GB2152656A

GB2152656A - Position sensing apparatus

Info

Publication number: GB2152656A
Application number: GB08323663A
Authority: GB
Inventors: David Edward Goodwin
Original assignee: GOODWIN ENGINEERING DEVELOPMEN
Current assignee: GOODWIN ENGINEERING DEVELOPMEN
Priority date: 1983-09-03
Filing date: 1983-09-03
Publication date: 1985-08-07
Also published as: GB8323663D0

Abstract

The invention relates to a position sensing for processes in which light is given off in degree dependant on the position of one element, such as the distance of a plasma arc torch 1 from a workpiece 8. The object of the invention is to provide a means whereby the light may be measured at a number of points by one or relatively few energy sensors. The invention is met by an optical element 3 which might generally take the form of a rod or ring in which the characteristic of the surface is that light is reflected and/or refracted so that a proportion travels along the element and continues to do so by the mechanism of total internal reflection. An energy sensor 5 or sensors is arranged to measure a proportion of that total light falling on the surface. <IMAGE>

Description

SPECIFICATION Position control apparatus This invention relates to position control systems. The invention may be applied to the control of the distance of a plasma cutting torch from the material being cut but can in general be applied to any process where electromagnetic radiation is emitted in degree dependant upon the separation between the working element and the workpiece.

In thermal metal cutting operations a cutting torch may be mounted on a slide above a workpiece. The torch may be for example an oxy-fuel cutting torch or a plasma cutting torch. Generally the torch is moved over the workpiece as cutting proceeds although sometimes the workpiece is moved under a stationary torch.

To maintain optimum cutting conditions and to allow for uneveness of the workpiece surface the slide may be moved to adjust the distance of the torch from the workpiece. In many cases the slide is motor driven, this motor being controlled by an operator who observes the cutting process. Should the distance of the torch from the workpiece need to be adjusted the operator activates the motor as required to move the slide and hence the torch closer to or farther away from the workpiece.

It is desirable to provide for some means of monitoring the distance between the torch and the workpiece and to provide for a means of automatically maintaining this distance within some pre-determined limits Previously height control systems as applied to plasma cutting torches have depended on the provision of an element which forms a capacitance or inductance. the value of this capacitance or inductance being dependant upon the distance of the element from the surface of the workpiece. The capacitance or inductance was often arranged to be part of a tuned circuit or oscillator. Thus the resonant frequency of this tuned circuit depended on the distance of the element above the surface of the workpiece. The change of resonant frequency with change in distance was monitored and arranged to provide an error signal which was applied to means to correct the change in distance.These systems have the disadvantage that the sensing elements must in practise be arranged to be as close as possible to the surface to be sensed and also close to the torch with the result that they are prone to mechanical and other damage.

Also with these systems the frequency changes with change in distance are small in relation to the resonant frequency of the circuit and it is difficult to provide circuits that respond to these small relative changes. Further, because these changes are small the oscillator circuits need to be stable. This is difficult to acheive since the stability of the circuit is affected by environmental factors such as heat and contamination with dirt or water (plasma cutting is often performed over water).

Systems have also been provided in the past whereby the surface to be sensed is contacted by mechanical sensors. These systems have the disadvantage that on passing over holes in the sensed surface or coming to the edge of the sensed surface the mechanism can become entangled and in any case can cause the torch to adjust downwards with the result that the torches and mechanical sensors are frequently damaged.

Systems are known wherein the voltage of the arc is monitored. The voltage of a plasma arc varies with arc length, therefore maintaining the voltage of the arc constant is said to maintain a constant torch height. The problem encountered with such systems is that in practise the voltage of the arc varies for reasons other than the distance between the torch and the workpiece such as the condition of the torch components, the plasma gas flow, cutting speed and workpiece thickness. Consequently arc voltage may give an inaccurate indication of torch height.

Simple optical systems are known wherein the light emitted by an arc is monitored by a sensor which provides a signal which can be used to adjust the position of the torch. The disadvantages of such systems in plasma cutting processes is that the amount of light collected by the sensor depends on the position of the sensor relative to the direction of cut. When the sensor is aligned with the direction of cut such that the light originates not only from the arc between the torch and the workpiece but also from the arc within the cut within the workpiece the sensor collects substantially more light than when it is aligned in any other direction. This means that such systems cannot be applied to the control of plasma torches cutting profiles since in general the direction of cut relative to the position of the sensor is frequently changing.

The object of this invention is to overcome the disadvantages of the previous known systems and provide for a system of position sensing which differs from previous systems in a new and novel way.

In plasma cutting the amount of light given off by the arc between the surface of the workpiece and the torch nozzle when cutting is generally proportional to the length of this arc.

Measurement of the amount of light given off by the torch therfore gives a measure of the distance between the torch nozzle and the workpiece. The length of arc visible and therefore the light intensity increases if it is sensed along the line of the cut. Therefore it is necessary to collect a proportion of the light given off in many or all directions in order that the varying intensities of light radiating in various directions from the arc are averaged so that the signal from the sensor or sensors is more nearly proportional to the distance of the torch from the workpiece.

The present invention comprises position control apparatus for use in processes wherein a workpiece is worked on by a working element and which processes result in the emission of electromagnetic radiation in many directions and in which the total emission in certain directions is dependant upon the separation of the working element from the workpiece, being apparatus whereby a proportion of the total emission in these certain directions is collected by an element and this proportion measured.

The coliecting element comprise a combination of reflective and/or refractive elements such that a proportion of the total incident radiation is collected and conveyed to one or more sensors. The working element may be for example a plasma cutting torch and the workpiece may be a sheet of metal.

The electromagnetic radiation may be of any wavelength and may contain a proportion of visible, infra red or ultra violet wavelengths.

The collected radiation from the process may be used to generate an electrical signal and this signal may be used to control a motor or other means which adjusts the position of the working element.

The radiation may be channelled to one or more sensors which do not directly view the source of the radiation by means of an optical element which collects light falling on it in a path surrounding the source of the radiation.

Further, the radiation may be directed onto such an optical element by means of one or more mirrors. The optical element may comprise a ring of material surrounding the source of the radiation wherein the inner surface of the ring refracts a proportion of the radiation falling on it in such a way that this radiation is transmitted around the ring by internal reflection. This ring may be provided with one or more sensors or the radiation may be chan nelled from such a ring to one or more sensors. Alternatively the optical element may comprise several turns of optic cable which may be monofilament polymer fibre optic cable with the sheath removed. The surface of such a monofilament polymer fibre optic cable may be roughened such that a proportion of the radiation falling on the cable is refracted and channelled along the cable by means of total internal reflection.This fibre optic cable may further comprise a means of channelling the radiation to one or more sensors.

The sensors may be photo-sensitive electronic devices such as for example photodiodes or photo transitors. The output signal of the sensors may be an electrical signal for example a voltage or it may be for example a mechanical signal such as a pneumatic or hydraulic pressure.

The Invention will now be described by way of example, with reference to the accompanying schematic diagrammatic drawings in which.

FIG 1 Shows one system FIG 2 Shows a different view of the same system.

FIG 3 Shows another system.

FIG 4 Shows another system.

FIG 5 Shows yet another system.

With reference to FIG 1 and FIG 2 a plasma cutting torch (1) mounted on an adjustable slide (2) above a workpiece (8) is surrounded by a ring (3) made of perspex or some other light transmitting material. The internal surface (4) of the ring is arranged to have the characteristic that it refracts radiation falling on it such that a proportion of the radiation travels around the ring by total internal reflection.

In this embodiment of the Invention the ring is formed from a light transmitting material with the Internal surface knurled or roughened which provides the required refracting characteristic. The ring is provided with a sensor (5) which collects a proportion of the radiation travelling around the ring. It is preferred that the sensor (5) be a photosensitive electronic device such as a phototransistor or a photo-diode. A radiation beam (6) from the plasma cutting torch (1) is refracted by the internal surface (4) of the ring (3) and travels around the ring (3) by total internal reflection to be detected by the sensor (5). There may be more than one sensor disposed about the ring at intervals. The signal from the sensor (5) is used to control means whereby the slide is adjusted thus adjusting the height of the torch above a workpiece in response to the sensor signal.

In the system of FIG 3 a plasma cutting torch (1) mounted on an adjustable slide (2) above a workpiece (8) is surrounded by a ring. The ring (3) is arranged to have a refracting internal surface (4) as previously described. An optic cable (7) is mounted in the ring in order to transmit to the sensor (5) a proportion of the radiation travelling around the ring by total internal reflection. A typical radiation beam (6) is shown.

It can be seen that in this and the previous embobiment the ring forms an element which collects a proportion of the total radiation falling on it which in turn is a proportion of the total radiation emission in various directions from the source of the radiation, As before the output signal of the sensor (5) is used to control means whereby the height of the torch (1) above the workpiece (8) is adjusted.

In the system of FIG 4 a plasma cutting torch (1) mounted on an adjustable slide (2) above a workpiece (8) is surrounded by a ring (10) which supports one or more turns of unsheathed optic cable (9). The optic cable used in this embodiment of the invention is preferably a monofilament polymer fibre optic cable.

The turns of unsheathed optic cable comprises an optic element which collects radiation from the source, in this case a plasma cutting torch. A typical radiation beam (2) is shown. A further length of the same optic cable with the sheath intact referred to in FIG 4 as sheathed optic cable (11) transmits radiation to a sensor (5). When the optic cable ismonofilament polymer optic cable the surface of the unsheathed portion of the cable may be roughened in order to enhance the refracting qualities of the surface. Again the output of the sensor (5) is used to control means whereby the height of the torch (1) above the workpiece (8) is adjusted.

In the system of FIG 5 a plasma cutting torch (1) mounted on an adjustable slide (2) above a workpiece (8) is surrounded by a ring (10). The ring is placed some way from the region of radiation emission. The ring (10) has provision to contain one or more turns of unsheathed optic cable (9). The optic cable is protected by a transparent cover (14). The torch is further provided with a shroud (12) which extends over the ring (10) onto the region where the radiation is being emitted At the lower end of the shroud (12) the internal surface (13) is arranged to be reflecting so as to comprise a mirror which reflects a proportion of the radiation emitted by the plasma stream and directs such radiation onto the unsheathed optic cable. A further length of the same optic cable with the sheath intact referred to in FIG 5 as sheathed optic cable (11) transmits radiation to a sensor (5). When the optic cable is a monofiiament polymer optic cable the surface of the cable may be roughened in order to enhance the refracting qualities of the surface. The output of the sensor (5) is used to control means whereby the height of the torch (1) above the workpiece (8) is adjusted.

Claims

1. A sensing element for a position control or measurement apparatus for a working element in which a working process emits light in degree generally dependant upon the separation of the working element from a workpiece, in which the sensing element, which is generally in the form of a rod or ring, has a surface such that a proportion of the total radiation falling on the surface is reflected and/or refracted along the length of the rod or ring and this radiation is conveyed along the rod or ring to an energy sensor or sensors.

2. A sensing element as in claim 1 in which the working element is a plasma cutting torch or other device in which an electric arc passes between the working element and the workpiece and there is light emitted from the arc.

3. A sensing element as in claims 1 and 2 in which the sensor is generally in the form of a ring which is positioned such that the electric arc is on or near the axis through the centre and perpendicular to the plane of the ring.

4. A sensing element according to any of the above claims, being made of an optically transparent material having a surface treated so as to cause light falling on it to be reflected and/or refracted such that some of the light passes to the other surfaces at such an angle that it continues to travel along the rod or ring by the mechanism of total internal reflection.

5. A sensing element as in claim 4 in which a sensor is linked to the rod or ring by fibreoptic cable such that a proportion of the light travelling along the rod or ring passes via the fibre-optic cable to the energy sensor.

6. A sensing element according to any of the above claims in which the sensing element consists of a fibre-optic cable in which the surface is treated such that a proportion of the light falling on the surface is reflected and/or refracted and passes along the cable to the energy sensor.

7. A sensing element as in the above claims in which one or more energy sensors are placed in the rod or ring so as to sense a proportion of the energy travelling along it.

8. A sensing element according to any of the above claims but which is not directly exposed to the source of radiation but receives radiation via other reflective and/or refractive elements.

9. A sensing element comprising of a multiple or any combination of elements according to the above claims.

1 0. A sensing element in accordance with any of the above claims in which a rod may be other than straight and a ring other than circular but which in any case is of such a shape as to permit the passage of light along its length.