Sensors That Work in the Iron and Steel Industry

Microwave radiation (radar) is electromagnetic in nature with wavelengths ranging from about one millimeter to one meter.  The radiation travels at the speed of light, 300,000 km/sec (or 186,000 miles/sec) and is generally unaffected by most environmental conditions.

The WADECO microwave sensors are non-contact, non-invasive, insensitive to the application area, intrinsically safe for personnel and possess solid-state reliability.  They are ideal for many applications because they are non-contact.  This means that a microwave sensor can continuously and reliably measure the distance to a target.  Typical operating ranges are 0.5 - 150 meters (1.5 - 500 feet). Applied to storage vessels the sensor can be isolated from the vessel by a window.  This non-invasive mounting permits access to all parts of the sensor without exposing personnel to the contents of the storage vessel.  When the application environment includes a lot of dust, steam, vapor, high temperatures, induced noise or intense ambient light, reliably sensing the target is very difficult for most technologies, but quite simple using microwaves.

WADECO sensors are designed around one of  two frequencies, 10 GHz or 24 GHz.  These frequencies, respectively, fall into the X-band (7 - 12.4 GHZ) or the K-band (18-26 GHz) ranges, which are commonly approved frequency bands in most countries.  To be certain that microwave sensors, when operating, do not interfere or cause problems with other devices operating in the same area, their output is regulated, to assure compliance with FCC Rule 15, covering field disturbance devices.  WADECO sensors are desgned to comply with these regulations.

With the ever-increasing demand for technologies in industry that are safe for personnel, microwaves are the logical choice.  With very low output power, always less that 10 mW, the sensor power is well below the American OSHA exposure specifications as stated in Section 1910.97.  This means that personnel operating or near operating WADECO sensors are subjected to no health hazards whatsoever from microwave radiation.

HOW DO MICROWAVES WORK?

Microwave energy is transmitted in a characteristic "beam". The antenna design and the transmitted power of the microwave energy determine the pattern of this "beam". The "beam" is not a straight line, but rather like an elongated balloon. It forms a 3-dimensional volume. Microwave beam patterns are different for each antenna design and each power rating. The area covered by the primary energy lobe is called the microwave 'foot-print'. An example of an antenna energy pattern, pictured in one plane, is shown in Figure 1.

Figure 1: Microwave Energy Pattern

In defining a beam pattern, the half-power beam width is generally used, which is known as the -3 dB point to a microwave engineer. It is defined as the angle between the half-power directions on either side of the line of the main energy direction.  At the point of half-power beam width, the area is defined at which at least half of the maximum power emitted will be seen by an object. The angle from the center line to this point determines the field of primary coverage. The maximum energy direction is the point of first null or where emitted power is no longer detected.

When a target is acquired in the volumetric area of microwave coverage, some of the energy is reflected back into the antenna and on into the controller. There the reflected energy is mixed with a portion of the transmitted energy producing an output signal. This signal is then processed by the control circuit, which in turns gives the user the desired data.

Figure 2: Rotary Microwaves

ROTARY MICROWAVES

WADECO also offers special engineering designs that other microwave sensor manufacturers cannot. Generally sensors used in the industry utilized plane polarized microwaves. The direction of oscillation of the electric field in an electromagnetic signal is called the polarization direction (or vector) and is perpendicular to the direction of propagation of the beam. That means the polarization vector is always fixed in one direction. If that direction were made to rotate, we have what is called circular polarized or rotary microwaves. WADECO has some proprietary designed sensors with rotary microwaves. A patented waveguide design has allowed the use of these special microwave sensors in applications where other types of microwave sensors fail. The basic advantage of rotary microwaves is simply explained. When a beam hits a strongly metallic target (e.g. liquid metal) the direction of rotation completely reverses. When a beam hits a non-metallic target like an insulator, slag, glass, ceramic, brick, crystalline material etc no reversal occurs. The discriminator circuits in microwave sensor have been designed to accept or reject the reverse rotation depending on the application. When a beam hits a surface at a grazing angle a small phase rotation of the polarization vector occurs. If this is then reflected back into the antenna, the spurious signal is rejected by the controller as it is not tuned to receive this type of signal. As a result the probability of false detection has been reduced to a negligible level.

BASIC MICROWAVE APPLICATIONS

The WADECO Range Finder  has been implemented with two output frequencies, 10 GHz and 24 GHz. This allows for better target resolution. For example, in the case of two closely separated targets, the 24 GHz implementation would be a better choice, since, by using the WADECO proprietary FFT software, clearly separate signals can be resolved. The software also has built in record, review, fast review features as well as an algorithm that optimizes the parameters of the sensor for the best operation in the individual application.

APPLIED TO THE IRON AND STEEL INDUSTRY

With the many inherent positive features of this technology, microwaves offer the potential to help solve today's existing problems in many areas in the iron and steel industry. Some of the more common problems that may be solved with microwave technology are accurate level sensing in torpedo cars, B.O.F., blast furnaces, collision avoidance of quenching and charging cars, edge sensing in hot strip mills and more. The very high temperatures of the metal or air column do not affect WADECO sensors. There is no need for any compensation factors that other technologies sometimes require. With high quality components, the operating temperature range of the board components in the controller box can be from - 10° C to +55° C. Internal vessel temperatures can be much higher, as long as the ambient air around the controller box is generally less than the electronic components maximum rating. The antennas are all metal and can be placed in environments up to 600° C.  Waveguide extensions can be utilized where required to reduce the temperature on the control circuit boards. The Range Finder is capable of operating in very confined and narrow spaces. Spurious signals due to the antenna confinement in a pipe, for example, are eliminated by software. These qualities all make the WADECO microwave sensors perfectly suited to the iron and steel industry. Microwaves retain their resolution regardless of range. This is not so with some other types that may lose resolution as the range to the target increases. WADECO sensors are designed for different range applications - from 5 m, 10 m and 20 m (by dip switch) for the MWS-CT/CR-l sensor, up to 150 m (500') for the Range Finder MWS-RF  crash avoidance modification. The Range Finder is able to measure the level of a molten metal surface at 50 m range with an error of only +1cm.

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