Programmable Limit Switch

Language:	[[::Programmable Limit Switch|English]]

PLS Overview

A programmable limit switch (PLS) monitors position feedback on both real and simulated axes. When the axis reaches any of the defined PLS positions,
the PLS toggles the state of a specified system output, either digital or simulated.
A PLS offers more flexibility than a limit switch, as it allows many trip points and can be reconfigured on-the-fly.

PLS positions are defined in an array and must have values that are monotonic increasing.
Any number of positions can be defined for a PLS.

Any number of PLS switches can be defined for an element (axis or group). However, the number of defined PLS switches will have a direct affect on system performance.

The PLS is scanned at a rate of once per motion bus cycle.
Implementation of PLS in the Product includes three types of functionalty:

• Axis-PLS: PLS positions defined for an axis
• Path-PLS: PLS positions defined according to a motion path
• High accuracy PLS: PLS positions are extrapolated and set based on system position, speed and acceleration. For use in high speed applications.

PLS Functional Description

Output State

A PLS toggles an output when a specified position is reached. The state is toggled each time the position is reached,
irrespective of the direction of motor travel.

PLS Polarity

The property PLSPolarity allows you to control the initial polarity of the PLS output, which is specified as 1 or 0. The default value is 0.
The output state is set when the PLS is enabled (PLSEnable).
The state of the output is determined by the axis position at the moment of PLSEnable, according to the following scheme:

If PCMD < PLSPosition[1]

Output = negative PLSPolarity (=1)

If PCMD >= PLSPosition[1] and PCMD < PLSPosition[2]

Output = PLSPolarity (=0)

If PCMD >= PLSPosition[2] and PCMD < PLSPosition[3]

Output = negative PLSPolarity (=1)

Consider the following example. The PLS positions are 10, 25, 30, 50 and 65. Polarity is 0. Once the PLS is enabled,
the output state will toggle on and off according to the location of the axis. Assuming PLS is enabled before position 10,
the output will toggle on at 10, remain on between 10 and 25, and toggle off between 25 and 30, and so on.

PLS Position Data

When a PLS is first defined, it does not have any associated positions. The CreatePLSData command is used to define the values for the PLS positions.
An unlimited number of PLS positions can be defined, but the array must be monotonic increasing.
PLS position values are always absolute (and not incremental). PLS positions are given in user units, and as such are affected by axis Displacement and PositionFactor properties.
Consider the following example. Positions 100 and 200 are specified, and the initial state of the digital output after enabling is 0.
The following table shows how the state of output changes as the position changes.

Position	Output State
0	0
100	1 (Toggle)
150	1 (No change)
200	0 (Toggle)
210	0 (No change yet)
200	1 (Toggle again)
150	1 (No change)
100	0 (Toggle again)

Hysteresis

If an axis stops at a specified PLS position, the state of the digital output may be continually toggled, due to slight oscillations in the actual axis position.
To accommodate this behavior, Product allows you to apply a hysteresis value around the output toggle positions.
Typically, 5 to 10 encoder counts (converted to position units) is sufficient.
The output state is toggled when the PLS position is reached and surpassed by the hysteresis value.
The following example shows a hysteresis value of 10; the output will toggle ON at position 110. It will toggle OFF at position 90.
If the axis driving the PLS stops at a PLS position within the hysterisis range, the PLS state does not change.
Hysteresis is necessary only if the system stops on or near a PLS position.

Repetition Interval

PLS positions may be repeated continually at a specified interval of encoder counts.
The repetition interval must be both positive and greater than the absolute value difference between the lowest and highest PLS position values. That is:

Repetition Interval > abs(PLS_Pos_Max - PLS_Pos_Min)

Consider the following example:

• PLS position values: -300, 100, 200
• Repetition Interval: 2000
• Polarity: 0

The output state is toggled at -300, 100, 200, and at 1700 (-300 + 2000), 2100 (100+2000), 2200, and at 3700 (-300 + 2000 + 2000), and so on in the position direction.
In the negative direction, the output state is toggled at -1800 (200 - 2000), -1900 (100 - 2000), -2300 (-300 - 2000), -3800, and so on.

Enable and Disable

When first defined, the PLS is disabled.
Once the PLS is enabled, the output is set according to the polarity and position, and the output pattern is generated.
The PLS must be disabled when modifying PLS properties.

PLS Commands and Properties

PLS Commands and Properties.

The following commands and properties are used for defining and using PLS functionality in a Product system.

MC-Basic	Command/Property	Description
COMMON | DIM] SHARED <PLS name> AS PLS <''axis''> <''output''><br>	Command	Declares a PLS variable. The name of the element and the output associated with the PLS are specified in the declaration.
CreatePlsData	Command	Creates a PLS data table containing the positions associated with a defined PLS.
DeletePls	Command	Deletes a defined PLS and all actions associated with that PLS.
PLSList	Command	Returns a list of the PLS names defined in the system.
WithPLS	Command	Nodal assignment of path-PLS. Enables path-PLS at the start of motion. The specified PLSes will be enabled during execution of the specified motion and disabled when the motion ends.
HWAssistance	Property	Enables/disables high-accuracy (hardware-enabled) PLS mode.
HWIOtime	Property	Returns the current contents of the hardware time buffers. Applicable only to high-accuracy PLS.
PLSAxisName/PLSElementName	Property	Returns the name of the axis or group for which the specified PLS is defined.
PLSDelayTime	Property	Returns the current contents of the PLS time buffers.
PLSEnable	Property	Enables or disables the PLS functionality.
PLSHysteresis	Property	Sets or queries the hysteresis level of a PLS.
PLSOutput	Property	Sets or queries the system output associated with a PLS.
PLSPolarity	Property	Sets or queries the initial polarity associated with a PLS.
PLSPosition	Property	Sets or queries an individual PLS position.
PLSPropagationDelay	Property	Expresses the elapsed time from the moment the position command is sent to the drive until it is set as a command value in the drive’s position loop.
PLSPropagationDisableDelay	Property	Propagation delay value, used when the PLS sets the external device to the disabled state.
PLSPropagationEnableDelay	Property	Propagation delay value, used when the PLS sets the external device to the enabled state.
PLSRelatedTo	Property	Flag used by PLSSource to define whether relative PLS position values are computed relative to the start of motion or the target position.
PLSRepeat	Property	Sets or queries the repetition interval of the PLS positions.
PLSSource	Propert	Defines certain attributes of PLS positions.

Axis-PLS Programming Sequence

Step 1 – Declare

Create the PLS using COMMON SHARED ... AS PLS in 'Config.Prg' or in the terminal.

The declaration must define a name for the PLS, specify the element that drives the PLS, and specify the output controlled by the PLS.
The PLS is disabled until activated by PLSEnable=1.

The following example sets up 'MyPLS' as a PLS driven by axis A1 connected to system digital output 1.

Common Shared MyPLS as PLS A1 System.Dout.1

By default, the PLS has the following properties:

• One PLS position, set to 0.
• PLSPolarity = 0 (initial polarity of the PLS output off).
• PLSRepeat = 0 (no repetition)
• PLSHysteresis = 0 (no hysteresis)

See: Input/Output.

Step 2 – Positions

Create the PLS data structure and define the PLS position data using CreatePlsData.
The position array is 1- based, and position values must be monotonic increasing.
An attempt to access an index outside the array will produce an error.
For example:

CreatePLSData 4 MyPLS
MyPLS.PLSPosition[1] = 1000
MyPLS.PLSPosition[2] = 1100

Alternately, use the short form PPOS instead of PLSPosition:

MyPLS.Ppos[3] = 2000
MyPLS.Ppos[4] = 2200

Step 3 – Initial Polarity

Set the initial polarity of the PLS output using PLSPolarity
.

PLSPolarity default is 0 (off), indicating the PLS output will toggle on at the first PLS position.

Change the initial polarity to 1 (on):

MyPLS.PLSPolarity = 1

PLSPOLARITY defaults to off, indicating the PLS state will be off after the first PLS position.

Step 4 – Repetition

To repeat the PLS function at recurring intervals, set the value of PLSRepeat to a non-zero, positive number.
For example, repeat the PLS at every 10,000 position units:

MyPLS.PLSRepeat = 10000

Step 5 – Hysterisis

Set PLSHysteresis if the application requires it. Otherwise, leave it set to 0. The value is in user units.

MyPLS.PLSHysteresis = 0.01

Step 6 – Enable

Enable the PLS:

MyPLS.PLSEnable = 1

Additional PLS Programming Instructions

PLS State

The PLS must be disabled in order to modify various PLS properties, modify PLS position values, and delete a PLS.
Disabling a PLS also helps conserve CPU resources.

Disable a PLS:

MyPLS.PLSEnable = 0

Enable a PLS:

MyPLS.PLSEnable = 1

Query the state of a PLS:

?MyPls.PLSEnable
if(MyPLS.PLSEnable = 0)

Drive an event via PLSEnable:

EventOn MyEvent MyPLS.PLSOutput = 1

Delete a PLS

A PLS can be removed from the system only when the PLS is disabled and there are no tasks in memory.
Delete a PLS:

DeletePLS MyPLS

Query the PLS Axis Name

Query the name of the axis that activates the PLS:

?MyPLS.PLSAxisName
MyPLS.PLSAxisName = AXIS1

Query the PLS Output State

Query the state of the PLS output by querying the digital output associated with the PLS.
For example, if PLS output is assigned to SYSTEM.DOUT.1, query the state of SYSTEM.DOUT.1:

?Sys.Dout.1
Sys.Dout.1 = 0

Axis-PLS Program Example – PLS Setup

The following program example demonstrates the set up of a PLS data structure.

NOTE: The PLS data object must first be defined in Config.prg or Terminal:

Common Shared <PLS Name> as PLS <Axis> System.Dout.<DOut number>

PLS SETUP PROCEDURE PROGRAM EXAMPLE