closed loop steppers and drivers?

[Knut]

Do any of the stepper drivers sold handle encoder feedback? Gecko doesn't mention it. Most pages describing the process seem to think you might as well go with servos and do it right. The new steppers with encoders on them are spendy, and the used servos with the proper gear boxes for my needs (Plasma table with a drilling head and plotter pen) are spendy and not easily found.

JerryFlyGuy mentioned he was finding size 23 gear boxes cheap, not sure if I could match that, or couple up with the right servos either. Would like to buy
direct from a surplus jockey and get a spare right up front.

copper is so expensive, ruining one sheet would pay for some closed loop stuff. With all the talk of tuning and backlash problems with servos, I have retreated to using steppers but would still like to know if it were keeping count. Is there any fix to my concern? Or should I just get some big steppers and forget about it? The basic Vicon table does just that.

Thanks

[mcpltd]

Generally dumb drivers do not have encoder feedback capabilities.
You can get other boards to handle the encoder feeback though.
The only driver i know of that handle encoder feedback are the drivers with the controller built in.
As an example the IMS IM483IE, panther, miclolynx and the Mdrive can handle encoder feedback.

[InspirationTool]

Mariss has said that Gecko is working on such a driver, but there has been no sighting in the wild yet.

The large stepper system seems to work for alot of people.

-Jeff

[bradodarb]

I am building a gantry mill and have decided that I cannot afford a servo system just yet. I found a company that sells a closed loop stepper system for under $1,200
at [URL="http://www.maxnc.com"] I'm not sure if their double stack motors will be enough to power my machine, but I'm certain that it will handle the loads applied by a plasma cutter just fine. Seems to be that the controller, drivers and motors are a complete package. The only downside I can see is that the controller interfaces through the parallel port of your CPU. Probably not the best setup when interpolating 4-5 axis but again would be more than adequate for a 2.5 axis setup.

[ger21]

If you get steppers with encoders, and use Mach3, you can get a board from www.rogersmachine.net that will monitor the position of your steppers, and stop or pause the machine if any errors occur.

As for the maxNC system. For not much more than half the price, you can get 3 Geckos and some 400-600 oz in motors and get 3 times the performance. You lose the closed loop, but should have plenty of power to not have to worry about it.

[Knut]

Thanks for all the tips. Looks like sticking to proven stepper techniques and readily available boards for my first table will make things less problematic.

That Rogers board looks like my only option, not sure how to back up and correct a mistake or miscount in Mach3, but perhaps that will become known once I do some study thereon.

Thanks again.l

[bradodarb]

Gerry, does your proposed nearly half the price solution include a controller/breakout? I have'nt used mach3 or whatever software maxnc is using, but from the screenshots mach3 looks quite a bit more sophisticated.

[Torchhead]

Properly designed stepper systems don't lose steps. The event of a loss of position comes when something gets beyond the ability of the motors. At that point even with encoder feedback the motor can't do any more. The best use of encoders on steppers would be to prevent ruining a part if lost steps do occur (stop the machine) or with more sophisticated electronics to actually slow the motors down to prevent lost steps (rather than adding in more power to try and correct). Slowing the motors down too much with plasma can have really bad cut results.

[kreutz]

Even properly designed stepper systems could lose steps. While the designer will leave a torque reserve on the motor as a safety margin nothing will stop the user from adding more load or speed reaching the limit. Stepper motor drivers are generally open loop, so lost steps will accumulate.
By adding the encoder you close the loop, so the controller is able to detect and correct the position error similar to what a servo does.

You can use the encoder as verification tool, detecting and flagging an error when you lose steps and stopping the system, or you can use a real closed loop controller that corrects the problem and saves your part.

Steppers don't work as servo motors in the sense that you can accelerate the servo to catch up when the positioning error increases, but you can't do the same with the stepper because it will probably stall.

The stepper motor's torque decreases with rotational speed, so the faster it goes the less torque you have available. Using PID in that case is frustrating due to the non linearity of the stepper response. What many closed loop stepper controller designers do instead is accumulate the error and provide those extra steps at the end of the movement.

See http://digital.ni.com/public.nsf/all...0?OpenDocument

[Torchhead]

Perhaps I should have said "properly designed and used stepper systems". I have an open loop stepper system for engraving and light routing that has never lost position unless I did something stupid like ramming into a holddown. Pushing a design past it's limits will result in some form of failure...even in servo's.

Adding in steps at the end of a move is fine for "positional" applications where the load is moving to a given spot. I see this a lot in industrial postioning systems where something has to move to a very precise point but the path it takes is immaterial. In the real world of CNC cutting, mis-position along the toolpath (cutpath) should not be made up somewhere later and the forces that caused the mistake ususally cannot be corrected.

Stepper closed loop operation has little value in toolpath operations unless it has a way to actually lower the speed (all the way to zero if necessary) if the position is outside the error range of the control. At lower speeds steppers have more torque and could possibly power through the issue (perhaps while ruining the cutting tool!). It's the inverse of the typical closed loop.

The analogy is in cruise control of an auto. It works fine if your running straight on the highway but functions poorly if you have a blow out. At that point it needs to do something else besides adding in more gas pedal!

[kreutz]

There is a renewal interest on closed loop stepper control, there are several advantages including low cost and higher torque at low rpm than equivalent size servos, the problem so far has been controller complexity.

Steppers show a tendency to resonate, lose torque and stall in what is called midband instabilities. There are some known techniques to compensate but so far it has been proven complex to design a rugged closed loop control system on similar basis as the servo controllers. Sometimes reducing speed will take the stepper over one of those resonance prone zones. The new interest is in using the called flux-vector control method with or without external encoder feedback in order to overcome those limitations.

New published results are promising, as shown in some published research papers, flux-vector control method allows magnetic field optimization control to compensate for the voltage loss due to BEMF and extend the motor’s speed range almost 100% eliminating midband resonance effects. Hopefully DIY controllers based on this technologies will appear in the next few years.

Google: "CLOSED LOOP CONTROL OF STEPPER MOTORS WITHOUT POSITION SENSOR" and "NEW BENEFITS OF VECTOR CONTROL OF HYBRID STEPPER MOTORS WITH ENCODER" research papers for more information.

[Korellibopper]

Have you checked out how you can daisy chain the PMDX-131 to make a closed loop stepper system. You add a another PMDX board and it has the encoders hooked up to it then you connect that to the PMDX-131's grnd power supply and a couple inputs. Its in the intructions manual you can download them from PMDX.