Method for absolute position detection and an apparatus therefore

There are provided a method and an apparatus for absolute position detection, in which position detection is performed with high accuracy by interpolation between feedback pulses generated as an object of position detection moves, and the power-on command displacement can be calculated with high accuracy, the method and the apparatus entailing only small electric power consumption. A stored value in an absolute counter (2), which is indicative of an approximate absolute position of an object of position detection, is updated each time a feedback pulse is generated from a feedback pulse generating circuit (1), and is retentively stored in the counter even after the power is turned off. As the object of position detection moves, moreover, the count value of interpolation pulses, generated during the time interval which elapses from the instant that the power is turned on again until the first one of the feedback pulses is generated, is temporarily stored in a correction counter (5). The power-on absolute position of the object of position detection is calculated by means of a numerical control unit (20), in accordance with the stored value, the count value, and the moving direction designated by a movement command delivered when the power is turned on. Further, the apparatus calculates the command displacement on the basis of the calculated absolute position and a command target position.

TECHNICAL FIELD 
The present invention relates to absolute position detection in which 
position detection is performed with high accuracy by interpolation 
between feedback pulses generated as an object of position detection 
moves, and more particularly, to a method and an apparatus for absolute 
position detection, in which the command displacement upon application of 
power supply can be calculated with high accuracy, the method and the 
apparatus entailing only small electric power consumption. 
BACKGROUND ART 
In an absolute positioning system in which pulses of a quantity 
corresponding to a command displacement, which is obtained by subtracting 
the present absolute position from a command absolute position, are 
delivered to effect positioning control, if the present position upon 
application of power supply is unknown, a reference point return operation 
must be performed, since the command displacement cannot be precisely 
calculated on the basis of the command absolute position only. In order to 
obviate such an awkward situation, as is conventionally known, an absolute 
position detector, e.g., an absolute pulse encoder, may be subjected to 
battery-backup so that a detected absolute position upon termination of 
power supply is retentively stored even after the power is turned off, and 
the reference point return operation when the power is turned on again can 
be omitted. As is also conventionally known, feedback pulses, which are 
generated from a pulse encoder as an object of position detection moves, 
may be divided by means of interpolation pulses, thereby improving the 
detection resolution. 
However, if an absolute counter and an interpolation counter for counting 
the feedback pulses and the interpolation pulses, respectively, are both 
subjected to battery-backup, in order to obtain the advantages of both 
these prior art arrangements, substantial electric power consumption is 
required, that is, the system is uneconomical. 
DISCLOSURE OF THE INVENTION 
The object of the present invention is to provide a method and an apparatus 
for absolute position detection which permits high-accuracy calculation of 
a command displacement when the power is on, and entails less electric 
power consumption. 
In order to achieve the above object, a method for absolute position 
detection according to the present invention comprises steps of: (a) 
generating a feedback pulse having a polarity corresponding to the moving 
direction of an object of position detection each time the object of 
position detection moves for a first predetermined distance; (b) updating 
a stored value indicative of an approximate absolute position of the 
object of position detection each time the feedback pulse is generated; 
(c) generating an interpolation pulse each time the object of position 
detection moves for a second predetermined distance shorter than the first 
predetermined distance; (d) storing and retaining the stored value even 
after the power is turned off; (e) counting the interpolation pulses 
generated during the time interval which elapses from the instant that the 
power is turned on again until the first one of the feedback pulses is 
generated, and temporarily storing the count value obtained; (f) 
calculating the absolute position of the object of position detection 
reached when the power is turned on again, in accordance with the stored 
value retentively stored in the step (d), the count value of the 
interpolation pulses stored in the step (e), the first and second 
predetermined distances, and the moving direction designated by a movement 
command delivered when the power is turned on again. 
An apparatus for absolute position detection according to the present 
invention comprises: a first pulse generating means for generating a 
feedback pulse having a polarity corresponding to the moving direction of 
an object of position detection each time the object of position detection 
moves for a first predetermined distance; second pulse generating means 
for generating an interpolation pulse each time the object of position 
detection moves for a second predetermined distance shorter than the first 
predetermined distance; an absolute counter subject to battery-backup and 
adapted to update a stored value each time the feedback pulse from the 
first pulse generating means is inputted; an interpolation counter for 
counting interpolation pulses supplied from the second pulse generating 
means and temporarily storing the count value obtained; and a correction 
counter adapted to receive and temporarily store the count value stored in 
the interpolation counter when the feedback pulse is generated. 
Preferably, the position detecting apparatus is used with built-in or 
external calculating means, which calculates the absolute position of the 
object of position detection reached when the power is turned on again, 
when the first one of the feedback pulses is generated after the power is 
turned on again, in accordance with the stored value in the absolute 
counter, the count value stored in the correction counter, the first and 
second predetermined distances, and the moving direction designated by a 
movement command delivered when the power is turned on again. 
According to the present invention, as described above, the power-on 
absolute position of the object of position detection is detected in 
accordance with the respective count values of the feedback pulses and the 
interpolation pulses. Thus, the power-on absolute position of the object 
of position detection can be calculated with high accuracy, so that the 
command displacement can be calculated directly on the basis of this 
calculated absolute position and a command absolute position, and 
positioning control can be effected without performing a reference point 
return operation. Also, only the count value of the feedback pulses is 
retentively stored when the power is off, so that there may be provided an 
absolute position detecting apparatus which requires only small electric 
power consumption, and is low-priced and small-sized.

BEST MODE OF CARRYING OUT THE INVENTION 
Referring first to FIG. 1, the principle of absolute position detection 
according to the present invention will be described. 
Each time an object of positioning control, i.e., an object of position 
detection, moves for first and second predetermined distances k and m 
(where m=k/N (N is the number of interpolation pulses generated within the 
predetermined distance k)), feedback pulses and interpolation pulses 
generated are counted. Only the count number of the feedback pulses, 
approximately indicative of a power-off absolute position P of the object 
of position detection, is retentively stored even after the power is 
turned off, so that the accurate power-off absolute position P temporarily 
becomes unknown. 
Thereafter, the power is turned on again, and a command is delivered to 
move the object of position detection to a target absolute position X of a 
value larger than that of the detected approximate position, that is, in 
the positive direction, for example. Thereupon, the interpolation pulses 
generated as the object of position detection moves in response to the 
movement command and counted until the first feedback pulse B+1 after the 
power is turned on is generated, and the number a of the generated pulses 
is stored. Using the stored value a corresponding to the number b (where 
b=N-a) of interpolation pulses generated during the time interval which 
elapses from the instant that the last feedback pulse B in the preceding 
positioning control is generated until the power is turned off, the number 
b being indicative of the difference between the absolute position P and 
the detected approximate position, that is, according to a predetermined 
calculation equation P=Bk+(N-a)m, the absolute position P of the object of 
position detection obtained when the power is turned off or when the power 
is turned on is calculated accurately. Based on the calculated value P and 
the target position X, the number of command pulses can be calculated 
accurately. 
Meanwhile, if the movement command delivered when the power is turned on 
again is intended to move the object of position detection in the negative 
direction to a target absolute position Y, the power-on absolute position 
P is calculated according to a second predetermined calculation equation 
P=Bk+bm, on the basis of the interpolation pulse number b obtained when 
the first feedback pulse B is generated. 
FIG. 2 shows an absolute position detecting apparatus according to an 
embodiment of the present invention. This position detecting apparatus 
comprises a position detecting unit 10 of, e.g., an optical type, which is 
operatively coupled to an object of position detection (not shown). The 
detecting unit 10 is arranged so as to deliver a feedback wave (not shown) 
composed of two sine-wave signals which are generated individually with a 
period corresponding to the movement of the object of position detection 
for the first predetermined distance, e.g., 2 .mu.m, and whose phases are 
deviated at an angle of 90.degree. from each other. The respective 
feedback wave input terminals of a feedback pulse generating circuit 1 and 
an interpolation pulse generating circuit 3 are connected to a feedback 
wave output terminal of the detecting unit 10. The feedback pulse 
generating circuit 1 is provided so as to generate feedback pulses (two of 
which are designated by symbols B and B+1 in FIG. 3) of polarity 
corresponding to the moving direction of the object of detection, with the 
same period for the feedback wave, that is, each time the object of 
detection moves for 2 .mu.m. On the other hand, the interpolation pulse 
generating circuit 3 is provided so as to generate interpolation pulses 
(designated by symbols C0, C1, C2, - - - , C19 in FIG. 3) of polarity 
corresponding to the moving direction of the object of detection, each 
time the object of detection moves for the second predetermined distance, 
e.g., 0.1 .mu.m. 
The position detecting apparatus further comprises an absolute counter 2, 
an interpolation pulse counter 4, and a correction counter 5. The absolute 
counter 2 has a feedback pulse input terminal connected to the output 
terminal of the feedback pulse generating circuit 1, and serves to count 
feedback pulse supplied from the circuit 1. The interpolation pulse 
counter 4 has a reset signal input termiinal and an interpolation pulse 
input terminal connected to the input terminals of the circuits 1 and 3, 
respectively, and serves to count interpolation pulses. The correction 
counter 5 has a strobe signal input terminal and an interpolation pulse 
input terminal connected to the input terminals of the circuits 1 and 3, 
respectively, and serves to temporarily store the number of interpolation 
pulses supplied from the circuit 3 until the feedback pulses are 
generated. The absolute counter 2 enjoys battery-backup. Furthermore, the 
position detecting apparatus is connected to a numerical control unit 20, 
as external calculating means, which receives count values of counters 2, 
4 and 5, and calculates the power-on absolute value of the object of 
position detection, as mentioned later. Since the elements 1 to 5, 10 and 
20 can be composed of conventional means, a detailed description thereof 
is omitted herein. In order to obtain required operation timings, suitable 
circuit means (not shown) are provided between the elements 1 and 4 and 
between the elements 1 and 5. 
The following is a description of the operation of the position detecting 
apparatus of the aforementioned construction. 
Here let it be supposed that the power is off, the object of position 
detection is in the absolute position designated by symbol P in FIG. 3, 
and the absolute counter 2 is stored with a count value B, e.g., 100, 
which indicates that the absolute position P is within the region between 
B and B+1 (200 .mu.m&lt;P&lt;202 .mu.m). In addition, the stored values in the 
interpolation pulse counter 4 and the correction counter 5, which are not 
subject to battery-backup, are reset to a value "0" when the power is off. 
When the power is turned on in such a state, the numerical control unit 20 
starts executing the movement command for moving the object of position 
detection to the target position, e.g., for moving the object in the 
positive direction to the absolute position X (e.g., X=220 .mu.m) whose 
value is greater than that of the present approximate absolute position 
(e.g., 200 .mu.m), and also reads the stored value B (e.g., B=100) in the 
absolute counter 2. As the object of position detection moves 
correspondingly, two sine-wave signals are outputted from the detecting 
unit 10 of the position detecting apparatus. Each time the object of 
position detection moves for the second predetermined distance, e.g., 0.1 
.mu.m, an interpolation pulse is delivered from the interpolation pulse 
generating circuit 3 which receives those sine-wave signals. Every time 
each interpolation pulse is supplied, the count values in the 
interpolation pulse counter 4 and the correction counter 5, which 
individually receive the interpolation pulse, are incremented. 
Thereafter, when the first feedback pulse (designated by symbol B+1 in FIG. 
3) is generated from the feedback pulse generating circuit 1, which 
receives the sine-wave signals from the detecting unit 10, after the power 
is turned on, this feedback pulse is supplied to the reset signal input 
terminal of the interpolation pulse counter 4 and the strobe signal input 
terminal of the correction counter 5 with a required timing. Thus, the 
counted value (designated by symbol a in FIG. 3) stored in the correction 
counter 5 when the feedback pulse is generated is latched in the 
correction counter, and the interpolation pulse counter 4 is then reset. 
When the numerical control unit 20 discriminates the generation of the 
first feedback pulse B+1, it calculates the power-on absolute position P 
of the object of position detection, in accordance with the predetermined 
calculation equation (in this case, the calculation equation P=Bk+(N-a)m 
corresponding to the positive moving direction) corresponding to the 
moving direction of the object of position detection, which is determined 
by the relationship between the movement target position X (=220 .mu.m) 
and the absolute position (=200 .mu.m) corresponding to the absolute 
counter value B read when the power is on, and with use of the value a 
read from the correction counter 5 and the preset parameter values k, N, 
and m. If the values a, k, N, and m are 11, 2 .mu.m, 20, and 0.1 .mu.m, 
respectively, the calculated value of the power-on absolute position P of 
the object of position detection is 200.9 .mu.m. The numerical control 
unit 20 operates so as to deliver command pulses of quantity corresponding 
to the differences (19.1 .mu.m) between the command position X (=220 
.mu.m) and the calculated absolute position P (=200.9 .mu.m). In response 
to the feedback pulses and interpolation pulses generated as the object of 
position detection moves, the stored values in the absolute counter 2 and 
the interpolation pulse counter 4 are updated, respectively, and the 
stored value in the counter 4 is reset with the generation of each 
feedback pulse. Based on a resultant output from the two counters 2 and 4, 
the numerical control unit 20 monitors the actual absolute position of the 
object of position control, and determines the arrival at the command 
position. 
Substantially the same position detection is executed also when the 
movement command is delivered to move the object in the negative direction 
to the absolute position Y (e.g., Y=150 .mu.m) whose value is smaller than 
that of the present approximate absolute position (e.g., 200 .mu.m). 
Thus, when the numerical control unit 20 discriminates the generation of 
the first feedback pulse B, it calculates the power-on absolute position P 
(=200.8 .mu.m) of the object of position detection, in accordance with the 
second predetermined calculation equation P=Bk+bm corresponding to the 
negative moving direction of the object of position detection, which is 
determined by the relationship between the movement target position Y 
(=150 .mu.m) and the absolute position (=200 .mu.m) corresponding to the 
absolute counter value B read when the power is on, and with use of the 
values B (=100) and b (=8) read from the absolute counter 2 and the 
correction counter 5 and the preset parameter values k (=2 .mu.m) and m 
(=0.1 .mu.m), and delivers command pulses of a quantity corresponding to 
the difference (50.8 .mu.m) between the command position X (=150 .mu.m) 
and the calculated absolute position P. 
In the apparatus of the embodiment described above, the absolute position 
of the object of position detection, when and after the power is turned 
on, is calculated by means of the numerical control unit 20, on the basis 
of the outputs from the corresponding counters. Alternatively, however, 
calculating means (not shown), which is contained in the absolute position 
detecting apparatus and having the required calculating function, may be 
used for this purpose.