Sensors and Actuators PDF

Introduction Definitions, classifications, general requirements  Example Direct sensor actuator link (not always possible) Two transduction steps (sound-electrical and vice versa) Note: sensor/transducer are one and the same  Example...

Introduction Definitions, classifications, general requirements  Example Direct sensor actuator link (not always possible) Two transduction steps (sound-electrical and vice versa) Note: sensor/transducer are one and the same  Example Direct sensor actuator link Does not work: Sound is converted into change of resistance No transduction takes place (no change of energy)! Must add power to affect transduction Cannot work in opposite direction either  Example Transduction: pressure to current A telephone system has two of these!  Our definitions: Sensor A device that responds to a physical stimulus. Transducer A device that converts energy of one form into energy of another form. Actuator A device or mechanism capable of performing a physical action  Our definitions: Stimulus The quantity that is sensed. Sometimes called the measurand.  Transducer Physical Physical quantity Transducer quantity A transducer is an element that converts one physical quantity into another physical quantity - Mercury thermometer (temperature – displacement) - Accelerometer (acceleration – voltage) - Electrode in a battery (ion – electrical charge) - Motor (electrical current – mechanical moment) - LED (electrical current – light) 1.11  Sensor - actuator A sensor is a transducer that converts a physical quantity into an electrical quantity: - Resistance thermometer (temperature - resistance) - Photodetector (light - current) Measured Electrical quantity quantity Sensor (e, I, U, R, …) An actuator is a transducer that converts an electrical quantity into a non-electrical quantity - Piezo actuator (charge - displacement) - Resistive heater (current - heat) - LED (current - light) Electrical quantity Actuator Physical quantity 1.12  Sensors Noise Measured Sensor Electrical quantity quantity x y = f (x) Ex: temperature T dy Ex: Resistance R Accelleration a Sensitivity S= Voltage V dx Sensitivity S: response in magnitude Transfer function: frequency response Noise: sensitivity to perturbations (internal and external) 1.13  Classification of Sensors and Actuators Based on physical laws Based on any convenient distinguishing property Possible to a certain extent - some devices defy classification 1. Active and Passive sensors 2. Contact and non-contact sensors 3. Absolute and relative sensors 4. Other schemes  1. Active and passive sensors Active sensor: a sensor that requires external power to operate. Examples: the carbon microphone, thermistors, strain gauges, capacitive and inductive sensors, etc. Other name: parametric sensors (output is a function of a parameter - like resistance) Passive sensor: generates its own electric signal and does not require a power source. Examples: thermocouples, magnetic microphones, piezoelectric sensors. Other name: self-generating sensors Note: some define these exactly the other way around  2. Contact and noncontact sensors Contact sensor: a sensor that requires physical contact with the stimulus. Examples: strain gauges, most temperature sensors Non-contact sensor: requires no physical contact. Examples: most optical and magnetic sensors, infrared thermometers, etc.  3. Absolute and relative sensors Absolute sensor: a sensor that reacts to a stimulus on an absolute scale: Thermistors, strain gauges, etc., (thermistor will always read the absolute temperature) Relative scale: The stimulus is sensed relative to a fixed or variable reference. Thermocouple measures the temperature difference, pressure is often measured relative to atmospheric pressure. 3. Absolute and relative sensors  4. Other schemes (cont.) Classification by specifications Accuracy Sensitivity Stability Response time Hysteresis Frequency response Input (stimulus) range Resolution Linearity Hardness (to environmental conditions, etc.) Cost Size, weight, Construction materials Operating temperature Etc.  Requirements for interfacing Needs: Matching (impedances, voltages, currents, power) Transformations (AC/DC, DC/AC, A/D, D/A, VtoF, etc.) Matching of specifications (temperature ranges, environmental conditions, etc.) Alternative designs Etc.  Connection of sensors/actuators The processor should be viewed as a general block – Microprocessor – Amplifier – Driver – Etc. Matching: between sensor/processor and processor/actuator  Example - Temperature control Sense the temperature of a CPU Control the speed of the fan to keep the temperature constant  Temperature control - implementation Sometimes the A/D and signal conditioning are separate from the processor The whole circuitry may be integrated into a “smart sensor” Match: impedance at input to amplifier and at processor  Temperature control - Alternative design Simpler (uses an integrated sensor that contains some of the necessary circuitry). May still require an A/D The performance of this design is not the same (range is 0-85°C while the previous design was -200 to 2000 °C or more)  Units SI units in most cases Standard units when understanding warrants it (e.g. psi for pressure) Will avoid mixed units (a common problem in sensors and actuators)  The decibel (dB) Often a physical quantity spans a very large range. Or the quantity only has a value wrt a reference value. – Human eye can see in luminance from 10-6 cd/m2 to 106 cd/m2 – Amplification or attenuation of signals.  The decibel (dB) Quantities are given as ratios on a logarithmic scale: – Given a quantity divide it by Ref value. – Take the base 10 log of the ratio. – If quantity is power-related multiply by 10. – If quantity is field-related multiply by 20. P V P = 10 log10 [dB] ⌫ = 20 log10 [dB] P0 V0  1.6 Units 2 EXAMPLE 1.8 Use of decibels Exercise A power sensor for detection of cellular phone transmissions is rated for an input power range of !32 dBm to 18 dBm. Calculate the range and span of the sensor in terms of power. Solution: The fact that the range is given in dBm means that the reference value is 1 mW. Starting with the low range value, we write P p ¼ 10log10 ¼ !32 dBm: 1 mW Dividing both sides by 10 gives P log10 ¼ !3:2: 1 mW Now we can write P ¼ 10!3:2 ! P ¼ 10!3:2 ¼ 0:00063 mW: 1 mW For the upper range, we have P P p ¼ 10log10 ¼ 20 ! log10 ¼ 2 ! P ¼ 102 ¼ 100 mW: 1 mW 1 mW The range is therefore 0.00063 mW to 100 mW for a span of 99.99937 mW. Note that the span in decibels is 50 dBm.  Quick quiz https://forms.office.com/r/v4n0ByHqe2  For worked out examples Go to the course repository: https://github.com/agmarrugo/sensors-actuators