MC_SetPosition

The function block sets the current axis position to a parameterizable value.

In absolute mode, the actual position is set to the parameterized absolute Position value. In relative mode, the actual position is offset by the parameterized Position value. In both cases, the set position of the axis is set so that any potential lag error is retained. The Options.ClearPositionLag switch can be used to clear the lag error.

Relative mode can be used to change the axis position during the motion.

As of TwinCAT 3.1.4024.51 and Tc2_MC2 3.3.56, the position offset can be deleted and the position can be reset to the originally referenced coordinate system via the Options.ClearPositionOffset switch. From this version onwards, the offset can also be read via ADS and displayed in the UserData variable of the AXIS_REF structure so that the value is available for the current cycle.

Inputs

VAR_INPUT
    Execute  : BOOL;
    Position : LREAL; 
    Mode     : BOOL; (* RELATIVE=True, ABSOLUTE=False (Default) *)
    Options  : ST_SetPositionOptions; 
END_VAR

Name	Type	Description
Execute	BOOL	The command is executed with a rising edge.
Position	LREAL	Position value that the axis position will be set to. In absolute mode, the actual position is set to this value, in relative mode it is shifted by this value.
Mode	BOOL	The axis position is set to an absolute value set if Mode = FALSE. Otherwise the axis position is changed relative to the specified Position value. Relative mode can be used for changing the position of an axis during motion.
Options	ST_SetPositionOptions	Data structure containing additional rarely used parameters. The input can normally remain open. ClearPositionLag: can optionally be used to set the set and actual positions to the same value. In this instance, the following error is cleared. SelectEncoderIndex: can optionally be set if an axis with several encoders is used and the position of a certain encoder is to be set (Options.EncoderIndex). EncoderIndex: indicates the encoder (0..n) if SelectEncoderIndex is TRUE. ClearPositionOffset: Can be set to delete the position offset selected with MC_SetPosition and thus reset the position to the originally referenced coordinate system; available from TwinCAT 3.1.4024.51 and Tc2_MC2 3.3.56.

Inputs/outputs

VAR_IN_OUT
    Axis : AXIS_REF;
END_VAR

Name	Type	Description
Axis	AXIS_REF	Axis data structure that unambiguously addresses an axis in the system. Among other parameters it contains the current axis status, including position, velocity or error state.

Outputs

VAR_OUTPUT
    Done    : BOOL;
    Busy    : BOOL;
    Error   : BOOL;
    ErrorID : UDINT;
END_VAR

Name	Type	Description
Done	BOOL	TRUE, if the reset was executed successfully.
Busy	BOOL	TRUE, as long as the function block is called with Enable = TRUE.
Error	BOOL	TRUE, if an error occurs.
ErrorID	UDINT	If the error output is set, this parameter supplies the error number.

Reading the MC_SetPosition offset

As of TwinCAT 3.1.4024.51, the position offset set by MC_SetPosition can be read out via ADS:

Function	ADS Read
Port	501 (dec)
Index Group	0x4100 + axis ID	0x5100 + axis ID
Index Offset	0x00n10017	0x0017
Data	REAL64

Merging the MC_SetPosition offset into the AXIS_REF

The AXIS_REF axis structure contains a UserData variable that can be configured so that the current value of the MC_SetPosition offset is displayed here from TwinCAT version 3.1.4024.51 onwards. The value is then available for the current cycle.

The UserData variable must be configured as follows via ADS:

Function	ADS Write
Port	501 (dec)
Index Group	0x4000 + axis ID
Index Offset	0x010D
Data	0x00000000: Deactivated (default) 0x00010017: Activates the MC_SetPosition Offset value

Requirements

Development environment	Target system type	PLC libraries to include
TwinCAT v3.0.0	PC or CX (x86 or x64)	Tc2_MC2