And high-performance laser scanners make the ROMER Absolute Arm an all-purpose 3D. These users appreciate how the ROMER Absolute Arm is designed.
I've worked with a CMM in the past, and I'm used to calibrating or qualifying various head angles (5-axis machine) before measuring parts. I would check each set of angles with a double-check program that would log erroneous data. However, I'm at a different company now, and they have this Romer arm. I understand that these devices are not highly accurate, but we're not getting the accuracy we need for the parts we're trying to inspect. I've created another double-check program that can be used to check repeatability against variable handling (wrist angle, arm angles, point positions). The results make me sad.My Question:Is it possible to calibrate various wrist angles before an inspection to tighten the accuracy threshold?Romer is an Infinite 2.0Software is PC DMIS (vs 11?).
That makes sense to me and that's what I was doing with the CMM. But, the technical focal for the Romer says there is no need for a calibration because 'it knows' where the probe is since every joint is a 'known' length. At the very least, there is a 'homing' routine you must perform when starting PC DMIS. This only consists of rotating each joint (not even to their zero's) randomly. It would make sense that every time you restart the Romer, you need to find or double check the center of the probe tip. The trick to my question is where (if it exists) can such a routine be found and executed for a PC DMIS machine.
To the OP:Based on the info that Hexagon has on their website, and bearing in mind that Hexagon handles both PC-DMIS and Romer, you should have absolutely no worries about the Romer arm's accuracy. As I've often said about a particularly bad musical performance, 'tuned at the factory'. Their claim is that no complex homing is required for initialization, and the absolute encoders always know where they are. What could possibly be the issue?;-)I don't think you'll get any support from Hexagon on your perceived need for a special calibration routine, since that would be flying in the face of their whole approach to marketing that system.You mentioned that you ran some repeatability study on the arm you have. Which model/size is it, and what results did you get? That would be entertaining, I would guess.
Are you seeing results that are out of the specifications for the system, or are you hoping it's better than spec? I suspect that you will need to come up with something on your own, which might involve some long gage standards for checking extended distances within the volumetric envelope. @Josh:The romer does have interchangeable tips, they screw into the wrist. So, I guess that is a 'known' as well.@specfab:The romer just came back from an official calibration, we checked it with the calibrated bar and it was within the same values we got back from the lab.I checked repeatability using a known sphere. It seems that accuracy is most hindered by the compilation of multiple points to a single feature. In other words, feature-to-feature seems to be accurate (so long as the feature was measured correctly). This gets into basic measurement/software training, ensure that feature-points are taken with relatively the same arm position and avoid changing wrist angles.
Based on my results I think that the physical hit-taking is the weakest link, then any remaining error is how the software interprets the points.My results on our Romer Infinite 2.0 (4ft reach):Repeated measurement of the same feature, same wrist angles (both fixed) Total Deviation = 0.0005' +-0.00015'Repeated measurement of the same feature, first wrist joint fixed, and the second wrist joint incremented Total Deviation = 0.007' +-0.0015'Repeated measurement of the same feature, random wrist angles, Total Deviation = 0.011'In all reality, the fixed position repeatability is fairly tight. However, 1) it doesn't comply with the 1/4th of your lowest tolerance rule of thumb for our shop (+-0.00025'), 2) fixed position angles aren't always an option for a single feature (5-axis machine work).After finishing the tests and trying to think of a solution, I came up with the idea to add a programmatic 'check' in PC-DMIS to verify that the vectors of the taken hits match (close enough) the vectors of the theoretical hits.
So, I came up with about half of the code and thought it would be cool to have some kind of LED indicator that turns on or turns green when the correct vector is reached. It could be done with the VBScript option in PC-DMIS and an Arduino, but then I realized it would be much easier to just get a CMM that takes the points at the right angle and can be qualified before every part.
Tbm0115, your observations are just another reinforcement of the 'bad idea' characteristic of using rotary encoders at the vertex of an angle of rotation with a substantial length lever arm attached. Your remediation approach sounds like it's about the best that you'll be able to manage for unrealistic performance expectations. As you note, using a real CMM might get you where you want to be, although making that case to management, especially to management that may have been involved in the Romer purchase, may be a tough sell, especially at the likely $60-100K price tag. Good luck with that;-). Although making that case to management, especially to management that may have been involved in the Romer purchase, may be a tough sell, especially at the likely $60-100K price tag. Good luck with that;-)We're actually pretty lucky here.
Management has a pretty rational approach to machine purchasing now (previous owner retired). The trick is fitting a CMM into this year's budget. We'll get there, but the first step was determining whether we could 'get by' with the Romer which doesn't seem plausible if customers require complex inspections. Now I at least have something to prove the arms worth.Anyone looking to buy a Romer arm? (kidding, I don't have that authority). I was just at Hexagon for a demo of a Romer.
I don't do inspection and metrology for a living so feel free to take this with a large grain of salt.The interest was in the scanner attached to the arm to inspect smaller plastic parts max 3' x 5' x 1' trying to verify tat the mold maker gets the geometry within the general overall tolerance. Lots of points vs manually picking individual, it was fairly impressive and easy to use. You can also use probes instead of the scanner. If your looking for sub 001 inspection this is probably not the right tool and they freely admit that, also the variability will be dependent on how much you move the arm - work within a small envelope like a 5' cube and your probably ok to sub thousandth, but work the same envelope and flop the joints of the arm around and you will get a larger error.As I said I don't do this stuff for a living but the lack of a home calibration does not seem significant. Mspformat pc tool box. I view this application more like a test indicator, your measuring from point a to be as the arm is portable.
So if the accuracy and repeatability is fine from point a to point b your good to go.Also got a demo on one of the vision measuring systems, those are good to a couple tenths. Seems like the application I am looking at could benefit from using both, general geometry verification using scanning with the Romer arm then automatics inspection of key areas with the vision system.They told me ballpark pricing on the small vision system is about $65k and the Romer arm about $75k. I was checking this for the company I work for, I came away wondering what outside inspection houses charge - buy these myself and start subcontracting inspections.No connection with Hexagon outside of just visiting for a demo.Paul. @Josh:The romer does have interchangeable tips, they screw into the wrist. So, I guess that is a 'known' as well.@specfab:The romer just came back from an official calibration, we checked it with the calibrated bar and it was within the same values we got back from the lab.I checked repeatability using a known sphere.
It seems that accuracy is most hindered by the compilation of multiple points to a single feature. In other words, feature-to-feature seems to be accurate (so long as the feature was measured correctly). This gets into basic measurement/software training, ensure that feature-points are taken with relatively the same arm position and avoid changing wrist angles. Based on my results I think that the physical hit-taking is the weakest link, then any remaining error is how the software interprets the points.My results on our Romer Infinite 2.0 (4ft reach):Repeated measurement of the same feature, same wrist angles (both fixed) Total Deviation = 0.0005' +-0.00015'Repeated measurement of the same feature, first wrist joint fixed, and the second wrist joint incremented Total Deviation = 0.007' +-0.0015'Repeated measurement of the same feature, random wrist angles, Total Deviation = 0.011'In all reality, the fixed position repeatability is fairly tight.
However, 1) it doesn't comply with the 1/4th of your lowest tolerance rule of thumb for our shop (+-0.00025'), 2) fixed position angles aren't always an option for a single feature (5-axis machine work).After finishing the tests and trying to think of a solution, I came up with the idea to add a programmatic 'check' in PC-DMIS to verify that the vectors of the taken hits match (close enough) the vectors of the theoretical hits. So, I came up with about half of the code and thought it would be cool to have some kind of LED indicator that turns on or turns green when the correct vector is reached. It could be done with the VBScript option in PC-DMIS and an Arduino, but then I realized it would be much easier to just get a CMM that takes the points at the right angle and can be qualified before every part.@OP: Your Romer arm came with a length standard bar (it should be in your arm case). It's a metal bar with 3 'cups' or conical seats on it.
You use this to verify the calibration of the arm at whatever frequency you wish. Daily, weekly, monthly, etc.) All you need to do is put the 15mm ball probe on (that's the master probe that they use at the factory to actually calibrate the arm with - the other probes are then calibrated against this master probe). After inserting the 15mm ball probe, open up your Cimcore Arm Utilities (should be on your desktop). Select 'Length Checkout' from the top menu buttons. Measure the LONG length of the bar 10 times- the 2 cups near the ends of the bar, not the one in the middle - one point on each cup, going back and forth between them.
(one point on left cup, one point on right cup, then back to the left cup, and back to the right cup - you'll have measured 5 points on each cup by the time you're done. When you're done, it will come up with a window that says how accurate and repeatable your arm is. You can do this test as you said, with a 'Fixed wrist' orientation (you'll get much better numbers this way) or with 'multiple wrist' movements (Flipping wrist over, 45 degrees, 90 degrees, spinning, etc.).
The Range/2 number that comes up is basically your 'volumetric length repeatability', or +/- repeatability. (Not to be confused with accuracy!!!) You also need to look at the label on the bar which will tell you the nominal distance between the two conical seats you measured. Compare that number with the Minimum and Maximum lengths measured, and that will give you your +/- ACCURACY. Think of a target for a rifle - you take 10 shots, and they are all within.25' of each other. GREAT REPEATABILITY - well for a rifle anyways.
HOWEVER, lets say that group of 10 shots is a FOOT away from the bullseye. SH!TTY ACCURACY! You're repeating your shot every time, but you aren't hitting the bullseye.
Don't confuse the two. Anyhow, that's how to check your Romer arm. Any further questions, just PM me.Cheers! XNoticeThis website or its third-party tools process personal data (e.g. Browsing data or IP addresses) and use cookies or other identifiers, which are necessary for its functioning and required to achieve the purposes illustrated in the cookie policy.
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