Distance measuring apparatus

Details Distance measuring apparatus Distance measuring apparatus

2001-07-09

2002-07-16

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

Triangulation ranging to a point with one projected beam

C396S098000, C396S106000, C396S120000, C396S125000

Reexamination Certificate

active

06421115

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to distance measuring apparatus for measuring the distance to an object to be measured and, more particularly, to distance measuring apparatus of an active type suitably applicable to cameras and others.
2. Related Background Art
The distance measuring apparatus of the active type used in the cameras and others is configured to project light from an infrared emitting diode (which will be referred hereinafter as “IRED”) toward an object to be measured, receive reflection of the projected light by a position sensitive device (which will be referred to hereinafter as “PSD”), arithmetically process signals outputted from this PSD by a signal processing circuit and an arithmetic circuit to provide distance information, and determine the distance to the object by a CPU. Since distance measurement with only one light projection can produce an error, it is common practice to perform a plurality of light projections to obtain a plurality of distance information and integrate the plurality of distance information by an integrating circuit to average the information.
The conventionally known distance measuring devices of this active type include those described in Japanese Patent Applications Laid-open No. H08-94919 and No. H08-94920.
FIG. 22
is a block diagram of the distance measuring apparatus described in these applications, which will be referred to as the distance measuring apparatus according to the first prior art.
In the distance measuring apparatus shown in
FIG. 22
, a driver
112
drives the IRED
114
under control of CPU
110
to make it output infrared light, and the infrared light is projected through a projection lens (not shown) toward an object to be measured. The infrared light reflected by the object is converged through a receiving lens (not shown) on the PSD
116
, and the PSD
116
outputs two signals I
1
and I
2
according to a position where the reflection of the infrared light is received. A first signal processing circuit
118
removes a stationary light component of noise included in the signal I
1
and a second signal processing circuit
120
a stationary light component of noise included in the signal I
2
.
An arithmetic circuit
132
calculates an output ratio (I
1
/(I
1
+I
2
)), based on the signals I
1
and I
2
obtained after the removal of the stationary light components, and outputs an output ratio signal according to the distance to the object. An integrating circuit
134
integrates a multiplicity of output ratio signals outputted in this way from the arithmetic circuit
132
to improve an S/N ratio. This integrating circuit
134
outputs a signal (hereinafter referred to as “AF signal”) according to the distance to the object. Then the CPU
110
executes a predetermined operation to obtain a distance signal, based on the AF signal outputted from the integrating circuit
134
, and controls a lens driving circuit
136
, based on this distance signal, to move a lens
138
to an in-focus position.
FIG. 23
is a drawing showing a relation between the AF signal outputted from the integrating circuit
134
of the first prior art and the distance to the object. In the graph shown in this drawing, the abscissa represents the inverse (1/L) of the distance L to the object and the ordinate the output ratio (I
1
/(I
1
+I
2
)) or the AF signal. As shown in this figure, the output ratio is substantially in a linear relation to the inverse (1/L) of the distance L in the range not more than a certain distance L
4
. Thus the output ratio becomes smaller as the distance L increases (or as 1/L decreases). In the range not less than the distance L
4
, however, the influence of the noise component becomes larger to the contrary with increase in the distance L. When the noise component is denoted by In (In≧0), the output ratio is given by (I
1
+In)/(I
1
+In+I
2
+In). In the range larger than the distance L
4
, the output ratio varies so as to increase (i.e., toward the output ratio of 50%). In addition, since In occurs at random, the output ratio becomes unstable depending upon measurement conditions. The reason is that with increase in the distance L the intensity of the reflected light received by the PSD
116
becomes smaller and the noise component In becomes relatively larger. With occurrence of this phenomenon, the distance L to the object cannot be uniquely determined from the output ratio.
For this reason, as shown in
FIG. 24
, a clamping circuit
130
is interposed between the second signal processing circuit
120
and the arithmetic circuit
132
to compare the far signal I
2
outputted from the second signal processor
120
, with a clamp signal Ic and output the clamp signal Ic when the far signal I
2
is smaller than the clamp signal Ic. Even in this structure, however, the distance output is fixed at a certain distance on the far side, as shown in
FIG. 27
described hereinafter, and there occurs great deviation from designed values.
The distance measuring devices giving a solution to this problem include those described below.
FIG. 25
is a block diagram of the distance measuring apparatus according to the second prior art. This figure shows only the structure on the photoreceptive side. In the distance measuring apparatus shown in this figure, the signals I
1
and I
2
outputted from the PSD
140
are supplied to respective stationary light removing circuits
142
and
144
to remove the stationary light component therefrom and thereafter the resultant signals are given to respective arithmetic circuits
146
and
148
. The arithmetic circuit
146
performs an operation of I
1
/(I
1
+I
2
) to obtain an output ratio, based on the signals I
1
and I
2
after the removal of the stationary light components, and the integrating circuit
150
integrates the output ratio. On the other hand, the arithmetic circuit
148
performs an operation of I
1
+I
2
to obtain the quantity of light and the integrating circuit
152
integrates the quantity of light. Then a selection part
160
selects either one of the output ratio and the quantity of light and calculates the distance to the object, based on the selected. The selection part
160
is a process in the CPU.
FIG. 26
is a block diagram of the distance measuring apparatus according to the third prior art. This figure also shows only the structure on the photoreceptive side. In the distance measuring apparatus shown in this figure, the signals I
1
and I
2
outputted from the PSD
170
are supplied to respective stationary light removing circuits
172
and
174
to remove the stationary light component therefrom and thereafter either of the resultant signals is given to one end of switch
176
. Under control of the CPU, this switch
176
supplies either of the outputs from the stationary light removing circuits
172
and
174
into the integrating circuit
178
. The integrating circuit
178
integrates either one of the input signals I
1
and I
2
. An arithmetic part
180
executes an operation of I
1
/(I
1
+I
2
) to obtain an output ratio, based on the integration result, while an arithmetic part
182
does an operation of I
1
+I
2
to obtain the quantity of light. Then a selection part
184
selects either one of the output ratio and the quantity of light and calculates the distance to the object, based thereon. The arithmetic parts
180
,
182
and the selection part
184
are processes in the CPU.
These distance measuring devices (FIG.
25
and
FIG. 26
) according to the second and third prior arts are constructed both to calculate the distance L, based on the output ratio (I
1
/(I
1
+I
2
)), when the distance L to the object is small, but calculate the distance L, based on the light quantity (I
1
+I
2
), when the distance L is large, whereby the distance L can be uniquely determined.
As described above, the distance measuring devices according to the second and third prior arts (FIG.
25
and
FIG. 26
) are the apparatus that can give a solution to the problem

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