Exercise: Select a Motor and Driver

This exercise turns motor selection into a concrete engineering decision. You will calculate the motion requirement, compare actuator types, choose a driver and supply, and define tests that prove the axis is safe before it runs at full speed.

Learning Objectives

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

• convert linear motion requirements into force, torque, speed, and power;
• decide whether a DC motor, stepper, or servo is the best fit;
• choose a driver class and supply with current and protection margin;
• produce a verification plan for motion, temperature, limits, and faults.

Prerequisites

Before starting, review:

• force, torque, speed, and power;
• DC motor stall current and torque-speed curves;
• stepper current limiting and missed-step risks;
• servo feedback, following error, and supply-current bursts;
• basic motor-driver protection, fuses, limit switches, and emergency-stop behavior.

You also need a calculator or spreadsheet. Use real datasheets if you want to turn this exercise into a hardware purchase.

Concrete Task

Design the drive for a small horizontal belt axis that moves a 1.2 kg carriage. The pulley radius is 12 mm. Desired maximum speed is 0.4 m/s. Peak acceleration is 1.5 m/s^2. Assume friction force is 4 N and use a safety factor of 2.

Deliver:

1. required linear force;
2. required pulley torque;
3. pulley speed in RPM;
4. minimum mechanical power at the pulley;
5. selected motor type and reason;
6. selected driver class;
7. supply voltage and current estimate;
8. protection and verification plan.

Required Calculations

Acceleration force:

Fa = m a = 1.2 x 1.5 = 1.8 N

Design force with friction and safety factor:

Fdesign = SF x (Fa + Ff) = 2 x (1.8 + 4) = 11.6 N

Pulley torque:

T = F r = 11.6 x 0.012 = 0.139 N m

Angular speed:

omega = v / r = 0.4 / 0.012 = 33.3 rad/s

RPM:

RPM = omega x 60 / (2 pi) = 33.3 x 60 / (2 pi) = 318 RPM

Minimum pulley power:

P = T omega = 0.139 x 33.3 = 4.6 W

This 4.6 W is mechanical output at the pulley. The motor electrical input must be higher because the motor, driver, belt, bearings, and gearbox are not perfectly efficient.

Implementation Sheet

Create a spreadsheet with these fields and replace candidate values with real datasheet numbers:

• Load mass: 1.2 kg; document the measured or specified value.
• Friction force: 4 N; document whether it is measured or assumed.
• Design force: 11.6 N; candidate force must be at least this value.
• Pulley torque: 0.139 N m; candidate torque must be available at 318 rpm.
• Pulley speed: 318 rpm; candidate speed must meet or exceed it.
• Mechanical power: 4.6 W; candidate continuous output must include margin.
• Driver current: use the motor datasheet; driver must survive acceleration and the chosen stall strategy.
• Supply voltage: commonly 12 V or 24 V; it must be compatible with motor and driver.
• Feedback: home switch minimum; use an encoder if lost position is unsafe.
• Protection: fuse, limits, and emergency stop must be tested before full-speed operation.

Selection Reasoning

A current-limited stepper system is a reasonable first choice if:

• the torque-speed curve still exceeds 0.139 N m at 318 rpm;
• missed steps are not dangerous or are detected by homing and process checks;
• the controller can generate the required step rate with acceleration ramps.

A servo is a better choice if:

• position loss is unacceptable;
• load changes are unpredictable;
• the axis must report following error or torque limit;
• higher speed and acceleration are required.

A brushed DC gearmotor can work if:

• encoder feedback is added for position;
• the gearbox output speed and torque meet the requirement;
• the H-bridge can handle startup, reversal, and jam current.

Expected Behavior

A correct selection should accelerate without stalling, reach 0.4 m/s, stop repeatably at the home switch, stay within motor and driver temperature limits after repeated moves, and enter a safe state when a limit switch, overcurrent, or emergency stop is triggered.

Verification Steps

1. Inspect the axis with power off: belt tension, pulley alignment, free travel, end stops, and guards.
2. Set motor-driver current limit below both motor and driver ratings.
3. Command motion at 25% speed and acceleration.
4. Confirm direction, home switch polarity, and limit-switch stop behavior.
5. Increase acceleration in small steps while watching missed steps, following error, or current limit.
6. Measure supply voltage during acceleration and reversal.
7. Measure motor and driver temperature after ten full-travel cycles.
8. Trigger emergency stop and confirm the axis removes drive power or torque as designed.
9. Record final speed, acceleration, current, temperature, and observed margin.

Common Failure Symptoms

• Hums but does not move: likely wrong winding pair, low current, or jammed axis; check wiring, current limit, and free travel.
• Moves then loses position: likely too much acceleration, resonance, or low torque margin; reduce acceleration and inspect the torque curve.
• Driver overheats: likely current too high, poor cooling, or undersized driver; check heat sinking, airflow, and current setting.
• Belt skips teeth: likely belt tension, pulley alignment, or acceleration too high; inspect mechanics and lower acceleration.
• Supply resets controller: likely supply droop or shared ground path; separate power paths and add bulk capacitance.
• Servo following error: likely load too high, bad tuning, or encoder direction wrong; lower the command and inspect feedback.

Debugging Guidance

Reduce acceleration first, then speed. If the axis improves, the system is torque-limited or resonance-limited. If it still fails at low speed, inspect wiring, driver enable, winding pairing, mechanical binding, limit-switch polarity, and supply voltage at the driver terminals. Do not keep repeating full-power failed starts; they heat the motor and driver quickly.

Extension Challenge

Repeat the selection for a vertical lift using the same mass, speed, and pulley radius. Gravity adds:

Fg = m g = 1.2 x 9.81 = 11.8 N

Decide whether a brake, counterweight, self-locking leadscrew, or powered holding strategy is required when power is removed. Explain why relying only on motor holding torque may be unsafe.

Concise Explained Solution

The horizontal axis needs 11.6 N design force, 0.139 N m pulley torque, about 318 rpm, and at least 4.6 W mechanical output at the pulley. A current-limited stepper can work if its torque-speed curve at 318 rpm still exceeds the required torque with margin and the system can tolerate or detect missed steps. A servo is preferred when losing position is unacceptable. A brushed DC gearmotor needs encoder feedback for position and an H-bridge sized for startup or jam current. Use a compatible 12 V or 24 V supply, set current limits, add fuse, limit switches, emergency stop, and thermal checks, then ramp speed and acceleration only after low-speed verification passes.

Further Reading

• Oriental Motor, motor sizing and torque-speed curve tutorials.
• Pololu, stepper and brushed DC motor driver current-limit guides.
• Kollmorgen, servo sizing fundamentals.
• Texas Instruments, motor-driver protection and current-sense application notes.

Mind Map