Industrial sensors and control--The basics--Part VI

PROCESS CONTROL SENSOR FOR ACCELERATION

The principle of operation of the process control acceleration sensor is illustrated in Fig. 6.30. The sensor element, consisting of a small cantilever and a photo-luminescent material, is attached to the end of a single multimode fiber. The input light of wavelength ?s is transmitted along the fiber from a near-infrared LED source to the sensor element. The sensor element returns light at two different wavelengths—one of which serves as a signal light and the other as a reference light—into the same fiber. The signal light at wavelength ?s is generated by reflection from a small cantilever. Since the relative angle of the reflected light is changed by the acceleration, the returned light is intensity-modulated. The reference light of wavelength ?r is generated by photoluminescence of a neodymium-doped glass element placed close to the sensor end of the fiber.

The optoelectronic detector module has two optical filters to separate the signals ?s and ?r, and also two photodiodes to convert the signal and the reference light into separate analog voltages. The signal processing for compensation is then merely a matter of electrical division. A measuring range of 0.1 to 700 m/s2 and a resolution of 0.1 m/s2 is obtained over the frequency range of 5 to 800 Hz.

AN ENDOSCOPE AS IMAGE TRANSMISSION SENSOR

An imaging cable consists of numerous optical fibers, typically 3000 to 100,000, each of which has a diameter of 10 ?m and constitutes a picture element (pixel). The principle of image transmission through the fibers is shown in Fig. 6.31. The optical fibers are aligned regularly and identically at both ends of the fibers. When an image is projected on one end of the image fiber, it is split into multiple picture elements. The image is then transmitted as a group of light dots with different intensities and colors, and the original picture is reduced at the far end. The image fibers developed for industrial use are made of silica glass with low transmission loss over a wide wavelength band from visible to near infrared, and can therefore transmit images over distances in excess of 100 m without significant color changes. The basic structure of the practical optical-fiber image sensing system (endoscope) is illustrated in Fig. 6.32. It consists of the image fiber, an objective lens to project the image on one end, an eyepiece to magnify the received image on the other end, a fiber protection tube, and additional fibers for illumination of the object.

Many examples have been reported of the application of image fibers in process control. Image fibers are widely employed to observe the interior of blast furnaces and the burner flames of boilers, thereby facilitating supervisory control. Image fibers can operate at temperatures up to 1000°C, when provided with a cooling attachment for the objective lens and its associated equipment. Another important application of the image fiber bundles is observation, control, and inspection of nuclear power plants and their facilities.

Conventional image fibers cannot be used within an ionizing radiation environment because ordinary glass becomes colored when exposed to radiation, causing increasing light transmission loss. A high-purity silica core fiber is well-known as a radiation-resistant fiber for nuclear applications.

The endoscope has demonstrated its vital importance in medical and biochemical fields such as:

• Angioplasty

• Laser surgery

• Gastroscopy

• Cystoscopy

• Bronchoscopy

• Cardioscopy

SENSOR NETWORK ARCHITECTURE IN MANUFACTURING

In fiber-optic sensor networks, the common technological base with communication is exploited by combining the signal generating ability of sensors and the signal transmit- ting capability of fiber optics. This combination needs to be realized by a suitable network topology in various manufacturing implementations. The basic topologies for sensor net- working are illustrated in Fig. 6.33. These topologies are classified into six categories:

• Linear-array network with access-coupled reflective sensors (Fig. 6.33a).

• Ring network with in-line transmissive sensors (Fig. 6.33b).

• Star network with reflective sensors (Fig. 6.33c).

• Star network with reflective sensors; one or more sensors can be replaced by a separate star network, in order to obtain a tree network (Fig. 6.33d).

• Star network that can also be operated with transmissive sensors (Fig. 6.33e).

• Ladder network with two star couplers. A star coupler is replaced by several access cou- plers, the number required being equal to the number of sensors (Fig. 6.33f ).

Topological modifications, especially of sensor arrays and ladder networks, may be desirable in order to incorporate reference paths of transmissive (dummy sensors) or reflective sensors (splices, open fiber end). The transmit and return fibers, or fiber highway, generally share a common single-fiber path in networks using reflective sensors.

When a suitable fiber-optic network topology is required, various criteria must be considered:

• The sensor type, encoding principle, and topology to be used

• The proposed multiplexing scheme, required number of sensors, and power budget

• The allowable cross-communication level

• The system cost and complexity constraints

• The reliability (i.e., the effect of component failure on system performance)

NEXT:  Power Line Fault-Detection System for Power Generation and Distribution Industry

About the Author

Sabrie Soloman, Ph.D, is the Founder, Chairman and CEO of American SensoRx, Inc.

Excerpted from Sensors Handbook, 2nd Edition by Sabrie Soloman (McGraw-Hill; 2010) with permission by McGraw-Hill.

If you liked this article, go to the Industrial Control DesignLine home page for the latest in industrial control design, technology, trends, products and news.

Also, get a weekly highlights update delivered directly to your inbox by signing up for our weekly industrial control newsletter here.