Signal Conditioning PDF

Chapter 2.3: Signal Conditioning CHAPTER OUTLINE WHEATSTONE BRIDGE CURRENT-TO-VOLTAGE CONVERTER COLD-JUNCTION COMPENSATION //Chapter 2.3 Signal Conditioning 2 At th...

Chapter 2.3: Signal Conditioning CHAPTER OUTLINE WHEATSTONE BRIDGE CURRENT-TO-VOLTAGE CONVERTER COLD-JUNCTION COMPENSATION //Chapter 2.3 Signal Conditioning 2 At the end of the chapter, the students are expected to: ▪ Recognize the need for signal Chapter conditioning; ▪ Determine the appropriate Objectives signal conditioning mechanism; and ▪ Use “black-box” circuits in performing signal conditioning. //Chapter 2.3 Signal Conditioning 3 Signal Conditioning Prepares signals for accurate data acquisition in ABE. Ensures reliable measurements of biological and environmental data. Includes amplification, filtering, and noise reduction. //Chapter 2.3 Signal Conditioning 4 Electrical output sensing devices advantages over mechanical devices 1. Ease of transmitting the signal from measurement point to the data collection point 2. Ease of amplifying, filtering, or otherwise modifying the signal 3. Ease of recording the signal //Chapter 2.3 Signal Conditioning 5 Why perform Signal Conditioning? 1. Enhances measurement precision. 2. Filters out unwanted interference. 3. Ensures data reliability and consistency. https://terpconnect.umd.edu/~toh /spectrum/SmoothingWhiteNoise. png //Chapter 2.3 Signal Conditioning 6 Why perform Signal Conditioning? https://ez.analog.com/resized- image/__size/1398x568/__key/communityserver-blogs-components- weblogfiles/00-00-00-03-16/5736.DNL_5F00_INL.png //Chapter 2.3 Signal Conditioning 7 Signal Amplification Signals: millivolt range are common, few in microvolts. Difficulty: Losses over wires of great length and many systems require 1 to 10 V input. Solution: Amplifier Purpose: Boosts weak signals to usable levels. Types: Includes voltage, The low-voltage signal, Vi, is current, and power amplified to a higher voltage, amplification. Vo. //Chapter 2.3 Signal Conditioning 8 Recall: Signal Amplification Gain is the factor by which a device, like an amplifier, increases the strength of an input signal. Common instrumentation amplifiers gain: 1 to 1000 It is often expressed as a ratio or in decibels (dB) and defined as: 𝑉𝑜 𝐺𝑑𝐵 = 20 log10 𝐺 𝑤ℎ𝑒𝑟𝑒 𝐺 = 20 log10 𝑉𝑖 Example: G =10 would have a decibel gain (GdB) of 20 dB while G =1000 would have a Gdb of 60 dB. If a signal is attenuated the decibel gain will have a negative value. //Chapter 2.3 Signal Conditioning 9 Recall: Signal Amplification (cont.) Amplifiers are designed to operate effectively within specific frequency bands (e.g., audio, RF). The range of frequencies an amplifier can handle without significant loss of gain or distortion. Application-Specific: Different amplifiers target different ranges, such as audio amplifiers (20 Hz - 20 kHz) or RF amplifiers (MHz to GHz). Always use the op-amp at its operating bandwidth //Chapter 2.3 Signal Conditioning 10 Recall: Signal Amplification (cont.) Phase angle is the time shift between input and output signals in an amplifier, often caused by circuit design and signal frequency. If the voltage input signal to the amplifier is in the form of a sine wave and expressed as Vi(t)= Vmi sin (2πft) where, f is the frequency and Vmi is the amplitude of the input sine wave. The output signal will be Vo(t) = GVmi( 2πft +φ ) where, φ is called the phase angle. //Chapter 2.3 Signal Conditioning 11 Wheatstone Bridge A circuit with four resistive arms, used to measure unknown R1 R3 resistance accurately. a b Balances voltages in two circuit DC Vdc + V0 - branches, detecting small changes in resistance. R2 R4 Applications: Common in load cells and pressure sensors to measure weight, force, or pressure Converts resistance changes from in agricultural systems. sensors (like strain gauges) into measurable voltage changes. //Chapter 2.3 Signal Conditioning 12 Wheatstone Bridge (cont.) V0 = Va − Vb  R2  R1 R3 Va = Vdc    R1 + R2  a V0 b DC Vdc + -  R4  R2 Vb = Vdc   R4 R  3 + R4   R2 R4  A Wheatstone bridge itself does not V0 = Vdc  −  amplify a signal; rather, it converts small  R1 + R2 R3 + R4  resistance changes into a measurable voltage difference (𝑉𝑜 ). //Chapter 2.3 Signal Conditioning 13 Wheatstone Bridge (cont.)  R2 R4  V0 = Vdc  −  R1 R3 R  1 + R 2 R 3 + R4 V0 R2 R4 a b = − DC Vdc + V0 - Vdc R1 + R2 R3 + R4 R2 R4  V0 R2  ( R3 + R4 )  −  = − R4  Vdc R1 + R2    Solving for R3   1 R3 = R4  − 1  R V0    2 −     R1 + R2 Vdc   //Chapter 2.3 Signal Conditioning 14 Strain Gauge The most common application of Wheatstone bridge is the conversion of the change in resistance of a strain gauge into voltage. In the figure below, recall that: Definition of strain,  Force Force L L L = L //Chapter 2.3 Signal Conditioning 15 Strain Gauge (cont.) Recall: Wire total resistance (R) is a function of length (l), cross-sectional area (A) and material resistivity () l = length Resistance R = l A Under axial strain, l increases and A decreases so the A resistance R increases //Chapter 2.3 Signal Conditioning 16 Strain Gauge (cont.) Gauge factor, GF R R0 R R0 GF = = L L  R1 R3 R3 = R0 + R = R0 (1 + R R 0 ) a b Vdc V0 R3 = R0 (1 + GF  ) DC + - R2 R4 //Chapter 2.3 Signal Conditioning 17 Strain Gauge (cont.)  R2 R4  V0 = Vdc  −  R1 R3 R  1 + R2 R3 + R4 a b DC Vdc + V0 -  R2 R4  R2 R4 V0 = Vdc  −   R1 + R2 R0 (1 + GF  ) + R4  R3 = R0 (1 + GF  ) Solving for      R4  1  1 = −1 −  GF R0  R2 V0   GF  −     R1 + R2 Vdc   //Chapter 2.3 Signal Conditioning 18 Arduino-Based Load Cell Using an HX711 amplifier module, Arduino Uno, 16x02 LCD and a Load Cell, one can easily make a weighing machine. //Chapter 2.3 Signal Conditioning 19 I2V I2V (Current-to-Voltage Conversion) converts a current signal into a proportional voltage signal, typically using an operational amplifier (op- amp) Applications: Ideal for sensors https://www.hackatronic.com/wp- like photodiodes and biosensors content/uploads/2020/10/I-to-V- converter-.jpg in ABE. Benefit: Increases sensitivity and simplifies data processing. //Chapter 2.3 Signal Conditioning 20 VCIS (Transresistance Amplifier) VCIS means Voltage Controlled Current Source and is an amplifier used for I2V Most signal processors, such as amplifiers, have headers that only utilize VOLTAGE SIGNAL //Chapter 2.3 Signal Conditioning 21 VCIS (cont.) Transresistance Amplifiers are used for low-power applications to produce an output voltage proportional to the input current. VCIS are commonly used for photodiodes and phototransistors (used in the production of solar power)which are commonly modeled as current sources //Chapter 2.3 Signal Conditioning 22 VCIS Calculations //Chapter 2.3 Signal Conditioning 23 Thermocouple Thermocouples are a popular type of temperature measurement device. A thermocouple is a cable of two wires made from two dissimilar conductors (usually alloys) that are soldered or welded together at one end All thermocouple operate thru the thermoelectric or Seebeck effect //Chapter 2.3 Signal Conditioning 24 Seebeck Effect Seebeeck Effect is when a conductor generates a voltage when it is subjected to a temperature gradient Nickel-Chromium The relationship + Alloy between The voltage difference of the temperature two dissimilar metals can be difference and voltage varies measured and related to the VS = SΔT with materials corresponding temperature gradient - Copper-Nickel Alloy //Chapter 2.3 Signal Conditioning 25 Measuring Temperature To measure temperature using a thermocouple, one can’t just connect the thermocouple to a voltmeter since the voltmeter will only measure voltage due to temperature difference between the primary junction (hot) and the junction where the voltage is being measured (cold junction) To determine the You need to know absolute the temperature temperature at at the cold the hot junction… junction! How can we determine the temperature at the SOURCE: http://www.pcbheaven.com/wikipages/images/thermocouples_1271330366.png reference junction? //Chapter 2.3 Signal Conditioning 26 Cold-Junction The cold junctions are formed since they connect to some form of terminal block that transitions from the thermocouple alloys into the traces used on the printed circuit board (PCB), which is usually copper. Thermocouples do not measure an absolute temperature and only measure the temperature difference between hot and cold junctions. Vmeasured = VHot – Vcold //Chapter 2.3 Signal Conditioning 27 Cold-Junction Compensation Thermocouples measure the voltage difference between two points To know the absolute temperature at the hot junction, one must know the temperature at the Cold junction VHot = Vmeasured + Vcold //Chapter 2.3 Signal Conditioning 28 Cold-Junction Compensation (cont.) In many applications, the temperature at the cold junctions are measured using a diode, thermistor, or RTD. As with any form of cold-junction compensation, two conditions must be met to achieve accurate thermocouple measurements: SOURCE: http://www.industrial- 1. Junctions B and C must remain electronics.com/DAQ/images/10_13.jpg isothermal or be held at the same temperature. 2. The isothermal temperature of junctions B and C must be accurately measured //Chapter 2.3 Signal Conditioning 29 Cold-Junction Compensation (cont.) The thermocouple temperature measurements involve two important factors: 1. measurement of the thermal gradient from the cold junction available from the thermocouple based on the Seebeck effect 2. The actual cold junction measurement. The thermocouple output coefficient (S) is a few µV/ºC and VS = SΔT is highly non-linear //Chapter 2.3 Signal Conditioning 30 Cold-Junction Compensation (cont.) Thermocouple measurements depend on the temperature difference detected by the thermocouple (Seebeck effect) and (2) accurately measuring the cold junction temperature. Thermocouples produce a small output of only a few microvolts per degree Celsius (µV/°C). For accurate readings, the system must detect very small voltage changes in microvolts. A low-noise amplifier can boost the signal, improving measurement accuracy. //Chapter 2.3 Signal Conditioning 31 Thermocouple Temperature Conversion Equation The thermocouple temperature conversion equation relates the voltage output of a thermocouple to temperature using a polynomial or piecewise function, depending on the thermocouple type: T = a0 + a1V + a2V2 + …. + anVn //Chapter 2.3 Signal Conditioning 32 Using Look-up Table for Type T Thermocouple Voltage difference of the hot and cold junctions: VD = 3.409 mV What is the temperature of the hot junction if the cold junction is at 22 oC? At 22 oC, the reference junction voltage is 0.870 mV The hot junction voltage is therefore 3.409 mV + 0.870 mV = 4.279 mV The temperature at the hot junction is therefore 100 oC //Chapter 2.3 Signal Conditioning 33 Acquiring Data Using Thermocouple //Chapter 2.3 Signal Conditioning 34 Arduino-Based Temperature Measurement Using Thermocouple Arduino and ThermoCouple K MAX6675 //Chapter 2.3 Signal Conditioning 35 Comparison Among Temperature Sensors > //Chapter 2.3 Signal Conditioning 36 Moving Average Smooths out signal fluctuations by averaging recent data points. Calculates the average over a sliding window of data points. Noise Reduction: Reduces random noise, making the signal cleaner. Window Size Effect: Larger window = more smoothing smaller window = more responsiveness. //Chapter 2.3 Signal Conditioning 37 Moving Average The formula for the moving average of a signal 𝑥 over a window of 𝑁 data points is: 𝑡 1 MA𝑡 = ෍ 𝑥𝑖 𝑁 𝑖=𝑡−𝑁+1 MAt is the moving average at time t, N is the window size (number of data points to average), xi is the value of the signal at time i. //Chapter 2.3 Signal Conditioning 38 Arduino MA function float calculateMovingAverage(float newReading) { // Update total by subtracting the oldest reading and adding the new one total -= readings[index]; readings[index] = newReading; total += readings[index]; // Update the index and wrap around if necessary index = (index + 1) % windowSize; // Calculate and return the moving average return total / windowSize;} //Chapter 2.3 Signal Conditioning 39 Single-Channel Kalman Filter Provides a powerful way to filter out noise and estimate the true state of a single variable (e.g., temperature, position) over time. Combines predictions from a mathematical model with actual noisy measurements, adjusting the estimate based on the uncertainty of each. Delivers smooth and accurate signal estimation, even with noisy input, by continuously updating based on new data. Ideal for real-time tracking and measurement in ABE, like smoothing soil moisture readings or tracking the position of agricultural equipment. //Chapter 2.3 Signal Conditioning 40 Single-Channel Kalman Filter https://user-images.githubusercontent.com/7277887/49056311-4699b880-f1c9-11e8- 9150-a6d52251f731.png //Chapter 2.3 Signal Conditioning 41 Arduino SKF function float kalmanFilter(float measurement) { // Prediction step (in this simplified case, no control input is considered) // Estimate remains the same, so we skip state transition here // Update error covariance for prediction step errorCovariance = errorCovariance + processNoise; // Calculate Kalman Gain kalmanGain = errorCovariance / (errorCovariance + measurementNoise); // Update estimate with the measurement estimate = estimate + kalmanGain * (measurement - estimate); // Update error covariance errorCovariance = (1 - kalmanGain) * errorCovariance; return estimate; // Return the filtered (estimated) value} //Chapter 2.3 Signal Conditioning 42 MA vs SKF Window size: 3, fast response but does not effectively filter noise //Chapter 2.3 Signal Conditioning 43 MA vs SKF Window size: 10, smoother but lags //Chapter 2.3 Signal Conditioning 44 References Lecture 4 Measurement Systems with Electrical Signals (Chapter 3). MECH 373 Instrumentation and Measurements Lecture Notes. My Concordia Portal. ENGR240 Lecture Slides. Wheatstone Bridge Texas Instrument. Cold-Junction Compensation Temperature Measurements. Engineering 80 Lecture Notes. Spring 2015. Harvey Mudd College.