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Robotics Sensors and Actuators

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Module 5: Introduction to motors

  1. Q1i. A variable reluctance stepper motor is constructed of material with salient poles: (i) Paramagnetic (ii) Diamagnetic (iii) Ferromagnetic (iv) Non-magnetic20242m

    Module 5: Introduction to motors

    A variable reluctance stepper motor is constructed of material with salient poles:
    (i) Paramagnetic
    (ii) Diamagnetic
    (iii) Ferromagnetic
    (iv) Non-magnetic

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    Worked Solution

    Answer

    Correct Option: (iii) Ferromagnetic

    Explanation

    Variable Reluctance (VR) stepper: Rotor and stator salient poles made of soft ferromagnetic material (e.g., silicon steel, soft iron) with high permeability.

    • Ferromagnetic: Strongly attracted to magnetic field, low reluctance → torque via reluctance variation.
    • Paramagnetic: Weakly attracted, insufficient.
    • Diamagnetic: Repelled.
    • Non-magnetic: No interaction.

    Rotor has no windings or magnets; torque produced by tendency to minimize reluctance (align poles).

    Final Answer

    Ferromagnetic → Option (iii).

  2. Q1j. A stepping motor is a _____ device. (i) Mechanical (ii) Electrical (iii) Analogue (iv) Incremental20242m

    Module 5: Introduction to motors

    A stepping motor is a _____ device.
    (i) Mechanical
    (ii) Electrical
    (iii) Analogue
    (iv) Incremental

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    Answer

    Correct Option: (iv) Incremental

    Explanation

    Stepper motor moves in discrete angular steps (e.g., 1.8° per pulse) in response to digital pulses. It is incremental / digital device.

    • Each pulse → one step, position = count × step angle.
    • Open-loop position control without feedback.
    • Also electromechanical (electrical + mechanical), but “incremental” distinguishes it from continuous analog motors.

    Final Answer

    Incremental (digital) → Option (iv).

  3. Q6b. Define Servo motor and explain in brief any type of servo motor, also write its application, advantage and disadvantage.20247m

    Module 5: Introduction to motors

    Define Servo motor and explain in brief any type of servo motor, also write its application, advantage and disadvantage.

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    Definition

    Servo motor: Closed-loop actuator with motor + feedback sensor (encoder/pot) + driver that controls position/velocity precisely via error correction e=θref−θactuale=\theta_{ref}-\theta_{actual}.

    Types (explain one: DC Servo)

    DC Servo: DC motor + encoder + PID controller. Input PWM/analog, encoder feedback, driver H-bridge controls direction/speed. Types: DC, AC, brushless.

    Working: Controller compares desired vs actual via encoder → PID → H-bridge → motor rotates → encoder updates until error zero.

    Applications

    • Robot joints, CNC, robotic arm, gimbal.
    • Conveyor, 3D printer.

    Advantages

    • High precision, fast response, high torque with gear.
    • Closed-loop, no step loss (vs stepper).

    Disadvantages

    • Costly, needs tuning (PID), complexity, requires feedback, hunting.

    Final Answer

    Servo = motor+feedback closed-loop; DC servo via encoder PID; advantages precision, disadvantages cost and tuning, used in robot joints/CNC.

  4. Q7a. Write and construct in detail of Variable Reluctance (VR) Stepper Motor, application, advantage and disadvantage.20247m

    Module 5: Introduction to motors

    Write and construct in detail of Variable Reluctance (VR) Stepper Motor, application, advantage and disadvantage.

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    Variable Reluctance (VR) Stepper Motor

    Construction: Stator 6 salient poles with windings (3 phases), rotor 4 soft-iron salient poles, no magnets/windings, air gap small. Ferromagnetic.

    MermaidDiagram
    Rendering diagram…

    Working: Energize phase A → stator A poles become electromagnets → rotor minimizes reluctance aligns with A. Sequentially energize B, C → rotor steps 30° (360/(6*4/2)). Step angle =360/(Ns⋅Nr/(Ns−Nr))=360/(N_s \cdot N_r/(N_s-N_r)).

    Advantages: Simple, cheap, no magnets, high speed, no detent.

    Disadvantages: Low torque vs PM/hybrid, vibrations, low damping.

    Applications: Low-cost positioning, printer carriage, clock.

    Final: VR stepper soft-iron rotor steps by reluctance minimization, simple low torque.

  5. Q7b. Write and construct in detail Series motors: Component, application, advantage and disadvantage.20247m

    Module 5: Introduction to motors

    Write and construct in detail Series motors: Component, application, advantage and disadvantage.

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    Series Motor (DC Series)

    Components: Field winding series with armature, commutator, brushes, stator/rotor iron.

    Construction: Field coil in series → same current IaI_a flows through field and armature. Flux ϕ∝Ia\phi \propto I_a, torque T∝ϕIa∝Ia2T \propto \phi I_a \propto I_a^2.

    Working: Supply → series field → armature → torque high at low speed, speed N∝V/ϕN \propto V/\phi.

    Advantages: Very high starting torque, suitable for traction.

    Disadvantages: Speed dangerously high at no load, poor speed regulation, sparks.

    Applications: Electric train, crane, hoist, starter motor.

    Final: Series motor field series with armature T∝Ia2T\propto I_a^2, high starting torque for traction.

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