Servo Motors

A servo is a motor system with feedback. Instead of simply applying voltage or counting open-loop steps, a servo compares a command with measured motion and drives the error toward zero. This makes servos valuable when the machine must know where it is, reject load disturbances, or stop on a fault.

Learning Objectives

By the end of this lesson, you should be able to:

• distinguish hobby servos from industrial servo axes;
• explain closed-loop position, velocity, and torque control;
• estimate torque, speed, power, and supply current requirements;
• understand hobby-servo PWM command timing and its limits;
• recognize hunting, following error, overload, backlash, grounding, and tuning problems.

Servo System Blocks

The controller subtracts feedback from the command. The resulting error is used to command motor current, voltage, or torque. A small hobby servo hides this loop inside the plastic case. An industrial servo axis exposes the motor, encoder, drive, controller, brake, and tuning parameters as separate design choices.

Hobby Servo Commands

Most hobby servos use a repeated control pulse. A typical frame is about 20 ms; pulse width sets the target angle.

These values are conventions, not universal guarantees. Some servos accept wider ranges and some are damaged if driven into mechanical end stops. Always test pulse limits gently with the linkage disconnected.

title "Hobby servo command pulse, illustrative"
time start=0 end=20 unit=ms divisions=10
MIN: pulse label="1.0 ms command" low=0 high=5 at=1 width=1 unit=V color=#2563eb
CTR: pulse label="1.5 ms command" low=0 high=5 at=7 width=1.5 unit=V color=#16a34a
MAX: pulse label="2.0 ms command" low=0 high=5 at=14 width=2 unit=V color=#dc2626
marker FRAME at=20 label="next frame"

Torque, Speed, and Power

The load determines required torque:

$$
T_\text{load} = F r
$$

Rotational mechanical power is:

$$
P_\text{mech} = T\omega
$$

where T is torque in N m and omega is angular speed in rad/s. Servo datasheets often use kgf cm; convert it before comparing values:

$$
1\ kgf\cdot cm \approx 0.0981\ N\cdot m
$$

For an arm that needs 15 N at 40 mm radius:

$$
T = 15 \times 0.04 = 0.60\ N\cdot m
$$

In hobby-servo units:

$$
T \approx \frac{0.60}{0.0981} = 6.1\ kgf\cdot cm
$$

After friction, acceleration, shock load, and safety margin, a 6 kgf cm servo is not enough. Choose a larger servo or redesign the linkage.

Closed-Loop Behavior

A servo loop can be too weak or too aggressive. Low gain gives sluggish response and position error. Excessive gain can cause overshoot, buzzing, oscillation, or hunting. Mechanical backlash, flexible belts, loose gears, and soft mounts make tuning harder because the sensor and load do not move as one rigid body.

Industrial drives often report following error when measured position cannot keep up with command. Treat following error as a useful fault, not an annoyance to suppress without understanding the cause.

Power and Grounding

Servos draw burst current during acceleration, stall, and reversal. A microcontroller board regulator is rarely suitable for powering more than a very small unloaded servo.

• Size the supply for peak current, not only average current.
• Place bulk capacitance near groups of servos.
• Share control-signal ground with servo power ground unless an isolated interface is used.
• Keep high-current servo returns away from sensitive analog sensor returns.
• Fuse branches where a stalled servo or damaged cable can overheat wiring.

Hobby, Smart, and Industrial Servos

• Analog hobby servo: internal potentiometer, pulse-width command, compact and cheap, but little feedback is exposed to the user.
• Digital hobby servo: internal sensor and MCU, pulse-width or serial command, stronger hold and features, but still sensitive to supply quality.
• Smart servo: internal feedback exposed over a serial bus, useful for reading position and faults, but tied to a vendor protocol.
• Industrial servo: encoder or resolver feedback with fieldbus, step-dir, or analog commands, high performance, safety options, higher cost, and more tuning work.

Practical Checks

1. Move the mechanism by hand and confirm travel stops are not binding.
2. Test command limits with the linkage disconnected.
3. Measure supply voltage during fast reversals.
4. Check servo case temperature after repeated duty cycles.
5. Watch for hunting, buzz, overshoot, and missed target position.
6. Confirm fault behavior: disable output, brake, limit switch, or emergency stop.
7. For industrial axes, save tuning, current limit, encoder direction, and homing parameters.

Common Mistakes

• Powering a servo from a weak microcontroller regulator.
• Assuming 1 ms to 2 ms pulse limits are safe for every servo.
• Comparing kgf cm and N m without conversion.
• Ignoring backlash and linkage geometry.
• Forgetting the common ground for a non-isolated signal.
• Masking following-error faults instead of finding the mechanical or tuning cause.

Summary

A servo combines motor power with feedback. Select it by torque, speed, travel, feedback quality, duty cycle, peak current, and fault behavior. Verify the complete mechanism under load because the control loop, power supply, linkage, and sensor all determine the final motion.

Further Reading

• Pololu, hobby servo control and power guides.
• Kollmorgen, servo motor sizing and tuning fundamentals.
• Texas Instruments, motor-control feedback application notes.
• ODrive and ClearPath documentation for practical servo-axis setup concepts.

Mind Map