IMPROVEMENTS IN DATA DERIVATION FROM MOVABLE SURFACES

Abstract: Positional and/or movement information concerning a movable member (4) can be derived by using an implanted code pattern (17) which is implanted into a surface layer of the movable member (4). The code pattern (17) is selectively readable by a reader (9) which transmits signals to computer means (20) for calculation of the positional and/or movement information in relation to a given datum.

Description: IMPROVEMENTS IN DATA DERIVATION FROM MOVABLE SURFACES FIELD OF THE INVENTION BACKGROUND OF THE INVENTION This invention relates to improvements in apparatus and method for deriving data from surfaces and relates particularly to a method and apparatus for deriving data from the surface of a moving work piece. The invention will be described with particular reference to its application in respect of linear and rotary actuators and transducers. However, it will be appreciated that the invention may be applied to any moving and movable member from which data is to be derived. Linear and rotary actuators comprise a major group of working devices which carry out useful work by relative movement of a load. Such actuators typically comprise hydraulic and pneumatic cylinders, solenoids and other electromagnetic devices, motors and the like. In many cases, such actuators are required to be monitored so that their exact positional status or position is known. The positional data may be used to allow control of the actuator or its function or for reporting or for feedback purposes. For example, it is often necessary in an actuator for a robotic to measure or know the precise location of the actuator at any given time so that the robot has "knowledge" of its positional status. In many cases, particularly with hydraulic actuators, the positional status of the actuator is determined by measurement of hydraulic fluid flow to and/or from the actuator. This measurement method, however, is subject to error and, therefore, often requires the use of constant recalibration when a actuator reaches one or other of its movement limits. DESCRIPTION OF PRIOR ART It is known to mark actuators with physical markings which are able to be read or otherwise sensed to provide an indication of the actuator location or position. Such markings may be etched into the surface of a working part of the actuator, such as the cylinder rod of a hydraulic or pneumatic cylinder. The etching is then able to be read by some form of optical reader. However, etched markings physically damage the surface and give rise to problems of wear both on the etched surface and any other bearing or sealing surfaces in contact therewith. Actuators are often provided with a marking or other physical attribute which allows the setting of predetermined limits for actuator movement. These arrangements provide very limited "ON/OFF" condition sensing when a predetermined position is achieved. Analogue signal generators produce a signal whose parameter amplitude is a function of displacement. Such generators can provide a technique for measuring the position of an actuator with high accuracy. However, such devices are prone to errors by variations in reference signals, particularly due to drifts in signals generated or transmitted to the device or in signals superimposed on an output signal constituting noise. Analogue signal generators are usually of limited length, in the case of linear actuators, by limitations of cost-effective manufacture. Pulse generators are also able to function in association with an actuator to produce a pulsed or cycled output, each pulse or cycle in the signal being a function of or being proportional to the actuator movement. Such devices can provide accurate measurement of movement but a major disadvantage is that the output is incremental. Further, should an event occur, such as a signal interruption, recalibration of the device must occur to restore the ability to determine an absolute position from a datum. In producing readable markings which can be used for pulse generation, techniques are used including etching of a moving surface, printing, use of laser beam energy to alter magnetic properties of selected zones on magnetic material, and the like. Such techniques, however, are not always possible on certain types of actuators and are prone to wear, distortion of the actuator and other difficulties. Positional information may also be derived using encoders of which rotary types are known. Such encoders may include linear scale sets, in which for each determined amount of movement a different pattern of code is presented to a fixed reader device. For linear devices, the Gray Scale is known, the Scale being a binary code in which, for each increment of one unit, only one character in the code pattern is changed. This allows for checking of the device integrity during movement. Bar Code is also known and can be applied to provide markings to represent numbers or characters, with, again, the ability to check for integrity of the information during use. The known devices suffer drawbacks particularly with linear actuators, and also with actuators of other forms, in providing suitably accurate, non-degradable, readable markings or other species which do not inhibit or in any way affect the normal operation of the actuator. It is known to mark a bright metal surface with a laser beam pulse. International Patent Application No PCT FI93/00433 (Publication No WO 94/11146) discloses laser marking of a surface with overlapping pulses of a laser beam which produces a change in colour of the surface material. Such a change in colour results from a change in the surface structure which is undesirable in actuators. It is also known to use high energy beams, such as a laser beam, for providing security and identification codes on glass, plastic and other materials. German Patent No DE 4126626- A- 1 discloses the use of a high energy beam converging under the surface of a material which is transparent to the beam, such as plastic or glass, to form an opaque region by ionizing the material at the point of convergence. It is also known to use a laser beam to provide bar code information for nuclear fuel rod identification. United States Patent No 4822987 discloses a two- stage fuel rod identification system where a bar code is applied directly on the fuel rod cladding tube by use of a laser beam. However, the bar code is effectively engraved into the surface of the tube thus destroying surface integrity. It is therefore desirable to provide a method of marking an actuator in such a way as to maintain surface integrity and yet provide markings which are able to be sensed or read so as to provide positional information in relation to the actuator. It is also desirable to provide an improved method and apparatus for deriving data from an actuator which obviates at least some of the problems of known methods and apparatus as outlined above. It is also desirable to provide improved apparatus for deriving data in relation to an actuator movement which is relatively simple and economical to implement. SUMMARY OF THE INVENTION It has been discovered that it is possible to embed data in the form of indicia, markings or the like within a surface layer of a member without measurably altering the surface dimensions or surface structure. One form of implantation technology uses high energy beams of one or more selected elements, or a selected isotope. The beams are directed at the surface and the elements penetrate the surface of the target material to a depth of approximately 200 molecules. The ion beam implantation technology allows the selected atomic species to be accurately placed within the surface layer of most materials whereby the implanted material is able to be detected by an appropriate reader. In accordance with one aspect of the present invention there is provided a method for deriving positional and/or movement information in respect of a movable member comprising the steps of implanting a code pattern in a surface layer of the movable member, determining a datum, and selectively reading the code pattern to determine positional and/or movement information therefrom in relation to the datum. The invention also provides a device for deriving positional and/or movement information in respect of a movable member comprising means for detecting a code pattern disposed in a surface layer of the movable member. Preferably, the code pattern is disposed in the surface layer by ion beam implantation. The invention also provides an actuator having a movable member implanted with a code pattern which is disposed in a surface layer of the movable member, the code pattern being able to be read by reader means. In one form of the invention, the cylindrical rod of an actuator is subjected to ion beam implantation along its length whereby the implantation provides "bar codes" at one millimeter intervals. The implantation is effected by bombarding the surface of the rod with high energy beams of tungsten or carbon, the rod being formed of a chrome steel. The ion beam bombardment implants atoms of the selected elements, i.e. tungsten or carbon within a surface layer of the rod up to approximately 200 molecules deep. The implantation is controlled insofar as the width and length of each code pattern marking is concerned to produce a marking which is able to be read by an appropriate reader. If the reader is an optical reader, the elements implanted will generally be carbon or other material which will absorb incident light. In the case of a ceramic rod, a magnetic reader may be utilized and the ceramic rod embedded with iron or cobalt which will exhibit magnetic properties. Although ion beam implantation is one method of performing the invention, other methods for achieving a doping of the surface layer of a substrate with a species of readable markings or pattern are also included within the scope of this invention. For example, the rod of a linear actuator may be formed of a plurality of co-axial cylinders one of which contains, is formed with or exhibits a code pattern in a readable form. In a further form of the invention, the code pattern may be formed by implanting or embedding, or otherwise dosing the surface layer of an actuator with a radioactive isotope. This enables the code to be detected and read by radiation detectors. The use of radioactive isotopes in bearing surfaces, such as the piston rod of an actuator cylinder, can also allow the detection of wear in the bearing surface when evidence of the isotopes in a lubricating oil, or actuating oil, is detected. In one particular embodiment of the invention, ion beam implantation provides a marked pattern code set on a moving member of an actuator which code set is detectable by an optical reading device either fixed in relation to the actuator or moving relative thereto. The reading device produces an output signal code which provides an indication of the code set read on the moving member. As the implanted markings are permanent and are not prone to wear by movement of the actuator rod through a bearing or seal, and as there is no alteration in the surface profile of the marked member, such as would risk wear to the seal and bearing, an actuator in accordance with the present invention avoids disadvantages of known actuators which use etched rods or have surfaces otherwise marked. Where the actuator comprises, for example, a linear hydraulic cylinder rod having a hard chrome plates surface layer, the selected element for ion beam implantation is preferably carbon. The reader may be an optical, matching single or multi row array reader with an in-built illumination source. With such a reader, each sensor is able to discriminate the presence of light absorbed or reflected from the code pattern presented. With this form of the invention, tests have shown the capability to apply a code set to actuators of up to 6 meters which is able to provide a measurement accuracy of +/- 0.01 mm and better using multi array reader units utilizing CCD's (Charged Couple Devices). Because of the inherent accuracy of measurement of the apparatus of the invention, it may be necessary to incorporate temperature compensation into the calibration and calculations. In order that the invention will be more readily understood and put into practical form, embodiments thereof will now be described with reference to the accompanying drawings. DESCRIPTION OF THE DRAWINGS Fig. 1 is a side elevational view of a linear actuator in accordance with the invention; Fig. 2 is a schematic view of an actuator rod having an embedded marking readable by an analogue reader; Fig. 3 schematically illustrates a control system incorporating the actuator of Fig. 2; and Fig. 4 illustrates a modified form of actuator in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the specific embodiments illustrated relate to linear actuators, it will be understood that the invention can be applied to actuators of any type as well as to transducers where the position or movement of a movable member needs to be determined. In the embodiment shown in Fig. 1, a hydraulic cylinder 1 comprises a cylinder body 2 in which a piston 3 drives the actuating rod 4 when working fluid, i.e. hydraulic fluid or pneumatic fluid, is directed into either of the ports 5 and 6. Such actuators are used in a large number of different types of applications including aeronautical, robotic, machine tool, production line and related applications. In a large number of those applications, it is essential that the exact position of the cylinder rod 4 relative to the cylinder body 2 is known. In accordance with the present invention, the rod 4 is marked by the use of ion beam implantation along the movement axis (and possibly also in other directions) with a pattern of markings 17 which provides a code for a code reader 9 to determine an attribute of the position and/or movement, velocity or acceleration of the rod 4. In the illustrated form, the pattern is marked at set increments along a surface layer 21 of the rod, for example, 1 mm, and each mark is of a code suitable for the data being sought, for example, Binary Gray Scale for full Scale displacement measurement of each 1 mm of movement. The ion beam implantation is effected with a high energy beam of a selected element which, in the present embodiment, is carbon. The implantation is effected by focusing the ion beams below the surface of the rod so that the bombarded ions are implanted in the surface up to a depth between approximately 50 and 200 molecules. The implantation enables extremely accurately placed markings to be made in the surface layer of the actuator rod 4 at the desired spacings. It will be understood that the rod 4 moves outside the cylinder body 2 to undertake useful work. This requires the provision of seals 7, to prevent leakage of the working fluid, and of a support bearing 8 to hold the rod 4 against lateral forces and to assist in low friction for the rod movement. The marking 17 is of itself of a suitable atomic species that will allow detection by the reader 9. For example, carbon may be used as the marking element in a steel rod which allows the use of an optical sensor which can detect the dark carbon lines. In other applications, a magnetic sensor may require the use of a specific magnetic element implanted near the surface of a non-magnetic material, such as ceramic. Radioactive sensors and radioactive elements may also be used in the performance of the present invention. Referring again to Fig. 1, to determine the position of the rod 4, that is, to read the coded pattern 17 on the rod 4 from a fixed position in respect to the cylinder body 2, a reader device 9 is inserted into a cavity 12 in the reader bearing housing 10 at the rod end of the cylinder body 2. This cavity 12 may be in the housing 10 itself or mounted externally by a coaxial, slide-on member (not shown) which engages with the cylinder body 2 and is coaxial with the rod 4. The position of the cavity 12 determines a datum in relation to which position of the rod 4 is compared. The reader device 9 consists of a lens or optical fiber array 11 which presents light reflected from the light surface 18, as will occur when the surface 18 is illuminated by a light source 14. The light is reflected to the sensor array 13 which provides a coded signal set output representative of the pattern 17 presented to the reader 9. The output signals of the sensor arrays 13 may be sent either directly from the reader 9 through the outlet plug 15 or to an electronic or optical processing system 16 which is able to generate other forms of output transmitted through the outlet plug 15. The outlet signal may comprise a digital signal data string which is able to be processed by suitable computer means 20, a switched output, or set of switched outputs, which may indicate limits switch outputs, an analogue signal output or an optical signal output. The output signals are totally dependent on the reader device used and form no part of this invention. Referring to Figs. 2 and 3, there is schematically illustrated an embedded coded pattern comprising markings 31 embedded in the surface layer and having a length which increase from one end 32 of a cylinder rod 33 to the other end 34 of the rod. The markings 31 are able to be read by an analogue reader 35 located in a bearing housing 36 to provide an accurate indication of the position of the rod 33. The reader 35 generates an output signal which is transmitted to the computer means 38. The output signal is a function of the lengths of the specific markings 31 which are read by the reader 35 and this signal is therefore directly indicative of the locational position of the cylinder rod 33. The computer means 38 which is pre-programmed to cause the cylinder rod 33 to perform a specific function, and the signals from the reader 35 enable the computer means 38 to control the hydraulic valve 39 which controls the supply of hydraulic fluid to one or other end of the hydraulic cylinder 37. The computer means 38 is able to receive inputs 40 to vary its function in accordance with any desired parameter, including work done by the cylinder rod 33. Referring to Fig. 4, there is illustrated a form of cylinder rod 43 which is formed of an inner rod 44 on the surface of which is printed or otherwise marked with a code pattern 45. The inner rod 44 is then encased within a sleeve 46, such as by shrink-fitting. The casing sleeve 46 being of a thickness or material that the markings 45 are able to be read through the casing. The casing sleeve may be of a bearing material, such as chrome steel or the like and the inner rod 44 may be formed of any other material suitable for the purpose for which the cylinder rod 43 is required. In this embodiment, the code patterns 45 may be marked on the surface of the inner rod 44 in any suitable manner, including printing and the like. The markings 45 may each be a simple line, the lines having a predetermined spacing or they may each be distinctive bar codes. With the individual line markings, positional data is determined by factors such as the number of lines counted, the length of the line or the like. Line length measurement will provide a specific determination of position without the need for counting or the like. Bar code information in individual lines can also provide specific positional information without the need for counting and can also be used for providing other information, such as angular movement of the rod 43 in addition to linear movement. It will be understood that an actuator whose movable member must pass through a seal and/or bearing system requires any marking thereon to be totally unaffected by or unable to cause effect to the bearing or seal and, as such, the present invention provides a unique actuator and reader. The invention is suitable for actuators having the moving members thereof formed of metal, ceramic or other materials which are able to be doped or have embedded therein the marking or code material. It will also be understood that a reader may read codes other than positional or movement codes. Thus, other information may be embedded in the codes and readers may output information concerning actuator manufacturers, messages concerning wear, repair and maintenance information and any other form of message which is desired to be communicated. Such actuator cylinders may comprise intelligent cylinders where the information stored by the implanted markings is in the form of a program which can be read by the reader and processed to cause the actuator to perform the specific function as set out in the program. Such programmable actuators may also be used in conjunction with programming means whereby the coded markings are able to be selectively changed as desired or as pre-programmed in a computer system.

Claims: CLAIMS: 1. A method of deriving positional and/or movement information in respect of a relatively movable member comprising the steps of implanting a code pattern into a surface layer of the movable member, determining a datum, and selectively reading the code pattern to determine positional and/or movement information therefrom in relation to the datum. 2. A method according to claim 1 wherein said step of implanting a code pattern includes the step of using high energy ion beams of one or more selected elements or isotopes directed at the surface of the movable member to implant said ions in the surface layer. 3. A method according to claim 1 wherein said code pattern is implanted in the surface layer up to a depth of 200 molecules. 4. A method according to claim 1 including the steps of directing a light source at said surface and using an optical array reader to read the pattern of reflected light. 5. A method according to claim 1 including the step of implanting a bar code pattern into the surface layer to provide a direct output of positional information coded into the bar code. 6. A method according to claim 1 including the step of implanting a magnetically readable code pattern into the surface layer of a non-magnetic movable member, and magnetically reading the code pattern. 7. A method according to claim 1 including the step of providing a code pattern reader adjacent the code pattern on the movable member, and transmitting signals from the reader to computer means to calculate positional information from the reader signals. 8. Apparatus for deriving positional and/or movement information in respect of a movable member comprising a marking of a code pattern in a surface layer of said movable member, reader means for selectively detecting said marking, and calculation means to calculate the positional and/or movement information from the detected code pattern. 9. Apparatus according to claim 8 wherein said marking is embedded in the surface layer of said moveable member up to a depth of 200 molecules. 10. Apparatus according to claim 8 wherein said marking comprises a bar code pattern implanted in the surface layer by ion beam implantation. 11. Apparatus according to claim 8 wherein said movable member is a cylinder rod of a hydraulic or pneumatic cylinder and said marking comprises an axially extending array of circumferentially extending and axially spaced lines. 12. Apparatus according to claim 11 wherein said lines vary in length along the length of the array. 13. Apparatus according to claim 8 wherein said reader means comprises an optical reader and includes a light source. 14. Apparatus according to claim 8 wherein said markings are formed of a magnetic material, said movable member is substantially of non-magnetic material and said reader means comprises a magnetic reader. 15. Apparatus according to claim 8 wherein said movable member comprises an inner member and an outer sleeve, said markings comprising a code pattern printed or otherwise formed on the surface of said inner member, and said reader means being able to read the markings through said sleeve. 16. Apparatus according to claim 8 wherein said calculation means comprises computer means to compute positional information from output signals of said reader means. 17. An actuator have a movable member implanted with markings of a code pattern disposed in a surface layer of said movable member, said markings being adapted to be read by reader means to derive positional and or movement information in respect of said movable member in relation to a predetermined datum. 18. A method of deriving positional and/or movement information in respect of a relatively movable member comprising the steps substantially as hereinbefore described with reference to the accompanying drawings. 19. Apparatus for deriving positional and/or movement information in respect of a movable member substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: DONALDSON JOHN HEDLEY, DONALDSON, JOHN, HEDLEY
Applicants: DONALDSON JOHN HEDLEY, DONALDSON, JOHN, HEDLEY
Application year: 1995
Main classification: G06K1
IPC codes: G06K1