Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2002-04-24
2004-04-13
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S221000, C327S514000
Reexamination Certificate
active
06720545
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a photoelectric sensor using light pulses for detecting the presence or absence of a target object, its distance, its size and its characteristics. In particular, this invention relates to such a photoelectric sensor provided with means for preventing erroneous operations even in an environment where there are periodic noise pulses appearing at a timing which coincides with the timing for judging received light.
For detecting the presence or absence of an object, its distance and its characteristics in a non-contacting manner, it has been known to utilize photoelectric sensors using light pulses which are variously called a photoelectric sensor, a distance sensor or a displacement sensor. Photoelectric sensors using light pulses are generally composed of a light transmitter for transmitting light pulses to a target area and a light receiver for receiving light pulses coming from the target area. They may be broadly divided into the transmitting type and the reflecting type. With a photoelectric sensor of the transmitting type, the light pulses transmitted from the light transmitter fail to be received by the light receiver if it is interrupted by a target object for detection. In the case of a photoelectric sensor of the reflecting type, by contrast, the light pulses transmitted from the light transmitter is received by the light receiver by being reflected by a target object.
These sensors may be divided also into an integrated type having both its light transmitter and receiver contained in a common housing structure and a separated type having them contained in individual housing structures. The integrated type is advantageous in that the light transmitter and receiver can be more easily correlated or synchronized. Many of photoelectric sensors of the reflection type and those of the transmitting type having separated heads such as those using fiber optics are structured as an integrated type. Many of the sensors of the transmitting type with the non-separated type of heads are structured as a separated type.
Where a photoelectric sensor is installed, noise of different kinds such as light and electromagnetic waves is expected to be present in addition to light pulses emitted from the sensor itself. Because of the noise of these kinds in the environment, noise pulses appear in the output line passing through the capacitor connecting a converter such as a photoelectric converter element in the light receiver, mixed in either through the converter or through the power source line. Some of such noise pulses appear periodically, while some appear at random.
Many different methods have been developed for preventing erroneous operations of a light receiver caused by such noise pulses. One of the methods is based on the synchronization technology whereby the timing of light transmission from the light transmitter and that of light reception by the light receiver are synchronized. Another method is based on the identification of a pulse array. The sensor output will not be switched on unless more than a specified minimum number of pulses are detected continuously. Once the sensor output is switched on, it is not switched off unless more than another specified number of pulses are missed. Still another method is a combination of both of these technologies such that the synchronization technology is used in the first stage to eliminate the noise pulses not in accordance with the received light level and then the pulse array is identified in the later stage to eliminate noise pulses which happened to be in synchronism with the judgment timing, or the timing of light transmission.
Such prior art methods are effective against noise pulses which appear randomly in the reception signal. If the noise pulses appear periodically and if the timing of their appearance coincides with the judgment timing, or the timing of light transmission, they can hardly be effective. Examples of such a situation occur in factories and storehouses where fluorescent lamps inclusive of the general frequency types and inverter types are commonly used. Since the timing of light transmission from such sensors cannot be varied too widely in view of the level of response required of the sensor, there is also a limit within which the effects of noise pulses can be avoided by varying the timing of light transmission.
SUMMARY OF THE INVENTION
It is therefore an object of this invention in view of the situation described above to provide a photoelectric sensor using light pulses with which erroneous operations can be effectively avoided even where noise pulses appear periodically and at a timing that coincides with that of the transmission of light.
It is another object to develop basic technologies for such improved photoelectric sensors.
In one aspect, this invention relates to a method for controlling a photoelectric sensor which transmits pulsed light repetitively by driving a light transmitting element at a specified light transmission timing and compares with a specified threshold value the level of what is herein referred to as the reception signal which is an electrical signal obtained by processing light received by a light receiving element at a timing (referred to as the level judging timing) with a slight delay from the aforementioned light transmission timing. If an AC waveform corresponding to noise is included in the reception signal, the light transmission from the sensor is controlled such that a zero-cross timing of this AC waveform and the level-judging timing will coincide, and a sensor output is generated on the basis of the result of the aforementioned comparison.
In the above, the “specified threshold value” may be determined on the basis of known level of reception signals based on light emitted by the sensor itself. The “slight delay” for defining a timing in such a situation is a commonly known method of correlating (or synchronizing) light transmission and light detection. Thus, the invention does not require this slight delay to be non-zero, that is, the comparison with this threshold value may be initiated at the same timing as the beginning of light transmission.
“Noise” may be of different origins. Noise due to light mixed in through the light receiving element such as light from an inverter type fluorescent lamp, as well as electromagnetic wave noise mixed in through a power line, may be included. Thus, “AC waveform”, referred to above, includes not only sinusoidal waveforms with regularly changing signal levels but also various waveforms with output polarity changing frequently through an AC zero-level.
The zero-cross timing and the level-judging timing need not be matched exactly. Some degree of mismatching is allowed. The allowable range of mismatching may be determined, depending upon the level-judging timing such as the time during which the sampling gate is left open in the case of a sensor having a sampling gate, the output characteristics of the AC waveform corresponding to noise, and the threshold value for the judgment.
According to a method of this invention, the level of reception signal is judged at the zero-cross timing of the AC waveform corresponding to the noise (referred to as the “noise output”) or when the noise output has its minimum value even if the sensor is being used in an environment where an AC waveform corresponding to noise is likely to appear in the reception signal such as immediately below an inverter type fluorescent lamp. Thus, an erroneous operation, such as outputting a judgment of presence of light (transmitted from the sensor itself) although no light pulse transmitted from the sensor itself is being received, can be avoided according to this invention.
In the method embodying this invention, it is preferable to select a zero-cross timing at which the polarity of the AC waveform corresponding to noise is changing to become opposite the polarity of waveform of reception signal corresponding to pulsed light transmitted from the sensor itself. The “polarity of the waveform of reception signal corres
Koya Toshiaki
Mizuhara Susumu
Nakamura Arata
Nakanishi Hiroaki
Beyer Weaver & Thomas LLP
Lee Patrick J.
Omron Corporation
Porta David
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