Method of sensing position of movable body and apparatus therefor

The present invention relates to improvements in a method of sensing a position of a movable body by directly or indirectly converting a unit movement of the movable body into digital pulse signals and an apparatus therefor. A reference position is selected at a point, at which said movable body abuts against and presses a stopper provided at a specific position within the moving range thereof. The pulse signals are counted as the movable body moves from the stopper using the reference position as a reference number. The coincidence of the number of the pulse signals counted with a first predetermined pulse number corresponding to a predetermined position is sensed so as to sense the fact that the movable body has reached a predetermined position and position sensing signals are emitted. Further, as necessary, the position sensing signals that have been emitted are sustained for the duration of a second predetermined pulse number corresponding to a predetermined stroke preset so that the position sensing signals indicate that the movable body has reached the predetermined position and is within a predetermined stroke from the predetermined position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of sensing a position of a 
movable body and an apparatus therefor, and more particularly to 
improvements in a method of sensing a position of a movable body by 
directly or indirectly converting a unit movement of the movable body into 
digital pulse signals and an apparatus therefor, suitable for sensing that 
a workpiece and a tool have reached predetermined relative positions so as 
to switch the control of approach means, particularly for use in a machine 
tool, industrial machine or the like. 
2. Description of the Prior Art 
Since the approach speed of the tool and the like are controlled in 
accordance with the positional relationship between a workpiece and a 
tool, i.e., the progress of working in tool machines and industrial 
machines in general, it is very important to accurately sense the position 
of the tool or the like. Consequently, heretofore, as shown in FIG. 1 for 
example, in a machine tool including: a slide base 14 fixedly provided for 
working a workpiece 12 fixed on a jig 10; a slide table 18 supporting 
spindles 16 and movable in the forward and backword directions on the 
slide base 14; a DC motor 24 for moving the slide table 18 forward or 
backword by rotating a feed shaft 22 threadably coupled to a feed nut 20 
solidly secured to the undersurface of the slide table 18; a DC motor 
driving circuit 26 for controlling the rotational direction and the 
rotational speed of the DC motor 24; a speed change setting circuit 28 for 
supplying a setting value of an approach speed properly suitable for the 
forwarded positions of the spindles 16 to the DC motor driving circuit 26; 
and a mechanical sequence circuit 30 for controlling the DC motor driving 
circuit 26; the slide table 18 is provided at predetermined positions on 
the side surface thereof with dogs 32 and the slide base 14 is provided at 
predetermined positions thereof with mechanical limit switches 34 adapted 
to be ON-OFF operated by the dogs 32, whereby the limit switch 34 is 
ON-OFF operated by the dogs 32 to sense the forwarded position of the 
slide table 18, i.e., the spindles 16, so that the DC motor driving 
circuit 26 can be controlled through the mechanical sequence circuit 30, 
thereby enabling to control the approach of the slide table 18. In the 
drawings, designated at 35 is a bearing, 36 a stopper solidly secured to 
the front end face of the slide table 18, 37 a dead stopper solidly 
secured to the inner wall surface of the slide base 14, CR.sub.1 and 
CR.sub.2 normal-opposite rotation switching contacts for switching the 
rotational direction of the DC motor 24, and LS.sub.1.about.LS.sub.4 
approach speed changing contacts for changing the approach speed in 
accordance with the progress of working. 
This method has such characteristic features that it is very simple to 
effect the method and it suffices to provide the mechanical limit switches 
34 and dogs 32, both of which substantially correspond in number to 
required signal outputs at positions where controls are required to be 
switched. However, the mechanical limit switches are low in mechanical 
strength and not satisfactorily protected from the atmospheric conditions 
at the sites of factory such as oil, water, dust and the like, and hence, 
tend to malfunction due to the presence of chips, dust, cooling water and 
the like, thus lacking in reliability. Furthermore, the positional 
relationship between the limit switches and the dogs tends to go out of 
order with age and it is difficult to adjust the dogs. Further, there is 
little disadvantage in the use of a single purpose machine in which a 
single workpiece is worked on by use of a single spindle. However, in the 
use of a general purpose machine having been demanded in recent years in 
which a plurality of workings are effected on a single workpiece or a 
plurality of workpieces, there is presented such a disadvantage that the 
positions of the dogs and limit switches should be changed each time the 
workpieces or spindles are replaced with new ones, thus lacking in 
general-purpose properties. 
With the machine tools as described above, in the case position sensing 
singles emitted from the limit switches are directly used in controlling 
the approach means or the like, the position sensing signal should be 
emitted not only at the instant the slide table reaches the setting 
position but also should be emitted when the slide table has reached the 
setting position and stays within a predetermined stroke from the setting 
position, the stroke being suitable for controlling the approach. 
Consequently, in the case point dog is used for switching the limit switch 
34 ON for a very limited predetermined period of time for example, 
heretofore, a self-sustaining circuit has been additionally used so as to 
sustain the position sensing signals emitted from the limit switches for a 
predetermined period of time corresponding to a predetermined stroke, or 
another point dog has been provided at a terminal point of the 
predetermined stroke so as to ON-OFF operate a single limit switch by use 
of two point dogs. As a result, the relay sequence has become complicated, 
reliability has been lowered, and moreover, the costs for manufacture have 
been increased. On the other hand, in the case a long dog is used which 
can operate the limit switch 34 for a comparatively long predetermined 
stroke, there are encountered such disadvantages that not only the long 
dog is decreased in ridigity but also it is difficult to adjust the long 
dog, and the change in signal sustaining time requires the long dog to be 
replaced. The abovedescribed disadvantages are true of the case a 
proximity switch is used of the limit switch. 
On the other hand, with numerically controlled machine tools and the like 
which have recently been manufactured, digital servomechanisms each 
provided therein with a positional feedback system have been adopted in 
many cases. As shown in FIG. 2, this digital servomechanism includes: a 
pulse generator 40 for directly or indirectly transducing a movement of a 
movable body 38 such as a slide table into a digital pulse signal; a 
reversible counter 42 for reversibly counting pulse signals emitted from 
the pulse generator 40 corresponding to the position of the movable body 
38 and supplying a deviation between preset command signal pulses and the 
pulse signals thus reversibly counted; a D/A converter 44 for converting 
an output from the reversible counter 42 into an analog signal; and a 
servoamplifier 46 for amplifying an output from the D/A converter 44 and 
supplying an output to a servomotor 48 for controlling the position of the 
movable body 38. The digital servomechanism as described above can monitor 
the position of the movable body 38 from time to time, and hence, it 
should lead to that the approach control with a high degree of accuracy 
can be effected in the ideal condition. However, in actual practice, in 
the abovedescribed servomechanism, the parts are large in number, the 
construction is complicated, the manufacturing costs are very high, the 
components are low in reliability and backlash and the like in 
transmission system contribute to unstable operation of the 
servomechanism. Further, there is presented such a disadvantage that a 
considerable scale of reconstruction is required before the servomechanism 
having the positional feedback system can be fitted to the conventional 
machine tool. 
SUMMARY OF THE INVENTION 
The present invention has been developed in view of the abovedescribed 
disadvantages of the prior art and has as its first object the provision 
of a method of sensing a position of a movable body and an apparatus 
therefor, wherein the position of the movable body can be reliably sensed 
without using mechanical limit switches being low in reliability or a 
complicated servomechanism, the apparatus can be readily assembled into a 
machine tool or the like, and moreover, the manufacturing costs can be 
reduced. 
The present invention has its second object the provision of a method of 
sensing a position of a movable body and an apparatus therefor, wherein 
the position of the movable body can be reliably sensed without using 
mechanical limit switches being low in reliability or a complicated 
servomechanism, and position sensing signals can be sustained for a 
predetermined stroke. 
The present invention has achieved the aforesaid first object by providing 
a method of sensing a position of a movable body by directly or indirectly 
converting a unit movement of the movable body into digital pulse signals, 
wherein said method includes: a first step of selecting as the reference 
position a point, at which the movable body abuts against and presses a 
stopper provided at a specific positon within the moving range thereof and 
counting the pulse signals using the reference position as the reference 
number; and a second step of sensing the coincidence of the number of the 
pulse signals counted in the first step with a first predetermined pulse 
number corresponding to a predetermined position so as to sense the fact 
that the movable body has reached the predetermined position. According to 
the present invention, the position of the movable body can be sensed in 
noncontact manner with a high degree of accuracy for a long period of time 
without using mechanical limit switches, a complicated servomechanism is 
not required, the apparatus as a whole is rendered compact to reduce the 
manufacturing costs, and the change of control switching points from one 
to another is facilitated, so that a multiple types of controls can be 
performed by one and the same apparatus. 
In the aforesaid first step, in resetting the counted number of the pulse 
signals at the reference position as the reference number, when the 
movable body returns to the reference position, irrespective of what the 
counted number of the pulse signals, the movable body is adapted to abut 
against and press the stopper under a substantially constant force and 
continue its movement until the pulse signals cease from being supplied, 
and the counted number of the pulse signals at this time is reset as the 
reference number, thus enabling to further increase the accuracy in 
sensing the position. 
Or, in the aforesaid step, in resetting the counted number of the pulse 
signals at the reference position as the reference number, when the 
counted number of the pulse signals at the time of being ceased from being 
supplied is out of the preset tolerance limits, off signals are adapted to 
be issued, thus facilitating to locate a fault. 
Furthermore, the present invention has achieved the aforesaid second object 
by providing a method of sensing a position of a movable body by directly 
or indirectly converting a unit movement of the movable body into digital 
pulse signals, wherein said method includes: a first step of counting the 
pulse signals; a second step of sensing the coincidence of the counted 
number of the pulse signals counted in the first step with a first 
predetermined pulse number corresponding to a predetermined position 
preset so as to issue position sensing signals; and a third step of 
sustaining the position sensing signals emitted in the second step for the 
duration of a second predetermined pulse number corresponding to a 
predetermined stroke preset so as to sense the fact that the movable body 
has reached a predetermined position and stays within a predetermined 
stroke from the predetermined position. According to the present 
invention, the fact that the movable body has reached the predetermined 
position and stays within the predetermined stroke from the predetermined 
position can be reliably sensed without using mechanical limit switches 
low in operating reliability, self-sustaining circuit, long dog or the 
like, and further without using a complicated, expensive servomechanism. 
Moreover, the change of the predetermined positions and the change of the 
predetermined strokes can be performed by rewriting of Random Access 
Memory (hereinafter referred to as "RAM") or the like of the limit 
detecting circuit, thus rendering general purpose properties. 
According to the present invention, in an apparatus for sensing a position 
of a movable body for use in a machine tool including: a slide base 
fixedly provided; a slide table movable on the slide table in the forward 
and backward directions; a DC motor for moving the slide table forward or 
backward by rotating a feed shaft threadably coupled to a feed nut solidly 
secured to the slide table; a DC motor driving circuit for controlling the 
rotational direction and rotational speed of the DC motor; a speed change 
setting circuit for supplying a proper approach speed setting value in 
accordance with the forwarded position of the slide table to the DC motor 
driving circuit; and a mechanical sequence circuit controlling the DC 
motor driving circuit, wherein said apparatus further includes: a dead 
stopper solidly secured to the inner wall surface of the slide base; a 
rotary encoder for detecting a rotation of the feed shaft corresponding to 
a movement of the slide table; and a limit detecting circuit for sensing 
an approach position of the slide table in accordance with an output from 
the rotary encoder and supplying same to a mechanical sequence circuit. 
Additionally, the rotary encoder includes a pair of photoelectric elements 
spaced apart from each other through 90.degree. in phase. 
Or, the aforesaid limit detecting circuit is made to include: an encoder 
input circuit for receiving an output from the rotary encoder; an input 
circuit for receiving a command during an output emitted from the 
mechanical sequence circuit; an output circuit; an indicator circuit; a 
peripheral interfence capable of programming; a central operation 
processing unit; a Read Only Memory; a Random Access Memory in which a 
predetermined pulse number is written and stored; and a bus for connecting 
the abovedescribed various components to one another.

DETAILED DESCRIPTION OF THE INVENTION 
Detailed description will hereunder be given of embodiments of the present 
invention with reference to the drawings. 
A first embodiment of the present invention is applied to the conventional 
machine tool shown in FIG. 1. As shown in FIG. 3, the first embodiment 
includes: a rotary encoder 50 for detecting a rotation of a feed shaft 22 
corresponding to a movement of a slide table 18 from a gear coupling 49; 
and a limit detecting circuit 52 for detecting an approach position of the 
slide table 18 in accordance with an output from the rotary encoder 50 and 
supplying an output to a mechanical sequence circuit 30; in place of dogs 
and limit switches. Other respects are similar to those of the 
conventional example as shown in FIG. 1, so that detailed description 
thereof will be omitted. 
For example, the rotary encoder 50 includes a pair of photoelectric 
elements provided about a gear of a gear coupling of the DC motor 24 being 
spaced apart from each other through 90.degree. in phase, and adapted to 
supply two pulse signals different in phase depending on the rotational 
direction of the feed shaft 22, i.e., the approach direction of the slide 
table 18 as shown in FIG. 4 to the limit detecting circuit 52. 
The limit detecting circuit 52 includes: an encoder input circuit 54 for 
receiving an output from the rotary encoder 50; an input circuit 56 for 
receiving a command during an output emitted from the mechanical sequence 
circuit 30; an output circuit 58; an indicator circuit 60; a peripheral 
interface (hereinafter referred to as "PPI") 62; a central operation 
processing unit (hereinafter referred to as "CPU") 64; a Read Only Memory 
(hereinafter referred to as "ROM") 66; a RAM 68; and a bus for connecting 
the abovedescribed components to one another. In RAM 68 of this limit 
detecting circuit 52, first preset pulse numbers P.sub.1 .about.P.sub.4 
corresponding to control change positions P.sub.1 .about.P.sub.4 are 
written in and stored by a program module separately prepared. 
Description will hereunder be given of action with reference to FIGS. 5 and 
6. A DC motor 24 rotates a feed shaft 22 through a gear coupling 49 to 
move a slide table 18 secured thereto with a feed nut 20 forward or 
backward. The slide table 18 is caused to rapid-approach to P.sub.1 
.about.P.sub.2 in response to limit signals LS.sub.1 .about.LS.sub.4 
obtained from the limit detecting circuit 52, to first-slow-approach to 
P.sub.2 .about.P.sub.3, to second-slow-approach to P.sub.3 .about.P.sub.4, 
and finally, to rapid-return to P.sub.4 .about.P.sub.1 as shown in FIG. 5 
for example. These approach speeds are preset in a speed change setting 
circuit 28, and one of these speeds is selected by opening or closing of a 
relay in the mechanical sequence circuit 30. 
When the feed shaft 22 is rotated by the DC motor 24, the rotary encoder 
50, being connected to the gear coupling 49, supplies two output pulses 
different in phase from each other according to the rotating direction, 
normal or reverse, as shown in FIG. 4, and received by the encoder input 
circuit 54 of the limit detecting circuit 52. 
As shown in FIG. 6, the limit detecting circuit 52 receives command signals 
during its operation from the mechanical sequence circuit 30 through the 
input circuit 56, and is at rest during its non-operation. In the case the 
mechanical sequence circuit 30 is supplying output signals, the two pulse 
signals being different in phase through 90.degree. from each other which 
are emitted from the rotary encoder 50 are discriminated whether they are 
in the forward direction (the normal rotation of the encoder) or in the 
backward direction (the reverse rotation of the encoder). In the case the 
encoder rotates in the normal direction and the slide table 18 moves in 
the forward direction, abnormal outputs to be described hereinafter from 
the limit detecting circuit 52 are cleared and the pulse signals from the 
rotary encoder 50 are summed. While, in the case the encoder supplies the 
reversed signals and the slide table 18 moves in the backward direction, 
the number of output pulses supplied from the encoder in accordance with 
the rearward movement of the slide table 18 is subtracted from the total 
sum of the pulse signals counted till then. These summed or subtracted 
result is compared by CPU 64 with the first preset pulse numbers P.sub.1 
.about.P.sub.4 corresponding to the predetermined positions P.sub.1 
.about.P.sub.4. When the former and the latter coincide with each other, 
ON-OFF condition of the limit signals LS.sub.1 .about.LS.sub.4 is changed 
and an output is supplied to the output circuit 58 through PPI 62. By 
this, the mechanical sequence circuit 30 can be controlled in accordance 
with the position of the slide table 18. When the working on and a stopper 
36 at one side of the slide table 18 reaches an original position where it 
comes into abutting contact with a dead stopper 37 secured to the end face 
of a slide base 14, the forward movement of the slide table 18 becomes 
impossible irrespective of what the rotating torque is, so that no output 
pulses can be obtained from the rotary encoder 50. Consequently, the 
condition of the outputs from the rotary encoder 50 disappearing for a 
certain period of time is determined to be the original position and the 
counter is reset. At this time, before resetting, it is descriminated 
whether the counted value of the counter at the time of returning to the 
original point comes to be about zero or not. More specifically, when the 
counted value of the counter is above the lower limit of the 
discrimination value and below the upper limit of the discrimination 
value, it is discriminated to be a normal operation, the counted value of 
the counter is reset at zero, rapid return is performed and the succeeding 
working is started. On the other hand, when the counted value of the 
counter is less than the lower limit of the discrimination value or over 
the upper limit of the discrimination value, it is determined that the 
counted value of the counter is shifted from the actual position of the 
slide table 18 due to an abnormality in the mechanical or electrical 
system. Then, an abnormality is indicated by the indicator circuit 60, and 
an abnormal output is supplied to the mechanical sequence circuit 30 to 
stop the working. 
Additionally, change of the limit signal (the control change signal point) 
can be readily effected by correcting the data (corresponding to the 
distance from the original point) of RAM 68 in the limit detecting circuit 
52 by a program module separately prepared. 
A second embodiment of the present invention will now be described. In this 
second embodiment, in RAM 68 in the limit detecting circuit 52, there are 
written and stored not only first preset pulse numbers P.sub.1 
.about.P.sub.4 corresponding to control change positions P.sub.1 
.about.P.sub.4 but also second preset pulse numbers t.sub.1 .about.t.sub.4 
corresponding to predetermined numbers of stroke T.sub.1 .about.T.sub.4 
for holding limit signals LS.sub.1 .about.LS.sub.4 which are position 
detecting signals corresponding to respective control change positions 
P.sub.1 .about.P.sub.4 when the slide table 18 reaches the respective 
control change positions P.sub.1 .about.P.sub.4 as in the aforesaid first 
embodiment. Other respects are similar to those of the first embodiment, 
so that detailed description thereof will be omitted. 
In this second embodiment, limit signals from the limit detecting circuit 
52 are supplied according to a flow chart shown in FIG. 7. More 
specifically, the counted number pi output pulses supplied from the rotary 
encoder 50 to CPU 64 through the encoder input circuit 54 and PPI 62 
becomes a preset pulse number P.sub.1 corresponding to a predetermined 
position P.sub.1 preset, the limit signal LS.sub.1 is turned ON and this 
ON condition is held for the duration of a predetermined pulse number 
t.sub.1 corresponding to a predetermined stroke T.sub.1 preset in RAM 68. 
Consequently, a DC motor driving circuit 26 is controlled by a 
predetermined stroke T.sub.1 in response to this limit signal LS.sub.1 
through the mechanical sequence circuit 30. In the case, the counted 
number pi counted by CPU 64 coincides with a preset pulse number p.sub.2 
corresponding to a predetermined position P.sub.2 preset, the limit signal 
LS.sub.2 is turned ON, and this ON condition is held for the duration of a 
predetermined pulse number t.sub.2 corresponding to a predetermined stroke 
t.sub.3. Likewise, in the case the counted number pi coincides with a 
preset pulse number p.sub.4, the limit signal LS.sub.4 is sustained for 
the duration of a predetermined pulse number t.sub.4. The mechanical 
sequence circuit 30 is controlled by these limit signals in accordance 
with the position of the slide table 18. Other respects are similar to 
those of the first embodiment, so that detailed description thereof will 
be omitted. 
In each of the abovedescribed embodiments, such an instant is not 
discriminated as the original point that the stopper 36 on the side of the 
slide table 18 comes into abutting contact with the dead stopper 37, but 
such a point is discriminated as the original point that the stopper 36 is 
continuouslly pressed against the dead stopper 37 for a predetermined 
period of time under a substantially constant force by the torque of the 
DC motor 24. Hence, errors due to a backlash, return and the like can be 
minimized. Additionally, in each of the abovedescribed embodiments, the 
reference position is selected to be the position of work being completed 
at the forward-most position of the slide table. However, the reference 
position should not be limited to this but the reference portion may be 
selected to be the position of return being completed at the rearward-most 
position of the slide table. Furthermore, in each of the abovedescribed 
embodiments, the reference positions are selected to be travel-ending 
positions (i.e., stroke ends) of the slide table 18, however, the 
reference position should not be limited to these positions, but the 
reference positions may be selected to be intermediate positions or 
intermediate working positions where certain working are completed. 
Furthermore, in each of the abovedescribed embodiments, two pulse signals 
having a difference in phase through 90.degree. are adapted to be supplied 
from the rotary encoder 50, the types of the output signals from the 
rotary encoder should not be limited to the above, but, in the case a row 
of single pulses are used as the output signals from the rotary encoder, 
the mechanical sequence circuit 30 may supply forward and backward 
commands for discrimination of rotating direction, or the rotary encoder 
itself may generate pulse signals different from each other in the case of 
normal or reverse rotation. In this case, the operation of the limit 
detecting circuit may be more simplified. 
Furthermore, in each of the abovedescribed embodiments, the limit signals 
are incorporated in the mechanical sequence circuit consisting of the 
relay sequence circuit, but, needless to say, the limit signals may be 
incorporated in a sequencer which has been widely used in recent years. 
Further, in each of the abovedescribed embodiments, the present invention 
is applied to a machine tool, in which a slide table is controlled in its 
approach by a DC motor, the scope of applications of the present invention 
should not be limited to this, but may be likewise applied to generally 
used machine tools in which the slide table is controlled in its approach 
by a hydraulic or pneumatic cylinder and the approach speed is changed by 
switching the flow rate of hydraulic oil or air. 
From the foregoing description, it should be apparent to one skilled in the 
art that the abovedescribed embodiment is but one of many possible 
specific embodiments which can represent the applications of the 
principles of the present invention. Numerous and varied other 
arrangements can be readily devised by those skilled in the art without 
departing from the spirit and scope of the invention.