Optics: measuring and testing – Position or displacement
Reexamination Certificate
1999-09-24
2002-01-15
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Position or displacement
C356S003040, C396S106000, C396S120000
Reexamination Certificate
active
06339475
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for measuring a distance to an object of distance measurement 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 a beam of light toward an object of distance measurement, and a position sensitive detector (PSD) for receiving the emitted and reflected light beam. The signal outputted from the PSD is processed in signal processing and arithmetic circuits to be inputted therefrom as distance information into a central processing unit (CPU), by which a distance to the distance measurement object is determined. Because an error may occur in distance measured based on one-shot light emission alone, multi-shot light emissions are generally employed to obtain multiple pieces of distance information. The obtained information is typically integrated and averaged in an integrating circuit.
FIG. 1
shows a circuit diagram illustrating a configuration of the integrating circuit in the above distance measurement system. The integrating circuit generally shown at
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
, a comparator
8
, and a switch
9
. The inverting terminal (−) of the operational amplifier
5
is connected through the switch
1
to the output terminal of an arithmetic circuit (not shown) and grounded through the integrating capacitor
2
. Also, the amplifier inverting terminal (−) is connected through the switch
3
to the constant current source
4
, through the switch
9
to the terminal of a power supply voltage V
CC
, and through the switch
6
to the output terminal of the operational amplifier
5
. The non-inverting terminal (+) of the operational amplifier
5
is connected to the reference power supply
7
, which provides a reference voltage V
REF
. The comparator
8
is connected to the junction between the inverting 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 not shown central processing unit (CPU) receives the signal outputted from the comparator
8
and controls the on-off operation of the switches
1
,
3
,
6
and
9
.
FIG. 2
shows timing chart
40
explaining changes in voltage level of the integrating capacitor
2
with time in the prior distance measurement system. In the integrating circuit
16
, when a release button is half- or partially-depressed after the main power source is turned on, the switch
9
is put in an “on” state only for a constant period of time under the control of the aforementioned CPU to cause the integrating capacitor
2
to be excessively charged to the power source voltage V
CC
. Even after the switch
9
is turned to the “off” state, the switch
6
is maintained in an “on” state for another period of time so that the integrating capacitor
2
is charged up to the reference voltage V
REF
provided by the reference power source
7
. After the charging up, the switch
6
is turned to the “off” state.
Then, the IRED emits pulsed infrared light and the switch
1
is turned to the “on” state for each constant time during the emission duration. As a result, the integrating capacitor
2
accepts from the arithmetic circuit an output signal thereof as a negative voltage, which corresponds to each emitted infrared light pulse. Thus, as shown in the timing chart
40
of
FIG. 2
, the voltage of the integrating capacitor
2
decrementally changes step by step in value corresponding to a distance. This is called a “first integration”.
After the predetermined number (e.g., 256) of negative voltage inputs (discharges) into the integrating capacitor
2
are completed, the switch
3
is turned to. the “on” state by the control signal from the CPU, whereby the integrating capacitor
2
is charged at a constant rate defined by the rating of the constant current source
4
. This is called a “second integration”.
During the period of the second integration, the comparator
8
always compares the voltage level of the integrating capacitor
2
and the reference voltage V
REF
to find out which is higher and when determined that they are coincident with each other, causes the switch
3
to be turned to the “off” state. This causes the charging of the integrating capacitor
2
to be stopped and the CPU to commence determining a time required to perform the second integration. As the charging by the constant current source
4
is uniform in rate, the sum of the signal voltages inputted in the integrating capacitor
2
during one distance measurement, that is, the distance to the object of the distance measurement can be determined from the aforementioned time required to perform the second integration. 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 in the open state, unless the constant current source
4
is provided in use for a long time.
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 circuit
16
because of the requirements for decrease of production cost. 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. 3
immediately after the start of the first charging. Because of this, in
FIG. 3
, when a switch SW is turned to “off” after the first charging, the voltage drop due to a resistance element Rx is observed. Such a phenomenon is called “dielectric absorption”, which may constitute one of the factors causing an error in distance measurements.
Thus, in the aforementioned active distance measurement system, the CPU instructs at the start of the distance measurement that the switch
9
is turned to “on” for the predetermined period of time to overcharge the integrating capacitor
2
to the voltage level higher than the reference voltage so that the voltage drop due to the dielectric absorption forcedly occurs in the integrating capacitor
2
. Because the system operates in such a manner, no voltage drop occurs in the integrating capacitor
2
due to dielectric absorption during the distance measurement, thus preventing the occurrence of the distance measurement error with the result that the dielectric absorption problem can be solved.
SUMMARY OF THE INVENTION
However, the above distance measurement system must accomplish, under the instruction from the CPU, not only the operations to turn the switch
6
to the “on” state to charge up the integrating capacitor
2
to the reference voltage V
REF
, to control the on-off action of the switch
1
to perform the first integration, and to turn the switch
3
to the “on” state to perform the second integration, but also the operation to turn the switch
9
to the “on” state to overcharge the integrating capacitor
2
. As a result, complicated wiring is required to transmit the control signals representative of the instruction from the CPU. Furthermore, when the aforementioned signal processing circuit, arithmetic circuit and integrating circuit are consolidated into an integrated circuit, terminals in the integrated circuit increase in number.
The present invention has been made in order to overcome the above drawbacks and has for its object to provide a distance measurement system, which can measure a distance with high accuracy and can decrease the numbers of terminals and lines.
With the above object in view, the invention provides a distance measurement system comprising:
No affiliations
Font Frank G.
Fuji Photo Optical Co., Ltd.
Leydig , Voit & Mayer, Ltd.
Nguyen Sang H.
LandOfFree
Distance measurement system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Distance measurement system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Distance measurement system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2817868