Sensors are the eyes and ears of embedded systems, allowing them to interact with the physical world. From temperature and pressure to motion and proximity, different sensor types measure various physical quantities, converting them into electrical signals for processing.

Understanding sensor characteristics is crucial for choosing the right sensor for your application. Sensitivity, accuracy, resolution, range, and response time all play a role in determining how well a sensor will perform in your embedded system design.

Sensor Types

Analog and Digital Sensors

• Analog sensors output a continuous voltage signal proportional to the measured physical quantity (temperature, pressure, light intensity)
• Digital sensors output a discrete digital signal, often in the form of a binary code or a pulse-width modulated (PWM) signal
• Analog sensors require an analog-to-digital converter (ADC) to interface with digital systems, while digital sensors can be directly connected to digital circuits
• Examples of analog sensors include potentiometers, thermistors, and strain gauges
• Digital sensors include encoders, Hall effect sensors, and digital temperature sensors (DS18B20)

Temperature, Pressure, and Proximity Sensors

• Temperature sensors measure the ambient temperature or the temperature of a specific object
  • Thermistors are resistive temperature sensors that change resistance with temperature (negative temperature coefficient (NTC) or positive temperature coefficient (PTC))
  • Thermocouples consist of two dissimilar metals joined together, generating a voltage proportional to the temperature difference between the junction and the reference point
  • Resistance temperature detectors (RTDs) are highly accurate temperature sensors that measure the change in resistance of a metal (usually platinum) with temperature
• Pressure sensors measure the force applied to a specific area, converting it into an electrical signal
  • Piezoresistive pressure sensors use a diaphragm that deforms under pressure, changing the resistance of a piezoresistive material bonded to the diaphragm
  • Capacitive pressure sensors measure the change in capacitance between a fixed plate and a movable diaphragm that deflects under pressure
• Proximity sensors detect the presence or absence of objects without physical contact
  • Inductive proximity sensors detect the presence of metallic objects by monitoring the change in inductance of a coil
  • Capacitive proximity sensors detect the presence of both metallic and non-metallic objects by measuring the change in capacitance between the sensor and the object
  • Optical proximity sensors use light (infrared or visible) to detect the presence of objects by monitoring the reflection or interruption of the light beam

Motion Sensors: Accelerometers and Gyroscopes

• Accelerometers measure the acceleration and tilt of an object along one or more axes
  • Capacitive accelerometers consist of a proof mass suspended between fixed plates, forming a capacitive divider that changes with acceleration
  • Piezoelectric accelerometers use a piezoelectric material that generates a charge proportional to the applied acceleration
  • Accelerometers are used in applications such as motion sensing, vibration monitoring, and tilt detection (smartphones, gaming controllers)
• Gyroscopes measure the angular velocity or rotation rate of an object around one or more axes
  • Mechanical gyroscopes use a spinning rotor to maintain a fixed orientation, measuring the angular velocity by the torque required to precess the rotor
  • MEMS (Microelectromechanical Systems) gyroscopes use vibrating structures to detect the Coriolis effect, which causes a secondary vibration perpendicular to the original vibration when the device is rotated
  • Gyroscopes are used in applications such as inertial navigation, attitude control, and image stabilization (drones, cameras)

Sensor Characteristics

Sensitivity, Accuracy, and Resolution

• Sensitivity is the ratio of the change in the sensor's output to the change in the measured physical quantity
  • A higher sensitivity means that the sensor can detect smaller changes in the measured quantity
  • Sensitivity is often expressed as the slope of the sensor's transfer function (output voltage vs. measured quantity)
• Accuracy is the degree to which the sensor's output matches the true value of the measured quantity
  • Accuracy is affected by factors such as linearity, hysteresis, and temperature drift
  • Calibration is the process of adjusting the sensor's output to minimize the error between the measured and true values
• Resolution is the smallest change in the measured quantity that the sensor can detect
  • Resolution is determined by the sensor's sensitivity and the resolution of the ADC used to digitize the sensor's output
  • A higher resolution allows the sensor to distinguish between smaller changes in the measured quantity

Range and Response Time

• Range is the minimum and maximum values of the measured quantity that the sensor can accurately detect
  • The sensor's output should be linear and accurate within the specified range
  • Operating the sensor outside its range may result in inaccurate readings or damage to the sensor
• Response time is the time required for the sensor's output to reach a specified percentage (usually 63.2% or 90%) of its final value after a step change in the measured quantity
  • A faster response time allows the sensor to track rapid changes in the measured quantity more accurately
  • Response time is affected by factors such as the sensor's internal capacitance, the impedance of the signal conditioning circuitry, and the sampling rate of the ADC
• The choice of sensor for a particular application depends on factors such as the required sensitivity, accuracy, resolution, range, and response time, as well as the environmental conditions (temperature, humidity, vibration) and the available space and power budget