Comment by cpgxiii
20 hours ago
Motor current is a workable, but generally unsatisfactory, proxy for torque when using heavily geared motors. Far better to measure output torque directly if you can, which is what's being done here using what is essentially a series elastic mechanism (itself a very common way of implementing torque sensing).
To elaborate more, current sensing is only "better" in an ease-of-implementation sense, in that a lot of motor drivers already have current sensing built in/easily added. For some applications this is good-enough, but in terms of estimating "real number" torque from current, it can take a lot of work to characterize for geared motors.
Motor current is an excellent way to measure torque and is what is done in real-world settings all the time. As a hobbyist you might have to settle for relative torque if you aren't in a position to characterize a motor or you don't have manufacturer's data for it.
I wouldn't say excellent. Motor current->torque is well correlated for motors in isolation, or with very low gear ratio. Not so with higher gear ratios. Can the current value be used to make control decisions for the motor? Yes. Does the current give you torque in Nm? No.
In the robotics world this is sometimes distinguished between actuators that report "effort" (i.e. a current-derived estimate) and actuators that report torque (i.e. actual torque sensing or direct-drive with current sensing). Both can be useful, but "effort" is not torque.
Current will never directly give you torque in Nm which is a straw man argument - you will always have to know the motor constant, input voltage, coil resistance, and gear ratio to back out torque. That's characterizing the motor. Yeah, that's a lot of work for a hobbyist, but it's not at all unreasonable to consider a motor an excellent torque transducer - that's what it does.
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There was a news story few days ago that Sony is launching an Ethernet controlled smart motor with integral reducer for robotics applications, and it has encoders on both sides of the reducer plus torque on output shaft. There's nothing so far on the English side of the public Internet about it, at all.
... so I doubt motor torque be end all be all. Especially when Sony does it like that.