An Electric Servo is a motor combined with a feedback sensor and a control loop that drives the shaft to a commanded position, velocity, or torque and holds it there against disturbance. Industrial robotics depends on it — every joint of a 6-axis arm is a servo. The drive compares commanded position to encoder feedback, runs a PID loop, and outputs current to correct the error. The outcome is sub-arcminute positioning under load, the kind of accuracy a FANUC LR Mate 200iD holds at ±0.02 mm repeatability across millions of cycles.
Electric Servo Interactive Calculator
Vary encoder line count, decode mode, and ballscrew pitch to see servo feedback resolution and a live closed-loop diagram.
Equation Used
The calculator multiplies encoder line count by the decode factor to get counts per motor revolution. For a direct-drive ballscrew, one revolution moves by the screw pitch, so pitch divided by counts per revolution gives the theoretical linear motion per encoder count.
- Incremental encoder counts are multiplied by the selected decode factor.
- Motor shaft directly drives the ballscrew with no gearing, slip, or compliance.
- Resolution is theoretical encoder quantization, not full machine repeatability.
How the Electric Servo Works
A servo is not a single part — it's three parts wired into a loop. You have the motor (usually a brushless permanent-magnet rotor today, sometimes a brushed DC unit on cheap hobby gear), a position sensor on the shaft (an incremental encoder, absolute encoder, or resolver), and a servo drive that runs the closed-loop position control. The drive subtracts measured position from commanded position, that difference is the following error, and a PID loop turns the error into a current command. More current means more torque via the torque constant Kt. The motor accelerates, the encoder reports new position, the error shrinks, and the loop settles.
Why bother with the loop at all? Because an open-loop motor has no idea whether it actually got where you told it to go. Load it down, hit a hard stop, drop the supply voltage — an open-loop motor just lies. A servo cannot lie. If commanded position and measured position disagree, the drive pushes harder until they agree or it faults out on a following-error limit. That's the whole point.
Get the tuning wrong and you'll know immediately. Too much proportional gain and the axis screams and oscillates around the target — you'll hear it as a high-pitched buzz. Too little integral gain and the axis settles 50 µm short of target every time and sits there, unable to push through static friction. Encoder resolution sets the floor on what the loop can even see — a 2,500-line incremental encoder quadrature-decoded gives 10,000 counts per rev, which on a 10 mm-pitch ballscrew is 1 µm per count. Common failure modes are encoder coupling slip (the axis drifts), drive overcurrent faults from sudden mechanical jams, and thermal shutdown when the holding torque demand exceeds the motor's continuous rating but stays under its peak rating long enough to cook the windings.
Key Components
- Brushless Servo Motor: A permanent-magnet rotor inside a 3-phase stator. Torque constant Kt typically runs 0.1 to 2 Nm/A depending on frame size. A NEMA 23 servo like the Teknic ClearPath delivers about 1.0 Nm continuous, 3.0 Nm peak.
- Encoder or Resolver: Reports shaft position back to the drive. Incremental encoders give 1,000 to 10,000 lines/rev (4× that after quadrature decoding). Absolute encoders like the Renishaw RESOLUTE hit 32-bit single-turn resolution. Resolvers survive harsh environments where optical encoders fail.
- Servo Drive (Amplifier): Runs the PID loop and switches current to the motor windings via IGBTs or MOSFETs at 8 to 20 kHz PWM. Modern drives like the Yaskawa Sigma-7 close the position loop at 4 kHz, giving servo bandwidth around 500 Hz to 1 kHz.
- Command Interface: Accepts position commands from a controller via EtherCAT, CANopen, step/direction pulses, or analogue ±10 V. Cycle time for EtherCAT command updates is typically 250 µs to 1 ms — the lower number lets you run smoother coordinated multi-axis motion.
- Power Supply and Bus Capacitor: Provides DC bus voltage, usually 24 V on small units, 320 V or 565 V on industrial AC servos. The bus capacitor absorbs regenerative energy when the motor decelerates a load — undersized caps blow the over-voltage fault on a hard stop.
Who Uses the Electric Servo
Servos show up wherever you need a shaft to land at a commanded position under variable load and hold it. Anywhere that open-loop steppers lose steps, brushed DC motors lack repeatability, or pneumatics can't hold mid-stroke, a servo is the answer. The cost is real — a small industrial servo axis runs $800 to $2,000 all-in — but on a machine that prints 100,000 parts a year, the math closes fast.
- Industrial Robotics: Every joint of a FANUC LR Mate 200iD or KUKA KR 6 R900 is a brushless AC servo with absolute encoder feedback, holding ±0.02 mm repeatability.
- CNC Machine Tools: Haas VF-2 vertical machining centre uses Yaskawa servos on X, Y, Z axes feeding a 10 mm-pitch ballscrew with 1 µm encoder resolution.
- Semiconductor Manufacturing: ASML lithography stages use linear servos with sub-nanometre interferometric feedback to position wafers under the optics.
- Packaging Machinery: Bosch Sigpack form-fill-seal lines run Beckhoff AX5000 servos on the cross-seal jaws to coordinate with film velocity at 200 packs/minute.
- Medical Devices: Intuitive Surgical's da Vinci Xi system uses miniature Maxon EC-i 40 servos with optical encoders inside each instrument arm for surgeon hand-tracking.
- RC and Hobby: A standard hobby servo like the Hitec HS-422 uses a brushed DC motor, plastic gearbox, and potentiometer feedback — same closed-loop principle scaled down to $12.
The Formula Behind the Electric Servo
Sizing a servo starts with continuous torque demand at the load. The formula below converts mechanical load and motion profile into the torque the motor must produce continuously without overheating. At the low end of typical loading — say 30% of motor rated torque — the servo runs cool, holds tight, and lasts decades. At nominal 60-70% loading you're in the sweet spot for thermal headroom and dynamic response. Push past 90% continuous and you cook windings, lose torque margin for acceleration, and start tripping thermal faults during summer heat waves on a shop floor with no air conditioning.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Trms | Root-mean-square torque demand over the motion cycle | Nm | lb·in |
| Ti | Torque required during motion segment i (accel, constant velocity, decel, dwell) | Nm | lb·in |
| ti | Duration of motion segment i | s | s |
| tcycle | Total cycle time including dwell | s | s |
Worked Example: Electric Servo in a label-printing rotary indexer
A specialty label printer in Lyon is sizing the servo for a 12-station rotary indexer carrying empty glass perfume bottles past a UV-cure print head. The dial plate is 600 mm diameter, total inertia (plate + 12 bottles + tooling) is 0.15 kg·m². Each index moves 30° in 0.25 s, then dwells 0.75 s while the print head fires. Cycle time is 1.0 s per station. The team is choosing between three Yaskawa SGM7J frame sizes — 200 W, 400 W, and 750 W — and wants to know which sits in the sweet spot.
Given
- Jtotal = 0.15 kg·m²
- θ = 30° = 0.524 rad
- tmove = 0.25 s
- tdwell = 0.75 s
- tcycle = 1.0 s
Solution
Step 1 — assume a triangular velocity profile (accel for half the move, decel for half). Peak velocity and acceleration:
Step 2 — compute peak accel torque at nominal operating conditions:
Step 3 — RMS torque across the full 1.0 s cycle. Accel and decel each last 0.125 s at 5.03 Nm, dwell is 0.75 s at near-zero holding torque:
Now the three operating points. At the low end of the typical range — pick the 750 W SGM7J (rated 2.39 Nm continuous, 7.16 Nm peak) — Trms sits at 105% of continuous rating. That's over-temperature territory, but barely; it'll trip on a hot day. At nominal — the 400 W SGM7J (1.27 Nm continuous, 3.82 Nm peak) — peak demand of 5.03 Nm exceeds the 3.82 Nm peak. The drive faults on overcurrent. At the high-end thermal-margin choice, you'd step to the 1 kW SGM7G (3.18 Nm continuous, 9.55 Nm peak): Trms is 79% of continuous, peak is 53% of peak rating. Comfortable, cool, plenty of headroom for tooling changes that add inertia later.
Result
The RMS torque demand is 2. 51 Nm with peak demand of 5.03 Nm during the index. The 1 kW SGM7G is the right pick — it sits at 79% continuous loading, which feels right on a shop floor: motor case runs warm but never hot to the touch, no thermal faults, and roughly 20% headroom if a future bottle design adds mass. The 400 W is undersized (drive faults on every index) and the 750 W is on the edge of over-temperature, the kind of choice that works in February and trips in August. If your axis trips during commissioning at the predicted torque, look first at coupling misalignment adding parasitic load (a 0.1 mm parallel offset on a bellows coupling can add 0.5 Nm continuous drag), second at regen resistor undersizing causing bus over-voltage on every decel, and third at encoder cable EMI causing phantom following-error faults — symptoms look like torque problems but the root cause is signal integrity.
When to Use a Electric Servo and When Not To
The main competitors to an electric servo are the stepper motor (open-loop or with closed-loop add-on) and the brushed DC gearmotor with simple velocity control. Each wins in different parts of the design space.
| Property | Electric Servo (brushless AC) | Stepper Motor (open-loop) | Brushed DC Gearmotor |
|---|---|---|---|
| Position accuracy | ±5 arcsec with absolute encoder | ±3 arcmin if no missed steps | ±0.5° with potentiometer feedback |
| Continuous torque at rated speed | 100% of rating up to 3000-6000 RPM | Drops sharply above 500 RPM | Drops with brush wear over life |
| Closed-loop bandwidth | 500 Hz to 1 kHz | Not applicable (open loop) | 10-50 Hz typical |
| Cost per axis (motor + drive) | $800-$2,500 industrial | $150-$600 | $50-$300 |
| Lifespan | 20,000+ hours, no brushes | 20,000+ hours | 2,000-5,000 hours (brush wear) |
| Stall behaviour | Holds position, faults on following error | Loses steps silently, position is wrong forever | Stalls and overheats |
| Tuning complexity | PID tune required, 30-60 min per axis | Plug and play | Simple velocity loop |
Frequently Asked Questions About Electric Servo
That's classic over-tuned proportional gain (Kp). The loop is correcting too aggressively — every time the encoder reports the axis crossed the target, the drive overshoots correcting it, then overshoots again coming back. Drop Kp by 20% and add a touch of derivative gain (Kd) to damp the oscillation.
If lowering Kp doesn't kill the hum, check mechanical compliance. A long bellows coupling, a worn ballscrew nut, or a flexible mount creates a resonance the loop excites. The fix is mechanical, not electrical — stiffen the coupling or add a notch filter at the resonant frequency in the drive.
Ask one question: what happens after a power cycle? With an incremental encoder, the drive doesn't know shaft position until you run a homing routine — drive to a limit switch, then to an index pulse. That's fine on a CNC where you home every morning. It's a problem on a robot arm hanging in mid-air with a workpiece in the gripper.
Absolute encoders cost roughly 2-3× more but eliminate homing, survive E-stops without losing position, and let you start a job mid-cycle after a power blip. For any vertical axis or any axis carrying a load through power-down, pay for absolute.
The integrator usually is, and here's why. RMS torque calcs assume your motion profile, inertia, and friction are all known precisely. In reality, friction grows as bearings age, tooling gets swapped for heavier versions, and operators run cycles faster than spec. A 750 W choice on 400 W demand gives you 50% headroom for that drift over a 10-year machine life.
Servo motors also lose torque at the top of their speed curve as back-EMF approaches bus voltage. If you're running near rated speed, the actual available torque can be 70-80% of the catalogue continuous figure. Size by torque-speed curve at your operating point, not the headline number.
Following error scales with velocity for proportional-only loops and with acceleration for loops missing feedforward terms. Catalogue specs are quoted with full velocity feedforward and acceleration feedforward enabled — most drives ship with these set to zero, expecting you to tune them.
Turn on velocity feedforward (start at 100% if your drive uses percent, or set Kvff = 1/Kv if it uses gain units), then acceleration feedforward at roughly Jtotal × Kt-1. Following error during constant velocity should drop to nearly zero, and accel-phase error should drop by 5-10×.
Yes, and almost every modern drive supports it as a built-in gantry or master-slave mode. Both drives close their own current and velocity loops, but they share a single position command and one drive runs a torque-balance loop that biases the slave to keep the rails square.
The trap: if you run two independent drives without gantry mode, thermal drift and tuning differences will fight each other. The motors push against the structure, you'll see one drive at +30% torque and the other at -30% just holding still, and the rails will rack within months. Use gantry mode or skip the dual-drive arrangement entirely.
When a servo decelerates a moving inertia, the motor acts as a generator and pushes current back into the DC bus, raising bus voltage. If the bus capacitors can't absorb it fast enough, voltage climbs past the trip threshold (typically 400 V on a 320 V bus, 800 V on a 565 V bus) and the drive faults to protect itself.
The fix is a regen resistor — an external power resistor the drive switches across the bus to dump excess energy as heat. Sizing rule: peak regen power equals ½ × J × ω² / tdecel. A 0.15 kg·m² load decelerating from 3000 RPM in 0.1 s dumps about 750 W peak — you need a 100 Ω, 200 W continuous-rated resistor with that kind of peak handling.
References & Further Reading
- Wikipedia contributors. Servomotor. Wikipedia
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