Motion Command question

Hi guys

I got a question anout MRP, Motion Redefine Position:

we could use this command to update the actual position of Axis, if we use MOV command to change the Tag(Axis.ActualPosition)value, what is the difference between thes two ways to change the position?

thanks

angi said:

Hi guys

I got a question anout MRP, Motion Redefine Position:

we could use this command to update the actual position of Axis, if we use MOV command to change the Tag(Axis.ActualPosition)value, what is the difference between thes two ways to change the position?

thanks

Click to expand...

angi,

Have you tried the MOV method? I don't believe that will work. I believe that the compiler will tell you that the Axis.ActualPosition is a read-only value, hence the need for the MRP.

I don't have the software in front of me to test this out at the moment, but somebody else will probably come along who can confirm or deny this.

Cheers,
Dustin

angi said:

Hi guys

I got a question anout MRP, Motion Redefine Position:

we could use this command to update the actual position of Axis, if we use MOV command to change the Tag(Axis.ActualPosition)value, what is the difference between thes two ways to change the position?

thanks

Click to expand...

angi,

I don't think the MOV command will work. I believe that Axis.ActualPosition is a read-only tag, hence the need for the MRP for access to the axis properties. I don't have the software here in front of me to try this though, so I can't swear to it.

Cheers,
Dustin

simply

MRP redefines the current position (does not move the axis)
MOV moves the axis to the location (moves the axis)


There is more however this is a simple start

Thanks guys!

that means I could ues MOV to update the ActualPosition, right?

No the ActualPosition attribute is read-only, you will need to use MRP or MAH,mode set to passive, to update the position of an axis.

Thanks KIU
But how to understand that the MRP will not move the Axis?

MRP has no movement attributes, i.e. speed accel decel, associated with it, you are simply redefining the axis position. A MAH can move an axis or I have seen it used to redefine a position in passive mode. My preference would be to use the MRP as that is the job it is designed for and saves someone following you from having to determine the attributes used to home the axis. But to each his own they both will work when defined correctly.

angi said:

But how to understand that the MRP will not move the Axis?

Click to expand...

You may have already looked at this, but the Logix5000 help files have pretty good explainations of all of the motion instructions...

" Use the Motion Redefine Position (MRP) instruction to change the command or actual position of an axis. The value specified by Position is used to update the Actual or Command position of Axis. The position redefinition can be calculated on an Absolute or Relative basis. If Absolute is selected the Position value is assigned to the current Actual or Command position. If Relative is selected the Position value is added as a displacement to the current Actual or Command position. The process of redefining the current axis position has no affect on motion in progress as the instruction preserves the current servo following error during the redefinition process. As a result, axis position can be redefined on-the-fly without disturbing axis motion. "

angi said:

Hi guys

I got a question anout MRP, Motion Redefine Position:

we could use this command to update the actual position of Axis, if we use MOV command to change the Tag(Axis.ActualPosition)value, what is the difference between thes two ways to change the position?

thanks

Click to expand...

The MRP command DOES NOT change the physical position, it just changes the indicated position from the feed back by changing an offset.
The MAM will physically move the actuator or motor. Normally you only use the MRP command to 'zero' or 'home' a position by moving against a stop and then executing the MRP command with the desired position. The should be little need for a MRP command to redefine position in a typical application.

thanks guys!