Position Trigger Object

Position Trigger objects monitor movers until they have crossed over a specified location on track, even if they do not stop there. Position Triggers will always point to the most recent registered mover which crossed over the Position Trigger's track position. When using track management the position trigger will only trigger if the mover crosses the specificed location and has a matching TrackId.

Setup & Execution

Setup

// Declaration
PositionTriggerA        : PositionTrigger;

// Initialization
PositionTriggerA.Position       := 2000;
PositionTriggerA.TriggerDirection   := mcDirectionPositive;

Execution

A position trigger must be checked each scan for movers that have crossed it's position. Then the mover must be acknowledged before another mover can be detected at this position.

// test for mover crossing this position trigger
IF (PositionTriggerA.MoverPassedPosition) THEN
    // get the most recent mover and change it's velocity
    PositionTriggerA.CurrentMover.SetVelocity(500);
    // acknowledge the mover calling the MuteCurrent method
    // this re-arms the position trigger for the next mover
    PositionTriggerA.MuteCurrent();
END_IF;

Usage notes

Position Triggers must also be added to the Mediator object. By default, this is handled already in the MAIN.Initialize ACTION.

// Example implementation
Mediator.AddPositionTrigger( PositionTriggerA );

Methods

Cyclic

Cyclic()

Position Triggers require a cyclic call in the Main Program because they need to constantly monitor the positions of registered movers.

This is automatically called by the Mediator when using the pre-defined position triggers in MAIN.

MuteCurrent

MuteCurrent()

Clears a Mover reference from the CurrentMover property, and resets MoverPassedPosition to be false. This can be done without the need to unregister a mover from a position trigger

Since Position Triggers uniquely latch their CurrentMover output, it is necessary to clear this value manually in order to prepare to process the next mover event.

IF PositionTrigger[0].MoverPassedPosition THEN
    PositionTrigger[0].CurrentMover.MoveVelocity( 200 );
    PositionTrigger[0].MuteCurrent();
END_IF

In this way, it is possible to reset the position trigger without having to completely unregister and re-register a mover from the trigger.

This method will produce a Log Event to aid in troubleshooting.

Reset Statistics

ResetStatistics()

Resets the Statistics for this station. See the Statistics property.

Set track

SetTrack()

PositionTrigger[1].Position := 1000.0;
PositionTrigger[1].SetTrack(Track[2]);

Properties

.MoverPassedPosition

BOOL

Status boolean indicating that at least one mover has passed over the threshold position since being registered

Position Triggers are unique in that the Current Mover output latches even though the mover may not still qualify under the trigger condition

.CurrentMover

REFERENCE TO Mover

Reference variable that refers to the most recent registered mover to cross over the track position since being registered

If no movers are tracked, or no tracked movers have yet satisfied the objective, then .CurrentMover is an invalid reference. In this case, an ErrorMover will replace the invalid reference. The user will receive notifications when this happens in the TwinCAT event logs. See Diagnostics > ErrorMovers for more information.
For this reason, it is recommended that all evaluations are nested inside IF checks for .MoverPassedPosition.
It is possible that multiple registered movers have crossed over a threshold position. Only the most recently valid mover handle is provided by .CurrentMover.
- You must compare your PLC scan time, maximum mover velocity and minimum mover gap to ensure that all movers can be detected at a position trigger regardless of their velocity.

.Position

LREAL

Current placement of the Position Trigger threshold along the track

.Track

REFERENCE TO Track

Track that the station is assigned to when using track management. See the Track object.

.TriggerDirection

Tc3_Mc3Definitions.MC_Direction

Sets the direction of travel required for the mover to trigger the position trigger.

Options are:

MC_PositiveDirection: The mover's position is increasing as it crosses the trigger. (Default)
MC_Negative_Direction: The mover's position is decreasing as it crosses the trigger.

.Statistics

A set of timers, counts and values that track the throughput of the position trigger and velocity of movers at the trigger.

Member	Type	Description
InterarrivalTime	TIME	Current amount of time between mover events
LatestMoverVelocity	LREAL	Mover velocity of the most-recent trigger event
InterarrivalTimeHistory	ARRAY[0..63] OF TIME	Ring buffer of recent interarrival times
MoverVelocityHistory	ARRAY[0..63] OF LREAL	Ring buffer of recent mover velocities
AverageInterarrivalTime	TIME	Average of all nonzero Interarrival times in the ring buffer
AverageMoverVelocity	LREAL	Average of all nonzero mover velocities in the ring buffer
AverageMoversPerMinute	LREAL;
ProcessedMoverCount	UDINT	Total number of movers processed by this position trigger

Extra Examples

// Basic Position Trigger implementation

// Begin monitoring Mover 1
IF bInit THEN
    Mover[1].MoveToPosition( 1000 );
    PositionTrigger.Position    := 800;
    PositionTrigger.RegisterMover( Mover[1] );
    bInit   := FALSE;
END_IF

// When Mover 1 crosses position 800, slow it down to 100mm/s
IF PositionTrigger.MoverPassedPosition THEN
    PositionTrigger.CurrentMover.SetVelocity( 100 );
    // MuteCurrent clears MoverPassedPosition and CurrentMover so that
    // Line 12 does not continually evaluate as true
    PositionTrigger.MuteCurrent();
END_IF