Comment by dbcurtis | Hacker News Reader

Comment by dbcurtis | Hacker News Reader

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Steppers skip steps if you are asking for more torque than they can deliver. No other reason. It might be: 1) too small of a stepper motor, so it can not deliver sufficient torque for maximum load, 2) too small of a driver that can not deliver the current needed for the motor to reach its rated torque, 3) software bug that tries to accelerate the motor faster than it is capable of under the load at hand, 4) software bug that throttles the driver.

They all boil down to commanded torque greater that the system is capable of delivering. Fix your design. Be suspicious of software trying to accelerate too aggressively under load.

I have cut a lot of metal on a Tormach PCNC 1100 Series 3 machine, with steppers. NEVER had an issue. Correctly designed stepper systems DO NOT miss steps.

That said, servos systems typically are capable of greater accelerations for a motor of a give volume and current load, because of the closed-loop control. Use servos for speed, not because you are afraid of skipped steps from a stepper.

Correctly designed stepper based systems use encoders or other feedback mechanisms to detect missed steps and correct for them. Open loop systems will always miss steps, and most hobbyist aimed gear is open loop because it is super cheap.

FWIW I designed and built CNC equipment for a living.

As for the Tormach machines, they use 3 phase, not 2 phase steppers, and use current sensing on the stepper outputs to give them a feedback mechanism, and an encoder to close the loop completely these drivers and stepper motors are better than the ordinary two phase kind that you find in regular hobbyist aimed gear.

https://www.damencnc.com/en/nema-34-closed-loop-stepper-syst...

They are properly classed as a hybrid servo system rather than just a stepper on account of that feedback mechanism.

You're not wrong, and when the part being cut is worth 5 figures for the raw stock, it's probably a criticality, but it's important to not gatekeep this process, lest it become a no-true-scotsman kind of thing.

I built a couple of 3d printers from scratch BECAUSE the various components were cheap and approachable. Haven't done CNC because my interests haven't taken me there...but the thing about advancement is: While one fella is saying 'You can't do it that way!' someone comes around and does it that way, and the first person is left in the dust.

You can't really stop Laser printers from dropping to many thousands of dollars to $70...all you can do is ride the wave.

The probability of any given step being missed is proportional to the amount of value lost of the step is missed. Pretty weird how that works ;)

Servos and closed loop systems are almost mandatory on large commercial mills due to the sheer scale and mass involved, but they’re not as mandatory on the hobby level.

In practice, hobby level machines don’t have near enough rigidity or spindle power to warrant high movement forces in the first place.

Nearly all of the hobby CNC machines on the market use open-loop steppers. They’re definitely not losing steps during normal operation.

The topic of steppers vs servos has been covered over and over on every hobby CNC forum for the past two decades. Closed loop systems are great if someone has extra budget to spare, but they’re unnecessary for machines using hobby-level spindle power.

That's not necessarily true, it all depends on what your desired outcome is. Stepper systems can be open loop as long as you're aware of what the torque threshold is and make sure you don't exceed that. Obviously a crash would exceed that, but there's ways to detect that (e.g., stallguard on Trinamics stuff) without adding an encoder. The whole point of open-loop stepper systems is that you make sure you stay under the torque limit by a margin (say, 30% or so) and you are fine. A properly designed system should never lose steps ever, besides in a crash event. Or if you load it up with too much side force, but you can get that on any machine.

Stepper motors also have resonances that you can excite with typical step pulse trains, that can also lead to missed steps while technically not overstepping the torque budget.

You can work around those resonance points quite well if you know what you are doing, the trick is to realize that the back-EMF around those resonance points can be so high that your stepper is momentarily generating power rather than consuming it. If you try to force it through that point by adding more current then the stepper will stall completely. A properly designed driver that is matched to the load of the stepper will use a complex voltage/current model that will drive the stepper just right to avoid this problem.

This technology originated with the Berger-Lahr company in concert with an Italian driver manufacturer for five phase stepper motors, which were the first to be driven past this resonance point, the tech was then perfected and adapted to other, cheaper steppers as well.

My guess is that the tormach detects skipped steps in the controller ("sensorless stall detection")

So it probably does skip steps, but corrects for it

That is exactly what it does. This is also a setup that will work fine, but it is actually much closer to how a servo would operate and negates quite a bit of the cost difference due to the far more expensive drivers, with encoders and current sensing on the stepper wires.

The encoders will tell you when to increase power because you're about to miss a step, the expected movement is lagging compared to the amount of input current. The current sensing will help you to detect tool strike situations before damage is done to the motors due to overcurrent.

If you detect missed steps, and apply feedback, isn't that now a servo mechanism?

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