SACA Mechatronics 2 — Questions and Answers

Question 1: Which type of motor is most commonly used in precision positioning applications within mechatronic systems due to its ability to move in discrete steps?

DC shunt motor
Stepper motor (Correct answer)
Universal motor
Wound rotor induction motor

Correct answer: Stepper motor

Stepper motors divide a full rotation into equal discrete steps, enabling precise open-loop position control without encoders — ideal for applications like 3D printers, CNC routers, and pick-and-place machines.

Stepper motors operate on the principle of electromagnetically rotating a toothed rotor through discrete angular steps (typically 1.8° for a 200-step/rev motor). By controlling the sequence of phase energization, a microcontroller can command exact positions. While they lack the torque density and high-speed performance of servo motors, their simplicity (no encoder needed for most applications) and low cost make them ubiquitous in light-duty automation. Microstepping drivers improve resolution to 1/256 step.

Question 2: In mechatronics, what is the purpose of a planetary gearbox attached to a servo motor?

To convert rotary motion to linear motion
To increase torque and reduce speed while maintaining compact dimensions (Correct answer)
To provide electrical power regeneration during deceleration
To add magnetic braking capability

Correct answer: To increase torque and reduce speed while maintaining compact dimensions

A planetary gearbox multiplies torque by the gear ratio while proportionally reducing speed, in a compact coaxial arrangement — matching high-speed, low-torque motor output to low-speed, high-torque application requirements.

Planetary gearboxes have a central sun gear, planet gears orbiting it, and an outer ring gear. This arrangement distributes load across multiple gear meshes simultaneously, achieving high torque density in a compact package. Gear ratios of 3:1 to 100:1 are common. In servo systems, the gearbox inertia ratio (load inertia reflected to motor shaft vs. motor inertia) should be kept below 10:1 for optimal control performance. High-precision planetary gearboxes offer arcminute-level backlash.

Question 3: What does a Hall-effect sensor measure in a brushless DC motor controller?

Winding temperature
Rotor magnetic pole position for commutation timing (Correct answer)
Bearing vibration amplitude
Bus voltage level

Correct answer: Rotor magnetic pole position for commutation timing

Hall-effect sensors detect the rotor's magnetic field orientation, providing the controller with rotor position information needed to correctly time the switching of stator phase currents (electronic commutation).

Brushless DC (BLDC) motors require electronic commutation to replace the mechanical brushes and commutator of brush-type motors. Three Hall sensors, placed 120° apart, detect the rotor's permanent magnets as it rotates, generating a 3-bit binary pattern that advances through 6 states per electrical revolution. The motor controller uses this pattern to energize the correct stator windings in sequence, producing continuous rotation. Without Hall sensors, the controller must use sensorless back-EMF detection.

Question 4: In a mechatronic system, what is the role of a digital-to-analog converter (DAC)?

To measure analog sensor signals and convert them for PLC input modules
To convert digital control commands from a microcontroller into analog voltage or current signals (Correct answer)
To digitize audio signals for human-machine interface sounds
To convert encoder pulse trains into speed measurements

Correct answer: To convert digital control commands from a microcontroller into analog voltage or current signals

A DAC converts the digital output of a microcontroller or DSP (a numeric value) into a proportional analog voltage or current signal, which is used to drive analog-input actuators such as proportional valves or servo amplifiers.

Many actuators (proportional hydraulic valves, analog servo amplifiers, variable frequency drives with analog inputs) require a 0-10V or 4-20mA command signal. Since microcontrollers and PLCs compute digitally, a DAC bridges this gap. Resolution (8-bit, 12-bit, 16-bit) determines the number of discrete output levels. A 12-bit DAC over a 0-10V range gives 4096 steps of ~2.4mV resolution — sufficient for most motion control applications.

Question 5: What is 'backlash' in a mechanical transmission system and why is it problematic in precision mechatronics?

Excessive spring force causing rapid return motion
The dead zone between gear teeth where no motion is transmitted, causing positioning error (Correct answer)
Vibration caused by motor cogging at low speeds
Thermal expansion of ball screws at high duty cycles

Correct answer: The dead zone between gear teeth where no motion is transmitted, causing positioning error

Backlash is the clearance (play) between mating gear teeth or lead screw/nut threads. When direction reverses, the motor moves through the backlash gap without moving the load, creating positioning errors and reduced control loop stability.

Backlash causes a hysteresis effect in positioning: when reversing direction, the output shaft doesn't move until all the gap between gear teeth is taken up. In a 1mm leadscrew pitch system with 0.05mm backlash, reversal moves cause 0.05mm errors. Solutions include anti-backlash gears (spring-loaded split gears), preloaded ball screws, harmonic drives (near-zero backlash), or backlash compensation in the motion controller (measuring and adding an offset on direction change).

Question 6: Which sensor technology uses the interference of light waves to measure displacement with nanometer-level resolution in precision mechatronic systems?

Capacitive displacement sensor
Laser interferometer (Correct answer)
LVDT (Linear Variable Differential Transformer)
Magnetostrictive position sensor

Correct answer: Laser interferometer

Laser interferometers exploit the wave nature of coherent laser light — measuring displacement as a count of interference fringes (each fringe = half a wavelength, ~316nm for a HeNe laser) — achieving sub-nanometer resolution.

Laser interferometers (Michelson or Fabry-Pérot types) split a laser beam, send one path to a fixed mirror and another to a moving mirror on the stage, then recombine them. As the moving mirror displaces, the optical path length changes, creating constructive and destructive interference (fringes). Each fringe count corresponds to λ/2 displacement. At 632.8nm (HeNe laser), resolution can reach 0.31nm with interpolation. They are used in semiconductor lithography, metrology, and ultra-precision machine tools.

SACA Mechatronics 2

Which type of motor is most commonly used in precision positioning applications within mechatronic systems due to its ability to move in discrete steps?