phut

I think this is looking at it from the wrong side. By the time you see the filament stall you've already ground out the filament and the damage is done. The mechanics really need to be right for the electronics and control system to handle any failures gracefully. From sensing I think it would wise to start by sensing the filament diameter indirectly, many of the pinch-roller upgrades with spring loading would be easily adapted. From the drive perspective, it would be useful to slip the drive mechanism via a controlled limiter (e.g. variable motor current) rather than relying solely on the greatly variable filament as a fuse.

It's up to the specifics of the application, even industrial machinery quite often uses stepper motors. For laser cutting machines up to ~200W for example it's almost ideal cost/benefit wise, there is only the mass of the head and gantry to contend with and steppers are cheap and predictable as long as the loading is predictable. In fact there are many parallels between the Ultimaker's mechanism and the one you would find inside many laser cutting machines - it's just that one is optimised for the delivery of laser light and the other optimised for delivery of plastic and electricity over a cable. As said there seems to be a huge variation of what servo actually means to people. 'Servo' in general describes a complete control system that operates via feedback. If the stepper motors that we used had a position feedback system, then this would make it a stepper servo and such things do actually exist for various reasons. The cheapest way of going servo is really the same way that some RepRaps are done with DC motors and encoder strips. I'm not sure about the exact resolution, but something like 0.08mm is probably within reach for those methods. With the same drive (belt) ratios the RS-555 equivalent DC motor used in most printers wouldn't quite match the holding torque offered by the current NEMA17 - you would have to go to a RS-755 and also put up with occasional 20 ampere spikes in current when your head really does lock up dead or you have the need for speed and want to negotiate a full reversal at full power for whatever reason. Inkjet printers get away with it because they usually decelerate 'off the page' - not really a facility available for our extruders where we don't have the luxury of microsecond flow control. Even then there's probably going to be issues related to the overall rotation speed of the motors, which determines the intrinsic cooling available to the windings (limiting long term power at low speeds).

My thoughts were much the same - to measure the forces somewhere at the extruder - but that only works well if you're after the forward forces only; the pressure gauge would obviously work better in the cases where you're doing retractions because of the play in the bowden assembly. A simplex way of measuring pressure at the extruder would be to change the motor out to an encoded PM motor and look at the control currents - which as long as you have minimised the friction with ball bearings at the pinch point and geartrain would be a fair indication of pressure. Of course changing the extrusion motor to PM also brings along a host of its own problems.

For me it seemed that part of the belt crept and lengthened over time, initially I couldn't get anything near the noise suggested by the video for the belt tension with the block tensioner alone (the sound was always a bit too high once I put in part C - but my deflection assessment suggested it was too low) but after a few nights the pitch settled to slightly below the suggested adjustment. I don't know if it's my mistake or something else happened. From assembly it looks like the X and Y rods should probably have their own clamp and location hole but they must have had a reason to do it the way they did. I personally pushed the rods in from the bottom up. If you look really closely the laser cut pieces are all tapered - for anything else it's two tenths of nothing but on an interference fit like the Z rods you can really feel it. It shouldn't require much force to push them in from the bottom up (where you're pushing into the taper of the laser cut pieces) just get it square with the piece and a small rotation as you go in. If it does require any more force or seizes after a few mm, sand it as recommended as there is a risk of splitting the wood.