electric transducers


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SENSORSa.k.a.Interfacing to the Real World:Review of Electrical Sensors and Actuators : 

ECE 480, Prof. A. Mason SENSORSa.k.a.Interfacing to the Real World:Review of Electrical Sensors and Actuators Andrew Mason Associtate Professor, ECE Teach: Microelectronics (analog & digital integrated Circ., VLSI) Biomedical Engineering (instrumentation) Research: Integrated Microsystems (on-chip sensors & circuits)

Transducers : 

ECE 480, Prof. A. Mason Transducers Transducer a device that converts a primary form of energy into a corresponding signal with a different energy form Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical, etc. take form of a sensor or an actuator Sensor (e.g., thermometer) a device that detects/measures a signal or stimulus acquires information from the “real world” Actuator (e.g., heater) a device that generates a signal or stimulus real world sensor actuator intelligent feedback system

Sensor Systems : 

ECE 480, Prof. A. Mason usable values Sensor Systems Typically interested in electronic sensor convert desired parameter into electrically measurable signal General Electronic Sensor primary transducer: changes “real world” parameter into electrical signal secondary transducer: converts electrical signal into analog or digital values Typical Electronic Sensor System real world analog signal primary transducer secondary transducer sensor sensor input signal (measurand) microcontroller signal processing communication sensor data analog/digital network display

Example Electronic Sensor Systems : 

ECE 480, Prof. A. Mason Example Electronic Sensor Systems Components vary with application digital sensor within an instrument microcontroller signal timing data storage analog sensor analyzed by a PC multiple sensors displayed over internet µC signal timing memory keypad sensor sensor display handheld instrument PC comm. card sensor interface A/D, communication signal processing sensor e.g., RS232 PC comm. card internet sensor processor comm. sensor processor comm. sensor bus sensor bus

Primary Transducers : 

ECE 480, Prof. A. Mason Primary Transducers Conventional Transducers large, but generally reliable, based on older technology thermocouple: temperature difference compass (magnetic): direction Microelectronic Sensors millimeter sized, highly sensitive, less robust photodiode/phototransistor: photon energy (light) infrared detectors, proximity/intrusion alarms piezoresisitve pressure sensor: air/fluid pressure microaccelerometers: vibration, ∆-velocity (car crash) chemical senors: O2, CO2, Cl, Nitrates (explosives) DNA arrays: match DNA sequences

Example Primary Transducers : 

ECE 480, Prof. A. Mason Example Primary Transducers Light Sensor photoconductor light  R photodiode light  I membrane pressure sensor resistive (pressure   R) capacitive (pressure  C)

Displacement Measurements : 

ECE 480, Prof. A. Mason Displacement Measurements Measurements of size, shape, and position utilize displacement sensors Examples diameter of part under stress (direct) movement of a microphone diaphragm to quantify liquid movement through the heart (indirect) Primary Transducer Types Resistive Sensors (Potentiometers & Strain Gages) Inductive Sensors Capacitive Sensors Piezoelectric Sensors Secondary Transducers Wheatstone Bridge Amplifiers

Strain Gage: Gage Factor : 

ECE 480, Prof. A. Mason Strain Gage: Gage Factor Remember: for a strained thin wire DR/R = DL/L – DA/A + Dr/r A = p (D/2)2, for circular wire Poisson’s ratio, m: relates change in diameter D to change in length L DD/D = - m DL/L Thus DR/R = (1+2m) DL/L + Dr/r Gage Factor, G, used to compare strain-gate materials G = DR/R = (1+2m) + Dr/r DL/L DL/L dimensional effect piezoresistive effect

Temperature Sensor Options : 

ECE 480, Prof. A. Mason Temperature Sensor Options Resistance Temperature Detectors (RTDs) Platinum, Nickel, Copper metals are typically used positive temperature coefficients Thermistors (“thermally sensitive resistor”) formed from semiconductor materials, not metals often composite of a ceramic and a metallic oxide (Mn, Co, Cu or Fe) typically have negative temperature coefficients Thermocouples based on the Seebeck effect: dissimilar metals at diff. temps.  signal

Fiber-optic Temperature Sensor : 

ECE 480, Prof. A. Mason Fiber-optic Temperature Sensor Sensor operation small prism-shaped sample of single-crystal undoped GaAs attached to ends of two optical fibers light energy absorbed by the GaAs crystal depends on temperature percentage of received vs. transmitted energy is a function of temperature Can be made small enough for biological implantation GaAs semiconductor temperature probe

Example MEMS Transducers : 

ECE 480, Prof. A. Mason Example MEMS Transducers MEMS = micro-electro-mechanical system miniature transducers created using IC fabrication processes Microaccelerometer cantilever beam suspended mass Rotation gyroscope Pressure 5-10mm

Passive Sensor Readout Circuit : 

ECE 480, Prof. A. Mason Passive Sensor Readout Circuit Photodiode Circuits Thermistor Half-Bridge voltage divider one element varies Wheatstone Bridge R3 = resistive sensor R4 is matched to nominal value of R3 If R1 = R2, Vout-nominal = 0 Vout varies as R3 changes VCC R1+R4

Operational Amplifiers : 

ECE 480, Prof. A. Mason Operational Amplifiers Properties open-loop gain: ideally infinite: practical values 20k-200k high open-loop gain  virtual short between + and - inputs input impedance: ideally infinite: CMOS opamps are close to ideal output impedance: ideally zero: practical values 20-100 zero output offset: ideally zero: practical value <1mV gain-bandwidth product (GB): practical values ~MHz frequency where open-loop gain drops to 1 V/V Commercial opamps provide many different properties low noise low input current low power high bandwidth low/high supply voltage special purpose: comparator, instrumentation amplifier

Basic Opamp Configuration : 

ECE 480, Prof. A. Mason Basic Opamp Configuration Voltage Comparator digitize input Voltage Follower buffer Non-Inverting Amp Inverting Amp

More Opamp Configurations : 

ECE 480, Prof. A. Mason More Opamp Configurations Summing Amp Differential Amp Integrating Amp Differentiating Amp

Converting Configuration : 

ECE 480, Prof. A. Mason Converting Configuration Current-to-Voltage Voltage-to-Current

Instrumentation Amplifier : 

ECE 480, Prof. A. Mason Instrumentation Amplifier Robust differential gain amplifier Input stage high input impedance buffers gain stage no common mode gain can have differential gain Gain stage differential gain, low input impedance Overall amplifier amplifies only the differential component high common mode rejection ratio high input impedance suitable for biopotential electrodes with high output impedance input stage gain stage total differential gain

Instrumentation Amplifier w/ BP Filter : 

ECE 480, Prof. A. Mason Instrumentation Amplifier w/ BP Filter instrumentation amplifier With 776 op amps, the circuit was found to have a CMRR of 86 dB at 100 Hz and a noise level of 40 mV peak to peak at the output. The frequency response was 0.04 to 150 Hz for ±3 dB and was flat over 4 to 40 Hz. The total gain is 25 (instrument amp) x 32 (non-inverting amp) = 800. HPF non-inverting amp

Connecting Sensors to Microcontrollers : 

ECE 480, Prof. A. Mason Connecting Sensors to Microcontrollers Analog many microcontrollers have a built-in A/D 8-bit to 12-bit common many have multi-channel A/D inputs Digital serial I/O use serial I/O port, store in memory to analyze synchronous (with clock) must match byte format, stop/start bits, parity check, etc. asynchronous (no clock): more common for comm. than data must match baud rate and bit width, transmission protocol, etc. frequency encoded use timing port, measure pulse width or pulse frequency µC signal timing memory keypad sensor sensor display instrument

Connecting Smart Sensors to PC/Network : 

ECE 480, Prof. A. Mason Connecting Smart Sensors to PC/Network “Smart sensor” = sensor with built-in signal processing & communication e.g., combining a “dumb sensor” and a microcontroller Data Acquisition Cards (DAQ) PC card with analog and digital I/O interface through LabVIEW or user-generated code Communication Links Common for Sensors asynchronous serial comm. universal asynchronous receive and transmit (UART) 1 receive line + 1 transmit line. nodes must match baud rate & protocol RS232 Serial Port on PCs uses UART format (but at +/- 12V) can buy a chip to convert from UART to RS232 synchronous serial comm. serial peripheral interface (SPI) 1 clock + 1 bidirectional data + 1 chip select/enable I2C = Inter Integrated Circuit bus designed by Philips for comm. inside TVs, used in several commercial sensor systems IEEE P1451: Sensor Comm. Standard several different sensor comm. protocols for different applications

Sensor Calibration : 

ECE 480, Prof. A. Mason Sensor Calibration Sensors can exhibit non-ideal effects offset: nominal output ≠ nominal parameter value nonlinearity: output not linear with parameter changes cross parameter sensitivity: secondary output variation with, e.g., temperature Calibration = adjusting output to match parameter analog signal conditioning look-up table digital calibration T = a + bV +cV2, T= temperature; V=sensor voltage; a,b,c = calibration coefficients Compensation remove secondary sensitivities must have sensitivities characterized can remove with polynomial evaluation P = a + bV + cT + dVT + e V2, where P=pressure, T=temperature T1 T2 T3 offset linear non-linear

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