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High‐power fiber optic cable with integrated active sensors for live process monitoring
SPIE Photonics West conference in San Francisco, January 2012
‐ Submitted version ‐
Ola Blomster, Mats Blomqvist, Hans Bergstrand, Magnus Pålsson
OPTOSKAND
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ABSTRACT
In industrial applications using high‐brilliance lasers at power levels up to and exceeding 20 kW and similarly direct diode lasers of 10 kW, there is an increasing demand to continuously monitor component status even in passive components such as fiber‐optic cables. With fiber‐optic cables designed according to the European Automotive Industry fiber standard interface there is room for integrating active sensors inside the connectors. In this paper we present the integrated active sensors in the new Optoskand QD fiber‐optic cable designed to handle extreme levels of power losses, and how these sensors can be employed in industrial manufacturing. The sensors include photo diodes for detection of scattered light inside the fiber connector, absolute temperature of the fiber connector, difference in temperature of incoming and outgoing cooling water, and humidity measurement inside the fiber connector. All these sensors are connected to the fiber interlock system, where interlock break enable functions can be activated when measured signals are higher than threshold levels. It is a very fast interlock break system as the control of the signals is integrated in the electronics inside the fiber connector. Also, since all signals can be logged it is possible to evaluate what happened inside the connector before the interlock break instance. The communication to the fiber‐optic connectors is via a CAN interface. Thus it is straightforward to develop the existing laser host control to also control the CAN‐messages from the QD sensors.
Keywords: high power lasers, fiber optic cables, sensors, fiber interlock, closed loop system, automotive standard * [email protected]; phone +46 31 706 27 63; fax +46 31 706 27 78; www.optoskand.se
1 INTRODUCTION
The optical fiber connector from Optoskand is designed for best possible transmission through the optical fiber using an industrial and robust design. Up to now the state of the art optical design used by Optoskand consisting of AR‐coated quartz cylinder, mode stripper and cooling water directly surrounding the optical fiber is well known and has been on the market since 1993.
Optoskand introduced sensors inside the optical fiber connector already in 2001 when the power losses inside the connector was monitored by measuring the temperature difference between inlet and outlet of the cooling water to the connector, aΔT. The temperature difference gave a direct feedback of the power losses taken care of by the connector. When the ΔT solution was introduced, the communication interface was also introduced and implemented to the optical fiber connector. The information from the ΔT circuit board was transferred through a bCAN‐bus interface to external accessories like a computer, which processed the results.
In 2007, Optoskand introduced the auto alignment equipment as a tool for the user to find the best possible alignment position for the optical fiber. The auto alignment equipment used sensors inside the incoupling unit as feedback information to the regulation system.
Today Optoskand introduces active sensors implemented inside the QD connector (European automotive standard) for monitoring, quality and safety purposes. The QD fiber itself is proven to handle extreme high power losses, which now
are possible to monitor using the sensors built into the QD connector. The sensors are built into the connector as standard and can easily be reached through the CANopen interface. The board hosting the sensors is a stand‐alone board, which can supervise the safety and be set up to switch off the interlock function in case of unexpected circumstances.
When working with optical fibers with high power lasers and material processing there are different kinds of light in the system to detect and to monitor from. Protective systems can monitor the light inside the optical fiber and act accordingly. Process systems can monitor the signals from the process and give feedback to the control and qualification system to compensate for changes in the process. A second step to take is to build up a closed loop regulation system around the sensors for compensation and active regulation.
a ΔT (deltaT) was introduced by Optoskand in 2001 and is a differential measurement in temperature between inlet and outlet of the cooling water to the optical fiber connector. b Controller Area Network, an industrial communication bus protocol
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2 INTEGRATED ACTIVE SENSORS
The fiber optical design from Optoskand is complemented with a number of sensors integrated into the connector to secure, maintain and help the user make transmission easy. The sensors used inside the connector are there to identify and lower the losses when coupling the laser beam into the optical fiber. The target for the sensors is transmission optimization and to reach that target, the sensors primary function is to identify losses in any shape and form. Working with lasers, the losses will manifest as light‐losses or an increase in temperature inside the connector.
Inside the connector Optoskand has integrated four sensors to detect these kinds of losses. The four sensors are:
1. Photodiodes that will detect what intensity and wavelength of light is inside the connector. A wide spectrum can be detected using photodiodes in the range UV, visible light and IR.
2. ΔT sensors, which measure the difference in water temperature between inlet and outlet cooling water to the connector. An increase in coolant temperature will indicate an increase of losses inside the connector. The sensor can measure temperature differences down to 0.01°C.
3. Absolute temperature, which gives status information about general condition of the connector.
4. Humidity sensor, which gives information about environmental climate inside the connector.
Apart from using the sensors for optimization when coupling the light into the optical fiber, the sensors also have the advantage of giving good feedback from the application to the control system. This feedback can either be used as a quality control or be implemented into the system as a closed loop.
To understand and argue for the philosophy behind implementing active photo‐sensors inside the optical fiber connector, the type of light existing in a system during action needs to be presented.
2.1 Light‐signals to be detected
To distinguish between different types of light in this report, a nomenclature is needed. The light generated by the laser source and going in the forward direction away from the laser source is called the “beam‐light”. The light going in the other direction towards the laser source from the application is called the “process‐light”. The process‐light consists of a wide spectrum of light, which is more or less limited by the transmission cut‐off from the quartz material used in the lens design and in the optical fiber. The wide spectrum is due to the material used in processes and how well the application is set up. Laser light not absorbed by the process will be reflected back through the optics and back into the optical fiber cable. This light is part of the so‐called process‐light but originates straight from the beam‐light, which is not absorbed in the application.
Figure 1. Schematic view of an optical fiber cable with two connectors in a process where light path is shown.
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2.2 Alignment and Protective system
It is difficult to arrange a perfect coupling of light into the fiber. In practice there are always losses associated with coupling and these losses can be detected either primary as light or secondarily as heat as soon as the power is absorbed by the surrounding material. These losses can be used when aligning the fiber and they can also be used to protect the system in case a failure occurs. During alignment a minimum signal from the light sensors will indicate a “well‐aligned fiber”, whereas a high signal will indicate losses around the in‐coupling. Apart from the signal from the photodiodes, there is also a ΔT sensor (see figure 2) implemented to detect the power absorbed by the surrounding
material and the cooling water. This ΔT sensor has got a resolution of 0.01C, which is in practice means that losses around 0.7 W are visible for the sensor using the flow rate 1.0 l/min.
Figure 2. Sensors inside the optical fiber connector.
By monitoring the losses in the connector, a protective system can be built up around the optical fiber. Light, which is not expected can be detected quickly and used to shut down the system before an accident occurs or to protect the system from damage. The four different sensors are all mounted on a circuit board together with a micro controller. The micro controller is able to switch off the safety system if any of the sensor signals should rise above the set threshold level. (See further chapter 3.)
2.3 Process feedback to the system
The active sensor signals can also be used for process monitoring. The process light returning from the application contains useful information, which can be used for quality control or to be implemented for a closed loop regulating system. Raw data values can either be sent out via the communication interface or be stored at the board inside the connector for future analysis. Through the communication interface, the signals from the sensors can be analyzed on the fly and compared with previous results. The quality of the process can be secured by using the sensor information implemented as a standard inside the Optoskand QD fiber.
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3 COMMUNICATION WITH INTEGRATED FIBER SENSORS
A threshold level can be set and stored on the board via a simple CANopen interface. Because the board is responsible for the comparison between the measured levels from the sensors and the threshold levels, the response time to shut down the interlock and the system is very fast and is done without any surrounding components.
The threshold levels can be set as absolute values or derivative levels. If signals from the sensors change too fast, the system can react to that change and switch off the system. (See figure 3)
Figure 3. Communication to the electronic board inside the optical fiber connector and threshold levels.
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4 EXPERIMENTAL
The experiments presented below will show how the photo sensor signal can give feedback to the system depending on the parameters set for an application and its properties.
The tests made in this report were performed together with Permanova Lasersystem AB in their application lab. A QD fiber (European automotive standard) from Optoskand was installed into an optical tool from Permanova. The optical tool consisted the following: Collimating lens with focus f=200 mm, focus lens with focus f=200 mm mounted inside a motorized focus unit, crossjet.
The tool was mounted on an ABB robot and the QD fiber was installed into a 2‐way beam switch, which was connected to a 6 kW IPG laser.
By adjusting focus positions in Z‐direction and adjusting power from the laser as well as speed from robot, the sensors are expected to give good feedback to the system with reference to the adjustments made.
Two tests were performed and during both these tests, the results from the photo diodes are of main interest for fast process feedback. The water flow inside the fiber connector at the output side was during the tests always 2.4 l/min.
4.1 Welding sheet metal together with a straight line
The first test made consisted welding two metal sheets together with a linear weld seam. Several trials were executed with different materials and the parameters to vary during the experiment were:
Power from the laser, 1 kW to 2 kW
Speed on the robot, 2 m/min up to 12 m/min
Focus position on the two sheets, 0 mm (in focus) to +12 mm
Different materials to process; galvanized steel sheet, black steel sheet and UC‐bor‐steel sheet.
4.2 Welding sheet metal together using a sophisticated pattern
The second trial a specific pattern (see figure 4) was used to weld the two sheets together. The pattern consists of eight corners, long and short straight lines and turns. Similar to the first experiment, the same parameters could be changed to achieve different results, which will give variations in the signals back to the sensors. When the robot follows the pattern presented in figure 4, it will have to reduce speed in order to turn around the corners. In the last straight line between point 8 and STOP position, the speed will increase. The change in speed along the pattern is expected to be visible in the photo sensor signal.
Figure 4. Pattern used in the second experiment.
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5 RESULTS
The results from the experiments described above will prove how the sensors can be used. The results below show clearly how the signals from the photodiodes give a good feedback to the system where the performance can be concluded.
5.1 Welding sheets together with a straight line
The figures below (figure 5 and figure 6) show two results where:
Figure 5 shows two sheets welded together. The material is UC‐bor‐sheet 1.3 mm and they were bonded together using 2000 W from the laser. The robot speed was set to 2 m/min and the focus position was adjusted to the top surface of the top sheet (perfect in focus). From the backside it is possible to see good constant penetration.
The feedback signal from the photodiodes located inside the both connectors (input side and output side) are presented in the diagram (see figure 7). The signal from the diode on the circuit board inside the output side connector is stable around 0.13 during the welding process.
Figure 6 also shows two sheets welded together. The same material is used (UC‐bor‐sheet 1.3 mm) and they were bonded together using the same parameters except the focus position, which in this case changed to +6 mm over the sheet. The result shows no penetration to the backside.
Looking at the diagram in figure 8 and the photodiode signal from the output side connector it has increased compared to the signal in diagram in figure 7. The same parameters were used for these two experiments except for the focus position in Z, which has change +6 mm. The photodiode signal has increased from 0.13 to 0.15 where it is stable during the process. No penetration is found on the backside of the sheet.
Figure 5. Good penetration. Sheets bonded well together. Figure 6. Not penetrating. Weak bonding between sheets.
The signals from the diodes are of great interest and when the welding results from above are compared there is clearly a difference between the feedback from the photodiodes referring to the penetration in the welding between the two experiments. Similar results were achieved for the other steel sheet metal materials.
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Figure 7. Result from figure 5 Figure 8. Result from figure 6
5.2 Welding buckled sheets together with a straight line
To evaluate if the photodiodes were able to see imperfections in the welding, a buckle was made on purpose. The sheets have some distance between them due to this buckle. During this experiment the two galvanized buckled steel sheets were welded together with a straight line. The power from the laser was 2000W, the robot speed was 2 m/min and the focus position was placed +3mm.
Figure 9. Buckled galvanized steel sheets welded together with a straight line
Figure 10. Photodiode signals when welding two buckled galvanized steel sheets welded together with a straight line.
Figure 10 shows how the imperfection from the steel plate affects the signal from the photodiode. The photodiode inside the exit side connector is far from stable and gives a good feedback of the imperfection from the welding result.
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5.3 Welding sheets together using a sophisticated pattern
The third experiment was carried out to see if it was possible to record a pattern from the photodiode signal and compare this to the robot program. The same pattern was executed twice using the same parameters except for the focus position, which in the second experiment changed from 0 mm to +3 mm.
1. UC‐bor‐sheet, 1.3 mm, 2000 W, 12 m/min, Focus position = 0 mm
2. UC‐bor‐sheet, 1.3 mm, 2000 W, 12 m/min, Focus position = +3 mm
During the first trial above, the penetration was good all the way through the pattern. The second trial however showed only limited penetration around the corners. Figure 11 and figure 12 below shows the result of the second trial. Figure 12 shows the backside of the metal sheet and it is possible to see only some penetration around the corners.
Figure 11. Front side of the metal sheet.
Figure 12. Back side of the metal sheet. Weak penetration in the corners.
The signals from the photodiodes in both trials are presented in the diagram in figure 13. The difference between the two trials is obvious. The first trial, which consistently penetrated the metal sheets, shows a stable, flat and low signal. The second trial however is very different. Lots of information can be drawn from this trail. For example, it is possible to calculate the number of corners in the pattern used.
Between point 3 and point 4, there is a long turn in the pattern, which is clearly visible in the diagram below. Between point 8 and the STOP position, it is possible to see the robot increasing and then decreasing speed while processing the long straight line.
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Figure 13. Shows two trials. The first in focus with good penetration and the second with only weak penetration, +3mm out of focus.
6 CONCLUSIONS
Above Optoskand has presented a solution with active sensors integrated inside the optical fiber connector. The sensors are monitoring the light, differential temperature, absolute temperature and humidity and communicating these values through a CANopen interface. The board inside the connector is a stand‐alone board, which works independently from any external accessories.
An important task for the circuit board is the possibility to shut down the interlock circuit for the optical fiber in case of abnormal values from the sensors. It is possible to download threshold values from the CANopen interface to board inside the connector. The board can compare the threshold values against the sensor values. If the sensor values are higher than the threshold values, the board can be set to switch off the interlock system.
It is above clearly shown that the sensor values communicated through the CANopen interface can be analyzed by external accessories and give a very good feedback to the system for quality control. The sensor values can also be a part of a closed loop regulating system, which then can secure the process for the user. The feedback from the sensors gives a good feedback to the system even if the process result shows very small differences. If a welding is made slightly out of focus, the sensors will give a good feedback about the situation. If there are imperfections between two welded parts, this will be visible for the sensors as well as welded patterns, which can be recorded and later compared with similar applications made over time.
When aligning the optical fiber in a laser system, the sensor values will detect any losses around the incoupling and help the user to optimize the fiber for best possible position. This is part of the low losses and industrial optical fiber design introduced by Optoskand.
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Authors: Ola Blomster, Mats Blomqvist, Hans Bergstrand, Magnus Pålsson ” High‐power fiber optic cable with integrated active sensors for live process monitoring”,
Proceedings of SPIE (2012)
Copyright 2012 Society of Photo‐Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or
modification of the content of the paper are prohibited.
Article authors:
Ola Blomster*, Mats Blomqvist, Hans Bergstrand, Magnus Pålsson Optoskand AB
Aminogatan 30, SE‐431 53 Mölndal, Sweden
* [email protected]; phone +46 31 706 27 63; fax +46 31 706 27 78; www.optoskand.se