WO2005083364A2

WO2005083364A2 - Position detecting system

Info

Publication number: WO2005083364A2
Application number: PCT/GB2005/000688
Authority: WO
Inventors: Frederick Mark Hudman; Derek John Rimmer
Original assignee: Newall Measurement Systems Limited
Priority date: 2004-02-24
Filing date: 2005-02-24
Publication date: 2005-09-09
Also published as: GB2411475B; GB0404100D0; GB2411475A; WO2005083364A3

Abstract

A position detecting system (10) is described, for example for use on a machine tool to monitor the position of the tool head relative to a workpiece. The system (10) comprises a number of position detectors (D1, D2, D3…..) each generating information about the position of the tool head along, or relative to, an axis. The position detecting devices are linked by a communications network (12). Each position detecting device receives information from each other position detecting device. Processing means associated with each position detecting device adjusts the positional information generated to take account of any changes detected by the other position detecting devices. Preferably, the system also includes a number of sensors (S1, S2, and S3….) for sensing environmental changes and linked to the communications network (12). In this way, each position detecting device also receives information from the environmental sensors and the positional information can be adjusted to take account of environmental changes.

Description

POSITION DETECTING SYSTEM

The present invention relates to a system to measure the position of an object within a 1 or 2 dimensional or volumetric (3 dimensional) space. The system is particularly suited, but not limited, to machine tool applications in which it is desired to detect the position of a tool head relative to a workpiece. Various types of position detectors are known which measure position along an axis. Many measure position relative to a datum point which must be established each time the detector is switched on. Such detectors can only detect position along one axis relative to the datum which has been set. GB 2377497 describes an inductive position detector having a sequence of detectable reference markers fixed within the device itself. This allows the detector to recognise its position along an axis according to the pattern of reference markers that it detects. Thus, a datum is effectively set within the device itself reducing the possibility of errors in setting the datum and increasing the efficiency of the device in use. However, this position detector still detects position only along a single axis and takes no account of movements on any other axis.

Additionally, with any type of position detector, errors can be introduced due to environmental factors such as temperature. A position detector such as the type described in GB 2377497, which includes an elongate magnetic element, will suffer expansion and contraction with temperature in the order of 12 parts per million/k. Thus for each degree temperature change a 12μμm difference in length can occur. In a typical machine shop environment the temperature may vary by several degrees over the course of a working day and therefore errors of say 40μμm can easily occur, which is simply not acceptable for high precision machining operations.

The present invention provides a position detecting system for detecting the position of an element, comprising a plurality of position detecting devices, each generating information about the position of the element along or relative to an axis, a communications network operable to link the position detecting devices whereby each position detecting device receives information from each other position detecting device, and processing means associated with each position detecting device operable to adjust, the positional information generated by the position detecting device to take account of any changes in position detected by the other position detecting devices prior to outputting the positional information.

In accordance with this system, the position detecting devices can continuously self calibrate and correct for changes in position along the other axes too in order to provide improved volumetric compensation and provide information about the position of the element with extremely high precision.

Preferably, the system includes three position detecting devices arranged to detect position along three mutually perpendicular axes. In this way, an element's position within a three dimensional volumetric space can be determined.

In a preferred embodiment, a plurality of sensors for sensing environmental changes are also provided, wherein the communications network is operable to link the position detecting devices and the sensors, whereby each position detecting device also receives information from each sensor and is operable to adjust the positional information also to take account of any sensed environmental changes

Most commonly, the sensors will be arranged to detect one or more of temperature, humidity and vibration. Additionally, sensors may be provided to detect one or more of forces acting on the element, the dimensions of the element, and/or the hardness of a body contacted by the element .

A sensor may also be provided to sense whether an auxiliary device, such as a cooling or heating device, is operating or not .

The communications network linking the sensors and the position detecting devices may consist of a physical link or a non-physical link such as an infrared (IR) or radio frequency (RF) link.

Each position detecting device may comprise an inductive position detector comprising a first member having a longitudinal axis and a body of magnetic material extending in the direction of the longitudinal axis and having a periodically varying dimension in the direction perpendicular to the longitudinal axis, and a second member movable relative to the first member along the longitudinal axis and comprising means to induce a magnetic field in the body of magnetic material, wherein at least one detectable reference marker is provided on the first member distinct from the body of magnetic material , and at least one marker detector is provided in the second member.

Furthermore, preferably a plurality of markers are provided which are positioned in a sequence comprising a series of segments of predetermined length, in each of which segments the arrangement of markers is unique. It is further preferable if the number of detectors provided is sufficient to detect the arrangement of markers in any one segment without relative movement between the detectors and the markers .

Position detecting devices of this type are particularly suitable for the system of the present invention due to their precision and innate processing capability

The present invention also provides a machine tool incorporating a position detecting system of the type described above.

In this case, the element may be the cutting head of the machine tool . In another aspect, the present invention also provides a method for detecting the position of an element, comprising the steps of detecting the position of an element along or relative to each of a plurality of axes with a plurality of position detecting devices to generate positional information, communicating the positional information from each position detecting device to each other position detecting device, adjusting the positional information produced by each position detecting device to take account of the positional information from each other position detecting device and generating an output representative of the position of the element.

The method further comprises detecting one or more environmental changes, communicating information concerning any environmental changes to each position detecting device and adjusting the positional information obtained from each position detecting device also to take account of any sensed environmental changes .

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a position detecting system in accordance with the present invention; and Figure 2 illustrates one example of a position detector for use in the system of the present invention.

A position detecting system 10 in accordance with one embodiment of the present invention is shown schematically in Figure 1. As described in more detail below, the system

10 comprises a number of position detecting devices Dl, D2 , D3.... a communications bus 12 and, as a preferred embodiment, also a number of environmental sensors SI, S2 , S3....

Preferably three position detecting devices Dl, D2 and D3 are provided, arranged to detect position along three mutually perpendicular axes X, Y and Z. Each detector Dl- D3 may be linked to, for example, a cutting head of a machine tool in order to measure its movement along one axis .

A plurality of environmental sensors SI, S2 and S3... are preferably also provided to monitor on any environmental factors which may affect the accuracy of the positional information generated by the position detecting devices Dl- D3. These sensors may principally be temperature sensors on various parts of the machine, including for example sensors on the tool head itself, on the workpiece itself and/or on the position detecting devices themselves. Sensors may also be provided to monitor whether some auxiliary device such as a coolant pump is operating or not, as many machine tools are provided with a pump to supply coolant fluid to the tool and workpiece. This will result in the temperature of the tool and workpiece being significantly lower than other parts of the machine, or coolant fluid may wash over the detectors themselves altering their temperature. Thus, sensors may be specifically targeted to monitor whether the coolant pump is operating, as well as monitoring temperature at specific locations, which may include the coolant itself.

Sensors may also be provided to monitor factors such as humidity, vibration, the hardness of the workpiece, forces acting on the tool head and any resultant deflection of the tool head, changes in tool dimensions, e.g. due to wear, whether small or large cuts are being taken from the workpiece and so on. This list is by no means exhaustive and sensors may be provided to monitor any environmental factor of concern.

The detectors Dl, D2 , D3... and environmental sensors

SI, S2 and S3... are networked together by a communications bus 12. Each sensor sends information to the bus 12, either continuously, or at intervals, or whenever there is a change in the environmental factor being monitored.

Each detector is in two way communication with the bus 12. Thus, each detector sends its own positional information to the other detectors via the bus 12 as well as receiving information from each other detector. In this way, each position detecting device is able to continuously self calibrate to take account of the information it receives from the other detectors to ensure that the positional information it generates is accurate as possible in light of any positional changes detected by the other detectors . In addition, when environmental sensors SI, S2 , S3 are provided, each detector Dl, D2 , D3 also receives information from each sensor via the bus 12. Therefore, each detector is able to continuously self calibrate to take account of the information it receives from the sensors, as well as the other detectors. In this way the positional information it generates is as accurate as possible in the light of both positional and environmental changes which may occur.

The positional information output from each detector can then be provided, in any desired format, via the bus 12 or other communications protocol to an external device such as the control system of a CNC machine tool, which may use the information to control the movement of the tool head. Although Figure 1 illustrates the detectors and sensors networked via a communications bus providing a physical link, the link between them need not be a physical, hardwired one. Communication by non-physical means, such as IR or RF is equally possible.

By way of example only, each position detecting device D may be an inductive position detector of the type described in GB 2377497. This is illustrated schematically in Figure 2. The detector D consists of a fixed scale 14 and a transducer 16 which encircles the scale

14 and is movable along its length. The scale 14 consists of a tube 18 of non-magnetic material housing a train of magnetic balls 20 in point contact and constrained against relative movement . A series of detectable reference markers 22 is provided in the tube 18, each aligned with a point of contact between a pair of balls 20, although not all pairs of balls 20 have a corresponding marker 22. The markers 22 are thus provided in a predetermined pattern along the scale 14, made up of a plurality of unique pattern segments. The transducer 16 carries one or more sensors 24 to detect the reference markers 22, as well as transmission and pick-up coils (not shown) . The transmission coils are used to induce a magnetic field along the line of contact of the balls 20 and the pick-up coils detect variations in the magnetic field as they are moved relative to the balls 20. Detector circuitry within the transducer 16 analyses the signalling to indicate, with high precision, the position over the pitch of one ball 20 in the scale. At the same time, the sensor (s) 24 recognise (s) which segment of the pattern and markers 22 it is detecting, to provide a relatively low precision indication of where it is along the scale 14. This is combined with the high precision information obtained from the transmission and pick-up coils to provide an overall high precision of measurement of position.

The particular advantage of employing position detectors of this type in the system of the present invention is that the transducers 16 already possess a high degree of processing capability in order to deal with information from the pick-up coils and reference markers sensors 24. Therefore, it is relatively straightforward to provide the slightly increased processing power necessary also to deal with information from the environmental sensors and other position detectors in the network, and to self calibrate the device based upon this information.

However, the invention is not limited to this type of detector and any type of position detector, for example optical types, could be employed in the present invention provided they are adapted to have the necessary processing power. This adaptation may be providing separate processing means for each position detector, or processing means associated with more than one detector. Use of the system of the present invention allows the position of an element within a working volume to be detected as accurately as possible independent of changes in environmental conditions. This leads to high accuracy, reliability and repeatability. Although the system has been described with reference to machine tools and particular types of position detectors, these are merely examples and many other ^• applications and embodiments of the system are possible.

Claims

1. A position detecting system for detecting the position of a element, comprising a plurality of position detecting devices, each generating information about the position of the element along or relative to an axis, a communications network operable to link the position detecting devices whereby each position detecting device receives information from each other position detecting device, and processing means associated with each position detecting device operable to adjust the positional information generated by the position detecting device to take account of any changes detected in position by the other position detecting devices prior to outputting the positional information.

2. A position detecting system as claimed in claim 1, comprising three position detecting devices arranged to detect the position along three mutually perpendicular axes.

3. A position detecting device as claimed in claim 1 or claim 2, further comprising a plurality of sensors for sensing environmental changes, and wherein the communications network is operable to link the position detecting devices and the sensors, whereby each position detecting device also receives information from each sensor and is operable to adjust the positional information also to take account of any sensed environmental changes .

4. A position detecting device as claimed in claim 3, wherein the sensors are operable to detect one or more of temperature, humidity and vibration.

5. A position detecting system as claimed in claim 3 or claim 4 , wherein the sensors are operable to detect one or more of forces acting on the element, the dimensions of the element, and the hardness of a body contacted by the element .

6. A position detecting system as claimed in any of claims 3 to 5, wherein one of the sensors detects whether or not an auxiliary device is operating.

7. A position detecting system as claimed in claim 6, wherein the auxiliary device comprises a cooling device.

8. A position detecting system as claimed in any of claims 3 to 7 , wherein the communications network comprises a physical link between the position detecting devices and the sensors .

9. A position detecting system as claimed in any of clams 3 to 7, wherein the communications network comprises a non-physical link between the position detecting devices and the sensors .

10 A position detecting system as claimed in claim 9 wherein the communications network comprises an infrared link between the position detecting devices and the sensors.

11. A position detecting system as claimed in claim 9, wherein the communications network comprises a radio frequency link between the position detecting devices and the sensors.

12. A position detecting system as claimed in any preceding claim, wherein each position detecting device comprises an inductive position detector comprising a first member having a longitudinal axis and a body of magnetic material extending in the direction of the longitudinal axis and having a periodically varying dimension in a direction perpendicular to the longitudinal axis, and a second member movable relative to the first member along the longitudinal axis and comprising means to induce a magnetic field in the body of magnetic material , wherein at least one detectable reference marker is provided on the first member, distinct from the body of magnetic material , and at least one marker detector is provided on the second member.

13. A position detecting device as claimed in claim 12, wherein a plurality of detectable reference markers are provided, positioned in a sequence comprising a series of segments of predetermined length, in each of which segments the arrangement of markers is unique, and wherein the number of marker detectors provided is sufficient to detect the arrangement of markers in any one segment without relative movement between the detectors and the markers.

14. A machine tool incorporating a position detecting system in accordance with any preceding claim.

15. A machine tool as claimed in claim 13, wherein the element comprises the cutting head of the machine tool.

16. A method of detecting the position of an element, comprising the steps of detecting the position of an element along or relative to each of a plurality of axes with a plurality of position detecting devices to generate positional information, communicating the positional information from each position detecting device to each other position detecting device, adjusting the positional information produced by each position detecting device to take account of the positional information from each other position detecting device and generating an output representative of the position of the element.

17. A method as claimed in claim 16, further comprising detecting one or more environmental changes, communicating information concerning any environmental changes to each position detecting device and adjusting the positional information produced by each position detecting device to take account of any sensed environmental changes prior to generating the output.

18. A position detecting system substantially as hereinbefore described and with reference to the accompanying drawings .

19. A method of detecting the position of an element substantially as hereinbefore described and with reference to the accompanying drawings .