Light Beam Interrupt Detection Apparatus for Use in a Vibrating Environment

The present invention relates to a light beam interrupt detection apparatus for use in a vibrating environment. The invention relates further to a light beam interrupt detection apparatus for use in a vibrating environment wherein a partial interrupt of the light beam is detected when an object smaller than the beam passes therethrough.

Light beam interrupt detection apparatus are used in a wide variety of applications to detect the motion and position of objects without physically disturbing the object whose movement or position is detected. Under certain circumstances, namely when either the light source creating the light beam or the light receiver detecting the light beam or both are connected to a surrounding object or environment which is vibrating or being vibrated, the reliability of the interrupt detection is compromised. Obviously, a small vibration causing a change in angle of the light source placed at a great distance from the light receiver will cause the light beam to miss the light receiver.

When operating in a vibrating environment, it is known to use a light beam which is sufficiently large that as the beam wanders during vibrations, sufficient light is received by the receiver such that an interrupt by an object can be detected. A larger beam however requires a larger light source and will consume more power at the light source for generating a larger beam.

Also, using a smaller receiver with a wider beam, which will help keep the beam on the receiver in order to operate in a vibrating environment, will reduce the amount of useable signal from the light receiver since less light is received, possibly decreasing the signal to noise ratio at the receiver and lowering the reliability of the interrupt detection.

It is therefore an object of the invention to provide a light beam interrupted detection apparatus for use in a vibrating environment which is able to use a minimum power light source without compromising the reliability of the interrupt detection.

According to the invention, there is provided a light beam interrupt detection apparatus for use in a vibrating environment comprising: a light source for producing a beam of light; a light receiver for receiving and detecting the beam, the receiver having a detector surface sufficiently larger than the beam such that the beam strikes the detector surface as the beam moves during vibration. The receiver produces an output signal which is substantially constant as the beam moves over the detector surface without the beam being broken. The output signal of the receiver is to be fed to signal processing means for detecting an interrupt of the beam by an object.

Preferably, the signal processing means detect a variation in the output signal caused by a partial interrupt of the beam, the object being so dimensioned as to block only a part of the beam as the object passes through the beam.

Preferably, the light source may comprise a laser diode and the receiver may comprise a single silicon photocell. The invention is preferably used in conjunction with a mechanical loom to detect broken threads.

FIG. 1 is a block diagram of the light beam interrupt detection apparatus according to a preferred embodiment;

FIG. 2 is a schematic block diagram of the light detector and preamplifier circuit according to the preferred embodiment; and

FIG. 3 is a graph showing an example of the output signal according to the preferred embodiment.

As shown in FIG. 1, the apparatus according to the preferred embodiment comprises a laser source (10) which produces a laser beam (32) having minimal divergence. At a substantial distance from the laser source (10), a light receiver (30) is positioned to receive light beam (32). Receiver (30) includes a light filter (14) which allows only light of a frequency substantially equal to that of the beam (32) to enter the housing of the receiver (30) is cylindrical in shape and is made of aluminum which is black anodized on its inside. Positioned towards the rear of receiver (30) is a silicon photocell sensor (12) which produces an electrical output signal which is substantially constant as the beam (32) moves over the surface of cell (12). A preamplifier circuit (16) to be described in detail hereinbelow is included in the light receiver (30) in close proximity to cell (12). The signal generated by preamplifier (16) can then be transmitted over shielded cable to a remote computer (18).

In the preferred embodiment, photocell (12) is a standard silicon photocell having a receiver surface free of non-transparent electrodes and of equal efficiency over its surface, which is hand-selected to respond highly at approximately 672 nanometers. The sensitivity is chosen such that a 1.0 mW laser diode beam of 672 nanometers wavelength having a diameter of 3.0 mm produces an output of 0.08 V. Filter (14) may comprise red plexiglass (No. 2423 by Rohm & Haas). Beam (32) is approximately 5 to 6 mm in diameter and has a power level of approximately 1 mW. The laser source (10) may use a laser diode such as the Toshiba T0LD9211 which uses a maximum of 5 mW.

As shown in FIG. 2, silicon photocell (12) is connected to preamplifier (16) which utilizes a two stage AC amplifier using amplifiers (22) and (24). The output of the second stage amplifier (24) is fed to the positive input of amplifier (28).

If beam (32) is completely blocked by an object or if a vibration should cause beam (32) to move completely away from cell (12), the output level on amplifier (26) will rise, indicating that the voltage level detected at cell (12) is below the minimum value (Vref). A false detection of an interrupt of beam (32) can be prevented by using amplifier (26). Amplifier (28) serves the purpose of maintaining the output signal low when receiving a signal from amplifier (26) and also of boosting the current level of the amplifier voltage signal coming from amplifiers (22) and (24).

The advantages of the preamplifier (16) construction shown in FIG. 2 can best be understood by referring to FIG. 3 which shows the output of amplifier (28) as thread (20) passes through beam (32) and as a full break of beam (32) is caused to happen by, for example, placing a hand in front of beam (32). Preamplifiers (22) and (24) operate such that their output is usually high at 10 V, and such that the variation in the voltage generated by cell (12) as thread (20) passes through beam (32) will cause the output of amplifier (24) to drop to zero.

As a thread (20) passes through beam (32), the DC level discriminator (26) will indicate that the voltage generated by cell (12) does not drop below the threshold value (Vref), while the two stage AC amplifier (22) and (24) will amplify the rapidly dropping signal to produce a zero output at amplifier (28). In the case of a larger object blocking the laser source (10), amplifier (26) will detect that the level of the signal emitted by cell (12) has dropped below the threshold reference, and will cause amplifier (28) to set the output signal to zero. As the signal at cell (12) goes to zero, the AC amplifier (22) and (24) will generate a zero output signal for a brief period of time and then will return to producing the 10 V output signal given that the voltage at cell (12) remains constant with beam (32) blocked. This means that without the threshold detector amplifier (26) the blocking of beam (32) would cause the output signal to drop from 10 V to zero and then return back to 10 V which would not be distinguished from a thread 20 passing through. With the addition of the threshold detection amplifier (26), the output signal from amplifier (28) is kept at zero even while the output from amplifier (24) returns to 10 V. When the hand is removed from beam (32) as shown in FIG. 3, the fluctuation in the voltage of cell (12) will cause amplifiers (22) and (24) to drop their output signal from 10 V down to zero and then back up to 10 V.

In the preferred embodiment, the length of the zero voltage state of the output signal is measured by computer (18) to determine whether the output signal represents a thread passing through beam (32), i.e. an alarm signal to shut down the mechanical loom, or a false alarm interrupt of beam (32) as would be caused by a hand or a failure of laser source (10). The previous two conditions are represented by short and long zero state pulses respectively. When a longer duration zero voltage state is detected, the following rise, drop and rise in the output signal is ignored so as to avoid triggering a false alarm. Although a false alarm detection is not used to cause an immediate shut down of the loom, an operator is warned by the computer of the condition. The output signal which leaves the preamplifier is essentially a digital HI/LO signal which the computer (18) can easily process.

Although in the preferred embodiment, the preamplifier circuit (16) includes a threshold detector, it is of course possible to amplify the signal of cell (12) and pass it directly to computer (18) for analysis. Although in the preferred embodiment, small AC fluctuation in the voltage generated by cell (12) are used, it can be appreciated by one skilled in the art that it is also possible to use the DC signal from cell (12) in order to detect an interrupt or partial interrupt of beam (32), especially in cases where a substantial part of light beam (32) is blocked as the object (20) passes therethrough. It can of course be appreciated that the detection of thread (20) having a diameter as small as 0.02 mm requires very high sensitivity.

Although the invention has been described above with reference to the preferred embodiment, it is to be understood that such description is not intended to limit the scope with the invention as defined in the appended claims.