Sensing and Sensors: Fundamental Concepts

Sensing and Sensors: Fundamental Concepts 2015 Sensitivity Range Precision Accuracy Resolution Offset Hysteresis Response Time Source: sensorwebs.jpl.nasa.gov

Human Physiology in Space" by Barbara F. Abuja and Ronald J. White, 1994

frequency = de / h h: Planck s constant 6.626068 10-34 m 2 kg / s

Sonar measures acoustic location in air or water. In echolocation, ranging is achieved by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. Echo-locating animals make use of the fact that they have two ears positioned slightly apart. The echoes returning to the two ears arrive at different times and at different loudness levels, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive direction and distance. With echolocation the animal can see not only where it's going but can also discern size and texture of objects in its path even when other sensors (such as vision) utterly fail (on a dark night). Most insectivorous bats call with dominant frequencies between 20 khz and 60 khz. Lower frequencies are avoided because echoes from insect-sized targets are weak when the wavelength is longer than the insect wing length. Bat echolocation calls: adaptation and convergent evolution Gareth Jones* and Marc W. Holderied, Proc. R. S oc. B (2007) 274, 905 912

A sensor is a device that receives a stimulus and responds with an electrical signal. A transducer is a converter from one type of energy into another one. (Loudspeaker: electric signal -> variable magnetic field -> acoustic wave / sound ) A direct sensor: converts a stimulus into an electrical signal by using a physical effect (photo effect, for example). A complex sensor: requires additionally one or more transducers before the sensor can generate an electrical signal.

Sensing Natural! Human! Synthetic! Natural systems Sensing mostly reflexive (as far as we humans know) Human beings Sensing intuitive, reflexive and intimately linked to cognition and every aspect related to cognition (philosophy, culture) Synthetic systems Inspired by natural systems, modeled by laws of physics, implemented in mechanical and electronic media, commonly known as sensors and transducers

Commonly Detectable Phenomena Biological Chemical Electric Electromagnetic Heat Magnetic Mechanical (displacement, velocity, acceleration, etc.) Optical Radioactive

Common Conversion Methods Physical thermo-electric, thermo-elastic, thermo-magnetic, thermo-optic photo-electric, photo-elastic, photo-magnetic, electro-elastic, electro-magnetic magneto-electric Chemical chemical transport, physical transformation, electro-chemical Biological biological transformation, physical transformation

Commonly Measured Quantities Stimulus Quantity Acoustic Biological & Chemical Wave (amplitude, phase, polarization, velocity), Spectrum, Fluid Concentrations (Gas or Liquid) Electric Charge, Voltage, Current, Electric Field (amplitude, phase, polarization), Conductivity, Permittivity Magnetic Magnetic Field (amplitude, phase, polarization), Flux, Permeability Optical Refractive Index, Reflectivity, Absorption Thermal Temperature, Flux, Specific Heat, Thermal Conductivity Mechanical Position, Velocity, Acceleration, Force, Strain, Stress, Pressure, Torque

Sensors do not operate in isolation; they are always part of a larger system.

Classifying Sensors passive sensors A passive sensor needs no additional power; it generates an electric signal directly in response to an external stimulus. >> examples: photodiode, piezoelectric sensor active sensors An active sensor requires external power for its operation (an excitation signal. This signal, in turn, is modified by the sensor to produce an output) >> examples: thermistor (temperature sensitive resistor) contact sensors A contact sensor detects change through direct physical contact with a target object. >> examples: limit switches, safety switches non-contact sensors A non-contact sensor creates an energy field and reacts to disturbances in that field; no physical contact is required >> examples: ultrasonic sensors

Basic Concepts Sensitivity the minimum input (of a physical parameter) that will create a detectable output Range the minimum and maximum values of a given parameter the sensor can measure. Precision..the ability of the sensor to reproduce the same results in repeated tests of identical conditions Accuracy the maximum difference between the actual value and the value indicated by the sensor Resolution the smallest detectable incremental change of input that can be detected in the output signal Offset..the output existing in the absence of input Hysteresis..the effect of direction of the input on the output Response Time the time required for a sensor to change from a previous state to a new state

Accuracy Accuracy is measured as the highest deviation of a value represented by the sensor from the ideal of true values at its input, (with a specific level of uncertainty). Accuracy defines how well results from a measurement system reproduce reality. Example: A linear displacement-sensor should generate 1 mv per 1 mm displacement. However, measurement shows that a displacement of s = 10.5 mm produced and output of S = 10.0V > inaccuracy = (0.5mm/10mm)* 100% = 5% The deviation from the ideal transfer function is given as δ (+-δ) in terms of the measured value or as percentage of the full span input or in terms of the output Accuracy ratings are performed to find the real performance of a sensor. They include combined effects of variations, hysteresis, dead band, calibration and repeatability errors. Precision, as opposed to accuracy, defines how well a measurement system reproduces its results (independent of whether they correspond to accurate facts).

Repeatability Repeatability is the ability of a system to recreate the same result under the same condition at different times. It is often expressed as the maximum difference between output readings from two calibrating cycles: δr = (Δ / FS)*100% (FS: full scale) Dead Band: An insensitivity of a sensor in a specific range of input signal (blind spot).

Reliability Reliability is the ability of a sensor to perform a required function under stated conditions for a stated period of time. It is often expressed in statistical terms as a probability that the device will function without failure for a number of uses. Because of this definition, reliability of a class or batch of devices is evaluated by observing the behavior of a large number of devices. Devices that have been fabricated with the same technology, same materials, and same processes are then assumed to behave in the same way (assumed reliability).

Dimensionality A sensor can have more that one dimension if the sensor's output is influenced by more than one input stimulus. example: infrared sensor V = G*( T b4 T s4 ) 4 th order parabola where Ts = temp of sensor surface, Tb = temp of an object of measurement and G a constant

Hysteresis Hysteresis refers to systems which have memory ; the effects of the current input to the system are not felt at the same instant. Such a system may exhibit path dependence. In a deterministic system with no hysteresis it is possible to predict the system's output at an instant in time, given only its input at that instant in time. In a system with hysteresis, this is not possible; there is no way to predict the output without knowing the system's current state. Hysteresis phenomena occur in magnetic and ferro-magnetic materials, in which a lag occurs between the application and the removal of a force or field and its subsequent effect.

Saturation A given sensor can not operate under all conditions and situations. Where the response no longer matches the transfer function, the sensor operates in saturated mode.

Resolution Resolutions is the Smallest increment of stimuli which can be sensed. Resolution is often expressed as a percent of full scale (FS). Example: an angular sensor has a FS of 270 deg. A 0.5 deg resolution would be specified as 0.181% FS. When there are no measurable steps in the output of a sensor it is said to be continuous. Often the resolution is not constant over the whole input range. In digital sensors, the resolution is specified as the number of bits used to represent the output, as in '8-bit resolution', where the result is represented by 8 bit long data for FS.

Excitation and Dynamic Characteristics Excitation is the electric signal often needed to make a sensor active for operation. It is often listed as a range of voltage and or current. In some sensors, the frequency of excitation is also important (and must be specified). Sensors react with a certain delay to inputs. Sensors are time dependent and show a dynamic characteristic. Some sensors need a warm-up period before they can operate reliably. Including time dependency leads to a different kind mathematical description of sensor behavior, one based on differential equations. Zero order sensor Out(t) = a + b * i(t) a: static sensitivity First order sensor b 1 *(dout(t)/dt) + b 0 *Out(t) = i(t) Second order sensor b 2 *(d 2 Out(t)/dt 2 ) + b 1 *(dout(t)/dt) + b 0 *Out(t) = i(t)

Output Impedance A: interface for sensor with voltage output B: interface for sensor with current output To minimize output distortions sensors need to be matched with their connecting circuitry. For voltage generating sensors, a lower impedance (Zout) is preferable and the circuit should have a high input impedance (Zin) A current generating sensor should have an output impedance as high as possible and the circuit's impedance should be as low as possible.

Orders of Sensors Temperature sensor for which the energy storage is thermal capacity (stays hot for n sec) Often such sensors are described by a frequency response: how fast they can respond to a changed input. A temperature sensor is of order one. A second order response is typical for a sensor that responds with a periodic signal such as an accelerometer. Typically, the operating range of a sensor is selected below or above the resonant frequency. For some special sensors, however, the resonant frequency IS the operating point (that is where they will have the strongest response). Damping Damping is the suppression of oscillation in a sensor of order > 1; a time dependent behavior. When the sensor's response is maximally fast without overshoot, the response is called critically damped. If an overshoot occurs, the sensor is said to react in under-damped response. If a sensor reacts slower than its max response, it is said to react in over-damped response.

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