Servo Enable (Servo ON / OFF)

Switching a motor drive into a state where it can actively output torque (enable), or cutting that output off (disable).

Servo enable means switching a motor drive from powered-but-idle into closed-loop operation, actively outputting torque according to commands — commonly called ‘enabling’; the reverse, cutting off output, is ‘disabling.’ Take the widely used CiA 402 fieldbus drive profile as an example: the drive has an internal state machine, and the host must send Shutdown, Switch On, and Enable Operation commands in sequence through a control word; only once the drive reaches Operation Enabled does it accept position, velocity, or torque setpoints, and an error drops it into a Fault state that must be reset before continuing. This explicit enable step exists to prevent the motor from jerking unexpectedly the instant power is applied. Once disabled, the motor no longer outputs torque, so a joint without a holding brake will drop under gravity — which is why humanoid and legged robots are usually suspended from a gantry when being enabled or disabled.

ExampleWhen bringing up an EtherCAT joint module, the host first reads the status word to confirm it is in Switch On Disabled, then sends Shutdown, Switch On, and Enable Operation in order; once the status word reports Operation Enabled, it starts sending periodic target positions. Before finishing, the joint is returned to a safe pose before being disabled.

Also called

Enable Operation, Servo ON, Servo OFF

Related

CANopen / CiA 402 Drive Profile · Servo Drive (Motor Driver) · Holding Brake · Safe Torque Off (STO) · Cyclic Synchronous Position / Velocity / Torque Modes (CiA 402) · Safety Gantry (Suspended Start)

Sources

ethercat_driver_ros2: CANopen over EtherCAT for electrical drives（CiA402 状态机） (Chinese)