Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2003-05-29
2004-12-07
Gregory, Bernarr E. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
Reexamination Certificate
active
06829042
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement of measurement accuracy in the case of determining a distance from an object according to a Time of Flight Method, and particularly to a distance measuring apparatus and method using a pulsed electromagnetic wave, by which a precise distance measurement can be performed with a high-speed response even when the distance is small.
2. Disclosure of the Prior Art
A method of determining a distance from an object according to a time period between a projection time, at which a pulsed electromagnetic wave is projected to the object, and a reflection-wave reception time, at which the electromagnetic wave reflected from the object is received is known as a Time of Flight Method.
For example, U.S. Pat. No. 5,054,911 discloses a light wave distance measuring instrument of the pulse type. According to this instrument, a part of the light wave projected to an object is used as a reference light wave, and allowed to pass alone an optical path having a predetermined length without interacting with the object. A reflection wave obtained when the light wave is reflected off the object and the reference wave are received by a single receiver. A distance between the object and the instrument is determined according to a time difference between a reference-wave reception time, at which the reference wave is received by the receiver, and a reflection-wave reception time, at which the reflection wave is received by the same receiver.
By the way, as shown in
FIG. 24
, when the distance between the instrument and the object is small, the reference wave “S” may be positioned closely adjacent to the reflection wave “R” on the time axis. In this case, it becomes difficult to clearly separate the reference wave “S” and the reflection wave “R” from each other, so that there is a problem in that the measurement accuracy is lowered. To avoid such a problem, it was proposed to introduce an optical fiber having a predetermined length into an optical path for the reflection wave, and delay the reception of the reflection-wave by a delay time “Td” such that the reflection wave “R” is spaced away from the reference wave “S” on the time axis, as shown by the dotted line in FIG.
24
. Thereby, the distance can be determined according to a time difference “TS-R” between the receptions of the reflection wave and the reference wave and the delay time “Td.”
However, the reflectivity of the object changes according to the color of the object surface, i.e., whether the surface color is black or white. In addition, the reflectivity changes according to the surface condition of the object, i.e., whether the object surface is a mirror reflection surface or a diffuse reflection surface. Therefore, when the reflectivity is small, there is a concern that a considerable loss of light can result from allowing the reflection wave to pass through the optical fiber. Needless to say, as the length of the optical fiber increases, the problem of light loss becomes more serious. In other words, it reduces the S/N ratio. For example, to prevent this S/N ratio deterioration, the gain of an amplifier or an amount of the light wave projected to the object can be increased. However, these solutions to the S/N deterioration problem lead to an increase in cost of the distance measuring instrument. In addition, there is another problem in that the distance measuring instrument becomes a huge and complex structure.
SUMMARY OF THE INVENTION
Therefore, in consideration of the above, a primary object of the present invention is to provide a distance measuring apparatus using a pulsed electromagnetic wave, by which a precise distance measurement can be performed with a high-speed response even when a distance between the apparatus and an object is small.
That is, the distance measuring apparatus includes:
a single projector for projecting the electromagnetic wave to an object;
a branch means placed between the projector and the object to obtain a reference wave branched from the electromagnetic wave;
a single receiver for receiving the reference wave and a reflection wave obtained when the electromagnetic wave is reflected off the object;
a delay means introduced into an optical path for the reference wave extending from the branch means to the receiver without interacting with the object to provide a delay time for delaying a reference-wave reception time, at which the reference wave is received by the receiver, such that a first time period between the reference-wave reception time and an electromagnetic-wave projection time, at which the electromagnetic wave is projected from the projector, is longer than a second time period between the projection time and a reflection-wave reception time, at which the reflection wave is received by the receiver; and
a processor for calculating a time difference between the reference-wave reception time and the reflection-wave reception time from outputs of the receiver, and determining a distance between the distance measuring apparatus and the object according to the time difference and the delay time.
According to the distance measuring apparatus of the present invention, since the reference-wave reception time is delayed than the reflection-wave reception time by the delay time, it is possible to prevent the occurrence of an inconvenience that the reference wave is overlapped with the reflection wave, as shown in
FIG. 24
, so that the distance can not be measured with accuracy. In particular, since maintaining a high strength of the reference wave is relatively easy, a light loss of the reference wave caused by allowing the reference wave to pass through the delay means such as an optical fiber to delay the reference wave reception time does not wield a large influence over the accuracy of measuring the distance.
When the electromagnetic wave is light, it is preferred that the delay means is provided by an optical fiber having a predetermined length or a plurality of mirrors arranged so as to prolong the optical path for the reference wave. In the case of using the optical fiber, it is possible to obtain a desired delay time according to the length of the optical fiber. In the case of using the mirrors, it is possible to stably provide the delay time even in the presence of disturbance factors such as changes in ambient temperature
In addition, it is preferred that the delay means includes a reference-wave receiving device used only to receive the reference wave from the branch means; a delay circuit for delaying an output signal provided from the reference-wave receiving device; and a reference-wave emitting device for providing an output of the delay circuit to the receiver. This embodiment is effective to downsize the distance measuring apparatus of the present invention.
It is preferred that the delay means includes a delay-time adjuster for changing the delay time. In this case, since an adequate delay time can be selected in accordance with the distance to be measured, it is possible to accurately determine the distance with an improved response speed.
It is also preferred that the distance measuring apparatus further includes a temperature compensating unit for compensating for fluctuations of the delay time, which is caused by changes in ambient temperature. In this case, it is possible to stably maintain the measurement accuracy without the influence of ambient temperature.
When the electromagnetic wave is light, it is preferred that the distance measuring apparatus further includes a light-amount adjuster for adjusting a light amount of at least one of the reflection wave and the reference wave received by the receiver. Since adjusting the light amount of the reflection wave and/or the reference wave is effective to reduce noise components, it is possible to improve the S/N ratio and to more accurately determine the distance.
It is preferred that the processor allows the projector to make a plurality of projections of the electromagnetic wave to the object, and calculates an average time diff
Honda Tatsuya
Nakamura Kuninori
Yoshimura Kazunari
Andrea Brian
Gregory Bernarr E.
Matsushita Electric & Works Ltd.
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