# Chain sag inaccuracy

I’m guessing the orientation of the chain would also play a role in the amount of sag. At the same length and tension, there should be more sag the closer it is to horizontal, right?

What I was trying to say earlier is that the current required by the motor will need to vary as the tension varies. In that case, we should be able to estimate the tension in the chain/cable at any point in time based on how much current the motor needs. I’m certainly not an electrical engineer though so I could be way off on this one.

I’m guessing the orientation of the chain would also play a role in the amount
of sag. At the same length and tension, there should be more sag the closer it
is to horizontal, right?

actually I’m not sure how much that hurts (although it may be that what matters
is the horizontal distance of the chain, not the total distance)

the bigger factor is that the closer to horizontal, the less force on it (or
more precisely, the closer to vertical the other chain is, the less force is
left to act on the long, close to horzontal chain)

it may be that both need to be taken into account.

What I was trying to say earlier is that the current required by the motor
will need to vary as the tension varies. In that case, we should be able to
estimate the tension in the chain/cable at any point in time based on how much
current the motor needs. I’m certainly not an electrical engineer though so I
could be way off on this one.

in theory yes, but since the worm gear prevents back-driving the motor, 0
current will tell you nothing about the tension on the chain.

you could possibly correlate the power to the motor and how much it’s moving,
but we’ve found that these motors are not that consistant, and the power to
generate a specific amount of force varies drastically based on how fast you are
moving (that’s the reason the PID loops were introduced, prior to that it tried
to generate tables to calibrate electrical power to motor movement)

The idea is a sound one, but we don’t ‘currently’ have a way to read the value.

Can you relate power to pwm frequency and percent, and compare it to actual speed from the encoder?

An interesting idea. That info is readily available.

This thread has gotten TLDR for me, so ignore this if it has been covered. Could you just add a wait measurement to the calibration prosses? Most people have a bathroom scale. Step on the scale with the sled and step on the scale without. Now you have the sled weight (within a pound or 2) which should be close enough to cover stretch conditions. Or am I wrong?

That would work. I use a fishing scale (never caught a fish as big as a Maslow sled, though ). I confirmed the accuracy on the bathroom scale as you described.

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the tension on the cables/chains is not just the weight of the sled, but also effected by the angle of your frame/workpeice the ammount of friction between the sled and the workpiece and the change of speed of the motors.
just saying…

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very true, but we know theangle, and speed (we don’t know the friction)

we assume that friction has no (or only momentary) effect on the position.
between the vibration from the motor and the effect of gravity swinging the
sled. We work to keep the chain angles such that there is sufficient force to
move the sled.

It’s seemed to me that the big unknown was the weight

this thread is making me reconsider how big an unknown that is, but until I get
some time to play around with a chain sag calculator, I don’t know.

@dlang beat me to it.

If we’re only looking at about 1mm of error due to chain sag and that is only at the very far corner I’m betting even a rough estimation of the wight would probably bring us to within our .4mm target pretty easily, right?

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if it is really so hard to calculate the chain sag, and it really makes 1mm of difference, couldn’t you add a piece that accurately measures the distance like the goliath?

So I can see it the chain sag it self is a minimal problem. The challenge is that the chain angle becomes so low to the low outer corners that we lose pull torque.
How much chain sag will depend on material / cut depth / router bits / friction / ++
Until we find a way to stretch the chain to the edge it will be impossible to calculate chain sag since there are too many deviating variables

The tension in the chain should account for all of the variables listed above. Another way we could quantify that tension in real time would be a strain gauge. From what I can tell, the other two major variables (length and orientation of chain) should be easy to quantify based on sled position.

Is gravity strong enough to counteract the side forces generated by the variables listed above when the longest chain feeds out and shortest chain has as little as 12-8 degrees?

A curve that hangs 33mm in the middle equals 1mm at 3000 chord length

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We could, but at what additional cost?

Is gravity strong enough to counteract the side forces generated by the variables listed above when the longest chain feeds out and shortest chain has as little as 12-8 degrees?

Yes, that’s how the machine moves in that area, but your max acceleration and
speed are lower

A curve that hangs 33mm in the middle equals 1mm chord length

the thing is that this can be calculated, and feed out a little extra chain to
compensate

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Load cells can range from a few dollars to several hundred depending on the application. I’m guessing the Arduino should be able to handle it but I have no idea.

Wouldn’t these problems also be minimalised by making the maslow bigger? ie put the motors further from the workpiece.

where would you put the load cells? if you put them on the sled, how do you get
the wires to the arduino? how many analog inputs does the arduino have?

keep in mind that you need to have this on each chain. this means that it needs
to be right at the end of the chain (hooking to the linkage or ring) so it needs
to mvoe a fair bit