Unit 3: Electrical and Electronics Elements - Subjective Questions

Construction: A DC motor consists of a stator, rotor or armature, permanent magnets or field windings, commutator, and brushes.

Working principle: When current flows through a conductor placed in a magnetic field, it experiences a force. The interaction of the magnetic field and armature current produces torque, causing the rotor to rotate.

The approximate torque relationship is:

where is motor torque, is the torque constant, and is armature current.

Characteristics:

• Provides relatively high starting torque.
• Speed can be controlled by varying the applied voltage.
• Direction can be reversed by reversing the polarity.
• Brushes and the commutator cause mechanical wear.

Applications: DC motors are used in mobile robots, conveyor mechanisms, robotic arms, and wheel-drive systems.

A servo motor is a motor used in a closed-loop control system to achieve accurate control of position, speed, or torque.

Main components:

• Motor, usually a DC or brushless motor.
• Gearbox for increasing torque and reducing speed.
• Position sensor such as a potentiometer or encoder.
• Control circuit or servo amplifier.

Working: The desired position is supplied as a reference input. The position sensor measures the actual shaft position. The controller calculates the error:

where is the desired position and is the actual position. The controller drives the motor until the error becomes sufficiently small.

Servo motors are widely used in robotic joints, grippers, camera platforms, and steering mechanisms because they provide precise and repeatable motion.

A stepper motor is an electromechanical actuator that converts electrical pulses into discrete angular movements.

Construction: It generally contains a toothed permanent-magnet or variable-reluctance rotor and a stator with multiple energized windings.

Working: The controller energizes the stator windings in a particular sequence. The rotor aligns itself with the energized magnetic field. Each input pulse causes the rotor to move through a fixed step angle:

where is the number of steps per revolution.

Advantages:

• Simple open-loop position control.
• High holding torque at low speed.
• Good repeatability.
• Easy digital interfacing.

Limitations:

• May lose steps under excessive load.
• Torque decreases at high speed.
• Can produce vibration and resonance.
• Requires continuous power to maintain holding torque.

Stepper motors are used in 3D printers, CNC machines, positioning tables, and small robotic joints.

A DC motor is suitable for simple continuous rotation. A servo motor is preferred where accurate position or speed control is required. A stepper motor is appropriate for low-cost incremental positioning when the load is predictable.

A brushless DC motor uses permanent magnets on the rotor and electronically controlled windings on the stator.

Construction: The main parts are a permanent-magnet rotor, multiphase stator windings, position sensors or sensorless detection circuitry, and an electronic inverter.

Electronic commutation: Instead of brushes and a mechanical commutator, semiconductor switches energize the stator phases in sequence. Hall sensors may detect rotor position and provide signals to the controller for correct switching.

Advantages:

• High efficiency.
• Low maintenance because there are no brushes.
• High power-to-weight ratio.
• Low electrical noise and good speed capability.
• Longer operating life.

Applications: BLDC motors are used in aerial robots, cooling fans, electric vehicles, robotic joints, hard-disk drives, and high-speed spindle systems.

The need for an electronic controller makes a BLDC system more complex than a basic brushed DC motor.

A motor driver is an electronic circuit that allows a low-power controller to operate a motor requiring higher voltage and current. It also provides switching, direction control, speed control, and protection.

An H-bridge consists of four electronic switches arranged around a motor. The switching states determine motor direction:

• Turning on the upper-left and lower-right switches makes current flow in one direction.
• Turning on the upper-right and lower-left switches reverses the current.
• Pulse-width modulation, or PWM, controls the average motor voltage and speed.
• Turning on both switches in the same leg must be avoided because it causes shoot-through current.

For an ideal PWM drive, the average voltage is approximately:

where is the duty cycle and is the supply voltage.

Motor drivers protect the microcontroller from motor current, voltage spikes, and electrical noise.

Different motors require different driver circuits because their winding arrangements and control requirements differ.

• DC motor driver: Uses an H-bridge or half-bridge. It supports direction control and PWM-based speed control.
• Stepper motor driver: Uses current-controlled chopper circuits to energize the phases. It may support full-step, half-step, or microstepping operation.
• BLDC motor driver: Uses a three-phase inverter with six semiconductor switches. Commutation may be based on Hall sensors or sensorless back electromotive force detection.

Selection criteria:

• Motor rated voltage and maximum current.
• Required continuous and peak torque.
• Switching frequency and PWM compatibility.
• Heat dissipation and efficiency.
• Direction, braking, and regenerative requirements.
• Protection against overcurrent, overvoltage, and overheating.
• Compatibility with the embedded controller's logic levels.

Correct driver selection prevents motor malfunction and protects both the actuator and controller.

An encoder is a position-sensing device that converts mechanical motion into an electrical signal.

Incremental encoder: It generates pulses as the shaft rotates. The number of pulses indicates relative displacement, while two phase-shifted channels, commonly called A and B, indicate direction. An index pulse may provide one reference position per revolution. The controller must establish an initial reference or perform homing after startup.

Absolute encoder: It provides a unique digital code for every shaft position. The position is available immediately after power-up, so homing is generally unnecessary. Absolute encoders may use parallel, serial, magnetic, optical, or capacitive sensing methods.

If an incremental encoder produces pulses per revolution and counts pulses, the angular displacement is approximately:

Encoders are used for feedback in servo systems, mobile robot wheels, and robotic manipulators.

Consider a wheel of diameter connected to an encoder that produces counts per revolution. The wheel circumference is:

One complete encoder revolution therefore corresponds to a linear travel of . The linear displacement represented by one encoder count is:

If the encoder records counts, the estimated linear displacement is:

For a quadrature encoder with four decoded transitions per cycle, the effective count number becomes , provided that all transitions are reliably detected. In that case:

In practice, wheel slip, unequal wheel diameters, encoder quantization, and backlash introduce errors. Calibration and sensor fusion are therefore often required for accurate odometry.

A robotic power supply system must deliver stable electrical power to actuators, controllers, sensors, and communication devices.

Major components:

• Primary source such as a battery or regulated external supply.
• Fuses, circuit breakers, or electronic current protection.
• DC-DC converters for generating required voltage levels.
• Voltage regulators for sensitive electronics.
• Power distribution board or wiring harness.
• Filtering capacitors and electromagnetic interference suppression.
• Battery-management circuitry when rechargeable batteries are used.

Important requirements:

• Sufficient voltage range and current capacity.
• Low voltage ripple for sensors and controllers.
• Protection against short circuits, overcurrent, and reverse polarity.
• Isolation or grounding where necessary.
• Adequate thermal management.
• Separation of noisy motor-power paths from sensitive signal paths.

The supply should be rated for peak actuator demand, not only the average operating current.

Battery selection depends on voltage, capacity, peak current, mass, operating temperature, cost, and safety requirements. The stored energy can be estimated as:

where is energy in watt-hours when is in volts and is capacity in ampere-hours.

Sensors convert physical quantities into electrical signals that an embedded controller can process.

• Light sensors: Photodiodes, phototransistors, or LDRs respond to light intensity. They are used for line following, light tracking, and object detection.
• Temperature sensors: Thermistors, semiconductor sensors, and thermocouples produce signals related to temperature. They support thermal monitoring and protection.
• Sound sensors: Microphones convert acoustic pressure into electrical signals. Robots use them for voice commands or sound localization.
• Inertial sensors: Accelerometers measure linear acceleration, while gyroscopes measure angular velocity. A combination is called an IMU and is used for attitude estimation and motion detection.

Sensor selection depends on range, sensitivity, resolution, response time, noise, cost, and environmental conditions.

A proximity sensor detects the presence or distance of an object without requiring physical contact.

• Inductive sensor: Detects changes in an electromagnetic field caused by metallic objects. It is reliable in dusty or oily environments but usually detects only metals.
• Capacitive sensor: Detects changes in capacitance caused by metallic or non-metallic objects. It can detect liquids and powders but is affected by humidity and material properties.
• Ultrasonic sensor: Transmits sound waves and measures the echo time. If the measured time is and sound speed is , distance is:

The factor of accounts for the outgoing and returning path.

• Infrared sensor: Uses emitted and reflected infrared radiation. It is compact and inexpensive but can be affected by surface color, ambient light, and object reflectivity.

The appropriate sensor depends on target material, range, accuracy, response time, and environmental conditions.

Position sensors measure the location or angular displacement of a robot component.

• Potentiometer: Produces a voltage proportional to shaft angle. It is inexpensive but has limited life because of mechanical contact.
• Optical encoder: Uses a coded disk and light source-detector arrangement to measure rotation with high resolution.
• Magnetic encoder: Uses Hall-effect or magnetoresistive sensing to detect the position of a rotating magnet. It is durable and suitable for dusty environments.
• Linear variable differential transformer: Measures linear displacement using transformer coupling and has high precision.
• Limit switch: Provides a discrete signal when a mechanism reaches a defined endpoint.

Position sensors provide feedback for joint control, homing, collision prevention, and calibration. Their resolution, accuracy, repeatability, range, and environmental robustness must be considered during selection.

Force sensors measure an applied force, load, or torque. They enable a robot to interact safely and accurately with its environment.

A common force sensor uses a strain gauge bonded to an elastic member. When force is applied, the member deforms and the gauge resistance changes. The fractional resistance change is related to strain by:

where is the original resistance, is the resistance change, is the gauge factor, and is strain.

Strain gauges are often connected in a Wheatstone bridge to convert small resistance changes into a measurable voltage. Force sensors are used for:

• Grasp-force control.
• Assembly and insertion tasks.
• Contact detection.
• Collision monitoring.
• Balancing and load measurement.

Calibration is necessary to relate sensor output to actual force.

Signal conditioning modifies a raw sensor signal so that it is suitable for measurement and processing by an embedded controller.

Functions include:

• Amplification: Increases a small signal to match the input range of an ADC. Instrumentation amplifiers are commonly used for differential sensor outputs.
• Filtering: Removes unwanted noise. A low-pass filter can suppress high-frequency noise from a slowly changing sensor.
• Isolation: Prevents dangerous voltage or ground-loop currents from reaching sensitive electronics. Optical or transformer isolation may be used.
• Level shifting: Adjusts the signal to the required voltage range.
• Linearization: Corrects nonlinear sensor characteristics through analog circuits or software.
• Excitation: Supplies a stable voltage or current to sensors such as strain gauges.
• ADC conversion: Converts an analog voltage into a digital number for controller processing.

Good signal conditioning improves accuracy, noise immunity, and system reliability.

A first-order RC low-pass filter consists of a resistor in series with the input and a capacitor connected from the output node to ground. The output is taken across the capacitor.

The transfer function is:

The cutoff angular frequency is:

and the cutoff frequency in hertz is:

For frequencies much lower than , the signal passes with little attenuation. For frequencies much higher than , the signal is attenuated. The filter therefore reduces high-frequency noise and electrical interference in sensor measurements.

However, a very low cutoff frequency can delay the signal and reduce the system's response to rapid changes. The filter must be selected according to the sensor bandwidth and control-loop requirements.

An embedded controller is a dedicated computing unit designed to monitor inputs, execute control logic, and generate outputs for a specific robotic application.

Main components:

• Microprocessor or microcontroller: Executes the control program.
• Memory: Stores firmware, variables, and data.
• GPIO: Interfaces with switches, sensors, and digital actuators.
• ADC: Reads analog sensor signals.
• Timers and PWM modules: Generate accurate timing and motor-control signals.
• Communication interfaces: Include UART, SPI, I2C, CAN, USB, or Ethernet.
• Interrupt system: Responds quickly to time-critical events.
• Power and clock circuits: Provide stable operation.

The controller repeatedly samples sensors, processes the data, calculates a control action, and commands motor drivers. Real-time behavior is important because delayed or irregular control actions can cause instability or inaccurate motion.

The embedded controller uses a motor driver and a position or speed sensor such as an encoder.

Direction control: The controller sets the H-bridge input state to select the polarity applied to the motor.

Speed control: The controller generates a PWM signal. Changing the duty cycle changes the average motor voltage and therefore the approximate speed.

Feedback control: The encoder measures actual speed. The controller compares it with the desired speed:

A proportional-integral-derivative controller may calculate the command:

The resulting command is limited to a safe range and converted into PWM duty cycle and direction signals. Feedback compensates for load changes, friction, and battery-voltage variations.

Open-loop control: The controller sends a command without measuring the actual output. A stepper motor operated only by commanded pulses is an example.

Closed-loop control: The controller measures the actual output using a sensor and corrects the command according to the error. A DC motor with encoder feedback is an example.

Advantages of feedback:

• Reduces position and speed errors.
• Compensates for load disturbances.
• Detects missed motion or stalled motors.
• Improves repeatability.
• Allows accurate torque, speed, and position control.
• Supports safety functions such as overload and collision detection.

Closed-loop systems require additional sensors, signal conditioning, computation, and tuning. Open-loop systems are simpler and less expensive, but they are suitable only when the load and operating conditions are sufficiently predictable.