Servo Motion Inputs
Enter values for the heaviest expected movement. The calculator estimates torque, mechanical power, electrical power, and supply current.
Example Data
A 1.5 kg load sits 0.18 m from a servo pivot. The arm moves 90 degrees in 0.60 seconds.
| Input | Example value | Purpose |
|---|---|---|
| Load mass | 1.50 kg | Creates gravity torque at the arm. |
| Center distance | 0.18 m | Defines the gravity lever arm. |
| Travel and time | 90° in 0.60 s | Defines angular speed and power. |
| Mechanism efficiency | 85% | Accounts for transmission losses. |
| Safety factor | 1.75 | Provides capacity for uncertainty. |
Formula Used
Gravity torque: τg = m × g × r × |sin(θ)|
External-force torque: τf = F × rf
Load torque: τload = τg + τfriction + τf
Shaft torque: τshaft = τload ÷ ηmechanism
Inertia torque: τinertia = I × α
Peak design torque: τpeak = (τshaft + τinertia) × safety factor
Power: Pmechanical = τ × ω, then Pelectrical = Pmechanical ÷ ηservo
Here, τ is torque in newton-metres, ω is angular speed in radians per second, I is rotational inertia, α is angular acceleration, and η represents efficiency.
How to Use This Calculator
- Measure the mass that the servo arm must move.
- Measure the pivot-to-center distance for that mass.
- Use 90 degrees when gravity produces the greatest torque.
- Add measured friction and any external force.
- Enter the travel angle and required movement time.
- Choose realistic efficiency values and a conservative safety factor.
- Compare the recommended peak torque and current with manufacturer data.
Selecting Servo Power for Reliable Motion
Servo Motion Power Basics
A servo moves a load through a chosen angle. The motor must create enough torque. Torque overcomes gravity, friction, and outside forces. Power describes how quickly that torque performs work. A slow movement can need high torque. A fast movement can need modest torque but higher power. Selecting only by torque can therefore cause problems. Selecting only by watts can also mislead. The correct choice considers torque, speed, voltage, duty cycle, and control requirements together.
Loads and Lever Arms
A load on an arm produces turning force around the pivot. The load mass is multiplied by gravity and center distance. The arm angle changes the gravitational component. A horizontal arm usually creates the highest gravitational torque. A vertical arm can create little gravitational torque. Friction adds resistance at every position. Springs, cables, gears, and external pushing forces add further torque. Enter values that match the hardest part of the movement. This makes the result more useful for real actuator selection.
Speed Changes Power
Angular speed is the travel angle divided by move time. Convert that value into radians per second. Mechanical shaft power equals required shaft torque multiplied by angular speed. Double the speed while torque stays constant. Required mechanical power then doubles. A servo may show enough stall torque while still moving too slowly. Check the manufacturer speed specification at your planned voltage. Compare it with your required travel time. Real loads also accelerate and decelerate, so leave practical time margin.
Efficiency and Electrical Demand
Mechanisms lose energy through gears, belts, joints, and bearings. Mechanism efficiency adjusts the load torque into shaft torque. Servo electrical efficiency estimates input power from mechanical output power. These values vary during motion. They are best used for planning, not certification. Electrical current is estimated from input power divided by supply voltage. It does not replace a manufacturer stall-current rating. Choose wiring, connectors, regulators, and batteries for expected peak current. Sudden starts can demand far more current than average travel calculations suggest.
Safety Factors Matter
A safety factor protects against unknown friction, changing loads, imperfect alignment, and wear. Start with a factor near 1.5 for predictable systems. Use larger margins for lifting, impacts, or uncertain conditions. The calculator reports a recommended design torque. Match that torque against the servo rating at your actual voltage. Do not compare it with a rating measured at a different voltage. Check whether the rating is stall torque or continuous torque. Continuous operation near stall can overheat a servo quickly.
Using Results Wisely
Treat the calculated power as a design estimate. Build a small prototype when possible. Measure movement time, supply voltage, and current under the real load. Watch servo temperature during repeated cycles. Increase capacity when motion stalls, chatters, or drops position. Reduce arm length or load mass where practical. Add gearing when more torque is needed. Gearing lowers output speed, so recalculate travel time. Good servo design balances torque, speed, power, heat, cost, and dependable control. Document every result before final component selection.
Frequently Asked Questions
1. What does servo power mean?
Servo power is the rate at which the servo delivers mechanical work. It depends on torque and angular speed. A higher power requirement can result from greater torque, faster movement, or both.
2. Is torque the same as power?
No. Torque is turning force. Power combines torque with speed. A servo can have high torque but low power when it moves slowly. It can also require more power when it moves quickly.
3. Why does arm length matter?
A longer arm increases the distance from the pivot. The same load then produces more torque. Reducing arm length can lower the required servo torque significantly.
4. Which torque rating should I compare?
Compare the calculator’s recommended peak torque with the servo torque rating at your actual supply voltage. Also inspect continuous torque capability, speed under load, and the manufacturer thermal guidance.
5. Should I use a safety factor?
Yes. A safety factor covers friction changes, imperfect assembly, voltage drop, wear, and unmeasured loads. Many systems start near 1.5 to 2.0, then use higher margins for lifting or impact loads.
6. How does friction affect the result?
Friction adds torque that the servo must overcome throughout movement. Include bearing drag, seals, gear losses, cable resistance, and contact forces. Measure it when practical for a stronger estimate.
7. Why include rotating inertia?
Inertia resists changes in rotational speed. It creates extra torque during acceleration and deceleration. Fast moves with large arms, disks, or payloads can need substantial inertia torque.
8. Does calculated current equal stall current?
No. The calculated value estimates motion current from power and efficiency. Stall current is manufacturer-specific and can be much higher. Size the power supply, wiring, and protection for documented peak and stall conditions.
9. What efficiency should I enter?
Use measured values when available. Otherwise, select conservative estimates. Mechanism efficiency often reflects gears, belts, and joints. Servo efficiency describes electrical input converted into mechanical shaft output.
10. Can this calculator size the battery?
It provides energy per movement and duty-cycle power estimates. Use those values with expected cycles, voltage limits, battery discharge ratings, and regulator losses to develop a complete battery design.
11. What should I test after choosing a servo?
Test the real load at the intended voltage. Measure speed, current, voltage sag, positioning accuracy, and temperature. Repeat the test at the highest expected duty cycle and the hardest operating angle.