GB2087598A - Automatic Control of Machine Tools - Google Patents

Abstract

Conventional automatic machine tool control installations 6 are updated by the local installation of an adapter unit MTLU able to receive a modern form of digital control data (from computer 7) indicative of required tool feed rates along respective tool drive axes, to decode this data to form a speed signal and vector signals indicative of distances to be moved along the respective axes between successive incremental tool positions and to form under the control of these signals respective pulse trains of which the repetition rate and phase are appropriate to drive directly the machine tool. <IMAGE>

Description

SPECIFICATION Automatic Control of Machine Tools This invention relates to the automatic control of machine tools.

The development of automatic control equipment for machine tools has been quite rapid recently with the result that much equipment in use now is relatively old-fashioned. It is kept however because the machine tool itself is a relatively expensive long-life item and may not be easily adaptable to interface with modern control equipment. By way of example, what is known as the Ferranti Mark 1V/Copath tape system was widely sold around 1960. A Copath tape is a four track magnetic tape having analogue, phaseencoded control signals stored thereon and produced by very specialised processing equipment attached to a computer. Original examples of such equipment are now old enough to require more and more maintenance but there is little desire to renew them because the system as a whole is now somewhat outmoded.

A more modern control system might comprise a central mainframe computer well away from the machine shop environment or even at a different site and operable to control even a large number of machine toolswhich use directly the digital control signals issued by the computer. At the machine tool itself, extensive facilities might be provided-for example a terminal via which an operator can call up selected programmes from the mainframe as he requires them or can intervene in the machining process if necessary.

Further, there might be provided a feed-back facility whereby each machine tool is able to signal back to the mainframe computer information on its progress. As suggested earlier, older machine tools may be adaptable to interface with modern control systems but this may not be easy or economically viable. For example, such adaptation may involve the replacement of the original tool transport systems by "rack and resolver" feedback systems and such constitutes a major and extensive refurbishment of the machine.

According to the present invention, there is provided a machine tool logic unit, for use in association with and connected to a numerically controllable machine tool installation and operable for receiving computer generated data in digital form and for translating such data to a different form adapted to suit said machine tool installation.

It will be appreciated that although the embodiment of the invention described herein is designed for interfacing with a Ferranti Mark IV/Copath tape installation, the invention is not limited thereto. The data requirements of particular machine tools is not completely standardised, even for modern tools, and thus a suitably constructed machine tool logic unit is useful in say a situation where a plurality of different machine tools are to be linked into a central mainframe control system of the kind described earlier.

For a better understanding of the invention, an embodiment thereof will now be described, by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram for explaining a Ferranti MKIV/Copath tape system, Figure 2 is a block diagram of a machine tool logic unit which also illustrates how it is introduced into the system of Figure 1, Figure 3 is a simplified circuit diagram of an interface arrangement used in the unit of Figure 2, Figure 4 is a logic diagram for a synchronisation circuit used in the interface arrangement, and Figure 5 shows some waveforms produced in the interface arrangement.

As shown in Figure 1, an installation for preparing and using Copath tapes might comprise a mainframe computer 1 into which control programmes are loaded and a processor 2 which receives digital control signals from the computer 1 and adapts them to a form usable by the Copath system. These signals are recorded on magnetic tape by a recorder 3. This tape is then replayed to a Copath tape preparation unit 4 which converts the recorded signals to an analog form and rerecords such analog signals onto a four track tape in the form of three phase-encoded control signals and a phase-reference signal. The four track tape is then taken to the machine shop and loaded into the relevant MKIV controller/machine tool installation 5 and 6.

As shown in Figure 2, a machine tool logic unit MTLU according to this invention may be coupled, at the machine tool site, to the MKIV controller 5 and is operable to receive from a local or remote computer 7 standardised digital control signals of the kind issued by the computer 1 in Figure 1. It translates these signals into a form directly usable by the tape interface unit 8 of the controller 5 to control the machine tool 6. It also provides for modern facilities such as a local operator terminal 8a to allow programme call-up and manual intervention in the programme.

The machine tool logic unit comprises a standard form of microcomputer system 9 including a central processor 10, for example an LSI 11/23, interconnected by way of data bus 11 with a U.V. erasable PROM memory 12 having a storage capacity of say 32K bytes, a random access memory 13 having a capacity of say 32K bytes, and an arrangement of one or more interface units 14 via which the system 9 is connected to the computer 7 and the local terminal 8a (i.e. a keyboard with or without a VDU). Data bus 11 also connects to an interface unit 1 5 which is shown in more detail in Figure 3.

The signals required by the controller 5 comprise four square-wave alternating signals of which one is a reference signal at a frequency of around 1 OOHz and of which the other three have frequencies differing from that of the reference signal by amounts which determine the speed of relative movement of the tool and workpiece in a particular one of the X, Y and Z coordinate directions. Thus, if a particular coordinate signal has a frequency equal to that of the reference signal, then the tool does not move in the relevant direction. If these frequencies differ, the tool moves forward or backward along that direction, depending upon whether the coordinate signal frequency is greater or less than the reference frequency, and at a speed dependant upon the magnitude of the difference.

The function of the microprocessor system 9 is to control the reception of data from the computer 7. Such data, in this case, comprises a series of coded signals giving tool feed rates along respective axes of three coordinates. The system 9 thus decodes this data and forms therefrom a series of vector signals X, Y and Z indicative of respective distances through which the tool is to move along the respective coordinate from one to another position. It also outputs a common scaling factor K indicative of the required feed rate, acceleration and deceleration distance signals A and D having regard to the machine tool capabilities and the required feed-rate, a critical axis distance CAD i.e.

the longest one of the X, Y and Z distances, and command signals CSR such as stop and start signals. These signals are made available to the interface unit 1 5 which, as shown in Figure 3, comprises address decoders 1 6 and 1 7 for recognising the relevant signals from among those present on bus 11 (which include for example signals for driving terminal 8a). The decoders distribute the signals X, Y, Z, K, A, D, CAD and CSR to respective buffer registers 18 to 25 from whence they are available to corresponding read-out registers 26 to 33.

Signals X, Y, and Z held by registers 26, 27 and 28 each comprise a sign bit indicative of the direction of movement along the X, Y or Z axis and a magnitude part indicative of the amount of such movement. The magnitude part of the signals are passed to respective ones of three pulse rate multipliers 34, 35 and 36 while the respective sign bits are fed to synchronisation circuits 37, 38 and 39.

A reference signal generator 45 which includes a 40 KHz oscillator and frequency dividing circuits produces two 10 KHz pulse trains , and (ii2 each having a mark to space ratio of one to three and being out of phase with respect to one another as shown in Figure 5 at (4) and (5). Figure 5(1) shows the 40KHz pulse train formed in the generator 45 and Figures 5(2) and 5(3) show the waveforms A and B of this train following division by two and four, these waveforms having a 1:1 mark/space ratio. By way of example, the trains (b, and (ii2 could be obtained by appropriate logical association of these waveforms A and B.

Waveform B is made available at an output of generator 45 and is fed to each of the synchronisation units 37, 38 and 39 along with pulse train QI,. Pulse train Q), is also fed to a reference signal frequency divider 49 and to a synchronisation signal generator 60. Pulse train 02 is passed along with command signals from register 33 to a feedrate override circuit 61 which has respective outputs connected to the synchronisation signal generator 60 and a binary rate multiplier 44. The synchronisation signals from generator 60 are supplied to a clock signal generator 40 along with a control signal from register 33.Register 33 also controls a selector switch 41 to pass to a critical axis counter 42 the pulses from that one of the multipliers 34, 35 and 36 which corresponds to the critical axis for a particular movement to be executed. The output from clock generator 40 is passed to a reversible counter 43 of which the content is available to control the binary rate multiplier 44. This multiplier receives a pulse train consisting of the signal 2 normally divided by two in the feedrate override circuit 61 which can in fact be implemented as another binary rate multiplier.

The division ratio effected by multiplier 44 is determined by the content of counter 43. The output of multiplier 44 is passed to another binary rate multiplier 47, of which the division ratio is controlled by the scaling factor signal K in register 29, and the output of this multiplier is passed to each of the multipliers 34, 35, and 36. The divider 49 has a division ratio of 100 and hence provides a 100Hz reference signal for the controller 5. The outputs of the synchronisation circuits 37,38 and 39 are divided by 100 in respective further dividers 50, 51 and 52 to become the X Y Z signals for controller 5.

When the tool of the machine tool is to execute an increment of movement for which the distance along say the X-coordinate is the greatest, then this X-coordinate becomes the critical axis and command register 33 causes switch 41 to select the output of multiplier 34 for transfer to counter 42. The content of this counter is available to a comparator 53 for comparison with, in turn, the signals registered by registers 30, 31 and 32.

Under the control of a mode selector logic unit 55, the signals in registers 30, 31 and 32 are, in turn, made available to comparator 53 via a selector unit 54. Unit 55 also controls reversible counter 43, and the unit 55 is, in turn, controlled by comparator 53.

Initially, the signal in register 30, i.e. the acceleration distance signal A, is available to comparator 53 via selector unit 54. Under the control of mode selector 55, counter 43 is started and begins to count the pulses from clock generator 40. The repetition rate of the pulses from generator 40 is set by register 33 to give a constant acceleration and deceleration of the machine tool movement whatever the value of the feedrate during the constant speed mode. As the content of counter 43 increases, binary rate multiplier 44 allows pulses from divider 45 to pass at an increasing repetition rate to multiplier 47 which scales them according to the signal K.

In turn, the multipliers 34, 35, and 36 each divide the repetition rate of the pulse signal from multiplier 47 by a ratio dependent upon the respective X, Y and Z distance signals from registers 26, 27 and 28. Thus, in effect, these distance signals are converted into relative speed signals which, during the initial acceleration mode, are each increasing. When multiplier 34 has produced a number of pulses equal to the content of register 30, comparator 53 responds to switch mode selector unit 55 to give a constant speed mode. In this mode, the content of counter 43 is held constant and hence also the repetition rate, at that time, of the pulses from multiplier 44.

Meanwhile, comparator 53 begins to receive via selector unit 54 the deceleration distance signal D from register 31. This signal indicates the distance along the critical axis at which the tool is to start decelerating and when it is equalled by the content of counter 42, mode selector 55 is entered into a deceleration mode. Here, the output of register 32, i.e. the critical axis distance signal CAD, is passed to comparator 53 while counter 43 is reversed and begins to count down.

Thus, the repetition rate of the pulses from multiplier 44 begins to reduce and, correspondingly, so also do the repetition rates of the pulses from multipliers 34, 35 and 36. When the number of pulses produced by multiplier 34 equals the critical axis distance signal CAD, the relevant tool movement increment is judged complete whereupon the mode selector unit outputs a signal to registers 26 to 33, an edge of which signal triggers these registers into making available a new set of signal values. The mode selector unit 55 also controls via interrupt unit 62 the transfer of further signal values along bus 11.

As mentioned earlier the repetition rates of the pulse signals from multipliers 34, 35 and 36 represent speeds of tool movement along the respective coordinate axes. The function of the synchronisation units 37, 38 and 39 is to provide respective synchronised signals of which the frequencies equal the 10 KHz reference signal , supplied by generator 45 plus or minus (which being dependent upon the value of the sign bit supplied by the respective one of registers 26, 27 and 28) the repetition rate of the signals supplied by multipliers 34, 35 and 36. After division, the respective signals become the signals, at 100 Hz plus or minus the frequency appropriate to speed along the relevant axis, required by the controller 5. As mentioned earlier, the feedrate override circuit 61 normally effects division of signal 2 by two so the multiplier 44 receives a signal at 5KHz.

If this multiplier and each of the multipliers 47 and 34 to 36 effect no division, i.e. if the input/output frequency ratio is unity, a 5 KHz signal is passed to synchronisation units 37 to 39.

Thus, in the example given, the frequency at the outputs of the units 37 to 39 will vary between 5 and 1 5 KHz and correspondingly that at the outputs of dividers 50 to 52 will vary between 50 and 1 50 Hz, these values corresponding to maximum speed of movement in one or the other direction along the corresponding axes.

The normal feedrate is overridden by a command sent from register 33 to cause the unit 61 to divide the frequency signal 2 by a factor other than two or not at all whereupon the frequency of the signal fed to multiplier 44 is varied from its normal 5 KHz. When the normal feedrate is overridden, unit 61 also passes a command signal to synchronisation signal generating circuit 60 to cause it to vary the frequency of the synchronisation signals passed to clock generator 40 and hence to vary the output frequency of the clock signals so as to modify the acceleration and deceleration times in accordance with the new feedrate.Sync. signal generating circuit 60 may comprise a counter say, which normally divides the frequency of signal , down by say four but from which a different stage output and hence a different division ratio can be selected by command from unit 61.

As shown in Figure 4, each synchronisation unit comprises an And gate 70 of which respective inputs receive the sign-bit from the corresponding one of the registers 26, 27 and 28 and the pulse train from the corresponding one of the binary rate multipliers 34, 35 and 36. The multiplier output pulse train is also supplied to one input of a two-input Nor gate 71 while the sign-bit is also supplied to one input of a two input Or gate 72. The output of And gate 70 is fed to the set input D of a bi-stable latch 73 while the reset input G of this latch receives waveform B (see Figure 5(3)) from reference signal generator 45. The output Q of latch 73 is fed to the second input of Or gate 72 while its complementary output 0 is fed to one input of a three-input Nand gate 74.A second input of gate 74 receives pulse train (iii' which pulse train is also received by the second input of gate 71, while the third input of gate 74 is connected to the output of gate 72.

The output of gate 72 is further connected to one input of an Or gate 75 of which a second input is connected to the output of gate 71. The outputs of gates 74 and 75 are connected to respective inputs of a Nand gate 76. To assist interpretation by those skilled in the art, Figure 4 is drawn according to the system whereby each logic element shows its general function within the overall circuit taking into account changes of logic convention.

Figure 5(6) shows, for illustration of Figure 4 operation only since it is not a typical output containing as it does no acceleration or deceleration phase, a series of pulses from a binary rate multiplier 34, 35 or 36 while Figure 5(7) shows a possible sign-bit waveform. Each rising edge of the product of these waveforms, shown at Figure 5(8) and formed by gate 70, sets latch 73. The latch is reset by each rising edge of waveform B (Figure 5(3)) so the Q output of the latch consists of a train of pulses as shown at Figure 5(9) whereof pulses are only initiated while the sign-bit is high but each of which is completed even if the sign-bit goes low while the pulse is being produced. Thus the output of Or gate 72 consists of the sign-bit high value extended to the end of the latch pulse being produced when the original sign-bit goes low as shown in Figure 5(10).While the extended signbit is high, the output of gate 75 stays high while gate 74 is enabled to pass conjunctions of reference signal , and the Q output of latch 73.

Since this Q output is low for a time following each pulse from the binary rate multiplier, the output of gate 74 and hence also of gate 76 consists of the reference signal Q), from.which each pulse next following a binary rate multiplier pulse is deleted as shown at Figure 5(11) during times to-ti and from t2. When the extended signbit is low, gate 74 output stays high while gates 75 and 76 pass both the reference signal Q1, and the pulses from the binary rate multiplier as shown at Figure 5(11) during time t1-t2.

It will be appreciated that the overall arrangement shown in Figure 2 could be varied.

For example, instead of being on-line to computer 7, the unit 6 could be provided with facilities for reading a stored programme medium, e.g. floppy discs, prepared off-line by a suitable processor.

This may still be simpler than say the Copath tape system. Alternatively, between mainframe computer 7 and a series of machine tools installations, there could be installed a subsidiary computer called a sub-work centre which distributes programmes to the respective installations and provides for local control, e.g.

control at the same site even if not in the actual machine shop.

As mentioned, instead of receiving digital control signals directly from the computer 7 along bus 7a the machine tool logic unit may receive data from a data storage medium reader such as a floppy disc reader or from the punched tape reader 8b in Figure 2, the punched tape (or other medium) 8c being prepared separately, for example by a tape punch unit connected to the computer 7 or another computer, and then taken to the machine shop and run through the reader 8b. The storage medium reader may be provided in addition to the direct iink rather than instead of it. Thus, bus 7a may connect to both the computer 7 and the reader 8b or only one of these items.Where the reader 8b only is provided (or some other kind of storage medium reader) the system shown in Figure 2 is still advantageous over the known Copath system shown in Figure 1 because of the aforementioned simpler operation and because the possibility for real-time control of the machine tool direct from the standardised form of digital control data along with the concurrent possibility of a local operator input via terminal 8a, is still available whether that standardised digital data is received direct from the main computer or from a storage medium produced by the computer.In the prior system of Figure 1, however, the processor 2 has to adapt the standard data to the very specialised form needed by the Copath unit 4 and when the fourtrack Copath tape has been prepared by unit 4, usually well in advance of the actual machining operation, no modification of the machining operation by the local operator is possible except by going through the complete process of obtaining a new Copath tape.

Finally it will be appreciated that although the machine tool logic unit illustrated in Figure 1 has been developed primarily for installation local to the machine tool to be controlled, it might be possible to use the logic unit, perhaps with comparatively minor modifications, as an improved replacement for the Copath tape preparation unit 4 of Figure 1, the outputs from the logic unit being simply connected up to a suitable four-track tape recorder.

By way of example further details of operation of the illustrated system may be as follows. The data presented to the MTLU by the computer 7 or tape reader 8b may take the particular form of a feed rate value, and X, Y, Z, distance values. The distance values may be either absolute or relative.

Preferably, the MTLU always converts any absolute distances to relative ones.

A vector is defined to mean a set of information passed to the interface unit of Figure 3. This involves an X, Y, Z value, a feed value (in fact this is 1/time value), some acceleration and deceleration information. The acceleration information takes the form of a percentage of feedrate to start accelerating with (note that 100% implies no acceleration). The deceleration information takes the form of a distance along the critical axis at which to start decelerating. Note that the acceleration and deceleration rates are fixed by the interface and not the software.

The software determines the velocity profile for each vector by examining the feedrate and direction of the previous and next vectors. For example if the previous vector was in a markedly different direction from the vector being calculated, then the feed must decelerate to zero at the end of the previous vector, and therefore accelerate from zero for this vector The problem is not quite as simple as explained above, because the limiting factor is not just direction, but direction and feed combined (the servo loop of the Ferranti MK IV will tolerate only a certain error before it trips out and if too great a change of velocity (step velocity) is applied to the system then it will trip).

The Interface must be supplied with data when requested (this happens under interrupt) or an error will occur. Therefore sufficient data must be buffered ready to output to take into account fluctuations in calculation time due to the complexity of the calculation algorithm, and the multiplicity of paths through the algorithm.

In addition to processing the X, Y, Z and feed data to the form needed to drive the hardware, the software also handles the keyboard and screen of the VDU 8a to provide an operator interface to the MTLU.

Claims (11)

Claims

1. Machine tool control apparatus characterised in that it is in the form of a machine tool logic unit for use in association with a numerically controlled machine tool installation and operable for receiving computer generated data in digital form and for using said data to form drive signals suitable for feeding to said machine tool installation to control the tool translation drive motors thereof.

2. Apparatus according to claim 1, characterised in that the machine tool logic unit is operable for receiving computer generated data indicative of required feed-rates of a tool of said machine tool along respective tool feed axes, and comprises decoding logic means for decoding said computer generated data to form a series of vector signals indicative of respective distances through which said tool is to move along said respective axes between each of a series of incremental tool positions and the next following position in said series and a common scaling signal (K) indicative of a desired tool feed rate, the machine tool logic unit further comprising interface means which includes register means for registering successive values of said signals, and pulse generation means for forming in response to each set of contemporaneous registered values a series of output pulse trains for driving said machine tool to move the tool thereof along said respective axes, the repetition rates of the respective pulse trains being ail dependent upon the respective registered value of said common scaling signal and respectively dependent upon the respective registered values of said vector signals.

3. Apparatus according to claim 2, characterised in that said interface means includes synchronisation means for synchronising the pulses of said output pulse trains with the output of a common reference signal generator.

4. Apparatus according to claim 2 or 3, characterised in that said interface means includes control means for ensuring that the repetition rates of said output pulse trains are respectively ramped up and down at the beginning and end respectively of each train to give an acceleration and deceleration phase of the tool movement between said incremental tool positions.

5. Apparatus according to claim 2, 3 or 4, characterised in that said decoding logic means is operable for selecting, from each set of contemporaneous values of said vector signals that value which represents the largest distance and for making available to said interface unit a corresponding critical axis distance signal (CAD), said control means being operable in response to the critical axis distance signal for controlling the ramping of said pulse train repetition rates.

6. Apparatus according to any one of claims 2 to 5 wherein said decoding logic means comprises a microprocessor system including a central processing unit arranged to be controlled in accordance with instructions stored in memory means of the system.

7. Apparatus according to any preceding claim, characterised in that said machine tool logic unit comprises input means for receiving said computer generated data in digital form along a line connected to the data generating computer and/or from a storage medium reader, such as a punched tape reader, operable for reading said computer generated data from a data storage medium.

8. Apparatus according to any preceding claim, characterised in that said machine tool logic unit comprises input means for receiving local operator generated data from a local operator terminal and for responding to said local operator generated data to modify the drive signals formed thereby, for example to vary the machine-tool feed rate from a value commanded by said computer generated data.

9. A machine tool and machine tool control installation including a machine tool which responds to a plurality of alternating drive signals to drive the tool of the machine tool along respective feed axes in dependence upon the repetition rates of said drive signals, for example a machine tool adapted for use in the Ferranti Copath machine tool control system or a like derivative thereof, characterised in that said installation includes a machine tool logic unit connected to the machine tool and operable for receiving computer-generated data in digital form, either directly from the computer and/or from a data storage medium, and providing realtime control of the machine tool by using the computer-generated data to form said plurality of alternating signals and feeding them directly to the machine tool.

1 0. Apparatus for receiving computergenerated digital numerical control data indicative of rates at which a tool of a machine tool is to be fed along respective coordinate axes and for using said data to form machine tool control signals according to the Ferranti MKIV Copath system or a like or similar derivative thereof, said apparatus comprising decoder logic means for decoding said computer-generated digital data to form vector signals, each of which comprises a series of successive values indicative of distances to be moved by said tool along the respective coordinate axes between successive incremental tool positions and to form also a common scaling signal indicative of a desired tool feed-rate, and interface means including register means for registering successive values of said vector and scaling signals, and pulse generating means for forming in response to each set of contemporaneous registered values a plurality of pulse trains for controlling said machine tool to move the tool thereof along said respective coordinate axes, the repetition rates of the respective pulse trains being all dependent upon the respective registered value of said scaling signal and respectively dependent upon the respective registered values of said vector signals.

11. Machine tool control apparatus substantially as hereinbefore described with reference to the accompanying drawings.