CNC machine, how to accurately measure coordinates?

Question

Microcontrollers

I'm learning the Arduino platform. LED flashing is over, webcam rotation is over. Now I want to assemble something really useful: I probably haven’t grown up to a quadrocopter yet, but it seems to me that I can assemble a CNC machine ( for a start, so that it draws something ).
If you collected one, how do you calculate the coordinates? Greater accuracy is needed, and a servo error of ± 3 degrees can greatly affect (I thought about connecting through a gearbox in order to reduce the error, but I decided to ask anyway). I read an article where you can connect to the outputs of a digital caliper - this is generally cool, there the measurement accuracy is ± 0.05mm - excellent accuracy, but before this article (end of 2011), people also assembled home-made machines: how were the coordinates counted?
But maybe there are elegant ways to measure coordinates? Interested in actually applied methods, not dreams and conjectures (I myself know how).

Answer 1

Usually there is a stepper motor in the drive, the coordinates are counted from it (without feedback)

Answer 2

Stepper motors actually measure the number of revolutions BEFORE the gearbox, respectively, there will be errors accumulated in the gearbox itself (and this is their main source, in addition to hinge play and deformation of the entire structure under physical stress, inertia, own weight and temperature changes).
This means that it is necessary to measure the angle of rotation of the last hinge in the gearbox (or the penultimate one, then you can combine the measurement of the angle with the number of revolutions) or even the distance that the tool itself moves.
To change the angle, it is easiest to use a disk with a large number of notches in the glass / slots in the opaque / just stripes and consider them as an ordinary photocell (a pair to determine the direction of rotation). Now I looked, Wikipedia has a completely understandable article about encoders:en.wikipedia.org/wiki/Rotary_Sensor
When it comes to 'do it yourself' and low budgets, then in this case, the larger the disk radius, the easier it is to make it yourself, and the strips can generally be printed on a printer. You can also develop the idea, and instead of a disk and a photocell, you can put a mirror / flashlight and catch a light spot with a webcam at a greater distance - this is not about obtaining real-time coordinates, but about constant automatic compensation for measurement errors (until this light spot is recognized ....)

Answer 3

If you do not use a stepper motor, then you need to use an encoder (there are a lot of options, the simplest strip on the film and a ready-made pair of LED phototransistor), a gearbox with a high gear ratio and a rather sophisticated engine speed control algorithm in order to position accurately.
In self-assembly machines, stepper motors are usually used, which is much simpler, sometimes they make feedback on the unused winding of the stepper motor to catch potential step skips, or again they use an encoder.

Answer 4

I will add that it is still worth looking at the software that you are going to control the machine - some do not know how to feedback. And then only stepper motors and no feedback. Yes, even with a heavy load, steppers can skip steps, and then the error accumulates.
It makes sense to set the current to about 30% more than steps start to be skipped at the heaviest load on the machine.