STO - Safe Torque Off function
General function description
The STO function is the most common and basic drive integrated safety-related function. If STO is activated, no more power, which can cause rotation or motion, is applied to the motor. The power stage of the drive will not provide any energy to the motor which can generate torque or force in case of a linear motor.
An active STO function results in a subsequent start-up/restart inhibit (see section below).
The STO function prevents an unintended start according to the standard EN 60204-1, section 5.4. With the designated STO function activated, the power stage of the drive is deactivated. The drive is then torque-free. This state is monitored internally in the drive.
Requesting/monitoring by the safety-related FB/Safety Module
Immediately with the STO activation the drive is set torque-free and the axis then coasts down until speed is zero.
When requesting STO, the safety-related function is executed immediately. STO can be requested by switching the signal at input S_STO_Request to SAFEFALSE (if input Activate = TRUE) or via the hard-wired link (if Safety Module device parameter HW_STO = Activated). Refer to the section below.
The torque-free status of the drive is indicated by switching the function block output S_STO_SafetyActive to SAFETRUE.
After indicating the active STO function, the axis coasts down. Therefore, S_STO_SafetyActive = SAFETRUE does not necessarily imply the standstill of the axis. The coast down time depends on physical properties of the used components, such as weight, torque, friction, etc.
WARNING
UNINTENDED EQUIPMENT OPERATION
Make certain that no hazards can arise for persons or material during the coast down period of the axis/machine.
Do not enter the zone of operation during the coast down period.
Ensure that no other persons can access the zone of operation during the coast down period.
Use appropriate safety interlocks where personnel and/or equipment hazards exist.
Failure to follow these instructions can result in death, serious injury, or equipment damage.
Application
As the STO function of the drive disables the power stage to the motor, it is suitable to be used in applications where the axis comes to a stop by itself in a sufficiently short time through its load torque or friction or when the "coast down" of the axis has no safety-related relevance.
STO as general functional safe state: The STO function is defined as the default functional safe state. Therefore, the STO function is the final fallback function of the other safety-related functions described in this documentation.
STO request via the function block or hard-wired
Depending on your requirements and the implementation of the system, the STO function can be requested in two ways:
Via the S_STO_Request input of the function block (transmitted via the SERCOS bus)
Additionally by the hard-wired signal link. For that purpose, the HW_STO parameter must be set to Activated (see section below).
Further Information
Refer to the topic "STO hard-wired" for detailed information.
Start-up inhibit: STO following the function block activation
After its start-up the Safety Module automatically enters the STO functionally-safe state. According to the relevant IEC 60204-1 standard, the STO function executes stop category 0. This stop category implies a subsequent start-up inhibit. The start-up inhibit is only removed if there is a positive signal edge at the Reset input of the safety-related function block.
Restart inhibit following an STO
After removing an STO function request by switching the respective function block input from SAFEFALSE to SAFETRUE, a restart inhibit is automatically activated to prevent the unintended restart of the axis. According to the relevant IEC 60204-1 standard, the STO function executes stop category 0. This stop category implies a subsequent restart inhibit. The restart inhibit is only removed if there is a positive signal edge at the Reset input of the safety-related function block.
Relevant Safety Module device parameters
How to edit the relevant safety-related device parameters: In the EcoStruxure Machine Expert - Safety 'Devices' window, ...
Left-click the Safety Module in the devices tree.
In the Device Parameterization editor on the right, scroll to the relevant parameter section (see table heading below).
Specify the parameters listed in the table below for this safety-related function.
For detailed information on the value range and default value for the parameter, refer to the corresponding chapter for the safety module used in the "Safety Module Parameters and Process Data Items" guide.
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Parameter section: Basic |
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HW_STO |
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WARNING
NON-CONFORMANCE TO SAFETY FUNCTION REQUIREMENTS
Verify that the device parameters for the Safety Module correspond to your risk analysis.
Be sure that your risk analysis includes an evaluation for setting incorrectly device parameter values.
Validate the overall safety-related function with regard to the set device parameter values and thoroughly test the application.
Failure to follow these instructions can result in death, serious injury, or equipment damage.
Relevant FB inputs/outputs and bits in status word
Function request via FB input S_STO_Request = SAFEFALSE
Function status indication via FB output S_STO_SafetyActive (SAFETRUE = safety-related function activated)
In the DWORD output at AxisStatus
bit 0: HW_STO (TRUE = Safety Module parameter HW_STO is set to Activated)
bit 1: STO status (TRUE = safety-related function activated)
