steppers with encoders?

[rolveram]

Hello everybody:
Can somebody tell me what is the advantage to install encoders in steppers?
Is this the way to get certainty to don´t lose steps?
rolveram

[Pres]

Hello everybody:
Can somebody tell me what is the advantage to install encoders in steppers?
Is this the way to get certainty to don´t lose steps?
rolveram

Nope, it won't have any effect on losing steps.

But it can quickly stop the machine so you can correct whatever has caused it to lose steps rather continuing to machine the part incorrectly.

Otherwise, you might not know it lost any steps until the program is finished and went back to target.
Only then do you find out it has moved. Yeech! - more scrap?

Where and when it lost steps might still be a mystery until you closely examine the (scrapped?) part.
However, it could be a bit too late to save the part.

Losing steps can easily occur in subsequent parts due to the cutter getting dull, inadequate axis lube producing extra drag, or excess/unplanned stock removal due to sloppy saw cuts or casting irregularities, etc.

IMHO, encoders are an essential ingredient in most any stepper powered machine.

Encoders DO NOT correct any stepper position problem, but it can simply stop the machine so you can correct the actual problem causing it.

At least it can give you a chance to save the part when the problem occurs.

HTH,
Pres

[John3]

Why in the world would someone put an encoder on a stepper motor?

I thought the only advantage of using steppers was that you didn't need encoders.

If you're going to the trouble of adding encoders to the motion controller why not take the additional step and use real servo motors???

John

[Pres]

Why in the world would someone put an encoder on a stepper motor?

Sorry about the failure of my communicative ability but I thought I answered that question in sentence #8 of my previous reply. I repeat it:
"Losing steps can easily occur in subsequent parts due to the cutter getting dull, inadequate axis lube producing extra drag, or excess/unplanned stock removal due to sloppy saw cuts or casting irregularities, etc."

I thought the only advantage of using steppers was that you didn't need encoders.

Not quite - overall expense is a major consideration.

If you're going to the trouble of adding encoders to the motion controller why not take the additional step and use real servo motors???

With my failure to communicate in anwering your first question I won't even try to elaborate on following error and axis response considerations.
Suffice it to say I've made alot of elliptical corner radiuses by using too high of feedrates, axis binding/drag, etc without any indication of a problem from my servo system.
Not until part inspection did it become apparent.

Servos are not a magic elixir to the myriad of machining problems one can experience.
However, prompt error detection with a encoder/stepper configuration could help to minimize many of them.

Pres

[rolveram]

Thank you both for your advise.

Press argument is by itself sure enough to consider install them, but Press will you please tell me how to do it. Does the exit pulses must be sent to both (steppers and encoders) at the same time? or must go first to the encoder and the encoder send the signal to the stepper?

John argument is considering that install servos is better solution than install steppers, probably is right, but brush DC servos require more maintenance and brushless DC servos, AC servos and drives are considerably more expensive than steppers.

I think to install steppers is the cheapiest way to make any retrofit, but probably is not the better way. The better way is for sure much more expensive. For which of this two alternatives you get more for each invested Dollar?

rolveram

[CAMCRASH]

Here is a link from National Instruments for their control which appears to have error detection
http://digital.ni.com/public.nsf/all...2572CF00805A51
The linked page is to a knowledgebase page which has product #s on it

[John3]

...... With my failure to communicate in anwering your first question I won't even try to elaborate on following error and axis response considerations.
Suffice it to say I've made alot of elliptical corner radiuses by using too high of feedrates, axis binding/drag, etc without any indication of a problem from my servo system.
Not until part inspection did it become apparent.

Servos are not a magic elixir to the myriad of machining problems one can experience.
However, prompt error detection with a encoder/stepper configuration could help to minimize many of them.

Pres

Couldn't / don't servo systems have the same prompt error detection if you monitor the error step count?

I have some (more than 10) 20 year old servo motors running daily, I think the brushes may have been changed once in their entire life. I'd argue that Brush motor maintenance isn't a serious consideration compared to missed steps and lower high speed performance of steppers.

That said, I see your point about steppers being simpler, I am considering a stepper drive on a single axis saw control.

On the other hand with great products like the Gecko 320/340 servo drives, why not use dc servos?

John

[Pres]

.... , but Press will you please tell me how to do it. Does the exit pulses must be sent to both (steppers and encoders) at the same time? or must go first to the encoder and the encoder send the signal to the stepper?

Check out: http://www.rogersmachine.net/encoderinfo.html for a description and system overview.

That board & software can stop a Gecko/Mach 3 machine if it loses a pulse.

Pres

[Pres]

Here is a link from National Instruments for their control which appears to have error detection
http://digital.ni.com/public.nsf/all...2572CF00805A51
The linked page is to a knowledgebase page which has product #s on it

Good example of why you want to match the encoder step with stepper pulse on a 1 to 1 basis -no more, no less.

[Pres]

Couldn't / don't servo systems have the same prompt error detection if you monitor the error step count?

The magnitude of the "following count" i.e. position error, since a servo machine ALWAYS has a following error, is dependent upon axis inertia, servo power and speed the axis is moving at.

The higher the speed the greater is the following error.

If you reduce the following error too much then the machine can overshoot its terminal position or, worse yet, go into oscillation.

Servo "tuning" is most important for the kind of machining you want to optimize for. i.e. high speed contouring vs lower speed/higher precision operation.

High speed/high precision is an oxymoron in stepper language.
In other words, do you want high speed OR high precision with a stepper system?

Home/benchtop cnc machines for metal machining are, or at least should be, run at relatively slow speeds.
For this use, a stepper system can do it at the lowest cost.

Besides, most have primitive (at best) lube systems that are not designed for high speed operation.

Having a 400ipm rapid on a 10" axis move is of little real value to the home user when compared to the cost of a drive system/machine capable of providing that.

An encoder attached to the stepper is to simply stop the machine in the event of a position loss. The user has to then figure out what caused the problem and correct it before proceeding with machining.

Pres

[rolveram]

Thank you Pres for your comments; tell me please what are the "relative low speeds" you are taking about with the steppers. Does it depends of stepper size or the rating torque, or what more else can be?
I am runing a 2250 oz-in stepper system in full size Matsuura at a 150 ipm jog rate and it seems very stable. Is this relative low speed?

rolveram