Servo motor signals and power source

cheers for that last one!
Try this one out, basically i need to know what methods are used to control a servo motor in a PLC such as signals and power sources but i am yet again completley baffelled!!
Thanks again!!!

Whats the application? the type of servo drive? which PLC? a lot of questions before we get the right answer.

Basically you can use a high speed output signal from a PLC to control the servo drive. Again whats you want to control ? position? velocity?

Instructions like PTO and others are used for doing this. and CCW and CW digital ouputs for controlling the direction. The PTO is a frequency output command. It will be used for controlling the traverse speed and distance.

High speed output??? What kind of high speed output?

Lark, you are thinking of stepper motors. 'Klaus again' is asking about servo motors. Servo motors do no use step counts and a direction although we do have a drive here that is a servo drive with a stepper like interface. In other words the drive converts the step and direction signals into a target or reference position for the servo motor to follow.

Normally a servo card in the PLC generates + or - 10 volts that goes to a drive ( amplifier ). The PLC servo card does not drive the servo directly because it doesn't supply enough current. The analog output is updated every 500 microseconds to 1 millisecond. That is much faster than a PLC can update the analog. If the PLC could update the analog output card that quickly the analog card itself would not respond fast enough.

Usually, the feed back is an incremental encoder. However, MDT and SSI feedbacks are also available. Other digital signals that you may see are
Positive and Negative Limit Switchs.
Home and Z inputs.
Enable output for enabling the drive
A fault input for letting the controller know the drive has faulted.

PLCs are NOT fast enough to PWM the control outputs. A PLC servo card could have PWM as an output be then it could only control servos that accept PWM. PWM is also quite noisy. Fortunately any PWM signals coming from the PLC servo controller would be low current. Still it would be more challenging to the the CE mark.

Not true, Peter. I am refering to servo systems.
Most of the servo drives available toay (Siemens/ AB// Mitusbishi) can be controlled in various methods, including PWM.
A PWM/ PTO signal can be definately used to control a servo drive, and I dont mean s stepper motor. The Servo Drive in turns controls the Servo Motor. SO the problem of current is taken care by the servo drive. For having more complicated 'moves' you need a external motion controller or a PLC with high pseed capabilities in addition of a Servo drive and Motor and encoder/ resolver assembly.

A servo drive will accept a PTO or PWM frequency input along with CW or CCW inputs, or it will accept the 0-10 Volts inputs, or it may communicate digitally with the PLC for the control signals (Kinda SERCOS systems). And the PLCs that have motion/ high speed capabilities built into them have dedicated circuits that do the work, and this does not interfere with the anaolg or discrete signal processing in any manner.

And yes, the PLCs available today in the lower range do have high speed outputs for PWM use. But they are not "That" fast. (Upto 20 KHz only.) So you need to really select what kind of response u need. There are any number of dedicated cards available that will go right inside the PLC in case u need a response of lets say 100 KHz.

Hope this hepls

I might be confusing something here.

Many, if not most, true servo drives can operate either by a pulse train or by an analog voltage signal. There are many different PLC cards to control servos: some of them produce pulse train output and some of them produce analog (Omron, for example, has both kinds in its mid-range C200 and CS1 series). There must be a difference.

I always thought that a servo drive will accept the PTO signal when configured for position-control. Which means that both the velocity and the position control loops are being closed within the drive's "brain" and all the math, motion profile, corrections, gains etc. is remaining within the drive. The PLC card simply generates the pulse train based on the required position command and monitors the servo drive rather passively, just making sure that the motor does not deviate too far from the target position, there are no errors etc. You can run "point-to-point" positioning applications this way, you can do registration moves, you can do constant-velocity feeds. You probably can do some rather simple types of "master-slave" following... Can you do electronic camming or interpolating moves this way? Not sure... maybe... very slowly.

For that you need a motion control PLC module, i.e. the one that produces analog velocity command signal. The servo drive then configured accordingly for speed control. The velocity control loop is still being closed within the drive (i.e. maintaining the speed that the drive is being told to run at). However the position control loop is now being resolved within the PLC card's "brain". Now the card is capable of coordinating several axes, camming, interpolations and the whole lot of other nice things.

I did not even mention such thing as torque control, when one needs to maintain a certain motor torque, like in "unwind-rewind" applications. I don't think that can be done with a pulse-train signal. I do beleive though. that PTO PLC modules and the ability of servo drives to run pulse-train signals is a legacy thing, intended for easier "drop-in" replacement of the older stepper systems.

By the way, since I have mentioned Omron: the PTO cards are called "positioning modules", while the analog ones - "motion modules". Feel the difference?

So the answer is: it all depends on the application. For simple "point-to-point" moves one may do perfectly well with PTO module. For serious motion you need analog. I did not mention the ever more popular network-based systems (SERCOS, Mechatrolink, Profibus etc.) - these are expensive and fancy versions of "analog" systems.

Well, that is what I think. I might be wrong here.

Lark, your previous post did not mention PWM

Pulse trains are for steppers. PWM and analog is for servos. Also, as LadderLogic and I have pointed out there are some servo drives that provide a stepper like interface form simple controllers like PLCs.

In any case Klaus again was asking about inputs and outputs and controls from a PLC not a high end servo. Lark, show me a PLC that can issue PWM with a direction output for direction. A frequency of 20K does not cut it. That leaves a resolution of only 20 steps if the PWM is updated every millisecond.
To simulate a 16 bit DAC one needs 65536*1000 or 65+ MHZ counter. I haven't seen PLCs that can update anything every millisecond, not to mention a high speed counter. I don't call 20khz high speed no matter what the PLC manufacturers say.

Does PTO mean the same as PWM to you guys? If so then what do you call the pulse traing output to a stepper? To me a pulse train does not imply a duty cycle.

Motion controller tangent.

I always thought that a servo drive will accept the PTO signal when configured for position-control. Which means that both the velocity and the position control loops are being closed within the drive's "brain" and all the math, motion profile, corrections, gains etc. is remaining within the drive. The PLC card simply generates the pulse train based on the required position command and monitors the servo drive rather passively, just making sure that the motor does not deviate too far from the target position, there are no errors etc. You can run "point-to-point" positioning applications this way, you can do registration moves, you can do constant-velocity feeds. You probably can do some rather simple types of "master-slave" following... Can you do electronic camming or interpolating moves this way? Not sure... maybe... very slowly

Click to expand...

I said in my previous post that we have a drive like that. It isn't what I call the norm. The problem with these types of drives is that the PLC must generate the motion profile for the drive to follow. This requires a lot of work.

Motion controller modules can be issued commands that will generate the motion profile as well as close the loop. Many have both a position and velocity 'loop'. The 1756-M02AE has a velocity loop and a position loop. Other controllers like ours combine the two filters into one so there a position loop or velocity loop depending on the requirements and feed back of the moment.

I do beleive though. that PTO PLC modules and the ability of servo drives to run pulse-train signals is a legacy thing, intended for easier "drop-in" replacement of the older stepper systems.

Click to expand...

I agree.

By the way, since I have mentioned Omron: the PTO cards are called "positioning modules", while the analog ones - "motion modules". Feel the difference?
[/qoute]
Yes. It is a marketing thing. However a high quailty module will always be doing position control if it wants to do camming or gearing or fancy motion profiles. The better modules will have a trajectory generator so the ideal postion, velocity and acceleration is always known. I do not get hung up on position versus motion module. Motion sounds arbitrary to me. Position does not.

So the answer is: it all depends on the application. For simple "point-to-point" moves one may do perfectly well with PTO module. For serious motion you need analog. I did not mention the ever more popular network-based systems (SERCOS, Mechatrolink, Profibus etc.) - these are expensive and fancy versions of "analog" systems.

Click to expand...

You forgot PWM. It is different than a pulse train.

Isochronous (constant time) bus systems are interesting. Profibus is not ready for prime time yet. At least not in a isochronous application. I havn't heard of Maechatrolink. Sercos, USB, and Firewire can be isochronous and provide ways of making an expandable system without analog.

Well, that is what I think. I might be wrong here

Click to expand...

I think you got it right. I just want everyone to know there is a difference between a PTO and a PWM. A pulse train does not imply a duty cycle.

Click to expand...

Peter:

Mechatrolink is Yaskawa's motion and I/O bus(4 to 10 mbit/sec), twisted-pair serial, up to 14 servos per drop, 0.5 to 1 ms cycle. The reason I mentioned it is because Omron's latest motion module for CS1 is Yaskawa's (it is available only in Europe now, AFAIK). It runs the enchanced version called Mechatrolink II. It is not trying to be an all-out standard as SERCOS but worth mentioning.

Yaskawa's MP series PLCs easily do 2 ms fixed scan time. Not as good as 0.5 to 1 ms of a dedicated motion controller, but good enough for lots of real-life applications. Not to mention that MP920 is a lot cheaper than ControlLogix and easily outperforms it as far as motion control is concerned, being at par regarding the logic functionality.

http://www.yaskawa.de/english/products/control/mp920.htm

To simplify, a servo running on a pulse input is essentially a stepper motor that will never miss a step. This setup has its uses, though I haven't done it in a while.