional to the magnetic field intensity adjacent the surface of the rod, and said signals being trans¬ mitted to a pattern recognition device comparing said signals with preregistered signal patterns representing specific positions of said rod. In this manner it is possible to utilize the state which the body happens to be in, and accordingly it is not necessary in advance to provide the surface of the rod with a specific magnetic structure.

Furthermore according to the invention the sensor arrangement may be completely or partially circumferential with the result that the detection of the position of the rod is very accurate.

In addition, the sensor arrangement may according to the invention com¬ prise approximately 360 equidistantly arranged sensors.

Brief Description of the Drawings

The invention is described in greater detail below with reference to the accompanying drawings, in which

Fig. 1 illustrates a hydraulic cylinder with a sensor arrangement according to the invention for detecting the position of the piston rod,

Fig. 2 is a side view of the sensor arrangement,

Fig. 3 is a sectional view through the sensor arrangement of Fig. 2 taken along the line A-A,

Fig. 4 is a sectional view through the sensor arrangement of Fig. 1 taken along the line B-B,

Fig. 5 illustrates an associated signal processing circuit, and Fig. 6 illustrates a device for calibrating the sensor arrangement.

Best Mode for Carrying Out the Invention

The hydraulic cylinder 21 shown in Fig. 1 is provided with a sensor ar¬ rangement 12 according to the invention for detecting the position of the piston rod 13. The sensor arrangement 12 is circumferential about the piston rod 13 and is secured to the terminal face of the cylinder 21 , cf. Fig. 3. It comprises a plurality of sensors 31 allowing a measuring of the micromagnetic field intensity in a large number of small zones adjacent the surface of the piston rod 1 3. In this manner the rod 1 3 need not be magni- tized in a specific code as the sensor arrangement 12 utilizes the magnetic state which the piston rod 1 3 happens to be in.

The measured field intensities, viz. the magnetic field pattern, are encoded in a pattern recognition device in a computer in form of a personal com¬ puter 51 together with the position of the piston rod 1 3. The magnetic field patterns measured later on can then by means of the pattern recogni¬ tion device be used for determining the position of the rod 13.

The use of only one sensor results in a very coarse indication of the posi¬ tion of the rod 13. Several sensors placed around the piston rod 1 3 increase the measuring accuracy. The measuring accuracy is substantially proportional to the number of sensors and is substantially inversely propor¬ tional to the size of each sensor.

It is possible to use several sets of circumferential sensor arrangements arranged at a regular interval in the longitudinal direction. As a result, the measuring accuracy can be further improved.

Fig. 3 is a sectional view of the sensor arrangement taken along the line A- A of Fig. 2. It is formed by an annular body surrounding the piston rod 1 3. The piston rod 1 3 is displaceable relative to the circumferential body. A plurality of sensors, such as 360 sensors, are arranged along the inner side of the circumferential body. In a predetermined position of the piston rod 13 in the annular body, the sensors 31 , such as Hall elements, emit an electric signal proportional to the magnetic field intensity adjacent the surface of said piston rod 13. The measured field intensities correspond to the state of the material. Cleaning rings 32 are provided on the inner side of the annular body.

Fig. 4 is a sectional view of the sensor arrangement taken along the line B- B and illustrating the conduit through which the signals are detected. A cavity 33 is provided between the sensors and the outermost portion of the annular body, in which cavity circuit boards with electric components can be inserted.

Fig. 5 illustrates an associated signal processing circuit. The signals from each of the sensors 31 are transmitted to an amplifier 53, and the ampli¬ fied signal is transmitted to a multiplexer 54. The multiplexer 54 comprises a number of inputs, such as 360 inputs, and is controlled by means of a timer 52. The multiplexed signal is transmitted to an analogous gate 55 and subsequently to an A/D converter 56. The A/D converter 56 transmits the digitized signals to a memory 57 in which the digitized signals are stored in such a manner that they can be compared with preregistered signal patterns.

The preregistered signal patterns are provided by means of a calibration device, cf. Fig. 6. The signal patterns of a large number of positions of the piston rod 13 are encoded by means of the calibration device together with the positions in question. The signal patterns matching the various positions are provided by means of an encoder 62 and transmitted to a personal computer 51 through a digital gate 58. The position in the longi¬ tudinal direction is controlled by means of a motor 61 which in turn is controlled by the personal computer 51 through an analogous gate 1 51 . The distance between the individual positions can for instance be 1 mm, 0.1 mm or 0.01 mm.

Measuring results are also encoded for the selected positions in the longi- tudinal direction of other angular positions of the piston rod 1 3. Such angular positions can for instance be values within a range of _+. 20° from the standard angular position of the piston rod 13 with an angular distance of 0.5°. The angular position signals are emitted by an encoder 64 and transmitted to a personal computer 51 through a digital gate 59. A motor 63 for providing a turning movement so as the change the angular position of the rod is controlled by an analogous gate 1 50. This function can alter¬ natively be performed by means of a shift register which can perform the turning by means of software.

The calibration device is used with one side of the hydraulic cylinder being secured to a holder 65. The cylinder eye 66 may be positioned by means of a nut 67 and a threaded spindle 68. The threaded spindle is driven by means of a motor 69 controlled by the personal computer 51 through an encoder 62. The turning of the piston rod 1 3 relative to the cylinder 1 1 is performed by means of the motor 63 through a gear wheel arrangement 69 and a coupling 70, said motor 63 being controlled by the personal com¬ puter 51 through an additional encoder 64.

When the personal computer 51 performs a pattern comparison at a speci¬ fic position in the longitudinal direction of the piston rod 1 3, it also per¬ forms a comparison at various angular positions of the piston rod 13 relative to the cylinder 1 1 so as to find the actual angular position by means of a program. Subsequently, the position in the longitudinal direc¬ tion can be found.

The digital values stored in the personal computer 51 are stored in form of values relative to the highest value, i.e. in form of relative values.

A longitudinal position scanning can, if desired, involve several individual scannings so as to achieve a high accuracy.

The pattern recognition device can be adapted in such a manner that it also reacts when only an essential correspondence exists with the preregistered signal patterns. In this manner it has been taken into account that the rod can have been subjected to effects. Furthermore, the pattern recognition device can be self-adjusting in such a manner that the preregistered signal patterns can be changed when the correspondence is almost, but not complete in agreement with one of said patterns. Limits can be encoded in advance as to how complete the correspondence shall be. A neural network can, if desired, be provided in connection with the pattern recognition device, said neural network automatically determining the degree of the correspondence.

Claims

Claims.

1. A sensor arrangement for detecting the position of a body, such as a rod, movable relative to a fixed position, ch aracte rised in that it comprises a plurality of magnetic sensors (31), each sensor being able to emit an electric signal proportional to the magnetic field intensity adjac¬ ent the surface of the rod (13), and said signals being transmitted to a pattern recognition device comparing said signals with preregistered signal patterns representing specific positions of said rod (13).

2. A sensor arrangement as claimed in claim 1, ch a racterised in that it is completely or partially circumferential.

3. A sensor arrangement as claimed in claim 1, c h aracte ri sed in that it comprises approximately 360 equidistantly arranged sensors (31).

4. A sensor arrangement as claimed in one or more of the preceding claims, characterised in that it comprises several sets of circum¬ ferential sensor arrangements.

5. A sensor arrangement as claimed in one or more of the preceding claims, c h a r a ct e r i s e d in that the pattern recognition device is adapted to react even when only a partial correspondence exists with one of the preregistered signal patterns.

6. A sensor arrangement as claimed in claim 5, c h aracte ri sed in that the preregistered signal patterns can be amended when the correspondence therewith is not complete, whereby the pattern recogni¬ tion device is self-adjusting.