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Safe torque off

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Motor velocity decaying to zero after STO has been activated.

Safe torque off a.k.a. STO is a motor drive function that disables drive's ability to produce torque with motor and also prevents unintentional motor start-up. STO is built with redundancy a way that guarantees the removal of torque even in the case of single failure of any component of the system.

Usefulness[edit | edit source]

Built-in STO in drive provides following

Pros
  • Eliminate the need of power cutting relay or contactor
  • Can provide dynamic braking of motor under STO condition instead of letting motor free-wheel which usually increases safety
  • Reduce cost of safety circuity
  • Simplify wiring
Cons
  • Most STO implementations not safe with brush DC motors and also may cause AC motor to lock-up in certain magnetic pole angle (thus motor can still produce torque but cannot rotate more than 1/2 revolutions)

STO in Argon servo drive[edit | edit source]

Argon (servo drive) has 4-way STO function making it one of the most fail safe implementations available. Safety is being provided by four simultaneous actions:

  • Removing AC input power from power stage by the means of internal safety rated relay
  • Cutting gate drive voltage from power stage rendering power stage inactive
  • Activating dynamic braking of motor
  • Software (firmware) disabling motor control

The cutting of AC power makes Argon safe even with DC motor and does not allow AC motor lock up.

STO in IONI servo/stepper drive[edit | edit source]

IONI Servo & Stepper Drive has one dedicated digital input for STO. When STO2 line is logic low, drive's power stage control is de-energized making it unable to drive any current or voltage to motor coils. In addition to this, drive firmware reads STO2 status and disables motor control signals.


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