Safe Stop 2 (SS2)

The Safe Stop 2 (SS2) instruction initiates and monitors the motor or axis deceleration within set limits to ensure the motor is brought to an operational stop. Once stopped, SS2 continues to monitor the operational stop of the motor.

Available Languages

Ladder Diagram

Function Block

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Structured Text

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Safe Stop 2 Application

Safe Stop 2 is used with a CIP safety drive that supplies speed and position of a motor or axis and a Safe Feedback Interface (SFX) instruction to scale the feedback. During operation, the SS2 instruction signals when the axis speed is at or below the Standstill Speed. When standstill is reached, SS2 then initiates SOS (Safe Operational Stop) to continue standstill monitoring.

Operands

The following table provides the 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 the 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

Operation

Normal Operation

The SS2 function begins if it has been previously reset and the Request input is asserted ON(1). At this point the Stop Monitor Delay Timer will begin. When the Stop Monitor Delay Timer expires the current axis speed is captured and the Stop Delay timer begins. As the Stop Delay Timer runs, the speed of the axis is monitored in real time according to the Speed Limit function, S(t), starting with Stop Delay Timer:

Speed Limit Function

S(t) = S0 + St - (Sr/ Ts)(t)

Where:

S(t) = Speed Limit

S0 = Speed captured at the end of Stop Monitor Delay

St= Decel Speed Tolerance

Sr = Decel Ref Speed

Ts = Stop Delay

t = the Stop Delay Timer value

When the SS2 Standstill Speed is reached then Safe Operating Stop (SOS) monitoring function within the SS2 function begins. Note that SS2 Standstill speed is reached before the Stop Delay timer expires in normal operation.

When the SOS monitoring begins, the Check Delay timer is started. After the check delay timer expires the position is captured. Either the speed or position, provided by an SFX instruction, is compared to the SOS Standstill Speed or Standstill Deadband according to the Mode setting. After Check Delay expires, Standstill output will be set to ON(1) as long as the speed is below the SOS Standstill Speed and the function is not faulted. The SOS monitoring remains active as long as it is not faulted and the Request input is ON(1). If the speed of the monitored axis exceeds the Standstill limit then the SOS function will Fault.

Position values used in the SS2 instruction are in Position Units. Speed values used in the SS2 instruction are in Position Units / Time Unit. A position unit is user defined according to the particular application and is configured in the SFX instruction. Time units are also configured in the SFX instruction and may be selected as seconds or minutes.

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 SS2 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 SS2 Active, SOS Active, SOS Standstill, SS2 Fault and SOS Fault 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:

Normal Operation, Automatic Restart

The following diagram shows a timing diagram for normal operation with Automatic Restart. In normal operation the SS2 Active output will remain ON(1) as long as the SS2 function has not been reset. For automatic restart operation, the SS2 function will be reset when the Request is removed OFF(0) provided no faults have occurred. When the SS2 function is reset the output O1 will be set to ON(1) indicating the function is ready to operate.

In the diagram, the Speed Limit function is shown as a solid red line ramping towards zero speed. The speed must stay below the Speed Limit function to maintain normal operation. After the SS2 Standstill Speed is reached the SOS Active Output is ON(1) indicating that the SOS function within SS2 is active and remains ON(1) as long as the Request remains ON(1).

Normal Operation, Manual Restart

When manual restart is configured, the SS2 function reset before subsequent operation. The Reset Required output indicates that the Reset input must make an OFF(0) to ON(1) transition to reset the instruction after the Request input is removed OFF(0). The following diagram shows normal operation with manual restart.

Faulted Operation

Faulted Operation, Deceleration Fault

The following diagram, a timing diagram of SS2 where a Deceleration Fault occurs, shows the axis speed exceeding the Speed Limit Function, resulting in a Deceleration Fault. Note that the timing diagram is shown for Manual Restart. For Automatic Restart the timing is similar except that the Reset Required [RR] output will not turn ON(1) until a fault occurs.

Faulted Operation, Standstill Speed Fault

The following diagram shows SS2 where a Standstill Speed Fault occurs. As shown, the axis speed reached SS2 and SOS Standstill Speed but during the SOS function the Speed increased until the SOS Standstill Speed was exceeded, resulting in a fault. Note that the timing diagram is shown for Manual Restart. For Automatic Restart, the timing is similar except that the Reset Required [RR] output will not turn ON(1) until the fault occurs.

Fault Codes and Corrective Actions

SS2 Fault Codes

SOS Fault Codes

Diagnostic Codes and Corrective Actions

Example