Brushless DC: distance sensor instead of hall

Hey everyone,

I am planning a CNC style machine (1-2m edge) that follows 3D movement live, so it has to be quite fast. I was told in this forum that steppers are rather slow and I really don't need to be that precise. This is why I was told to use BLDC motors.

Today I had an idea: Since the motor will travel on a linear path (per axis), I could measure the the distance from the corner to the platform instead of a sophisticated hall sensor. This way, I imagine it would be faster to compute as well as cheaper. Do you agree?

What kind of sensor could I use for this? Infrared?

I made a quick sketch of one axis and the setup I am talking about. Not my best work, but I hope it makes sense.

IMG_20161106_151157.jpg

blar1234:
Hey everyone,

I am planning a CNC style machine (1-2m edge) that follows 3D movement live, so it has to be quite fast. I was told in this forum that steppers are rather slow and I really don't need to be that precise. This is why I was told to use BLDC motors.

Steppers can be fast, if driven properly, their advantage is open-loop motion control (no encoders or
servo loops needed). They are very noisy.

To move fast use a belt drive, not a leadscrew. To move with high force use a leadscrew (but move
more slowly).

Phrases like "quite fast" and "rather slow" are almost free of information content - how fast do you
actually want to go? With what load?

Today I had an idea: Since the motor will travel on a linear path (per axis), I could measure the the distance from the corner to the platform instead of a sophisticated hall sensor. This way, I imagine it would be faster to compute as well as cheaper. Do you agree?

You mean use a linear encoder, not a rotary encoder? Replace a relatively easy to source cheap component
with something more obscure and expensive? Why? What "sophisticated hall sensor" are you meaning
anyway?

What kind of sensor could I use for this? Infrared?

You need a noise-free position feedback signal or your feedback loop will be extremely jerky.
IR or ultrasonics will not work well here.

I made a quick sketch of one axis and the setup I am talking about. Not my best work, but I hope it makes sense.

Have a look at this bespoke XY camera platform design, it seems to be a reasonable approach,
belt drive gantry layout:

Hi Mark, thank you for the detailed response.

I did not go into my exact requirements and setup, because I have already gotten great feedback on that. This time, I am really only interested in the encoder aspect. (I want to achieve at least 2 m/s, with a platform weight of around 1 kg.)

I can't really defend my idea. On the forum, I read that those rotary encoders are expensive and hard to control, which kind of put me off, so I wanted to use components that I have experience with instead. Now it just sounds stupid though.

You are saying if I use a BLDC, I should go for a rotary encoder and forget about my idea, right?