Differences Between DISTANCE and POSITION Moves.

This article refers to G&L PiC application note document AN000003 and describes the difference between DISTANCE and POSITION moves.

DISTANCE:

DISTANCE moves are incremental from the current position when the move becomes active in the que.

In DISTANCE moves the command delta is calculated based on the input DIST value.  The endoint position is based on the command delta and the current command position.  The equations:

            Command Delta = DIST*(FU/LU)

                        DIST - distance value at DISTANCE move function DIST input

                        FU - feedback units entered in servo setup.

                        LU - ladder units entered in servo setup.

            Command Endpoint = Command Delta + Current Command Position

The axis moves the incremental integer amount calculated  in Command Delta.  Values to the right of the decimal are truncated.

POSITION:

POSITION moves are absolute to the position specified at the POS input of the function.

POSITION moves calculate the Command Endpoint based on the value at the POS input.  The Command Delta is then calculated based on t he Current Command Position and the Command Endpoint.  The Equations:

            Command Endpoint = POS*(FU/LU)

                        POS - position value at POSITION move function POS input.

                        FU - feedback units entered in servo setup.

                        LU - ladder units entered in servo setup.

The axis moves to the Command Endpoint.  Values to the right of the decimal are truncated.
A problem can occur when using DISTANCE moves and making repeated short moves and FU/ LU is a non integer number.

Example difference between DISTANCE and POSITION.

TABLE 1

FU = 25,400    LU = 10,000   FU/LU = 2.54                           Initial position 1.0000

DIST (LU)	Current Command Position (FU)	Calculated DIST (FU)	Command Endpoint (FU)	Endpoint (LU)
10	25400	25	25425	1.0010
10	25425	25	25450	1.0020
10	25450	25	25475	1.0030
10	25475	25	25500	1.0039
10	25500	25	25525	1.0049
10	25525	25	25550	1.0059
10	25550	25	25575	1.0069
10	25575	25	25600	1.0079
10	25600	25	25625	1.0089
10	25625	25	25650	1.0098

TABLE 2

FU = 25,400    LU = 10,000  Initial position 1.0000

POS(LU)	Current Command Position (FU)	Calculated DIST (FU)	Command Endpoint (FU)	Endpoint (LU)
1.0010	25400	25	25425	1.0010
1.0020	25425	25	25450	1.0020
1.0030	25450	26	25476	1.0030
1.0040	25476	25	25501	1.0040
1.0050	25501	26	25527	1.0050
1.0060	25527	25	25552	1.0060
1.0070	25552	25	25577	1.0070
1.0080	25577	26	25603	1.0080
1.0090	25603	25	25628	1.0090
1.0100	25628	25	25654	1.0100

Notice that the values in the Endpoint columns of table 1 and 2 differ.  Table 1 loses .0001 in two places.  At 1.0040 it becomes 1.0039 and at 1.0100 it becomes 1.0098.

Solution:

A DISTANCE function style of move can be done by doing a READ_SV of Command Position, (variable 3), then add the DIST value to the Command Position for the POS input value of a POSITION function move.