Guidelines Design two complete, novel bioinstrumentation systems using biosignals that are relevant to an important health problem. Novel means not introduced in class and not able to be found on the internet. Each of your solutions should contain the following elements: 1) a labeled block diagram of a full instrument to acquire the signal, process it, and convert to a measurand. 2) block diagram must include a sensor, sensor circuit, processing circuitry (active and/or passive), and output (digital, analog, LED, alarm, etc.). 3) state a reason for each choice of bioinstrumentation element. 4) define signal range, instrument resolution, sensor sensitivity. 5) state guidelines for detecting healthy vs. unhealthy levels of the biosignal. How should a clinician interpret the measurand? 6) what modifications would be needed to make this a senior design project? What is the ideal team (student expertise? Faculty expertise? Experts outside the university)? Answers should be submitted in one clearly labeled word document, with your name and student ID, a title for each instrument, and responses to questions 1) through 6) clearly marked as such. The rubric below will be used to grade: Points given 0 1 2 3 1) No block diagram. Two or more blocks/labels are missing. One block is missing. Block diagram is complete. 2) No labels or only very generic labels of components. Lack of detail. Two errors or omissions prevent strong and clear signal from flowing from biosignal to measurand to output. One error or omission in specific components prevents strong and clear signal from flowing from biosignal to measurand to output. Instrument components work together, signal flows correctly. 3) No rationale provided or only very general and generic information provided. Three or four mistakes in the justification of use of instrumentation components One or two mistakes in the justification of use of instrumentation components. Each instrumentation component is well explained. Element # 4) No equations are provided, and no characteristics are given justifiable numeric values. Two characteristics and/or two equations to describe them are missing. One of the three characteristics is missing. One equation is missing. Range, resolution, and sensor sensitivity are defined and correct. Equations are given. 5) No ranges of values discussed, and false readings not discussed. Healthy and unhealthy ranges are provided but with no explanation. False readings mentioned but with limited explanation. Healthy and unhealthy ranges of output are defined. No discussion of errors (false readings), or only very general discussion without details. Healthy and unhealthy ranges of output are defined. The rates of false positives and false negatives are estimated, justified, and ways to reduce them are discussed. 6) Omitted. No discussion of resources and timeline, only general features of team discussed. Some efforts made to translate to senior design, but team members are not discussed in detail. Translation to senior design project (limited time and funds) is discussed. Essential features of the best team of students and faculty + outsiders is discussed. Guidelines Design two complete, novel bioinstrumentation systems using biosignals that are relevant to an important health problem. Novel means not introduced in class and not able to be found on the internet. Each of your solutions should contain the following elements: 1) a labeled block diagram of a full instrument to acquire the signal, process it, and convert to a measurand. 2) block diagram must include a sensor, sensor circuit, processing circuitry (active and/or passive), and output (digital, analog, LED, alarm, etc.). 3) state a reason for each choice of bioinstrumentation element. 4) define signal range, instrument resolution, sensor sensitivity. 5) state guidelines for detecting healthy vs. unhealthy levels of the biosignal. How should a clinician interpret the measurand? 6) what modifications would be needed to make this a senior design project? What is the ideal team (student expertise? Faculty expertise? Experts outside the university)? Answers should be submitted in one clearly labeled word document, with your name and student ID, a title for each instrument, and responses to questions 1) through 6) clearly marked as such. The rubric below will be used to grade: Points given 0 1 2 3 1) No block diagram. Two or more blocks/labels are missing. One block is missing. Block diagram is complete. 2) No labels or only very generic labels of components. Lack of detail. Two errors or omissions prevent strong and clear signal from flowing from biosignal to measurand to output. One error or omission in specific components prevents strong and clear signal from flowing from biosignal to measurand to output. Instrument components work together, signal flows correctly. 3) No rationale provided or only very general and generic information provided. Three or four mistakes in the justification of use of instrumentation components One or two mistakes in the justification of use of instrumentation components. Each instrumentation component is well explained. Element # 4) No equations are provided, and no characteristics are given justifiable numeric values. Two characteristics and/or two equations to describe them are missing. One of the three characteristics is missing. One equation is missing. Range, resolution, and sensor sensitivity are defined and correct. Equations are given. 5) No ranges of values discussed, and false readings not discussed. Healthy and unhealthy ranges are provided but with no explanation. False readings mentioned but with limited explanation. Healthy and unhealthy ranges of output are defined. No discussion of errors (false readings), or only very general discussion without details. Healthy and unhealthy ranges of output are defined. The rates of false positives and false negatives are estimated, justified, and ways to reduce them are discussed. 6) Omitted. No discussion of resources and timeline, only general features of team discussed. Some efforts made to translate to senior design, but team members are not discussed in detail. Translation to senior design project (limited time and funds) is discussed. Essential features of the best team of students and faculty + outsiders is discussed. Bioinstrumentation— Sensors and Actuators Definitions Transducer: a device which accepts energy of one form as an input, converts it to another form and transmits it as an output Sensor: accepts physical parameter as input, and converts it to an electrical signal -also called an input transducer -resistive -inductive -capacitive -piezoelectric Actuator: accepts electrical signal as input, and converts it to a mechanical output -output transducer -examples: valves, solenoids, pumps, motors Signal Conditioning—Conceptual Sensor Output (S) Mod Desired Output (D) D = (S-N)/Mod Noise(N) Measurand (M) Measurand (M) Measurand (M) Measurand (M) The Wheatstone Bridge Vi – i1R4 – i1R3 = 0 (1) KVL Vi – i2R2 – i2R1 = 0 (2) KVL V0 + i1R4 – i2R2 = 0 (3) KVL V0 – i1R3 + i2R1 = 0 (4) KVL i2 = (V0 + i1R4)/R2 (5) (3) rearranged Vi/(R1 + R2) = (V0 + i1R4)/R2 (6) (2) rearr. subst. into (5) Vi/(R1 + R2) = V0 /R2+ Vi R4/R2(R3 + R4) (7) (1) rearr. subst. into (6) V0 = Vi [R2/(R1 + R2) – R4/(R3 + R4)] (8) (R2 times (7), rearr.) R1/R2 = R3/R4 (9) (V0 = 0) The Resistive Temperature Detector • • • • Low sensitivity (dR/dT) High thermal inertia Susceptible to mechanical damage Resistance in the leads creates error WIRE RTD For a Platinum RTD: α1 = 3.96 x 10-3 (°C-1) α2 = -5.85 x 10-7 (°C-2) Range = 0 – 200 °C Ro = 100 Ω housing leads connectors glass Pt ceramic What is the sensitivity at 0 and 200°C? What is the resistance at 100 and 200°C? Use: RT = Ro (1 + α1T + α2T2 + …) Derive dR/dT THIN FILM RTD Resistive Temperature Detector: Solution An RTD is nonlinear… RT (Ω) RT (Ω) 1000 …but approx. linear over usable range 500 200 100 0 0 0 2000 4000 0 dRT/dT T (°C) 0.3961 0.396 0.3959 0.3958 0.3957 100 T (°C) What is the sensitivity at 0 and 200°C? What is the resistance at 100 and 200°C? 0 100 200 T (°C) dR/dT(0) = 0.396 Ω/°C dR/dT(200) = 0.3957 Ω/°C R(100) = 139 Ω R(200) = 177 Ω 200 RTD Instrumentation Circuit THREE LEAD COMPENSATION R Vi R • Rt is low, so contact resistance from the wires can interfere with the measurement. • Therefore, compensation with three leads introduced into the bridge circuit • At balance, L3 current = 0, and voltage drops through L1 and L2 are equal. • (R+L2)/(Rt+L1) = constant • The Sensitivity is ~ 1 mV/°C V0 = Vi [(R+L2)/(R+Rt+L1+L2) – R/(R + R)] R Vi R Assume L1 = L2 = L, L3 = 0 V0 = Vi [(R+L)/(2L+R+Rt) – R/(R + R)] V0 = Vi [(R+L)/(2L+R+Rt) – 1/2] (voltage dividing principle across the left side) (rearrange terms) Rt = R[(Vi – 2V0)/(Vi + 2V0)] – L [(4V0)/(Vi + 2V0)] Error term due to lead resistance -accounted for in the calibration Thermistors • • • • • • A thermally-sensitive semiconductor resistor Small size Low thermal inertia Nonlinear -30 to 200 °C Sensitivity of ~4%/°C RT = A exp(B/T) Derive dR/dT 1) Raise T 2) Increases # of active charge carriers by promotion into the conduction band 3) Conduction band ~ current 4) Higher current ~ lower resistance N-type: Fe2O3 doped with Ti P-type: NiO doped with Li Thermistor R vs. T and Sensitivity RT (Ω) 600 400 200 0 0 100 T (°C) 200 T (K) 300 -16030 -16040 dRT/dT -100 -16050 -16060 -16070 0 200 400 600 Thermistor Instrumentation Circuit Amplified voltage divider Bridge circuit See worked out example of a thermistor used in a bridge circuit The thermistor is nonlinear because the resistance-temperature curve is a decaying exponential Thermocouple • • • • • • Passive device that generates a voltage difference that varies with temperature Very small, inexpensive, rugged Low thermal inertia High sensitivity (5-80 µV/°C) Reference point is required nonlinear Contact Potential: When two dissimilar metals contact, a contact potential develops and drives electric charge from one metal to the other 𝛻V = -S(T)𝛻T S = Seebeck coefficient Seebeck or Thermoelectric Effect: Current flowing in a loop consisting of two metals, caused by temperature differences between the two junctions. Thermocouple Instrumentation Circuit • The reference junction could be placed in an ice-bath • This is known as a cold junction thermocouple 𝑇 • V= ‫𝑙𝑎𝑡𝑒𝑚𝑆(𝑒𝑠𝑛𝑒𝑠 𝑇׬‬1 (𝑇) − 𝑆𝑚𝑒𝑡𝑎𝑙2 (𝑇)) 𝑑𝑇hard to solve for 𝑟𝑒𝑓 each temperature • V = E(Tsense) – E(Tref)evaluate characteristic functions E, at two temperatures. • E(Tsense) = V + E(Tref)V is measured, Tref is set, so E(Tref) is known. Tsense is found from a look-up table of E(Tsense) • The above circuit is for a reference junction compensated thermocouple: E’s are voltages, Tref is reference temperature, Tsense is the temperature to measure Thermocouples are nonlinear, but can be approximately linear over a restricted range RTD vs. Thermistor vs. Thermocouple RTD Thermistor Thermocouple Material Metal (Pt) Ceramic metal oxide Dissimilar metal (Ni, Chromium, Rhodium, Constantan, Iron) Useful range 0-200 °C -50-130 °C -270-1260 °C Sensing principle ρ(T), so R(T) Charge carriers (T) -Ohm’s Law Thermoelectric (Seebeck) effect sensitivity quasilinear (quadratic) Nonlinear (exponential) ~linear (polynomial) thermistor R RTD TC T V Thermocouples are very versatile Bioinstrumentation— Sensors and Actuators Sensors and Actuators— Summary of Part 1 • Sensors behavior is governed by the ruling equations describing the physical principle of measurement • Sensors may be linear (y=Ax) or nonlinear (y = b0 + Ax + b1x2 + b2x3+…) • Sensor sensitivity S = Δ electric parameter / Δ physical parameter • This can be local at a specific input value x, and is the partial derivative of the electrical parameter with respect to x • DON’T confuse the sensitivity of a sensor with that of a sensor + circuit. They are two different numbers. • Sensor nonlinearity δ = nonlinear terms / linear terms • This can be local at a specific x, or global (determine x that maximizes nonlinear factor) Thermistor: RT = A exp(B/T) -semiconductor RTD: RT = Ro (1 + α1T + α2T2) -Pt wire or film -R = ρL/A Thermocouple: V=α(T1-T2) -dissimilar metals -Seebeck effect If the Wheatstone bridge resistor labels get confusing… 1) Draw the currents in the right directions. 2) KCL, 3) Ohm’s law, 4) use voltage divider equation twice to go halfway across the bridge, 5) subtract terms to get V0, 6) normalize by Vi Vi Vi Vi V0 V0 Vi V0 Temperature sensing during cardiac ablation to treat ahrhythmias Live Surgical Demonstration of Cardiac Ablation A surgeon’s testimonial of a multi-thermistor device Resistive Strain Gauges • • • • • A resistive material (fine wire or metal foil) is placed on a flexible backing The backing is placed on the object from which strain is to be measured Forces that stretch or contract the wire along the long axis change the resistance (Figure) A bridge circuit converts ΔR into ΔV, and amplifiers make the signal larger Force→strain→ ΔR→ ΔV (V versus force turns out to be approximately linear) • Note: 1 strain gauge = 1 axis strain and pressure; 2 perpendicular gauges = 2 axes; 4 in a square = 2-axis measurement with temperature compensation • See Class Notes for Derivation of gauge factor and use of the strain gauge in a circuit. Semiconductor Strain Gauges When energy is input to a semiconductor, its resistance eventually decreases because the added energy promotes charge carriers to a level that allows more charge to flow (current) In a metal wire, input of energy causes free charges to collide more often (more kinetic energy), thus eventually causing increased resistance and a barrier to increased current conductor semiconductor I I V V -V = reverse bias Self-heating +V = forward bias • High gauge factor K • Piezoresistive effect • Nonlinear S Semiconductor Transducers—General Semiconductor: has electrical properties in between conductors and insulators -silicon -gallium arsenide -germanium -cadmium sulphide n-type semiconductors: doped with impurities to have an excess of electrons p-type semiconductors: doped to have reduced electrons I=I0[exp(qV/kT)-1] I V I0 is the leak current A Semiconductor Pressure Sensor— Designed by Biomedical Engineers Semiconductors are not flexible—hard to make them Strain Gauges are Diverse! Additional Uses of Strain Gauges • Flow monitoring in medical pumps • Insulin • Medical weighing • Biomechanics research • Medical imaging instruments Capacitive Transducers C = ε0εairA/x (1) definition of capacitance dC = -(ε0εairA/x2 )dx (2) d/dx of (1) dC/C = -dx/x (3) plug (1) into (2) Capacitive Pressure Transducers De Sauty Bridge: 2 resistors + 2 capacitors Vi = V0[Ct/(Ct+C1) – R1/(R1+R2)] Ct/C1 = R2/R1 (at balance, V0=0) Piezoelectric Transducers The Piezoelectric effect: certain crystals, when deformed, generate an electrical charge -quartz -lithium sulfate – barium titanate • Frequency response depends on the RC time constant Pressure and Vibration Sensing V = SV*P*D SV is the voltage sensitivity (V*m/N) P is pressure (N/m2) D = transducer thickness – + [charge/force] Compare to V = SV*P*D SV = k/ε0εr Photodiodes Photodiodes types are application-specific Range: 1 pW/cm2 – 100 mW/cm2 Uses: spectroscopy, imaging (near infrared, visible, near UV), position sensing, laser profilometry, pulse oximetry. Photodiode Types: P/N PIN avalanche Photodiodes are semiconductor junctions bandgap Conduction band: +1.12 eV Charge carriers~T Charge carriers~ hν100 Bioinstrumentation— Op-Amps and Integrated Circuits A timer circuit is an integrated circuit that controls square wave duty cycle +5V 555 1 Ground Vcc 8 2 Trigger Discharge 7 3 Output Threshold 6 Control 5 4 Reset RA RB 8 Ra 7 3 Rb (a) Th = -ln(0.5)(R +Rln 𝑇𝐻 = a− b)C 0.5 𝑅𝐴 + 𝑅𝐵 𝐶 4 6 2 C 5 1 C 5V 0V (b) 𝑇𝐿 =T− ln 0.5 𝑅𝐵 𝐶 l = -ln(0.5)RbC (c) Duty cycle=DC=TH/(TH+TL) Figure 2.41 The 555 timer (a) Pinout for the 555 timer IC. (b) A popular circuit that utilizes a 555 timer and four external components creates a square wave with duty cycle > 50%. (c) The output from the 555 timer circuit shown in (b). 555 Timer Integrated Circuit Uses • Timing • Pulse Generation • Oscillator • Pulse width modulation • Frequency divider • Pulse generator • Logic clock • Tone generation • Simple ADC • Temperature sensing (if connected to thermistor) Composition • 25 transistors • 2 diodes • 15 resistors • Silicon chip Pins • 1 GND = ground • 2 TRIG = OUT pin goes high and timing interval starts when input falls below 1/3 Vcc • 3 OUT = output voltage signal • 4 RESET = resets the timing interval when switched transiently to ground • 5 CTRL = control access to the internal voltage divider (2/3 Vcc by default) • 6 THR = the timing interval ends when the voltage at THR is greater than 2/3 Vcc/CTRL • 7 DIS = open collector output from a transistor, may discharge a capacitor between intervals • 8 VCC = positive supply voltage Digital to Analog Converter: (3-bit) Input versus (voltage) Output 7/8 Vref Analog output 6/8 Vref 5/8 Vref 4/8 Vref 3/8 Vref resolution = 2/8 Vref 1 2 1/8 Vref n Vref 0V 000 001 010 011 100 101 110 111 Figure 2.42 The ideal static behavior of a 3-bit DAC. For each digital string, there is a unique analog output. Digital to Analog Converter— Circuit 1. The digital input is b2, b1, b0 2. Digital input controls switches through the 3-8 decoder 3. Switches control a voltage divider such that voltage is divided by: 4. Resolution = Vref/2n n is the bit-depth (3 in this example—the 3 bits are b2, b1, b0 b2 Vref R R R b1 b0 3-to 8 decoder 7 6 5 4 3 2 1 0 7/8 Vref 6/8 Vref Vo R R R 1/8 Vref 0 R R Figure 2.43 A 3-bit voltage scaling DAC converter. “Switch” closure vs. Vo of a DAC 0 X X X X O O O O 1 O O O O X O O O 2 O O O O O X O O 3 O O O O O O X O 4 O O O X X X X O 5 O O X O O O O O 6 O X O O O O O O 7 X O O O O O O O Vo 7/8 6/8 5/8 4/8 3/8 2/8 1/8 0 -0 is closest to ground; 7 is closest to Vref, as in slide 6 -Vo is a fraction of Vref Analog to Digital Converter: (Voltage) Input versus (3-bit) Output 111 110 Digital output 101 100 011 010 001 000 0/8 1/8 2/8 3/8 4/8 5/8 6/8 7/8 Vref Figure 2.44 Converting characteristic of 3-bit ADC converter. Analog to Digital Converter: Circuit Example Comparator: Compares two voltages, produces positive or negative square wave Vin 2. 0.5Vref Either < or > Than Vin Clock Clock + _ Digital control logic 3. “1” decrement “0” increment 1. V=0.5Vref Digital output DAC converter Figure 2.45 Block diagram of a typical successive approximation ADC. 1. 2. 3. 4. The loop starts with V = 0.5Vref (this is binary 100) V is compared to Vin If >, then the first bit = 1. If
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