Displacement measuring apparatus

The displacement measuring apparatus of a laser interference technology includes a probe 3 which delivers at a photodetector 17 an RF signal of which a modulated component proportional to a displacement of an object 2 under measurement. The apparatus has also a phase-demodulation circuit 9 in which an interference signal produced by shifting the frequency of the RF signal to an intermediate frequency is phase-demodulated on the basis of the carrier signal for supply to a light modulator 16. Thus the apparatus can detect a displacement of the object with a high resolution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a displacement measuring apparatus for 
measuring a displacement of an object by irradiating a laser beam to the 
object, and more particularly, to a displacement measuring apparatus 
adapted to optically modulate, using a carrier signal of a predetermined 
frequency, either of laser beams for irradiation to a reference mirror and 
an object whose displacement is to be measured, respectively, and measure 
a displacement of the object on the basis of an interference signal 
produced by interference between reflected laser beams from the reference 
mirror and object, respectively. 
2. Description of Related Art 
Heretofore, displacement measuring apparatuses have been proposed which 
make the use of the laser interference technology to detect a vibration 
and moved position of an object in a non-contact manner. As one of such 
displacement measuring apparatuses, a fringe counting type one is 
generally used. 
Referring now to FIG. 1, there is illustrated in the form of a schematic 
block diagram a conventional displacement measuring apparatus of the 
fringe counting type. 
In FIG. 1, the conventional displacement measuring apparatus is generally 
indicated with a reference 100. As shown, the apparatus 100 comprises a 
probe 102 to produce an RF signal by irradiating a laser beam to an object 
101 whose displacement is to be measured and modulating a laser beam 
component developed due to the displacement of the object, a mixer circuit 
103 to convert the RF signal to an IF signal, a bandpass filter (will be 
referred to simply as "BPF" hereinafter) 104 to filter the IF signal, a 
fringe detection circuit 105 to detect a fringe component of the IF signal 
filtered by the BPF 104, an up/down counter (will be referred to simply as 
"U/D counter" hereinafter) 106 to count the detected fringe component and 
produce a digital displacement signal, a digital/analog converter (will be 
referred to simply as "D/A converter" hereinafter) 107 to convert the 
digital displacement signal from the U/D counter 106 to an analog 
displacement signal, an carrier signal oscillator (will be referred to 
simply as "carrier OSC" hereinafter) 108 to produce a carrier signal, and 
a local carrier signal oscillator (will be referred to simply as "local 
OSC" hereinafter) 109 to produce a local carrier signal on the basis of 
the carrier signal from the carrier signal oscillator 108. 
The probe 102 comprises an interference optical system 110 incorporating a 
laser diode 111 to emit a laser light, beam splitter 112 to split the 
laser light into two beams and combine reflected split laser beams, 
reference mirror 113 and a light modulator 114 to optically modulate the 
laser beam, which interference optical system 110 producing an 
interference light by interference between a reflected laser beam from the 
reference mirror 113 and a one from the object 101, and further a 
photodetector 115 to detect the interference light and photoelectrically 
convert it. 
The laser diode 111 emits a laser light of a predetermined wavelength. The 
emitted laser light is split by the beam splitter 112 into two beams. One 
of the split laser beams is irradiated to the reference mirror 113 fixed 
inside the probe 102, while the other beam is irradiated to the object 101 
through the light modulator 114 which will optically modulate the laser 
beam irradiated to the object 101 using a carrier signal supplied from the 
carrier OSC 108. 
The beam splitter 112 works also to combine a reflected laser beam from the 
object 101 and a one from the reference mirror 113 to produce an 
interference light. The photodetector 115 detects the interference light 
produced by the beam splitter 112 and optically converts it. 
The probe 102 having the above-mentioned construction modulates, using the 
carrier signal, a laser beam component developed due to a displacement of 
the object 101 to produce an RF signal, and delivers it at the 
photodetector 115. 
The mixer circuit 103 is supplied with the RF signal from the probe 102, 
and converts it to an IF signal of an intermediate frequency on the basis 
of a local carrier signal from the local OSC 109. 
The BPF 104 is supplied with the IF signal from the mixer circuit 103 to 
filter an IF signal component in a predetermined frequency band. 
The fringe detector circuit 105 is supplied with the filtered IF signal 
from the BPF 104, and detects a fringe component of the IF signal on the 
basis of the carrier signal supplied from the carrier OSC 108. Namely, the 
fringe detector circuit 105 detects an interference fringe component of 
the interference light produced by interference between the reflected 
laser beams from the reference mirror 113 and object 101, respectively. 
The U/D counter 106 counts up or down the interference fringe component 
detected by the fringe detector circuit 105 to detect a displacement of 
the object 101, and provides the result of detection as a digital 
displacement signal. 
The D/A converter 107 converts the digital displacement signal to an analog 
signal. 
The conventional displacement measuring apparatus 100 of the fringe 
counting type having the aforementioned construction functions as will be 
described below: 
Since the reference mirror 113 is fixed inside the probe 102, the distance 
l.sub.1 between the beam splitter 112 and reference mirror 113 will not 
vary. On the other hand, since the object 101 moves in parallel with the 
irradiated direction of the laser beam, the distance l.sub.2 between the 
beam splitter 112 and object 101 will vary. 
Assume here that the light modulator 114 is not provided in the 
interference optical system 110. In this case, an interference light is 
produced as will be described below. For instance, when there is no 
optical-path difference between the distances l.sub.1 and l.sub.2, the 
reflected light from the reference mirror 113 is in phase with that from 
the object 101, so that the interference light will be bright. When the 
object 101 moves so that the optical-path difference between the distances 
l.sub.1 and l.sub.2 is .lambda./2 (.lambda. is the wavelength of laser 
beam), the reflected light from the reference mirror 113 is in phase 
opposition to that from the object 101, thus the interference light will 
be dark. Therefore, if the light modulator 114 is not provided in the 
interference optical system 110, an interference light will take place in 
which bright and dark interference fringes develop each time the object 
101 moves over a distance of .lambda./2. 
On the other hand, in the conventional displacement measuring apparatus 100 
having the light modulator 114 provided in the interference optical system 
110, the light modulator 114 modulates the phase of the laser beam. Thus, 
an interference light produced in this displacement measuring apparatus 
100 will cause bright and dark interference fringes corresponding to 
modulation frequencies even if there is no optical-path difference between 
the distances l.sub.1 and l.sub.2. The interference fringes of the 
interference light will change correspondingly to the optical-path 
difference between the distances l.sub.1 and l.sub.2 if the optical-path 
difference varies. 
Therefore, in the conventional displacement measuring apparatus 100, as the 
object 101 moves, an interference light can be produced in which bright 
and dark interference fringes corresponding to modulation frequencies 
vary. 
In the conventional displacement measuring apparatus 100, after such an 
interference light is detected by the photodetector 115, the interference 
fringes in the interference light are counted by the fringe detector 
circuit 105 to detect a moved distance, or displacement, of the object 
101. 
However, since the conventional displacement measuring apparatus 100 of the 
fringe counting type is adapted to count the interference fringes in the 
interference light, the resolution of displacement detection depends upon 
the wavelength of the laser beam. Therefore, the conventional apparatus 
can hardly detect a displacement of the object 101 with a high resolution. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention has an object to overcome the 
above-mentioned drawbacks of the prior art by providing a displacement 
measuring apparatus which can detect a displacement of an object under 
measurement with a high resolution utilizing a laser interference 
technology. 
Also, the present invention provides a displacement measuring apparatus 
which can reset a detection output to zero regardless of the position of 
an object under measurement utilizing a laser interference technology. 
The above object can be achieved by providing a displacement measuring 
apparatus comprising, according to the present invention: an interference 
optical system incorporating a laser source to emit a laser light for 
irradiation to a reference mirror and an object under measurement; and a 
light modulator to optically modulate either of laser beams for 
irradiation to the reference mirror and object, respectively, using a 
carrier signal of a predetermined frequency, which interference optical 
system being adapted to produce an interference light by interference 
between reflected laser beams from the reference mirror and object, 
respectively; a photo-detecting element to receive the interference light 
and detect an interference signal; and a displacement detecting means for 
detecting a displacement of the object by demodulating the phase of the 
interference signal on the basis of the carrier signal. 
In the displacement measuring apparatus according to the present invention, 
an interference signal produced by interference between the reflected 
light beams from the reference mirror and object, respectively, is 
converted to an electrical signal which in turn will be phase-demodulated 
to detect a displacement of the object. 
According to the present invention, the displacement measuring apparatus 
further comprises: means for dividing the frequency of the interference 
signal at a predetermined ratio to produce a frequency-divided 
interference signal; means for dividing the frequency of the carrier 
signal at such a ratio as to produce an output frequency identical to the 
center frequency of the frequency-divided interference signal, to thereby 
produce a frequency-divided carrier signal; and means for synchronizing a 
reset signal for supply to the carrier-signal frequency dividing means 
with the frequency-divided interference signal; the carrier-signal 
frequency dividing means producing, upon supply of the reset signal 
synchronized with the interference signal, a frequency-divided carrier 
signal having with respect to the frequency-divided interference signal 
such a phase difference that an output after phase-demodulated by the 
displacement detecting means has a predetermined value; and the 
displacement detecting means detecting a phase difference between the 
frequency-divided carrier and interference signals and demodulating the 
phase of the interference signal. 
As mentioned above, according to the present invention, upon supply of the 
reset signal synchronized with the interference signal, the 
frequency-divided carrier signal is produced which has with respect to the 
frequency-divided interference signal such a phase difference that the 
output after phase-demodulated has the predetermined value, and the phase 
difference between the frequency-divided carrier and interference signals 
is detected for phase demodulation of the interference signal. 
These objects and other objects, features and advantages of the present 
intention will become more apparent from the following detailed 
description of the preferred embodiments of the present invention when 
taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 2, there is illustrated in the form of a schematic 
block diagram the first embodiment of the displacement measuring apparatus 
according to the present invention. 
In FIG. 2, the displacement measuring apparatus according to the first 
embodiment is generally indicated with a reference 1. As shown, the 
apparatus 1 comprises a probe 3 to irradiate a laser beam to an object 2 
under measurement and produce an RF signal by modulating a displacement of 
the object 2, mixer circuit 4 to convert the RF signal to an IF signal, 
BPF 5 to filter the IF signal, interference signal counter 6 to count the 
leading edge, for example, of the IF signal filtered by the BPF 5 (this 
filtered IF signal will be referred to as "interference signal" 
hereinafter) to produce a frequency-divided interference signal, frequency 
multiplier circuit 7 to multiply the frequency of the carrier signal to 
produce a frequency-multiplied carrier signal, carrier signal counter 8 to 
count the leading edge, for example, of the frequency-multiplied carrier 
signal to produce a frequency-divided carrier signal, phase demodulator 
circuit 9 to demodulate the phase of the frequency-divided interference 
signal on the basis of the frequency-divided carrier signal to produce a 
displacement signal indicative of a displacement of the object 2, carrier 
OSC 10 to produce a carrier signal, and a local OSC 11 to produce a local 
carrier signal on the basis of the carrier signal. 
The probe 3 incorporates an interference optical system 12 comprising a 
laser diode 13 to emit a laser light, beam splitter 14 to split the laser 
light into two beams and combine the reflected split beams, reference 
mirror 15 and a light modulator 16 to optically modulate the laser beam. 
In the interference optical system 12, a reflected light from the 
reference mirror 15 and one from the object 2 are put into interference 
with each other to produce an interference light. The probe 3 further 
comprises a photo diode 17 to detect and photoelectrically convert the 
interference light. 
The laser diode 13 emits a laser light of a predetermined wavelength. A 
laser light emitted from the laser diode 13 is split by the beam splitter 
14 into two beams of laser light. One of the laser beams is irradiated to 
the reference mirror 15 fixed inside the probe 3, while the other is 
irradiated to the object 2 through the light modulator 16 which will 
optically modulate the laser beam irradiated to the object 2 with a 
carrier signal supplied from the carrier OSC 10. 
The beam splitter 14 works also to combine a reflected laser beam from the 
object 2 and one from the reference mirror 15 to produce an interference 
light. The photodetector 17 detects and optically converts the 
interference light produced by the beam splitter 14. 
The probe 3 having the aforementioned construction functions to produce an 
interference light as will be described below: 
Since the reference mirror 15 is fixed inside the probe 2, the distance 
l.sub.1 between the beam splitter 14 and reference mirror 15 will not 
vary. On the other hand, since the object 2 moves in parallel with the 
irradiated direction of the laser beam, the distance l.sub.2 between the 
beam splitter 14 and object 2 will vary. 
Assume here that the light modulator 16 is not provided in the interference 
optical system 12. In this case, an interference light is produced. For 
instance, when there is no optical-path difference between the distances 
l.sub.1 and l.sub.2, the reflected light from the reference mirror 15 is 
in phase with that from the object 2, so that the interference light will 
be bright. When the object 2 moves so that the optical-path difference 
between the distances l.sub.1 and l.sub.2 is .lambda./2 (.lambda. is the 
wavelength of laser beam), the reflected light from the reference mirror 
15 is in phase opposition to that from the object 2, thus the interference 
light will be dark. Therefore, when the light modulator 16 is not provided 
in the interference optical system 12, an interference light will take 
place in which bright and dark interference fringes develop each time the 
object 2 moves over a distance of .lambda./2. 
On the other hand, in the probe 3 in which the light modulator 16 is 
provided in the interference optical system 12, the light modulator 16 
modulates the phase of the laser beam. Thus, an interference light 
produced by the probe 3 will have developed therein bright and dark 
interference fringes corresponding to modulation frequencies even if there 
is no optical-path difference between the distances l.sub.1 and l.sub.2. 
The interference fringes of the interference light will change 
correspondingly to the optical-path difference between the distances 
l.sub.1 and l.sub.2 if the optical-path difference varies. 
Therefore, in the probe 3, as the object 2 moves, an interference light can 
be produced in which bright and dark interference fringes corresponding to 
modulation frequencies vary. 
As mentioned above, in the probe 3, after such an interference light is 
detected by the photodetector 17 to produce an RF signal by modulating, 
using a carrier signal, the modulated component proportional to a 
displacement of the object 2. 
The mixer circuit 4 is supplied with the RF signal from the probe 3 to 
convert the RF signal to an IF signal within an intermediate frequency 
band on the basis of the local carrier signal supplied from the local OSC 
11. 
The BPF 5 is supplied with the IF signal from the mixer circuit 4 to filter 
a component of the IF signal in a predetermined frequency band. The IF 
signal thus filtered by the BPF 5 is used as an interference signal. 
The interference signal counter 6 is supplied with the interference signal 
from the BPF 5. The interference signal counter 6 is designed as a 2.sup.n 
-ary forward counter to count the leading edge, for example, of an 
interference signal. It provides an output pulse from the most significant 
bit (MSB) as an interference signal. That is, the interference signal 
counter 6 works as a frequency divider to divide or the frequency of the 
interference signal at a predetermined ratio. 
The frequency multiplier circuit 7 is supplied with a carrier signal from 
the carrier OSC 10. It is composed of a PLL (phase-locked loop) circuit, 
for example, to provide a frequency-multiplied carrier signal produced by 
multiplying the frequency of the carrier signal at a predetermined ratio. 
The carrier signal counter 8 is supplied with the frequency-multiplied 
carrier signal from the frequency multiplier circuit 7. This carrier 
signal counter 8 is designed as a 2.sup.n -ary forward counter to count 
the leading edge, for example, of a frequency-multiplied carrier signal. 
It provides an output pulse from the most significant bit (MSB) as a 
frequency-divided carrier signal. That is, the carrier signal counter 8 
works as a frequency divider to divide or the frequency of the 
frequency-multiplied carrier signal at a predetermined ratio. 
The frequency division ratio of the interference signal counter 6, 
frequency multiplication ratio of the frequency multiplier circuit 7 and 
the frequency division ratio of the carrier signal counter 8 are set so 
that the center frequency of the frequency-divided interference signal 
resulted from modulation of a displacement of the object 2 is identical to 
the frequency of the frequency-divided carrier signal. 
Assume here that the carrier signal has a frequency of 4 MHz and the 
interference signal has a frequency of 2 MHz, for example. In this case, 
when the interference signal counter 6 is designed as a 3-bit octal 
counter, the frequency multiplier circuit 7 as a hexadecimal frequency 
multiplier and the carrier signal counter 8 as an 8-bit 256-ary counter, 
both the center frequency of the frequency-divided interference signal and 
frequency of the frequency-divided carrier signal can be 250 kHz. 
The phase demodulator circuit 9 is supplied with the frequency-divided 
carrier and interference signals to demodulate the phase of the 
frequency-divided interference signal using the frequency-divided carrier 
signal as a reference signal. The phase demodulator circuit 9 provides a 
displacement signal produced by demodulating the phase of the 
frequency-divided interference signal, as a result of the measurement of a 
displacement of the object 2. 
Next, the interference signal counter 6, carrier signal counter 8 and the 
phase demodulator 9 will further be described with reference to FIG. 3. 
As shown, the interference signal counter 6 is supplied with an 
interference signal of 2 MHz, for example. The interference signal counter 
6 is composed of a 3-bit octal counter to provide a frequency-divided 
interference signal of 250 kHz from the most significant bit Q.sub.2. 
Namely, the interference signal counter 6 divides by 8 the interference 
signal. 
The carrier signal counter 8 is supplied with a frequency-multiplied 
carrier signal of 64 MHz, for example. The carrier signal counter 8 is 
composed of an 8-bit 256-ary counter to provide a frequency-divided 
carrier signal of 250 kHz from the most significant bit Q.sub.7. Namely, 
it divides by 256 the frequency-divided carrier signal of 64 MHz. 
The phase demodulator circuit 9 comprises a first edge detector circuit 21 
to detect the leading edge of a frequency-divided interference signal, 
second edge detector 22 to detect the leading edge of a frequency-divided 
carrier signal, D flip-flop circuit 23 having a set input terminal (SET) 
and reset input terminal (RST), and a lowpass filter 24. 
The first edge detector circuit 21 detects the leading edge of a 
frequency-divided interference signal. It serves as a mono-multivibrator. 
The output of the first edge detector circuit 21 is supplied to the set 
input terminal (SET) of the D flip-flop circuit 23. 
The second edge detector 22 detects the leading edge of a frequency-divided 
carrier signal. The second edge detector 22 also serves as a 
mono-multivibrator. The output of the second edge detector circuit 22 is 
supplied to the reset input terminal (RST) of the D flip-flop circuit 23. 
The D flip-flop circuit 23 has both the clock terminal and input terminal 
(D) pulled up to VCC. Therefore, the output (Q) of the D flip-flop circuit 
23 stays high from a period of time since a pulse is supplied to the set 
input terminal (SET) thereof until a pulse is supplied to the reset input 
terminal (RST). That is, a time for which the output (Q) stays high 
indicates a displacement of the object 2. 
The lowpass filter 24 produces a displacement signal by removing a high 
frequency component such as carrier signal, etc. from the output signal 
supplied from the D flip-flop circuit 23. 
It should be noted that the D flip-flop circuit 23 in the phase demodulator 
circuit 9 may be replaced with any other flip-flop circuit which has a 
set/reset function. If the D flip-flop circuit 23 is of an edge-triggered 
type, the first and second edge detector circuits 21 and 22 may not be 
provided in the phase demodulator circuit 9. 
The phase demodulator circuit 9 using such a D flip-flop circuit 23 
provides a displacement signal which will be zero when the phase 
difference between the frequency-divided interference and carrier signals 
is 180 deg. The dynamic range of the output displacement signal from the 
circuit 9 is .+-.180 deg. with respect to the 180 deg. 
The phase demodulator circuit 9 may not be the above-mentioned one 
comprising the first and second edge detector circuits 21 and 22, and D 
flip-flop circuit 23, but it may be a one using an EX-OR (exclusive-or) 
circuit 25 indicated with a dash line in FIG. 3 and of which the output is 
high when the frequency-divided interference and carrier signals are 
coincident with each other, to detect a displacement of the object 2. 
The phase demodulator circuit 9 using the EX-OR circuit 25 provides a 
displacement signal which will be zero when the phase difference between 
the frequency-divided interference and carrier signals is 90 or 270 deg. 
In this case, the dynamic range of the output displacement signal from the 
circuit 9 is .+-.90 deg. 
The displacement measuring apparatus 1 having the aforementioned 
construction produces a displacement signal indicative of a displacement 
of the object 2 by modulating, by the phase demodulator circuit 9, the 
phase of an interference signal produced by modulating the displacement of 
the object 2. The displacement signal varies linearly with respect to the 
displacement of the object 2. Therefore, the displacement measuring 
apparatus 1 can measure a displacement of the object 2 with a considerably 
higher resolution than the conventional displacement measuring apparatus 
of the fringe counting type. 
Next, the second embodiment of the displacement measuring apparatus 
according to the present invention will be described herebelow with 
reference to FIG. 4 showing the second embodiment of the present invention 
in the form of a block diagram of a schematic block diagram. 
Note that the second embodiment has same components as those included in 
the aforementioned displacement measuring apparatus 1 according to the 
first embodiment. Such same components are indicated in FIG. 4 with same 
references as in FIG. 2, and so will not be described in detail any 
longer. 
In FIG. 4, the displacement measuring apparatus according to the second 
embodiment is generally indicated with a reference 30. The displacement 
measuring apparatus 30 comprises a probe 3 to irradiate a laser beam to an 
object 2 under measurement and produce an RF signal by modulating a 
displacement of the object 2, mixer circuit 4 to convert the RF signal to 
an IF signal, BPF 5 to filter the IF signal, interference signal counter 6 
to count the leading edge, for example, of the IF signal filtered by the 
BPF 5 (this filtered IF signal will be referred to as and "interference 
signal" hereinafter) to produce a frequency-divided interference signal, 
frequency multiplier circuit 7 to multiply the frequency of the carrier 
signal to produce a frequency-multiplied carrier signal, carrier signal 
counter 8 to count the leading edge, for example, of the 
frequency-multiplied carrier signal to produce a frequency-divided carrier 
signal, phase demodulator circuit 9 to demodulate the phase of the 
frequency-divided interference signal on the basis of the 
frequency-divided carrier signal to produce a displacement signal 
indicative of a displacement of the object 2, carrier OSC 10 to produce a 
carrier signal, local OSC 11 to produce a local carrier signal on the 
basis of the carrier signal, and a synchronization circuit 31 to 
synchronize a reset signal with the frequency-divided interference signal 
to produce a load pulse for supply to the carrier signal counter 8. 
The synchronization circuit 31 is supplied with a reset signal when the 
user presses a reset button or the like. The synchronization circuit 31 
synchronizes the reset signal with a frequency-divided interference signal 
output from the interference signal counter 6. For example, the 
synchronization circuit 31 synchronizes the reset signal with the leading 
edge of the frequency-divided interference signal, to produce a load pulse 
by differentiating the reset signal synchronized with the 
frequency-divided interference signal. The load pulse thus produced is 
supplied to the carrier signal counter 8. 
Next, the interference signal counter 6, carrier signal counter 8 and the 
phase demodulator 9 included in the displacement measuring apparatus 30 
according to the second embodiment will further be described with 
reference to FIG. 5. 
The load pulse produced by the synchronization circuit 31 is supplied to a 
load terminal (LOAD) provided at the carrier signal counter 8 which will 
then load a previously latched load data (D.sub.0 to D.sub.7) as an output 
data to output terminals (Q.sub.0 to Q.sub.7). Therefore, when the carrier 
signal counter 8 is supplied with a load pulse, it will clear the output 
data and provide a new load data. 
The carrier signal counter 8 latches as a load data in advance a data in 
which a displacement signal provided from the phase demodulator circuit 9 
becomes zero. That is, the carrier signal counter 8 will latch as a load 
data a data having such a predetermined phase difference with respect to a 
frequency-divided interference signal that a displacement signal provided 
from the phase demodulator circuit 9 is zero. 
The load data varies depending upon the demodulation method adopted in the 
phase modulator circuit 9. For example, if the phase demodulator circuit 9 
adopts the aforementioned D flip-flop circuit 23, the displacement signal 
output becomes zero when the phase difference between the 
frequency-divided interference and carrier signals is 180 deg. Therefore, 
if the carrier signal counter 8 is a 3-bit 256-ary counter, 0 (00000000 
when it is binary) is latched as a load data in advance. If the phase 
demodulator circuit 9 uses the aforementioned EX-OR circuit 25, the 
displacement signal output will be zero when the phase difference between 
the frequency-divided interference and carrier signals is 90 or 270 deg. 
Therefore, if the carrier signal counter 8 is an 8-bit 256-ary counter, 64 
or 192 (01000000 or 11000000 when it is binary) is latched as a load data 
in advance. 
FIG. 6 is a time chart of various signals in the displacement measuring 
apparatus 30. 
First, assume that there occurs a predetermined phase difference between 
the frequency-divided interference and carrier signals and the 
displacement signal is not zero (at time t.sub.-2), for example. When a 
reset signal is supplied (at a time between times t.sub.-1 and t.sub.0), 
it will be synchronized with the leading edge of the frequency-divided 
interference signal 9 (at the time t.sub.0). And the reset signal 
synchronized with the frequency-divided interference signal is 
differentiated to produce a load pulse. 
When the carrier signal counter 8 (at time t.sub.0) is supplied with the 
load pulse, it will provide a load data and a frequency-divided carrier 
signal provided from the most significant bit (MSB) vary. At this time, if 
the phase demodulator circuit 9 adopts the D flip-flop circuit 23, a 
frequency-divided carrier signal is provided of which the phase is shifted 
180 deg. from the frequency-divided interference signal. When the 
frequency-divided interference signal having the phase difference of 180 
deg. is supplied, the displacement signal output will be zero. 
In the displacement measuring apparatus 30 described above, the 
synchronization circuit 31 provides a load pulse produced by synchronizing 
the reset signal with the frequency-divided interference signal. Since the 
carrier signal counter 8 can provide a load data upon supply of the load 
pulse, the displacement signal can be zeroed irrespectively of the 
position of the object 2. 
Thus, the displacement measuring apparatus 30 can measure a displacement of 
the object 2 with reference to a fixed position of the object 2. 
The load data loaded to the carrier signal counter 8 in the displacement 
measuring apparatus 30 is such a value that the displacement signal output 
becomes zero. Note however that in the present invention, the load data 
value is not limited. According to the present invention, a load data may 
be loaded which has such a value that the displacement signal output would 
have a predetermined offset, for example. In this case, the value the 
displacement signal takes when the object 2 is at the reference position 
can freely be set by the user, which leads to an improved operability of 
the displacement measuring apparatus. 
In the foregoing, the present invention have been described concerning the 
displacement measuring apparatuses 1 and 30 in which an RF signal detected 
from the probe 3 is converted to an IF signal. However the present 
invention is not limited to these embodiments, but the displacement 
measuring apparatus according to the present invention may be adapted to 
directly count the RF signal provided from the probe 3 by means of the 
interference signal counter 6. Also, the present invention has been 
described concerning the displacement measuring apparatuses 1 and 30 in 
which a carrier signal is frequency-multiplied by the frequency multiplier 
circuit 7, and then demultiplied or divided by the carrier signal counter 
8. However the present invention is not limited to these embodiments, but 
the displacement measuring apparatus according to the present invention 
may be adapted to directly count the carrier signal the carrier signal 
counter 6. That is, in the present invention, if the center frequency of a 
frequency-divided interference signal provided from the interference 
signal counter 6 is identical to the frequency of the frequency-divided 
carrier signal provided from the carrier signal counter 8, the frequency 
division and multiplication ratios are not limited. 
Also in the foregoing, the present invention has been described concerning 
displacement measuring apparatuses 1 and 30 in which the laser diode 13 is 
used as the laser source. However the present invention is not limited to 
these embodiments, but the displacement measuring apparatus according to 
the present invention may use a helium neon laser as the laser source. 
Also, a laser light emitted from a laser diode or helium neon laser may be 
transmitted through an optical fiber and irradiated to the object 2 and 
reference mirror 15. 
Furthermore, the present invention has been described concerning 
displacement measuring apparatuses 1 and 30 in which the light modulator 
16 is disposed in the forward path of the laser light between the beam 
splitter 14 and object 2. However the present invention is not limited to 
these embodiments, but the light modulator 16 may be disposed in the 
backward path of the laser light between the beam splitter 14 and object 
2, or in the forward or backward path of the laser light between the beam 
splitter 14 and reference mirror 15. Alternatively, the light modulators 
16 may be disposed in both the forward and backward optical paths, 
respectively, near the object 2 or reference mirror 15. 
In the displacement measuring apparatus according to the present invention, 
an interference light produced by interference between a reflected light 
from the reference mirror and a one from an object under measurement, is 
converted to an electrical signal. The electrical signal is 
phase-demodulated on the basis of a carrier signal to detect a 
displacement of the object. Thus, the displacement measuring apparatus of 
the present invention can detect a displacement of the object with a high 
resolution. 
Also in the displacement measuring apparatus according to the present 
invention, when a reset signal synchronized with an interference signal is 
supplied, a frequency-divided carrier signal is produced which has with 
respect to a frequency-divided signal such a phase difference that an 
output signal after phase-demodulated takes a predetermined value, a phase 
difference between the frequency-divided carrier and interference signals 
is detected, and the interference signal is phase-demodulated. Thereby, 
the displacement measuring apparatus of the present invention can 
zero-reset or offset a detection output irrespectively of the position of 
the object, thus the apparatus is easily operable.