Author Topic: problems with calibration  (Read 17358 times)

Gopher

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Re: problems with calibration
« Reply #15 on: October 19, 2011, 10:39:17 PM »
Yes I know the RV uses open loop steppers, and needs limit sensors on the  arm and up/down axes, but rotation is always relative to the position the head is in when it picks up the tool, and has continuous 360 deg rotation, so why would you need an absolute reference for it?

Hmmm a hangover from a previous and subsequently altered design decision? Could it somehow make the maths easier when taking into account the robot arm choice rather than the considerably saner Cartesian one?
Is it just me or has rotational accuracy always been an RV weakpoint?

Mike

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Re: problems with calibration
« Reply #16 on: October 20, 2011, 12:02:04 AM »
Yes I know the RV uses open loop steppers, and needs limit sensors on the  arm and up/down axes, but rotation is always relative to the position the head is in when it picks up the tool, and has continuous 360 deg rotation, so why would you need an absolute reference for it?

Hmmm a hangover from a previous and subsequently altered design decision? Could it somehow make the maths easier when taking into account the robot arm choice rather than the considerably saner Cartesian one?
Is it just me or has rotational accuracy always been an RV weakpoint?
I think it uses a standard stepper, which gives 200 or 400 steps/rev depending whether it can half-step, and because of the arm arrangement there is a potential placement rotation error up to half a step either way (probably a little more due to accumulation of step errors on the two arm motors) as even if part isn't rotated relative to pick position, it will always be rotating to compensate the arm angle. I'd guess you'd only notice this on larger fine-pitch packages - rough calculation a QFP144 could have an error of about +/-0.2mm assuming 400 steps.
Whenever I watch the RV running I think about what a major headfuck it must have been for the software guy to figuring out all the corrections needed to get a part the same way round anywhere on the bed. Then again maths was never my strong point!

smt-user

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Re: problems with calibration
« Reply #17 on: October 24, 2011, 04:00:00 PM »
Hi

I manage to solve the problem .As it seems two connectors were switched between them . So the pickup tool sense gor +12 volts and the cameras were not getting
any power at all .I switched and it works ok now . To finalize after a lot of checking there are 4x 4pin headers in the arm .The first hold z axis photo switch , the second the
pick up tool sense . The third the rotation and the fourth  power to the cameras and leds.
   

Gopher

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Re: problems with calibration
« Reply #18 on: October 24, 2011, 04:47:49 PM »
Well done, having changed the arm board in mine I'm not sure the cables were long enough to accidentally switch them, I guess yours must be that tiny bit longer.