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Topic: Comparing in minutiae relative cylinder speeds (Read 1 time) previous topic - next topic

2Tricky

This is my first post and I've been a member for 2 minutes.   I can't say that I have no concerns with my concept of Arduino but I am hoping that it might just be the vehicle to help in a way that other beers can't! 

I have some experience in programming and have some electrical/electronics under my belt but find that the programmer in me confuses the sparky and so feel like a complete beginner at times.  I have no Arduino 'stuff' (2 days ago I would have associated it with some natural weather phenomenon) and so would like a feel of whether my ambition is too great with Arduino in mind and if not where to start.  I love the idea of using old unloved parts that gather dust otherwise but am also aware that I might have to invest to get a reliable/useful result.

I have an interest in comparing the rotation motion (speed initially) of cylinders in a printing machine… why… because lines (imperfections) are clearly visible in the one-colour printed image and this testifies to differing motions of the cylinders concerned and/or potentially varying axis points (vibration or material thickness variation could in theory be responsible).  The minimum requirement is to compare 2 cylinders and nirvana would be around 15.  As I see it, the difficult thing here could be the resolution of the information required and the potentially 'odd' ratio of the relative RPM's.  I would like to conduct the monitoring throughout a cylinder perifferal speed range and for a number of cylinder diameters (so for the same perifferal speed setting, a cylinder that becomes half the circumference will have twice the RPM - but only one cylinder in the basic implementation of 2 could change diameter, though in the 15, there could be 5 that change diamater ).

Some figures:
Known-smallest-changeable-cylinder periferral speed: 3.048m (=  18.46 RPM approx. for smallest changable cylinder and 10 RPM for largest changeable cylinder)
Estimated-largest-changeable-cylinder periferral speed: 54.864m (=  332.31 RPM approx. for smallest changable cylinder and 180 RPM for largest changeable cylinder)

The required resolution of data is debatable in the way that the first picture of the Mars surface verified some important points.  However, I don't imagine that there would be a need to go beyond a reading per 10 microns perifery change.

Some less abstract specifics:
Known-smallest changeable-cylinder circumference: 6.5" (165.1mm)
Estimated-largest changeable-cylinder circumference: 12" (304.8mm)
A known constant-cylinder circumference: 11" (279.4mm)

In addition there is at least one axis that has no cylinder as such attached and where the RPM is perhaps a more realistic term.

For anyone who has been able to read this or even follow it thanks very much and of course I would be very grateful for feedback.

Thanks a lot.

PeterH

I'm envisaging a solution where each cylinder/axis has some sort of position encoder attached which sends a train of pulses to the Arduino, and the Arduino determines the average pulse frequency over some time period and calculates a surface speed from that. Is that the sort of thing you're looking for?

I haven't quite figured out what units you're working in. What sort of range of rotational speed are you going to need to measure? How precisely do you need to know the speed, and how often do you need to measure it? (Average per revolution? Average per hour? Average per degree of rotation?)
I only provide help via the forum - please do not contact me for private consultancy.

2Tricky

One thing that I haven't muddied the water with yet is encoder versus resolver i.e. digital versus analogue.  As I see it, a resolver is just a poor mans encoder but on the other hand, depending upon the AD converter, maybe not - because perhaps the precision (bits) of the AD converter can be selected to 'suit' the resolution required - I don't know.

Your view is how I see it too.  Because the machine is imperial (hence the cylinder circumferences mentioned) the units are 'naturally' inches but of course metric is easier to deal with.

I'll take a middle of the road scenario (4 axis) because it includes all different RPM's ratios.  For cylinder A, the constant relationships are:
A:B = 1:1
A:C = 88:52 (smallest cylinder), 88:96 (largest cylinder)
A:D = 1:4 (This is actually unknown and so this is an educated guess - though I intend to find out at the earliest opportunity)

As for how often does it need to be measured, well if I go for my 'no doubt high enough' scenario of 10 microns perifferal movement, at a modest machine speed (30 feet per minute), then for the various cylinders:

For all cylinder A, B and C: 10 microns pass in 0.0000656 seconds  approx.
D which is actually just a drive shaft (treated as a cylinder earlier to try to make it easier), has an RPM relative to A of about 1:4 (A:D).

PeterH I hope that this is clearer now but if it is still not clear enough, please just say.

Thanks a lot 2Tricky

Tumbleweed

2Tricky.
Are you saying the cylinders actually can actually change their diameter? On the fly, by replacing, or just change in diameter from one to the next. Knowing the rpm with out knowing the present diameter wouldn't do much good, might need some kind of sensor on the diameter too.
Possibly a barcode like scanner on the actual cylinder circumference?
Just thoughts.
TomJ
Einstein once said you don't really understand anything until you can explain it to your Grandmother

2Tricky

#4
Aug 12, 2012, 06:23 pm Last Edit: Aug 12, 2012, 06:43 pm by 2Tricky Reason: 1
Tom the cylinders don't change on the fly, they are changed from job to job by the operator and so this can simply (?) be put into a/the formula.  I can see that this is a bit difficult to grasp from just my text but the long and the short is, except for one shaft, it is the periferal speeds that need 'watching'.  This is of course intimately related to relative RPM's and so have little or no distinction from assessing a shafts relative RPM but never the less, the end of the rainbow is the peripheral movement.

As I said at the beginning, I suspect it is the odd relative RPM's that are potentially awkward (not forgetting the high resolution) but if it is a sticking point, we can talk about just one scenario such as a relationship of A:C of 88:52 and forget 88:96.  I will be happy testing just one and hoping that some useful information comes back.  Ideally I have in mind an oscilloscope plot (or data that I can plot later) where one cylinder can be seen to blip out of a relatively linear speed while others don't - this would indicate skid etc.

So the RPM's we now have are A:B as 1:1, A:C as 88:52 (although there is a lower denominator, it is based on gear teeth and so I've stuck with it) and if necessary we can forget the shaft (which as I say I reckon is about 1:4 for A:D and I'll check ASAP).

After writing the above, I realised that I have presented it wrongly - the correct A:C RPM ratio is 52:88 whilst the others are still fine.

I hasten to add, if the ratio 52:88 is horrible (and I suspect it is) I can make it simpler such as 64:88 for instance - I suspect it depends upon finding suitable denominators for encoder/resolver resolutions.

Tom thanks for your input 2Tricky

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