Optical displacement measuring device

Details Optical displacement measuring device Optical displacement measuring device

2002-04-25

2004-12-07

Pyo, Kevin (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231160, C250S23700G, C356S616000

Reexamination Certificate

active

06828548

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical displacement measurement apparatus, more particularly relates to a high precision optical displacement measurement apparatus using a photoelectric transmission type linear encoder, used for a contact type digital displacement meter.
BACKGROUND ART
In the past, optical measurement devices using lasers or light emitting diodes (LED) and optical measurement devices using optical encoders have been known. Optical measurement devices can achieve a high precision since they use as units of measurement the wavelengths of the lasers or LEDs. Further, optical measurement devices are mainly used for measuring the length between two points, i.e., measuring relative position. Optical encoder measurement devices are comprised of a scale made of a glass plate, film, metal sheet, etc., an optical grid provided at a predetermined pitch from the scale, a fixed index grid arranged facing the scale across a predetermined distance (the phase of the optical grid and the phase of the fixed index grid being shifted 90 degrees), a fixed light source for emitting parallel light to the scale, and a light receiving sensor. When the scale moves, the optical grid and fixed index grid overlap with each other to produce differences in lightness are produced. The light receiving sensor detects the difference in lightness. Optical encoder measurement devices are being used commercially as digital displacement meters and are mainly being used for measuring the length between two points, i.e., measuring relative position.
Below, optical encoder measurement devices of the prior art will be explained with reference to the drawings.
FIG. 1
shows the state of use of a contact type digital displacement meter
40
including a photoelectric transmission type linear encoder. The contact type digital displacement meter
40
is used connected to a counter
41
by a connection cable
7
. The contact type digital displacement meter
40
is supplied with power from the counter
41
to perform measurement and outputs the measurement value to the counter
41
. The counter
41
processes the signal output from the contact type digital displacement meter
40
and digitally displays the obtained measurement value on a display unit
42
. Therefore, the displacement of an object measured by the contact type digital displacement meter
40
is displayed on the display unit
42
as a digital value.
The contact type digital displacement meter
40
has a frame
8
covered by an upper cover
9
A and a lower cover
9
B and has bearings
18
fastened at the two ends of the frame
8
. The bearings
18
support a spindle
5
. A contactor
6
is screwed into the front end of the spindle
5
.
When measuring the length, thickness, etc., the contactor
6
screwed into the front end of the spindle
5
is brought into contact with the measured object. Displacement of the measured object causes the spindle
5
to move in the arrow direction. The displacement of the spindle
5
is detected by the photoelectric transmission type linear encoder built into the contact type digital displacement meter
40
, the detection output is processed by the counter
41
, and the displacement of the measured object is displayed on the display unit
42
.
FIG. 2
explains the principle of a conventional photoelectric transmission type linear encoder built into the contact type digital displacement meter
40
explained in FIG.
1
. The spindle
5
into which the contactor
6
is screwed has connected to it a moving scale
3
made of a transparent member. The moving scale
3
is formed with an equal pitch optical grid
11
.
At one side of the moving scale
3
are provided a light source
1
and a condenser lens
2
. At the other side are provided a fixed scale
34
formed with equal pitch optical grids
47
and
48
and light receiving elements, i.e., photodiodes
28
. The light source
1
and the photodiodes
28
face each other across the moving scale
3
moving in accordance with displacement of the measured object and the fixed scale
34
fixed to a constant position.
The optical grid
11
provided at the moving scale
3
and the optical grids
47
and
48
provided at the fixed scale
34
have the same pitches and same line widths, for example, pitches of 20 &mgr;m and line widths of 10 &mgr;m. The two types of scales are fabricated to extremely high precisions.
At the time of measurement, the spindle
5
moves in the arrow direction. The amount of light passing through the scales becomes maximum when the transparent portions of the optical grid
11
of the moving scale
3
and the transparent portions of the optical grids
47
and
48
of the fixed scale
34
match. On the other hand, when the moving scale
3
moves by exactly ½ of the pitch of the optical grid from that state, the transparent portions and nontransparent portions of the optical grids overlap, so the amount of light transmitted becomes the minimum. That is, along with movement of the moving scale
3
, the signals output from the photodiodes
28
become sinusoidal signals. By counting the number of their cycles, the distance of movement of the moving scale
3
can be found.
In general, the fixed scale
34
is normally provided with two optical grids
47
and
48
. Corresponding to this, two photodiodes
28
are also provided. Further, one optical grid
47
is shifted by exactly ¼ pitch from the other optical grid
48
.
FIG. 3
shows the signals output from the two photodiodes
28
when the moving scale
3
moves. If expressing the light passing through one optical grid
47
of the fixed scale
34
as the signal A in
FIG. 3
, the signal B expressing the light passing through the other optical grid
48
of the fixed scale
34
is shifted in phase from the pitch
P
of the signal A by ¼ pitch. It is possible to determine the right or left direction of movement of the moving scale
3
by the advance or delay of the phase of the signal B with respect to the signal A.
FIG. 4
shows the configuration of a first prior art of a photoelectric transmission type linear encoder built into the contact type digital displacement meter
40
explained in
FIG. 1. A
cross-section of the linear encoder is shown. The linear encoder is mainly provided with two LEDs
1
used as light sources, a moving scale
3
, a spindle
5
, a fixed scale
34
, and two photodiodes
28
.
The frame
8
has an upper cover
29
A and lower cover
29
B and a linear encoder support base
30
screwed to it. The spindle
5
is supported by two bearings
18
fastened to the frame
8
. A contactor
6
for contacting the measured object is screwed into the front end of the spindle
5
. The moving scale
3
is positioned with and fastened to a moving scale support base
31
. The moving scale support base
31
is positioned with and fastened to the spindle
5
, so movement of the spindle
5
becomes movement of the moving scale
3
. The moving scale
3
is sandwiched between the light source LEDs
1
and condenser lenses
2
and the light receiving side fixed scale
34
and photodiodes
28
.
For a stopping mechanism of the spindle
5
, while not shown, a stopping rod is fastened to the spindle
5
at one end. The other end slides in a groove provided in the frame
8
to thereby function as a stop. Further, the rod is linked with the frame
8
by a tension spring and is set to apply a suitable contact pressure to the measured object.
At the light emitting side, the two LEDs
1
are fastened to the LED support base
32
. The condenser lenses
2
are fastened to the LEDs
1
. The LED support base
32
is positioned with and screwed to the linear encoder support base
30
so as to facilitate positioning with the light receiving side. The two LEDs
1
and condenser lenses
2
sandwich the moving scale
3
between them and face the light receiving side fixed scale
34
and two photodiodes
28
.
At the light receiving side, the two photodiodes
28
are set on a PCB (printed circuit board)
33
. The PCB
33
is fastened to the linear encoder support base
30
. The fixed sc

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