13.1 Sensor types and characteristics
4 min read•august 7, 2024
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. , , , , and all play a role in determining how well a sensor will perform in your embedded system design.
Sensor Types
Analog and Digital Sensors
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- Analog sensors output a continuous 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 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
Key Terms to Review (30)
Accelerometer: An accelerometer is a device that measures the rate of change of velocity of an object, allowing it to detect acceleration forces acting in multiple directions. These sensors are essential for various applications, including motion detection in smartphones, navigation systems in vehicles, and stability control in drones. The data collected by accelerometers can be used to interpret physical movements and changes in orientation, making them vital in many embedded systems.
Accuracy: Accuracy refers to how close a measured value is to the actual value or true state of what is being measured. In technology and engineering contexts, especially involving sensors and data collection, accuracy is crucial as it affects the reliability of the measurements and the overall performance of the system. High accuracy means minimal errors in readings, which directly impacts decision-making processes and system effectiveness.
Analog sensor: An analog sensor is a device that converts a physical phenomenon, such as temperature, light, or pressure, into a continuous electrical signal that represents the measurement. These sensors provide real-time data and can output varying voltage levels based on the intensity of the physical quantity being measured, allowing for precise monitoring and control in embedded systems.
Capacitive Proximity Sensor: A capacitive proximity sensor is an electronic device that detects the presence or absence of an object without physical contact, utilizing changes in capacitance to sense nearby objects. These sensors work by measuring the capacitance change in an electric field when an object approaches, making them suitable for detecting conductive and non-conductive materials, including liquids, plastics, and wood.
Capacitive Sensor: A capacitive sensor is a type of electronic sensor that detects changes in capacitance caused by the presence of an object, typically conductive or dielectric, within its vicinity. This change is measured to determine the position or presence of the object, making capacitive sensors widely used in touchscreens, proximity sensors, and other applications requiring non-contact detection.
Data acquisition: Data acquisition is the process of collecting, measuring, and analyzing physical phenomena using sensors and data acquisition systems. This involves converting the signals generated by sensors into a digital form that can be processed, stored, and analyzed. Different sensor types have unique characteristics that influence their suitability for specific applications, making understanding these characteristics crucial for effective data acquisition.
Digital sensor: A digital sensor is a device that converts physical signals, such as light, temperature, or motion, into digital data that can be processed by a computer or microcontroller. These sensors provide high accuracy and precision, allowing for the collection of data in a form that can easily be analyzed and utilized in various applications, including automation and control systems.
Gain adjustment: Gain adjustment is the process of altering the amplification level of a signal in order to enhance its quality or accuracy. This technique is particularly important in sensor systems where the signal strength can significantly impact the measurement output, allowing for more precise readings and better overall performance of the system.
Gyroscope: A gyroscope is a device that uses the principles of angular momentum to measure or maintain orientation and angular velocity. It is widely used in navigation systems, smartphones, and various other technologies to help stabilize and control movement. Gyroscopes play a crucial role in determining the orientation of an object in three-dimensional space, which is essential for applications requiring precise directional information.
Inductive proximity sensor: An inductive proximity sensor is a non-contact device used to detect metallic objects within its sensing range, utilizing electromagnetic fields for operation. These sensors are widely used in industrial applications for automation and safety, as they provide reliable performance without needing physical contact with the target object. They are crucial in scenarios where contact-based sensors may wear out or fail due to environmental conditions.
Interface protocols: Interface protocols are standardized rules and conventions that enable communication between different components in a system, particularly between sensors and microcontrollers or other processing units. They ensure that data is transmitted accurately and reliably, allowing devices with diverse designs to work together seamlessly. Understanding these protocols is essential for ensuring compatibility and functionality in embedded systems, especially when integrating various sensor types with specific characteristics.
Mechanical gyroscope: A mechanical gyroscope is a device that utilizes the principles of angular momentum to measure or maintain orientation. It consists of a spinning rotor or wheel, which maintains its axis of rotation regardless of the movement of its base, providing valuable information about orientation and rotational motion in various applications, including navigation and stabilization systems.
Mems gyroscope: A MEMS gyroscope is a micro-electromechanical system device that measures angular velocity using tiny mechanical structures. This technology enables accurate orientation and motion detection in compact devices, making it crucial for applications like smartphones, drones, and automotive systems. With its small size and low power consumption, a MEMS gyroscope plays a significant role in enhancing user experience and functionality across various electronics.
Optical proximity sensor: An optical proximity sensor is a type of sensor that detects the presence of nearby objects without physical contact, using light or infrared signals to sense changes in the environment. These sensors can accurately measure distances and detect the presence or absence of an object based on reflected light, making them essential for various applications like automation, robotics, and security systems.
Pascal: Pascal is a unit of pressure in the International System of Units (SI), defined as one newton per square meter. It is commonly used in various fields including physics, engineering, and meteorology to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The pascal serves as a fundamental metric to gauge how force is applied over an area, making it crucial in the understanding of sensor types and their characteristics.
Piezoresistive sensor: A piezoresistive sensor is a type of sensor that measures mechanical stress or pressure by changing its electrical resistance in response to deformation. This property is primarily found in materials like semiconductors, where the resistance varies significantly under applied mechanical strain, making them highly sensitive and ideal for applications like pressure sensing in various devices.
Pressure Sensor: A pressure sensor is a device that measures the pressure of gases or liquids and converts that physical force into an electrical signal. These sensors are essential in various applications, including industrial automation, automotive systems, and environmental monitoring, providing crucial data for system control and safety.
Proximity sensor: A proximity sensor is a device that detects the presence or absence of an object within a specified range without physical contact. These sensors are widely used in various applications, such as automotive systems, mobile devices, and industrial automation, to enhance functionality and improve user experience.
Range: Range refers to the maximum distance over which a wireless communication protocol can effectively transmit data or where a sensor can accurately detect or measure its target. In wireless communication, range is influenced by factors such as signal strength, interference, and environmental conditions, while in sensors, it relates to the limits within which a sensor can operate effectively, including sensitivity and resolution.
Resistance Temperature Detector (RTD): A Resistance Temperature Detector (RTD) is a temperature sensor that operates on the principle of measuring the change in electrical resistance of a material as its temperature changes. This type of sensor typically uses pure metal, often platinum, which exhibits a predictable and nearly linear relationship between resistance and temperature. RTDs are widely recognized for their accuracy, stability, and repeatability in temperature measurement across various applications.
Resolution: Resolution refers to the smallest change that can be distinguished by a system or device, particularly in the context of measurement and signal representation. In electronic systems, it determines the precision of analog-to-digital and digital-to-analog conversions, influencing the quality and accuracy of data captured from sensors, as well as the performance of timers and counters in embedded systems. Higher resolution allows for more detailed information capture and output, essential in applications like audio and video processing.
Response Time: Response time is the duration it takes for a system to react to an input or stimulus, often measured from the moment an event occurs until the system produces an output. This measure is critical for ensuring that systems behave predictably and meet operational requirements, especially under constraints where timely responses are essential for functionality.
Sensitivity: Sensitivity refers to the ability of a sensor to detect changes in the input signal, indicating how responsive it is to variations in the measured parameter. A high sensitivity means that even small changes in the input can produce significant changes in the output signal, which is crucial for accurate data acquisition. This characteristic is essential when considering factors such as signal noise and the need for proper signal conditioning to ensure reliable readings.
Sensor Fusion: Sensor fusion is the process of integrating data from multiple sensors to produce more accurate, reliable, and comprehensive information than can be achieved with individual sensors alone. This technique enhances the ability to interpret environmental data by combining the strengths of various sensor types, thereby improving overall system performance. Sensor fusion is essential in applications where data from different sources must be synthesized to inform decision-making and improve functionality.
Signal conditioning: Signal conditioning is the process of manipulating a signal in such a way that it meets the requirements for further processing or analysis. This includes filtering, amplifying, and converting signals from sensors to make them suitable for digital processing or control applications. It plays a vital role in ensuring accurate data capture and effective system response, particularly in time-based control systems, sensor characteristics, and interfacing techniques.
Temperature Sensor: A temperature sensor is a device that detects and measures temperature, converting it into a readable signal for monitoring or control purposes. These sensors are crucial in various applications, from industrial processes to consumer electronics, and they often require careful interfacing and signal conditioning to ensure accurate readings. Different types of temperature sensors have distinct characteristics that influence their performance in specific environments and applications.
Thermistor: A thermistor is a type of temperature sensor that changes its electrical resistance in response to changes in temperature. This sensitivity allows thermistors to be used in various applications, including temperature measurement and control systems. They are commonly made of ceramic materials that exhibit a significant change in resistance with small changes in temperature, making them useful for precise temperature monitoring.
Thermocouple: A thermocouple is a temperature sensor that consists of two dissimilar metal wires joined at one end, which generates a voltage proportional to the temperature difference between the joined end and the other ends of the wires. This sensor is widely used for temperature measurement due to its simplicity, robustness, and ability to operate over a wide range of temperatures.
Voltage: Voltage is the electrical potential difference between two points in a circuit, which drives the flow of electric current. It is a fundamental concept in understanding how electrical systems operate, particularly when it comes to sensors that convert physical phenomena into electrical signals.
Zeroing: Zeroing is the process of calibrating a sensor to ensure accurate readings by adjusting it so that it outputs a known value, typically zero, when there is no input signal. This adjustment is crucial for various sensor types, as it helps eliminate systematic errors and ensures the reliability of measurements in embedded systems. Proper zeroing enhances the sensor's accuracy, improves data integrity, and can significantly affect the performance of systems relying on precise measurements.