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calibrating a vision system

bigtallanddopey
Our company has just bought a second hand vision CMM. It is basically a small (500mmx400mmx350mm) bridge style CMM with a camera on it. Not pcdmis or hexagon related at all.

We recently had the machine calibrated where the company (machine manufacturers - Baty) came in and calibrated it to there own standard. They also calibrated it using a small 50/60mm wide plaque with shapes on it at different positions on the cmm bed. All measurements apparently came out ok and they have provided a certificate.

The problem I have is that when I compare it to a touch trigger calibration they didn't check the measurement volume of the machine. There was no measurement from point X on one side of the machine to point Y at the other side like you would get when a KOBA bar is used. I know that 95% of the time we will only measure O rings etc, but if we were to measure a part with two holes in it that was 500mm long would we know that the measurement is accurate?

Has anyone got any experience with the calibration of vision systems on a CMM? is there an international standard that should be being followed?

Any answers would be great.

B,T & D
  • I have an Optiv Performance 1z443, a dual vision/touch system.

    We were given a standard from Hexagon; it's a stand, with a glass calibration plate, a 20mm ring, and a gage ball artifact screwed into it.

    Here's my process for calibrating (are you familiar with standard calibration procedures?)

    First, I calibrate the camera probe offset on the ring, choosing YES the tool has moved.
    Next, I'll calibrate a probe I don't use for anything else, a 4x10, on the ring, RESET TO THEO'S, choosing NO the tool has not moved.
    Using that same probe, after calibrating on the ring, I will calibrate the sphere, DON'T RESET TO THEO'S, choose YES the tool has moved.
    After that, calibrate every tip on the sphere, RESET TO THEO'S, choose NO the tool hasn't moved
  • I have an Optiv Performance 1z443, a dual vision/touch system.

    We were given a standard from Hexagon; it's a stand, with a glass calibration plate, a 20mm ring, and a gage ball artifact screwed into it.

    Here's my process for calibrating (are you familiar with standard calibration procedures?)

    First, I calibrate the camera probe offset on the ring, choosing YES the tool has moved.
    Next, I'll calibrate a probe I don't use for anything else, a 4x10, on the ring, RESET TO THEO'S, choosing NO the tool has not moved.
    Using that same probe, after calibrating on the ring, I will calibrate the sphere, DON'T RESET TO THEO'S, choose YES the tool has moved.
    After that, calibrate every tip on the sphere, RESET TO THEO'S, choose NO the tool hasn't moved

    that's more probe/camera calibration though isn't it? We don't have a glass plate here (that I know of) so I may need to get one to do checks on, at the moment I just check a ring gauge.

    I was more trying to get at, what do Hexagon do when they come in and do there annual calibration? when we had our company calibrate they have measured something with the camera and basically said that the machine is measuring OK. But all they did was measure a small object, I could do that with a ring gauge and say the machine is operating OK.

    However when Hexagon calibrate our large CMM they measure things across the whole machine envelope at different elevations and angles etc.
  • Hmm I see what you're asking. I will do my best, as I don't nor ever have worked for Hexagon, only watched them work.

    We have a "Hexagon Optical Calibration Standard", P/N 25-1234-02 on our calibration plate.

    They screw the plate in and perform a lot of measurements using native PC-DMIS on this standard; as to what they are doing, they have one or two specialists that work with mostly (or strictly) vision systems; I believe these one or two individuals would hold more information on what they do.
    I'm not exactly sure they'd be keen on giving out their secrets, but when someone is here to calibrate it they are quite informative...
    The man left his signature on the calibration cert; I can't legibly give you a name unfortunately...

    As for the latter, they have lasers and a bunch of other uber-cool measurement devices to calibrate; perhaps because of how the camera operates compared to a touch probe, there's no need for laser calibration, accounting for the size of pixels and such..
  • unless you specifically ask for it or you have a 443 with a scanning probe, they typically don't do a volumetric test. Ex. when they calibrate my Optiv 321, they only do an X-Y scale test and don't bother with the touch trigger probe.

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