Safe torque off

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

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

STO in Argon servo drive

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