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Sensor Lecture Interfacing
- 1. Sensors p.1 SENSORS Interfacing tothe Real World: Review of Electrical Sensors and Actuators
- 2. Sensors p.2 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
- 3. Sensors p.3 usable values Sensor Systems Typicallyinterested 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 analo g signal primary transducer secondary transducer sensor sensor input signal (measurand) microcontroller signal processing communication sensor data analog/digital network display
- 4. Sensors p.4 Example ElectronicSensor 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 keypadsensor 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
- 5. Sensors p.5 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
- 6. Sensors p.6ECE 480,Prof. A. Mason Example Primary Transducers • Light Sensor – photoconductor • light ∆R – photodiode • light ∆I – membrane pressure sensor • resistive (pressure ∆ R) • capacitive (pressure ∆C)
- 7. Sensors p.7ECE 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
- 8. Sensors p.8ECE 480,Prof. A. Mason Strain Gage: Gage Factor • Remember: for a strained thin wire – ∆R/R = ∆L/L – ∆A/A + ∆ρ/ρ • A = π (D/2)2 , for circular wire • Poisson’s ratio, µ: relates change in diameter D to change in length L – ∆D/D = - µ ∆L/L • Thus – ∆R/R = (1+2µ) ∆L/L + ∆ρ/ρ • Gage Factor, G, used to compare strain-gate materials – G = ∆R/R = (1+2µ) + ∆ρ/ρ ∆L/L ∆L/L LD dimensional effect piezoresistive effect
- 9. Sensors p.9 Temperature SensorOptions • 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
- 10. Sensors p.10 Fiber-optic TemperatureSensor • 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
- 11. Sensors p.11 Example MEMSTransducers • MEMS = micro-electro-mechanical system – miniature transducers created using IC fabrication processes • Microaccelerometer – cantilever beam – suspended mass • Rotation – gyroscope • Pressure Electrodes Ring structure Diaphragm (Upper electrode) Lower electrode 5-10mm
- 12. Sensors p.12ECE 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
- 13. Sensors p.13ECE 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 20-100Ω – 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
- 14. Sensors p.14 Basic OpampConfiguration • Voltage Comparator – digitize input • Voltage Follower – buffer • Non-Inverting Amp • Inverting Amp
- 15. Sensors p.15ECE 480,Prof. A. Mason More Opamp Configurations • Summing Amp • Differential Amp • Integrating Amp • Differentiating Amp
- 16. Sensors p.16ECE 480,Prof. A. Mason Converting Configuration • Current-to-Voltage • Voltage-to-Current
- 17. Sensors p.17ECE 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 + = 3 4 1 12 d 2 R R R RR G total differential gain
- 18. Sensors p.18ECE 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
- 19. Sensors p.19ECE 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 keypadsensor sensor display instrument
- 20. Sensors p.20ECE 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 – I2 C = 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
- 21. Sensors p.21ECE 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