Comment by analog31 | Hacker News Reader

Comment by analog31 | Hacker News Reader

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Historically, hobbyists have chosen stepping motors because they can be controlled with a fairly simplistic controller, and there are lots of solutions out there... check out some of the step motor drive boards at Pololu Electronics. But stepping motors have limitations, namely speed and torque per unit motor size. Note that as a hobbyist, I'm still living in the stepper motor world, so I'm interested in this project.

Brushless servo motors offer the potential for much higher torque and speed in a small package, but require a different form of electronic control, and the gist of this project is that such controls haven't been available for hobbyists. So it's a matter of finding a gap in the existing project / product space, and filling it.

Note that e-bikes use brushless servo motors, so somebody has done the math on what motor technology is most efficient. I also think these motors are in electric cars, and even appliances such as washing machines.

I will be reading with interest. I'll also start keeping an eye out for what kinds of motors are available.

As for "industrial level," I'd say that it's a matter of pushing machine speeds up to a level similar to what you might see in a factory. Often, with stepper motor driven gadgets, you accept the fact that your machine will take a long time to do anything, but who cares, it's a hobby.

On the industrial side, you very quickly run into the situation where your motor is no longer the limiting factor affecting speed and precision, but the design and assembly of the mechanism is.

Stepper speed can be quite impressive with the right driver, still not up to servo levels but much higher than you might think possible, it is all about getting to and past the resonance points in the motor and the whole dynamic assembly. Once you can do that steppers can run quite fast.

As for torque, servos again have the edge, but there are pretty large steppers too (1" shaft...).

If you drive steppers with a standard H-bridge you will never get close to what that stepper can do, driving stepper motors to their potential is a black art where at some point during the operational domain you'll be taking energy out of the motor rather than putting it in in order to get past the resonance point. There are companies that specialize in such high speed drivers (Berger-Lahr and RTA for instance), they're not cheap and by the time you're done making this work you might as well install a servo system.

Pretty much this! People definitely do work some black magic here. (See for a simple example microstepping drivers, where the current profile isn't sinusoidal, but specfically tuned to match the motor profile).

One place where I did see this used frequently is in certain consumer electronics, such as inkjet printers. (Ever wonder why companies like Allegro sell weird combos like a chip for driving 2 steppers, a DC motor and a Buck converter....this is why)...When you are shipping a million printers a year, you save pennies using the cheap actuator and fix it in the controls, but for most projects, yeah totally not worth the hassle!

Indeed, and on the hobby side, microstepping has gotten very easy over the last few years. Here's my favorite driver du jour:

https://www.pololu.com/product/2970

I wonder how much these "alternative" driving modes are portable across different sizes and vendors/n. of poles

It's parametric, once you know the math it will transfer. There are (or were, not sure if they are still in use) 5 and 7 phase stepper motors to help combat these resonance problems.

I do motor design and research for a robotics and automation firm. In my experience, steppers often can provide significantly higher torque than a similarly sized BLDC. Their reputation comes from their use in low cost applications, where they are run open-loop and so the load torque must be kept well below what the motor is actually capable of providing, to avoid missing steps. Close the loop with an encoder and stepper performance is more like a geared BLDC. It was a big surprise to me when I first noticed this, because it didn't align with the reputation that stepper motors had.

ShopBots, the big CNC routers, use big steppers with closed loop control, and get considerable power and speed out of a small motor without missing steps. Tormach CNC mills are stepper-driven without feedback; the motor is powerful enough and geared down enough that missing steps isn't a problem.

Steppers do consume full power when not moving, of course, so they're not favored for battery powered systems.

The Tormach CNC actually uses a special driver that can detect missing step without feedback.

Shopbot has a closed loop stepper on some models [1], but Tormach says they don't.[2] Tormach just has a big enough stepper and enough gear reduction to avoid missed steps by brute force.

[1] http://www.shopbottools.com/mProducts/ShopBotdesign.htm [2] https://www.tormach.com/technical_questions.html

I have compiled a small list of motors that I think are suitable for robotics applications, along with some useful calculations, here: https://docs.google.com/spreadsheets/d/12vzz7XVEK6YNIOqH0jAz...

You can take a copy of the spreadsheet and enter in your own motors that you find. Also do note there is a second tab on the sheet that lets you explore cycle times on trapezoidal trajectories.

Yeah, brushless is definitely the way to go for efficiency, and good smooth motion. Stepper for an e-bike or washing machine (Not a BLDC either, but...) would be nuts, and in general steppers are really for cheap semi-decent control, not for anything where precision is really needed.

My colleague had a motor from a newer washing machine out on his desk, and it did appear to be a BLDC. Alternating magnets with three phases of coils as you'd expect. Driven from rectified 120V AC!

Very cool! So I should perhaps qualify my comment to say that some newer appliances may be moving that way. (I've done work on motor drives for consumer electronics, robots and some industrial devices in the semiconductor world...no appliances, but it's cool to hear some of them are going that way!)

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