Distance measurement system

Details Distance measurement system Distance measurement system

1999-09-22

2001-11-06

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

With photodetection

C396S106000, C396S120000

Reexamination Certificate

active

06313907

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for measuring a distance to an object to be measured and more particularly to an active distance measurement system favorably applied to various types of cameras.
2. Related Background Art
Such an active distance measurement system applied to cameras generally includes an infrared-emitting diode (IRED) for emitting an infrared beam toward an object to be measured, and a position sensitive detector (PSD) for receiving the object-reflected infrared beam. The signal output from the PSD is a signal responsive to a position where the object-reflected infrared beam is received. A signal processing and arithmetic unit determines a distance to the object to be measured from this signal. Because a large error may occur at once measurement, averaging of multiple pieces of distance information is generally performed to obtain more accurate distance information.
FIG. 4
shows a circuit diagram illustrating a configuration of an integrating unit used for obtaining the average of the distance information in the distance measurement system. This integrating unit
16
comprises a switch
1
, an integrating capacitor
2
, a switch
3
, a constant current source
4
, an operational amplifier
5
, a switch
6
, a reference power source
7
, and a comparator
8
. The negative input terminal of the operational amplifier
5
is connected through the switch
1
to the output terminal of an arithmetic unit
15
and grounded through the integrating capacitor
2
. Furthermore the negative input terminal of the operational amplifier
5
is connected through the switch
3
to the constant current source
4
, and connected through the switch
6
to the output terminal of the operational amplifier
5
. Also, the positive input terminal of the operational amplifier
5
is connected to the reference power source
7
, which provides a reference voltage V
REF
. The comparator
8
is connected to the junction between the negative terminal of the operational amplifier
5
and the integrating capacitor
2
and compares the potential of the junction and the reference voltage V
REF
to find out which is higher. The comparator
8
outputs a signal corresponding to the comparison results. A central processing unit (CPU)
19
receives the signal output from the comparator
8
and controls the on-off operation of the switches
1
,
3
and
6
.
As an example of the distance measurement system using such an integrating unit
16
is a distance measurement system mounted in a camera. When a shutter release button is half- or partially-depressed after powering on the camera, the CPU
19
turns on the switch
6
to charge the integrating capacitor
2
. As the result, the integrating capacitor
2
is charged, as generally shown in
FIG. 5
, to the reference voltage V
REF
provided by the reference power source
7
. After the charging up, the switch
6
is turned off and retained in such a state.
Then, the IRED emits infrared pulses and the switch
1
is turned on. As a result, output signals (distance information) from the arithmetic unit
15
are input into the integrating capacitor
2
asnegative voltages. AS shown in
FIG. 5
, the voltage across the integrating capacitor
2
decrementally changes step by step in value corresponding to each distance measurement information. This is called a “first integrating”.
After the predetermined number (e.g., 256) of negative voltage inputs (discharges) into the integrating capacitor
2
are completed, the switch
1
is turned off and the switch
3
is turned on in response to control signals from the CPU, whereby the integrating capacitor
2
is charged at a fixed speed defined by the power rating of the constant current source
4
. This is called a “second integrating”.
All the while of the second integrating, the comparator
8
compares the voltage level of the integrating capacitor
2
and the reference voltage V
REF
, If the comparator
8
estimates that they are coincident with each other then the comparator
8
turns the switch
3
off to stop charging the integrating capacitor
2
, i.e. finish the second integrating. The CPU
19
counts a charging time of capacitor
2
(length of time spent in the second integrating). As the charging speed by the constant current source
4
is uniform, the sum of the signal voltages input into the integrating capacitor
2
during the first integrating can be determined from the aforementioned charging time of capacitor
2
. The distance to the object can be determined based on the resultant sum. On the basis of the obtained distance to the object, the CPU
19
controls a driving of lens to focus. In the subsequent distance measurement, as the required charging of the integrating capacitor
2
has been realized by the constant current source
4
, the switch
3
may be retained open, unless the constant current source
4
is provided in use for a long time.
SUMMARY OF THE INVENTION
In the active distance measurement system as explained above, it is desired to use a low-cost ceramic condenser as an integrating capacitor for the integrating unit
16
because of the requirements for decreasing the cost of manufacturing. However, the ceramic condenser encounters the problem of a drop in charged voltage due to dielectric absorption. That is, the capacitor
2
forms an equivalent circuit shown in
FIG. 6
immediately after the start of the first charging. Because of this, when a switch SW is turned off after the first charging, the voltage drop due to a resistance element Rx in
FIG. 6
may be observed. Such a phenomenon is called “dielectric absorption”.
Because of such a dielectric absorption occurring by using a ceramic condenser as the integrating capacitor
2
, a relatively large voltage drop &Dgr;V occurs as shown in
FIG. 5
when the switch
6
is opened in the first distance measurement, and then the first integrating starts. Therefore, a time delay &Dgr;t corresponding to the voltage drop &Dgr;V is caused in a length of time required to charge in the second integrating. This time delay At results in an error in distance measurement. It is noted that although a film condenser causes a voltage drop due to dielectric absorption, an amount thereof is very little so that: substantially no influence is exerted on the distance measurement. However, not only its cost of manufacturing is too high, but also it is rather bulky. This requires a large mounting space, preventing the system from being small sized.
As a distance measurement system solving such problems, a system is known, which is disclosed in Japanese Laid-Open Patent Publication No. 8-110222. In the distance measurement system disclosed in the above publication, after the main power is supplied and before the first distance measurement starts, an integrating capacitor is preliminarily charged for a predetermined period so that the voltage drop due to the dielectric absorption forcedly occurs in the integrating capacitor. This prevents the occurrence of the voltage drop in the integrating capacitor due to dielectric absorption during the first distance measurement. Otherwise, in the first distance measurement after the main power is supplied, the integrating capacitor may be charged for a sufficient length of time to prevent the occurrence of the voltage drop due to the dielectric absorption. In this manner, no voltage drop develops due to the dielectric absorption in the distance measurement, so that the occurrence of the distance measurement error can be evaded.
However, even with the distance measurement system disclosed in the above publication, the integrating capacitor drops in voltage level in the case where the distance measurement does not start for a considerable lapse of time after the main power source is powered on and in the case of a standby mode where a supply of power is stopped when no manipulation is carried out for a certain period. Thus, a distance measurement error due to the dielectric absorption of the integrating capacitor may occur in the subsequent distance measurement

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