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Hello to all,

I would ask for an advice about setup for stone cutting machine project. Machine cutts big stone boulders to 2-10 cm thick plates with required precision of  0 to +1-3 mm depending of thickness. Costumer has a problem with x -axis movement (this movement defines the thickness of plates), because it is not precise enough. The setup i have conatins:

 -Induction motor for x-axis movement,

-VFD for induction motor,

-Incremental encoder for traveled distance feedback hooked to PLC.

-Vision PLC .

I have a few projects with only PLC or VFD setup, but first of this kind, so i would ask someone with more experience for advice how to achive precise positioning depending the given target value by operator and best setup for this project . The possible problems are inertia (the X -axis holds big hydraulic cutting motor) and also the lowest acceleration and deceleration value that i can set at VFD is 0.1sec. Supouse that I have to lower the speed of motor movement depending how close to target value it gets to achieve precise stop. What is the best possible way to do it through ladder ( PTO or simple math functions)? 

Any kind of help will be appreciated,

Best regards,

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Best solution - use closed loop VFD (or servo drive) with positioning option. In this type of system PLC used only for transfer target position.

In case Visilogic PLC - maybe use Unitronics servo drive+motor - most cost efective. It is few positioning option in new Visilogic servo demo project.

If you need to setup precise positioning via VFD - you need to run motor in two mode - FAST and SLOW.   FAST= (SLOW to MAX speed) and SLOW (5--10% of MAX speed).

PLC must calculate distance and sw FAST to SLOW when diatance  is more than FAST to SLOW  ramp distance . If not - only SLOW speed is used.

SLOW to STOP ramp time and distance is more stable and can be corrected via setup parameter.

PLC send to VFD  FAST and SLOW setpoint and two Command - RUN and CHANGE SPEED. Main problem - is it setpoint change descreet input present on VFD.

If not - PLC must change FAST and SLOW analog setpoint via ladder and operate only RUN command.

I build many STOP+CUT application based on  VFD   with +-1mm precision.

Unitronics VFD have setpoint source change input and can be used in this type of application with different combination of analog and Modbus setpoints.

 

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I have a machine with similar setup. I have it in open loop with an absolute encoder 1000pulse/rev. The gearbox motor drives a 1" 10 start ball screw. Two MIs for fast and slow speed I've used the within range block to switch from fast MI to slow MI. It is open loop, but still achieve 0.005"accuracy. I've used a limit switch at the home position to reset the MIs to 0 so they never get away from me.  I've used a NO contact with the run command to store the MIs all the time and reload with SB2 at startup. This way it will remember the las position at startup. 

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Thank You for feedback,

Its really helpful,

@kratmel,I know that closed loop setup would be best and easiest solution, but like every costumer he wants to get as much as possible with lowest possible investment. This whole setup is already at machine,except PLC (there is HS counter that is bad,) and costumer wants to have all commands valves open -close , start ,stop etc... at one place so I want to use HMI PLC to hook everything on it. FAST and SLOW speed seems like a good solution, on this VFD there is not dicrete input for set point change, but it has analog input for speed control so I can use it to control speed from PLC. Other thing,i, supouse it is foolish question, but what with VFD acc.-deacc. time, can it make problem in this setup?(when PLC gets info from encoder that given distance is traveled, and  sends stop command to VFD - so can this deacceleration time  be problem here in order to overshoot target distance ?

@TozoM8 , us i understand You have used within range block in ladder to send FAST and SLOW speed to motor depending how close is to target value.

On 2/8/2021 at 3:28 PM, TozoM8 said:

 I've used a NO contact with the run command to store the MIs all the time and reload with SB2 at startup. This way it will remember the las position at startup. 

Can You give a little more detailed explanation about this two last sentences?

Best regards,

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On 2/8/2021 at 8:52 PM, MateK SB said:

lowest acceleration and deceleration value that i can set at VFD is 0.1sec

are you saying that is the shortest ramp time possible?  In some ways you need to decide whether to ramp it with the plc, or it's innate systems.  I also feel sure that the drive will have a digital input for stop, which might make things easier.

The info that you have to learn from the machine itself is how it reacts (speed and position) to various speed change instructions from the plc.  I do this by making a small program that stores various settings and subsequent results.  You can actually make it do the changes itself if you want to save yourself lots of button pressing!  I would suspect that if it has a stop input, that will be a surefire way to get consistency from a given low speed, which is ramped down to like Tozo and Kratmel suggest.  As Tozo says, the limit switch or other means of defining home consistency is a must if doing open loop.

cheers, Aus

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32 minutes ago, Ausman said:

are you saying that is the shortest ramp time possible?  In some ways you need to decide whether to ramp it with the plc, or it's innate systems.  I also feel sure that the drive will have a digital input for stop, which might make things easier.

Hello Aus,

Yes, ofc, VFD has digital input for STOP ( to this point drive recieved  this signal from Counter, now should  recieve it form PLC)

VFD settings allow the shortest possible ramp time of 0.1 s( range from 0,1-600 s),its an old Vee Elvovert VFD.

About home position, there should be no problem, I have limit switches installed.

Cheers,

 

 

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5 minutes ago, Pavel_setnicar said:

Maybe .........

I assume  that you are using interrupt routine , right?

Yes, i will to use 2.5mS interrupt routine for STOP function.

I saw this kind of machines that work OK with open looop systems, so I am just looking for advice from someone with more expierience  what is  best and easiest possible way to do it. 

Cheers,

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I test interrupt and no interrupt solution in my last VFD positioning setup.

If i use 64mm weel on 1000imp encoder, SLOW speed with 2-5Hz, 1450 rpm motor and 1:40 gear and 200mm steel feeder weels- result is maybe the same (+-0.5mm).

If SLOW speed is 10-15Hz - interrupt solution have more stable result (+-1mm) - no interrupt (+-2mm).

Maybe main problem in open loop solution is to correct position if stone cutting machine lost some part of inertia after some numbers thick plates is cutted.

VFD_calibration1.JPG.fe76bcf120e73c9635e8dfac80a183f8.JPG

In my application inertia is stable and positioning result is stable also.

P.S.

Stone cutting machine do not needed to set home position. It must be set to zero after first cut by PLC ladder in position selected by operators.

And after positioning - next new 0 home position generated after next cut.   Only limit sw must be installed for stop drive if new move is not possible.

 

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10 hours ago, Ausman said:

Don't forget there is also a 1.25ms interrupt for enhanced models, which yours should be.

cheers, Aus

I forget about it, 

ty, cheers

@kratmel,  valuable informations,

Yes, only limit switches, first cutting position (start of cutting procedure) set operator manually, depending of stone boulder size and position on table.

I have a similir setup, and will try with similar approach.

Ty :)

 

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