Hello Paul - Readers - Complex concepts trigger warning

Your project is interesting and tough on so many levels. Lets talk about geometric resolution or accuracy to begin with. The geometric control system that most CNCs use is called deterministic. What you are describing is deterministic control. This means that the structure determines where the tool centre point (TCP) is. The machine actually does not know where the TCP is we assume that it is at the correct place because we told it to go there. You mention 1mm (so I assume that means +/-0.5mm) over 15m. This means the control system and the structure have to be good to 0.1mm which means it has to be made to 0.01mm. This is typically done by machining lands to the required accuracy using bigger machines then the machine your building (called a mother machine) In your case this is not possible as its a multi piece lightweight structure and its very big. You cannot control a TCP from within a deterministic system unless the entire system is built to the required accuracy. In your case this is impossible. So lets talk about humans. Our bodies are indeterminate and very complex. We use several systems to be able to touch our nose or pick up a glass of water. Particularly we use a system called Proprioception (PP). This is an electromagnetic referential system that is external to our mechanical deterministic system. The PP gives feedback to our motion systems and steers (reactively and predictively) our limbs so they get to where we want them to get to. Plus it not only handles position it also handles velocity and forces. Amazing. You are going to need a machine perception system.

I have been involved in a similar system that repair welds large mining equipment particularly extremely large excavator buckets, much bigger than you are talking about lets say 20m by 30m by 10m high....The company has gone down that road and stalled and started a stalled a couple of times. But size does not matter. The issues are the same as if it's on a benchtop or on a mining site.

So since you can't solve this issue in the deterministic universe your machine (and others) will have to have an external laser, lidar, precision gps or ultrasonic system (the perception system. Or machine perception system MPS) that measures where the TCP is at all times absolutely and then compares that to the deterministic information, create an error differential then feed that back to the controller for correction. This is done on some machines via glass scales and on very large mills its done with lasers these days. So I imagine you will have to have a scaffold or flagpole above your machine (as it needs to be separate from the deterministic machine), a transmitter/receiver on the machine head and a lidar scanner looking down on your job doing the MPS work. Your cameras will do the local work within a local geometric reference frame at the machine head and this will be integrated into the global reference frame and corrected in real time.

Then there is the issue of being a large lightweight machine and it will vibrate. Plus thermal expansion/contraction will be very large and I'm not sure how you will compensate for that, perhaps the MPS can cope...

All of this is not trivial... as you may have already guessed. Peter

as a note a lunar lander has literally fallen over on the moon because they did not spend the M$$$ to check the laser inertial system, they assumed that it being a commercial system it would work. Plus they left the system initialisation as a manual switch that had to be human operated prior to launch and it was not on a checklist with a big RED TAG so of course it was overlooked, so the mission which costs say $80M has basically failed. Inertial control is tricky. Musk has had heaps of rockets blow up trying to land the critters but his over the hump on that now. Why do I say this? because Paul many of these issues you are about to get entangled with are all the same stuff just smaller scale... Control, Control, you must learn control... Yoda

re: tension in washing line. The apparent stiffness increase is due to the foundations of the line not the line itself. If your lines are attached to tree twigs there is no gain in stiffness. If they are embedded in tonnes of concrete, then it's the foundation stiffness that improves not the line stiffness. Yoda probably had something to say about that as well.