Omron pulse train help

[Abdullah Seliem 0]

I have made a program to drive stepper controller to control stepper motor using CP1E PLC pulse train output ,my application need high precision position control and i have use plc instruction to make that (PULS,ACC,SPED,INI) but the positioning not accurate.ie, there is random offset in the position,i want to know how can i obtain precise control using CP1E or another Omron PLC

[Michael Walsh 268]

I would guess that it is not the PLC. If programmed properly, it will send out the correct number of pulses every time. If I were to need to do precise positioning, I would not think to use a stepper, but a servo. A stepper motor could be cogging and therefore not moving the appropriate distance and there is no way for the PLC to know.

[ParaffinPower 13]

Hello What are you using the INI command for? If you try to stop a move with this, the position will not be repeatable - that's what the PULS instruction is for. Michael is correct, though; it's not the PLC but more likely the stepper. If high precision is required, then use a servo. Pp

[Sergei Troizky 26]

Not that obvious. Stepper positioning is more repeatable than servo, because the latter has certain positioning done tolerance. Edited 6 Jun 2011 by Sergei Troizky

[ParaffinPower 13]

Hello Sergei, I hear you; let me re-phrase. If you need the load to end up where you told it to go, use a servo. Pp

[Abdullah Seliem 0]

I would like to thank everyone for the reply Can i use encoder with the stepper motor to increase its accuracy thank you

[Sergei Troizky 26]

No, it is impossible to increase stepper accuracy, as it is always within one step of the drive (which may be full, half or micro step). Encoder may be used for positioning confirmation, and you will have to perform additional positioning should correction be necessary.

[Abdullah Seliem 0]

can i correct this random displacements in positioning by the plc program

[Sergei Troizky 26]

Yes, this is what I meant by additional positioning for correction. However, this would be fighting consequence. Better find and eliminate the reason.

[Abdullah Seliem 0]

but now i am trying to find a method to make a stepper motor braking for immediately stopping

[pfort 20]

Not just a matter of deceleration ramp ajustment?

[ParaffinPower 13]

I can use this phrase every day! Thank Sergei

[Sergei Troizky 26]

I still cannot understand, is your problem the stepper motor behavior understanding or the PLC PTO control. A stepper motor stops immediately itself, as soon as the pulsetrain stops (assuming the drive has no intelligence and is an amplifier only). If, on immediate stop, the motor and load consolidated inertial torque exceeds the motor holding torque, the motor will slip forward and loose expected position. Edited 7 Jun 2011 by Sergei Troizky

[Abdullah Seliem 0]

My problem is how to stop the stepper motor without exceeding the specified position, either increasing or decreasing at high speeds.

[Sergei Troizky 26]

Well, first of all, forget independent mode for positioning. Use @PLS2 for positioning, it supports both absolute and relative moves. Start with low acceleration/deceleration and increase them only when you see the positioning is correct. Remember, steppers cannot start/stop immediately over certain frequency (so called self-starting frequency). For 200steps/revolution motor, 100-200Hz should work for self-starting. Edited 8 Jun 2011 by Sergei Troizky

[Abdullah Seliem 0]

Thank you 'Sergei', i tried to use a combination of two instructions and It has given me an accurate position. i have used @ACC in continuous mode and then @pLS2, as shown in attach.