Making Sense of Smart Infrared Sensors
Fine Tune Process Control with Smart Infrared Sensors
by
Ronald Cook
Keeping up with continuously evolving process technologies is a major challenge for process engineers. Add to that the challenge of keeping up with rapidly evolving methods of monitoring and controlling those processes, and the combined tasks can become quite intimidating. However, by employing the latest computer-related hardware, software, and communications equipment, as well as leading-edge digital circuitry, infrared temperature sensor manufacturers are giving users the tools they need to meet those challenges. And one of those tools is the next generation of infrared thermometer: the smart sensor.
Today’s new smart infrared sensors are a joining of two rapidly evolving sciences, combining infrared temperature measurement with high-speed digital technologies usually associated with the computer. These instruments are dubbed smart sensors because they house microprocessors programmed to act as transmitters and receivers, which allow bi-directional, serial communications between sensors on the manufacturing floor and computers in the control room. And since 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.
Integrating Smart Sensors into Process Lines
While the widespread implementation of smart infrared sensors is new, infrared temperature measurement is not. It has been successfully used in process monitoring and control for decades. 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 while in place, which could be dangerous or cause delays. If a manufacturer wanted to upgrade a sensor, it usually required buying a new unit, calibrating it to the process, and installing it. This often left process lines inactive for long periods of time. For example, in a wire galvanizing plant, sensors 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--a hot and often dangerous undertaking. For safety reasons, this type of 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 configure, monitor, address, upgrade, and maintain their infrared temperature sensors remotely. Smart infrared sensors, with bi-directional communications capabilities, simplify integration into process control systems. Once a sensor is installed on a process line, engineers can tailor all sensor parameters when changing conditions warrant modifications--all from a personal computer in the control room. For example, if ambient conditions change, such as room temperature fluctuations, or if the process itself 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 sensor failure occurs due to high ambient temperature conditions, a cut cable, or failed components, a smart sensor’s fail-safe conditions engage automatically. The sensor activates an alarm to trigger a shut down, keeping the product or machinery from becoming damaged. If ovens or coolers fail, high and low alarms can also signal that there is a problem and/or shut down the line.
For smart sensors to be compatible with the thousands of different types of processes, they have to 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.
Extending Sensor Life
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 newer 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.
Figure 1 is an example of a smart infrared temperature sensor’s configuration screen. The Windows™ graphical interface is intuitive and easy to use. In this particular 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 serial connection.
Figure 1: Configuration Window
The first two columns are for user input. The third shows returned sensor parameters, some that can be changed through other screens or through custom programming. Parameters that can be changed by user input include the following:
Using Smart Sensors
You can use smart infrared sensors in any manufacturing process where temperatures are critical for quality products. For example, Figure 2 shows a generic web/converting process with an oven, a corrugating/embossing die, and a cooler.
Figure 2: Process Line Example
In Figure 2, six infrared temperature sensors monitor 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, and the data can be graphed for easy monitoring, or it can be archived to document process temperature data. Set points, alarms, emissivity, and signal processing information can be downloaded to each sensor using remote addressing features. The result is tighter process control.
Remote Online Addressibility
On a continuous process, similar to the process in Figure 2, every smart sensor can be connected to each other or to other displays, chart recorders, and controllers on a single network. Sensors may be arranged in multidrop or point-to-point configurations, or just by themselves.
In a multidrop configuration multiple sensors (up to 15 in some cases) can be combined on a network-type cable. Each sensor can have its own "address" so each can be configured separately with different operating parameters. Because smart sensors use RS-485 or FSK (Frequency Shift Keyed) communications, they can be up to 1200 meters (4000 feet) away for RS-485, or 3000 meters (10,000 feet) away for FSK from the control room computer. (Some use RS-232 communications, but cable length is limited to only a few hundred feet.)
In a point-to-point installation, a smart sensor can be connected to chart recorders, process controllers, and displays, as well as 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--all without shutting down the process line.
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 you purchase it, or you can use custom software.
Field calibration software permits calibration of smart sensors and uploading of the new parameters directly to the sensor’s circuitry. Calibration software also allows downloading a sensor’s current parameters to the computer, which can be stored as data files, so a complete record of calibration and/or parameter changes can be kept. An example of one set of calibration techniques can include one-point offset, two-point, and three point with variable temperatures.
If working with a single temperature in a process and there is a need to "offset" the reading at the temperature to make it match a "known temperature," use one-point offset calibration. 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.
If there is a need to match readings at two specific temperatures, use the two-point calibration (Figure 3). This technique uses the calibration temperatures to calculate a gain and an offset. This gain and offset is applied to all temperatures throughout the entire temperature range. For example, steel and glass industries sometimes need to calibrate to two points along sections of a production line.
Figure 3: Two-Point Calibration Example
If working with a wide range of temperatures in a process, and there is a need to match readings at three specific temperatures, use three-point variable temperature calibration (Figure 4). This technique uses the calibration temperatures to calculate two gains and two offsets. The first gain and offset is applied to all temperatures below a midpoint temperature, and the second set of gain and offset is applied to all temperatures above the midpoint temperatures. Three-point calibration is not as common as one- and two-point, but occasionally manufacturers might need to perform this calibration technique to meet specific standards.
Figure 4: Three-Point Calibration Example
Field calibration software also allows the running of routine diagnostic tests on smart sensors, including power supply voltage tests, detector heater temperature tests, and relay tests. The results let process engineers know if the sensors are performing at their optimum and also provides an easy troubleshooting tool.
Conclusion
With the new generation of smart infrared temperature sensors, process engineers can keep up with changes brought on by newer manufacturing techniques and with increases in production. They are now able to configure as many sensors as necessary to their specific process control needs, and to extend the life of those sensors far beyond that of earlier non-smart designs. Production increases mean a need to decrease equipment downtime. To be able to monitor equipment and to fine tune temperature variables in process lines helps keep the lines running efficiently and product quality high. A smart infrared sensor’s digital processing components and communications capabilities provide a level of flexibility, safety, and ease-of-use never available until now.