STO (Safe torque off)

STO (safe torque off) is a safety function according to DIN EN 61800-5-2 and corresponds to an uncontrolled stopping according to EN 60204-1, stop category 0.

In the event of a requested STO, the motor is safely, immediately and automatically separated from the energy supply in the inverter if the IGBT control is safely interrupted. The motor is prevented from starting unexpectedly. If the STO is actuated while moving, the drive coasts to stop.

The function is suitable up to performance level (PL) d according to EN 13849-1 if the signals STO and STO_GND are interrupted in two channels with a switching device.

If the STO signal is looped through several drives, the STO state applies to all drives in this group. If a drive is in an error state due to a drive error (SBM = 0), it has no effect on the STO state of the other drives in this group.

 

STO with hybrid cable looped through in the drive train

 

Standards and characteristic values

Standards

Topic

Classification

EN 61800-5-2

Safety function

STO

Safe torque off

EN 60204-1

Stop category

0

EN ISO 13849-1

Performance Level (PL)

d

Category

3

IEC 61508

Safety integrated Level (SIL)

2

EN 61131-2

Interface STO

X08.1 and X08.2 (signal STO / STO_GND)
Digital input type 3

 

Key values according to EN ISO 13849-1

Value

Notice
PFH

1,4 E-8 1/h

1,4 % von SIL 2
PFDavg (T = 20a)

1,2 E-3

12 % von SIL 2
MTTFd

654 a

high
DCavg

74,2 %

low

 

Reaction times

Value

STO is triggered by hardware input

< 5 ms

Software reacts on error (error reaction)

< 20 ms

Diagnostic message is displayed

< 200 ms

Dynamization STO: The signal to control STO must be dynamic with a pulse duration

≤ 1,5 ms

 

Notes on operation

For your safety

DANGER

Danger to life from touching electrical connections!

In the status 'Safe torque off (STO)', the pulses for controlling of the output stage are blocked, the device remains connected to the mains (no automatic mains separation). Electrical terminals and connectors carry further voltages that may cause death or serious injury upon contact.

Steps to prevent:

  • Provide shock-hazard protection
  • Prior to any work on the device: Turn off the main switch to disconnect the power supply, and secure switch against being turned on again.
  • Wait at least 5 minutes for components to discharge.
  • Connection or disconnection of terminals is only allowed if they are free of voltage.
  • Measure the terminals voltage to verify that the terminal is de-energized. One suitable measuring point is the DC bus between the UZP and UZN terminals.

 DANGER

Danger to life due to unexpected movements!

The drive will be torque-free in the status 'Safe torque off (STO)', in case of mains failure or in case of faulty drive controller. External application of force on the drive axis may result in life-threatening movements (e.g. hanging axes can fall down).

Steps to prevent:

  • Install an external, mechanical brake that prevents a movement.
  • Install a counterweight in order to maintain the axis in balance.

DANGER

Danger to life by jerky movement of the motor shaft!

If defects in the power output stage or the control thereof at the same time can occurs a short circuit in the circuit with the following effects:

 

  • Mains fuse is triggered if safety function STO is active.

  • Rest movement! Jerky align of the motor (180° / pole pair). A rotating field will not be reached.
    Example 10 pole motor:
    10 pole → 5 pole pair
    180° / 5 pole pair = 36° (Rest movement)

 

Behavior of the motor holding brake at STO

The STO state has no effect to the motor holding brake. The motor holding brake is supplied by connection X08.1 and X08.2 (signal 24V, 0V).

Hybrid cable

Cables like hybrid cable contains power, signal and communication wires as well as safety-relevant cables. Always install the safety related cables in accordance with EN ISO 13849-2 Table D.4 permanently (fixed) and protected against external damage, e.g. in the cable channel or armored tube. An energy chain can also be a suitable protection against external damage.

 

Consider possible causes of damage to cables and take appropriate preventive measures:

  • Driving over cables by the machine itself
  • Driving over doe to vehicles or other machines
  • Touching the cable with the machine construction or other nearby parts during the movement
  • Running in or out of cable baskets or cable reels
  • Acceleration forces and wind forces on cable garland systems or overhead lines
  • Excessive friction due to line pickup
  • Exposure to excessive radiant heat

 

Further information on correct cable routing can be found in the standard EN 60204-1.

 

To prevent failure due to wear, replace cables when the maximum permissible number of bending cycles has been reached (relevant for cables that are subjected to cyclic bending, for example in a drag chain).

The shield must be placed on PE on both sides.

Assembly

Observe the installation instructions for heat dissipation so that the safety function is operated within the permissible temperature range.

Siehe 'Assembly instructions'.

Function test STO

Check STO safety function!

The achievable safety categories are only valid if the customer will performs the following checks:

  • When startup, you must checked the operation of STO safety function and the signal transitions 'STO inactive' → 'STO active'

  • Annually review of the operation and signal transitions 'STO inactive' → 'STO active'

Following states must be checked::

Further Information

Siehe '[X08] Supply voltage 24 VDC and STO supply line'.

Siehe '[X09] Transmission supply voltage 24 VDC and STO'.

Siehe 'Connection examples for operation with STO'.

Siehe 'Diagnosis STO'.