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Smart Infrared Temperature Sensors
Keeping up with continuously evolving process technologies is a major challenge for
ocess engineers. Add to that the demands of staying current with rapidly evolving methods of monitoring and controlling those processes, and the assignment can become quite intimidating. However, infrared (IR) temperature sensor manufacturers are giving users the tools they need to meet these challenges: the latest computer-related hardware, software, and communications equipment, as well as leading-edge digital circuitry. Chief among these tools, though, is the next generation of IR thermometers—the smart sensor.
Today’s new smart IR sensors represent a union of two rapidly evolving sciences that combine IR temperature measurement with high-speed digital technologies usually associated with the computer. These instruments are called smart sensors because they incorporate microprocessors programmed to act as transceivers for bidirectional, serial communications between sensors on the manufacturing floor and computers in the control room (see Photo 1). And because the circuitry is smaller, the sensors are smaller, simplifying installation in tight or awkward areas. Integrating smart sensors into new or existing process control systems offers an immediate advantage to process control engineers in terms of providing a new level of sophistication in temperature monitoring and control.
I Integrating Smart Sensors into Process Lines While the widespread implementation of smart IR sensors is new, IR temperature measurement hasbeen successfully used in process monitoring and control for decades (see the sidebar, “How Infrared Temperature Sensors Work,” below). In the past, if process engineers needed to change a sensor’s settings, they would have to either shut down the line to remove the sensor or try to manually reset it in place. Either course could cause delays in the line, and, in some cases, be very dangerous. Upgrading a sensor usually required buying a new unit, calibrating it to the process, and installing it while the process line lay inactive. For example, some of the sensors in a wire galvanizing plant used to be mounted over vats of
molten lead, zinc, and/or muriatic acid and accessible only by reaching out over the vats from a catwalk. In the interests of safety, the process line would have to be shut down for at least 24
hours to cool before changing and upgrading a sensor.
Today, process engineers can remotely configure, monitor, address, upgrade, and maintain their IR temperature sensors. Smart models with bidirectional RS-485 or RS-232 communications capabilities simplify integration into process control systems. Once a sensor is installed on a process line, engineers can tailor all its parameters to fit changing conditions—all from a PC in the control room. If, for example, the ambient temperature fluctuates, or the process itself undergoes changes in type, thickness, or temperature, all a process engineer needs to do is customize or restore saved settings at a computer terminal. If a smart sensor fails due to high ambient temperature conditions, a cut cable, or failed components, its fail-safe conditions engage automatically. The sensor activates an alarm to trigger a shutdown, preventing damage to product and machinery. If ovens or coolers fail, HI and LO alarms can also signal that there is a problem and/or shut down the line.
II Extending a Sensor’s Useful Life
For smart sensors to be compatible with thousands of different types of processes, they must be fully customizable. Because smart sensors contain EPROMs (erasable programmable read only memory), users can reprogram them to meet their specific process requirements using field calibration, diagnostics, and/or utility software from the sensor manufacturer.
Another benefit of owning a smart sensor is that its firmware, the software embedded in its chips, can be upgraded via the communications link to revisions as they become available—without removing the sensor from the process line. Firmware upgrades extend the working life of a sensor and can actually make a smart sensor smarter.
The Raytek Marathon Series is a full line of 1- and 2-color ratio IR thermometers that can be networked with up to 32 smart sensors. Available models include both integrated units and fiber-optic sensors with electronic enclosures that can be mounted away from high ambient temperatures.
Clicking on a sensor window displays the configuration settings for that particular sensor. The Windows graphical interface is intuitive and easy to use. In the configuration screen, process engineers can monitor current sensor settings, adjust them to meet their needs, or reset the sensor back to the factory defaults. All the displayed information comes from the sensor by way of the RS-485 or RS-232 serial connection.
The first two columns are for user input. The third monitors the sensor’s parameters in real
time. Some parameters can be changed through other screens, custom programming, and direct PC-to-sensor commands. Parameters that can be changed by user input include the following:
· Relay contact can be set to NO (normally open) or NC (normally closed).
· Relay function can be set to alarm or setpoint.
· Temperature units can be changed from degrees Celsius to degrees Fahrenheit, or vice versa.
· Display and analog output mode can be changed for smart sensors that have combined one- and two-color capabilities.
· Laser (if the sensor is equipped with laser aiming) can be turned on or off.
· Milliamp output settings and range can be used as automatic process triggers or alarms.
· Emissivity (for one-color) or slope (for two-color) ratio thermometers values can be set. Emissivity and slope values for common metal and nonmetal materials, and instructions on how to determine emissivity and slope, are usually included with sensors.
· Signal processing defines the temperature parameters returned. Average returns an object’s average temperature over a period of time; peak-hold returns an object’s peak temperature either over a period of time or by an external trigger.
· HI alarm/LO alarm can be set to warn of improper changes in temperature. On some process lines, this could be triggered by a break in a product or by malfunctioning heater or cooler elements.
· Attenuation indicates alarm and shut down settings for two-color ratio smart sensors. In this example, if the lens is 95% obscured, an alarm warns that the temperature results might be losing accuracy (known as a “dirty window” alarm). More than 95% obscurity can trigger an automatic shutdown of the process.
III Using Smart Sensors
Smart IR sensors can be used in any manufacturing process in which temperatures are crucial to high-quality product. Six IR temperature sensors can be seen monitoring product temperatures before and after the various thermal processes and before and after drying. The smart sensors are configured on a high-speed multidrop network (defined below) and are individually addressable from the remote supervisory computer. Measured temperatures at all sensor locations can be polled individually or sequentially; the data can be graphed for easy
monitoring or archived to document process temperature data. Using remote addressing features, set points, alarms, emissivity, and signal processing, information can be downloaded
to each sensor. The result is tighter process control.
IV Remote Online Addressability
In a continuous process similar to that in Figure 2, smart sensors can be connected to one another or to other displays, chart recorders, and controllers on a single network. The sensors may be arranged in multidrop or point-to-point configurations, or simply stand alone.
In a multidrop configuration, multiple sensors (up to 32 in some cases) can be combined on a network-type cable. Each can have its own “address,” allowing it to be configured separately with different operating parameters. Because smart sensors use RS-485 or FSK (frequency shift keyed) communications, they can be located at considerable distances from the control room computer—up to 1200 m (4000 ft.) for RS-485, or 3000 m (10,000 ft.) for FSK. Some processes use RS-232 communications, but cable length is limited to <100 ft.
In a point-to-point installation, smart sensors can be connected to chart recorders, process controllers, and displays, as well as to the controlling computer. In this type of installation, digital communications can be combined with milliamp current loops for a complete all-around process communications package.
Sometimes, however, specialized processes require specialized software. A wallpaper manufacturer might need a series of sensors programmed to check for breaks and tears along the entire press and coating run, but each area has different ambient and surface temperatures, and each sensor must trigger an alarm if it notices irregularities in the surface. For customized processes such as this, engineers can write their own programs using published protocol data. These custom programs can remotely reconfigure sensors on the fly—without shutting down the process line.
V Field Calibration and Sensor Upgrades
Whether using multidrop, point-to-point, or single sensor networks, process engineers need the proper software tools on their personal computers to calibrate, configure, monitor, and upgrade those sensors. Simple, easy-to-use data acquisition, configuration, and utility programs are usually part of the smart sensor package when purchased, or custom software can be used.
With field calibration software, smart sensors can be calibrated, new parameters
downloaded directly to the sensor’s circuitry, and the sensor’s current parameters saved and stored as computer data files to ensure that a complete record of calibration and/or parameter changes is kept. One set of calibration techniques can include one-point offset and two- and three-point with variable temperatures:
• One-point offset. If a single temperature is used in a particular process, and the sensor reading needs to be offset to make it match a known temperature, one-point offset calibration should be used. This offset will be applied to all temperatures throughout the entire temperature range. For example, if the known temperature along a float glass line is exactly 1800°F, the smart sensor, or series of sensors, can be calibrated to that temperature.
• Two-point. If sensor readings must match at two specific temperatures, the two-point calibration shown in Figure 3 should be selected. This technique uses the calibration temperatures to calculate a gain and an offset that are applied to all temperatures throughout the entire range.
• Three-point with variable temperature. If the process has a wide range of temperatures, and sensor readings need to match at three specific temperatures, the best choice is three-point variable temperature calibration (see Figure 4). This technique uses the calibration temperatures to calculate two gains and two offsets. The first gain and offset are applied to all temperatures below a midpoint temperature, and the second set to all temperatures above the midpoint. Three-point calibration is less common than one- and two-point, but occasionally manufacturers need to perform this technique to meet specific standards.
Field calibration software also allows routine diagnostics, including power supply voltage and relay tests, to be run on smart sensors. The results let process engineers know if the sensors are performing at their optimum and make any necessary troubleshooting easier.
VI Conclusion
The new generation of smart IR temperature sensors allows process engineers to keep up with changes brought on by newer manufacturing techniques and increases in production. They now can configure as many sensors as necessary for their specific process control needs and extend the life of those sensors far beyond that of earlier, “non-smart” designs. As production rates increase, equipment downtime must decrease. By being able to monitor equipment and fine-tune temperature variables without shutting down a process, engineers can keep the process efficient and the product quality high. A smart IR sensor’s digital processing components and communications capabilities provide a level of flexibility, safety,
and ease of use not achieved until now.
How Infrared Temperature Sensors Work
Infrared (IR) radiation is part of the electromagnetic spectrum, which includes radio waves, microwaves, visible light, and ultraviolet light, as well as gamma rays and X-rays. The
IRrange falls between the visible portion of the spectrum and radio waves. IR wavelengths are usually expressed in microns, with the IR spectrum extending from 0.7 to 1000 microns. Only the 0.7-14 micron band is used for IR temperature measurement.
Using advanced optic systems and detectors, noncontact IR thermometers can focus on nearly any portion or portions of the 0.7-14 micron band. Because every object (with the exception of a blackbody) emits an optimum amount of IR energy at a specific point along the IR band, each process may require unique sensor models with specific optics and detector types. For example, a sensor with a narrow spectral range centered at 3.43 microns is optimized for measuring the surface temperature of polyethylene and related materials. A sensor set up for 5 microns is used to measure glass surfaces. A 1 micron sensor is used for metals and foils. The broader spectral ranges are used to measure lower temperature surfaces, such as paper, board, poly, and foil composites.
The intensity of an object's emitted IR energy increases or decreases in proportion to its temperature. It is the emitted energy, measured as the target's emissivity, that indicates an object's temperature.
Emissivity is a term used to quantify the energy-emitting characteristics of different materials and surfaces. IR sensors have adjustable emissivity settings, usually from 0.1 to 1.0, which allow accurate temperature measurements of several surface types. The emitted energy comes from an object and reaches the IR sensor through its optical system, which focuses the energy onto one or more photosensitive detectors. The detector then converts the IR energy into an electrical signal, which is in turn converted into a temperature value based on the sensor's calibration equation and the target's emissivity. This temperature value can be displayed on the sensor, or, in the case of the smart sensor, converted to a digital output and displayed on a computer terminal。
