mpakravanan

Hi, can anyone explain to me what is the difference between absolute and relative move blocks, when I want to use them for controlling stepper motors?

Hello.

The question is very generic. I am having a difficult time to decide what exactly you want as an answer. There is a document that has been very helpful to me; the current version is:
SIMATIC STEP 7 S7-1200 Motion Control V6.0 to V7.0 in TIA Portal V16 

I will use REL and ABS as shorthand in some quick introductory notes..

REL and ABS starts a task if triggered with a boolean trigger that transition from FALSE to TRUE. It will continue the task until finished or a process interrupts / override it. (see manual) Note that the target speed cannot change while the REL/ABS block is doing a movement.

REL is for moving a distance. The starting position is not of importance. Only the current position is of importance. A negative distance is always in the reverse direction.

ABS is movement to an absolute position. The reference position is set by using the MC_HOME instruction. No movement is allowed if the reference / homing position is not set. The reference is also lost when the enable (MC_POWER) is lost. +/- on the position request does not indicate the direction. The target position is always measured against the reference position.

For your stepper motor..
The control is open loop. It does not matter which instruction you use, there is no feedback on the actual position. The control system assumes the stepper (or servo) does go where it is commanded to go. You need to develop your own code if you wish to utilize measured position and make a closed loop control.

What of these notes did you already know?
What is it specifically you are looking for?

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thank you.

I have a stepper system with two stepper motors and drives, they must change the position of two individual laser lights, first they must go to home position then starts to move the lasers forward and when necessary backward. and always move them. I want to know when I want to control the system, which function I should use?

I mean in general when should we use absolute positioning and when should we use ralative positioning?

as I know before moving the axes in absolute positioning mode we should home/reference them, then starts absolute positioning but in relative positioning it is not necessary why?

mpakravanan

I mean in general when should we use absolute positioning and when should we use ralative positioning?

as I know before moving the axes in absolute positioning mode we should home/reference them, then starts absolute positioning but in relative positioning it is not necessary why?

REL moves an object a distance from the point where the instruction was triggered. The actual position is of no importance. It can be driven to exceed a physical hardware limit.

There is some study involved in understanding why you use motion commands. Those are dictated by the application. You are not explaining your application clearly. I am not equipped to teach you everything but will put some notes down. 

In describing motion some points to are required..
* coordinated movement vs single axis motion vs dependant motion
* count of movement axis
* count of measurement axis 
* open vs closed loop motion
* type driver
* type motion: angular vs rotary vs linear vs virtual vs speed
* absolute vs relative position measurement 
* accuracy and speed
* backlash: inherent inaccuracies AND the compensation thereof 

The S7-1200 is not suited for all types of motion types. It is good for single motion control and you can cheat dependant motion. Coordinated movement is best done with a dedicated motion controller.

Profile cutting where coordinated movement is done, is best done with speed control. The interaction of the cutting position is regulated by the controller. The S7-1200 is not suited for this (without doing much development).

Cutting in a straight line at a single speed is easy with ABS motion. Interruptions can be handled easily. REL can also do the job but the program needs to track where it is going according to the physical limitations of the motion.

Simple profile cutting ("open cut") can be done with REL motion better than ABS motion because the relative distance from each node point is important. But it is still open for the designer to choose.

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