Distance measurement system

Details Distance measurement system Distance measurement system

1999-09-24

2001-11-27

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

Triangulation ranging to a point with one projected beam

C396S106000, C396S120000

Reexamination Certificate

active

06323940

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
However, with such a distance measurement system, the charging speed of the integrating capacitor
2
with the constant current source
4
in the second integrating may change because of such as a power voltage-lowering due to battery drain. In this case, a value of distance to the object obtained based on the time required for the second integrating may be inaccurate. Furthermore, when a period required for the second integrating becomes longer, a time lag from release-button manipulation to light exposure also increases. Thus, when it is desired to shoot such as a moving subject (object to be measured), a photograph having a desired composition may not be obtained.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a distance measurement system, which can detect abnormal charging speed of an integrating capacitor in the second integrating and which can perform suitable processing.
To achieve this object, the present invention supplies an active distance measurement comprising: (1) a light source for emitting a predetermined series of light pulses toward an object to be measured; (2) a position sensitive detector (PSD) for receiving object-reflected light pulses and outputting signals each corresponding to a position where the object-reflected light pulse is received; (3) an arithmetic unit for outputting signals each corresponding to the distance to the object in response to the signals output from the PSD; (4) an integrating unit including an integrating capacitor, and a comparator comparing a charged voltage in said capacitor with a predetermined reference voltage and outputting a compared signal represented compared result, said integrating unit performing a first integrating during which said integrating capacitor is discharged or charged in response to the signals output from said arithmetic unit, and a second integration during which said integrating capacitor is charged or discharged at a fixed speed until the time when the charged voltage in said integrating capacitor reaches to the predetermined reference voltage or until a predetermined maximum period elapses; and (5) a distance detection unit for detecting the distance to the object based on the period if a period of second integrating is enough to charge or discharge said integrating capacitor to the reference voltage.
With the distance measurement system according to the present invention, a series of light pulses are emitted toward the object to be measured from the light source. These pulses are reflected by the object to be measured. Such object-reflected pulses are received by the PSD. The position where the object-reflected pulse is received changes responsive to the distance to the object. The PSD outputs the signal dependent on the light-receiving position. The arithmetic unit calculates the distance to the object based on this signal and outputs the signal corresponding to the distance. The integrating unit accumulates the signals output from the arithmetic unit (first integrating) by charging or discharging the integrating capacitor in response to the

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