OPERATING PRINCIPLES
Detection principle of new triple beam area reflective series
$ Optical triangulation sensing method
The light beam projected by the LED, passing through the condensing lens of the light projector, is applied to the sur- face of the target to be detected. One part of the diffused reflected light rays passes through the light receiver lens producing a spot on the position sensi- tive device. When the detectable target is at a position A that is at a compara-
tively near distance as shown in Fig. 1, a spot is produced at (a). When the tar- get is at a position B that is far, a spot is produced at (b). Accordingly, if any spot position on the position sensitive device is detected, the distance to the target can be determined. This is the principle of optical triangulation range measurement.
Fig. 1 Optical triangulation range measurement method
Other examples of this method are the automatic focusing and camera range measurement systems.
$ Triple beam type range sensing principle
The MQ photoelectric sensor area reflective type has adapted this optical triangulation range measurement prin- ciple, but in order to improve the relia- bility of the detection of the sensor, a more elaborate method has been devised. First, light receivers are posi- tioned symmetrically on either side of the axis of the light projector, compos- ing a triple beam arrangement. As shown in Fig. 2, when the moving detectable target is at the edge of the projected light beam, the spot reflected from the target is at a location different
from the point where the spot is pro- duced when the beam is completely intercepted, generating a range mea- surement difference of +∆X, but with the triple beam composition, the sym- metrically arranged position sensitive device (2) has a spot produced con- versely at a
Fig. 2 Triple beam range measure- ment method
By averaging the range measure- ment signals of the 2 light receiving systems, the range measurement difference is cancelled.
$ Optical system of the triple beam photoelectric sensor
In the MQ photoelectric sensor area reflective type, it is necessary to give special consideration to the lens. In order to improve the precision, an aspherical lens, having limited spherical and coma aberration, is used.
In addition, as shown in Fig. 4, a photo diode having 2 output terminals is used. By comparing the output currents from the 2 PSDs, there is no relationship to the level of the incident light. Thus, the ratio of reflection from the detectable target exerts no influence and the range measurement and detection can be interpreted accuracy.
Fig. 3 Light beam trace of the lens
(a)is a spherical lens, and (b) is the aspherical lens. Because there is no aberration in (b), range measure- ment precision is high.
Fig. 4 PSD (position sensitive device) construction
By making a calculation of the ratio of I1 and I2, the light spot position can be detected.
$ Operating principle of area reflective method
The operation of the MQ photoelectric sensor area reflective type is explained in Fig. 5. After the output from the 2 PSD elements is added, the I/V value is converted and the logarithm deter- mined. By subtraction, the distance sig- nal in (I1/I2) is obtained. This can be optionally set, and with the distance adjustment control, comparison with the produced value can be made to detect whether the target is or is not within the distance range.
Fig. 5 Signal processing circuit block diagram
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