Exercise: Select a Motor and Driver

This exercise turns motor selection into a concrete engineering decision. You will size the actuator, choose a driver, check supply current, and define verification tests before hardware is purchased.

Learning Objectives

By the end of this exercise, you should be able to calculate required torque and speed, compare DC, stepper, and servo options, select a compatible driver, plan protection, and write a verification checklist.

Prerequisites

You should already understand force, torque, speed, power, DC motors, stepper motors, and servo motor basics. You also need a calculator or spreadsheet.

Task

Design the drive for a small belt axis that moves a 1.2 kg carriage horizontally. The pulley radius is 12 mm. Maximum desired 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. selected motor type;
5. selected driver class;
6. supply voltage and current estimate;
7. protection and verification plan.

Calculations

Acceleration force:

$$
F_a=ma=1.2\times1.5=1.8\ N
$$

Total force with friction and safety factor:

$$
F_\text{design}=2(F_a+F_f)=2(1.8+4)=11.6\ N
$$

Pulley torque:

$$
T=Fr=11.6\times0.012=0.139\ N\cdot m
$$

Angular speed:

$$
\omega=\frac{v}{r}=\frac{0.4}{0.012}=33.3\ rad/s
$$

RPM:

$$
RPM=\omega\frac{60}{2\pi}\approx318\ RPM
$$

Mechanical power:

$$
P=T\omega=0.139\times33.3\approx4.6\ W
$$

Buildable Selection Sheet

Use this CSV-style table in a spreadsheet and replace the candidate values with real datasheet numbers.

item,value,pass rule
required force,11.6 N,candidate force >= required
required torque,0.139 N m,candidate torque >= required at 318 RPM
required speed,318 RPM,candidate speed >= required
supply,24 V,driver and motor rated for supply
current limit,per motor datasheet,driver can regulate current
feedback,home switch minimum,encoder preferred if missed motion is unsafe
protection,fuse plus E-stop plus limit switch,required before full-speed test

Expected Behavior

A suitable system should accelerate without stalling, reach 0.4 m/s, stop repeatably at the home switch, and remain within motor and driver temperature limits after repeated moves.

Verification Steps

1. Confirm free mechanical travel with power off.
2. Set driver current limit below motor and driver ratings.
3. Run at 25% speed and acceleration.
4. Increase acceleration while watching missed steps, following error, or current limit.
5. Measure motor and driver temperature after ten full travel cycles.
6. Trigger end-stop and emergency-stop inputs.
7. Record final speed, acceleration, current, and temperature.

Common Failure Symptoms

• Axis hums but does not move: wiring, current limit, or jammed mechanics.
• Moves then loses position: insufficient torque, too much acceleration, or resonance.
• Driver overheats: current too high, poor cooling, or undersized driver.
• Belt skips: pulley tension, acceleration, or torque margin problem.

Debugging Guidance

Reduce acceleration first, then speed. If the axis improves, the motor is torque-limited. If it still fails at low speed, inspect wiring, driver enable, coil pairing, mechanical binding, and supply voltage during motion.

Extension Challenge

Repeat the selection for a vertical lift where gravity adds mg to the required force. Decide whether a brake, counterweight, or leadscrew is required for safety when power is removed.

Concise Explained Solution

The design force is 11.6 N, required pulley torque is about 0.139 N m, required speed is about 318 RPM, and power at the pulley is about 4.6 W. A small stepper with a current-limited driver can work if its torque-speed curve still exceeds 0.139 N m at 318 RPM; a servo is better if position loss is unacceptable. Use a 24 V supply when compatible because it improves current rise at speed, and verify by ramping acceleration while monitoring temperature and position.

Further Reading

• Oriental Motor, torque-speed curve interpretation.
• Pololu, stepper driver current limit guides.
• Kollmorgen, servo sizing fundamentals.

Mind Map