IR   Thermometry Gets Smart

Thanks to advances in optics, digital electronics, and PC software, the new generation of IR thermometers can go where no sensors have gone before.

Photo 1. Noncontact IR sensors measure temperature by capturing emitted radiation using optics, a detector, an amplifier, and electronics. Advances in these components, as well as in PC software, have resulted in a new generation of IR sensors.

Okay, here's your choice: You can inch your way precariously across a catwalk 10 ft above a 1500°C process to adjust sensor measurement parameters. Or you can use a smart IR device.

IR thermometers offer significant advantages in industrial manufacturing processes where traditional contact thermometers are difficult or impossible to use, especially in applications where materials are moving, very hot, or inaccessible or where the product would be contaminated by contact measurement (see Photo 1). While the basic principles of IR thermometry haven't changed over the years, recent technological advances have made it possible to build IR temperature sensors that are easier to install, use, and maintain (see Photo 2, below).

Photo 2. Noncontact temperature measurement and remote adjustment of sensor parameters is desirable for hot, continuous processes. In this installation, two IR temperature sensors in rugged ThermoJacket enclosures measure the temperature of a steel pipe as it is heated and bent to precise specifications. The pipe will be used in a high-pressure, high-temperature power plant boiler application, where pipe strength is critical.

Changes in optics, although not as rapid as changes in electronics, have taken the form of improved manufacturing techniques that allow vendors to provide higher performance components at lower cost. The cost of producing an IR optical system continues to decrease, due in large part to the development of inexpensive plastic fresnel lenses. This cost reduction has resulted in low-priced IR sensors.

An aspherical mirror (i.e., a mirror that deviates slightly from a perfect sphere's convex interior/concave exterior, such as a parabolic mirror) can also be used to collect and focus IR energy onto a detecting element. Low-cost, high-performance mirrors are being used in some portable IR units. These mirrors, created by precise plastic-injection molding, are coated with aluminum or gold and deliver optical resolution comparable to or better than some relatively expensive lens systems.

Because most materials that transmit IR energy have high indices of refraction, they have a higher degree of surface reflection than optical-quality crown glass. This causes multiple reflections within lenses, which produce a fuzzy image on the detector and reduces the available resolution of the optical system. However, with the use of coated optics (antireflection coatings), the reflected fuzzy portion of the image is reduced to an acceptable level, and the sensor can accurately focus on much smaller targets. The price of high-quality coated optics has continued to drop, removing previous price barriers to optimum optical performance. Some high-performance IR thermometers offer optical resolution as high as 300:1.

Another improvement: IR manufacturers have started using low-cost diode lasers in their products. An option on many fixed industrial sensors, the laser defines the center of the IR target, allowing quick alignment for installation of the IR sensor.

Digital Electronics Support Fast, Smart Sensors
Initially, IR sensors were analog systems that amplified the IR signals collected by the device's optics. These simple sensors provided a nonlinear output, either as a current loop or a voltage. With a few modifications, these analog systems were even able to provide linearized output and simple signal processing algorithms, such as a variable averaging filter. Improvements in the electronics industry have improved signal processing techniques (e.g., linearization and filtering) in all areas of instrumentation, and IR is no exception.

Today's IR sensors are smart transducers with fast response times. The integration of advanced electronics has also resulted in a class of sensors that are easily compatible with digital communication protocols.

In the past 20 years, digital electronics has taken over the functions of linearization and signal processing. The signal is typically acquired via an instrumentation amplifier, taken through an A/D converter, and then handed over to a microprocessor or to digital signal processors (DSPs). High-speed DSPs have enabled fast, smart IR sensors, with simultaneous digital and analog outputs as fast as 1 ms. For example, if a target changes temperature by 1°C, the output of the sensor will change by 1°C 1 ms later.

Photo 3. Sensor setup, monitoring, and data analysis is made easy with PC software and a smart IR sensor with an interface and two-way communications. The Raytek PC software screen displays four views of the process: current temperatures with high and low alarm indicators; a configurable graph of temperatures using one- or two-color measurement modes; statistical values, such as maximum, minimum, mean, and standard deviation; and a configurable histogram.

The standardization of PC operating systems and graphical user interfaces (e.g., Windows) have provided familiar work environments that allow sensor manufacturers to develop software that is easy to use in plant environments and that facilitate the training of new users. Unfortunately, vendors of industrial instrumentation are still locked in fierce competition among numerous communications standards. But despite their differences, all the standards support smart devices by providing bidirectional digital communications and the ability to address multiple sensing devices.

One common way to work around conflicting communications standards is for the sensor manufacturer to provide an addressable RS-485 output with a device-specific control protocol. This standard digital signal can then be patched into an existing network of sensors via software device drivers or can be used with companion software from the sensor manufacturer. The latest generation of smart IR sensors are fully controllable via such digital schemes.

Screen 1. A DSP in the sensor and standardized PC software allows onsite calibration via serial communications. An easy-to-use software interface guides users through the calibration or diagnostics process. (Click magnifier for full-size image.)

Because the signal processing is largely digital, calibration (see Screen 1) and firmware upgrades can be made in the field via a serial communications line. You just start up the software and follow the instructions. Sensors can even provide diagnostics or be set to a fixed output to calibrate meters in the current loop. And for those engineers who need to go further than using basic signal processing (e.g., peak hold or averaging), custom algorithms for calculating temperature can be programmed into the sensor on site.

When combined with advanced optics and the latest electronics, instruments using these detectors can be used in applications previously beyond the reach of IR systems. A typical application for high-speed/low-temperature measurement could be found in induction heat treating of a coating on rapidly moving, small metal components, such as a heat treated drill bit or metal screw.

Surface-mount technology has also had an impact on the IR sensor. For years, high-performance IR sensors consisted of either a sensing head coupled by a cable to a signal processing box, or a large enclosure containing both electronics and optics. In applications in which the sensor is used to provide a temperature signal for a control loop, a large housing or a separate box for the processing electronics can be a nuisance. But thanks to miniature surface-mounted electronic components, today's most advanced sensors combine optics and electronics in a compact housing that is easier to install and maintain than older designs.

Dual-wavelength thermometers, or ratio-thermometers, simultaneously measure IR radiation at two wavelengths and calculate temperatures via a ratio of the collected energy. High-performance ratio-thermometers accurately measure targets obscured by other structures, dust, smoke, or particulates. To control dust or other materials that build up on the lens or viewport of a smart ratio-thermometer, software programs allow engineers to adjust the alarm to trigger at an unacceptable level of obscuration.

Fiber-optic IR thermometers benefit not only from the integration of smart electronics and software but also from the lower costs of fiber optics.

Advancements in optics, electronics, software, and digital communications has resulted in a new generation of IR thermometers.

The fibers conduct the IR energy to the IR detector the same way light is conducted in fiber-optic phone lines. These fibers—which can easily be run for distances greater than 10 m—can survive relatively unprotected in harsh environments, such as high temperature or high electromagnetic fields in heat-treating applications. And because the diameter of the fiber is relatively small and quite flexible, it can be routed to a target that may not be in a clear line of sight, a situation that would preclude the use of a standard IR thermometer.

The combination of advancements in optics, electronics, software, and digital communications has resulted in a new generation of IR thermometers that offer many performance advantages. This new class of sensor technology provides an unprecedented set of features in one instrument—fast response, smaller spot sizes, and wider temperature ranges; bidirectional digital communications for remote sensor setup, calibration, and diagnostics; and software programs for remote monitoring and data analysis. These enhancements can have a dramatic effect on ease of use, maintenance, and long-term cost of ownership.

Smart IR sensors are being used in numerous industries, frequently driven by the need for statistical process control or archiving data for ISO-9000 programs. With the digital output of the sensor tied directly to a PC, monitoring, archiving, and reporting critical process temperatures has never been easier. And for personal safety, making adjustments to sensor measurement parameters without having to walk 10 ft above a 1500°C process makes a smart IR device very desirable.

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