The document describes the design of an upgraded dual-head LightGage Metrology System that can simultaneously measure both sides of a part and report thickness. Key aspects include using invar posts and vee pads to minimize thermal drift, pneumatic isolation for vibration stability, and Dyadic linear actuators integrated with TMS software for automated coarse motion and positioning of the dual sensor heads.
2. [3]5 V=11.8k45k+11.8k24 V<br />The basic functionality implemented by the TMS software includes the ability to set the actuators to move at one of three preset speeds, move to a relative position, move to an absolute position, turn the servos on/off, home the actuators, and finally to receive feedback from the controller. The actuator controllers are able to report their absolute position, on/off status, whether motion is in progress or completed, and a variety of error codes which could be implemented by the TMS software to respond accordingly. These error codes include the deactivation of the ILK line, indicating that the emergency shutoff has occurred. All of this functionality is used to write scripts in the TMS software, which are in turn used to control the timing and motion of the actuators.<br />scripts<br />Tropel’s TMS software package is very flexible, so integrating the Dyadic actuators into the measurement routine was very straightforward. In the “recipe” for each measurement, a script operator is inserted, which calls a desired function within a VB script defined by the user. The software then runs the appropriate function when the operator is called—either before or after the measurement (the user decides). For this project only 2 scripts were written: one that raises the head before the measurement, and one that lowers the head once the part has been placed in the tool. The basic order of operations during a measurement is as follows:<br />1. User presses Measure Button<br />2. Head to Load Position (send motor collection to absolute position 0). This raises the top head to the load position so user can place part in tool.<br />3. User is prompted by a message box to place part in tool. User must press OK to proceed.<br />4. Head to Measure Position (send motor collection to absolute position 95). This lowers the top head onto the invar posts, allowing the actuators to retract so they are no longer in contact with the lift plate.<br />5. User is prompted once again to verify that fringes are present in the live video. User must press OK to proceed.<br />6. System Measures Part.<br />7. Head to Load Position. Top head is once again raised after the measurement so that the user can remove the part.<br />The beauty of TMS is that the user can very easily tell the software to turn off all motions. So if the user would like to measure the same part multiple times, they can simply disable the script operators and the motors will not move the next time a measurement is taken.<br />fine motion: micrometers<br />While the customer had originally desired an entirely automated system, budget and time constraints meant that the fine adjust portion of the head motion would have to be manual. Because the instrument is so sensitive, in order to finely adjust the head position such that fringes could be found, it was necessary to use micrometers—the most sensitive manual adjustment method within the team’s means. Thus, in order to ensure that the two fizeaus were parallel with one another, the user has to adjust the micrometers manually to find fringes. Originally it was thought that this would be challenging, as each fringe represents roughly 0.4 microns of tilt, meaning that in order to see fringes, the system would have to be within 5 microns of parallel. However, it was found that by setting all three micrometers to the same height (ruling), one was able to find fringes by simply moving the left and right micrometer up and down slightly. This is due to the system components being very precisely fabricated.<br />part detection: keyence sensor<br />While using actuators significantly simplifies the operation of the system, it poses another problem: what if the system is not set up correctly and the actuators crash the top head into the part? This situation would prove very costly, as both fizeaus would be damaged and need to be replaced. Thus, in order to solve this problem, the team decided on a method that has been proven by both Tropel and industry in general: a Keyence part detection sensor. Conveniently, Tropel used this exact method on another tool and the team was able to acquire the thrubeam mounts and fibers from Tropel. Dr. Raisanen had a spare Keyence sensor that he allowed the team to use, allowing us to complete this subsystem without using up any of our budget. <br />The sensor is wired through a relay directly to the motor controller kill switch. Both the sensor and the actuators run on 24V, so it was possible to configure the sensor in a way that when the beam is clear, 24V is flowing through the sensor to the motors; basically the system operates as if the sensor is not there and the actuators are allowed to move as instructed. However, when the beam is broken, the sensor switch is tripped and the voltage to the actuators drops to zero, causing the motors to perform maximum deceleration and cancelling all motion before the head can strike the part. The thrubeam is mounted 4mm below the top fizeau in order to give the actuators sufficient distance to stop.<br />Figure 13: Keyence Sensor DiagramFigure 14: Sensor Assembly<br />Results and discussion<br />final product<br />As written in the problem statement, the final deliverable for P09701 is a working prototype of a dual-head LightGage system for use in Tropel’s engineering and applications research. The product being delivered to the customer is capable of:<br />1.Characterizing both sides of a part’s surface over the period of one measurement.<br />2.System features actuators that lift the top head between measurements so that user can place or remove part from system. <br />3.Kinematic mounts ensure system returns to same position after lifting for every measurement.<br />4.Motions are fully integrated into TMS—no external software is needed to make system operate. It can be used just like any other Tropel Metrology Instrument.<br />5.Actuators have integrated motion controllers, connect to computer via USB, and run on COTS 24V power block. There is no external motion controller and no need for an expensive power supply.<br />6.Finding fringes is simple using micrometers, and only need to be adjusted for first measurement. There is no detectable drift in the micrometers.<br />7.Invar posts reduce the possibility of thermal drift. There is no detectable thermal drift during the measurement.<br />how it works<br />Though it has been mentioned previously, the system is very easy to operate. The measurement process has been made as simple as possible for the end-user. The measurement process is as follows:<br />1.User opens DefaultLG_P09701.rcp recipe using TMS and presses Measure button.<br />2.System will lift top head to load position. If head is already lifted, it will stay there. User is then prompted to place part on system. User must then press “OK” button to continue.<br />3.Top head is then lowered to measure position. Top head is now resting on Invar posts and lift pads have disengaged from the lift plate. User is then prompted to verify fringes are present in live video. If no fringes are present, user must find them, and then press “OK” button to continue.<br />4.Part surfaces are measured sequentially—bottom surface is measured first, and then top surface.<br />5.When data collection is complete, top head lifts back up to load position so part can be removed.<br />6.Data is processed and displayed to user. Measurement complete. The measurement process takes about two minutes from start to finish.<br />Figure 15: Scan Results<br />how we know it works<br />fringes<br />Building a dual head LightGage system as proposed includes two variables (risks) that could not be modeled before assembly: micrometer drift and ambient air mixing effects. While it was known that the invar posts would provide the needed thermal stability in the support structure, it was not known how much, if any drift there would be in the micrometers, especially after lowering the top head to the measure position. Lubricating fluid in micrometer threads can be especially frustrating, as it tends to settle when loaded—that is, the fluid migrates up the threads, causing the micrometer to continue to settle for a period of time. During this time, any measurement would be meaningless. Furthermore, because the LightGage is sensitive to thermal effects, there was no way to quantify the amount of unmixed air in the room (air currents of different temperatures swirling in the room), which could also negatively affect the measurement data.<br />The micrometer drift was to be explored by placing a weight on the tips for a period of time and then measuring the difference in the micrometers after the weight has been added. The desired specification of overall mechanical drift for the system was stated as 6nm. This would require the team be able to accurately measure the micrometer length and change down to the nm level. This is nearly impossible and infeasible for the team given that the equipment available was not accurate enough, nor could the measurement be outsourced due to the budget constraints. Therefore the amount of drifting done by the micrometers, or the system in general, could only be tested once the system was completed.<br />To test this design’s viability, the only solution is to take a measurement using the top head and qualitatively analyze the measurement data. The top head is used for this verification because the upward looking head design has already been commercialized by the Customer and is known to work under various conditions. The downward looking head is the challenge, as it is mounted more than 100mm above the bottom head, and keeping the two fizeaus perfectly parallel is crucial. As can be seen below, it is indeed possible to take a reasonable measurement using the top head.<br />After a verification of the top head’s ability to see a part below and have fringes available the drift was addressed. A very simple way to qualitatively see drift is to set the micrometers such that there are 3 to 5 fringes showing in the live video. If the fringes move to one direction or another after a period of time there has been a significant mechanical drift. For perspective, if the fringes move one fringe to the left or right (0.4 microns), the top head has drifted nearly 5nm. It can be noted that the fringes in the top head live video did not move and the micrometer drift was acceptable. <br />Though it cannot be shown in this paper, the effects of air mixing on the system can be easily seen in the live video as well. If the room is properly ventilated, the fringes in the live video should not move. However, if there is a current of warm air circling the room, the fringes will “dance,” looking almost like waves on a pond. In this case, either the system should be moved into a more stable place, or an enclosure should be put around it. This depends mainly on where the machine is to be placed.<br /> repeatability<br />The system designed will eventually be sold to manufacturers that will be able to use it continuously with few adjustments. To achieve this level of use the lifting of the top LightGage head must be repeatable to avoid continuous changes to the micrometers. <br />To attain a maximum throughput for the measurements in a day the lifting action of the motors must be synchronized and smooth. This will ensure the same placement of the top assembly each time and therefore have a greater probability of consistency in measurements without readjustments every time. To check this a part can be loaded and measured, but not removed. The motors would then lift and settle as if a new part was being measured, but the same part would be measured for a second time. This measurement would be recorded, along with more measurements depending on the tolerance required by the customer. The measurements would be analyzed to assess their consistency of the measurements and problems in the system could be addressed and modified. Due to time constraints and the final acquisition of the last motor at such a late date, the repeatability tests were unable to be completed. These tests are suggested for further prototypes as this is merely a proof of concept. <br />Conclusion<br />meeting customer needs<br />The system that the group will be delivering to the customer is a working proof-of-concept of the dual-head LightGage system. It is able to measure both sides of a part during the same measurement cycle; it can be assembled on existing LightGage granite, making it possible to install as a bolt-on upgrade to single-head systems; and it uses low-cost, compact, and easy to set up actuators that have been fully integrated into Tropel’s measurement software package. The team has demonstrated that the system is virtually fool-proof, very easy to operate, and successfully measures both sides of a part. The product that the team is delivering is mechanically complete, and will be 100% functional once the customer works out bugs inherent in their system that only they have the ability to fix.<br />It is not known, however, how meaningful the measurement data is. There are various reasons for this:<br />1.The team is not familiar with the optical set up of the LightGage head. Adjustments were made to the alignment in the heads such that measurements could be taken, but it is not known whether or not this will affect the accuracy of the data collected.<br />2.Though familiar with the TMS software, the team did not have the depth of understanding or the capability to fix bugs that were found in the software. Thus, a true thickness measurement could not be taken because the software would not allow the user to input the correct part feature height. This problem was not resolved before delivery. The customer made it known that they were unsure what effect their more recent software revisions would have on our ability to get the system working properly.<br />3.The team was unable to reach an acceptable level of measurement repeatability due to the conditions inside the metrology lab. The temperature in the room was rarely stable—often ranging in temperatures between 65 degrees and 75 degrees, and various levels of humidity. Furthermore, the air currents constantly flowing through the room caused the fringes in the live video to “dance” further eroding any attempt at repeatable measurements. The customer, however, has appropriately environmentally controlled space where the system will reside once delivered. This should significantly increase measurement repeatability. <br />Overall, however, it is believed that the team is delivering a successful product. While the customer wanted a dual-head LightGage, it was just as important to them that they see how a team operates with very few resources. Indeed, the team had less than $3,000 to develop the next generation LightGage system (not including any Tropel-sourced LightGage hardware). The customer was interested in finding actuators that could be connected to a computer via USB without the need for an external motion controller. The team found actuators that fit their description and were able to power them using an off-the-shelf 24V power pack. Furthermore, we were able to deliver a very robust and cost-effective system that re-uses many of Tropel’s stock parts and uses a minimal amount of custom fabricated components. .<br />4.The system is not without fault. The ability to measure both sides simultaneously is not yet within reach. The software limits the action as does the brightness of each LightGage. The light from one head can over saturate the other and ruin a measurement. This requires the LightGage heads to measure one at a time in order to have useful output.<br />5.The system is operational and proves the concept of a dual-head LightGage; however, there are improvements that can be made. This system was created on a modest budget and with more funds could have been made to work better. The worry of ambient temperature fluctuations affecting the LightGage, for example could have been avoided by the creation of an enclosure that had the option of temperature control. This would eliminate the issue of temperature no matter the environment. <br />Vibration was addressed by the air-table, although previously addressed by a granite slab generally used for other LightGage operations. The advantage of the granite was the pre-drilled holes in a strategic pattern for the placement of fixtures for the actuators and base plate. If the pre-drilled hole configuration could be placed onto the face of the air-table in lieu of the pre-drilled grid, the system would be more stable and the dismantle/re-build would be more accurate and repeatable if moving was an issue. <br />The system does not need constant adjusting between measurements, although some adjustment via the micrometers is necessary. The adjustment is not always intuitive and takes some skill to know where to move the micrometers and which micrometer to adjust first. For a customer, learning this task can be time consuming and the time lost to a learning curve can be expensive. This can be avoided by the replacement of the manual micrometers and replacing them with motors. This automated addition would simplify the process for the customer and help to avoid lost time while learning a new process by allowing for software to automatically adjust the fine motion. <br />Small adjustments to improve the system include a better computer to run the software. This would ensure quick measurements and faster changing times. The hardware of the entire system should be of one type. Currently some parts are English while the majority is in metric. The material for the lift structure could also be made of a more robust material to avoid wear from bending under the weight of the top assembly when lifted.<br />Acknowledgments<br />Senior design team P09701 would like to thank Corning Tropel for all of their support, technically and financially. We would also like to express our gratitude for our advisor Dr. Raisanen and all of his help throughout the project.<br />References<br />[1] Thomas J. Dunn, “Frequency-Scanning Interferometry: Advances Precision Component Manufacturing and Assembly”, Photonics Spectra, June 2005, pp. 96-100<br />[2] Thomas J. Dunn, Nestor O. Farmiga, Andrew W. Kulawiec, Joseph C. Marron, “Frequency-Scanning Interferometry”, Corning Tropel White Paper<br />