Servo
Works with a feedback sensor and servo drive to control motion. The system compares measured position, speed or torque with a command and corrects the difference within its capabilities.
The servo drive compares a command with feedback, often from an encoder, and adjusts motor drive to reduce the error. Feedback at the motor shaft cannot automatically detect every error in a gearbox or load; a fully closed-loop arrangement measures further along the mechanism.
In plain terms
Like a driver glued to a speedometer who never lets the needle drift from the target — the eye reads, the foot adjusts, and any deviation is corrected before it grows.
Why designers use it
- Controlling position, speed or torque using feedback.
- Detecting and responding to following error.
- Meeting motion requirements with an appropriately sized motor, drive and mechanism.
Best for
- CNC machines
- Industrial robots
- Pick-and-place
Key specifications
- Torque and speed: Check continuous, effective and peak requirements
- Feedback: Sensor resolution, location and total mechanical error
- Motion and tuning: Load inertia, resonance and control response
- Thermal and regeneration: Check duty cycle and braking-energy handling
When not to use it
- Simple unloaded indexing under 100 positions/min — a stepper motor is half the cost and needs no feedback.
- Constant-speed pump or fan drives — a VFD-controlled induction motor is far cheaper and accurate enough.
Common mistakes
- Assuming encoder resolution guarantees equal positioning accuracy at the load.
- Ignoring both effective/RMS torque and peak torque requirements.
- Ignoring regenerative energy during deceleration.
- Using excessive control gain or overlooking mechanical resonance.
- Treating a maintenance interval as a guaranteed operating lifetime.
Where you will find it
- A Mazak Integrex i-200 mill-turn machine uses Yaskawa Sigma-7 SGM7G brushless servos on each of its 5 axes: 24-bit absolute encoders feed a Yaskawa drive running at 16 kHz, and the linked motion controller holds tool-tip position to under ±2 µm during simultaneous-axis turbine-blade contouring.
- A FANUC R-2000iC industrial robot uses αi-series brushless servos at each of six joints: the integrated 24-bit serial encoders feed FANUC's R-30iB controller running its proprietary torque-control firmware, achieving 50 µm path accuracy at 2 m/s tool-tip speed under 165 kg payload.
- An ASML TWINSCAN NXE EUV lithography stage uses dual-axis voice-coil servos with sub-nanometre laser interferometer feedback to position the silicon wafer beneath the exposure beam: the position loop runs in the megahertz range, and any servo error directly translates into chip-overlay error in the printed pattern.
A short history
A servomotor pairs a motor with a position-detecting component, such as an encoder, and operates together with a servo drive to form a closed-loop control system. The drive compares feedback on position, speed, or torque against a commanded value, then corrects the motor's operation in real time so the system can meet required performance, enabling precise, flexible positioning to control machine operation.
Good to know
- A motor-shaft encoder cannot directly observe every downstream backlash or flex error.
- Deceleration can return energy to the drive, so braking is also an electrical design problem.