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Sensors unite for process improvement.

Engineers are using physical connectors and computerized tools such as signal conditioners to integrate varied sensor functions and improve the measurement, and thus the quality, of industrial processing.

Manufacturers are adding more sensors to their process lines to pinpoint and eliminate production problems. Moreover, to provide plant operators with a more detailed picture of their production lines, engineers are integrating sensor functions, either by designing multifunction sensors, or by developing computer networking systems to link data from multiple sensors.

An important consideration when integrating expanded sensor systems is to make the data compatible with the data-acquisition equipment already in use at a manufacturing plant, said Richard Miller, president of Richard K. Miller and Associates Inc. (Norcross, Ga.), a technology and market research firm that follows trends in sensor development. Engineers are designing signal conditioners, semiconductor chips, and a variety of networking media to integrate sensors while using as much of the existing computer hardware as possible. For example, a specially designed signal conditioner was instrumental in digitizing information from a hybrid sensor developed by Dallas-based Texas Instruments Inc. (TI) to take two different measurements of the same point.

TI engineers combined force and displacement sensors into a single unit to test the function switches the company used to manufacture for its digital watches. Due to the small size of the function switches, less than 1/8-inch in diameter and 3/8-inch long, the TI research team needed sensors able to measure very small expenditures of force. They found enough sensitivity in the miniature FTA-G low-range force transducers manufactured by Lucas Schaevitz Inc. (Pennsauken, N.J.).

These devices can measure 10 grams or less of force by means of the displacement of a core located between two stainless steel flexures, or springs, on the front and back of the transducer. When the transducer comes into contact with the target area, any force applied to the target will cause the springs to flex, moving the transducer's stainless st The displacement of the core creates an electrical signal that is proportional to the amount of force. A signal conditioner transforms that signal into digital information for a computer.

TI used another Lucas Schaevitz product, the 050MHR Linear Variable Differential Transformer (LVDT) displacement sensor, to measure displacement of the digital watch function switches. The LVDT sensor is comprised of a free-moving core made of hydrogen-annealed nickel-iron, set within two symmetrically spaced coils. The core moves in relation to the surface of the object being measured. The LVDT produces an electrical output that is proportional to the displacement of the core.

According to Leonard Leopold, an applications engineering coordinator at Lucas Schaevitz, TI first determined the appropriate position for the force transducer to measure the function switches of their watches. "They then linked the LVDT core to the front spring of the force transducer by means of a stainless steel rod, with one end of the core pointing toward the target. As the core changed position due to displacement of the target it would move the spring of the force transducer, thus providing both force and displacement measurement for the operators," explained.

While TI was linking the two sensors for the function switch testing, Lucas Schaevitz was designing the electronics to condition the two different signals generated by the disparate sensors. "We developed a single-box, two-channel signal conditioner to digitize the data from the sensors so it could be sent to a computer," aid Leopold. TI used the outputs of their hybrid sensor to measure corresponding force and displacement parameters for their function switches. These parameters included peak force and peak displacement of the function switches, as well as safety limits for both force and displacement. The company was able to establish preferred tolerances on the function switches, which permitted higher quality-control standards and fewer field failures.

Combination Sensors

Enhance Composite Curing

Leopold said that the hybrid sensor TI fashioned is a relatively uncommon configuration for Lucas Schaevitz sensors. "Today, with improved signal conditioning, an industrial processor is more likely to connect separate sensors operating at different points on the process line," he said. However, he said that switch manufacturers use hybrid sensor systems like the one designed by TI to take two distinct readings of the same point. One of the latest hybrid sensors is the combination pressure and temperature curite sensor designed by Interlink Electronics (Carpinteria, Calif.). Curite sensors are being used by aerospace firms to measure the curing of composite aircraft parts by autoclave processing and vacuum bagging, which involves evacuating the air from a plastic membrane around the part to force the composite material against the tool to give the part its final form. The sensors are also used in resin transfer molding, which involves injecting resin into a male/female mold to form a composite part.

There are two primary reasons operators of aerospace composite lines want to know pressure and temperature simultaneously during the curing process, said Richard Siegel, director of marketing at Interlink. "First, composite manufacturers want to know the localized cure conditions to verify that the composite materials will be completely cured for maximum strength and integrity. Secondly, to provide a record of the actual cure conditions to meet quality assurance audits and the like."

The curite combination sensor is comprised of an Interlink force-sensing resistor to determine pressure, and an iron-constantan thermocouple that measures temperature. Curite pressure sensors are based on polymer thick-film technology. They consist of a polymide layer printed with a semiconductive compound and another polymide layer printed with interdigitating electrodes, making it conductive. A spacer adhesive separates the conductive and semiconductive substances. As these layers are brought into contact with one another due to pressure, a very high resistance can be measured. The greater the compressive force on the layers, the less the measured resistance. The changes in resistance of the sensor reflect the pressure of the target area.

Interlink designed its pressure sensors to withstand operating temperatures up to 500 [degrees] F. They are 0.3-millimeter thick, ensuring minimal distortion of surrounding areas caused by sensor placement.

The welded bead thermocouple is located adjacent to the pressure sensor, between the two wires that carry the pressure signals. As the thermocouple contacts the target surface, the change in the electrical potential of the junction between the iron and constantan wires is measured by a voltmeter, and can be converted to a thermal value electronically. Interlink engineers chose the welded bead thermocouple for its small size (the wire has a 0.01-inch diameter) and quick response.

The combination sensor is connected to a signal conditioner which converts the sensors' signals into a voltage output that can be read by existing data-acquisition devices such as laboratory and production process control equipment. Each curite signal conditioner is designed to accommodate eight combination pressure and temperature sensors, or eight separate pressure and temperature sensors respectively, if single point readings are not required.

Sensor analyst Miller noted that in addition to collecting more data, signal conditioners can also be used to electronically compensate thermal sensors for changes in ambient temperature. The curite signal conditioner is capable of this because it is internally calibrated to allow compensation for any changes in the external or ambient temperature that might compromise the accuracy of its readings.

Intelligent Chips

Lighten Computer Burden

For process lines with multiple sensors taking readings at multiple points, sensor integration is largely a matter of data integration. Miller said semiconductor chips could be used in multiple sensor integration to perform computer tasks that might otherwise require a more powerful computer to be installed. An example of this is the LonWorks technology designed by Motorola Inc. to collect and combine the information received from separate sensing points. The technology incorporates semiconductor chips designed by Motorola's Semiconductor Products Sector (Phoenix) and a computer protocol designed by Echelon Corp. (Palo Alto, Calif.).

Each LonWorks system is made up of nodes that collect sensory data and communicate with one another through a variety of communications media using a common message based control protocol. The nodes consist of a sensor, a Motorola Neuron Chip, a power source, a transceiver for communicating over the network medium, and circuitry for interfacing to the process line being controlled or monitored.

The silicon neuron chip in the node is an MC143120. According to Jeff Koonce, senior applications engineer for LonWorks, the chip is an integrated component that performs the network and application-specific processing within a node. "It is specifically designed for nodes that require smaller application programs," Koonce said. Depending on the costs involved, the node power source can be twisted-pair copper wire or batteries.

A single computer chip typically makes up the node transceiver. This is an integrated circuit separate from the neuron used to communicate with the network. LonWorks networking media include powerlines, radio frequencies, fiber-optic cables, and ultrasonic waves, depending on the needs of the installer. "In a retrofit situation, powerlines or radio frequencies could save on installation costs because you don't have to tear down any walls," explained Koonce. Optical fibers are more resistant to tapping, and therefore effective if security is an issue. Koonce added that if a processor wishes to economize, ultrasonic media usually cost less than the others.

A second type of neuron chip, the MC143150, was designed for large-scale applications; for example, master controllers integrated with a personal computer that must collect data gathered from hundreds of nodes in a processing plant.

The LonTalk communications protocol developed for Motorola neuron chips by Echelon represents a departure from standard protocol design, said Koonce. "Other protocols developed by software companies or standards committees result in documents used by end-users who then have to write the microcomputer code." Standards are sometimes written so loosely that they may not be compatible, he added. Echelon imbeds the LonTalk protocol into the Motorola neuron chips, saving the processor the time needed to develop a protocol.

Motorola does not sell the LonWorks technology directly to industrial processors. Computer network designers, such as Action Instruments Inc. (San Diego), use the LonWorks components to develop a sensor-integration system for industrial facilities. Action Instruments specializes in providing computerized process-control systems. In June, the company devised its first prototypical LonWorks system for an aluminum manufacturing plant.

This system integrates the measurement of flow, temperature, and pressure on a process line making aluminum strips for a variety of markets, including automotive and appliances. The process line uses noncontact infrared radiation thermometers to measure the temperature of the molten aluminum being poured into strip form. After the strip has cooled, thermocouples contact the aluminum to gauge its temperature.

"The manufacturers use differential pressure cell sensors to monitor the pressure supplied by the rolling press that compresses the aluminum strip," said Frank Williams, vice president of marketing and sales at Action Instruments. Strain gage sensors located underneath the conveyor are used to measure the displacement of the belt. This displacement corresponds to the weight of the aluminum strip.

Action Instruments' Action Pak Model 5200 data collectors contain the components of the LonWorks nodes. The Action Paks are translucent plastic boxes measuring 4-by-2-by-4 inches. Twisted-pair wires carry the signals 4000 feet from the Action Paks to an Action Pak Model 5900 network manager, or gateway. This device converts the sensory data to an RS-232 signal sent to the computer in the plant manager's office.

Williams said that the LonWorks system can improve the aluminum process line by, for example, checking pressure to prevent a buildup of material. "The plant manager can also change the performance of the sensor nodes to meet different parameters by changing the parameters on his computer," Williams added. For example, a temperature sensor programmed to measure 0 [degrees] C to 1500 [degrees] C can be directed to measure 0 to 500 [degrees] C if necessary.

Making Hot Die Forging

A Reality

A hot die forging system conceived by the SIFCO Forge Group (Cleveland) was begun a year ago to make aircraft parts. This project made use of a sensor-integration system developed by the advanced manufacturing center of Cleveland State University. The hot die forging is jointly funded by SIFCO and the Aeronautical Systems Division of the U.S. Air Force as part of the Air Force's Industrial Modernization Incentive Program (IMIP).

A total of 53 sensors were integrated to measure velocity, position, force, pressure, and temperature on a hydraulic screw press that has been extensively renovated and upgraded as part of the project.

In the SIFCO system, induction coils were wrapped around the dies to heat them. When they reached the desired temperature, parts made of titanium, waspaloy, or inconel alloys were induction heated and manually placed on the press. (A conveyance system is currently under development.) The press was cycled with the required force and forged the part.

According to Frank Schossler, a mechanical engineer and research associate at the advanced manufacturing center, five pressure transducers were placed in the hydraulic system at various points to measure the hydraulic pressure and detect cavitation caused by a system malfunction or improper press operation. Engineers at the manufacturing center selected strain gage-based force sensors that were installed on the legs of the press to measure the force imparted to the metal part being forged.

Velocity and position sensors were also located on the legs of the press to account for the stretch in the press after impact. "The stretch is only about 1/8 of an inch, but it can affect the dimensional accuracy of the part," Schossler said.

The velocity and position sensors were magnetostriction devices that measure the change in magnetic fields to detect movement and position. The magnets that set up the fields were attached to the forging ram itself, while the tube and transducer assemblies of the velocity and position sensors remained on the legs of the press. Schossler said that since the sensors and magnets did not come into contact, the vibration of impact did not interfere with their reading.

Temperature measurement was integral to the design of the hot die system, whose thermal characteristics represent a departure from conventional forging. "Normally, parts are forged at a differential of temperature between the press and the dies. This difference in temperature causes the surface of the material to cool, affecting the grain structure of the part and requiring further machining later," explained Bill Elderkin, manager of the IMIP project for SIFCO. He said that reducing machining would also lower the amount of scrap produced. This is a significant cost-saving measure due to the expensive alloys used. By lowering the temperature differential, SIFCO engineers hope to reduce the distortion of the grain structure and extra machining that forging requires.

Noncontact infrared radiation thermometers were selected to measure the temperature of the metal parts being placed on the press. These devices convert the natural radiation being emitted from a heated object and convert it into a corresponding temperature. "Since the environment in which the press operates is dirty, we use a two-color infrared thermometer that measures the ratio between two adjacent wavelengths, rather than absolute intensity, eliminating the inaccuracy dirt can cause to the measurement," added Schossler.

Contact measuring thermocouples are positioned within the press frame to ensure that the press does not overheat from contact with the dies. Thermocouples were also installed within the dies themselves to prevent undue wear. "The sensors in the dies will warn us if the temperature gradient between the inside and the outside of the dies becomes excessive. Extreme temperature gradient causes thermal stresses that will fatigue the dies," explained Schossler.

Information from all of the SIFCO forge press sensors is transmitted by shielded twisted-pair wires to a data-acquisition interface box located in the plant's control room. All of the data-acquisition components within the interface box are made by Keithley Instruments Inc. (Cleveland), including the signal conditioning boards needed to manipulate the information received for further processing and to supply power to the sensors. Sensor information is sent from the interface box to two places: the programmable logic controller that controls the motion of the press, and the personal computer of the operator.

The Keithley Asyst Viewdac software package handles the data acquisition process. This package was designed exclusively for the 80386 and 80486 chip family of 32-bit PCs. Viewdac is capable of analysis functions including curve fits and statistics.

The SIFCO operator oversees the function of the press, making any fine adjustments; for example, increasing ram force by 5 percent directly at the keyboard. All data received by the operator's PC is automatically and permanently stored on an optical disk. These are called WORM disks, for Write Once, Read Many times. Once written on, these disks cannot be altered, thus providing an ideal medium for long-term data archiving.

The SIFCO press has been undergoing field tests since June. It is being further developed by SIFCO with the assistance of Cleveland State University Advanced manufacturing center. "By knowing what is going on at every point of the process, through the sensor information, SIFCO can control quality and continue to refine the capabilities of the system," Elderkin said. One benefit sensor integration will bring to the hot-die system is cutting the time needed to heat the dies, he said.

More Measurements,

Better Software

Fiber optics will likely play a larger role in future sensor integration, sensor analyst Miller said. "Transmitting information in the form of light, via fiber-optic cables rather than in electrical form along twisted-pair wire, will eliminate electromagnetically induced noise, such as the high electric fields generated by welding, that would compromise sensor accuracy," said Miller.

In general, he said that manufacturers would continue to increase the number of measurements taken of their process lines to improve their products. "For example, manufacturers are starting to perform tolerance measurement at the end of each machining stage, rather than at the end of the finishing process, which is accepted practice."

John Tucker, an applications engineer for Keithley Instruments, said that enhanced software, such as the Echelon protocol imbedded in Motorola's neuron chip, will be a key element of future sensor integration. "Manufacturers are telling us that they do not have the time to program their computer networks anymore. They want process-control software, complete with computer windows and graphical user interface, to integrate and implement sensor data as swiftly as possible."
COPYRIGHT 1992 American Society of Mechanical Engineers
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Valenti, Michael
Publication:Mechanical Engineering-CIME
Article Type:Cover Story
Date:Sep 1, 1992
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