Commonly Used Sensors - Details

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A displacement sensor can be constructed using a potentiometer. A moving object is mechanically coupled to the pot wiper, whose movement causes the resistance change Advantage: low cost, low tech, easy to use Disadvantage: wear due to moving parts, low accuracy, low repeatability, and limited frequency response.

Potentiometric Sensor

As the metal target moves across the two sensing coils the ratio of the current induced in the two sensing coils changes. This ratio can be calibrated to a exact target location.

Inductive Position Sensors

Advantage: ▪ No moving parts and no contact makes this sensor extremely reliable. ▪ It has no temperature sensitivity. ▪ Since all sensing elements can be enclosed the sensor is able to operate in harsh environments. Disadvantage: ▪ Limited sensing range. ▪ Target object must have the ferrous material machined into it or attached to it.

Inductive Position Sensors Advantage and Disadvantage

▪ The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender.

Laser range sensor

▪ Advantage: High resolution, speed and accuracy as well as non-contact reliability. ▪ Disadvantage: not all objects work – based on reflectance and surface roughness. Expensive, limited range.

Laser range sensor Advantage and Disadvantage

▪ Ultrasonic displacement sensors emit ultrasonic waves from the sensor head and receive the waves reflected off the target object. ▪ A piezoelectric transducer is used for both emitter and receiver.

Ultrasonic Range sensor

▪ Output is a series of square wave pulses generated by a photocell arrangement and a coded disk with evenly spaced transparent and dark.

Incremental Encoder

▪ provide a unique output code for every single position of rotation indicating both position and direction. ▪ Their coded disk consists of multiple concentric “tracks” of light and dark segments.

Absolute Encoder

▪ MEMS (micro electro mechanical system) based accelerometers are widely used for many applications. ▪ The most common type is a capacitive accelerometer. ▪ The proof mass moving will change the distance between two set of conductive plates

MEMs Accelerometer

▪ A piezoelectric accelerometer uses a piezoelectric ceramic and an attached proof mass. The inherent high pass filter means that the accelerometer wont measure constant acceleration such as gravity. ▪ Used in Seismic monitoring and machine vibration.

Piezoelectric Accelerometer

Use an inductive displacement sensor with a spring with a known spring coefficient.

LVDT force sensor

1. Absolute pressure sensor - pressure relative to perfect vacuum. 2. Gauge pressure sensor - pressure relative to atmospheric pressure. 3. Vacuum pressure sensor - pressures below atmospheric pressure. 4. Differential pressure sensor - the difference between two pressures, one connected to each side of the sensor 5. Sealed pressure sensor - pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).

▪ Types of Pressure Sensors:

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure, resistance increasing as pressure deforms the material. ▪ Advantage: ▪ Excellent linearity and low hysteresis ▪ High Sensitivity ▪ Low cost MEMS solutions ▪ Disadvantage ▪ Temperature dependent

Piezoresistive Pressure Sensors

▪ Uses a capacitive transducer to measure the strain in a diaphragm or bellow. ▪ The stiffness and strength of the material can be chosen to provide a range of sensitivities and operating pressures. By choosing materials for the capacitor plates that have a low coefficient of thermal expansion, it’s possible to make sensors with very low sensitivity to temperature change

Capacitive Pressure Sensors

▪ Advantage: ▪ Excellent linearity and low hysteresis ▪ High Sensitivity ▪ Low cost MEMS solutions ▪ Less Temperature dependent than Piezoresistive ▪ Disadvantage ▪ Requires more sophisticated circuitry ▪ Non-linear respone

Capacitive Pressure Sensors Advantage and Disadvantage

▪ Inductive pressure sensors measure the strain of the diaphragm by means of LVDT or Hall Effect ▪ Advantage: ▪ High Sensitivity ▪ Minimal Temperature dependent ▪ Disadvantage ▪ More expensive

Inductive Pressure Sensors

An optical position sensor similar to those covered are used to measure deflection of a diaphragm. ▪ Advantage: ▪ High Sensitivity ▪ Minimal Temperature dependent ▪ Minimal EMF sensitivity ▪ Disadvantage ▪ Expensive

Optoelectronic pressure sensors

▪ When measuring vacuum close to absolute using a capacitive or resistive sensor will not provide the required accuracy. ▪ Other properties such as heat transfer of the gas are used to indirectly measure the absolute pressure. ▪ A Pirani gauge works by using two heated thermistors.

VACUUM SENSOR

MEMS based flow meters can also be created using a capacitive sensor and a flexible membrane.

MEMS Flow

▪ Another method of measuring speed based on pressure differential is by using a pitot tube. ▪ A pitot tube has a port facing the flow and one perpendicular to the flow. ▪ The pressure difference created by the flow velocity follows Bernoulli's equation. ▪ Pitot tubes are used in many applications including airspeed of an aircraft, air flow in stacks and chimney

Pitot tube

A "variable area meter" measures fluid flow by allowing the cross-sectional area of the device to vary in response to the flow, causing some measurable effect that indicates the rate.

Rotameter or variable area meter

Thermal mass flowmeters generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid. This type of flow meter can provide mass flow reading without any pressure and temperature compensation.

Thermal mass flowmeters

▪ A fine wire is electrically heated to some temperature above the ambient. ▪ The wire is cooled by the air flowing past it. ▪ Measuring resistance of the wire provides this temperature indirectly. ▪ A relationship can be obtained between the resistance of the wire and the flow speed.

Hotwire anemometer

Ultrasonic anemometers measure wind speed based on the time of flight of sonic pulses between pairs of piezoelectric transducers. ▪ 2 or even 3-dimensional wind measurements are possible using pairs of transducers. ▪ The fast response of ultrasonic anemometers makes them ideally suited for measuring turbulent flow. ▪ Same principle can be used to measure flow in a pipe.

Ultrasonic anemometers

▪ Vortex flow meters use a an obstacled (called a shedder bar) in the path of the fluid to create vortices. ▪ The frequency at which these vortices switch sides is proportional to the fluid flow. ▪ A piezoelectric element is used to measure the pressure after the shredder bar.

Vortex flow

▪ Electromagnetic flow meters are non-contact nonobstructive flow meters ideally suited for a conductive fluid. ▪ The principle of operation is: ▪ Two coils or permeant magnets generate a magnetic flux through the tube. ▪ Two electrodes are placed perpendicular to the tube ▪ The magnetic flux separates positive and negative charged particles in the fluid creating a voltage

Electromagnetic flow meters

▪ Coriolis flow meter provides a measure of mass flow rather than volume flow or velocity. ▪ Coriolis effect is a force that acts on an object that are in motion within a moving frame of reference.

Coriolis flow meter provides

Use thermoelectric effect to create a voltage across two dissimilar metals proportional to the temperature gradient across them.

Thermocouple Temperature Sensor

Thermoresistive properties of metal alloys are used to measure absolute temperature. RTDs have the highest accuracy (0.025˚C) of any common temperature sensor

Resistive Temperature Sensor (RTD)

▪ Thermistors are another type of resistor with strong thermoresistive properties. ▪ Most commonly thermistors are made of powdered metal oxides.

Thermistor

▪ Infrared thermometers use a photoelectric pyrometer. ▪ Sensor turns blackbody IR radiation to a measurable signal. ▪ A laser beam is included to help aim the thermometer.

Infrared Thermometer

▪ Alternative approach is to use an acoustic temperature senor. ▪ Uses the principle that temperature of a media affects the speed of sound through it.

Acoustic Sensor

▪ A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes. ▪ The metal oxide’s electrical capacity changes with the atmosphere’s relative humidity. ▪ The capacitive type sensors are the only type to be able to measure relative humidity from 0% to 100%.

Capacitive humidity sensor

▪ Advantage: the only sensors that can measure to 0% RH. ▪ Disadvantage: ▪ The sensor will require a circuit to create the signal at the optimal frequency and measure the capacitance. ▪ Regular re-calibration is also required for optimal performance.

Capacitive humidity sensor advantage and disadvantage

▪ The principle behind resistive humidity sensors is the fact that the conductivity in non – metallic conductors is dependent on their water content. ▪ The relationship between resistance and humidity is inverse exponential. ▪ The low resistivity material is deposited on top of two electrodes ▪ The electrodes are placed in comb shaped pattern to increase the contact area.

Resistive humidity sensor

▪ Advantage: low cost, small size and minimal requirement for recalibration. ▪ Disadvantage: highly sensitive to chemical vapors and other contaminants.

Resistive humidity sensor adv and dis

Measures the thermal conductivity of both dry air as well as air with water vapor. ▪ Consist of two matched negative temperature coefficient (NTC) thermistor elements in a bridge circuit. ▪ One is hermetically encapsulated in dry nitrogen and the other is exposed to the environment.

Thermal conductivity humidity sensor

Changes in composition of the electrolyte will result in measurable current change between the working electrode and return electrode. ▪ Electrolyte is encapsulated in a casing ▪ Gas permeates through a hydrophobic membrane ▪ Voltage is supplied to the working electrodes. ▪ In the absence of the target gas a negligible current flows through the sensor to provide the baseline zero. When target gas is present the oxidization of the gas produces or consumes electrons that facilitate higher charge transport (current).

Electrochemical chemical sensor

Solution ▪ Manufacturers aim to minimize cross-sensitivity by selecting optimal cathode and electrolyte solution. ▪ Filter placed on the inlet of the sensor ▪ Use multiple sensors in a reduction matrix

Electrochemical chemical sensor selectivity issue

▪ MOS gas sensors are less expensive to produce and used widely for CO, propane, and other hazardous gas detection. 1. When semiconductors (e.g. tin dioxide) are heated in air at high temperature, oxygen is adsorbed on the sensing surface by capturing free electrons. 2. The reduction in free electrons increases the resistance of the sensor. 3. In the presence of reducing gases, the surface density ofadsorbed oxygen decreases as it reacts with the reducing gases. 4. Electrons are then released into the tin dioxide, allowing current to flow freely through the sensor.

Metal Oxide Gas Sensors (MOS)

1. An infrared (IR) lamp directs waves of light through a tube filled with a sample of air toward an optical filter in front of an IR light detector. 2. The IR light detector measures the amount of IR light that passes through the optical filter. 3. The IR wavelength must match the absorption band of the target gas. For example 4.2 micron band for CO2. 4. As the IR light passes through the length of the tube, the target gas molecules absorb the specific band of IR light while letting other wavelengths of light pass through.

Nondispersive Infrared (NDIR)

5. At the detector end the remaining light hits an optical filter that absorbs every wavelength of light except the target wavelength (e.g. 4.2 micron for CO2). 6. Finally, an IR detector reads the remaining amount of light that was not absorbed by the CO2 molecules or the optical filter. 7. The higher the concentration of absorbing gas the lower the amount of light detected

Nondispersive Infrared (NDIR)

1. UV lamp is used to emit high energy photons. 2. Photons break up molecules into positively charged ions and electrons. 3. The electrons are attracted to the + electrode and the ions to the - electrodes. 4. The higher the gas concentrations, the more electrons and ions are released and the higher the current through the electrodes.

Photoionization Detector (PID)

▪ Various compounds will require a different minimum photon energy to ionize (ionization potential).

Photoionization Detector (PID)

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