Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2000-11-29
2004-02-17
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005100, C250S2140AG, C250S2140LS
Reexamination Certificate
active
06693703
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a distance measuring device, in which sensitivity of a photodetection unit of a distance measuring device is easily adjustable, and the invention also relates to a method for adjusting the photodetection unit of the distance measuring device.
In distance measurement based on a light wave, a modulated measuring light beam is used, and a distance is determined from a phase difference of the reflected light beam.
FIG. 4
is a drawing to explain a basic type of a distance measuring unit
1
of a distance measuring device, and
FIG. 5
is a block diagram to explain the control unit
13
of the distance measuring device.
When a distance measuring light beam
4
is emitted from a light emitting element
3
which is modulated and driven by an oscillator
2
, the distance measuring light
4
is turned to a parallel beam by an objective lens
5
and this beam is projected to a reflection prism
6
, which is an object to be measured. The distance measuring light
4
reflected by the reflection prism
6
enters the objective lens
5
again and is received by a photodetection element
7
. A part of the distance measuring light beam emitted from the light emitting element
3
is reflected by a reflection mirror
9
and it is received by the photodetection element
7
as an internal reference light beam
12
.
Based on photodetection signals of the received distance measuring light beam
4
and the received internal reference light beam
12
, the distance is calculated from the phase difference by a distance measuring circuit
8
.
The distance to the object to be measured (the reflection prism
6
) as obtained by the distance measuring circuit
8
is displayed on a display unit
15
as a numerical value indicating the distance based on the control by a CPU
14
. Distance measuring instruction, data, etc. are inputted from a keyboard unit
16
.
FIG. 6
is a drawing to explain the distance measuring unit
1
more concretely.
On the side of the light emitting element
3
, a chopper
17
is provided to cut off the distance measuring light beam
4
coming from the light emitting element
3
such as LED, LD, etc., and the chopper
17
is rotated by an optical path changing motor
18
. On the side of the photodetection element
7
, a density filter
20
is provided to filter a reflected distance measuring light
4
′, and the density filter
20
is rotated by a density adjusting motor
21
. As the photodetection element
7
, APD (avalanche photodiode) is used depending on the conditions such as photodetection sensitivity.
The chopper
17
is provided with such a pattern that the light beam on the inner side and the light beam on the outer side are alternately cut off. When the chopper
17
is rotated by the optical path changing motor
18
, the optical path is changed alternately. The density filter
20
is provided with such a pattern that the density is gradually changed in rotating direction. When a rotating position of the density filter
20
is selected by the density adjusting motor
21
, the received light amount of the photodetection element
7
can be adjusted. The light amount of the reflected distance measuring light beam
4
′ entering the photodetection element
7
is increased or decreased according to the distance between the reflection prism
6
and the distance measuring device. By properly selecting the position of the density filter
20
, the received light amount of the photodetection element
7
can be maintained on a constant level.
Based on the photodetection light amount (i.e. the received light amount) of the photodetection element
7
, the density adjusting motor
21
is rotated so that the light amount of the reflected distance measuring light beam
4
′ and the internal reference light beam
12
entering the photodetection element
7
is maintained at a constant level, and the position of the density filter
20
is controlled. In synchronization with a reference signal issued by the distance measuring circuit
8
, the optical path changing motor
18
is rotated and controlled, and the reflected distance measuring light beam
4
′ and the internal reference light beam
12
entering the photodetection element
7
are switched over.
The avalanche photodiode (APD) as described above has highly accurate photodetection sensitivity and has a wide dynamic range, and it is used as a photodetection element. However, the characteristics of APD widely vary according to each individual element, and adjustment is necessary when it is incorporated in a distance measuring device. In particular, the adjustment is indispensable in case of a distance measuring device with high accuracy.
By changing a bias voltage, APD has an ability to adjust a gain multiplication of an electric current by receiving the light beam. The characteristics of the bias voltage and the output current of APD are shown in FIG.
7
. As shown in
FIG. 7
, APD has a wide range of the gain multiplication, and it can be 150 times or more. When the bias voltage exceeds a certain level (Va), an avalanche phenomenon occurs. Therefore, in the adjustment of APD, the bias voltage is set to a value lower than the bias voltage to induce the avalanche while maintaining the needed gain multiplication.
Description will be given now on a conventional adjustment mode of APD.
In the past, a special-purpose adjustment tool has been required for the adjustment.
As shown in
FIG. 8
, a conventional type adjustment tool comprises a combination of an LED
22
for adjustment and a density filter
23
.
First, a light beam is emitted from the LED
22
, of which output light amount is determined at a constant level, and it is projected toward the photodetection element
7
at a predetermined position. The emitted light beam is modulated in the same manner as the actual distance measuring light beam
4
.
APD directly receives the light beam from the LED
22
, and the adjustment is made from a value obtained by photoelectric conversion. The gain multiplication in this case is assumed as 1×.
Next, a {fraction (1/150)} density filter
23
is inserted between the photodetection element
7
and the LED
22
. The photodetection light amount of the APD is turned to {fraction (1/150)} of the initial value. When the photodetection light amount is decreased to {fraction (1/150)}, the bias voltage is increased in such manner that the output current from APD is equalized to the value when the density filter
23
is not used. By the equalized bias voltage, the gain multiplication of 150 times is obtained.
However, even with the photodetection element
7
thus adjusted, it sometimes happens that the accurate output current as set to 150 times may not be obtained depending on the frequency of the use of the distance measuring device, the course over time, etc. In such case, the product must be adjusted again. For this reason, in the past, the distance measuring device has been re-adjusted periodically or after the use of a predetermined period of time. As described above, a special-purpose adjustment tool and a measuring instrument are required for the adjustment, and the user of the distance measuring device cannot perform adjustment as desired, and the re-adjustment has been usually requested to the manufacturer. This means that complicated procedures are required each time the distance measuring device has to be sent back to the manufacturer for re-adjustment. Also, the distance measuring device cannot be used during the re-adjustment, and this causes much inconvenience to the user.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a distance measuring device and a method for adjustment, by which it is possible to adjust the photodetection element (APD) in an easy and convenient manner, to eliminate complicated procedures for the re-adjustment, and to exclude the limitation on the use of the distance measuring device due to the re-adjustment.
To attain the above object, the present invention provides a distance measuring device for measuring
Kumagai Kaoru
Ohishi Masahiro
Yoshino Ken-ichiro
Buczinski Stephen C.
Kabushiki Kaisha TOPCON
Nields & Lemack
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