Your application looks super cool! I think you can get some really fast prints, especially if you stick something like this on it.
Hehe yes please please do not use the pen demo as a reference for what is possible, we threw that together with very barebones mechanics over 2 years ago. Specifically, the carbon tubes bend a lot, so yeah it’s not a good example.
The tracking error depends on many factors. Ignoring any mechanical compliance or deflection, it depends on the balance of three things:
Magnitude and frequency spectrum of disturbance forces. Larger disturbances means larger errors, but also there could be some frequencies more sensitive than others: very high frequencies will get filtered by the moving mass, very low frequencies will get corrected easily by feedback: there is usually a sensitive area around 50Hz-100Hz (depending on feedback bandwidth).
Sources of disturbance can be motor cogging torque, and stick-slip and other non-linearities in your axis, and of course unmodelled dynamics in the deflection of the mechanics.
Moving mass: larger mass gives more filtering to the above forces
Feedback bandwidth. Higher bandwidth means we can actively act to correct any error caused by disturbance, pushing up the sensitive area mentioned in point 1. The limit to the bandwidth we can use is given by the encoder resolution.
I think your choice of a high pole count motor (lower cogging torque) and encoder (8k CPR resolution) are good choices. For the heavier axis that has more filtering mass you could indeed use the larger (but fewer pole count) motors. Overall I think your system should perform very well.
If you are able to send feasable trajectories, I think you should be able to stay within 0.5 deg tracking error, but of course it depends on how much disturbance is coming in from your mechanics. Will you be using igus bushings on their precision aluminium rails?
As for figuring out torque/power, I would suggest that you take a copy of the ODrive motor guide and play with things like the Bus Voltage, and you can see what you get. You can also trace the equations to see how it works. To answer your question: higher voltage means higher base speed, torque doesn’t change. The torque rating of a motor is proportional to
rated_current / kv, and the rated speed of the motor is proportional to
rated_voltage * kv.