SENSORS a.k.a. Interfacing to the Real World: Review
SENSORS a.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) ECE 480, Prof. A. Mason Sensors p.1 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 ECE 480, Prof. A. Mason intelligent feedback system Sensors p.2 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 primary
analo secondary usable or digital values real values world transducer g signal sensor transducer Typical Electronic Sensor System input signal (measurand) sensor sensor data analog/digital microcontroller signal processing communication
ECE 480, Prof. A. Mason network display Sensors p.3 Example Electronic Sensor Systems Components vary with application digital sensor within an instrument microcontroller signal timing data storage sensor C sensor signal timing memory keypad display
handheld instrument analog sensor analyzed by a PC sensor sensor interface e.g., RS232 A/D, communication signal processing PC comm. card multiple sensors displayed over internet internet sensor processor comm. sensor bus PC
sensor bus comm. card ECE 480, Prof. A. Mason sensor processor comm. Sensors p.4 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 ECE 480, Prof. A. Mason Sensors p.5 Example Primary Transducers Light Sensor photoconductor light R photodiode light I membrane pressure sensor resistive (pressure R) capacitive (pressure C) ECE 480, Prof. A. Mason Sensors p.6
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 ECE 480, Prof. A. Mason
Sensors p.7 Strain Gage: Gage Factor Remember: for a strained thin wire R/R = L/L A/A + / A = (D/2)2, for circular wire D L Poissons ratio, : relates change in diameter D to change in length L D/D = - L/L Thus R/R = (1+2) L/L + / dimensional effect piezoresistive effect Gage Factor, G, used to compare strain-gate materials G = R/R = (1+2) + / L/L
L/L ECE 480, Prof. A. Mason Sensors p.8 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 ECE 480, Prof. A. Mason Sensors p.9 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 ECE 480, Prof. A. Mason Sensors p.10 Example MEMS Transducers MEMS = micro-electro-mechanical system miniature transducers created using IC fabrication processes Microaccelerometer cantilever beam suspended mass Electrodes Rotation
Ring structure gyroscope Pressure Diaphragm (Upper electrode) Lower electrode ECE 480, Prof. A. Mason 5-10mm Sensors p.11 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 ECE 480, Prof. A. Mason Sensors p.12 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 ECE 480, Prof. A. Mason Sensors p.13 Basic Opamp Configuration Voltage Comparator digitize input Voltage Follower buffer Non-Inverting Amp Inverting Amp ECE 480, Prof. A. Mason Sensors p.14 More Opamp Configurations Summing Amp Differential Amp
Integrating Amp Differentiating Amp ECE 480, Prof. A. Mason Sensors p.15 Converting Configuration Current-to-Voltage Voltage-to-Current ECE 480, Prof. A. Mason Sensors p.16 Instrumentation Amplifier Robust differential gain amplifier gain stage Input stage high input impedance
input stage buffers gain stage no common mode gain can have differential gain Gain stage differential gain, low input impedance total differential gain Overall amplifier amplifies only the differential component 2 R2 R1 R4 Gd R1 R3 high common mode rejection ratio high input impedance suitable for biopotential electrodes with high output impedance ECE 480, Prof. A. Mason
Sensors p.17 Instrumentation Amplifier w/ BP Filter instrumentation amplifier HPF non-inverting amp 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. ECE 480, Prof. A. Mason Sensors p.18 Connecting Sensors to Microcontrollers Analog sensor
C sensor signal timing memory keypad display instrument 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 ECE 480, Prof. A. Mason Sensors p.19 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 ECE 480, Prof. A. Mason Sensors p.20 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 r linea
T= temperature; V=sensor voltage; a,b,c = calibration coefficients Compensation offset T = a + bV +cV2, T1 ar non-line T2 T3 remove secondary sensitivities must have sensitivities characterized can remove with polynomial evaluation P = a + bV + cT + dVT + e V2, where P=pressure, T=temperature ECE 480, Prof. A. Mason Sensors p.21
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