WO1988002473A1

WO1988002473A1 - Device for determination of the position of a movable body and method for using the device

Info

Publication number: WO1988002473A1
Application number: PCT/NO1987/000064
Authority: WO
Inventors: Jens M. Hovem; Kurt-Even Kristensen
Original assignee: Stiftelsen For Industriell Og Teknisk Forskning Ve
Priority date: 1986-10-01
Filing date: 1987-10-01
Publication date: 1988-04-07
Also published as: NO863906D0; NO863906L

Abstract

A device for determining the position of a movable body (11), particularly its angular displacement. A reflector element (12) is fastened to the body and a row of acoustic transmitters/receivers (15a...n) are arranged in a circular arc with centre of curvature coinciding with the body's rotation axis (16). These components are joined respectively to a signal generator (18) for sequential pulsing of the transmitter elements and a signal-processing circuit (19) to determine the reflection maxima and the angular displacement.

Description

Device for determination of the position of a movable body and method for using the device

The invention concerns a device presented in accordance with the introduction to Claim of Patent 1, to determine the position of a moveable body which is moved along a predetermined path, particularly by rotation.

In different connections there is a need to monitor the position of rotating or displaced objects, eg spindles in valves. For valves in oil/gas pipes, electro-magnetic angle meters are normally used. These have a short life-time, amongst other things because of the moveable parts. In addition, laying electric cables in explosive 1 iquids /gases involves a risk.

It is known from earlier devices that electromagnetic or similar radiation has a deterministic relationship to the moveable object. However, devices based on such relationships have not proved to be useful for determining the position as far as small movements are concerned, eg the angular displacement of a rotating spindle. The main purpose of the invention is to design a device for determining the position of moveable parts, in particular a spindle which rotates or which can be turned through a certain angle. It is a particular aim to create such a device without moving parts, and thus reduce the need for bringing electric wires into the area with the consequent risk of explosion. One application might be, for example, on connection with off-shore installations.

The invention comprises a device which is described in the characterising part of the Claim of Patent 1. Such measuring equipment is particularly suited for determining the angular position or rotation position of spindles, eg on valves and other operating units where a component is turned or is to be turned through an angle of less than 360º. The invention can, however, also be used for measuring the position of machine parts etc which are moved rectilinearly.

Additional features of the invention are described in the attached Claims of Patent.

Example:

The invention will now be described in more detail, with reference to the illustrations, where: fig.1 shows a schematic portrayal of a measurement arrangement with an attached block diagram for the signal processing circuit. fig.2 shows a vertical cross-section in schematic rendering through a measurement unit in accordance with the invention intended for mounting on a spindle, and fig.3 shows the cross-section through a mechanism with rectilinear movement developed in accordance with thre imventiαn..

In the example, a spindle 11 is shown which can belong to a arbitrary machine part which is intended to rotate through a limited angle, eg 60º. It could be, for example, a spindle on a valve, or a manipulation tool. The aim here as to measune t he spindle's movement or position, measured for example in degrees, from a starting position. On the spindle 11 a reflector 12 is fastened on a suitable mounting 13. The reflector 12 is designed for reflection of acoustic waves. The reflector 12 can be made of rust-free or inert material with a rectangular frontal cross-section. The mounting can, for example, be screwed to the spindle 11. Near the reflector 12, in its movement area, a row 14 of electro-acoustic conversion elements, 15a-n, is placed (rotational elements or transducers). Row 14 has geometric circular arc form with a centre which coincides with the spindle ll's rotational axis 16. The conversion elements 15 a...n, which can be piezoelectric, are directional and have maximum sensitivity direction towards the rotation axis 16. The conversion elements are each connected to a coupling circuit 17, which is connected to a signal generator 18 on the one side, and to a signal processing circuit 19 on the other. To run the system and for callibrating the measured results, a terminal 20 is shown in this example connected to the signal processing circuit 19, which can be micro-processor based. The measurement of the spindle ll's angular displacement is based on the principle that each one of the convertors 15a...n sends out a signal and that the reflected signal which is registered in the other converters is transmitted to the signal processing circuit 19.

Determining the position requires two operations: first the angle interval which the reflector 12 finds itself in is determined, ie which of the conversion elements 15a...n the reflector finds itself between. Thereafter an exact position in this interval is determined.

After pulses are sent from the conversion elements 15a...n one after the other, the position of the relector is found between the elements in the pair of elements which have the highest level of received signal. In order to determine the position exactly, one can either measure the amplitude of the reflected signals, and use the directionality of the oscillations and the reflector, or measure the phase of the received signal. Both methods are possible in practice, but an amplitude measurement is less sensitive to inaccuracies in the system geometry and installation. Thus it will be better to use the oscillations and the reflector's directionality. There are several known methods for combining the signal level with two or more neighbouring converters, which give an unambiguous position determination within an interval.

The accuracy is dependent on the relative distance between the elements, the directionality of the elements and the reflector, the signal/noise-ratio and the accuracy of the digitisation in the signal processing. In a combination which gives a size which is approximately linearly dependent on angle/position, the position will be given the more accurately the greater the slope in the linear approximation, and the accuracy will be the same over the linear area.

In a laboratory model with relative distance between the elements of 3º and a frequency of 500kHz, 0.5% accuracy will be attained over the entire angle interval. With another combination of signal levels, an accuracy of better than 3.1% over an interval of ± 1° around the elements 15a....n could be achieved. Subintervals are also possible with very good resolution. Fig.2 shows an example of an performance graph of the invention designed for placing on a spindle. In a box-shaped casing 21 a rotatable rod is mounted, which is supported on bearings 23 and 24 respectively on opposite sides of the casing. The rod 22 is taken through the lower bearing 24 in the figure and located with a coupling flange 25 outside the casing. The casing 21 is also located with two anchor pads, 26 and 27 respectively, for mounting on a machine not shown, valve or similar. As an alternative to rod 22 being taken through the lower bearing 24, magnetic coupling, for example, between the spindle, the axle for which the position is to be decided, and the mesuring unit's axle could be considered. Thus disturbance of the axle is avoided and the complexity of the mechanical arrangement of the measuring system can be reduced. The casing 21 is coated inside with a lining of absorbant material, eg reinforced foam rubber. In one corner of the casing 21 a space, 29, is provided outside the lining 28 for the parts of the signal processing circuits etc which it is expedient to site at the place of use.

The main elements in the equipment are a reflector 30 which is mounted on a radially-spoked supporting axle 31 fastened to the rod 22. A row 14 with conversion elements 15a...n is fastened to the casing 21's wall internally on the lining 28, and radially out from the reflector 30.

The interior of the casing 21 is filled with a suitable liquid, eg castor oil 32.

Alternatives:

The reflector can be replaced by a pulsating element fastened to the spindle or the piston rod, and which emits acoustic signals to the detector rows. These will then be used only as receiver elements. The signal processing procedure for determining the position will be as in the case with the reflector, while the analogue electronic-circuits which are necessary for sending out and receiving pulses, will be simpler. However, the system's reliabilty is reduced and also its safety, because an electric cable must be carried out to a moving part. The invention can also be used to measure linear displacement as illustrated in fig.3 Here a section of a hydraulic cylinder 33 with a piston rod 34 is shown. A rectilinear row of conversion elements 35 is built into the wall, while a reflector 36 is attached to the piston rod 34. The position of the reflector 36 can be found by the corresponding method, as described above.

Claims

CLAIM OF THE PATENT

1. A device for determining the position of a moveable body which is moved along a pre-determined path, particularly by rotation, with one transmitter which emits an acoustic signal towards the body and with one receiver to detect the reflected signal from the body, together .with a signal-processing circuit to callibrate and indicate a geometric dimension in the sender-body relationship, c h a r a c t e r i s e d by having a reflector element (12) attached to the body, with rotating bodies radially outside the body's rotational axis, and a row (14) of transmitter-receiver components (15a...n), with rotating bodies arranged in circular arc form with centre of curvature which coincides with the body's rotation axis (16), and that the components are joined respectively to a signal generator (18) for sequential pulsing of the transmitter elements and a signal-processing circuit (19) to determine the reflected maxima and specify the angular displacement.

2. A device in accordance with Claim of Patent 1, c h a r a c t e r i s e d by the reflector (12;36); being fastened to a mounting (13;31) fixed to the moveable body (11;22;34).

3. A device in accordance with Claim of Patent 1 or 2, c h a r a c t e r i s e d by comprising a revolvable rod (22) mounted in a closed casing (21), in that this rod is shaped for coupling and turning with a body where the angular displacement is to be measured, and where the rod acts as a mount for Lhe reflector (30), in that the row (14) of conversion elements (15a...n) is mounted on the wall of the casing radially outside the reflector's rotation path.

4. Method for use of the invention in accordance with Claim of Patent 1 for measuring the angular movement of a moveable body which is moved along a predetermined path, particularly by rotation c h a r a c t e r i s e d by the angle of the body being determined from the amplitude measurement or phase measurement of the signal which is received by the receiver units (15a...n) by first determining the angle interval which the reflector finds itself in and thereafter determining a more exact position by combining the signals to two or more neighbour receivers while simultaneously taking into account the directionality to the receivers in relation to the reflector and thereby obtaining a measure which is a precise function of position, ie angle.