MCV_CamSwitch

The MCV_CamSwitch function block realises the output of a cam-controlled switch that is assigned to an axis. The cam's range can be defined freely.

The function blocks internal calculations are based on a modulo coordinate system with the value range [0, 3600000] 0,1 µm or 10^-4°.

The prerequisite that no more than half the modulo value range is covered by the axis in each PLC cycle must also be fulfilled.

Block diagram

Parameters of the function block

VAR_IN_OUT
	Axis	AXIS_REF	Axis reference.
VAR_INPUT
	Enable	BOOL	Activates the cam-driven switch.
	FirstOnPosition	DINT	Position of the axis at which the output of the cam-driven switch changes from FALSE to TRUE, if the axis is rotating in a positive direction. Default unit [0,1µm or 10^-4 °]
	LastOnPosition	DINT	Position of the axis at which the output of the cam-driven switch changes from TRUE to FALSE, if the axis is rotating in a positive direction. Default unit [0,1µm or 10^-4 °]
	CompTime	DINT	Compensation time by which the switch-on phase of the cam-driven switch is advanced or delayed. Unit [1ms]
	PosType	INT	Selection whether command position or actual position of the axis should be used for comparison with the input variables and for calculation of the switch-on position / switch-off position that is changed by the compensation time. 0: Actual position of the axis (ACTIVE_POS) 1: Command position of the axis (CURRENT_POS)
VAR_OUTPUT
	Active	BOOL	Indicates the current state of the cam-driven switch.
	State	BOOL	Output of the cam-driven switch. The output is TRUE, if: Case 1: ‘FirstOnPosition’ < ‘LastOnPosition’ The value of the axis position lies between ‘FirstOnPosition’ and ‘LastOnPosition’. Case 2: ‘ FirstOnPosition’ > ‘LastOnPosition’ The value of the axis position lies at time t₀in front of and at the following time t₁behind the range spanned by ‘FirstOnPosition’ and ‘LastOnPosition’ The signal remains for at least one PLC cycle at TRUE.
	Error	BOOL	Indicates if an error has occurred.
	ErrorID	WORD	Error identification.
	Pulsed	BOOL	For visualization purposes. If "State" = TRUE, "Pulsed" is assigned TRUE for one second.

Behavior of the function block

Input values for ‘FirstOnPosition’ and ‘LastOnPosition’ that are less than the low limit of the modulo coordinate system are set to the low limit and input values that are more than the high limit are set to the high limit. This is done internally in the function block without acknowledgement.
Specification of a compensation time leads in the function block to modification of the switching positioned defined by ‘ FirstOnPosition’ and ‘LastOnPosition’. If values should result for the modified switching positions that are outside the value range of the modulo coordinate system, these are converted to values within the range. This is done internally in the function block without acknowledgement.
Modification of the switching positions by specification of a compensation time is done directly in the PLC cycle by they are passed on at the "CompTime" input and fully take effect immediately.
Specification of the compensation time "CompTime" acts equally on the ‘FirstOnPosition’ and ‘LastOnPosition’. Positive values for "CompTime" lead to a situation in which the "State" output changes after a delay to reach a switching position. Negative values lead to a situation in which the "State" output changes before the switching position are reached.

MCV_CamSwitch 2: — Figure 3-1: Impact of the compensation time on the switching times

Behavior of the ‘State’ output of the cam-controlled switch and the positions that are passed on to the ‘FirstOnPosition’ and ‘LastOnPosition’ inputs. The axis moves in positive direction from the low limit of the modulo coordinate system to the high limit.

Case 1: ‘FirstOnPosition’<‘LastOnPosition’

MCV_CamSwitch 3: — Figure 3-2: Behavior of the "State" output when ‘FirstOnPosition’ < ‘LastOnPosition’

Case 2: ‘FirstOnPosition’>‘LastOnPosition’

MCV_CamSwitch 4: — Figure 3-3: Behavior of the "State" output when ‘FirstOnPosition’ < ‘LastOnPosition’

The function block takes two special cases into account, which result from the fact that the PLC system is a discreet system. These are:

The axis position at the time t_n is before the switch-on position and the axis position at the time t_n+1 is after the switch-off position. Therefore, the switching range is less than the distance covered by the axis in one PLC cycle.
If the axis position is before the switch-on position at the time t_n, specification of a compensation time may result in a situation in which the axis position at the time t_n+1 is after the switch-off position due to modification of the switch-on points.

In these special cases, the "State" output becomes TRUE for at least one PLC cycle although it was not detected within one revolution of the axis that the axis position was between the switching positions.