Stepper vs. Brushed DC for fast rig

Hi everyone,

I am planning a project that is similar to a CNC setup, but has to be much faster due to its interactive nature: A platform (<1 kg) inside a rig (1 x 1 x 1 m) should follow tracked movements live.

Most of the CNC machines are either lathes or printers and go quite slowly in terms of movement. I wonder what the bottleneck is: the materials having to be cut, or because the stepper motors cannot go faster.

I read that the NEMA17 can go at around 10-45 RPS. One YouTube comment suggested that brushless DC motors with an encoder are the fastest and most responsive (apart from linear motors which are expensive).

I do know how to calculate torque and linear speed, but first I need to know which type of motor is better fit for the task, as I feel like I shouldn’t follow CNC setups too closely.

Joe

A lot depends on what you want in addition to speed. Stepper motors have the advantage that the programming to control their position is simple.

Controlling any sort of NON-stepper motor to get precise positioning will be a great deal more complex - especially if you want to stop at a specific place and hold that position. It may be best to buy an off-the-shelf motor and controller system - but they are not cheap.

Many (most) ink jet printers use cheap DC motors and very precise optical encoders. But they do NOT require the print head to stop at a precise position. They just need to know "we are now passing point 237564, give a little squirt of blue".

Nema17 just describes the size (1.7 inches) of the front face of the motor. You can get weak and powerful, fast and slow Nema17 motors.

If you want to build a machine that responds quickly I think you need to get out your spreadsheet and do lot of calculations to figure out the forces required.

...R
Stepper Motor Basics
Simple Stepper Code

In general, all else being equal, a BLDC will have higher speed and lower torque - this is to
some extent a difference gearing can hide though. A good brushless servomotor will
give excellent performance once tuned, and use less power than any stepper can.

Incidentally the high torque of a stepper comes from the very small separation of the poles
(standard steppers have 50 pole pairs effectively, BLDCs are usually 2, 4, 6 poles). Smaller
separation for the same magnetic flux means higher torque (but lower speed for the
same drive frequency). This is the fundamental reason its hard to get steppers spinning
fast and why high voltages are needed to do so.

Thank you both for those insights.

This sounds like I need to calculate my desired torque and speed first and that I can consider both types of motor for the job, where stepper is more on the torque-heavy side, whereas brushless DC is more on the speedy side.

I will write back when I have the numbers…

torque and angular velocity. Or force and linear speed. Don't forget inertial forces.

MarkT:
torque and angular velocity. Or force and linear speed. Don't forget inertial forces.

Throw in the decreasing torque of the stepper motor as it increases in speed.

Woo calculus! This is gonna be fun!

A DC motor will also show reduced torque as its speed increases. Othewise a no-load motor would run at infinite rpm.

...R

I suggest that a pneumatic system as the prime mover might be a better approach - the power involved is significant - I've designed such things ..

or a linear electric motor as used in music consoles

regards
Allan

Thanks for the feedback everyone.

If I have a pulley with a radius of 2.5 cm and a platform weight of 1 kg or 10 N, I need a minimum torque of 25 N-cm, right?

If I require a speed of 2 m/s (same pulley radius 2.5 cm), I need a minimum 764 RPM on my motor, right?

This one NEMA17-Stepper that I found has the following specs:
Holding Torque 4.8 kg·cm
Rated Torque 4.2 kg·cm
Maximum Speed (w/1063 Motor Controller) 600 RPM
Maximum Speed (w/1067 Motor Controller) 4688 RPM

So there seems to be enough excess 'power' to compensate for not considering inertia, friction etc., right?

Again, I do not want this particular stepper confirmed, but rather how big of a motor (-investment) I need for the project. I have never dealt with these kinds of items and formulas before.

I always use SI units throughout to avoid silly conversion errors, so:

holding torque = 0.47Nm,
rated torque 0.41Nm, (this will be at slow speed only, note, stepper torque drops off
with speed a lot).
radius = 0.025
velocity = 2

angular velocity is 80 rad/s, torque required 0.245Nm, so 20W mechanical power not allowing
for losses or inertia.

I doubt any NEMA17 without reduction gears can hold 0.25Nm throughout its speed range, note, but with
the faster controller there's a better chance. Look at the torque v. speed graphs of particular
motor/drive-voltage combinations - the holding torque is often several times the usuable torque.

I'd tend to go for something like a 0.08Nm 4000rpm motor and 5:1 gearing as a BLDC or DC
motor matching that load, that's 32W which gives headroom to help overcome losses. (Yes,
DC and BLDC motors can be overdriven for short periods of time within reason).

Thanks Mark.

You mentioned Watts which I find interesting, because it describes the tradeoff between speed and torque better and lets me compare motor sizes better. I am not bound to be using a NEMA17 or stepper at all, but I do get the feeling that my required speed is a problem for steppers altogether, is that right?

I had no illusions about my requirement for both speed and torque would result in big motors no matter what. At this point, it seems like I need to build a miniature first…