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Description  |
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FIELD OF THE INVENTION
This invention relates to signal transmission systems, e.g. for use on
machine tools, coordinate measuring machines, inspection robots, and the
like (hereinafter referred to as "machine tools").
DESCRIPTION OF PRIOR ART
Various probes are known for the inspection of workpieces on such machines.
They include trigger probes which provide a trigger signal when they
contact or attain a predetermined relationship with a workpiece surface,
and measurement probes which provide a digital or analog output concerning
the position of the surface.
Particularly when the probe is to be interchangeable with other tools, as
in a machine tool, it is known to provide a wireless transmission system
for transmitting the probe output signal back to an interface with the
machine. For example, U.S. Pat. No. 4,509,266 describes an optical (infra
red) transmission system. Such systems are also commercially available
from Renishaw Metrology Ltd, of Wotton-under-Edge, Gloucestershire, United
Kingdom. Similar systems can also be used to transmit signals from other
sensors, e.g. relating to the presence or position of workpieces on the
machine bed or on a conveyor or pallet, or to the status of a device such
as a vise, a gripper or a robot. See, for example, U.S. Pat. No.
4,545,106.
These known systems include a battery power supply within the probe or
other sensor, or within an optical transmission module attached thereto.
To conserve battery power, it is obviously desirable that the optical
transmission should only be switched on when required, and the above U.S.
Pat. No. US 4,509,266 describes a two-way transmission system in which a
receiver in the probe switches on the battery's power supply upon receipt
of a high intensity infra red flash from a machine-mounted transmitter
associated with the machine interface. The commercially available systems
from Renishaw Metrology Ltd achieve a similar effect by transmitting a low
intensity infra red signal from the machine-mounted interface module
(which receives the signal transmitted by the probe) to a probe-mounted
receiver. The low intensity infra red signal for this purpose is modulated
on and off at a given frequency which can be picked out from ambient
radiation by the receiver in the probe. This low intensity signal is, in
effect, a short burst of infra red pulses at the given frequency.
Such systems have been found to work well in practice. However, it is
increasingly the trend for machine tools to be provided with several
probes for different inspection jobs, which are either usable
simultaneously, or which are interchangeable and stored in a tool magazine
when not in use. When one probe is to be used, it would be desirable to be
able to switch it on without at the same time switching on other probes on
the machine tool, or on adjacent machine tools. The same problem exists if
such a transmission system is used to transmit signals from other sensors.
SUMMARY OF THE INVENTION
The present invention provides a machine tool signal transmission system
comprising:
a plurality of battery-powered wireless sensor signal transmitting means,
each having means for receiving a given signal and means for switching on
the battery power upon receipt of that signal,
means remote from the sensor signal transmitting means for generating and
transmitting said given signals for each sensor signal transmitting means,
each given signal having a unique characteristic for each sensor,
said receiving means on each sensor signal transmitting means having means
responsive to the unique characteristic associated with that sensor for
switching on the battery power in response thereto, but not reacting to
the given signal for other said sensor(s).
In a preferred embodiment, the given signals are transmitted optically.
Preferably the unique characteristic is a given frequency of modulation of
the transmitted signal for each sensor. Each sensor receiver then has a
filter for the given frequency associated with that sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now be described, by
way of example, with reference to the accompanying drawings, wherein
FIG. 1 is a schematic diagram showing an arrangement of a probe signal
transmission system on a machine tool,
FIG. 2 shows a first transmission and receiving circuit in an interface
module,
FIGS. 3 and 4 show two alternative receiving circuits for a probe,
FIG. 5 shows a second transmission and receiving circuit for an interface,
and
FIG. 6 shows a further transmission circuit for a interface, transmitting
to a plurality of probes.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows the bed 10 and tool holding spindle 12 of a machine tool. The
spindle 12 can be moved in X,Y and Z directions relative to the bed 10, in
order to perform machining and inspection operations upon a workpiece 14
clamped to the bed 10. To machine the workpiece, the spindle 12 can pick
up any of a variety of cutting tools (not shown) stored in a tool magazine
16, under the program control of a computer numerical control (not shown).
To perform inspection operations, the spindle 12 can pick up any of a
plurality of probes 18A,18B,18C which are also stored in the magazine 16.
The probes shown in FIG. 1 are touch trigger probes, which have a circuit
22 (FIG. 3) for providing an output signal upon contact of a stylus 20
with a surface of the workpiece 14 in a well-known manner. To transmit
these probe signals back to the machine, each probe is provided with a
signal transmitter circuit as indicated at 23 in FIG. 3. The probe circuit
22 and transmitter 23 are powered by a battery 24 in the probe. The
transmitter includes an infra red light emitting diode 26 on the surface
of the probe as shown in FIG. 1, or if desired more than one such light
emitting diode can be spaced around the circumference of the probe to
provide omni-directional signal transmission. In use, the infra red signal
transmitted from the light emitting diode 26 is received by a receiver and
transmitter module 28 mounted on the machine. This module 28 has circuits
as shown in FIG. 2, including a receiver photodiode 58 and signal
processing circuit 60, from which the probe signal is passed on a line 62
via an interface (not shown) to the machine's computer numerical control.
Circuits for transmitting the probe signal from the probe and for
receiving and processing it in the module 28 are conventional, and need
not be described further.
To conserve power, the battery 24 in each probe is connected to the probe
circuit 22 and transmitter 23 via an electronic changeover switch 30. The
switch 30 is switched (so as to supply power to the circuit 22 and
transmitter 23 via a line 54) upon receipt of a given signal transmitted
from the machine receiver and transmitter module 28. The given signal is
produced in the machine module 28 by the remaining circuit shown in FIG.
2, and is received by a photo diode 32 on each probe, which is part of a
receiver 34 of the probe.
As shown in FIG. 2, the transmitter 35 in the machine module 28 includes an
infra red light emitting diode 36. The machine module circuit of FIG. 2
also comprises a plurality of square wave oscillators 38A,38B,38C,
corresponding to the maximum number of probes 18A,18B,18C which are to be
controlled. Each oscillator has a different frequency
f.sub.1,f.sub.2,f.sub.3, such as 5 kHz,6 kHz,7 kHz. When a given probe 18
has been picked up in the spindle 12 and is to be switched on, a signal to
this effect is transmitted from the computer numerical control of the
machine to a control circuit 40 within the module 28. The control circuit
40 then acts on a selector switch 42 to select a corresponding output of
one of the oscillators 38A,38B,38C. It next triggers a monostable 44 to
produce a pulse of a given duration, such as 250 ms. The output of the
monostable 44 controls a gate 46 which passes the output of the selector
switch 42, so that the transmitter 35 receives a 250 ms burst of square
wave pulses at the frequency f.sub. 1,f.sub.2 or f.sub.3, as selected.
This burst of pulses is transmitted optically to the receiver 34 of the
probe, the infra red output of the light emitting diode 36 being
effectively modulated (switched on and off) at the frequency
f.sub.1,f.sub.2 or f.sub.3.
The probe (FIG. 3) contains a band pass filter 48 which receives the output
of the receiver 34. The filter 48 in each of the probes 18A,18B,18C is
uniquely tuned to the appropriate frequency f.sub.1,f.sub.2,f.sub.3,
corresponding to that probe. Therefore, the signal transmitted to switch
on the probe is passed by the band pass filter 48 only to that probe as
selected by the selector 42, and the remaining probes in the magazine 16
are unaffected.
The output of the band pass filter 48 of the selected probe is taken to a
conventional detector circuit 50, which detects the alternating signal by
rectifying and smoothing it. The result is a pulse of a length
corresponding to the 250 ms of the monostable 44 which is used to switch
over the electronic switch 30, connecting the battery 24 to the probe
circuit 22 and transmitter 23 and activating them.
The switch 30 is controlled by a timer 52, in order to switch off the probe
circuit 22 and transmitter 23 after a predetermined time, e.g. 1 or 2
minutes. The timer 52 is powered via the switch 30 and line 54. It is
reset for a further 1 or 2 minute period each time the probe circuit 22
changes state, i.e. when the stylus 20 makes or breaks contact with the
workpiece 14. This means that the probe circuit is deactivated after a
period of 1 or 2 minutes of non-use.
The receiver 34, filter 48 and detector 50 are powered from the battery 24
via the changeover switch 30 and a line 56. When the switch 30 provides
power to the line 54, it removes power from the line 56. This ensures that
the transmitter 23 cannot interfere with the receiver 34 of the probe,
since they are never both activated simultaneously.
FIG. 4 shows an alternative arrangement for the probe receiving circuits of
FIG. 3. Instead of using a timer 52, in this arrangement the probe circuit
22 and transmitter 23 are switched off by a further signal from the
machine transmitter module 28. The module 28 generates and transmits this
further signal in exactly the same way as the signals for switching on the
various probes, but desirably the frequency of the oscillator 38A, 38B or
38C which generates the modulation for the switch-off signal is at a
rather higher frequency than any of the switch-on signals. This ensures
that it is readily distinguishable from any of the switch-on signals, and
the same switch-off signal can be used for all probes.
The probe's receiving circuit in FIG. 4 includes the band pass filter 48
and detector 50, as in FIG. 3, for switching on the battery switch 30.
When switched on, this powers not only the probe circuit 22 and
transmitter circuit 23, but also a high pass filter 64 and corresponding
detector 66. The high pass filter 64 receives the input from the receiver
unit 34, and reacts only to the high frequency switch-off signal
transmitted by the module 28. Upon receipt of this signal, it is detected
by the detector 66, and used to switch off the switch 30, to conserve
battery power.
The advantage of the arrangement in FIG. 4 is that it is not necessary to
wait for the timer 52 in one probe to time out and switch the probe off,
before bringing another probe into use. One reason for making the
switch-off signal of a much higher frequency than the switch-on signals,
rather than the other way around, is that the operational amplifiers used
in the high frequency filter and detector circuits 64,66 tend to use
rather more current than their lower frequency counterparts 48,50. In the
interests of conserving battery power, therefore, it is better that they
should only be switched on when the probe is in use, rather than having to
be powered in a stand-by mode when the probe is not in use.
Although frequencies of 5 kHz,6 kHz,7 kHz have been quoted in respect of
FIG. 2 for the frequencies f.sub.1,f.sub.2 and f.sub.3, these frequencies
may in fact be varied over a wide range, governed by the frequency
response of the transmitting and receiving diodes 32,36. For example,
these frequencies may be anywhere within the range from below 1 kHz to 1
MHz, or even higher (say up to 10 MHz) with appropriate diodes. The
various frequencies chosen may advantageously be fairly widely spaced
apart in the frequency spectrum, so that sharp tuning circuits in the band
pass filters 48 are not necessary to exclude the signals for non-selected
probes. With the frequencies mentioned above, it may be necessary for the
band pass filters 48 to contain more than one stage of tuning.
FIG. 5 shows an alternative circuit for the receiver and transmitter module
28. In this case, the circuit 80 is intended as an expansion unit for an
existing optical transmitter interface circuit 68, designed to transmit
and receive signals to and from only a single probe. The existing
interface 68 therefore includes an oscillator 70, monostable 72 and gate
74, which produce a burst of pulses on an output line 76 intended to
switch on the single probe when instructed to do so by the numerical
control of the machine on a line 78. In this respect, the circuits
70,72,74 act in a similar manner to one of the oscillators 38A, monostable
44 and gate 46 shown in FIG. 2. The interface 68 also includes the
receiver signal processing circuit 60, similarly to FIG. 2.
The expansion unit 80 receives the burst of pulses on the line 76, and
passes them through a low pass filter 82 in order to recreate a single
pulse of a length corresponding to that generated by the monostable 72.
This is used to enable the output of a selector circuit 84. The selector
84 is separately controlled by binary input lines 86, also from the
machine's numerical control. These select one of three inputs 88, feeding
the selected input through to the output line 90 to produce a burst of
pulses whenever enabled on the line 92. The inputs 88 are driven by square
waves at respective frequencies f.sub.1,f.sub.2,f.sub.3. Thus, the output
line 90 carries a burst of pulses at the selected frequency, whenever the
selector output is enabled by the pulse on the line 92.
Whilst these selector inputs 88 could come from separate oscillators of the
appropriate frequencies (as shown in FIG. 2), in the present circuit they
are derived from a single oscillator 94, by a divider 96. The divider 96
is based upon a readily available integrated circuit normally used for
electronic organs, and gives frequencies f.sub.1,f.sub.2 f.sub.3 of 3483.6
Hz, 5215.6 Hz and 7812.5 Hz respectively. These frequencies are chosen
because none of them coincides with the harmonics of the others,
particularly the second and third harmonics. They are nevertheless easily
derivable from the organ divider 96. Because the harmonics do not
coincide, it is easy to provide reliable filtering circuits such as those
shown in FIG. 3, in order to distinguish one switch-on signal from
another.
The burst of pulses at the output 90 of the selector 84 can of course be
used to drive a single transmitter unit 35, as previously, but we prefer
to drive two or more such transmitter units 35 in parallel via a suitable
driver circuit 98. The two or more transmitter units 35 can then be placed
at different locations on the machine tool, so as to ensure that there is
always a line of sight from at least one of the transmitters to the probe,
whatever the position of the probe on the machine tool.
Each of the transmitter units 35 has an associated receiver unit 58, to
receive the probe signal in parallel. Again, at least one of these units
58 will always be in a line of sight from the probe. The probe signals are
combined in a buffer 100, and fed on a line 102 back to the conventional
signal processing circuit 60 in the interface unit 68.
It will be understood that the receiving circuits in the probes, which
receive the signals from the expansion unit of FIG. 5, may be similar to
those shown in FIGS. 3 or 4.
FIG. 6 shows an alternative to the arrangement of FIGS. 2 to 5. In this
arrangement, the switch-on signals are differentiated from each other not
by the frequency of modulation of the light beam, but rather by the color
of the light beam.
A burst of pulses at a given frequency is generated by an oscillator 104,
monostable 106 and gate 108, upon receipt of a signal from a control
circuit 110. This is similar to the corresponding circuits in the
conventional interface 68 of FIG. 5. However, the burst of pulses produced
by these circuits is fed to one of three possible transmitter units,
35A,35B,35C, by a selector circuit 112, under the control of the control
circuit 110. The three transmitter units are arranged to transmit light
beams of three different colors, for example red, green and infra red, or
red and two different infra red wavelengths. These different color light
beams are received by probe receiver units 34A,34B,34C which correspond to
the receiver units 34 of FIG. 3 except that each is covered by a
respective optical filter 37A,37B,37C which makes the receiver responsive
only to one of the colors transmitted by the units, 35A,35B,35C.
The units 35A,35B,35C may be made to radiate light of the desired color by
the use of light emitting diodes 36B,36C which emit only the respective
desired colors. Alternatively, an appropriate optical filter 39A may be
placed over the corresponding light emitting diode 36A, so as to filter
out all but the desired color.
For fuller details of such multiple color transmission systems, reference
should be made to our co-pending concurrently filed patent application
U.S. Ser. No. 07/334,538, ABANDONED, claiming priority from UK patent
application no. 8808612.9, applicant's case number 105, which is
incorporated herein by reference. That co-pending application relates to
the use of multiple colors for transmission of the probe signals back to
the machine interface, rather than for transmission of the switch-on
signals from the interface to the probes.
The probes 18A,18B,18C described have been touch trigger probes, which give
a trigger signal upon contact with the workpiece 14. However, some or all
of the probes used may be of other types. For example, they may be probes
which provide a trigger signal upon attaining a predetermined proximity to
the workpiece surface. They may also be scanning or measurement probes
which provide an analog or digital measurement signal (as opposed to a
trigger signal) relating to the displacement of a stylus caused by contact
with the workpiece, or relating to the distance of a workpiece surface
from the probe, detected in any suitable manner (e.g. optically). If
desired, some or all of the channels may be reserved for sensors other
than probes, e.g. sensors which provide signals relating to the presence
or position of workpieces on the machine bed or on a conveyor or pallet,
or to the status of a device such as a vise, or gripper or a robot. When
the machine's numerical control requires information from such a sensor,
it turns the sensor on in the same manner as described above.
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