Safely-Limited Position (SLP)

The Safely-Limited Position (SLP) instruction monitors the position of a motor or axis to ensure that the position does not deviate above or below defined limits.

Available Languages

Ladder Diagram

Function Block

This instruction is not available in function block.

Structured Text

This instruction is not available in structured text.

Safely-Limited Position Application

Safely-Limited Position is used with a CIP safety drive that supplies speed of a motor or axis and a Safe Feedback Interface (SFX) instruction to scale the feedback. During operation, the SLP instruction sets the limit output when the motor position moves outside of the specified limits. During operation of the SLP function the limits may be adjusted. The output is used to initiate an application specific action such as STO, SS1, SS2 or STO.

Operands

The following table provides operands used for configuring the instruction.

The following table explains the instruction inputs.

1

ISO 13849-1 stipulates instruction reset function must occur on falling edge signals. To comply with ISO 13849-1 requirements, add the logic immediately before this instruction. Rename the ‘Reset Signal’ tag in this example to the reset signal tag name. Then use the OSF instruction Output Bit tag as the instruction’s reset source.

This table explains instruction outputs. The outputs are external tags (safety output modules) or internal tags used in other logic routines.

This table explains instruction outputs that are written to the user-specified tag.

Affects Math Status Flags

No

Major/Minor Faults

None specific to this instruction. See Index Through Arrays for array-indexing faults.

Execution

Ladder Diagram

Operation

Normal Operation

The SLP function begins if it has been previously reset and the Request input is asserted ON(1). At this point the Check Delay Timer begins. When the Check Delay Timer expires, position monitoring begins. The Actual Position, provided by an SFX instruction, is compared to the Positive and Negative Position Limits. If the position is not within these limits then the Limit Output is set to ON(1) and remains set until the SLP function is reset. The SFX instruction must be homed before the SLP function operates.

Position values used in the SLP instruction are in Position Units. A position unit is user defined according to the particular application and is configured in the SFX instruction.

During operation the Position Limits may be programmatically changed. If the limits are changed while the function is operating then the new limits will take effect immediately.

Pass-Through Tags

A Safe Motion Monitoring Drive has one or more motion axes that are controlled by a motion task. The Safe Motion Monitoring Drive also has one or more motion safety instances that support safety functions used in a safety task of a safety controller. Some of the tags associated with a drives motion safety instance are pass-through tags. The following table shows the pass-through tags and the corresponding axis tags for the SLP function:

1

module is the name for the drive module in Logix Designer I/O Configuration tree

2

instance is 1 or 2 for dual axis drives otherwise null

3

axis is the axis name in the Logix Designer Motion Group and is associated with module

When assigning the SLP Active, SLP Limit and SLP Fault instruction outputs to the motion safety instance pass-through tags, the corresponding Axis Safety Status and Axis Safety Faults tags automatically update in the motion controller. The motion control task of motion controller reads the Axis Safety Status and the Axis Safety Faults tags to coordinate operation between the safety task and motion task. The following is a typical sequence of events:

In many applications it is necessary for the SLP Positive Travel Limit or Negative Travel Limit to change dynamically. Changes to the SLP Travel Limits are range checked and then applied to the SLP function even if the function is active. It may also be necessary for the motion application to coordinate speed control with Active Limit changes. To accommodate motion coordination the safety controller tag list contains two general purpose 16 bit tags for each motion safety instance. These tags appear as module:SO.PassThruData[A|B][instance]. Axis tags named axis.AxisSafetyDataA and axis.AxisSafetyDataB are updated whenever the corresponding pass-through tags module:SO.PassThruDataA[instance] and module:SO.PassThruDataB[instance] change values.

Normal Operation, Automatic Restart

The following diagram shows normal operation with Automatic Restart. After Check Delay expires the Position is shown to be within the Positive and Negative Travel Limits. The diagram then shows the position moving outside of the limits and the Limit Output is set to ON(1). For automatic restart operation, the SLP function is reset when the Request is removed, OFF(0), provided no SLP faults have occurred.

Normal Operation, Manual Restart

When manual restart is enabled, the SLP function requires a reset before subsequent operation. The Reset output indicates that the Reset input must make an OFF(0) to ON(1) transition to reset the function after the Request input is removed OFF(0). The following diagram shows normal operation with manual restart.

Faulted Operation

Faults for SLP may be for invalid configuration, SFX Instruction not ready or not homed as described in the Fault Codes and Corrective Actions section.

Fault Codes and Corrective Actions

Diagnostic Codes and Corrective Actions

Example