Servo vs. steppers: Speed, Torque and Accuracy [video]

Absolute encoders can be really good for position critical applications that you don't want to re-home all the time. Linear stages, winches - if you don't want to re home it and it needs accurate position control you start looking at an absolute encoder.

Even for speed control, hall effect sensors are kind of a poor way to track position. What is nice about hall effect sensors is that you can use them as a signal to perform brushless commutation in your motor controller, and then also use them as a poor-man's encoder. Very useful if you don't need that much accuracy in your application, but you do need brushless motors for some reason. But one of the first things I would go to as a application engineer was recommend customers get an encoder mounted.

> They are called resolvers which are rotary transformers which output a sine and cosine signal. You input an AC sine wave and compare the phase offsets of the outputs to determine the rotor angle.

Right. Syncro resolvers are the classic way to do it. Modern versions use a permanent magnet and two Hall-effect sensors.

FWIW (having used Dynamixels for 13+ years for hobby stuff), there are cheaper alternatives with tradeoffs now, too. Though I've only played with e.g. AX-12-sized ones (specifically HiWonder HX-35HM sitting on my desk from early last year)

There's so much to do in robotics, that you won't catch me going back and modifying RC servos, ever. I'll do hard things elsewhere! (And I don't even do autonomous hobby robots...)

I have tried that one time to tap into the potentiometer (not sure if that's what you're talking about with hobby servos).

Eventually you do want to fork out money for more expensive servos as those cheap 9g blue ones don't cut it, start to get hot/melt when used in a more serious robot application. Granted at that time I was using one cell so maybe that was more current. At any rate when I switched from $2 to $10 servos (especially metal gear) it was noticeable. The price matters when your robot has 12 or 18 of these servos on it.

Thanks for the links, as my robotic projects get more serious I do want positional feedback, term I picked up from a fun book "proprioception"

It depends on the use case, I was curious on price. It is geometrically limited too, with regard to how to mount the magnet to the servo horn (vs. taking the body apart). I just like how you can use that tech, I've seen someone make a USB joystick with a magnet/hall effect sensor to use as a 3D mouse for example. Also have seen people taking hs and using them as rotary encoders on BLDC motors.

edit: I probably should have watched this video before my initial comment, servo I thought RC servo not these big ones. Even so the bigger one would be easier to work on than a smaller hobby one.

Eh, as a controls engineer, I get a bit of Gell-Mann amnesia watching his content on drives and motors. There's a complete lack of tuning described in the video, it seems a lot of his performance complaints are due to conservative out-of-the box defaults.

The stuff he builds very cheaply with little more than some wood, Python, and Raspberry Pis is impressive, and he deserves all the kudos for building cool shit and putting it on the Internet (I'm just a consumer and critic, not a creator in comparison). But serial control from an interpreted script instead of CAN/Ethercat messaging from a motion controller or PLC is not the way these products are typically used in industry, and most people don't run the defaults.

There's definitely a niche for hobbyist-grade, student-grade, or lab-grade motion products that name brands like Beckhoff, Omron, Fanuc, Rockwell, and Siemens largely ignore. You could get 4 axes from DMM for the price of 1 from most of those vendors. And while simple serial commands can make interesting things happen with the DMM unit, you'd have to scale a daunting cliff of a learning curve just to get a motion axis initialized in their massive, standardized, proprietary, legacy ecosystems.

Again, no disrespect intended: I've invested thousands of hours into building custom, multi-million-dollar machine tools at my day job and instead of challenging myself, going to the workshop and turning on a camera when I get home, I've vegetated on the couch, entertained, and sometimes educated by his content building a milling machine or lathe out of wood. But this "servos vs. steppers" debate really only applies to low-cost, simple, hobbyist-grade equipment, and isn't such a big topic in the industrial space.

If you're micro-stepping a stepper then you're applying different currents to the coils. You can also microstep/openloop a BLDC just like a stepper and you can run a stepper with a closed loop controller just like it's bigger BLDC cousin.

Steppers had a niche in situations where you want to take discrete/accurate steps with very little control circuitry, e.g. printers, disk drivers etc. they're much simpler to work with vs. a closed loop bldc system, require no tuning, less software, etc.

I used to think this but you can actually drive steppers with FOC (if they have appropriate feedback) and modulate the current based on required torque. It’s open-loop steppers that don’t do this.

When I was about 13 I took apart a broken servo from a hobby RC car I had. What a revelation to me. The circuit board was pretty mysterious to me, but I noticed the encoder (didn't know what it was called then) on the output shaft, and immediately realized its purpose. It looked to me a bit like a volume knob. I wondered why the servo motor didn't keep turning, but I realized this thing must be telling the motor "a little more" or "a little less", and it would have to keep making small oscillations back and forth to compensate for external forces, etc, which explained all the twitching noises it made. It was a great discovery at that age.

$30? That's steep

edit: granted it pulls 35oz