Panasonic MQ-W3 - Manual (Page 5)

MQ-W
270
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.
$ Triple beam type range sensing principle
$ Optical system of the triple beam photoelectric sensor
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 –
∆X difference position,
and by means of averaging both range
measurement signals, the correct range
measurement can be made. This
results in significantly improved
repeatability and background suppres-
sion.
Fig. 1 Optical triangulation range
measurement method
Other examples of this method are
the automatic focusing and camera
range measurement systems.
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.
Fig. 4 PSD (position sensitive
device) construction
By making a calculation of the ratio
of I
1
and I
2
, the light spot position
can be detected.
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.
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.
$ 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 (I
1
/I
2
) 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
08/2005