8.2 Sensors and Actuators

8.2 Sensors and Actuators

1. Types and Selection

1. Sensor Definition:

   • Device that detects physical quantity and converts it to measurable signal.

   • Acts as "measuring" component in control loop.

2. Actuator Definition:

   • Device that converts control signal into physical action.

   • Acts as "doing" component in control loop.

3. Sensor Classification:

   • Based on Quantity Measured:

     • Pressure, temperature, flow, level, position.

     • Force, torque, acceleration, vibration.

   • Based on Operating Principle:

     • Electrical: Resistive, capacitive, inductive.

     • Piezoelectric, thermoelectric, electrochemical.

     • Optical, magnetic, ultrasonic.

   • Based on Output:

     • Analog (continuous signal).

     • Digital (discrete signal).

     • Switching (on/off).

4. Actuator Classification:

   • By Energy Source:

     • Electrical (motors, solenoids).

     • Hydraulic (cylinders, motors).

     • Pneumatic (cylinders, valves).

   • By Motion:

     • Linear (push/pull action).

     • Rotary (rotational action).

5. Selection Criteria:

   • Technical Factors:

     • Range and accuracy requirements.

     • Response time and bandwidth.

     • Environmental conditions (temp, pressure, hazards).

     • Physical size and mounting constraints.

   • Operational Factors:

     • Reliability and maintenance needs.

     • Compatibility with existing systems.

     • Cost and availability.

   • System Integration:

     • Output signal type (analog/digital).

     • Power requirements.

     • Communication protocol.

2. Measurement of Pressure, Flow, Temperature

1. Pressure Measurement:

   • Primary Sensors:

     • Bourdon Tube: Mechanical, for high pressures.

     • Diaphragm/Capsule: Flexible element for moderate pressures.

     • Bellows: For low pressures and differential measurements.

   • Electrical Pressure Transducers:

     • Strain Gauge: Bonded to diaphragm, measures deformation.

     • Piezoelectric: For dynamic pressure measurements.

     • Capacitive: Diaphragm as capacitor plate.

   • Special Types:

     • Manometers: Liquid column, simple but limited.

     • Dead Weight Testers: Primary calibration standard.

2. Flow Measurement:

   • Positive Displacement:

     • Measures actual volume passed (e.g., piston, gear meters).

     • High accuracy, good for viscous fluids.

   • Differential Pressure:

     • Orifice Plate, Venturi, Flow Nozzle.

     • $Q = K \sqrt{\Delta p}$ relationship.

   • Velocity-Based:

     • Turbine Meters: Rotor speed proportional to flow.

     • Electromagnetic: Faraday's law, for conductive fluids.

     • Ultrasonic: Time-of-flight or Doppler shift.

   • Mass Flow Meters:

     • Coriolis: Direct mass measurement.

     • Thermal: For gases.

3. Temperature Measurement:

   • Contact Methods:

     • Thermocouples:

       • Two dissimilar metals, Seebeck effect.

       • $V = a(T_1 - T_2) + b(T_1^2 - T_2^2)$

       • Types J, K, T, E (base metal), R, S, B (noble metal).

     • Resistance Temperature Detectors (RTDs):

       • Platinum (Pt100, Pt1000) most common.

       • $R_T = R_0[1 + \alpha(T - T_0)]$

       • High accuracy and stability.

     • Thermistors:

       • Semiconductor, high sensitivity.

       • Negative (NTC) or Positive (PTC) temperature coefficient.

   • Non-Contact Methods:

     • Infrared Pyrometers: Measure emitted radiation.

     • Thermal Imaging Cameras: Spatial temperature distribution.

3. Hydraulic, Pneumatic, Electric Actuators

1. Hydraulic Actuators:

   • Characteristics:

     • High power density (high force from small size).

     • Precise control of position and speed.

     • Stiff system (low compliance).

   • Components:

     • Hydraulic Cylinders (linear).

     • Hydraulic Motors (rotary).

     • Control Valves (directional, flow, pressure).

   • Advantages:

     • Very high force/torque capability.

     • Smooth motion control.

     • Self-lubricating.

   • Disadvantages:

     • Requires pump, reservoir, piping.

     • Potential for leaks (fluid cleanliness critical).

     • Higher maintenance.

   • Applications: Heavy machinery, presses, aircraft controls.

2. Pneumatic Actuators:

   • Characteristics:

     • Moderate force, fast response.

     • Compressible fluid (air) - less precise than hydraulic.

     • Simple, clean, explosion-proof.

   • Components:

     • Air Cylinders (single/double acting).

     • Pneumatic Motors (vane, piston types).

     • Control Valves and Regulators.

   • Advantages:

     • Simple and low-cost installation.

     • Safe (non-flammable, exhausts to atmosphere).

     • Low maintenance.

   • Disadvantages:

     • Lower force than hydraulic systems.

     • Less precise position control.

     • Requires compressed air supply.

   • Applications: Packaging, clamping, material handling.

3. Electric Actuators:

   • Characteristics:

     • Clean, quiet operation.

     • Excellent positioning accuracy.

     • Easy integration with control systems.

   • Types:

     • DC Motors:

       • Simple speed control.

       • Brushed (maintenance) vs Brushless (long life).

     • AC Motors:

       • Induction (robust, simple).

       • Synchronous (precise speed control).

     • Stepper Motors:

       • Discrete steps, open-loop position control.

       • $Step Angle = \frac{360^\circ}{N}$ where N = steps/revolution.

     • Servo Motors:

       • Closed-loop position/speed control.

       • High dynamic response.

   • Advantages:

     • Precise control and positioning.

     • Easy to install and maintain.

     • High efficiency.

   • Disadvantages:

     • Lower power density than hydraulic.

     • Can generate heat.

     • May require gear reduction for high torque.

   • Applications: Robotics, CNC machines, valve automation.

4. Selection Guidelines:

   • Force/Speed: Hydraulic for highest force, Pneumatic for speed, Electric for precision.

   • Environment: Pneumatic for hazardous areas, Electric for clean rooms.

   • Control Complexity: Electric for complex control, Pneumatic for simple on/off.

   • Cost: Pneumatic often lowest initial cost, Electric lowest operating cost.