WO2018033704A1

WO2018033704A1 - Inspection apparatus and a method of operating an inspection apparatus

Info

Publication number: WO2018033704A1
Application number: PCT/GB2017/052353
Authority: WO
Inventors: Derek Marshall; Benjamin Jason Merrifield; James Arthur Hartley; Martin Peter SUMMERS
Original assignee: Renishaw Plc
Priority date: 2016-08-16
Filing date: 2017-08-09
Publication date: 2018-02-22
Also published as: TWI659214B; TW201812306A

Abstract

A method of operating a machine tool apparatus, comprising at least first and second spindles and at least first and second respective measurement systems, so as to measure a plurality of sets of nominally identical points on at least first and second nominally identical respective parts located in the machine tool, the method comprising: for each set of nominally identical points in turn, causing a first relative movement between the parts and the measurement systems so as to measure one of the parts using one of the measurement systems and subsequently causing a second relative movement between the parts and measurement systems so as to measure the other of the parts using the other measurement system.

Description

INSPECTION APPARATUS AND A METHOD OF OPERATING AN INSPECTION APPARATUS

This invention relates to an inspection apparatus and a method of operating an inspection apparatus, in particular a machine tool and a method of operating a machine tool.

Computer numerically controller (CNC) machines tools are well known in which a CNC controller has control over multiple machine axes, allowing a cutting tool to come into contact with a workpiece in a specified location within the machine volume. These axes can be controlled by instructions provided in a CNC part program loaded into the machine at the end-user site. Typically, if many nominally identical components are required as part of a mass-production exercise, an identical CNC part program may be run on many identical CNC machines set up in the same way.

In some cases, for efficiency of space on a factory floor, or for other reasons, it is not efficient to operate a single spindle per CNC machine. In these cases, some users prefer CNC machines which have multiple spindles within a single machine. On these machines, multiple spindles may be connected to a single CNC controller system operating so that each spindle on the machine performs nominally identical movements. In some cases, physical axes might be shared. For example, a common configuration consists of a large table controlled by X, Y, axes controls. Above the table there are two spindles, both controlled by the machine' s "Z" axis. This allows a single part program to cut two identical workpieces simultaneously.

In this configuration it is common to allow a temporary independent offset for each spindle in the "Z" direction (which is typically along the axis of the rotating tool) to allow for differences in the assembly of the cutting tool. However, because of the shared X, Y axes it is not possible to provide offsets to the position and rotation of one workpiece independently of another. Accordingly, such spindles are said to be "slaved" to each other.

On CNC machine tools, spindle probing is also commonly used. It is known to use spindle probing, e.g. as a Go / No-Go check and as a way to set machine offsets. In these cases, a program function can be provided by the CNC control manufacturers such that the machine moves until an electrical input is raised by the probing system (e.g. when the probe contacts the workpiece surface and triggers). At the exact point in time when the electrical input is received by the CNC control, the machine control records the position of its axes, and also starts the process of stopping the machine axes. Once the machine has stopped, the machine positions at the time of probe trigger are available in machine variables and can be used by a program for onward calculation, including the application of pre-recorded compensation data unique to the probe, feature dimensional calculations, etc.

The present invention relates to improvements in obtaining measurements of a plurality of parts on a machine tool having multiple spindles, in particular multiple slaved spindles. For example, as described in more detail below, the invention relates to a method of operating a machine tool apparatus comprising at least first and second spindles and at least first and second respective measurement systems. The method can be for measuring a plurality of points (e.g. nominally identical points) on at least first and second parts (e.g. nominally identical parts) located in the machine tool. The method can comprise performing a number of (e.g.

repeated) machine moves, one for each part. Each machine move can comprise driving the first and second parts and the first and second measurement systems relative to each other. Such relative motion of the parts and their respective measurement systems can occur together/simultaneously. The output(s) of the different measurement systems can be used for the different machine moves. According to a first aspect of the invention there is provided a method of operating an inspection/positioning (e.g. machine tool) apparatus, comprising (for example at least first and second spindles and) at least first and second respective measurement systems. The method can comprise measuring a plurality of sets of nominally identical points on at least first and second respective parts (e.g. located in the inspection/positioning apparatus, for example machine tool). The method can comprise for each set of nominally identical points in turn/in succession (e.g. so that the sets of nominally identical points are measured one after the other), causing a first relative movement between the parts and measurement systems so as to measure one of the parts using one of the measurement systems (e.g. a first part may be measured by a first probe). The method can comprise subsequently causing a second relative movement between the parts and measurement systems so as to measure the other of the parts using the other measurement system (e.g. a second part may be measured by a second probe). In this manner, a first point of each set of measurement points (e.g. a first point on the first object) may be collected during the first relative movement and the second point of each set of measurement points (e.g. a nominally identical point on the second object) may be collected during the second relative movement.

In this manner, each set of nominally identical points is acquired in succession. For example, a first set of nominally identical points is collected before a second set of nominally identical points. This method of collecting each set of nominally identical points in turn has been found to be quicker than measuring all required points for a first object and then repeating the same measurements for the second and any subsequent objects.

Accordingly, during the first relative movement a first one of the measurement systems could be said to be active/currently used whereas a second one of the measurement systems could be said to be non-active/not currently

used/suppressed and during the second relative movement the second

measurement systems could be said to be active/used whereas the first measurement systems could be said to be non-active/not currently used/ suppressed. The second relative movement can comprise a repetition of the first relative movement. In other words, the method may comprise repeating relative movement between the parts and measurement systems so as to measure the other of the parts using the other measurement system. In this manner, the first point of each set of measurement points (e.g. a point on the first object) may be collected during the first relative movement whilst the second point of each set of measurement points (e.g. a nominally identical point on the second object) may be collected during the second/repeated movement. Said repeated movement could comprise a nominally identical movement. For example, the same machine instruction could be used to cause said subsequent movement as the initial/first movement.

The first relative movement could comprise movement of the parts towards the measurement systems. The second relative movement could comprise movement of the parts away from the measurement systems. In this manner, the first point of each set of measurement points may be collected during motion of the

measurement systems towards the parts whilst the second point of each set of measurement points may be collected during motion of the measurement systems away from the parts. As will be understood, both parts and/or both measurement systems can be moved together/simultaneously, during said moves (e.g. during the initial and

subsequent/repeated move).

The at least first and second parts can comprise first and second tools, e.g.

mounted in at least first and second tool mounts of the positioning/inspection apparatus, for example at least first and second spindles. Example tools include cutting, grinding and/or milling tools.

The at least first and second measurement systems can comprise first and second tool setters. The tool setters may be non-contact tool setters. The tool setters may be contact tool setters. The at least first and second parts can comprise at least first and second workpieces. The at least first and second measurement systems can comprise first and second probes mounted in at least first and second tool mounts of the positioning apparatus, for example at least first and second spindles.

The first and second probes can comprise contact probes comprising a deflectable stylus. The probes could be configured to provide a signal indicative of stylus deflection. The probes could be configured to provide a signal indicative of the extent of deflection (e.g. could be an analogue or scanning probe). The probes could be configured to provide a signal indicating that deflection has occurred (e.g. that a threshold deflection has occurred). For example, the first and second probes may be touch trigger probes. The first and second probes may be of the same type. The first and/or second measurement systems could be configured to wirelessly communicate with a receiver. Separate receivers may be provided to communicate with the first and second measurement systems. Alternatively, the first and second measurement systems may communicate with a common/the same receiver (e.g. a plurality of measurement probes may communicate with a single receiver unit). The first and/or second measurement systems could be configured to

communicate with a receiver via radio signal and/or optical signals. Such optical signals could be visible or non-visible.

The at least first and second measurement systems could both be powered on/active during the moves (e.g. during the initial and repeated moves). The at least first and second measurement systems could both be configured to provide outputs (e.g. stylus deflection signals, such as trigger signals) during the moves (e.g. during the initial and repeated moves). Accordingly, as described in more detail below, a switching (or suppression) method/system can be used to switch between which of the outputs of the measurement systems to use during a move. The machine tool apparatus can comprise a controller configured to receive input from only one measurement system. In other words, the controller could have only one measurement system input. The input can comprise a SKIP signal input. Accordingly, the controller could have only one SKIP input. Optionally, the controller is configured to receive inputs from multiple (e.g. two or more) measurement systems. The controlled could have multiple (e.g. two or more) measurement system inputs. Accordingly, the controller could have multiple (e.g. two or more) SKIP inputs. The method can comprise switching between the measurement systems outputs. This can be such that the output of one of the measurement systems is used for said measurement (e.g. passed to a controller during the measurement) of said one of the parts, and such that the output of the other of the measurement systems is used for said measurement (e.g. passed to a controller during the measurement) of said other of the parts. In other words, the method can comprise switching between the outputs of the at least first and second measurement systems, such that the output of one of the measurement systems is passed to the controller during the initial move, and such that the output of the other of the measurement systems is passed to the controller during said subsequent/repeated move. As will be understood, such switching can operate mutually exclusively. The switch can be effected by the machine tool controller. It should also be noted that the first and second measurement systems may share some common elements. For example, signal processing or conditioning electronics may be provided by a common processing unit (e.g. circuitry for the first and second measurement systems may be housed in the same outer casing or provided on the same circuit board). If the controller comprises more than one SKIP input, the controller could select which SKIP input it uses (e.g. monitors).

The machine tool apparatus can comprise an interface to which the at least first and second measurement systems provide signals indicative of stylus deflection.

The interface could be configured to switch between providing measurement information (e.g. stylus deflection signals, such as a trigger signal) from one of the measurement systems and the other of the measurement systems, to the controller. For example, in line with the above paragraph, the interface can be configured to switch between the outputs of the measurement systems, such that the output of one of the measurement systems causes it to provide a trigger signal to the controller during the initial move, and such that the output of the other of the measurement systems causes it to provide a trigger signal to the controller during said subsequent/repeated move. As will be understood, such switching can operate mutually exclusively. The switch can be effected by the machine tool controller instructing the interface (e.g. via an M-code command).

The interface could be physically integral with or separate from the controller.

If the controller comprises more than one SKIP input, there could be provided one interface for each SKIP input. Each interface could be connected to/in

communication with more than one measurement system. Each interface could be configured to switch between the outputs of the measurement systems.

The machine tool apparatus can comprise a numerically controlled (NC) machine tool apparatus, for example a computer numerically controlled (CNC) machine tool apparatus.

The method can comprise moving at least first and second tool mounts (e.g.

spindles) of the positioning apparatus to cause said relative movement. The method can comprise moving the machine tool's table to cause said relative movement.

The method can comprise suppressing one of the measurement systems during the first (e.g. initial) move and then suppressing the other of the measurement systems during the second (e.g. repeated) move. Such suppressing could comprise, for example, ignoring the output of a measurement system, disabling a measurement system (e.g. the probe and/or its respective receiver), preventing the measurement system (e.g. the probe and/or its respective receiver) from issuing a signal on detection of the part (e.g. on stylus deflection), and/or a switching technique/deice such that measurement system used to signal a controller (e.g. the probe' s output used to issue a SKIP signal to a controller) is switched from one measurement system (e.g. probe) to the other.

The method can comprise monitoring for an output from the non- active/suppressed/not currently used measurement system. The output could be a measurement signal, a part detected signal, for example a trigger signal. For example, the method could comprise monitoring for deflection of the non- active/suppressed/non-currently used probe. The method can comprise taking action in response to such an output, e.g. in response to a determination that the suppressed measurement system has detected something, e.g. triggered, for example, deflected. Such action could, for example, comprise halting motion, and/or issuing an error/warning signal. Optionally, the suppressed measurement system issues a signal at intervals (e.g. predetermined/periodic/regular intervals) which indicates whether or not it has detected something/measured/triggered (e.g. deflected). The controller can comprise fewer signal inputs than the number of measurement systems present. An apparatus of the type described in WOO 1/55670 may be used to allow different measurement signals to be passed to the machine tool controller.

According to a second aspect of the invention there is provided a machine tool apparatus configured to operate in accordance with the method as described above.

Also described herein is a method of operating a machine tool apparatus, comprising at least first and second spindles and at least first and second respective measurement systems, so as to measure a plurality of sets of nominally identical points on at least first and second nominally identical respective parts located in the machine tool, the method comprising: for each set of nominally identical points in turn, causing a first relative movement between the parts and the measurement systems so as to measure one of the parts using one of the measurement systems and subsequently causing a second relative movement (e.g. a repeated movement) between the parts and measurement systems so as to measure the other of the parts using the other measurement system. The method may further comprise any of the additional features described herein. A machine tool apparatus configured to operate in accordance with the method may also be provided. Also described herein is a method for measuring the position of a measurement point on the surface of a part comprising the step of moving a measurement probe having a deflectable stylus relative to the part, wherein the measurement point is measured whilst the measurement probe is being moved away from the part. A machine tool apparatus configured to operate in accordance with the method may also be provided.

Embodiments of the invention will be described with reference to the following figures in which: Figure 1 is a schematic system diagram of a first embodiment of the invention;

Figure 2 is a schematic diagram of nominally identical first and second parts having a plurality of nominally identical points to be measured using the system of Figure 1 ;

Figure 3 is a schematic system diagram of a second embodiment of the invention,

Figures 4a and 4b show a further embodiment of the invention in which measurements are also taken when a measurement probe is moved away from a surface; and Figures 5 to 8 are schematic diagrams showing various system architectures for a machine tool in accordance with the present invention.

As will be understood, measurement probe providers typically provide inspection software with their devices, this software being written within the native language of the CNC controller platform. The software is designed to control the CNC machine tool to take best advantage of the probing system provided by the vendor, when combined with the capabilities of the CNC machine. For spindle probing, one function provided by this software relates to the measurement of features - for example, the measurement of a bore or a line feature comprises the inspection of several measured points and then the combination of those points to calculate the desired parameters of the feature, such as dimensions, offsets from nominal, form error, etc. Multiple features may be combined by the customer part program to represent the result from an entire workpiece e.g. to enable the alignment of a workpiece based on two measured bores.

Within the software another function provides the ability to measure individual probed points. This function in turn is used by the feature-measurement functions.

The point measurement function may support different measurement strategies depending on the capability of the CNC machine controller. E.g. for some CNC controllers a multiple-touch measurement strategy is required to achieve effective cycle time, whereas other CNC controllers do not require this.

The invention relates to a modification of the individual point or feature capture strategy which is specifically designed for use on multiple- spindle machines. In the modification, according to an example embodiment of the invention, the commands that control which input the CNC control is monitoring are embedded in the point capture function, such that the CNC commands required to capture each point are automatically repeated, once for each spindle. The function also switches the probing input to which the CNC control is responding, e.g. by using a SKIP sharing device (such as an interface as described below) or by using a built- in controller function. After each measured point on each spindle, the

measurement position is recorded (by the CNC controller) in a fixed set of CNC variables; the point measurement function can also copy these into known locations for use by feature-calculation or workpiece-calculation functions.

In this way, an individual data point for each spindle probe (mounted onto respective spindles) may be captured, with only the touch moves themselves being repeated for each point. By comparison to a strategy whereby the entire part program is repeated for each probe input, this represents a significant cycle time saving.

Each feature-calculation function must also take account of the fact that multiple sets of probe data are now available. E.g. the measurement cycle may report the average results from each spindle, or the range of results. However, this process remains the same regardless of how the data was captured.

A similar process applies to tool setting whereby the same nominally identical tool is loaded into each machine spindle.

Referring initially to Figure 5, there is shown a positioning apparatus in the form of a machine tool apparatus 100 comprising a machine tool 102, a controller 18, first 12 and second 14 receivers and an interface 16. Optionally a computer, e.g. PC 104, could be provided. The machine tool 102 comprises motors (not shown) for moving first 2 and second 4 spindles which respectively hold first 6 and second 8 measurement probes relative to respective first 20 and second 20' workpieces on a table 11. The location of the first 2 and second 4 spindles (and hence the first 6 and second 8 probes) is accurately measured in a known manner using encoders or the like. Such measurements provide spindle position data defined in the machine co-ordinate system (x, y, z). The controller 18 (e.g. a computer numerical controller "CNC") controls x, y, z movement of the first 2 and second 4 spindles within the work area of the machine tool in accordance with a program, and also receives data relating to the spindle position. The program which the CNC 18 follows to control the machine tool could be an automatically or manually generated program. The program could be generated on computer 104, the controller 18, or could be generated elsewhere and imported into the controller 18, or a combination thereof (e.g. part generated elsewhere and modified on the controller 18).

Figure 1 schematically shows the first 2 and second 4 spindles of the machine tool 102 and the respective first 6 and second 8 measurement probes mounted thereon. In this case the first and second measurement probes are each a contact probe having a deflectable stylus and is configured to issue a stylus deflection signal, e.g. a trigger signal, on deflection beyond a threshold (which could for example be mechanically or electrically determined). The first and second spindles could be slaved together e.g. as described above, such that for example they are fixed and moveable together in at least the x and y dimensions so as to drive the probes into workpieces located on the machine tool's table 11. Optionally the spindles could be held fixed and then the machine tool' s table 11 could be moved (e.g. so as to move workpieces into the probes). Optionally, a combination of spindle movement and table movement is possible.

The first 6 and second 8 measurement probes are in wireless communication with respective first 12 and second 14 receivers (in an alternative embodiment they could be wired). In the embodiment described and shown, separate receivers are provided. However, as will be understood a common/single receiver could be used for receiving signals from multiple measurement systems/probes. The wireless communication could be radio or optical (visible or non- visible) for example. The first 12 and second 14 receivers are connected to/in communication with the interface 16 which is connected to/in communication with a controller 18. The interface 16 relays a signal (e.g. a trigger signal) from the first 6 and second 8 measurement probes to the SKIP input on the controller 18. In this embodiment, the controller 18 has an output (labelled MODE) which can be used to tell the interface 16 whether to use the first 6 or second 8 probe as its source for the SKIP signal.

In use, the first 6 and second 8 probes are used to measure first 20 and second 20' nominally identical workpieces. In particular, the first 6 and second 8 probes are used to measure a plurality of sets (e.g. pairs) of nominally identical points, (e.g. first set of nominally identical points 22, 22', second set of nominally identical points 24, 24', third set of nominally identical points 26, 26' , and fourth set of nominally identical points 28, 28') on the first 20 and second 20' nominally identical workpieces (see Figure 2).

In this embodiment, the controller 18 has fewer SKIP inputs than the number of spindles/measurement systems (e.g. probes). In particular, in this embodiment the controller only has one SKIP input and is configured to stop the machine movement and record the encoder positions on receipt of a SKIP signal. The method according to one embodiment of the invention comprises measuring each set (e.g. pair) of nominal points in turn using a repeated machine movement, with the output of the first 6 and second 8 probes being used sequentially. For example, initially for the first set of nominally identical points 22, 22', the method comprises causing a double, or repeated, machine movement such that on the first machine move one of the workpieces 20, 20' is measured by using one of the first 6 and second 8 probes, and then on the second machine move the other of the workpieces is measured by using the other of the first 6 and second 8 probes. As will be understood, on both of the moves, both of the first and second probes are relatively moved with respect to their respective workpieces (and both may contact their respective workpiece, e.g. at the nominally identical point) but only one of the first and second probes is actually used for measurement on each move.

This could be achieved, for example, by sequentially turning the probes (and or their respective receivers) on and off as required. For example, initially turn the first probe 6 on, perform the move until a SKIP signal is received, turn the first probe 6 off, turn the second probe 8 on and repeat the move. However, such an implementation could be slow, especially if the probes take a long time to warm up. Optionally, the method could be implemented, for example, by the system being configured to suppress one of the measurement systems during the initial move, and then suppress the other of the measurement systems during the repeated move. Such suppression could be achieved for example, disabling the probe and/or its respective receiver, preventing the probe from and/or its respective receiver issuing a signal on stylus deflection, and/or a switching technique such that probe output used to issue a SKIP signal to the controller 18 is switched from one probe to the other. For example, a switching system, e.g. interface 16, could be configured to switch between which probe output is used to issue a SKIP signal to the controller 18.

Accordingly, in one particular example the interface 16 switches between which signals from the probes it uses. For example, with regard to the first set of nominally identical points 22, 22' on the initial move the interface 16 may be configured (e.g. on the basis of the MODE signal from the controller) to only use the stylus deflection signal from the first probe 6 so as to cause a SKIP signal to be received at the controller 18. Accordingly, during said initial move, the machine tool will be configured to drive the first 2 and second 4 spindles together/simultaneously (e.g. because they are slaved to each other) so that the first 6 and second 8 measurement probes are each driven toward their respective workpiece 20, 20' . When a signal is received by the interface 16 which indicates deflection of the first probe' s stylus (because the stylus has been driven into the first workpiece 20), it issues a SKIP signal to the controller 18. The machine tool then stops the movement and records the machine tool's encoder positions so that it can determine the point of measurement of the point 22 to be measured on the first workpiece 20, in the machine tool's coordinate measurement system. As will be understood, during the initial move, the second probe' s stylus may also have been driven into the second workpiece 20', and issued a stylus deflection signal to the interface, but the interface 16 will not pass this on to the controller.

The machine tool then causes the same machine movement to be repeated.

However, this time the interface 16 may be configured (on the basis of the MODE signal from the controller) to only use the stylus deflection signal from the second probe 8 so as to cause a SKIP signal to be received at the controller 18. When a signal is received by the interface 16 which indicates deflection of the second probe' s stylus (because the stylus has been driven into the second workpiece 20') it issues a SKIP signal to the controller. The machine tool then stops the movement and records the machine tool' s encoder positions so that it can determine the point of measurement of the point 22' to be measured on the second workpiece 20' , in the machine tool' s coordinate measurement system. As will be understood, during this repeated move, the first probe's stylus may also have been driven into the first workpiece 20, and issued a stylus deflection signal to the interface, but the interface 16 will not pass this on to the controller 18.

Since the nominally identical points are being measured in turn, once

measurement of both of the first set of nominally identical points 22, 22' are measured, then the machine tool moves on to measure the next set (e.g. pair) of nominally identical points (e.g. the second nominally identical points 24, 24') using the same above described repeated move technique. This is in contrast to using the first probe to measure some or all of the points on one of the first part 20 and then using the second probe to measure some or all of the points on the second part 20' . Although the process of measuring the nominally identical points in turn using repeated moves requires more switches between the first and second probes the cycle time can be significantly reduced by avoiding the need to move around the part multiple times. In the embodiment described above, separate receivers 12, 14 are provided.

However, as will be understood a common/single receiver could be used for receiving signals from multiple measurement systems/probes. For example, as illustrated in Figure 6, both the first 6 and second 8 measurement probes could communicate with the first receiver 12. This could be achieved in various ways, for example, by the probes operating on different frequencies, and/or by using different signal/code indicators. Additionally or alternatively, rather than a physically separate interface 16 being provided between the controller and receiver(s) 12, 14 as described in the above embodiments, the receiver(s), could communicate directly with the controller 18. In this case, for example, the interface could be parts of the controller. Optionally, for example, as illustrated in Figure 6, the receiver 12 could be plugged directly into the controller 18. In this case, the controller 18 could issue a MODE signal to the receiver to inform it which probe/trigger signal it wants to receive a SKIP signal from. Figure 7 shows an alternative embodiment in which the controller 18 has the same number of SKIP inputs as spindles, e.g. in this embodiment two SKIP inputs (SKIPl and SKIP2). However, even though the controller 18 has two SKIP inputs it might be that the controller 18 cannot handle simultaneous SKIP signals. For instance, it might be that the controller was designed to have two SKIP signal inputs so that it could handle SKIP signals from multiple systems where it was known that simultaneous SKIP signal handling would not be required. For example, multiple SKIP inputs could have been provided so that the controller 18 could have dedicated inputs for a probe signal receiver and also for a tool setter (e.g. such as that described in more detail below) where it was known that simultaneous SKIP signal handling would not be required. In the embodiment of Figure 7, the probing program could be programmed such that the SKIP input monitored/registered by the probing program switches from one to the other. For example, a macro could be provided such that on the first move (or move into the part as described in more detail below) the SKIPl input is monitored and such that on the second (e.g. repeated) move (or move out of the part as described in more detail below) the SKIP2 input is monitored.

A similar process can be used for tool-setting purposes on multi-spindle machines for which the controller has only one SKIP input. For example, Figure 3 illustrates an embodiment which comprises first 30 and second 40 tool setters, for use in setting first 50 and second 52 tools. Such tools could be cutting, milling, grinding tools or the like. In this embodiment, the tool setters are non-contact tool setters and in particular are what is commonly referred to as break-beam tool setters. As will be understood other types of tool setters, including contact tool setters could be used. Each tool setter comprises a transmitter 32, 42 for emitting a light beam, and a receiver 34, 44 for detecting the light beam. In this case, when the light beam is broken by its respective tool, the receiver issues a signal (e.g. a trigger signal) to the interface 16. In the same way as above, a repeated machine move can be used and the signals from each receiver can be used in turn (e.g. by using the MODE signal to tell the interface 16 which receiver signal to use as its source for the SKIP signal) so as to measure nominally identical points on each of the first and second tools. As will be understood, in this case, a first set of nominally identical points could for example be the tip positions of the tools, a second set of nominally identical points could be a first diameter measurement of the tool, and a third set of nominally identical points could be a second diameter measurement of the tool. As with the above, these nominally identical points can be measured in turn by repeating the machine move and switching between the tool setters using interface 16 between each move, rather than measuring all the points on one of the tools and then measuring all the points on the other of the tools. As will be understood, the repeated moves are "repeated" in the sense that the same point on each workpiece is nominally measured by the respective measurement system a plurality of times (e.g. twice). For example, in the above described embodiment of Figure 2, the probe 6 is brought into engagement with the measurement point 22 multiple times. As will be understood, the first and second (and any subsequent moves) need not be identical. For example, they could be performed at different speeds, or approach the part from different directions, e.g. have a different path. However, optionally at least the paths of the repeated moves are nominally identical. Optionally, the speed of the repeated moves can be nominally identical.

Figure 8 shows a further example embodiment of a machine tool according to the invention. As with the other described embodiments, the machine tool of Figure 8 comprises two spindles 2, 4. However, the first 2 and second 4 spindles are shown twice in Figure 8, to illustrate that at one moment in time they can each be loaded with a probe 6, 8 to measure objects 20, 20' (e.g. as described above in connection with Figure 1), and at another moment in time they can each be loaded with a tool 50, 52 which can be measured using first 30 and second 40 tool setters (e.g. as described above in connection with Figure 3).

Similar to the embodiment of Figure 7, the controller 18 of the embodiment of Figure 8 comprises two SKIP inputs (SKIPl and SKIP2). However, in this embodiment, SKIPl is connected to a first interface unit 16 for first 12 and second 14 probe signal receivers, and to a second interface unit 16' for first 30 and second 40 tool setters. In this case, a macro/program running on the controller 18 can select which SKIP input is monitored. Accordingly, during a probing routine, the controller 18 can be instructed to monitor for a signal on the SKIPl input.

Furthermore, in line with that described above in connection with Figure 1, a repeated move operation could be used to measure the first 20 and second 20' objects, and a MODE signal can be supplied to the first interface 16 so as to tell the first interface 16 whether to use the first 6 or second 8 probe as its source for the SKIP signal. For example, the MODE signal to the first interface unit 16 can be used such that on a first move (or on the move into the part as described in more detail below), the signal from the first probe 6/receiver 12 is used to issue a SKIP signal to the controller 18 and such that on the second (repeated) move (or on the move out of the part as described in more detail below) the signal from the second probe 8/receiver 14 is used to issue a SKIP signal to the controller 18. At a different point in time, during a tool setting procedure, the controller 18 can then be instructed to monitor for a signal on the SKIP2 input. Accordingly, for example, in line with that described above in connection with Figure 3, a repeated move operation could be used to measure first 50 and second 52 tools in the first 2 and second 4 spindle, and a MODE signal can be supplied to the interface 16' so as to tell the interface 16' which receiver (34 or 44) signal to use as its source for the SKIP signal on each move. Referring to figures 4a and 4b a further embodiment of the invention will now be described. Figures 4a and 4b again show first 6 and second 8 probes that are used to measure first 20 and second 20' nominally identical workpieces. In a similar manner to the examples described above, pairs of nominally identical points 122 and 122' on each workpiece are measured in turn.

In the examples described above, there is repeated motion of the first 6 and second 8 probes towards the first 20 and second 20' nominally identical workpieces to allow the nominally identical points of each pair of points to be measured one after the other. It is, however, also possible to use the first probe 6 to measure a first point 122 during motion of the probes toward the workpieces (i.e. by sensing when contact is first made with the first workpiece 20 surface) but to use the second probe 8 to measure the second point 122' during motion of the probes away from the workpieces (i.e. by sensing when contact is lost with the second workpiece 20' surface). In other words, the first probe 6 can be configured to measure points during motion towards the surface, whilst the second probe can be configured to measure points during motion away from the surface. This allows each pair of nominally identical points to be collected during a move towards and then away from the workpieces. This can often be faster than using two repeated moves.

Figures 4a illustrates how the first point 122 of a pair of nominally identical points is collected during motion of the probes towards the workpieces. In particular, figure 4a shows motion of the first 6 and second 8 probes towards the first 20 and second 20' nominally identical workpieces. During this movement, the first probe

6 is the "active" probe used for measurement. For example, as described above, an interface (not shown in figure 4a or 4b) may be provided which issues a SKIP signal to the machine tool controller when deflection of the stylus of the first probe 6 occurs. The machine tool stops the movement on receipt of the SKIP signal and records the machine tool' s encoder positions so that it can determine the position of measurement of the first point 122 on the first workpiece 20, in the machine tool's coordinate measurement system. Measurement of the first point 122 is thus performed in a similar manner to the measurement of the first point 22 described with reference to figure 1 above.

Figure 4b illustrates how the second point 122' of a pair of nominally identical points can be collected. After the machine tool has stopped following receipt of the SKIP generated by deflection of the stylus of the first probe 6, the probes are moved away (i.e. retracted or backed-off) from the workpieces. During this movement, the interface is instructed by the machine tool controller to select the second probe 8 to be the "active" probe that is used for measurement. The interface thus issues a signal to machine tool controller when there is no longer any deflection of the stylus of the second probe 8. For example, the SKIP signal from the second probe 8 could be inverted in the interface. Receipt of the inverted SKIP signal by the machine tool controller (i.e. which occurs when the second probe loses contact with the surface) would cause the machine to stop

momentarily at a retract trigger point. The machine tool controller then records the machine tool's encoder positions at the instant this inverted SKIP signal is received thereby allowing the position of the second point 122' on the second workpiece 20' to be determined, in the machine tool's coordinate measurement system. Once the necessary position data is captured, continued motion of the probes could occur immediately (e.g. making any pause in motion imperceptible to end users). It should also be noted that the interface may not need to invert the SKIP signal from the second probe 8 if the machine tool controller itself is capable of performing an analogous function. For example, the machine tool controller may be programmable (e.g. via a macro call command or parameter) to invert a SKIP input thereby allowing operation in the same manner as above (but without needing the interface to invert the SKIP signal). The skilled person would thus be able to implement such a function differently for different machine tool controllers. As will be understood, a check that the second probe 8 is deflected can take place before the move off the surface is performed. If not, then an error signal or warning could be issued. The check could comprise instructing the machine to move the probes by a small amount (e.g. less to ΙΟΟμιη, e.g. approx. 50μιη) and determining if the move was successful. If a "triggered" signal was issued by the active probe (i.e. in this case the second probe),e.g. because it was already off the surface (and in this case the SKIP signal has been inverted), then the move will not be successful and so it will be known that the probe was not on the surface of the object.

It should be noted that the first 6 and second 8 measurement probes will preferably have been calibrated for the type of measurement they will be required to perform. The calibration of the first measurement probe 6 may thus be performed by acquiring measurement points by driving the probe into a surface (e.g. of a calibration artefact) whilst the second measurement probe 8 is calibrated using measurement points acquired when moving the probe away from a surface. In this manner, accurate measurement points can be acquired for both motion into, and away from, a workpiece surface.

Although figures 4a and 4b describe a method that measures workpieces using spindle mounted probes, the various features of the other embodiments described herein could also be implemented in this embodiment. For example, the same technique could also be applied to the measurement of tools using tool setters. It should also be noted that although a dual spindle/workpiece arrangement is described, there is no reason why the technique could not be used on machine tools having three or more spindles. If more than two spindles are provided, measurements during motion into and out of the surface may be combined with repetitions of that motion to allow measurements of nominally identical points on three or more objects to be collected. In this manner, the cycle time required to measure multiple points on multiple objects can be reduced further.

As described above, during any given move one of the measurement systems is suppressed. Accordingly, if during a move the suppressed measurement system and part are in a position sensing relationship (e.g. a probe' s stylus is deflected), either it won't issue a SKIP/triggered signal or it' s SKIP/triggered signal will be ignored and so the machine will continue to move. There is therefore a risk of a damaging crash between the suppressed measurement system and a part, e.g. if the part is not where it is expected to be. To avoid this, a crash-detection mechanism could be provided, for detecting such a situation (e.g. for detecting deflection of the suppressed/non-active probe), and for taking action in response thereto (e.g. stopping motion of the machine and/or issuing an error/warning signal). For example, a state signal (e.g. a secondary deflection- state signal), which is separate from the SKIP signal, could be issued by the measurement system, even when it is suppressed. Such a state signal could be received by the machine tool apparatus (e.g. its controller), and action taken when the state signal indicates that the measurement system and part are in a position sensing relationship (e.g. when the probe's stylus has deflected). In one embodiment, the measurement system (e.g. the suppressed measurement system) could be configured to issue, at regular interval, a signal indicating whether or not it is in a position sensing relationship with something (e.g. whether or not the probe's stylus is deflected). Such a signal could be separate, and independent, to the SKIP signal. On receipt of such a signal indicating that the measurement system is in a position sensing relationship, the machine tool could, for instance, halt motion to avoid damage to the measurement system and/or part, and/or issue an error/warning signal.

Claims

CLAIMS:

1. A method of operating a positioning apparatus comprising at least first and second respective measurement systems for measuring a plurality of sets of nominally identical points on at least first and second nominally identical respective parts, the method comprising:

for each set of nominally identical points in turn, causing a first relative movement between the parts and the measurement systems so as to measure one of the parts using one of the measurement systems and subsequently causing a second relative movement between the parts and measurement systems so as to measure the other of the parts using the other measurement system.

2. A method according to claim 1, wherein the second relative movement comprises a repetition of the first relative movement.

3. A method according to claim 1, wherein the first relative movement comprises movement of the parts towards the measurement systems and the second relative movement comprises movement of the parts away from the measurement systems.

4. A method as claimed in any preceding claim, in which said at least first and second parts comprise first and second tools, optionally mounted in at least first and second tool mounts of the positioning apparatus.

5. A method as claimed in claim 4, in which said at least first and second measurement systems comprise first and second tool setters.

6. A method as claimed in any one of claims 1 to 3, in which said at least first and second parts comprise at least first and second workpieces.

7. A method as claimed in claim 6, in which said at least first and second measurement systems comprise first and second probes, optionally mounted in at least first and second tool mounts of the positioning apparatus.

8. A method as claimed in claim 7, in which the first and second probes comprise contact probes comprising a deflectable stylus, and are configured to provide a signal indicative of stylus deflection.

9. A method as claimed in any preceding claim, in which said first and second measurement systems are configured to wirelessly communicate with a receiver.

10. A method as claimed in any preceding claim, in which the positioning apparatus comprises a controller configured to receive input from only one measurement system.

11. A method as claimed in claim 10, in which said input comprises a SKIP signal input.

12. A method as claimed in any claim 10 or 11, comprising switching between the outputs of the measurement systems, such that the output of one of the measurement systems is passed to the controller for the measurement of said one of the parts, and such that the output of the other of the measurement systems is passed to the controller for the measurement of said other of the parts.

13. A method as claimed in any preceding claim, in which the positioning apparatus comprises an interface to which the at least first and second

measurement systems provide signals indicative of stylus deflection.

14. A method as claimed in claims 11 or 12 and 13, in which the interface unit is configured to switch between providing measurement information from one of the measurement systems and the other of the measurement systems, to the controller.

15. A method as claimed in any preceding claim, in which the at least first and second measurement systems are both powered on during the moves.

16. A method as claimed in any preceding claim, in which the positioning apparatus comprises a computer numerically controlled positioning apparatus.

17. A method as claimed in any preceding claim, in which the method comprises moving at least first and second tool mounts of the positioning apparatus to cause said relative movement.

18. A method as claimed in any of claims 4, 7 and 17, in which said first and second tool mounts comprise first and second spindles.

19. A method as claimed in any preceding claim, in which the method comprises moving the positioning apparatus' table to cause said relative movement.

20. A method as claimed in any preceding claim, comprising suppressing one of the measurement systems during the first move and then suppressing the other of the measurement systems during the second move.

21. A method as claimed in any preceding claim, in which the positioning apparatus comprises a controller comprising fewer signal inputs than the number of measurement systems present.

22. A method as claimed in any preceding claim, in which the positioning apparatus is a machine tool apparatus.

23. A method as claimed in any preceding claim, in which the positioning apparatus comprises at least first and second spindles.

24. A positioning apparatus configured to operate in accordance with the method as claimed in any preceding claim.

25. A positioning apparatus as claimed in claim 24, in which the positioning apparatus comprises a machine tool apparatus.