JP3505932B2 - Origin detection device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a machine tool,
Relative displacement between two members that are relatively displaced in precision measurement equipment
Appropriate origin detection applied to a scale device that detects the position
Equipment related. 2. Description of the Related Art Metal processing machines and precision measuring instruments
Used to detect the amount of displacement between two members that are relatively displaced.
Digital scale has an input that indicates the relative displacement.
Outputs a incremental signal (hereinafter referred to as a scale signal)
Realizing highly accurate and reliable measurement
Signal to indicate the absolute origin (hereinafter referred to as the origin signal).
) Is conventionally provided. [0003] Of these digital scales, magnetic
Digital scale (for example, Magnescale (this application
)), The scale material is generally coaxial
And its diameter is relatively small (for example, 2.
0mm), so the magnetic scale for the origin signal
Record on the same scale material as the magnetic scale for the signal
It is technically difficult. Therefore, magnetic digital
Scales for fixed-point signals are separate from the scale material for Kale
Is recorded, and the fixed point signal itself obtained from the
Signal or use the fixed-point signal
Select one scale signal from scale material
And use the scale signal as the origin signal.
are doing. FIG. 10 shows a magnetic digital scale.
The overall configuration of a well-known phase detection type displacement amount detection device
It is a block diagram shown. The scale section 1 has a coaxial
The wavelength λ for the scale signal along the length of the kale material
(For example, 200 μm)
1a and (n ± 1/4) λ (n is an arbitrary natural
Number of magnetic heads (eg, magnetic flux response)
Type magnetic heads) 1b and 1c. Magnetic head
1b and 1c are respectively the excitation winding and the output in the saturable core part.
It has a structure in which a winding is wound. Magnetic Digit
When using scales for machine tools, precision measuring equipment, etc.
Means that the scale 1a has one of the movable part and the fixed part.
One of the magnetic heads 1b,
1c is attached to the other one. The scale signal detection circuit 2 is not particularly shown.
Iga frequency f _c (Angular frequency ω _c = 2π · f _c ) Current signal
Signal generator and the signal from this oscillator
Frequency f _c / 2 signal to obtain a signal of
A phase shifter for shifting the phase of the signal from the wave number down converter by π / 4.
The signal from this frequency downconverter is
An excitation signal is applied to one of the excitation windings 1b and 1c.
And the signal from this phase shifter remains.
The excitation signal is supplied to one of the excitation windings as an excitation signal. this
As a result, the output windings of the magnetic heads 1b and 1c
According to the relative displacement x between the magnetic head 1b and the magnetic heads 1b and 1c.
The resulting balanced modulated signals are obtained. [0006] These balanced modulation signals are scale signal detection.
It is sent to the output circuit 2. The scale signal detection circuit 2
A mixer that mixes these balanced modulation signals, and from this mixer
Band-pass filter that passes only the second harmonic of the signal
From this band pass filter,
It has a phase amount according to the displacement amount x as in the equation (1), and
Twice the angular frequency ω of the excitation signal _c Phase modulation signal e _PM Get
Can be e _PM = E _PM sin (ω _c T + 2πx / λ) (1) where E _PM Is a constant and t is time. The scale signal detection circuit 2 performs this phase modulation.
Signal e _PM Pulse forming circuit (for example, Schmitt
Trigger circuit), and this pulsing circuit
Obtained frequency f _c Square wave signal (hereinafter referred to as S signal)
Is sent from the scale signal detection circuit 2 to the interpolation circuit 3
You. [0008] The interpolation circuit 3 is N times the S signal (for example,
200 times) frequency Nf _c Clock pulse for interpolation (hereinafter
Lower CKI signal) to connect the scale 1a and the magnetic
Resolution λ / N according to the direction of relative movement with respect to heads 1b and 1c
Output pulse signal (hereinafter referred to as UP / DOWN signal)
Circuit. Therefore, the UP / DOWN signal is output.
The number of force pulses is proportional to the amount of displacement on the scale 1a.
For example, when the displacement amount is λ, the number of output pulses is 200
It becomes. Next, the origin detection circuit will be described.
You. The fixed point signal generator 4 records a magnetic scale for the fixed point signal.
And a magnetic head 4b. Magnetic
Digital scale for machine tools and precision measuring equipment
At a specific position in the longitudinal direction of the scale 1a
The positional relationship with scale 1a is obtained so that a fixed point signal can be obtained.
In the set state, the magnetizing member 4a moves between the movable portion and the fixed portion.
And the magnetic head 4
b is attached to the other one. The signal from the magnetic head 4b is detected at a fixed point signal.
It is sent to the output circuit 5. The fixed point signal detection circuit 5 outputs the signal
Is shaped into a rectangular wave signal to obtain a rectangle as shown in FIG. 11A.
Pulse circuit that outputs a wave signal (hereinafter referred to as RG signal)
(For example, a Schmitt trigger circuit) and the RG signal
The rising edge is detected and a signal as shown in FIG.
And a differentiating circuit for outputting the same. This Z
The signal is a fixed-point signal corresponding to a specific position on the scale 1a.
Is sent from the fixed point signal detection circuit 5 to the control circuit 6.
You. On the other hand, in the scale signal detecting circuit 2, the S signal
Besides the phase modulation signal e _PM Same as angular frequency ω _c Under
The reference signal eREF for phase comparison as shown in equation (2) is
Waveform shaping with a shunt circuit (eg Schmitt trigger circuit)
Frequency f _c Square wave signal (hereinafter referred to as REF signal)
This REF signal scales with the S signal
The signal is sent from the signal detection circuit 2 to the λ signal generation circuit 7. eREF = EREFsin (ω _c T) (2) Here, EREF is a constant. The CKI signal from the interpolation circuit 3 is a λ signal.
The signal is sent to the signal generation circuit 7. The λ signal generation circuit 7
Digitally comparing the phase of the REF signal with the signal
, The displacement amount for the recording wavelength of the magnetic scale of the scale 1a
One pulse in the CKI signal for each λ while discriminating the moving direction
(Hereinafter, output from the λ signal generation circuit 7).
The pulse train to be performed is called a λ signal, an example of which is shown in FIG.
Shown). The λ signal is supplied from the λ signal generation circuit 7 to the control circuit.
Sent to 6. The control circuit 6 receives a signal from the fixed point signal detection circuit 5
Using the Z signal as a gate, for example, as shown in FIG.
The pulse P of the λ signal output first after the output of the Z signal is
This is a circuit that outputs as an origin signal. [0014] By the way, digital
Highly accurate and stable displacement detection using a scale
In order to achieve this, scale material must always be used during the entire detection period.
Must be able to detect a certain upper position as the absolute origin
It becomes. However, the fact that detection is carried out for a long time
Digital scale by interrupting detection halfway
If you ever need to turn off the power to the
It is fixed to a magnet that is different from the scale material, such as
Digital scale with recorded magnetic scale for point signal
Are used when the origin is detected before the interruption and when the detection is resumed after the interruption.
The position on the scale material at which a fixed point signal can be obtained at the time of origin detection
Although the position is slightly small (usually a magnetic scale
(Within a half of the recording wavelength λ)
Is inevitable to some extent. That is, for example, after detecting the absolute origin before interruption,
Vibration during work on metal processing machines, precision measuring equipment, etc.
The cause is that the position of the magnet
May move, causing the origin
A switch that provides a fixed-point signal at the time of detection and at the time of home position detection after interruption.
The position on the kale material fluctuates. Also, for example, inside
At the absolute origin after the interruption, the interruption before interruption due to temperature change
Magnetism expands or contracts slightly compared to the origin
It may have been
A fixed point signal can be obtained at the point detection and at the origin detection after the interruption.
The position on the scale material fluctuates. Therefore, the fixed point is separate from the scale material.
Conventional digital scale with recorded magnetic scales for signals
Now, before and after the detection is interrupted,
It was difficult to detect the position as the absolute origin. The present invention has been made in view of the above points.
To change the position on the scale material where the fixed point signal can be obtained.
Nevertheless, always scale before and after detection interruption
Since a certain position on the material is detected as the absolute origin,
It is intended to provide an origin detecting device which can be used. An origin detecting apparatus according to the present invention is provided.
Is a signal (λ) corresponding to the recording wavelength of the scale on the scale.
Signal generating means for generating a signal) and on this scale
Displacement signal generation that generates an output signal corresponding to the displacement of
Source and signal at a specific location on this scale
(Fixed-point signal).
In the digital scale origin detection device, a fixed point signal is generated.
Λ signal and the position on the scale where the raw means generated the fixed point signal
Distance from the position on the scale where the generation means generated the λ signal
The displacement signal generating means between those positions is generated
Measurement means for measuring using signals and this measurement means
Storage means for storing the measured distance, and the measurement means
The measured distance has already been measured by this measuring means and this storage means
Means for comparing the distance with the distance stored in the
Scale detected based on fixed-point signal generated by means
The upper origin position is corrected according to the comparison result of this comparison means.
And correction means for performing the correction. When the origin is detected before the interruption, a fixed point signal generating means is provided.
Is the position on the scale where the fixed point signal was generated and the λ signal
The distance from the position on the scale where the stage generated the λ signal
The measuring means uses the signal generated by the position signal generating means.
Is measured and stored in the storage means. Thereafter, the detection is interrupted.
When the origin is detected when restarting
Position on the scale that generated the fixed point signal and λ signal generation means
Is the distance from the position on the scale where the λ signal was generated.
Measured again by the step, this distance and before the interruption
Already measured by the measuring means and stored in the storage means
Is compared by the comparing means. Here,
The scale element at which the fixed-point signal was generated before and after the interruption
If there is a change in the position on the material, these two distances
The comparison result of being different is obtained. And the comparison result
Based on the fixed-point signal generated by the fixed-point
The origin position on the scale detected based on the
It is corrected by means. As described above, before and after the interruption of the detection,
The position on the scale material where the point signal was generated fluctuated
In that case, the home position after the interruption is
You. This allows the fixed point signal to be generated on the scale material.
Despite the position change, before and after the interruption of detection
Constant position on scale material is always detected as absolute origin
Will be done. Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.
Will be described in detail. FIG. 1 shows the displacement detecting device shown in FIG.
The present invention is applied to a magnetic digital scale
The same reference numerals are used for parts and signals corresponding to FIG.
And a detailed description thereof will be omitted. This displacement detector
Now, the UP / DOWN signal output from the interpolation circuit 3
And the Z signal output from the fixed point signal detection circuit 5 and the λ signal
Λ signal output from the signal generation circuit 7 is used as an origin detection circuit.
8 As shown in FIG. 2, the origin detecting circuit 8
Lecturer 10, CPU 11 and memory 12 are included.
The UP / DOWN signal from the interpolation circuit 3 is
It is supplied to each of the two Counter terminals. Fixed point
The Z signal from the signal detection circuit 5 is supplied to one of the selectors 10.
Λ from the λ signal generation circuit 7
The signal is supplied to the other selection input terminal S2 of the selector 10.
Supplied. The control input terminal C of the selector 10
A control signal output from the port terminal of U11 (for example,
Then, a binary signal of “1” or “0” is supplied. Sele
The selector 10 selects, for example, when the value of the control signal is “1”.
Select the input signal to the alternative input terminal S1 and set its value to '0'.
At this time, the input signal to the selection input terminal S2 is selected. Sele
The selection output signal of the
Supplied to IRQ. Under the control of the CPU 11, the memory 12
To write / read data. As memory 12
The power of the magnetic digital scale is turned off.
Even non-volatile, for example,
RAM (NV-RAM) or EEPROM (E
For use with electrical eraseable programmable ROM)
It has been. Next, the operation of the origin detecting device will be described.
You. Attached to metal processing machines or precision measuring equipment
Of the displacement on the magnetic digital scale
When the delivery is started, the CPU 11 controls the selector 10
Give signal '1'. In this state, the fixed point signal detection circuit 5
3A outputs a Z signal as shown in FIG.
Signal supplied to interrupt terminal IRQ via selector 10
Is done. When an interrupt is applied, the CPU 11
Switches the control signal given to the selector 10 to '0'
And UP / DO supplied from the interpolation circuit 3
The counting of the number of output pulses of the WN signal is started. After the interruption, the λ signal generation circuit 7
As shown in FIG. 3B, the first pulse P1 of the λ signal is output.
When the pulse is received, the pulse is supplied to the interrupt terminal via the selector 10.
Is supplied to the child IRQ. When the interrupt is again applied, the CPU
11 finishes counting the UP / DOWN signal and
Count value C ₀ Is written to the memory 12. And λ
Signal P1 is output as the origin signal, and the first origin
The origin detection processing at the time of detection is ended. The number of output pulses of the UP / DOWN signal is
Is proportional to the amount of displacement on the rule 1a.
The Z signal was obtained at the time of the first origin detection.
The position on the scale 1a and the scale 1a on which the λ signal was obtained
L to the position of ₀ (See FIG. 3), UP / DOWN signal
This means that it was measured in the form of a signal count value. Then, on a magnetic digital scale
After the displacement is detected and displayed, the detection is interrupted.
The power of the magnetic digital scale is turned off.
After the power is turned off, the power of the magnetic digital scale is turned on again.
When the detection of the displacement is restarted, the CPU 11
In the same manner as in the above processing, the UP / DOWN signal
Counting when the origin is detected after the interruption.
Position on the scale 1a where the Z signal was obtained and the λ signal
Is the distance L from the position on the scale 1a where _n (here
Is n is an integer of 2 or more.
Measured distance).
Then, the first pulse of the λ signal is applied to the interrupt terminal IRQ.
If interrupted by being supplied, a message
Count value of UP / DOWN signal written to memory 12
C ₀ Is read from the memory 12 and the processing as shown in FIG.
Execute the process. First, the count value C after the interruption _n Suspended from
Previous (or first) count value C ₀ Absolute value of the value after subtracting
｜ C _n -C ₀ Is the UP / DOWN equivalent to the displacement λ
The number of output pulses of the signal (here, as in the previous example,
100, which is half of the number of output pulses).
It is determined whether or not it is below (step S1). Here, the Z signal is obtained when the first origin is detected.
Origin detection after interruption for the position on the scale 1a
Sometimes the deviation of the position on the scale 1a where the Z signal was obtained is ±
Assuming that it is within the range of λ / 2 or less (the aforementioned
As shown in the figure,
Vibration or temperature during work on machine tools or precision measuring equipment
This deviation due to the degree change is usually within the range of ± λ / 2 or less.
), The absolute value | C _n -C ₀ | Is 100 or less
In this case, the scale at which the Z signal was
Z signal is obtained when detecting the position on the
The position on the scale 1a is the same pulse of the λ signal
It exists before the obtained position. That is, as shown in FIG.
C ₀ Is in the range of 101 to 200, the absolute value |
C _n -C ₀ Is less than 100 because Z signal after interruption
Is within the shaded area in the figure.
On the other hand, as shown in FIG. ₀ Is 1 to 10
0, the absolute value | C _n -C ₀ | Is 10
0 or less means that the position where the Z signal after interruption is obtained
Is within the range of the oblique line. Thus, the absolute value
｜ C _n -C ₀ Is less than 100, the first
Und value C ₀ Is any value from 1 to 200,
Z signal is obtained at the time of initial home position detection and after home position detection after interruption.
The position on the scale 1a is the same pulse P of the λ signal.
1 will be present before the position where it was obtained. Therefore, the result of the determination in step S1 is
Signal, the Z signal is supplied to the interrupt terminal IRQ.
Λ signal first supplied to the interrupt terminal IRQ after being supplied
Signal from the output terminal (not shown)
(Step S2). And return
Then, the origin detection processing ends. As a result, FIG. 5 and FIG.
As is clear from Fig. 6, when the first origin is detected,
The same position as the position on the scale 1a output as the origin signal
Is the origin. On the other hand, the result of the judgment in step S1 is
If no, the first count value C ₀ Medium than
Count value C after interruption _n To determine if is smaller
(Step S3). Here, the absolute value | C _n -C ₀ | Is 100
Count value C after exceeding and suspending _n Is smaller
Is the first count value C as shown in FIG. ₀ Is 101
Obtain Z signal in range of ~ 200 and after interruption
7 is within the range of the hatched portion in FIG. Therefore
In this case, when the Z signal was obtained at the first detection of the origin,
The position on the scale 1a is before the position of the pulse P1 of the λ signal.
Is present, the Z signal is
The position on the scale 1a is one position ahead of the pulse P1.
Exists before the pulse P0. Therefore, the result of the determination in step S3 is
Signal, the Z signal is supplied to the interrupt terminal IRQ.
After being supplied, the pulse of the λ signal is supplied to the interrupt terminal IRQ
After the position on the scale 1a by the distance λ
(Step S4). And return
Then, the origin detection processing ends. Thereby, from FIG.
As can be seen, the λ signal is used as the origin signal when the origin is first detected.
Origin is the same as the position on the scale 1a output as
It becomes. On the other hand, the absolute value | C _n -C ₀ | Exceeds 100
And the count value C after the interruption _n Is bigger
Is the first count value C as shown in FIG. ₀ Is 0-1
00 and the available position of the Z signal after the interruption
This is the case where the position is within the range of the oblique line in FIG. So this
In this case, the scale at which the Z signal was
Is located before the position of the pulse P1 of the λ signal.
Z signal is obtained when the origin is detected after the interruption.
The position on the scale 1a is a position one pulse behind the pulse P1.
It will be before Lus P2. Therefore, the judgment result in step S3 is NO.
Becomes, the Z signal is supplied to the interrupt terminal IRQ
After that, the pulse of the λ signal is supplied to the interrupt terminal IRQ.
Distance λ ahead of the position on the scale 1a
(Step S5). And return
Then, the origin detection processing ends. As a result, FIG.
As can be seen, the λ signal is used as the origin signal when the origin is first detected.
Origin is the same as the position on the scale 1a output as
It becomes. By the origin detection processing as described above, the Z signal
Range of the position on the scale 1a where
Within ± λ / 2 with respect to the recording wavelength λ of the magnetic scale of
As far as possible, despite the change,
The pulse of the λ signal is supplied to the interrupt terminal IRQ of the CPU 11.
The position on the supplied scale 1a (in the examples of FIGS.
(The position at which P1 was obtained) is always
Is detected as the origin. By the way, the flowchart of FIG.
Shows processing when λ signal is used as origin signal
However, in the origin detection processing, the Z signal is used instead of the λ signal.
The reason that the signal itself may be used as the origin signal is
I'm scattered. When the Z signal itself is used as the origin signal
In FIG. 9, instead of the interrupt processing of FIG.
It is assumed that the interrupt processing shown in FIG. First step S11 of this interrupt processing
Is the same as step S1 in FIG.
Is the λ signal after the Z signal is supplied to the interrupt terminal IRQ.
When the pulse is supplied to the interrupt terminal IRQ
Distance L from the position on the rule 1a ₀ Only equivalent to the forward position
(Step S12). And return. to this
As can be seen from FIGS. 5 and 6,
Scale 1a that outputs Z signal as origin signal during detection
The same position as the upper position is the origin. If the answer is no in step S11,
In step S13, the same determination as in step S4 of FIG. 4 is performed.
U. If yes, the Z signal goes to the interrupt terminal IRQ
After being supplied, the pulse of the λ signal is supplied to the interrupt terminal IRQ.
Distance λ-L from the position on scale 1a when fed
₀ (Step S14). Soshi
And return. This makes it clear from FIG.
The Z signal is output as the origin signal when the origin is first detected.
The position on the scale 1a becomes the origin. On the other hand, if the result of step S13 is NO
In this case, after the Z signal is supplied to the interrupt terminal IRQ, λ
When a signal pulse is supplied to the interrupt terminal IRQ
Distance λ + L from position on scale 1a ₀ Just forward position
(Step S15). And return
You. As a result, as is clear from FIG.
Scale 1 that outputs Z signal as origin signal at point detection
The same position as the position on a is set as the origin. As described above, when the first origin is detected, the CP
When the Z signal is supplied to the interrupt terminal IRQ of U11
The position on the ruler 1a is always set to the origin even when the origin is detected after the interruption.
Is detected as In the interrupt processing shown in FIGS. 4 and 9,
So that the CPU 11 can also determine the moving direction and the polarity.
The direction of movement is either positive or negative
Only (direction in which only UP signal is output or DOWN signal
Signal direction)
Not only when the movement direction changes between positive and negative directions
The same position on the scale 1a
Of course, it can be detected as In the above embodiment, the interruption of the CPU 11
Whether to supply the Z signal or the λ signal to the terminal IRQ
Although the selector 10 is provided for selection,
When using a CPU with two or more terminals IRQ
Separates the Z signal and the λ signal without providing a selector.
May be directly supplied to the interrupt terminal IRQ. And that
In the case of, the Z signal is supplied to the interrupt terminal IRQ and
From the first pulse of the λ signal to another interrupt terminal IRQ
Count the number of output pulses of the UP / DOWN signal until supplied.
The scale 1a from which the Z signal was obtained by counting
Between the above position and the position on the scale 1a where the λ signal was obtained.
Of course, the distance can be measured. In the above embodiment, UP / DOWN
CPU 11 executes signal count and count value comparison
But dedicated to making such counts and comparisons
Of course, hardware circuits may be provided.
You. In the above embodiment, the first origin detection is performed.
And the position on the scale 1a at which the Z signal was obtained and λ
The distance from the position on the scale 1a where the signal was obtained is indicated by UP
/ DOWN signal is counted.
The output corresponding to the displacement on the scale 1a is not limited to
An appropriate signal (for example, the displacement amount on the scale 1a)
A / B phase signal that produces a corresponding phase change)
The distance may be measured. In the above embodiment, a magnetic digital
Although the present invention is applied to a scale, an optical digital
The present invention may be applied to a small scale. In this embodiment, the scale material is
Digital recording of magnetic scale for fixed point signal on another magnet
The present invention is applied to a small scale. However, the fixed point signal
Magnetic scale for the scale signal is identical to the magnetic scale for the scale signal.
Digital scale recorded on scale material
The position of the magnetic scales on the scale material
Therefore, fixed point due to expansion and contraction due to temperature change
The position on the scale where the signal is obtained can fluctuate
I can't say it's not at all. So, such digital
The present invention may be applied to a small scale. The present invention is not limited to the above embodiment,
Various other configurations may be made without departing from the spirit of the invention.
Of course, it is possible. As described above, the origin detecting device according to the present invention is
According to the position, the position on the scale material where the fixed point signal is obtained
Before interruption of origin detection due to vibration, temperature change, etc.
Even if it fluctuates later, always before and after the interruption
Detect a certain position on the scale material as the absolute origin
be able to. Therefore, the digital value is determined based on the origin.
Performing highly accurate and reliable measurements using a scale
There is a benefit that can be. Further, a circuit for generating a λ signal and an UP /
Equivalent to the amount of displacement on the scale like the DOWN signal
A circuit that generates an output signal and a circuit that generates a fixed-point signal
On a digital scale with
The circuit configuration and processing make such high accuracy and reliability
There is the advantage that high measurements can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an example of a configuration of a displacement amount detection device employing the present invention. FIG. 2 is a block diagram illustrating an example of a configuration of an origin detecting device of FIG. 1; FIG. 3 is a diagram illustrating an example of a signal supplied to a CPU in FIG. 2; FIG. 4 is a flowchart illustrating an example of a process executed by a CPU in FIG. 2; FIG. 5 is a diagram illustrating an example of a signal supplied to the CPU in FIG. 2; FIG. 6 is a diagram illustrating an example of a signal supplied to the CPU in FIG. 2; FIG. 7 is a diagram illustrating an example of a signal supplied to the CPU in FIG. 2; FIG. 8 is a diagram showing an example of a signal supplied to the CPU of FIG. 2; FIG. 9 is a flowchart illustrating another example of the processing executed by the CPU in FIG. 2; FIG. 10 is a block diagram illustrating an example of a configuration of a conventional displacement amount detection device. 11 is a diagram showing an example of input / output signals of the control circuit of FIG. [Description of Signs] 1 scale section, 1a scale, 1b, 1c, 4b
Magnetic head, 2 scale signal detection circuit, 3 interpolation circuit, 4 fixed-point signal generator, 4a magnetizing body, 5
Fixed point signal detection circuit, 7 λ signal generation circuit, 10 selector, 11 CPU, 12 memory

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-81611 (JP, A) JP-A-7-253317 (JP, A) JP-A-51-138461 (JP, A) 282326 (JP, A) JP 50-25818 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01D 5/00-5/62 G01B ⁷ /00-7/34 G01B 21/00

Claims (1)

(57) Claims 1. A λ signal generating means for generating a signal (λ signal) corresponding to a recording wavelength of a scale on a scale, and an output corresponding to an amount of displacement on the scale. A displacement signal generating means for generating a signal; and a signal (fixed-point signal) at a specific position on the scale.
A fixed-point signal generating means for generating the origin, a position on the scale at which the fixed-point signal generating means generates a fixed-point signal and a position on the scale at which the λ signal generating means generates a λ signal. Measuring means for measuring a distance between the positions using a signal generated by the displacement signal generating means, a storing means for storing the distance measured by the measuring means, and a measuring means for measuring the distance. Comparing means for comparing the distance with the distance already measured by the measuring means and stored in the storage means; and an origin position on a scale detected by the fixed point signal generating means based on the fixed point signal at which the fixed point signal is generated. And a correcting means for correcting the difference according to the comparison result of the comparing means.