Machine tools abstract
A position transducer for machine tools and measuring machines,
arranged to measure relative displacements of a first part of the
machine with respect to a second part of the machine, is described.
The main feature of this position transducer is to operate by means
of the optical coupling of at least two photoelectric readers fixed
on the first part and at least two optical photoengraved rules rigidly
connected to the second part. Selector means are provided to present
in output the signals from one of the photoelectric readers.
Machine tools claims
What we claim is:
1. Position transducer for machine tools and measuring machines,
arranged to measure relative displacements of a first part with
respect to a second part, wherein said transducer is obtained by
means of the optical coupling of at least two photoelectric readers
fixed on said first part and at least two optical photoengraved
rules rigidly connected to said second part, and wherein selection
means are provided which are arranged to present in output the signals
from one of said photoelectric readers.
2. The position transducer of claim 1 wherein said photoelectric
readers are equispaced from each other at a distance which is slightly
smaller than the length of one of said optical rules.
3. The position transducer of claim 2 wherein the facing ends
of said optical rules are disposed at a distance which is equal
to or slightly smaller than the distance between the end photoelectric
readers.
4. The position transducer of claim 3 wherein the maximum measurable
relative displacement between said two parts is given by L.sub.o
.times. n.sub.L .times.n.sub.F, where L.sub.o is the length of said
optical photoengraved rules, n.sub.L is equal to the number of said
optical rules, and n.sub.F is equal to the number of said photoelectric
readers.
5. The position transducer of claim 1 wherein the length of one
of said optical photoengraved rules is slightly more than one meter.
6. The position transducer of claim 1 wherein said first part
is a movable carriage of said machine, while said second part belongs
to a fixed structure of said machine.
7. The position transducer of claim 1 wherein said selection means
comprise a selection circuit and control means for said circuit,
for presenting in output the signals coming from one of said readers.
8. The position transducer of claim 7 wherein said control means
comprise a plurality of sensors, the number of which is equal to
the total number of photoelectric readers multiplied by the total
number of the optical photoengraved rules and reduced by one unit,
and which are arranged to activate themselves when said readers
have substantially reached one end of said rules.
9. The position transducer of claim 8 wherein said sensors comprise
proximity switches disposed on said fixed part and activated by
templates belonging to said movable part.
10. The position transducer of claim 8 wherein said sensors comprise
proximity switches disposed on said fixed part and activated by
magnets rigidly connected to said movable part.
11. The position transducer of claim 7 wherein said control means
comprise reference notches photoengraved on said optical rules and
detectable by said photoelectric readers.
Machine tools description
BACKGROUND OF THE INVENTION
This invention relates to a position transducer for machine tools
and measuring machines, realized by means of optical coupling of
photoelectric readers and photoengraved rules in a new and particular
arrangement.
At the present time, in the measuring machines and machine tools
in which it is necessary to know with the best precision the position
of a movable part, as for example a carriage, with respect to fixed
reference positions, pinion/rack assemblies are used as position
transducers. For example, the rack is fixed to the machine in a
position parallel to the movement of the carriage, and meshing with
the rack is a pinion pertaining to the carriage. As the carriage
moves, the pinion rotates by an angle which is proportional to the
displacement of the carriage. The resolution, and conseqently also
the precision, obtained with this type of transducer is of about
1/100 of a millimeter. If it is desired to have a better resolution,
in the order of one micron, transducers are utilized which are formed
by an optical photoengraved rule and a photoelectric reader element.
Said rule is a bar isostatically fixed to the structure of the machine
in order that it will not be exposed to deformation, and having
thin parallel equispaced traces formed thereon by means of photoengraving
process. Said rules are of various lengths, but usually they are
not constructed for lengths more than 3 meters, because greater
lengths would involve problems of difficult thermal stabilization
during the photoengraving process, as well as not negligible problems
of transportation.
If the element on which the carriage slides is very long, for example
five or more meters, it is impossible to use photoengraved rules
and it is necessary to have recourse to the pinion/rack coupling,
with consequent reduction of the resolution.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a position transducer
for machine tools and measuring machines, comprising photoelectric
readers and optical photoengraved rules, which will allow being
utilized on carriage sliding elements of any length and at the same
time will ensure the best precision peculiar to photoengraved rules
of normally length.
According to the invention there is provided a position transducer
for machine tools and measuring machines, arranged to measure relative
displacements of a first part with respect to a second part, wherein
said transducer is obtained by means of the optical coupling of
at least two photoelectric readers fixed on said first part and
at least two optical photoengraved rules rigidly connected to said
second part, and wherein selector means are provided which are arranged
to present in output the signals from one of said photoelectric
readers.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention a particular embodiment
thereof will now be described by way of example with reference to
the accompanying drawings, in which:
FIG. 1 is a perspective view of a measuring machine having mounted
thereon the position transducer of the present invention;
FIG. 2 is a side elevational view of the machine shown in FIG.
1; and
FIG. 3 is a block diagram of the position transducer mounted on
the machine of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 show a measuring machine whose bridge crane-shaped
structure comprises two horizontal beams 1 and 2 each of which rests
on three vertical pillars 3. Disposed on the upper portion of the
beams 1 and 2 are rails 4 on which two carriages 6 of a bridge 8
are slidably mounted, said bridge 8 being therefore able to move
horizontally along the direction of the beams 1 and 2. Along the
bridge 8 namely along the direction orthogonal to the beams 1 and
2 may slide a carriage 9 having a vertically slidable column 10
mounted thereon, at the lower of which there is connected a measuring
tool 11.
In a known type of such measuring machine, to which reference is
made herein, the beams 1 and 2 have lengths such as to allow a working
stroke of six meters for the carriages 6.
Fixed on one of the rails 4 are two photoengraved rules 13 and
14 of known construction, each having a length of slightly more
than 1 meter (for example, 1.1m), disposed on straight line parallel
to the rails 4 at a distance of substantially 2 meters from each
other and 1 meter from the center of the beam 1. On the outer side
of the carriage 6 there is fixed a rod 12 which is horizontal and
parallel to the rails 4 having a length slightly greater than 2
meters, on which there are fixed, at a distance of 1 meter from
one another, three photoelectric readers 15 16 and 17.
These latter are known elements having two outputs, comprising
substantially a sensitive element of the photoelectric type, and
are positioned in such a way that said sensitive element, during
the travel of the carriage 6 passes in front of the rules 13 and
14.
FIG. 3 shows a block diagram of the checking circuit of the transducer
of the present invention.
The two outputs of the photoelectric readers 15 16 17 are connected
to the inputs of a selection circuit 18 formed for example by a
plurality of logic gates, having connected thereto a connection
21 from the reader checking circuit 22. The inputs of this circuit
22 are connected to five sensors 23 24 25 26 and 27 which may
conveniently be formed by means of microswitches actuated by a template
of the carriage 6 or by proximity switches actuated by a magnet
also fixed to the carriage 6 and are disposed along the rail 4
at a distance of 1 meter from one another. When the bridge 8 is
situated at the end of its travel towards the left-hand side, the
reader 17 is situated at the left-hand end of the rule 13; if the
carriage is moved towards the right-hand side, the reader 17 presents
at its outputs two signals, the first of which is formed by a number
of rectangular pulses equal to the number of notches on the rule
13 on which it has passed, whilst the second is substantially equal
to the first one, but is, with respect to this latter, advanced
or delayed by 90 electric degrees, according to the direction of
movement of the carriage 6. The two signals of the reader 17 are
presented in output by the selection circuit 18 which is connected
to a device, known per se and not shown in FIG. 3 for example an
electronic processor, which determines and supplies the position
of the bridge 8 with a resolution of for example 1 .mu.m.
As the bridge 8 continues moving towards the right-hand side, the
reader 17 arrives at the right-hand end of the rule 13 whilst the
reader 16 is situated at the beginning of the left-hand end (it
is to be noted that the distance between the readers 16 and 17 is
1 meter, whilst the length of the rule 13 is slightly more than
1 meter). At this point the sensor 23 is actuated and sends a pulse
such as to signal to the reader checking circuit 22 that the reader
16 has started reading. An instruction sent onto the connection
21 gives rise to a switching in the circuit 18 which presents in
output the signal of the reader 16; in this way, the reading is
never discontinued, although the optical rule does not extend the
whole length of the beam 4. As the bridge 8 continues moving towards
the right-hand side, upon instruction from the sensor 24 the reading
is effected by the reader 15. Successively, when reader 15 reaches
the right-hand end of the rule 13 reader 17 reaches the left-hand
end of the rule 14 and the reading is now effected, upon instruction
from sensor 25 by the reader 17.
Similarly, by means of the sensors 26 and 27 the output signals
from selection circuit 18 will pertain sequentially to the readers
16 and 15 until the end of the stroke at the right-hand side of
the carriage is attained. The same sequence as described above will
take place, in th inversed order, during the reverse movement of
the bridge 8 from the right to the left.
The maximum error which may take place in the passage of the reading
between two different readers is of one resolution, i.e. 1 .mu.m;
such error, which is only local, give rise to a deformation of the
output signal, as can be seen at the moment A of switching-over
in FIG. 3 and can be eliminated by adjusting by means of a micrometric
screw the distance between the readers. Moreover, such error is
not cumulative, since at the passage of the carriage in the reverse
direction, owing to the switching position, it acts with opposite
sign and equal modulus, so that it annuls itself.
Thus, the position transducer of the present invention as described
hereinabove, can be utilized for movable carriage travels of any
length, ensuring a resoltuion of 1 .mu.m, by simply varying the
length of the photoengraved optical rules and the number of the
photoelectric readers. Namely, the number of the photoelectric readers
multiplied by the length of an optical rule gives a fictitious length
value of the optical rule, determined by the sequential passage
of the various readers. Therefore, by dividing the total stroke
of the carriage by such fictitious length value we obtain the total
number of optical rules which are necessary. Thus, the distance
between the opposed ends of said rules must be equal to/or slightly
smaller than the distance between the photoelectric end readers,
which readers have to be disposed with respect to each other at
a distance equal to the length of the optical rule. Conveniently
such values are determined by assuming the length of the photoengraved
optical rule to be equal to 1 m, which is a value for which such
rule can be manufactured with a high precision. The economical advantage
obtained by reducing the length of the rules, owing to the increased
number of photoelectric readers, is quite substantial; in fact,
the cost of one meter of rule is substantially equal to the cost
of three readers. In a machine of the type described hereinabove,
the cost of the transducer may be reduced substantially to a half
of the cost of that of a known type having one reader only. There
is, moreover, a limit to the economical convenience of increasing
the number of the readers; in fact, from a certain number upwards
any further increase does not allow but small reductions of the
length of the photoengraved rules. Taking into consideration that
a high number of readers involves a greater complexity of the assembly
operations and a higher cost of the selection circuit, the optimization
of the cost in the case of embodiment shown is conveniently obtained
with three photoelectric readers and two photoengraved rules each
having a length of about 1 meter, for a working stroke of the carriage
of 6 meters. The position transducer of the present invention, which
has been described herein in its application for measuring displacements
along the greater axis of the machine, is suitable as well for measuring
displacements along the two axes orthogonal to the transducer or
for effecting measurements along circular surfaces.
Obviously, the position transducer of the present invention is
susceptible of modifications and variations which do not depart
from the inventive idea contained in it. For example, the sensors
may be substituted by notches photoengraved on the optical rules,
which notches are detected by the readers passing in front of them
and are suitable signalled to the reader checking circuit 22; there
may be for example two reference notches, one for each end of the
rule and spaced 1 meter from one another. In this case, the signal
which indicates that a reference notch has been encountered is obtained
simultaneously on two readers, one of which is the one whose output,
at that point, is present at the output of the circuit 18; at this
point, by means of the circuit 22 a switching takes place in the
circuit 18 which switching produces the output of the signal which
arrives from the other reader.
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