[jonnycowboy]

We actually have very cheap and pretty powerful position-controlled actuators (hobby servo motors). Attach any kind of spring and displacement measurement device (potentiometer, hall sensor, optical, LVDT, etc) and voila, instant torque controlled actuator.

You can look up Series Elastic Actuators for more info or use this article as guidance (any spring will do as long as the force range and spring constant is adequate).

https://www.sciencedirect.com/science/article/pii/S240589631...

[gene-h]

The servo they use is $493[0], at that price I wouldn't necessarily consider it a hobby servo. The control for that specific series elastic servo needs some work. There have been other attempts at making cheap series elastic actuators. An interesting one was the programmable spring work[2][3]. Although one problem with series elastic actuators is that they can be difficult to control because of the compliance. Force servos were also an interesting attempt at doing cheap force control[4] and by using load cells they avoided the compliance problem. Unfortunately, force servos did not have any position control.

[0]http://www.robotis.us/dynamixel-mx-106t/ [1]https://www.youtube.com/watch?v=sjFR4ACVLmk [2]http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.147.... [3]https://www.youtube.com/watch?v=_g79mOSvSsE [4]https://www.youtube.com/watch?v=sjFR4ACVLmk

[GistNoesis]

It is also possible to use current sensing to measure torque.

ODrive + brushless + encoder (~150€ per axis) can provide current measurements, though you may need to create some PID loop.

If the brushless solution is still too expensive. You can use 775 brushed motors + encoder + BTS 7960 driver + (optional gearbox) (They have current sensing by using additional resistance (Edit:and RC filter) ) (10€-50€ per axis from china). Although that's not powerful enough for jumping robots.