4 ms·

Assuming you're talking about AC servos typically used for industrial equipment (and not the DC kind used for robots or RC aircraft), there are kind of two rout

by Youden 4y ago

Assuming you're talking about AC servos typically used for industrial equipment (and not the DC kind used for robots or RC aircraft), there are kind of two routes you can go. On the easy hobbyist route, you can get the parts from a nice online store with email support like DamenCNC [0]. You'll need to read the spec sheet to find the speed/torque/resolution though. On the harder route, you collect a bunch of spec sheets, figure out what features you want, come up with a list of SKUs, find a seller on Alibaba, AliExpress or eBay and ask them for a quote. Not all of them will be responsive but I didn't find it too hard. For a vague idea of the price, you can usually get the order of magnitude from the listings on the sites I mentioned. Ultimately though, servos are treated as industrial equipment and take a lot of work for a hobbyist to figure out. If you're not buying a pre-made kit sold by a hobby store, my suggestion is that you should know the manual like the back of your hand and be able to clearly visualise the full installation process step-by-step before you buy anything, as there's a lot of stuff that can trip you up and a lot of ways you can even kill yourself. Servo drivers don't have nice AC wall plugs for example, they have screw terminals, so you need to be able to wire mains voltage electricity without giving yourself an electric shock or causing a fire. The manuals usually say you should get a professional electrician to handle it. All the cables are non-standard and usually per-series so you need to make sure you buy the right ones. The list goes on... "Surely I just feed it electricity and it spins" you might think but unfortunately it's not that simple. These motors are deceptively complex things. As others have mentioned though, Teknic is a thing. Those motors are much simpler, they're designed as drop-in replacements for stepper motors and they have an easy to use online store. I'm not a fan though, for a similar dollar amount and several orders of magnitude more stress and time, you can have high-end A3 servos from Delta with more features than you'll ever need. The encoder is 24-bit! You'll never be able to accurately use that resolution without precision engineering and temperature control but it's nice to know it's there. [0]: https://www.damencnc.com/en/ac-servo-drive-400w-asd-a2-0421-m/a829 https://www.damencnc.com/en/ac-servo-drive-400w-asd-a2-0421-...

abakker 4y ago
I'll say for completeness here (having mentioned Clearpath servos in another comment) that DC clear path servos are totally fine for "semi" industrial uses. I have them on a bed mill I am restoring, and with a good ballscrew I can easily move a 300lb Z axis at 200inches/minute without torque saturation. That said, they are simple because they are limited. They give you only 3 inputs: Enable, Step, Direction. they give you one output called "High level feedback" and BOY OH BOY is it high level. it is basically only useful for basic error detection. In the software you have pretty cool tuning options - a visual oscilloscope, the RAS feature (which is really good for DIY CNC machines), and encoder resolution selection. A DMM, or especially a Delta, will give you a lot more IO to the motors, and a lot more stuff to play with. E.g. direct access to the encoder position (even if the motor is not active), many more error states, warnings, etc. and some more condition specific tuning. Last bit on clearpaths. I've been talking about the SDSK series motors. those are controlled from an external controller through pulses like stepper motors. They have other series of the same motors that interface differently. Specifically, they have some motors designed to interface with microcontrollers and have a really nice set of C libraries for doing all the basic motor control stuff. If you are building things like conveyor belt automations or material handling stuff, the clearpaths are very much an "easy button" compared to nearly every other kind of motor on the market IMO.
- iancmceachern 4y ago
  I agree, love their motors for simple applications where you don't want to write software
rq1 4y ago
Thank you for this comprehensive comment. I appreciate it. I did the drawings of several drones and robot arms, and accurately computed the needed torque. I target specific precisions for several applications. I’m a mathematician so the math is straightforward to me though as you said, it’s super easy to overlook something without the appropriate experience, especially since it’s cross disciplinary (ME, EE, SE… etc. All at once yeah!) I found these for instance on eBay: * https://www.ebay.com/itm/393967889048 https://www.ebay.com/itm/393967889048 * https://www.ebay.com/itm/275442630163 https://www.ebay.com/itm/275442630163 I can get from them more than 120Nm with the appropriate planetary gearbox without sacrificing too much speed and even more with gas springs compensating the static loads… theoretically… but I’m a bit skeptic about the announced 12Nm and the 51200microsteps/rev resolution for instance. I think it’s better (in my case) to have access to maximum IO on the encoder side. I’ll double check the electrical installation with a pro for sure, I already did a mistake in the past that almost destroyed the house boiler in the middle of the winter… lesson learned.
- Youden 4y ago
  Those motors are just closed-loop steppers. I'd say they're inappropriate for robot arms because they can only apply their specified torque when the angle happens to be in proximity to a full step [0]. Proper AC servo motors like the A3 series from Delta are - as far as I understand - able to apply their specified torque with "infinite" resolution, in practice limited primarily by the encoder. In robotics though, BLDC motors are often used. You might want to look into ODrive [1]. Make sure you're choosing an appropriate gearbox too. A planetary gearbox has some amount of backlash associated with it, often around 10 arcmin from what I've seen in data sheets. If that's inappropriate for you, look into harmonic drive. Much more expensive but for a reason. [0]: https://www.machinedesign.com/archive/article/21812154/microstepping-myths https://www.machinedesign.com/archive/article/21812154/micro... [1]: https://odriverobotics.com/ https://odriverobotics.com/
  - rq1 4y ago
    Thank you for your reply. Proper servos with high torque are quite expensive. I wanted to keep the project under a certain amount to scale it. With the springs to compensate the static loads (gravity) I can use weaker servos but I didn’t yet completed the “sensitivity table” of the generated angular momentums. Yes indeed, the ~10arcmin should represent something like 5mm at about 1m distance. Plus the ~30mm of the 1.8deg/step again at 1m or 3mm with 10:1 gearbox. I thought I could tackle it by using more expensive parts (but weaker) at the end part of the arm for instance and write a software to orchestrate all the 6motors together. I didn’t know about the harmonic drive I’ll look into it. Thank you again for the references.

by Youden 4y ago

abakker 4y ago

I'll say for completeness here (having mentioned Clearpath servos in another comment) that DC clear path servos are totally fine for "semi" industrial uses. I have them on a bed mill I am restoring, and with a good ballscrew I can easily move a 300lb Z axis at 200inches/minute without torque saturation. That said, they are simple because they are limited. They give you only 3 inputs: Enable, Step, Direction. they give you one output called "High level feedback" and BOY OH BOY is it high level. it is basically only useful for basic error detection. In the software you have pretty cool tuning options - a visual oscilloscope, the RAS feature (which is really good for DIY CNC machines), and encoder resolution selection. A DMM, or especially a Delta, will give you a lot more IO to the motors, and a lot more stuff to play with. E.g. direct access to the encoder position (even if the motor is not active), many more error states, warnings, etc. and some more condition specific tuning. Last bit on clearpaths. I've been talking about the SDSK series motors. those are controlled from an external controller through pulses like stepper motors. They have other series of the same motors that interface differently. Specifically, they have some motors designed to interface with microcontrollers and have a really nice set of C libraries for doing all the basic motor control stuff. If you are building things like conveyor belt automations or material handling stuff, the clearpaths are very much an "easy button" compared to nearly every other kind of motor on the market IMO.

iancmceachern 4y ago
I agree, love their motors for simple applications where you don't want to write software

rq1 4y ago

Thank you for this comprehensive comment. I appreciate it. I did the drawings of several drones and robot arms, and accurately computed the needed torque. I target specific precisions for several applications. I’m a mathematician so the math is straightforward to me though as you said, it’s super easy to overlook something without the appropriate experience, especially since it’s cross disciplinary (ME, EE, SE… etc. All at once yeah!) I found these for instance on eBay: * https://www.ebay.com/itm/393967889048 https://www.ebay.com/itm/393967889048 * https://www.ebay.com/itm/275442630163 https://www.ebay.com/itm/275442630163 I can get from them more than 120Nm with the appropriate planetary gearbox without sacrificing too much speed and even more with gas springs compensating the static loads… theoretically… but I’m a bit skeptic about the announced 12Nm and the 51200microsteps/rev resolution for instance. I think it’s better (in my case) to have access to maximum IO on the encoder side. I’ll double check the electrical installation with a pro for sure, I already did a mistake in the past that almost destroyed the house boiler in the middle of the winter… lesson learned.

Youden 4y ago
Those motors are just closed-loop steppers. I'd say they're inappropriate for robot arms because they can only apply their specified torque when the angle happens to be in proximity to a full step [0]. Proper AC servo motors like the A3 series from Delta are - as far as I understand - able to apply their specified torque with "infinite" resolution, in practice limited primarily by the encoder. In robotics though, BLDC motors are often used. You might want to look into ODrive [1]. Make sure you're choosing an appropriate gearbox too. A planetary gearbox has some amount of backlash associated with it, often around 10 arcmin from what I've seen in data sheets. If that's inappropriate for you, look into harmonic drive. Much more expensive but for a reason. [0]: https://www.machinedesign.com/archive/article/21812154/microstepping-myths https://www.machinedesign.com/archive/article/21812154/micro... [1]: https://odriverobotics.com/ https://odriverobotics.com/
- rq1 4y ago
  Thank you for your reply. Proper servos with high torque are quite expensive. I wanted to keep the project under a certain amount to scale it. With the springs to compensate the static loads (gravity) I can use weaker servos but I didn’t yet completed the “sensitivity table” of the generated angular momentums. Yes indeed, the ~10arcmin should represent something like 5mm at about 1m distance. Plus the ~30mm of the 1.8deg/step again at 1m or 3mm with 10:1 gearbox. I thought I could tackle it by using more expensive parts (but weaker) at the end part of the arm for instance and write a software to orchestrate all the 6motors together. I didn’t know about the harmonic drive I’ll look into it. Thank you again for the references.