Servo control

aandiw · Jan 19, 2009

Did anyone experience use PLC to control servo motor? I've a problem when using PLC to control table position for cutting machine.
The problem is I can't stop exactly at position that I want to go. Sometimes the position can be reach exactly sometimes cannot.
I think the problem is delay. It can be delay from PLC or mechanical delay.

Machine condition :
The machine use two AC inductive motor one for High Speed and one for Low Speed.
Linear speed for high speed motor is about 7 cm/sec, and low speed about 3 mm/sec if we control with inverter.
Feed back sensor use encoder 1000 ppr, worm gear pitch is 10 mm.
I use high speed counter (HSC) and counter interrupt to stop motor when HSC reach count destination.
PLC : Thinget XC3-60
But even I use interrupt it still can't stop exactly at position I want, sometimes it can.
Should I change the drive system or the PLC type?

zai_jnr · Jan 20, 2009

Need to make sure,your PLC spec is suitable for positioning system..in PLC spec..u need make sure it is suitable for positioning function or not..make sure output card from PLC to servo is transistor type..cannot use relay type...due to trigger switching function is difference between relay and transistor type...

Mammoth · Jan 20, 2009

Than it is not a real servo system. A servo system consists on path planner (positioning controller), amplifier, mostly a AC synchron motor and a feedback device (encoder). With this configuration, slow and fast speeds for positioning are not necessary anymore.
The controller sends the signals to the amplifier according his inner control loops and the feedback.
I'm very familiar with Fanuc drives and never had such a problem.

aandiw · Jan 20, 2009

Cutting machine

I agree with Mamoth opinion, may be it is not real servo control. But as information, this is rebuild control machine.
The machine was build from Germany and old enough but the mechanic conditon still good. The original control was damaged. It's original control used microcontroler system with
several card. I'm curious why the original control can control
accurately although it use two speed induction motor with brake. Do I have to change the motor? Or may be there is a way in programming method to make precision control in such conditon with PLC?

Peter Nachtwey · Jan 20, 2009

aandiw said:
I agree with Mamoth opinion, may be it is not real servo control

Technically your system is a servo system. A servo system doesn't need to have a target profile generator but all good ones do. This avoids sudden jerks in the machinery as the set point is changed. Clever people can write there own target generator or tune their system in such a way that the system moves smoothly without a target generator.

I'm curious why the original control can control
accurately although it use two speed induction motor with brake. Do I have to change the motor? Or may be there is a way in programming method to make precision control in such conditon with PLC?

I/we see this all the time. There is a lot of knowledge that goes into the firmware of a motion controller. The people that wrote the firmware understood their system. I would also bet the motion controller had faster and more consistent loop updates. This is important if there isn't a velocity loop inside the drive.

The short answer is that you should get a motion controller for your system. You can probably write your own code but you will use far more time than the cost of the motion controller and it will never work as well.

REHJ · Jan 21, 2009

I may be missing something here but are the Decel times, stopping speed and scan rates being considered here? I use to work a lot with position controls and we used Inverters to position on conveyors. One of the biggest things that we had to consider when doing this was decel rates and the scan rate of the CPUs involved. even if you are interupting. Even with very fast scans. Every one who has never done the calculations always think it is bull until you make them do the calculations.

230grain · Jan 21, 2009

I would check it out mecahniclly also. If you haave ant backlash in your ball screw or whatever device you are using that will cause the same problem.

Otherwise I would look at decell times as someone also mentioned.

Also if your motor is under powerd for the load it may not be able to compensate for the momentum and stop it consistantly. That mey also be corrected by a parameter in your servo drive.

drawson · Jan 21, 2009

aandiw said:
The problem is I can't stop exactly at position that I want to go.

What does exactly mean? Are you trying to position it within 1mm, 10mm, 0.01mm?

wdreynolds1 · Jan 22, 2009

We just did this same application on our table unloaders in the finishing department. The control goes like this---> ControlLogix-->interposing relays--->profibus--->TI1 PLC--->ABB 800 series drive---> 7.5HP AC motor with encoder feedback through the same control. We were able to stop the table unloaders to within +/- .33 inches by tuning the decel--that's with ten foot stroke.

Walker

aandiw · Jan 23, 2009

I'm consider with PLC scan time, so I use interrupt counter.The machine accuracy is 0.01 mm. I suspect the brake mechanism is not precise. May be the brake gap change, so I'll try braking with inverter brake facilities. If it can't work, maybe I must use a motion controller like Mr. Peter suggest. Can any body suggest what kind motion controller suitable for this machine ? The motor is 3 phase with two speed, the high speed motor is 0.55 kW, 1.6 A 2800 rpm. The low speed motor is 0.04 kW 1.3 A 280 rpm. By the way thanks for all opinions here.

Mammoth · Jan 23, 2009

Have a look at this:
Motor: http://www.gefanuc.com/products/family/beta-i-motor
Controller: http://www.gefanuc.com/products/family/dsm324
PLC: http://www.gefanuc.com/products/family/series-90-30

or independent controllers w/o PLC:
http://www.gefanuc.com/products/family/versamotion

Mammoth

RobImb · Jan 23, 2009

Hi,
Another motion controller that is cheap and cheerful is the Giddings and Lewis MMC controller (4 axis) with indtramat drive and motor we used them very succesfully in packaging machine upgrades and the programming can all be done using ladder and a few special pre written motion blocks all supplied with the software.
try this link too http://www.glcontrols.com/

good luck

drawson · Jan 23, 2009

aandiw said:
The machine accuracy is 0.01 mm.

I really doubt you will consistently achieve this accuracy without using a motion controller.

Personally I highly recommend A Delta Motion Controller. They are really easy to use and the software is fantastic. Reliability has been very good for us. www.deltacompsys.com

If you are using a ControlLogix PLC I would look at ABs analog motion module. It has worked well for us. Have a look at the 1756-M02AE. It is integrated very tightly with the RSLogix 5000 software.

Roy Matson · Jan 23, 2009

Its several years since I worked on servo systems. The Allen Bradleys were special modules that included inputs from the encoder. All the PLC did was tell it where to go, the module looked after the rest.
Roy

aandiw · Jan 23, 2009

REHJ said:
I use to work a lot with position controls and we used Inverters to position on conveyors. One of the biggest things that we had to consider when doing this was decel rates and the scan rate of the CPUs involved. even if you are interupting. Even with very fast scans.

Respond to REHJ opinion
Would you share a little about your project that has been done successfully in the past? About the resolution, what calculation you made to reach the accuracy? The brake use mechanical or resistor braking? Did you use interrupt or not? The maksimum speed? Slow down speed? Actually my system error now is about 0.01~0.03 mm from my destination position. Thnks

Servo control

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