ok, so this is one of several ideas that I have been thinking of recently.
so, goal is to drive a stepper closed loop (similar to what can be done with a BLDC motor), preferably without any reliance on external sensors (IOW: preferably no encoders or similar), and ideally without modifying the motors.
I am wondering if anyone knows of "good" (AKA: cheapest possible) ways to do this?
or if anyone can clarify whether or not my ideas here could work?
why?: say, if someone is building something like a humanoid arm, a lot of motors will be needed, and encoders would add considerably to the cost. the goal would also be for it to be able to ideally have comparable speed and strength to a human arm (say, able to lift up to 50 or 75lbs, ...).
why not servos?: cost mostly. they tend to be either very small/weak or very expensive.
why not BLDC motors?: typically lower torque, so more reduction would be needed. also cost, as BLDC motors seem to cost a fair bit more than steppers (watts per $ is also a bit worse).
so, steppers seem to be one of the cheaper options to get well-controlled reasonably high-torque output.
however, for a useful arm, it would be useful for it to be able to move and respond gracefully to loads.
so, the idea is preferably, if possible, to find a purely electronic solution.
ATM, this is more of a hypothetical idea (I don't have money...).
one possibility seems to be that, during part of the cycle, the bridge could be disabled. at this point, the voltages across the phases could be observed, and a "flip" could be used to detect when to commutate. potentially, both phases could be monitored, with the controller alternately watching each phase.
however, monitoring one phase would require 7 IO pins per motor, and measuring both phases would require 9 or 10 IO pins.
pins would be:
bridges, A B C D;
enable: EnA, EnB
feedback: SnA, SnB, SnC, SnD
probably the sense pins would give a value relative to the mid-point between the positive and negative rails:
possibly, phase drives base of PNP, with emitter connected to a midpoint reference, and the collector is a pulldown;
the pulldown drives an NPN connected to a 3.3v pullup, which does the signaling.
issues: will it work? I really don't know.
some people seem to argue that you can't get usable EMF feedback of this sort from steppers, but I am not sure (there is the trick of linking steppers together, where turning one turns the other, implying it should be possible to get "some" sort of useable feedback out of them). had at one point used a stepper to drive an H-bridge to run some other steppers with manual controls, so at least this can be done.
another half-considered thought was if unipolar steppers were used, one possible hack would be to drive it asymmetrically, with one of the phases driven like in a bipolar stepper, and the other phase driven unipolar.
in this scenario, the other half of the unipolar phase could be useable to sense the movement of the rotor. its voltage should fluctuate as the rotor spins, and with any luck this could be converted to a useable signal.
this seems a little hacky though, and depends on the use of unipolar steppers, which would limit options (there are a lot less of them listed, where 4-wire steppers are most common, 6 wires less common, and 8 wire rather uncommon).
another possible hack could be partly rewiring it, noting that each phase has 4 sub-coils (for 8 coils in-total), but the motor should still work with 3 coils per phase. two of the coils could potentially be disconnected and used as sensor coils (the motor spinning inducing voltages on these coils). alternatively, tap wires could be installed (with a 3/1 split), with the tap wires used to help with sensing.
well, that or try installing a hall-effect sensor (with a piece of steel used to "read" the rotor at a particular spot).
but, I don't know...
otherwise: nevermind whether or not all of the electronics and similar could fit within the arm (chances are it would need to be largely moved using pulleys and cables, or be kind of large/bulky).
probable option would be to use a mix of NEMA-17 and NEMA-23 steppers, probably still with a bit of reduction (via pulleys and spools).
thoughts?...
so, goal is to drive a stepper closed loop (similar to what can be done with a BLDC motor), preferably without any reliance on external sensors (IOW: preferably no encoders or similar), and ideally without modifying the motors.
I am wondering if anyone knows of "good" (AKA: cheapest possible) ways to do this?
or if anyone can clarify whether or not my ideas here could work?
why?: say, if someone is building something like a humanoid arm, a lot of motors will be needed, and encoders would add considerably to the cost. the goal would also be for it to be able to ideally have comparable speed and strength to a human arm (say, able to lift up to 50 or 75lbs, ...).
why not servos?: cost mostly. they tend to be either very small/weak or very expensive.
why not BLDC motors?: typically lower torque, so more reduction would be needed. also cost, as BLDC motors seem to cost a fair bit more than steppers (watts per $ is also a bit worse).
so, steppers seem to be one of the cheaper options to get well-controlled reasonably high-torque output.
however, for a useful arm, it would be useful for it to be able to move and respond gracefully to loads.
so, the idea is preferably, if possible, to find a purely electronic solution.
ATM, this is more of a hypothetical idea (I don't have money...).
one possibility seems to be that, during part of the cycle, the bridge could be disabled. at this point, the voltages across the phases could be observed, and a "flip" could be used to detect when to commutate. potentially, both phases could be monitored, with the controller alternately watching each phase.
however, monitoring one phase would require 7 IO pins per motor, and measuring both phases would require 9 or 10 IO pins.
pins would be:
bridges, A B C D;
enable: EnA, EnB
feedback: SnA, SnB, SnC, SnD
probably the sense pins would give a value relative to the mid-point between the positive and negative rails:
possibly, phase drives base of PNP, with emitter connected to a midpoint reference, and the collector is a pulldown;
the pulldown drives an NPN connected to a 3.3v pullup, which does the signaling.
issues: will it work? I really don't know.
some people seem to argue that you can't get usable EMF feedback of this sort from steppers, but I am not sure (there is the trick of linking steppers together, where turning one turns the other, implying it should be possible to get "some" sort of useable feedback out of them). had at one point used a stepper to drive an H-bridge to run some other steppers with manual controls, so at least this can be done.
another half-considered thought was if unipolar steppers were used, one possible hack would be to drive it asymmetrically, with one of the phases driven like in a bipolar stepper, and the other phase driven unipolar.
in this scenario, the other half of the unipolar phase could be useable to sense the movement of the rotor. its voltage should fluctuate as the rotor spins, and with any luck this could be converted to a useable signal.
this seems a little hacky though, and depends on the use of unipolar steppers, which would limit options (there are a lot less of them listed, where 4-wire steppers are most common, 6 wires less common, and 8 wire rather uncommon).
another possible hack could be partly rewiring it, noting that each phase has 4 sub-coils (for 8 coils in-total), but the motor should still work with 3 coils per phase. two of the coils could potentially be disconnected and used as sensor coils (the motor spinning inducing voltages on these coils). alternatively, tap wires could be installed (with a 3/1 split), with the tap wires used to help with sensing.
well, that or try installing a hall-effect sensor (with a piece of steel used to "read" the rotor at a particular spot).
but, I don't know...
otherwise: nevermind whether or not all of the electronics and similar could fit within the arm (chances are it would need to be largely moved using pulleys and cables, or be kind of large/bulky).
probable option would be to use a mix of NEMA-17 and NEMA-23 steppers, probably still with a bit of reduction (via pulleys and spools).
thoughts?...