Here is where I’d like to discuss chains or cable for different machine configurations, uses and price points. I am developing the Maslow further for the open source market and would like to have contribution of the community and build solutions that people with existing systems can upgrade to if they’d like.
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The need is to have very precise positioning (<0.5mm over a 3.5m+ distance
As a result, any errors in the chain/cable need to be predicatable so that they
can be compensated for in software.
chains ‘stretch’ as the gap between the pins and the holes are taken up.
cables stretch based on the tension on them
chains are run over sprockets, resulting in very precise positioning (but
requiring take-up on the other size of the sprocket)
cables are hard to grip to measure and usually are anchored to a drum. The
amount of cable that a given amount of rotation of the drop will move is going
to depend on a lot of things (how many layers of cable are then on the drum, how
much tension was the cable under when the layers were spooled up.
cables develop a ‘set’ when left in a given position for a lengthy time period
Tension on the machine ranges by a factor of 10x or more (on the stock maslow
this ranges from 3 to 30 pounds)
David Lang
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Will test out a couple cable drive systems at some point but for now a stronger chain is a good place to start. Cable would allow for a rough electrical length measurement similar to how CMM’s work. I do not know if chains would have a predictable resistance to length.
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Ooo electrical measurement is a cool idea. How accurate can that technique be?
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Will test out a couple cable drive systems at some point but for now a
stronger chain is a good place to start.
why do you think we need a stronger chain?
Cable would allow for a rough electrical length measurement similar to how
CMM’s work. I do not know if chains would have a predictable resistance to
length.
remember, we are looking at sub-mm accuracy, electrical resistnce varies by temp
(among many other things), do you really think you are going to get measurements
that precise? think about how precisely you would have to measure the time to do
so.
This is pretty out there but can you use signal reflection to find the length of a cable? Like send a signal into the cable and measure how off phase it is when it reflects back?
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I think you would get separate reflections from each joint in the chain. The signal could be pretty noisy. On the other hand, it is joints of chain we’re counting…
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Initially would be happy to get down to 1 mm but it is possible to get better than .1mm
With the concerns of chain breakage already expressed by others it would be irresponsible to look at using larger motors without upgrading the chain.
only instance of chain breaking I’ve heard about is when the sled attachment was assembled wrong.
note that the signal is moving at ~200m/us, and we are needing to measure <0.1mm
in length. that is going to take VERY fast electronics to do.
and it will be an extremely noisy signal to analyze, you not only have the links
showing up, but also your contact to the chain, and the fact that the signal
goes both ways in the chain (is the short measurement towards the sled or away
from the sled? you have no way to know for sure)
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are you talking about machine resolution or measurement resolution?
the limits to machine resolution right now are the softare and calibration, our
precision (repeatability) is much better than our target of 1/64"(~0.4mm), but
our accuracy is not that good. There are a number of people who are reporting
getting in the 1mm range on accuracy, but many others are struggling, and the
big effort right now is trying to solve this.
David Lang
sorry, upgrading the chains won’t help the type of breakage we have had
reported.
look at the pictures of the breakage, you could go with a MUCH heavier chain,
and the particular joint that broke would not be significantly stronger. It’s
not a joint that is supposed to have any stress on it, and in normal operation,
it is not at all important (in fact, having that piece free to rotate would be a
slight advantage in many cases)
We have just been using the chain wrong on everything except the top mount
triangulation kit.
David Lang
If you built an inexpensive TDR that you could put on each chain and run it in real-time, then you could switch away from chains to something else… say a light wire. At that point, it doesn’t matter if the cable stretches. But I don’t know the feasibility of building an inexpensive TDR that can give you sub-mm accuracy… Not too sure about the feasibility of building an expensive TDR either.
That’s why I was mainly thinking of using it for a cable based system. Looking into driving cables like a lead screw with anti twist. Or only use it for continuity check.
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The reasons not to go with larger chain are
-
it’s heavier, so the chain sag problem will be larger, and at some point we
end up with the weight of the chain applying more pull on the sled than the
minimum we want -
it’s bigger, so the sprockets are bigger. which means you need more powerful
motors, higher resolution encoders, etc (or at least higher gear ratios) -
it’s probably more expensive (although using bicycle chain may be slightly
cheaper). the cost of the chain is currently not a large part of the machine
cost, somewhere around $1/ft when purchased in bulk, so <$25
David Lang
Anyone know of chain that only rotates in a 180° arc like this?
Anyone know of chain that only rotates in a 180° arc like this?
September 12The reasons not to go with larger chain are
- it’s heavier, so the chain sag problem will be larger, and at some point we
end up with the weight of the chain applying more pull on the sled than the
minimum we want
- it’s bigger, so the sprockets are bigger. which means you need more powerful
motors, higher resolution encoders, etc (or at least higher gear ratios)
- it’s probably more expensive (although using bicycle chain may be slightly
cheaper). the cost of the chain is currently not a large part of the machine
cost, somewhere around $1/ft when purchased in bulk, so <$25
David Lang
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September 12With the concerns of chain breakage already expressed by others it would be irresponsible to look at using larger motors without upgrading the chain.
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Since sag and flexibility act in different directions we may be able to brance and treat the moment arm as a solid while still having the flexibility of chain.
you mean something that will curve in one direction but not the other?
that exists, but is a specialty product that is a lot more expensive than plain
roller chain.