Hi,
This is a progress report on new developments in the "unstallable stepper project".
The goal of the "unstallable stepper project" is very simple. Imagine you have built / bought a 4-foot by 8-foot gantry-style router. You are doing 3D (x,y,z) routing with it when your drunken brother-in-law is over and gets the idea to see what the ride is like if he were sitting on the gantry. The result would be ordinary steppers or servos would immediately stall / fault as soon as the sobriety-challanged one clambered aboard.
Now imagine the motors simply slowed down or even stopped due to the "idiot aboard" overload but they never leave the 3D path in progress. Dislodge the load and the motors pickup to their original speeds and finish the work as if nothing happened. Not a gouge or mark on the finished work afterwards.
So how to get from here to there? A lot of parts that all have to come together is how.
Part 1A, Stepper Servo: You see BLDC servos, AC servos, PM DC brush-motor servos. You don't see step motor servos (Vexta Alpha Step and the like, step drives with monitoring encoders don't count because they are half-assed solutions). Why? Because technically it is very hard to servo-tame a step motor. Very, very hard in fact.
Begs the question; why bother what with all the other servo choices? The answer is steppers have a unique speed-torque curve that makes them perfect for 2-mode applications. Lots of torque at low speed work feed-rates and only enough torque for high-speed rapids. Nothing is wasted.
Part 1B, Stepper Servo: Open-loop step motor systems have to be seriously derated. They are running open-loop after all. Closed-loop, a mild-mannered Clark Kent type motor becomes Superman. It literally jumps on the bench from acceleration reaction torque on its way to 15,000 RPM. This from a NEMA-23 2A motor with a 24VDC power supply. It takes 0.03 seconds to go from a standstill to 3,000 RPM. 0.42 seconds later and it's at 15,000 RPM.
The biggest differece? It sets its own optimal rate of acceleration, it cannot resonate, it always gives 100% but cannot cross the line and stall while trying to give 101%. That's built-in.
Part 2, The G-Rex factor: Your 300-lb brother-in-law on the gantry requires way past 100% from the motors. They won't deliver more than 100% so something has to yeild. What has to give is the step pulse rate and that's where the G-Rex enters the equation.
The G-Rex from the get go was designed as a vector based pulse engine. What that means is the G-Rex generates axis step pulse frequency ratios naturally. Now imagine 3 motor axis tracing out a 3D path; the motor speeds are at fixed ratios along a 3D line. One or more servo steppers report an impending overload to the G-Rex and it responds by slowing down the vector velocity step pulse rate.
The motors respond by backing up their speed-torque curves to find more "grunt"; what was a near overload at one speed becomes a manegable load at a lower speed. Only step motors have this beneficial speed-torque curve. This works all the way down to zero velocity.
No matter what your brother-in-law does, he cannot disturb the router from its programmed path. He may be able to stop it but he cannot move it off of the intended path. That is the goal for the unstallable step motor project.
Part 1 is now progressing rapidly and it's consuming the effort currently being applied. Part 2 should follow after part 1 is finished.
Mariss