Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-11-16
2004-12-28
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227240, C250S221000, C356S073100, C385S031000
Reexamination Certificate
active
06835923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to proximity sensors. It also relates to proximity sensors in electronic devices using beam guides. More specifically the invention relates to a self monitoring of the operability of optical proximity sensors and a method of its operation. In particular the invention relates to a method of monitoring a sensor, a lightguide systems for use with the sensor and a device including the sensor.
2. Description of the Prior Art
The basic functioning of optical proximity sensors is described in GB2178840. The optical proximity sensors of British Patent 2,178,840 comprise two optical fibers. However in this patent there is no mention of a self-monitoring signal and it does not have any prisms that direct a self-monitoring signal from a transmitter to a detector.
To guarantee proximity sensor's functioning a self-monitoring of the system is very important. There are several ways to do this but in optical proximity sensors this is normally done by reflecting a signal back to the proximity sensor and using this reflected signal as a control signal for self-monitoring. In proximity sensors, where there are different lightguides for the transmitter and the detector, self-monitoring is normally done so that external prisms or reflectors reflect part of the signal coming from the transmitter back to the detector. A problem associated with this solution is that external parts are easily damaged and this could affect the functionality of the device.
In U.S. Pat. No. 5,382,785 a photodiode is used to self-monitor the function of a proximity sensor with a laserdiode. In this system part of the signal is reflected by a partially reflecting mirror of a laser diode and this reflection is used in control of the system. Further, this document does not mention the use of the prisms and two lightguides.
It is desired to create a self-monitoring signal that reflects changes in the proximity detector performance so, that all possible failure conditions are detected, but no false detection happen. An easy way to make this signal is by the provision of an external prism or other external reflecting features. These external features are most easily damaged and functionality of the device deteriorates.
FIG. 1
depicts a conventional version of an optical proximity sensor with a conventional two lightguide design. This picture is incorporated to describe the fundamental structure of an optical proximity sensor. The proximity sensor comprises a transmitter
2
, a transmitter lightguide
4
, a receiver lightguide
8
and a receiver
10
. The lightguide
4
directs light emitted from the transmitter
2
to an area in which an object is to be expected. If there is an object present the light from the transmitter is reflected and led by the receiver lightguide
8
to the receiver
10
. The lightguides
4
and
8
further comprise external prisms
6
a
and
6
b
that project over the surface of the casing
12
. The transmitter
2
and the receiver
10
are welded to a printed circuit board (PCB)
16
. The lightguides
4
and
8
are carried in hollow tubes
14
that are attached to a casing
12
. The hollow tubes
14
and the PCB
16
are connected flexibly.
FIG. 2
depicts a partially enlarged view of the lightguides of the optical proximity sensor of FIG.
1
. The lightguides
4
and
8
further comprise external prisms
6
a
and
6
b
that project over the surface of the casing
12
. These prisms direct a part of the light
20
emitted from the transmitter via a prism
6
a
and
6
b
directly from the transmitter lightguide
4
to the receiver lightguide
8
. This part of the light directed from prism
6
a
to prism
6
b
is called self monitoring beam
22
. The rest of the light
20
from the transmitter is directed as an object illumination beam
28
to an area in which an object is to be expected. The self monitoring beam
22
is directed by the prism
6
b
of the receiver lightguide
8
towards a receiver (not shown), and further to the receiver. In the area between the both prisms
6
a
and
6
b
the self monitoring beam
22
can also be called an overflow signal. So the receiver can receive a minimum radiation, even if no object is in the proximity. The receiver can therefore detect, if the transmitter is transmitting and or the receiver is receiving, even in the absence of an object. The prisms therefore can provide a self test signal. So the normal curve relating the proximity of an object with a signal from the receiver is superimposed with a non zero base signal.
FIG. 3
depicts an enlarged detail of
FIG. 2
illustrating the external prisms
6
a
and
6
b
. The route of the overflow signal
22
is presented in the picture. The main drawbacks of conventional optical proximity sensor design with lightguides and external prisms is depicted. The light
20
from a transmitter enters the prism
6
a
of the tx-lightguide
4
(lightguide of the transmitter) from below. Most of this light
20
is passed from the surface of the lightguide
4
as an object illumination beam
28
. The prism
6
a
of the tx-lightguide
4
(lightguide for emitter) reflects a small part of the light
22
in a direction to a prism
6
b
on the rx-lightguide
8
(lightguide for the receiver). The part of the light
20
intended to be reflected as beam
22
to the other prism is depicted as the ten rays with the arrowhead within the tx-lightguide
4
.
The prism
6
a
of the tx-lightguide
4
(lightguide for emitter) has some disadvantageous external influences. The prism
6
a
has a broken tip
42
and a scratch
44
as a result of wearing, as a prism
6
a
projects from the surface of the lightguides of the sensor. The prism further comprises two edges contaminated with soiling. The first soiling is a liquid pollution
46
and the second soiling is a solid pollution
48
. The above influences affect the transmission of the self monitoring beam
22
in different ways. The broken tip
42
, the liquid soiling
46
and the scratch
44
produce stray light
26
. The solid soiling
48
absorbs transmitter light
20
and part of the overflow signal
22
. These disadvantageous effects reduce the intensity of the self monitoring beam down to approximately 10%. This leads to a reflection intensity which is too low, so that the receiver detects a faulty transmission, being interpreted as a sensor failure. The failure is detected as the self monitoring beam
22
vanishes. The vanishing of the overflow signal
22
can be related to a real breakdown or a soiling of a small area (the prisms
6
a
,
6
b
) on the lightguides
4
,
8
.
So with a sensor as described above the sensor indicates a failure even if the surface of the lightguides may only be slightly contaminated.
In the earlier solution there were prisms provided on top of the lenses. They were subject to wearing and also collected dirt. This caused the self-monitoring signal to fall under a threshold even in conditions where detecting the object would happen reliably.
So it is desirable to have a proximity sensor that provides a self-monitoring signal in a reliable way even under wearing and dusty conditions, and that is easily manufactured.
SUMMARY OF THE INVENTION
According to one embodiment of the present invention a method is provided for self-monitoring the operation of a proximity sensor. The proximity sensor comprises at least a transmitter, a receiver, and a first and second lightguide. The method can be executed as follows: Producing a beam with the transmitter, and transmitting the beam into the first lightguide. The beam can be a radiation, for example infra red light, visual light or radio waves emitted from the transmitter. Within the first lightguide the beam is split into a first beam and a second beam. The second beam is transmitted into the second lightguide, and is directed towards the receiver. The receiver receives and analyzes the second beam to determine the operation of the proximity sensor.
This basic method describes the return of a part of the beam from the transmitter directly
Hämäläinen Tiina
Peili Vesa
Pirhonen Risto
Vähä-Ypyä Henri
Antonelli Terry Stout & Kraus LLP
Nokia Corporation
Pyo Kevin
LandOfFree
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