closed loop

[Art Ransom]

I need servo or stepper that knows if step count is off and correct it. I understand the therory but need advice on implementing it on Mach 3 controler. Barnds sites etc.

[Torchhead]

Let's say for the sake of argument you could determine if the step count is off and had the software make a correction. What would you have it do? If you are losing steps it's because something has forced the steppers outside of their operational capabilities. If you have them tuned correctly they have little or no reserve power. Just adding in more steps will not "correct" the problems. The actual answer is to move the torque up by slowing the motors down. Steppers gain torque with slower speed. If the loop were set right the motors would continue to slow until the positional error was fixed or they just stopped.

The other issue with any software-hardware solution is the loop gain and delay. It's no trivial task to build a software "servo" loop inside a package that is doing dozens of other tasks.

Mach3 does not support a closed loop stepper or servo. The current solutions are hardware based and in the servo world (where you CAN force the motor to give you more speed AND torque)

The current implementation of the encoder postional feedback with MACH is that through a macro you can read the actual postion via the encoders and compare it with the commanded position and issue a fault and stop the machine motion. It's not a true closed loop because no steps are added or subtracted but it does prevent a ruined piece of work. It stops the machine and allows you to correct the problem that is causing the motors to run outside their SOA.


Rogers Machinery sells a solution for MACH that does the above.

[unterhaus]

If you really want closed loop control with a pc control, your only option is EMC.

[derekj308]

Hi Guys

I'm pretty sure that Mach4 and the G-Rex from Gecko will (does?) support positional feedback with correction. I think Mariss dubbed it "the unstallable stepper".

Cheers

[ger21]

Originally Posted by derekj308 Hi Guys I'm pretty sure that Mach4 and the G-Rex from Gecko will (does?) support positional feedback with correction. I think Mariss dubbed it "the unstallable stepper". Cheers

Mach4 is no more. Mach3 now uses a plugin to control the G100, and at this time, there is no closed loop support.

The "unstallable stepper" is something completely different that Mariss is working on.

[derekj308]

Hi Gerry

I am disappointed that Mach4 is no longer developed or supported but I am relieved that at least there is a plugin to enable it to work with the G100. I am sure positional feedback will come. I don't follow G100 or MachX threads religiously so my finger isn't on the pulse of the development of this MachX/G100 system so please forgive me if I am way out of touch on this subject.

I am confused about your statement about the "unstallable stepper" being completely different. This thread, to me at least, revolves around correcting position with feedback, be it with an off the shelf servo or stepper drive that has that functionality or using a dedicated piece of hardware like a G100 with 'dumb' drives. Art Ransom has asked how to implement it with Mach3 and I assumed incorrectly that Mach4 and the G100 had all this worked out so I threw up the post.

My understanding of the "unstallable stepper" project is that it is based on the feedback loop existing within the G100 i.e. positional feedback with correction. My understanding of the role of MachX (or any other control software) tied to a G100 was to deliver the commanded position to the G100, not act as part of the feedback loop so as to relieve the PC of the burden of number crunching and to ensure clean, accurately timed pulses so the drives woulds hum. You would still feedback the position to the controller for secondary checking but if you had faith in the program on the G100 you could run without feedback to the controller and be confident that position was being maintained whilst feedrate was being slowed to cope with additional load on the motors.

http://www.cnczone.com/forums/showthread.php?t=20058

is the thread where I based my understanding of what the "unstallable stepper" project was about.

If you could point me to more relevant threads so I can catch up I would appreciate it.