Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-09-08
2002-12-03
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140LS, C250S221000, C327S514000
Reexamination Certificate
active
06489603
ABSTRACT:
The present invention relates to a method for calibrating a photoelectric cell.
Photoelectric cells are commonly used in a wide range of fields, and with different functions, all of which nevertheless can be said to relate to a switching between two states of a user, caused by the change of the luminous signal received by a photo-receiver of the photoelectric cell when an object (or a person) passes in the field of optical detection of the photoelectric cell, superimposing itself to the detection background and intercepting the light beam emitted by a photo-emitter of the photoelectric cell.
In fact, generally a photoelectric cell comprises a photo-emitter, which generates a luminous signal, a photo-receiver or photodetector, which receives—directly or after reflection—the luminous signal emitted by the photoemitter and converts it into an electrical signal, and a comparator, which compares the signal received to a triggering threshold and provides a binary output signal that represents the presence or the absence of an object, and which is used for driving a user.
For sake of brevity, the terms “Target” and “Background” in the following description shall be indifferently used to indicate respectively both the object and the signal detected in the presence of the object, and both the detection background and the signal detected in the absence of the object.
In the following description, and according to the current terminology, the photoelectric cell is said to operate “in the light” if the output is active when the sensor is in the operating condition in which it receives the maximum light, that is to say, above the triggering threshold. Vice versa, the photoelectric cell is said to operate “in the dark” if the output is active when the sensor is in the operating condition in which it receives the minimum light, that is to say, below the triggering threshold.
Since photoelectric cells are commonly provided with a normal output or Q, and a complementary output or Qneg, the operation “in the light” or “in the dark” can be selected using the output that meets the requirements of the particular user. Among the typical applications, in proximity applications, based on the reflection of the light by the object intercepting the light beam emitted by the photo-emitter, the most typical operation is that in the light, corresponding to the activation of the photoelectric cell in the presence of the object. On the other hand, in “barrier” applications, that is, based on the object interrupting the light beam emitted by the photo-emitter, the most typical logic of operation is that in the dark, again corresponding to the activation of the photoelectric cell in the presence of the object. Usually, barrier applications are further distinguished between “through-beam barrier applications”, wherein the photodetector and the photo-emitter are housed in separate devices which are mounted so as to face each other, and “retroreflex barrier applications”, wherein the light emitted by the photo-emitter is reflected by a prismatic reflector, which sends it to the photo-receiver, housed in the same device as the photo-emitter.
In any case, the photoelectric cells must be calibrated upon installation, that is to say, the triggering threshold must be set according to the type of object to be detected and according to the distance of the object itself.
A first calibration method is the so-called variable-resistance or trimmer acquisition method: during calibration, for example in the “proximity” case, the object is arranged into the detection field, and the trimmer is brought to the minimum position, after which it is rotated, thus increasing the sensitivity of the photo-receiver, until the in-the-light output turns on. At that point, the rotation is continued for a little more so as to have a triggering threshold corresponding to a greater distance of the target.
However, in photoelectric cells with key calibration, the calibration occurs through two different acquisition steps: Target acquisition and Background acquisition.
FIG. 1
illustrates a flow chart of the known dual-acquisition calibration method, which provides for a sequence of at least two pressures of the calibration key (alternatively, there can be two different keys, one for the Target and another for the Background, the logic of operation being totally equivalent). Thus, starting from an operating state
100
of the photoelectric cell, in a first block
102
a first pressure of the acquisition key is waited for, which must last for some seconds, as checked in a block
104
; otherwise, the operating state
100
is returned to, cancelling the acquisition in block
106
. Usually, furthermore, as indicated with reference numeral
108
, there is a visual indication of the pressure of the acquisition key. If the first key pressure lasts enough, the Target acquisition block
110
is entered, accompanied by the visual indication of acquisition in progress and successful acquisition. As indicated with reference numeral
112
, at this point it is waited for the key to be released for at least one moment, and afterwards, a second pressure of the key is waited for (block
114
), also lasting for a certain period of time (block
116
) and accompanied by a visual indication (block
118
). Should that be the case, the Background acquisition is carried out in block
120
, accompanied by the visual indication. Then, in a block
122
, it is checked whether the contrast between Target and Background is sufficient, as it shall be better described hereinafter, visually indicating the occurrence or the failure of the calibration in blocks
124
and
126
, respectively. In positive case, the photoelectric cell returns to the operating state
100
, whereas in negative case, block
102
is returned to, waiting for the first key pressure.
With reference to
FIG. 2
, which graphically illustrates the signal levels concerning the traditional calibration of a photoelectric cell, it must be specified that when both the Target and the Background are acquired, actually a certain number of readings is carried out so as to obtain a first series of data associated to the Target, and a second series of data associated to the Background. The maximum T
M
and the minimum T
m
reading signal for the Target and the maximum S
M
and the minimum S
m
reading signal for the Background are extracted from this series of data. Then, the traditional calibration algorithm provides for calculating the interval amplitudes between the maximum and minimum value detected, &dgr;
T
=T
M
−T
m
and &dgr;
S
=S
M
−S
m
, multiplying the greatest one (to which a suitable constant can be added in advance) by a safety constant, and finally, checking whether the distance &dgr; between the intervals is greater than the product thus obtained. This is the acquisition validity condition. More in particular, the distance &dgr; is calculated as &dgr;=T
m
−S
M
in the case (
FIG. 2
) of Target more luminous than the Background, as it happens in proximity applications, whereas it is calculated as &dgr;=S
m
−T
M
in the case (not shown) of Target less luminous than the Background, as it happens in barrier applications.
If the validity condition is met, the triggering threshold F is typically set exactly in the middle of the distance &dgr;, that is to say, in the case shown in
FIG. 2
, to F=S
M
+&dgr;/2=T
m
−&dgr;/2. In the case (not shown) of Target less luminous than the Background, the triggering threshold is typically set to F=T
M
+&dgr;/2=S
m
−&dgr;/2.
Afterwards, the triggering hysteresis is calculated, that is to say, the actual switching-on threshold F
on
and switching-off threshold F
off
of the photoelectric cell are respectively set by adding to and subtracting from (or vice versa, for the operation in the dark, not shown) the triggering threshold F an hysteresis amount, which can be fixed or proportional to the triggering threshold F.
It must be emphasized that all constants used in the calibr
Allen Stephone B.
Datasensor S.p.A.
Nixon & Vanderhye P.C.
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