Transducers and Sensors

Transducers

• Conversion of a primary energy form into a corresponding signal of a different energy form
• Primary energy forms include mechanical, thermal, electromagnetic, optical, and chemical
• Can take the form of a sensor or an actuator

Sensor

• Detects or measures a signal or stimulus
• Acquires information from the real world
• Example: Thermometer

Transducer Types

• Primary transducers sense a physical quantity and convert it to voltage or current
• Secondary transducers convert the initial current or voltage into another form of signal

Primary Transducers

• Light Sensors
  • Photoconductors change resistance as light levels change
  • Light increases, resistance increases in photoconductors
  • Photodiodes generate current in response to light levels
  • Light increases, current increases in photodiodes
• Membrane Pressure Sensors
  • Measures force per area
  • Resistive pressure sensors (using strain gauge, piezo resistor) exhibit resistance changes due to pressure
  • Capacitive pressure sensors exhibit capacitance changes due to pressure, where the distance changes with pressure

Temperature Sensor Options

• Resistance Temperature Detectors (RTDs)
  • Use platinum, nickel, and copper due to the effect of temperature on their resistance
  • Resistance varies with temperature change.
  • Positive temperature coefficients mean resistance increases as temperature increases
• Thermistors
  • Made from semiconductor materials, often a composite of ceramic and a metallic oxide
  • Negative temperature coefficients mean resistance decreases as temperature increases
• Thermocouples
  • Composed of two dissimilar metallic wires joined to form a junction
  • A small voltage is generated when the junction is heated or cooled
  • The generated voltage is directly dependent on temperature

Active and Passive Sensors

• Active Sensors
  • Require an external power supply to operate
  • The power supply is called an excitation signal, and is used to produce the output signal
  • Self-generating devices change their properties in response to an external effect
  • Active sensors also produce signal amplification
  • Examples: LVDT sensor, strain gauge
• Passive Sensors
  • Do not need any additional power source or excitation voltage
  • Generate an output signal in response to an external stimulus
  • Thermocouples generate their own voltage output when exposed to heat
  • Direct sensors change their physical properties like resistance, capacitance, or inductance

Analogue and Digital Sensors

• Analogue Sensors
  • Produce a continuous output signal or voltage
  • The signal is generally proportional to the measured quantity
  • Examples: Temperature, speed, pressure, displacement, and strain
• Digital Sensors
  • Produce discrete digital output signals or voltages
  • Output is a digital representation of the measured quantity
  • Binary output signal (logic 1 or logic 0), either ON or OFF
  • Discrete (non-continuous)
  • Single "bit" is serial transmission
  • Single "byte" is parallel transmission

Further Transducer Details

• Conventional Transducers
  • Large in size, generally reliable
  • Based on older technology
    • Thermocouple: Temperature difference.
    • Compass (magnetic): Direction
• Microelectronic Sensors
  • Millimeter-sized, highly sensitive, less robust
    • Photodiode/phototransistor: Photon energy (light)
    • Infrared detectors: Detect proximity or intrusion
    • Piezoresisitve pressure sensor: Senses air or fluid pressure
    • Microaccelerometers: Senses direction or velocity, example of car crash
    • Chemical sensors: O2, CO2, Cl, Nitrates (explosives)
    • DNA arrays: Match DNA sequences

Sensor Calibration

• Non-ideal sensor effects
  • Offset: Nominal output doesn't equal nominal parameter value
  • Nonlinearity: Output isn't linear to input
  • Cross parameter sensitivity: Output varies secondarily with another input
• Calibration Adjusts the output to coincide with the parameter.
  • Analog signal conditioning
  • Look-up tables
  • Digital calibration involves equations:
    • T = a + bV+cV^2
      • T= temperature; V=sensor voltage;
      • a,b,c= calibration coefficients
• Compensation:
  • Remove secondary sensitivities by knowing their characteristics
  • Can be removed with polynomial evaluation, such as:
    • P = a + bV+ cT+ dVT+ e V^2, where P=pressure, T=temperature

Signal Processing

• The output signal from a sensor must be processed to be suitable for the next operational stage
• This may require addressing various issues:
  • Too small and needs amplification
  • Containing interference that needs removal
  • Being non-linear and needing linearization
  • Being analogue and needing conversion to digital
  • Being digital and needing conversion to analogue

Signal Conditioning Details

• Protection
  • Prevents damage to the next element (e.g., microprocessor) from high current or voltage
  • Protection involves:
    • Series current limiting resistors
    • Fuses
    • Polarity protection
    • Voltage limitation circuits
• Right type of signal
  • Converting the signal to DC voltage or current
  • Example: Converting resistance change of a strain gauge into a voltage change
    • Utilizes a Wheatstone bridge and the out-of-balance voltage
  • Converting the signal to digital or analogue (A/D and D/A)
• Level Shifting
  • Thermocouple signals might be only a few millivolts
  • Signals need to be much larger (volts) when fed into an analogue-to-digital converter for a microprocessor
  • Operational amplifiers amplify the signal

Active vs. Passive Components

• Passive components
  • Resistors
  • Capacitors
  • Inductors
• Active components
  • Bipolar or metal oxide semiconductor (MOS) transistors
• Analog Circuits
  • Electronic circuits are studied where the inputs and outputs are continually changing

Discrete Amplifiers

• Active components: transistors
• Discrete amplifiers change in temperature and can be unstable

Operational Amplifiers

• Integration of more than one amplifier on the same chip
• Properties:
  • Open-loop gain: Ideally infinite, practical values are 20k-200k+, high open loop gain creates a virtual short between + and - inputs
  • Input impedance: Ideally infinite, CMOS opamps are close to ideal
  • Output impedance: Ideally 0, practical values of 20-100 Ω
  • Zero output offset: Ideally 0, practical value <1mV
• Commercial opamps provide different properties:
  • low noise
  • low input current
  • low power
  • high bandwidth
  • low/high supply voltage
  • special purpose like comparator, instrumentation amplifier