Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1999-10-13
2003-01-28
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005150
Reexamination Certificate
active
06512575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for measuring the distance of an object. In particular, the method and device of this invention further enable an optical code provided on the object to be read.
2. Discussion of Prior Art
A measurement of the distance of an object from a measuring device is of high utility and sometimes fundamental in several technical fields. This is the case, for example, of all those machining processes where a knowledge of the distance to a surface being machined from the machine tool is necessary for correct positioning of the tools and/or for exact programming of the machine; or all those instances where a knowledge of the distance parameter can allow instruments to be set for optimum processing (e.g., in optics and photography, where the distance parameter is closely related to the focusing problem).
Furthermore, a measurement of the distance to an object is often needed in object handling and sorting systems, wherein objects even markedly different in size may have to be identified and classified, and the object dimensions must be detected automatically in order to speed up and optimize subsequent object routing and storing steps.
Such systems typically include a belt conveyor, onto which the objects to be identified and sorted are placed, and one or more optical devices, usually of the laser light emitting type (commonly indicated as laser scanners) which are adapted to read optical codes and measure the object dimensions.
For improving the reliability of such reading and measuring operations, an indication of the distance between the object and the laser scanner is preferably provided first. In fact, a knowledge of the distance parameter is useful, on the one hand, to properly focus the emitted laser beam onto the object to be scanned, such that the optical code placed on the object can be read correctly, and on the other hand, to find out the object dimensions, such as its volume, for example. In addition, a knowledge of the distance parameter in real time advantageously allows the circuits to be “parametrized”.
Optical devices capable of providing an indication of the distance to an object have been known. For example, European Patent Application No. 0 652 530 of the same Applicant discloses a laser scanner with high-frequency modulated laser light emission, wherein the distance of the object is obtained as a function of the phase difference between the emitted signal by the scanner and the received signal. In particular, the scanner comprises a laser light emitting source which is amplitude modulated by a local oscillator, an optical scan means for directing the laser light toward an object to be scanned, and a light-receiving means for picking up light diffused by the illuminated object and generating an electric signal which is proportional to the intensity of the diffused light. The signal generated by the light-receiving means is sent to a phase demodulator which also receives a signal from the local oscillator; the demodulator measures a phase difference between said two signals, and produces an electric signal which is proportional to this phase difference.
A suitable calculating means then processes said electric signal to calculate a distance value as a function of said phase difference.
Other devices have also been known which measure the distance of an object on the basis of the so-called “flight time” of a pulse applied to an emission laser.
Specifically, the time taken by the pulse to travel along the optical path from the emitting means to the object, and from the object to the light receiving means, is measured. This time is proportional to twice the distance of the object from the device.
It has been found that the above-described devices cannot provide an adequately accurate measurement of distance, due to a number of drawbacks discussed in greater detail hereinafter.
A first drawback of modulated light devices is that variations in the device operative temperature bring about an uncontrolled variation in the transfer function distance signal/distance, specifically a variation in the phases of respectively the emitted and received signals by the device, which significantly affects the distance measurement. In fact, the laser is modulated by turning it on and off according to a given duty-cycle (e.g., for a 40 MHz modulation, the laser would be turned on/off 40,000,000 times per second), the duty-cycle being the ratio of the laser “on” duration and the period. In order for the system to maintain a constant average output power as temperature changes (a useful condition to keep the read performance of an optical code unvaried), a control circuit is provided whose side effect is that of varying the modulation duty-cycle according to the operative temperature variation; however, this variation in the duty-cycle brings about an uncontrolled variation in the phase of the signal emitted by the laser light source.
Likewise, with specific reference to avalanche silicon light receivers (wherein the light receiver gain is established by the reverse bias voltage), to provide the light receiving means with a constant gain as temperature changes (a condition which is also useful to keep the read performance of an optical code unvaried), a compensation circuit is provided and effective to vary the reverse voltage of the signal generated thereby (and hence, as a side effect, the capacity of the light receiving means too) as the operative running temperature changes; this variation implies an uncontrolled phase variation of the output signal from the light receiving means.
Throughout this specification and the appended claims, the term gain (or reception sensitivity)of the light receiving means, is used to indicate the ratio between the voltage actually generated by the light receiving means and the actual optical signal received.
The electronic components which are comprised into the device (specifically, the phase demodulator thereof) also introduce in the transfer function uncontrollable variations with temperature.
Another drawback of modulated light devices is associated to the value of the ratio between the signal detected by the light receiving means and noise (S/N), which ratio may be quite small for objects placed far or near enough and/or dark objects. In such circumstances, a sufficiently clean signal can be obtained only by an intensive signal filtering procedure
A further drawback is associated to the variation of the error of the distance signal according to the optical signal detected by the light receiving means; this is due to operational limitations and to the high sensitivity of the device to changes in reflectance of the objects.
In summary, it has been found that all of the above drawbacks affecting modulated light devices imply an overall error in the distance measurement which can be estimated at about ±5%. This percentage of error restricts the usability of the above-described devices to just those applications where the distance measurement need not to be highly accurate and repeatable.
It has been found, moreover, that pulse devices, while being immune from the aforementioned drawbacks, still have problems which cause accuracy and repeatability errors, to be estimated at about ±15cm, so that they are particularly suitable for distance measurements of large size objects and objects having relatively large scanned areas. In addition, also in this case the measurements are deeply affected by the reflectance of the objects, changing temperature, etc.
Measurement errors are also introduced, with both modulated light devices and pulse devices, by the ageing and the dimensional tolerances of the device optical and electronic components.
To measure a distance, devices incorporating LEDs, or devices provided with ultrasonic and/or radio-wave emitting means, could be used instead of laser devices. However, such devices are inadequate to provide reliable and accurate distance measurements. In addition, LEDs can only be used for measuring short
Buczinski Stephen C.
Datalogic S.p.A.
Nixon & Vanderhye P.C.
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