The times, they keep on changing. In the 1990s, noncontact level measurement technology was beginning to be deployed in powder and bulk applications. It was the heyday of ultrasonic technology. At that time, lasers were still rare and expensive measurement devices, used mainly in survey-distance measuring. Due to continuous advances in technology, laser diode prices dropped from $150 in the late 1990s to less than $20 today. This made laser technology an attractive and affordable option for noncontact measurement.
Today, we have three primary noncontact level and distance measurement technologies in use: ultrasonic, open-air radar, and laser. Laser measurement systems have two significant advantages: they can measure any solid surface at any angle, and they can measure with narrow beams.
With wavelengths of <1 µm, the surface of almost any dry material appears rough to a laser. The defused reflection emanates from an exact spot on the reflected beam. This is why lasers measure distance precisely; the natural reposing angles of dry bulk materials do not matter to a laser-based system. At the same time, the optical lens of the laser device can focus the outgoing beam to a size of 2 in./50 mm with beam divergence of less than 0.2°, resulting in a small footprint over long ranges.
Ultrasonic and open-air radar have beam divergences of 9° to 11° (some special designs can achieve 4°). With wavelengths from 2000 to 50,000 times that of a laser device, ultrasonic- and radar-based instruments measure surfaces perpendicularly, to assure a good return signal. Both operate well when aimed at smooth surfaces, such as powders.
However, lasers have been applied successfully in a broader range of level and positioning applications. For example, in petrochemical applications, granular plastics are stored in narrow, tall silos. This presents a challenge for most noncontact measurement technologies. Lasers are reliable and cost-effective as they easily install in existing silo nozzles, regardless of location. A built-in aiming laser targets the conical silo bottom. Because lasers can measure any surface at any angle with a narrow beam, level detection can also be applied to empty silos. This translates into accurate measurements of the last few grams of material and prevents cross-contamination of different types or grades of material as they are run through the silo. The same principle applies in the food industry.
Previously the domain of nuclear transmitters, laser-level measurement products have also been applied in high-temperature and high-pressure reactor vessels with industrial sight glasses. The laser’s ability to measure through sight glass provides benefits not available with nuclear transmitters: end-user convenience, low cost of ownership, and cost savings due to lower maintenance and administrative regulations.
Laser technology goes beyond single-point measurement. It can be used as an alternative to conventional belt scales. The laser scanner and its belt-speed sensor are self-contained, requiring only a 24-V-dc power source. There is no external control box or complicated field wiring. This makes it possible to commission and configure the device without stopping the conveyor.
Laser technology goes one step further—to three-dimensional material surface profiling. Because lasers have the unique ability to measure any solid surface at any angle with a narrow beam, it is a natural migration for lasers to be applied to sophisticated material profiling of complex shapes. Three-dimensional laser scanners bring volume accuracy of dry materials in silos and stockpiles to as low as less than 1%. These devices use a rotating laser head for a multipoint laser scan to measure uneven surfaces. A three-dimensional map of the surface is created and the onboard microprocessor calculates the material volume and mass. The benefit is what all end-users seek: fast and accurate product inventory measurement on demand, and, as a result, better process control.
Ivo Radanov is product manager for laser products at K-Tek Corp. (Prairieville, LA). He has 22 years of experience in research and development, engineering, and product development. He holds a degree in electronics engineering and a postgraduate degree in control devices and robotics systems. He can be reached at [email protected].