3 ms·

Haha -- yes indeed about the engineering headaches! (Although as I'm sure you know...the more I learn about any field of engineering, the more I learn about jus

Haha -- yes indeed about the engineering headaches! (Although as I'm sure you know...the more I learn about any field of engineering, the more I learn about just how-many trade-offs there are in building something!) As for gear ratio -- depends on which joint, but Harmonic Drive's standard gearboxes come in ranges of 50:1 to 320:1, and I'd say they were all within that range. As far as direct drive motors go, all I can say is that I was very aware of them, and very interested in looking at it. (I was on the embedded systems team doing motor control), but by the time I joined, the architecture had already been decided and robot's were being built. That said, there are certain other difficulties there, both with the control electronics for larger motors, and with the relative cost of the motors, especially since Rethink is going for the goal of reducing robot costs. I can't say they won't go that way some day. I think at the very least, it's worth exploring, but I'm not longer there so I don't have much say in that! Other fun stuff -- Even though we used a harmonic drive, it was coupled to what is basically a torsion spring between it and the output. This arrangement is called a series-elastic actuator, and it allows you to do some neat stuff with rapid torque sensing, both from the control and safety side of things. It's also a lot of fun for the control engineers trying to control something sitting at the end of a spring :-P

TFortunato 9y ago
Just wanted to add, that is a cool project on hack-a-day, but once you start looking at precision motion, or variable loads (eg. moving a robot arm), you start to become keenly aware of the physical imperfections of your motor and gear-box, and start having to think of things like cogging in the motor, or torque-ripple in the gearbox output and all that. The drive signals on those direct drive robots are NOT going to look like the perfect "3 sine-waves 120 deg out of phase" you see on wikipedia, and you need some damn good control software, and sensing of motor state to get good motion. I'm not trying to say it's impossible, or to knock anyones project or sounds argumentative. It's been done, as you pointed out! Just wanted to point out that while in theory it's much nicer to be able to do direct torque control on the motors, the headaches to become apparent in practice. (I still love the idea of direct drive :-D )

TFortunato 9y ago

Just wanted to add, that is a cool project on hack-a-day, but once you start looking at precision motion, or variable loads (eg. moving a robot arm), you start to become keenly aware of the physical imperfections of your motor and gear-box, and start having to think of things like cogging in the motor, or torque-ripple in the gearbox output and all that. The drive signals on those direct drive robots are NOT going to look like the perfect "3 sine-waves 120 deg out of phase" you see on wikipedia, and you need some damn good control software, and sensing of motor state to get good motion. I'm not trying to say it's impossible, or to knock anyones project or sounds argumentative. It's been done, as you pointed out! Just wanted to point out that while in theory it's much nicer to be able to do direct torque control on the motors, the headaches to become apparent in practice. (I still love the idea of direct drive :-D )