Method for controlling electric actuator and apparatus for the same

Rotary motion of a motor of an electric actuator is converted into rectilinear motion with a small sliding resistance by the aid of a ball screw and a feed nut. On the other hand, a control apparatus for controlling the electric actuator comprises memory groups composed of memories to indicate displacement positions of a displacement member. The memory groups include a starting point memory. If a controller selects the starting point memory, the control apparatus displaces the displacement member to a starting point position. Therefore, it is unnecessary to provide any dedicated signal line for displacing the displacement member to the starting point position. It is possible to decrease the number of signal lines connected to the control apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a control method and an apparatus for the 
same for controlling a displacement position of a displacement member 
provided in an electric actuator. 
2. Description of the Related Art 
An electric actuator has been hitherto used as a means for transporting a 
workpiece or the like. A control apparatus for controlling the electric 
actuator is provided with signal lines connected to a controller for 
selecting an arrival target position of a displacement member, and 
dedicated signal lines for displacing the displacement member to a 
position of a starting point. 
As described above, the control apparatus for the electric actuator 
concerning the conventional technique requires the dedicated signal lines. 
Therefore, a problem arises in that it is necessary to provide a large 
number of signal lines for the control apparatus. 
SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a method for 
controlling an electric actuator and an apparatus for the same which make 
it possible to decrease the number of signal lines connected to the 
control apparatus for the electric actuator. 
A principal object of the present invention is to provide a method for 
controlling an electric actuator and an apparatus for the same which make 
it possible to decrease the number of signal lines connected to a 
controller by using one of a plurality of memories for storing arrival 
target positions of a displacement member, as a starting point memory for 
storing a position of a starting point of the displacement member so that 
any dedicated signal line is unnecessary to restore the displacement 
member to the starting point. 
Another object of the present invention is to provide a method for 
controlling an electric actuator and an apparatus for the same which make 
it possible to prevent the electric actuator from increase in load and 
eliminate any fear of breakage of the electric actuator by detecting 
certain displacement of a displacement member when the displacement member 
is displaced to a starting point or when the displacement member is 
displaced to a displacement limit disposed on a side opposite to the 
starting point so that the electric actuator is stopped. 
The above and other objects, features, and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method for controlling the electric actuator and the apparatus for the 
same according to the present invention will be described in detail below 
with reference to the accompanying drawings, referring to a preferred 
embodiment. 
With reference to FIG. 1, reference numeral 10 indicates an electric 
actuator to be controlled by a control method according to the embodiment 
of the present invention. The electric actuator 10 comprises a body 12 
which has a lengthy size, and a slide table 14 which is arranged in 
parallel to the body 12 and which is formed of, for example, aluminum or 
synthetic resin. A part of the body 12 is formed to expand upwardly along 
the longitudinal direction. The body 12 has its upper surface which is 
engraved with four strips of grooves 16a to 16d. A magnetic detection 
switch 18, which serves as a displacement limit-detecting switch for 
detecting one displacement limit of the slide table 14, is arranged in any 
one of the grooves 16a to 16d. A plurality of magnetic detection switches 
18 may be provided in the grooves 16a to 16d, if necessary. 
A hole 20, which extends along the longitudinal direction of the body 12, 
is defined in the body 12 (see FIG. 2). A motor 22 such as a stepping 
motor is secured to one end of the body 12. A ball screw 26, which has, 
for example, a diameter of 8 mm and a length of 139 mm, is coaxially and 
rotatably attached to a rotary shaft of the motor 22 via a coupling member 
24. Therefore, the ball screw 26 is arranged along the longitudinal 
direction of the hole 20. A feed nut 28 meshes with the ball screw 26 via 
a plurality of ball members 27. A magnet 30 to be used for positional 
detection is provided on the feed nut 28. A cylindrical member 32, which 
is formed to have a cylindrical configuration with a lengthy size, is 
slidably fitted to the hole 20. The cylindrical member 32 surrounds the 
ball screw 26. The cylindrical member 32 is composed of a light metal such 
as aluminum or a synthetic resin. The feed nut 28 is secured to one end of 
the cylindrical member 32. A plate-shaped support member 34 is secured to 
the other end of the cylindrical member 32. The slide table 14 is 
connected to the support member 34. A displacement member 36 is 
constructed by the support member 34 and the slide table 14 (see FIG. 1). 
The support member 34 and the slide table 14 may be formed of a material 
such as aluminum. Alternatively, when the support member 34 and the slide 
table 14 are composed of a synthetic resin such as polyimide resin and 
polyacetal, then the electric actuator 10 has a light weight, and it is 
possible to decrease the load on the motor 22. For example, the body 12 
may be also formed of aluminum or synthetic resin. 
As shown in FIG. 3, guide members 38a, 38b are fastened by screws 40a 40b 
at the lower surface of the slide table 14 along the longitudinal 
direction. Projections 42a, 42b are formed on both side surfaces of the 
guide members 38a, 38b. Guide rails 44a, 44b are secured to the upper 
surface of the body 12, and V-shaped grooves 46a, 46b are defined on the 
guide rails 44a, 44b. The grooves 46a, 46b are slidably engaged with the 
projections 42a, 42b. Therefore, a guide means is constructed by the guide 
members 38a, 38b and the guide rails 44a, 44b. As shown in FIG. 4, an 
optical sensor switch 48, which serves as a starting point-detecting 
switch for detecting the position of the starting point of the slide table 
14, is provided at an end of the upper surface of the body 12. The optical 
sensor switch 48 comprises a light-emitting section 50a and a 
light-receiving section 50b which are opposed to one another. A shielding 
member 52, which is formed to have a plate-shaped configuration, is 
provided at the end of the guide members 38a, 38b opposite to the support 
member 34. When the guide members 38a, 38b slide to the end opposite to 
the support member 34, the shielding member 52 is inserted into the space 
between the light-emitting section 50a and the light-receiving section 50b 
of the optical sensor switch 48. 
Next, explanation will be made with reference to FIG. 5 and the followings 
for a control apparatus 60 according to the embodiment of the present 
invention for controlling the electric actuator 10. 
The control apparatus 60 is formed to have a substantially rectangular 
parallelepiped configuration, including a plurality of heat-releasing long 
holes 62a and heat-releasing fins 62b which are formed at its upper and 
side surfaces. An engagement groove 61 is formed at the back of the 
control apparatus 60. As shown in FIG. 6, the engagement grooves 61 are 
engaged with a rail member 63 to attach the control apparatuses 60 to the 
rail member 63. In this embodiment, a plurality of control apparatuses 60 
may be attached to the rail member 63. As shown in FIG. 7, connectors 64, 
66 are provided at the bottom of the control apparatus 60. Each of the 
connectors 64, 66 is connectable to the electric actuator 10 and the 
controller 70 (see FIG. 8). Serial communication connectors 72a, 72b are 
provided at the bottom of the control apparatus 60. The serial 
communication connectors 72a, 72b are connectable to other control 
apparatuses 60 or the like by using cables 73 (see FIG. 6). 
As shown in FIG. 8, the control apparatus 60 comprises therein a control 
input/output unit 74 to be connected to the connector 66. The control 
input/output unit 74 is provided with a plurality of resistors 76 and 
light emitting diodes 78 to serve as input interfaces, and a plurality of 
phototransistors 80 to serve as output interfaces. A motor control unit 82 
is connected to the control input/output unit 74. Signals are sent and 
received between the motor control unit 82 and the controller 70 via the 
control input/output unit 74. A motor drive unit 84, which functions as a 
driver for driving the motor 22, is connected to the motor control unit 
82. The motor drive unit 84 is connected to the motor 22 of the electric 
actuator 10 via the connector 64 so that the driving signal is supplied to 
the motor 22. A switch input unit 86 is connected to the motor control 
unit 82. The switch input unit 86 is connected via the connector 64 to the 
optical sensor switch 48, the magnetic detection switch 18, and a 
malfunction detection switch 88 of the electric actuator 10. The switch 
input unit 86 functions as an interface between the motor control unit 82 
and the optical sensor switch 48, the magnetic detection switch 18, and 
the malfunction detection switch 88. A power source unit 90 is connected 
to the connector 64. 
A positional information storage unit 92 is connected to the motor control 
unit 82. As shown in FIG. 9, the positional information storage unit 92 is 
comparted into eight memory groups 96a to 96h. Each of the memory groups 
96a to 96h comprises four memories 98a to 98d. The memory 98a of the 
predetermined memory group 96 is used as a starting point memory 99a for 
storing the position of the starting point of the slide table 14. The 
memory 98b of the memory group 96a is used as a positive direction 
movement memory 99b, the memory 98c is used as a negative direction 
movement memory 99c, and the memory 98d is used as a velocity information 
memory 99d. Arbitrary positional information for indicating an arrival 
target position of the displacement member 36 is stored in each of the 
memories 98a to 98d of the other memory groups 96b to 96h. That is, the 
control apparatus 60 can store positional information on 28 places. 
Any one of the memory groups 96a to 96h is selected depending on the signal 
inputted from the controller 70 into signal lines BNK1 to BNK3 of the 
connector 66. Any one of the memories 98a to 98d is selected depending on 
the signal inputted into signal lines POINT 1 TO POINT 4 of the connector 
66 (see FIG. 8). Accordingly, the controller 70 can select any one of the 
predetermined memories 98a to 98d of the predetermined memory groups 96a 
to 96h. 
In this embodiment, the signal lines BNK1 to BNK3 and the signal lines 
POINT 1 to POINT4 can be used to select the action of the slide table 14 
including, for example, restoration to the starting point, movement in the 
positive direction, movement in the negative direction, and velocity 
switching. Therefore, it is possible to decrease the number of signal 
lines as compared with a case in which dedicated signal lines are provided 
to restore the slide table 14 to the starting point, and move the slide 
table 14 in the positive and negative directions. If the number of signal 
lines is not decreased, the dedicated signal lines can be used as signal 
lines for indicating positional information. Therefore, it is possible to 
displace the slide table 14 to a larger number of positions. 
The control apparatus 60 for the electric actuator 10 according to the 
embodiment of the present invention is basically constructed as described 
above. Next, its operation will be explained below on the basis of flow 
charts shown in FIGS. 10 to 13, as related to the method for controlling 
the electric actuator 10 according to the embodiment of the present 
invention. 
At first, the method for restoring the slide table 14 to the starting point 
will be explained with reference to a time chart shown in FIG. 14. 
The controller 70 allows the signal lines BNK1 to BNK3 of the control 
input/output unit 74 to have "0" to select the memory group 96a of the 
positional information storage unit 92 (see Region 100a in FIG. 14, FIG. 
8, and FIG. 9). At this point of time, the motor control unit 82 checks 
whether or not all of the signal lines POINT1 to POINT4 have "0" (FIG. 10, 
step S1). If all of the signal lines POINT1 to POINT4 have "0", then the 
motor 22 is stopped (step S2), and the signal line BUSY is allowed to have 
"0" (step S3). Subsequently, the controller 70 allows the signal line 
POINT1 to have "1" to select the memory 98a, i.e., the starting point 
memory 99a (Region 100b). Since all of the signal lines POINT1 to POINT4 
do not have "0" in the step S1, the motor control unit 82 judges whether 
or not all of the signal lines BNK1 to BNK3 have "0" (step S4). In this 
case, since all of the signal lines BNK1 to BNK3 have "0", it is judged 
whether or not the starting point memory 99a is selected according to the 
signal lines POINT1 to POINT4 (step S6). In this case, since the starting 
point memory 99a is selected, the routine proceeds to a starting point 
position-restoring return routine (step S7). 
The operation is executed in the starting point position-restoring return 
routine as shown in FIG. 11. That is, the motor control unit 82 allows the 
signal line BUSY of the control input/output unit 74 to have "1" to 
indicate for the controller 70 that the control operation of the electric 
actuator 10 is started (step S21). The motor drive unit 84 is controlled 
in order to displace the slide table 14 to the starting point position. 
The motor drive unit 84 outputs a drive pulse signal to the motor 22 of 
the electric actuator 10. Accordingly, the motor 22 starts rotation (step 
S22). 
When the motor 22 is driven as described above, the ball screw 26 is 
rotated by the aid of the coupling member 24 (see FIG. 2). Accordingly, 
the rotary motion of the ball screw 26 is converted into the rectilinear 
motion by the aid of the feed nut 28, and the feed nut 28 is linearly 
displaced. As a result, the slide table 14 is moved along the guide 
members 38a, 38b in accordance with the rotation of the motor 22. In this 
embodiment, the end of the wall for constructing the hole 20 is closed by 
the cylindrical member 32. The ball screw 26 is surrounded by the 
cylindrical member 32, and it is not exposed to the outside. Therefore, it 
is possible to prevent the ball screw 26 from adhesion of dust or the 
like. 
The motor control unit 82 detects the signal from the optical sensor switch 
48 as the starting point switch to judge whether or not the slide table 14 
is restored to the starting point position (step S23). If the shielding 
member 52 is not inserted into the space between the light-emitting 
section 50a and the light-receiving section 50b of the optical sensor 
switch 48, then the optical sensor switch 48 is turned ON, and the motor 
control unit 82 detects that the slide table 14 is not restored to the 
starting point position. Accordingly, the starting point 
position-restoring return routine comes to an end, and the routine returns 
to the step S1 (see FIG. 10). 
The routine proceeds to the steps S1, S4, and S6 in the same manner as 
described above, and the routine proceeds to the starting point 
position-restoring return routine again. Accordingly, the signal line BUSY 
maintains "1" (step S21), the motor 22 continues the rotation (step S22), 
and the slide table 14 continues the movement. 
If the shielding member 52 is inserted into the space between the 
light-emitting section 50a and the light-receiving section 50b of the 
optical sensor switch 48 in accordance with the movement action of the 
slide table 14, the optical sensor switch 48 is turned OFF. The signal for 
this fact is transmitted from the connector 64 to the switch input unit 
86. The motor control unit 82 detects that the slide table 14 has been 
moved to the starting point position (step S23). The detection allows the 
motor drive unit 84 to stop the rotation of the motor 22 (step S24). 
During this process, the slide table 14 cannot be stopped immediately due 
to inertia, and it causes overrun to exceed the starting point position in 
some cases. Accordingly, the motor drive unit 84 reverses the motor 22 to 
displace the slide table 14 in the opposite direction in a certain range 
so that the overrun is absorbed to restore the slide table 14 to the 
starting point position (step S25, Region 100c). 
As described above, the slide table 14 is stopped at the starting point 
position on the basis of the signal outputted from the optical sensor 
switch 48. Therefore, for example, it is possible to avoid breakage of the 
motor 22, the ball screw 26, and the feed nut 28, which would be otherwise 
caused due to possible displacement of the slide table 14 in the same 
direction exceeding the starting point position. 
If the slide table 14 is displaced to the starting point position as 
described above, then the motor control unit 82 allows the signal line 
BUSY of the control input/output unit 74 to have "0", and it allows the 
signal line SET-ON to have "1" (step S26). Further, the same signals as 
those for the signal lines POINT1 to POINT4 are outputted to the signal 
lines OUT1 to OUT4 (step S27). Accordingly, the controller 70 detects that 
the slide table 14 has been moved to the starting point position (Region 
100d). 
Next, explanation will be made with reference to FIG. 15 for the method for 
displacing the slide table 14 to the arrival target position stored in any 
one of the memories 98a to 98d of the memory groups 96b to 96h of the 
positional information storage unit 92. 
At first, the controller 70 allows the signal lines BNK1 to BNK3 of the 
control input/output unit 74 to have a predetermined value of "0" or "1" 
to select any one of the memory groups 96b to 96h of the positional 
information storage unit 92 (see Region 102a in FIG. 15, FIG. 8, and FIG. 
9). Subsequently, if necessary information is stored in the memory 98b, 
the signal line POINT2 is allowed to have "1" to select the memory 98b 
(Region 102b). Since all of the signal lines POINT1 to POINT4 do not have 
"0" in the step S1, the routine proceeds to the step S4. Further, since 
all of the signal lines BNK1 to BNK3 do not have "0", the routine proceeds 
to a memory storage position-attaining movement routine (step S5). The 
motor control unit 82 allows the signal line BUSY of the control 
input/output unit 74 to have "1" to indicate for the controller 70 that 
the control operation for the electric actuator 10 is started (step S31 in 
FIG. 12). The motor drive unit 84 is controlled in order to displace the 
slide table 14 to the position stored in the memory 98b. The motor drive 
unit 84 outputs a drive pulse signal to the motor 22 of the electric 
actuator 10. Accordingly, the motor 22 starts rotation (step S32). At this 
time, the motor control unit 82 counts the number of pulses of the drive 
pulse signal outputted from the motor drive unit 84 to the motor 22. 
Therefore, it is possible for the motor control unit 82 to detect the 
displacement position of the slide table 14. 
The motor control unit 82 compares the displacement position of the slide 
table 14 with the position stored in the memory 98b (step S33). If the 
both are not coincident with each other, then the memory storage 
position-attaining movement routine comes to an end, and the routine 
returns to the step S1 (see FIG. 10). The routine proceeds to the steps S1 
and S4 in the same manner as described above, and the routine proceeds to 
the memory storage position-attaining movement routine again. Accordingly, 
the signal line BUSY maintains "1" (step S31), the motor 22 continues the 
rotation (step S32), and the slide table 14 continues the movement as 
well. 
If the signal line POINT2 is allowed to have "0" (Region 102c) before the 
slide table 14 arrives at the position stored in the memory 98b, the motor 
control unit 82 detects in the step S1 that all of the signal lines POINT1 
to POINT4 have "0" to stop the rotation of the motor 22 by the aid of the 
motor drive unit 84 (step S2). If the motor 22 is stopped, the motor 
control unit 82 allows the signal line BUSY to have "0" (step S3, Region 
102d). 
If the signal line POINT2 is allowed to have "1" again (Region 102e), then 
the motor control unit 82 allows the signal line BUSY to have "1" in the 
step S31, and the motor 22 is rotated in the step S32. Accordingly, the 
slide table 14 is moved again toward the position stored in the memory 
98b. 
If the displacement position of the slide table 14 is coincident with the 
position stored in the memory 98b (step S33), the motor control unit 82 
stops the rotation of the motor 22 (step S34, Region 102f). Accordingly, 
the slide table 14 stops at the position stored in the memory 98b. The 
motor control unit 82 allows the signal line BUSY to have "0" to indicate 
for the controller 70 that the slide table 14 has been displaced to the 
predetermined position (step S35). The same signals as those for the 
signal lines POINT1 to POINT4 are outputted to the signal lines OUT1 to 
OUT4 (step S36). 
Next, explanation will be made with reference to FIG. 16 for the method for 
continuously moving the slide table 14 in a predetermined direction 
regardless of the position stored in the positional information storage 
unit 92. 
At first, the controller 70 allows the signal lines BNK1 to BNK3 of the 
control input/output unit 74 to have "0" to select the memory group 96a of 
the positional information storage unit 92 (see Region 104a in FIG. 16, 
FIG. 8, and FIG. 9). Subsequently, the signal line POINT 2 is allowed to 
have "1" to select the memory 98b, i.e., the positive direction movement 
memory 99b (Region 104b). The motor control unit 82 detects in the step S1 
that all of the signal lines POINT1 to POINT4 do not have "0", and the 
routine proceeds to the step S4. Further, it is detected that all of the 
signal lines BNK1 to BNK3 have "0", and the routine proceeds to the step 
S6. Since the controller 70 does not select the starting point memory 99a, 
the routine proceeds to the step S8. Since the positive direction movement 
memory 99b is selected, the routine proceeds to a positive direction 
movement routine (step S9). 
The motor control unit 82 allows the signal line BUSY of the control 
input/output unit 74 to indicate for the controller 70 that the control 
operation for the electric actuator 10 is started (step S41 in FIG. 13). 
The signal line SET-ON is allowed to have "0" to indicate that the slide 
table 14 makes continuous movement regardless of the positional 
information stored in the positional information storage unit 92 (step 
S42). The motor drive unit 84 is controlled in order to displace the slide 
table 14 in the positive direction, i.e., in the direction to make 
separation from the starting point position. The motor drive unit 84 
outputs a drive pulse signal to the motor 22 of the electric actuator 10. 
Accordingly, the motor 22 starts rotation (step S43). 
If the motor 22 is driven as described above, the slide table 14 is moved 
in the positive direction along the guide members 38a, 38b in accordance 
with the rotation of the motor 22 in the same manner as described above. 
The motor control unit 82 judges whether or not the signal line POINT4 has 
"1" (step S44). If the signal line POINT4 has "1", it is judged that the 
velocity information memory 99d is selected. The motor control unit 82 
controls the velocity of rotation of the motor 22 to be a velocity of 
rotation stored in the velocity information memory 99d (step S45, Region 
104c). Accordingly, the velocity of movement of the slide table 14 is 
changed. 
Subsequently, the motor control unit 82 detects the signal supplied from 
the magnetic detection switch 18 as the displacement limit-detecting 
switch to judge whether or not the slide table 14 has arrived at the 
displacement limit position (step S46). If the magnet 30 provided on the 
feed nut 28 does not approach the magnetic detection switch 18, the 
magnetic detection switch 18 is not turned ON. Accordingly, the positive 
direction movement routine comes to an end, and the routine returns to the 
step S1 (see FIG. 10). 
If the signal line POINT2 is allowed to have "0", the motor control unit 82 
judges that the signal lines POINT1 to POINT4 do not have "0" in the step 
S1 to stop the rotation of the motor 22 (step S2, Region 104d). 
Accordingly, the movement of the slide table 14 is stopped. The motor 
control unit 82 allows the signal line BUSY to have "0" (step S2, Region 
104e). 
If the signal line POINT2 does not have "0", and the slide table 14 
continues movement in the positive direction, then the magnet 30 provided 
on the feed nut 28 approaches the magnetic detection switch 18, and the 
magnetic detection switch 18 is turned ON. The signal for this fact is 
transmitted from the connector 64 to the switch input unit 86. The motor 
control unit 82 detects that the slide table 14 has arrived at one 
displacement limit. At this time, the slide table 14 cannot make any more 
movement. Therefore, if the motor 22 continues the rotation, for example, 
there is a fear of increase in load on the motor 22, the ball screw 26, 
and the feed nut 28, resulting in breakage. Accordingly, if the motor 
control unit 82 detects that the magnetic detection switch 18 is turned ON 
in the step S46, the rotation of the motor 22 is stopped to avoid, for 
example, breakage of the motor 22, the ball screw 26, and the feed nut 28 
(step S47). The signal line BUSY is allowed to have "0" (step S48). 
If the slide table 14 is displaced in the negative direction, i.e., in the 
direction to approach the starting point position, the signal line POINT3 
is allowed to have "1" to select the negative direction movement memory 
99c. Accordingly, the motor control unit 82 judges that the negative 
direction movement memory 99c is selected in the step S10, and the routine 
proceeds to a negative direction movement routine (step S11). The negative 
direction movement routine is operated in the same manner as in the 
positive direction movement routine, detailed explanation of which will be 
omitted. 
The malfunction detection switch 88 is operated as follows. That is, for 
example, if the optical sensor switch 48 or the magnetic detection switch 
18 is turned ON when the slide table 14 of the electric actuator 10 is at 
a position different from those obtained during normal operation, it is 
judged that any abnormality occurs in the electric actuator 10 to output a 
signal indicating this fact to the motor control unit 82. In consequence, 
the motor control unit 82 stops the rotation of the motor 22. The signal 
line ERROR is allowed to have "1" to indicate this abnormality to the 
controller 70. 
According to the embodiment of the present invention, it is unnecessary to 
provide any dedicated signal line to be used for the starting point 
restoration signal for the control apparatus 60. Therefore, it is possible 
to decrease the number of signal lines to make connection between the 
control apparatus 60 and the controller 70. If the number of signal lines 
is not decreased, the dedicated signal line can be used as a signal line 
for indicating the positional information, making it possible to set a 
larger number of stop positions at which the displacement member 36 can be 
stopped by using the control apparatus 60. 
When the displacement member 36 arrives at the starting point or the 
displacement limit, the control apparatus 60 can detect this fact by the 
aid of the optical sensor switch 48 and the magnetic detection switch 18 
to stop the electric actuator 10. It is possible to avoid any increase in 
load on the electric actuator 10, and it is possible to eliminate any fear 
of breakage of the electric actuator 10.