Servo vs. steppers: Speed, Torque and Accuracy [video]
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Stepper motors can be controlled with closed loop controllers and an encoder. As I said in some other comment way down the only difference between a stepper motor and a brushless servo is that steppers have more poles. A by the way is you can also "step" a BLDC (i.e. run it in open loop just like a stepper motor).
Good closed loop stepper controllers are a huge step up (heh) from open loop control, but there's a big difference in characteristics of a stepper motor - which is a very special motor configuration making huge trade-offs specifically to allow repeatable open-loop operation - and the more standard servo motor choices. Because steppers are special you also tend to specifically say "closed loop stepper" rather than considering it a servo.
Be a bit careful with random closed loop controllers though. Cheap ones can be finicky, and poor tuning and broken failure modes is no fun when used in machinery that could injure you, which tends to be where such steppers are installed. Aggressive and erratic movement or randomly entering fault and powering down is quite a lot different from simply losing steps.
In the video he has what looks like a magnetic absolute encoder, I have honestly never seen that in industrial applications although I have seen "absolute" optical encoders that have a backup battery to store the home point, kind of weird imo.
I was surprised when I first encountered servos with just magnetic hall effect sensors but there are actually lots of applications where you want speed control but not accurate position control.
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.
Quadrature encoders don't skip counts unless your output is single ended and your wiring is poor leading to interference of the signal. PWM drives tend to make a lot of noise and single ended encoders will see lots of problems unless you take great care. I always use differential signal encoders in our machines here at work. They use an RS485 driver to achieve this in the encoder itself. Aerotech, the main vendor we use for motion control, uses differential signal by default. Never had positioning or counting problems unless the encoder was physically damaged.
Right. Syncro resolvers are the classic way to do it. Modern versions use a permanent magnet and two Hall-effect sensors.
Each angle has its value of the optical signal, down to the ellipse's symmetry. Two ellipses, painted at an angle between their axes, give a unique pair for each angle, even amenable to interpolation.
This must be quite resistant to both electromagnetic interference and to fast / jiggy rotation. It's more bulky though, and likely requires calibration.
Also a good engineering rule is to have your stepper provide double the maximum torque than you need by specification, especially if you use partial steps.
Common dc motors don't need as much tolerance, but it doesn't hurt anyway.
Also, you still need to debounce signals from optical encoders because of the inteference you mentioned, even if your physicist tells you that optical encoders cannot bounce.
To read encoders try to use timer interrupts instead of pin change interrupts. A bit unintuitive for software developers, but polling is safer because you have specified maximal speed and your µC program isn't disabled from magnetic interference as well. There are specialized µC for quadrature encoders, but you usually do not need them if you don't have high speed applications.
Rod Brooks' original insect robots used R/C servos where someone had wired in an extra wire to extract the analog error signal. This provided force feedback. So that's quite possible.
The general problem with servomotors for hobbyist use is price. Note that the OP was given those motors as an influencer. Industrial motors with encoders are expensive, and controllers are worse. Some years ago I was talking to a Maxon rep at a trade show. They'd just introduced their own controllers. He told me that the motor and the controller cost about the same to make, but the controller people were getting 90% of the profit because controllers had become cheap to make. So they built a controller to improve their margins.
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"
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...)
Order now, before tariffs go up.
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.
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.
Contrast with Teknic where I could get the servo drive up and running in a few hours because of an actually readable product manual and plenty of sample code and a Windows DLL to make everything easy.
There's definitely opportunity at the lower end of the market.
One key is find a hobby that is enough different from your day job you are not burned out of doing it. I can write code at home and sometimes I do - but most of the time I'm burned out after doing that for my day job. However I can still bend the sides of a ukulele, use CAD to design a new switch housing for some manual machine, practice trumpet, or other such tasks that are not related to my day job. I personally am not interested in editing a video (which takes a lot of time to do well) so you won't see me on youtube, that too is something I could do if I was interested in it.
Though with kids often all I have time for is cooking a meal before getting them to bed and then I'm off to bed myself. I wouldn't trade it for the world, but there is limited time and so there are a lot of things I want to do that I don't have time to do.
I assume these videos are targeted at hobbyists… I can’t imagine people in your position using him as a source of knowledge.
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.
It's a potentiometer most of the time, BTW. Encoders are on the spendy "digital servos"
Most common type uses optical sensors internally, although there are also magnetic types. A quadrature encoder is the most popular and provides two output signals, phase separated by 90 degrees. The phase separation allows the reader to determine which direction the shaft is moving.
A potentiometer used for position feedback is usually just called a potentiometer. I guess it's technically an encoder, but I don't think I've ever heard it referred to that way.
There are also resolvers, discussed somewhere else in this thread, but we won't go there!
HTH
I have absolutely seen potentiometers called encoders in industrial applications. The underlaying technology is irrelevant. Encoders and revolvers are just transducers of position.
I'm making a set of roller blinds, and it's a pretty fun/challenging project that incorporates 3d design and printing, circuit design, and "microprogramming" (using a pico w board). It's really neat seeing physical manifestations of running code!
edit: granted it pulls 35oz