Analog or Step/Direction servo drives for CNC tool room lathe?

I'm looking into buying 14" to 18" CNC toolroom lathe.

To keep costs down, it looks like the lower tier controllers (like a Siemens 808d) use step/direction servo drives and have the loop closed between the drive and motor.

As long as quality drives and motors are used (Siemens, Yaskawa, etc.), is there any real world disadvantage to going with step/direction drives/motors/controller as opposed to the higher tier controllers that use analog drives?

I would think a true turning center would benefit from feedback from the drive (real-time motor load), but for use in a low production application, is the lower tier stuff adequate?

I'd never own or recommend an open loop servo system. That's no better than Mach 3 or whatever the hobbyists are using now.

I have no interest in "hobby" stuff like Mach3. I'm talking about lower priced, CNC tool room lathes with industrial controls that have all the canned cycles.

No. Don't waste time with open loop controls. You want the machine to know where it is, not to hope it's where it told it to be.

jj80909 said:

I have no interest in "hobby" stuff like Mach3. I'm talking about lower priced, CNC tool room lathes with industrial controls that have all the canned cycles.

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Closing the loop does not ad that much to the total control cost. I can not see how any builder can construct a machine without using a closed loop servo system and end up with a machine that is trustworthy enough to actually use.

I'm also not a fan of stepper motor drives. The problem is that stepper drives even with a closed loop encoder system have no good way to compensate for the positioning error.

The analog and the digital variations of servo motors increase the output torque to compensate for positioning error up to the point of generating a positioning error fault.

The stepper motors even with an encoder are either on position or off. If they are off position the only solution is to increment the stepper motor. It either has enough torque to do it or it doesn't.

Axis acceleration and deceleration are also difficult to accomplish with the steppers as the only variable is the time between pulses.

When NC tape machines were in their infancy, using steppers was a common approach however these machines used encoders or resolvers and were mainly point to point positioning or straight line milling. They were incapable of doing circular interpolation unless extremely short segments were used requiring gobs of tape.

To me, no matter how you wrap up a stepper servo and especially with an open loop, you have a hobbyist machine.

I think there's some confusion on the 808D. It's running Siemens V60 servo drives, which are closed loop. There are no stepper motors. I think the confusion comes from calling it step/direction. Siemens calls it Pulse/Direction, which gets away from the stepper motor connotation.

If you go up to the 808D Advanced, you use V70 drives, which are controlled by a drive bus. You also get some more tuning/comp options.

I've never seen a good comparison performance-wise between and 808D/V60 setup (pulse/direction) and an 808D Advanced/V70 setup (bus). The 808D advanced has more tuning and comp options, but as far a performance, I haven't seen any comparisons.

DanielG said:

I think there's some confusion on the 808D. It's running Siemens V60 servo drives, which are closed loop. There are no stepper motors. I think the confusion comes from calling it step/direction. Siemens calls it Pulse/Direction, which gets away from the stepper motor connotation.

If you go up to the 808D Advanced, you use V70 drives, which are controlled by a drive bus. You also get some more tuning/comp options.

I've never seen a good comparison performance-wise between and 808D/V60 setup (pulse/direction) and an 808D Advanced/V70 setup (bus). The 808D advanced has more tuning and comp options, but as far a performance, I haven't seen any comparisons.

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Technically the V60 and V70 are stepper motors however they are using a micro-step type approach and they are using encoders.

I think the V70 drives would be superior for a machine tool application in that by using the Profibus for communications, much better total control can be achieved.

Getting back to the OP's original question, such a system will be adequate if not pushed to hard. As long as the machine is operated within the controls operational window, things will be fine. Just don't go outside of the window and expect satisfactory performance.

The step/dir systems are usually positional control, control loop only cares how to get from A to B. Anything that happens between the steps is not looked after if it stays in error margin.

So in the end it is a closed loop but there can be positional difference between the points but the last point is what it should be.

Have run analog and positional control and both work just as well if you stay withing the machines specs.
Doing a lot of radius or taper type turning then I would definitely go with the analog drives.
On basic turning cylindrical forms and threads and so on I personally dont see any difference.

Marko

The step - direction control signals to a servo drive work just fine in practice, yeah theres a allowable in position window that gets too large it errors out, good drives though will be doing exactly what a analogue control would and trying there hardest to keep that small as possible, the window is how far before it faults, its not what it consideres in position which seams to be the common misconception. but then in some ways so called analogue is no better and you still only have a control thinking the motor should be going this way or that. What matters far more for both is how well setup they are, thats what truly gets you nearest were you want to be.

The advantage of letting the drive do this rather than the control is your not using up the valuable control computing to do a task that’s easily handled by a very simple loop of logic in the drive. Drive simply has a counter in it for pulses, direction desides weather there added or removed and the control side of the drive then compares that count number to the actually encoder count number then powers the motor in the correct direction, Do to how servo's work they always tend to be in motion, thats were a stepper system is strangely more accurate, on a stepper at any given moment you always have anything upto its full tourque trying to hold it in position to that step

Ziggy2 said:

Technically the V60 and V70 are stepper motors however they are using a micro-step type approach and they are using encoders.

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Are you sure on that? The 808D catalog I have calls it "SINAMICS V60 servo drive" and a "SIMOTICS S-1FL5 synchronous feed motor" The data sheet for the S-1FL5 also calls it a servo motor.

The other nice thing about the 808D Advanced/V70 drives, beyond using a bus, is that you can use absolute encoders. I'm a big fan of the Siemens multi-turn absolute encoder motors. In the right application (i.e. your travel doesn't over-run the multi-turn amount) they can save a lot of hardware and startup hassle.

I understand [well mostly] how analog systems work, analog out to the drive, encoder to the control, so I would always tend to go that way, viewing the drives as long term expendable.

adama said:

The step - direction control signals to a servo drive work just fine in practice, yeah theres a allowable in position window that gets too large it errors out, good drives though will be doing exactly what a analogue control would and trying there hardest to keep that small as possible, the window is how far before it faults, its not what it consideres in position which seams to be the common misconception.

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Sure. But, when the drive faults, you're now lost. The machine has to go back to the home position to reconcile where it is with where it says it is. That's not how I want things to work.

The other issue is that the drive can only control one motor. If one axis has an issue, the other axes carry on like nothing has happened until a fault trips everything out. In a closed loop system, the control moderate all of the axes simultaneously. Maybe not an issue on a little tool room mill, but important in large machines making expensive parts.

There is a reason you will not find open loop step direction drives on a real industrial machine. That's for the hobbyists.

DanielG said:

Are you sure on that? The 808D catalog I have calls it "SINAMICS V60 servo drive" and a "SIMOTICS S-1FL5 synchronous feed motor" The data sheet for the S-1FL5 also calls it a servo motor.

The other nice thing about the 808D Advanced/V70 drives, beyond using a bus, is that you can use absolute encoders. I'm a big fan of the Siemens multi-turn absolute encoder motors. In the right application (i.e. your travel doesn't over-run the multi-turn amount) they can save a lot of hardware and startup hassle.

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Yes, Siemens does refer to them as "servo motors" which depending on your frame of reference technically applies to any mechanical force device being used to move an object. For many of us, we usually imply that "servo motors" are an analog type device and that "stepper motors" are always just steppers in a hobbyist environment.

If you dig down into the Siemens documentation, you will see that in essence the V60 and V70 are actually stepper motors with a rather sophisticated controller-driver.

This does not make them a hobbyist quality system but there are specific characteristics of the system that are strong points as well as weak points.

ewlsey said:

Sure. But, when the drive faults, you're now lost. The machine has to go back to the home position to reconcile where it is with where it says it is. That's not how I want things to work.

The other issue is that the drive can only control one motor. If one axis has an issue, the other axes carry on like nothing has happened until a fault trips everything out. In a closed loop system, the control moderate all of the axes simultaneously. Maybe not an issue on a little tool room mill, but important in large machines making expensive parts.

There is a reason you will not find open loop step direction drives on a real industrial machine. That's for the hobbyists.

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Again wrong, step direction drives properly tied together all fault at the same time, stop talking about facts like the times are still the 1970's things have moved on. The system is still very much closed loop, as a system it knows were it is more than a few modern drives don't lose postilion, they just need the blockage removed and they still will happily regain said position. Drives don't have to be dumb in the digital age, most are not dumb any more.

Steppers and fundamentally how steppers work let you have more torque in a given package at the expense of speed, if you need low speed direct drive high torque fundamentally stepper like motors are were your at, if you want high speed servo's are a better bet. its all down to how there designing and wired, to a degree a stepper is little more than a AC motor just only needs 2 phases. Steppers give up mechanically the notion of smooth rotation for large touque values, the magnet paths are a lot more direct than in a servo setup were oftern the rotor is skewed to make the motion as smooth as possible, just what you need for high speed movement.

FYI its also totally possible to use a step direction drive set-up with a separate source of error correction like glass scales hung directly of the axis or cleverer optical measurement set-ups, This point it returns to being much the same as your analogue function.

Better or best needs to be looked at from a system wide stand point, not a narrow minded one size fits all notion, more than a few modern drives are ditching both analogue and step direction and going straight to digital mod-bus type approaches were depending on setup data is flowing both ways and not just direction - position but also torque etc all flowing in as near to real time as digital ever gets. End of the day there’s lots of ways to skin this cat and more than a few of them when correctly applied are just fine in everyday practice. Equally a lot of them are becomeing ever more entangles with the kinda interfaces that are so cheap and easy to do these days.

adama said:

.....step direction drives properly tied together all fault at the same time, .....

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IME, this is where my bad experiences with step and direction controlled machine come from. The machine builder does not get everything integrated as well as could be and then, when problems occur, they are often like ewsley describes.

Ziggy2 said:

Yes, Siemens does refer to them as "servo motors" which depending on your frame of reference technically applies to any mechanical force device being used to move an object. For many of us, we usually imply that "servo motors" are an analog type device and that "stepper motors" are always just steppers in a hobbyist environment.

If you dig down into the Siemens documentation, you will see that in essence the V60 and V70 are actually stepper motors with a rather sophisticated controller-driver.

This does not make them a hobbyist quality system but there are specific characteristics of the system that are strong points as well as weak points.

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My understanding of stepper motors is that they have a toothed gear-like rotor. They have high torque at low speeds, but it drops off relatively quickly with increasing speed. They can be used open loop for motion control.

In modern, higher-end industrial automation, servo motors are generally brushless permanent magnet motors*. They need to run closed-loop for motion control. Torque is usually constant up to a few thousand rpm, then torque decreases until you hit the maximum rpm. You can operate at significantly higher torque for a short time, limited by thermal issues.

I'm looking at the data sheet for the S-1FL6 motor, which is used as with the V70 drive (and can be used with a V90 drive for non-CNC applications). It's listed as a permanent-magnet synchronous motor, and the torque curve (albeit with the V90 drive) looks like a servo torque curve, not a stepper torque curve.

What are the strong/weak points that you're talking about? Looking at the documentation I've seen, they look basically the same as every other servomotor. Where in the Siemens documentation are you seeing that they're stepper motors internally?

* there are also brushed servos, servo induction motors, etc., but in my experience they're a lot less common in the class of equipment we're talking about.

adama said:

Better or best needs to be looked at from a system wide stand point, not a narrow minded one size fits all notion, more than a few modern drives are ditching both analogue and step direction and going straight to digital mod-bus type approaches were depending on setup data is flowing both ways and not just direction - position but also torque etc all flowing in as near to real time as digital ever gets.

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This is very handy. The ability to view motor torques in real time is invaluable for troubleshooting. A lot of times you can tell that there's a mechanical problem before it becomes a serious one. It also makes setting up dual-drive systems much easier.

Bidirectional communication also allows the CNC or PLC to do anti-backlash on a rack-and-pinion or pinion-and-gear setup using two motors and two pinions.

DanielG said:

My understanding of stepper motors is that they have a toothed gear-like rotor. They have high torque at low speeds, but it drops off relatively quickly with increasing speed. They can be used open loop for motion control.

In modern, higher-end industrial automation, servo motors are generally brushless permanent magnet motors*. They need to run closed-loop for motion control. Torque is usually constant up to a few thousand rpm, then torque decreases until you hit the maximum rpm. You can operate at significantly higher torque for a short time, limited by thermal issues.

I'm looking at the data sheet for the S-1FL6 motor, which is used as with the V70 drive (and can be used with a V90 drive for non-CNC applications). It's listed as a permanent-magnet synchronous motor, and the torque curve (albeit with the V90 drive) looks like a servo torque curve, not a stepper torque curve.

What are the strong/weak points that you're talking about? Looking at the documentation I've seen, they look basically the same as every other servomotor. Where in the Siemens documentation are you seeing that they're stepper motors internally?

* there are also brushed servos, servo induction motors, etc., but in my experience they're a lot less common in the class of equipment we're talking about.

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I think we need to be careful in how we are viewing these.

What is the difference between an AC motor and a stepper motor in the winding-pole layout? Not much except for the number of degrees between the poles. The other difference that is not seen is how the power is switched.

Steppers are on/off +/-. An AC motor in a servo application is varying the frequency, voltage, and current to control the motor usually in a sinusoidal wave form. The DC servo motors are using a varying voltage and or polarity to control torque and direction.

AT various times all of these servo motors share similar characteristics. But then at other points on their performance curves, they are much different.

What Siemens is doing here is in a gray area between the traditional servo motor and a stepper motor. They are driving it with direction and step signals. The driver is driving the servo motor to the next position with each pulse.

There are some advantages to this method such as using PWM with the pulses to limit power dissipation etc. The downside is that this method usually has a max rotational or pulse interval limit and then the torque falls off severely depending on exactly how the motor is wound.

They could also probably use the same motor or very similar motor in a vector drive mode and get similar results however the vector drive method will probably function at a higher performance at the higher rpms and possibly cost more. All of this depends on the exact configuration the motor windings are.

All of this gets very complicated and the end product is a result of which corners the machine builder takes making much of this conversation moot points.

It all gets down to the exact hardware used, how the software is configured, and how many compromises in the cost/performance relationship are made.

For you, you are going to have to decide if this machine will perform to your expectations. Everything has limitations to what is the optimal performance window.

Feeding the servo drive pulse/direction as the control signal doesn't make it a stepper motor. A servo motor under position control is going to have a very different torque curve from a stepper motor, even a closed loop one. There are lots of servo drives, both from Siemens and others, like Yaskawa, that you can feed either pulse/direction, speed, or torque control signals into.

I think when talking about low end machine tools, it's important to make the distinction between a servo system (even one where the CNC talks to the drive via pulse/direction) and a stepper system (even a closed loop one). The torque curves of servo motors are generally far better for this application.

This has gotten pretty far OT. For the OP, as I said I haven't used the 808D, but I have used other low-end CNC's on toolroom lathes. If the lathe otherwise met my criteria, I wouldn't let the fact that it communicates with the servo drive via pulse direction stop me. How the builder puts the machine together and integrates the CNC is going to have a much larger effect.

Ziggy2 said:

Steppers are on/off +/-. An AC motor in a servo application is varying the frequency, voltage, and current to control the motor usually in a sinusoidal wave form. The DC servo motors are using a varying voltage and or polarity to control torque and direction.

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Thats just it, steppers are not simple on off's any more, yes thats how they use to be, these days its possible - hell its the norm to drive them in whats called micro-stepping, which to all intensive purposes approaches giving them a multi phase sine wave like power source. You no longer switch from one pole on to one off, but gradually switch one down and bring the other up to get smoother rotation. Lots of the better drives then go further and stick a encoder on the back and use this to get a position feed back and in effect become a servo. Do to how steppers are, do to the laminations and magnet arrangements (No its not just a winding difference, its far more in how the magnetic fields are controlled in the motor than how the coils are wound) you get far higher (double to tripple is common) low speed torques than a typical servo motor, trade of is a drop off at higher speeds, gotta remember there is no free lunch, a given motor volume will produce up-to a maximum amount of power based on its internal resistance and how well it can dissipate the heat it generates. Stepper, dc brushed servo, brush-less servo and AC induction all give you differing torque curves, as a machine builder you need to chose the right one to match the load and the way your trying to shift that load.

Really though im with DanielG above, how good the end machine is is a lot more down to how its working - setup system wise than any one feature. Loads of people on here like to bash the hobbyist systems, but correctly set-up its relatively easy to get the same or more capabilities than a circa 2000 commercial CNC machine, you won't find hobbyists dicking with drip feed, you won't find them using archaic interfaces or paying OEm's thousands for a extra MB of ram, replacing back up batteries or having any issues with backing up there control parameters. Rigid tapping is the norm in the hobbyist world these days and its not a multi thousand upgrade!!

By all means go out and spend serious coin on the industrial controls, but please, ask your self what your getting and then ask yourself just how its going to work for the application as a system, because on a CNC its the system wide integration that makes or breaks it, not any one feature.