Chapter 3 Outlines Basic Sensor Technology Sensor Systems Application Considerations Measurement Issues and Criteria Sensor Signal Conditioning Conditioning Bridge Circuits DAC for Signal Conditioning ADC for Signal Conditioning Signal Conditioning High Impedance Sensors 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 1 Basic Sensor Technology • What are Sensors? – A device which provides a usable output in response to a specified measurand Input Signal Output Signal Sensor – Convert a Signal or Stimulus (Representing a Physical Property) into an Electrical Output – Sensing technology uses sensors to acquire information by detecting the physical, chemical, or biological property quantities and convert them into readable signal • They embedded in our bodies(eg: eyes, ear…), automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications • Without the use of sensors, there would be no automation !! DMU, ECEG-5323: Instrumentation 06/21/2022 Engineering 2 Sensor Technology - Terminology • Sensors can improve the world through diagnostics in medical applications improved performance of energy sources like fuel cells and batteries and solar power improved health, safety and security for people sensors for exploring space improved environmental monitoring. • The seed technologies are now being developed for long-term vision that includes intelligent systems that are self-monitoring, self-correcting and repairing, and self-modifying • The ability for a system to see (photonic technology), feel (physical measurements), smell (electronic noses), hear (ultrasonics), think/communicate (smart electronics and wireless), and move (sensors integrated with actuators), is progressing rapidly and suggests an exciting future for sensors. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 3 Classification of Sensors o 1. Active and Passive sensors Active sensor: a sensor that requires external power to operate. Passive sensor: generates its own electric signal and does not require a power source 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 4 Classification of Sensors(…) o 2. Contact and noncontact sensors Contact sensor: a sensor that requires physical contact with the stimulus. Examples: strain gauges, most temperature sensors Non-contact sensor: requires no physical contact. Examples: most optical and magnetic sensors, infrared thermometers, etc. o 3. Absolute and relative sensors Absolute sensor: a sensor that reacts to a stimulus on an absolute scale. Examples: Thermistors, strain gauges, etc., Relative sensor: the stimulus is sensed relative to a fixed or variable reference. Examples: Thermocouple measures the temperature difference, pressure is often measured relative to atmospheric pressure o 4. Other schemes Classification by broad area of detection • Chemical sensors • Optical • Mechanical • Biological, etc… 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 5 Classification of Sensors(…) o 4. Other schemes(…) Classification by physical law • Photoelectric • Magnetoelectric • Thermoelectric • Photomagnetic, etc… Classification by area of happlication • Consumer products • Healthy • Transportation • Scientific, etc… 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 6 Sensor Systems • Deals with sensors and associated signal conditioning circuits • Sensors and their associated circuits are used to measure various physical properties such as temperature, force, pressure, flow, position, light intensity, etc • Their outputs must be properly conditioned before further analog or digital processing can occur. • Sensors do not operate by themselves. • Composition: signal conditioners, various analog or digital signal processing circuits. • Generally part of a larger system. • The system could be a measurement system, data acquisition system, or process control system, etc… 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 7 Sensor Systems(…) • Most sensor outputs are nonlinear with respect to the stimulus, and their outputs must be linearized in order to yield correct measurements. • Analog techniques may be used to perform this function. • The application of sensors in a typical process control system is shown in Figure below. • Assume the physical property to be controlled is the temperature. • The output of the temperature sensor is conditioned and then digitized by an ADC. • The microcontroller determines if the temperature is above or below the desired value, and • outputs a digital word to the digital-to-analog converter (DAC). • The DAC output is conditioned and drives the actuator, in this case a heater. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 8 Sensor Systems(…) • Notice that the interface between the control center and • The remote process is via the industry-standard 4–20mA loop. Figure: Typical industrial process control 06/21/2022 loop DMU, ECEG-5323: Instrumentation Engineering 9 Sensor Systems(…) • What are the selection criteria for sensors? o Accuracy-should be high o Stability-maintain under abnormal conditions o Environmental conditions -usually has limits for temperature & humidity o Range-Measurement limit of sensor o Calibration-essential for most of the measuring devices as the readings changes with time o Resolution-smallest increment detected by the sensor o Cost-should be low o Power consumption -low o Repeatability -the reading that varies is repeatedly measured under the same environment 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 10 Sensor Performance Characteristics • General characteristics of sensors are two, namely • Static characteristics The static characteristic of a sensor refers to the relationship between the output and the input of a sensor with respect to a static input signal. Because the input and output are independent of time at this time Examples: Accuracy, Drift, Dead Zone ,Static Error, Sensitivity, Reproducibility, Static correction, Scale range, Scale span, Noise, Dead Time, Hysteresis, Linearity. • Dynamic Characteristics dynamic characteristics of the sensor refer to the characteristics of the sensor's output when the input changes. Examples: Speed of response, Measuring lag , Fidelity and Dynamic error 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 11 Application Considerations • The highest quality, most up-to-date, most accurately calibrated and most carefully selected sensor can still give totally erroneous data if it is not correctly applied. • The instrumentation engineer must carefully analyze every aspect of the test to be performed, • Test article, • Environmental conditions • Analytical predictions (if available). • In most cases, this process will indicate a clear choice of acceptable system components What is the most difficult tasks facing an instrumentation engineer? Ans: The selection of the proper measuring system. Why do we need to select the proper measuring system? Ans: obtain accurate, reliable data on each and every measurement. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 12 Application Considerations(…) Sensor Characteristics • The prospective user is generally forced to make a selection based on the characteristics available on the product data sheet • Many performance characteristics are shown on a typical data sheet • Many manufacturers feel that the data sheet should provide as much information as possible. System Characteristics:• The sensor and signal conditioners must be selected to work together as a system • The system must be selected to perform well in the intended applications • Overall system accuracy is usually affected most by sensor characteristics such as environmental effects and dynamic chxs. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 13 Sensor Application Considerations(…) • Sensing Range • • • • Photoelectric controls: 0 to 1000 ft range Ultrasonic sensors: 0 to 11 ft range Inductive proximity sensors: 0 to 65 mm range Capacitive proximity sensors: 0 to 50 mm range • Target Material • • • • Inductive proximity: metal type will determines sensing range correction factor Capacitive proximity : Material type will determines sensing range dielectric constant Photoelectric: target color will determine sensing range excess gain Ultrasonic: Target porosity or density will determine sensing range • Environment • • • • 06/21/2022 Sealing: IP rating-IP65 dust tight, IP67 temp immersion, IP68 NEMA ratings: NEMA 1, 2, 2, 4, 12, 13, 6P 1200psi Rated Weld Field Immune: 20,000 Amps/ Sparks and Splatter Resistant Temperature ratings: Above 158 degree f, use fiber optic or high temp Hazardous Locations: Intrinsically safe/ with Zener diodes barriers DMU, ECEG-5323: Instrumentation Engineering 14 Sensor Application Considerations(…) • Mounting Side to side Face to face • Control Interface 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 15 Sensor Application Considerations(…) • Control Interface(…) 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 16 Sensor Application Considerations(…) • Response time AC applications that requires response time of less than 50 milliseconds(20Hz switching frequency) DC applications that require response time of less than 10 milliseconds(75 Hz switching frequency) may require special attention • Wiring Options Pre-Corded, or Allen-Bradley Cordsets 889N Mini • 3 and 5 Pin AC • 4 Pin DC 889 D DC micro • 4 Pin DC 889R AC micro • 3 Pin AC/DC 889P Pico • 3 & 4 Pin DC 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering Others • Accuracy • Shock and Vibration • Surrounding Temperature Conditions • Resistance to corrosion • Output repeatability • Lifespan 17 Instrument selection • Selecting a sensor/signal conditioner system for highly accurate measurements requires very skillful and careful measurement engineering. • Must be considered: All environmental, Mechanical Measurement conditions Installation must be carefully planned and carried out • The following guidelines are offered as an aid to selecting and installing measurement systems for the best possible accuracy. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 18 Instrument selection(…) o Sensor The most important element in a measurement system is the sensor. If the data is distorted or corrupted by the sensor, there is often little that can be done to correct it. o Will the sensor operate satisfactorily in the measurement environment? Check: Temperature Range Maximum Shock and Vibration Humidity Pressure Acoustic Level Corrosive Gases Magnetic and RF Fields Nuclear Radiation Salt Spray Transient Temperatures Strain in the Mounting Surface o Will the sensor characteristics provide the desired data accuracy? Check: Amplitude Linearity and Hysteresis 06/21/2022 Sensitivity Frequency Response Internal Capacitance Transverse Sensitivity Temperature Deviation Weight and size Calibration Accuracy etc. DMU, ECEG-5323: Instrumentation Engineering 19 Instrument selection(…) o Is the proper mounting being used for this application? Check: Is Insulating Stud Required? Ground Loops Calibration Simulation Is Adhesive Mounting Required? Thread Size, Depth and Class o Cable Will the cable operate satisfactorily in the measurement environment? Will the cable characteristics provide the desired data accuracy? o power supply Will the power supply operate satisfactorily in the measurement environment? Is this the proper power supply for the application? Will the power supply characteristics provide the desired data accuracy? 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 20 Instrument selection(…) o Amplifier The amplifier must provide gain, impedance matching, output drive current, and other signal processing Will the amplifier operate satisfactorily in the measurement environment? Is this the proper amplifier for the application? Will the amplifier characteristics provide the desired data accuracy? o Data acquisition o The followings are essential when choosing correct data acquisition hardware • Signal/signal type compatibility • Maximum sampling rate required • Minimum measurement resolution required • Does my DAQ system need resolution? Readout This is the display of a measuring system o Installation Even the most carefully and thoughtfully selected and calibrated system can produce bad data if carelessly or ignorantly installed 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 21 Types of Instruments to make Measuring Devices o PMMC type: o Moving iron m/s of dc voltage and current used in dc galvanometer Cannot be used for ac m/s Some errors are caused by temperature type Less expensive variations. Can be used for both dc and ac Reasonably accurate. o Electro-Dynamometer type Can be used for both dc and ac m/s. Free from hysteresis and eddy current errors. o Hot wire type Can be used for both dc and ac Unaffected by stray magnetic fields Readings are independent of frequency and waveform. oElectrostatic type oInduction type oRectifier type oThermocouple type 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 22 Permanent magnet moving coil This permanent magnet moving coil meter movement is the basic movement in most analog (meter with a pointer indicator hand) measuring instruments. It is commonly called d'Arsonval movement This type of meter movement is a current measuring device which is used in the ammeter, voltmeter, and ohmmeter. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 23 Permanent magnet moving coil … Suspension Galvanometer 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 24 Permanent magnet moving coil … o Deflection Toque It has been mentioned that interaction between the induced field and the field produced by the permanent magnet causes a deflecting torque, which results in rotation of the coil. Deflection torque is controlling the deflection and tries to stop the pointer at its final position. But due to inertia, the pointer oscillates around its final position before coming to rest. Hence damping torque is provided to avoid this oscillation and bring the pointer quickly to its final position Thus the damping torque is never greater than the controlling torque. In fact it is the condition of critical damping which is sufficient to enable the pointer rising quickly to its deflected position without overshooting 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 25 Permanent magnet moving coil … 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 26 DC Voltmeter 1. Multiplier Resistor A basic d’Arsonval movement can be converted into dc voltmeter by adding in series resistor multiplier as shown IM = full scale deflection current of the movement (Ifsd) RM = internal resistance of the movement RS = multiplier resistance V = full range voltage of the instrument Current in series; IS = IM DC Voltmeter The multiplier limitscircuit. the current through the movement so as not to exceed the value of the full scale deflection current (Ifsd) EX: A basic D’ Arsonval movement with a full-scale deflection of 50 µA and internal resistance of 500Ω is used as a DC voltmeter. Determine the value of the multiplier resistance needed to measure a voltage range of 0-10V. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 27 DC Voltmeter(…) 2. Multi Range Multiplier Resistor A DC voltmeter can be converted into a multirange voltmeter by connecting a number of resistors (multipliers) in series with the meter movement. A practical multi-range DC voltmeter is shown in Figure DC Multi-range Voltmeter circuit EX: Convert a basic D’ Arsonval movement with an internal resistance of 100Ω and a full scale deflection current of 1mA into a multirange dc voltmeter with voltage ranges of 0-15V and 0- 50V 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 28 DC Voltmeter(…) Example: A basic d’Arsonval movement with internal resistance, Rm=100Ω, and full scale current, Ifsd = 1 mA, is to be converted into a multirange dc voltmeter with voltage range of 0-10V, 0-50V, 0-250V, and 0-500V. The circuit arrangement of Figurer below is to be used for this voltmeter EX: Convert a basic D’ Arsonval movement with an internal resistance of 100Ω and a full scale deflection current of 1mA into a multirange dc voltmeter with voltage ranges of 0-15V and 0- 50V 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 29 DC Ammeter 1. Shunt Resistor The basic movement of a DC ammeter is PMMC galvanometer. Since the coil winding of a basic movement is small and light, it can carry only very small currents. When large currents are to be measured, it is necessary to bypass the major part of the current through a resistance, called a shunt. IM = full scale deflection current of the movement (Ifsd) RM = internal resistance of the movement(coil) RS = resistance of shunt I= full scale current of the ammeter including the shunt Voltages in parallel; Vshunt =Vmovement I S RS = I M RM 06/21/2022 Basic DC Ammeter ckt For each required of full scale meter current we can then solve for the value of the shunt resistance required. DMU, ECEG-5323: Instrumentation Engineering 30 DC Ammeter(…) 2. Ayrton Shunt The current range of the dc ammeter may be further extended by a number of shunts, selected by a range switch,. Such a meter is called a multi range ammeter 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 31 DC Ammeter(…) Example: Design an Ayrton shunt to provide an ammeter with current ranges of 1A, 5A and 10A. A d’Arsonval movement with an interval resistance Rm = 50 Ω and full-scale deflection current of 1mA is used in the configuration of Figure below. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 32 Ohmmeter 1.Series Ohmmeter R1 = Limited Current Resistor R2 = Zero Adjust Resistor Rx = unknown Resistance Rm = Meter Resistance Operation of Series Ohmmeter Individual Series Ohmmeter • When Rx = 0 ( AB terminal short), circuit the current in circuit is maximum and the pointer shown the full reading. Adjust the R2 until the full scale, IM. The pointer at full scale is mark as 0 ohm. • When Rx = infinity (AB terminal open), the current in circuit is 0. The unknown resistance must connect series with basic meter movement. This circuit use to measure higher resistance and the pointer is mark as infinity. When Rx connected; 06/21/2022 The pointer DMU, ECEG-5323: Instrumentation location Engineering 33 Ohmmeter(…) . 2. Shunt Resistor in Series Type Ohmmeter R1 = Limited Current Resistor R2 = Zero Adjust Resistor Rx = unknown Resistance RM = Meter Resistance Individual Shunt resistor in series type ohmmeter circuit the ratio between current reading when Rx is connected and full scale current. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 34 Ohmmeter(…) 3. Parallel (Shunt) Ohmmeter Individual Shunt Ohmmeter circuit. Operation of Shunt Ohmmeter • S1 is using for cut-off the battery (Vin) when not using the circuit. When Rx = 0 ( AB terminal short), no current in circuit and the pointer is mark as 0 ohm. • When Rx = infinity (AB terminal open), the current (I M) in circuit is maximum. Adjust R1 until the meter movement is full scale, and the pointer is mark as ∞Ω (Infinity) The pointer location 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 35 Alternating current indicating instruments •The d'Arsonval movement responds to the average or dc value of the current through the moving coil. •If the movement carries an alternating current with positive and negative half cycles, the driving torque would be in one direction for the positive alternation and other direction for the negative alternation. •If the frequency of the ac is very low, the pointer would swing back and forth around zero point on the meter scale. •At higher frequencies, the inertia of the coil is so great that the pointer cannot follow the rapid reversals of the driving torque and hovers around the zero mark, vibrating slightly. •To measure ac on d'Arsonval movement, we may need: – – – (a) unidirectional torque that does not reverse each half cycle, (b) rectification of the ac, (c) to use the heating effect of the alternating current to produce an indication of its magnitude. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 36 Alternating current indicating instruments(…) • Electrodynamometer Figure 1 shows the electrodynamometer movement. Unlike to the D’Arsonval movement, Electrodynamometer uses the current under measurement to produce the necessary field flux. The fixed coil is splitted into two halves and they are connected in series with moving coil. The movable coil movement is controlled by the springs The expression for the torque developed is the same as the D’Arsonval movement: where T = torque (Nm) B = flux density in the air gap (tesla) A = effective coil area (A = bxh) (m2) I = current in the movable coil (A) N = turns of wire on the coil. Since the flux density (B) depends on the current through the fixed coil, B is also directly proportional to current (I). Hence the torque is proportional to current squared (I2). 06/21/2022 Schematic diagram of an electrodynamometer movement. Eg: wattmeter, Power Measurement in 3 phase 3 wire system, Energy meters (1p/3p) DMU, ECEG-5323: Instrumentation Engineering 37 Alternating current indicating instruments(…) • Rectifier-type Instruments This method is very attractive, because it has a higher sensitivity than either the electrodynamometer or the moving-iron instrument. o Half wave rectifier o Full-wave rectifier 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 38 Alternating current indicating instruments(…) • Moving Iron(coil): these instruments use movable piece of iron placed in the pointer over the scale and hence moving iron instrument Eg: AC voltmeters, Ammeters & Ohmmeter 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 39 Alternating current indicating instruments(…) • Thermo instruments Schematic representation of a basic thermocouple instrument 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 40 Alternating current indicating instruments(…) • • Example: Thermo instrument was designed for voltage measurement with 0-5V, 0- 10V, and 025V ranges. Full scale deflection for the galvanometer is 50μA and internal resistance is 200Ω. The resistance of the heating elements is 200 Ω and current-temperature characteristics of heating element and temperature-potential difference characteristics of thermo instrument are shown in Figure. Find the potential difference across the thermo instrument and range resistors. Ifsd = 50μA Rm = 200Ω R = 200Ω Potential difference across the thermo instrument is: • If Figure c is examined it is found that 10 mV potential difference is resulted from the temperature increase of 200 ˚C. That increase is the direct effect of the current through the heating element. From Figure b 5mA current causes that increase so: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 41 Alternating current indicating instruments(…) • Example(…) Figure: Thermo instrument with 3 ranges 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 42 Alternating current indicating instruments(…) • Electrostatic Voltmeter ‣ Electrometer, is the only instrument that measures voltage directly rather than by the effect of the current it produces. ‣ This instrument has one distinguishing characteristic: ‣ It consumes no power (except during the brief transient Schematic representation of an electrostatic voltmeter. period of initial connection to the circuit) The instantaneous energy stored in the electric field is: • Peak Value Voltmeter All voltmeters covered so far measures the effective value. They are calibrated in rms values of the sinusoidal signal. If the measured signal is not sinusoidal form, the peak value is used for the measurement. Figure in page 44 shows the typical peak value voltmeter. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 43 Alternating current indicating instruments(…) • The instantaneous torque may be found by keeping e constant and permitting the movable plates to undergo a small angular displacement, ∂θ . The developed torque then is: • The average torque over an entire period T of the alternating voltage is Peak value voltmeter with differential amplification • Differential amplification is used to strength the sensitivity decreased due to the nonlinearity of the diode characteristics. The peak value voltmeter can measure the signals up to 1GHz. However leakage capacitors in the long cables decrease the output impedance in the high frequencies 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 44 Exercises 1. 2. 3. Design an ammeter to measure 100mA using a 50mA PMMC meter movement with internal resistance 3kW. State the ohmmeter scale when the current is 0A, ½ FSD, ¼ FSD and FSD An experimental AC voltmeter uses the circuit of Figure below, where PMMC movement has an internal resistance of 50 Ω and requires a DC current of 1mA for full-scale deflection. Assuming ideal diodes (zero forward resistance and infinite reverse resistance), calculate the value of the multiplier Rs to obtain full-scale meter deflection with 10 V ac (rms) applied to the input terminals 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 45 Exercises(…) 4. • Rsh shunt resistor is used in order to draw more current through the diode D1 and Diode D2 prevents the leakage current across D1 in the reverse direction. A meter movement has an internal resistance of 100Ω and requires 1mA dc for full scale deflection. Shunting resistor Rsh, placed across the movement, has a value of 100Ω. Diodes D1 and D2 have an average forward resistance of 400Ω each and are assumed to have infinite resistance in the reverse direction. For a 10V ac range, calculate (a) the value of multiplier Rs, (b) the voltmeter sensitivity on the ac range. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 46 Measurement Issues and Criteria • The requirements of the measurement will determine the selection and application of the sensor. • How then can we quantify the requirements of the measurement? • First, we must consider what it is we want to measure. Such as To, P, Flow. • Second, we must consider the environment of the sensor. • Third, we must consider the requirements for accuracy (uncertainty) of the measurement. • Last, but not least, the user must assure that the whole system is calibrated and traceable to a national standards organization (such as National Institute of Standards and Technology [NIST] in the United States). 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 47 Sensor Signal Conditioning • Typically a sensor cannot be directly connected to the instruments that record, monitor, or process its signal, • Because the signal may be incompatible or may be too weak and/or noisy. • The signal must be conditioned-i.e., cleaned up, amplified, and put into a compatible format. Functions of Signal Conditioner: Signal amplification (OpAmp) Current – voltage change circuits, Filtering (OpAmp, RLC) resistance change circuits (Wheatstone Interfacing with P (ADC) bridge) Protection(Zener & photo isolation) error compensation…. etc. Linearization • The followings are the important aspects of sensor signal conditioning. • Analog-to-Analog Conversion Signal Conditioning • Conditioning Bridge Circuits 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 48 Analog-to-Analog Conversion Signal Conditioning • Current-to-Voltage Converter Example: Find a value R that converts a 4 mA to 20 mA current signal into a 1 – 5 V output Voltage. Vo Vo I in R R I in Input current converted to voltage by R. Since V+ = VVo = Iin∙R 06/21/2022 Vo 1 V @ I in 4 mA R 1V 250 4 mA Check output at 20 mA Vo 250 (20 mA) 5 V DMU, ECEG-5323: Instrumentation Engineering 49 Analog-to-Analog Conversion Signal Conditioning(…) • Voltage-to-Current Converter • V-to-I Converter (Transconductance Amps) V+ = V- =Vin and Iin =0 So Io – IR =0 or Io=IR Ungrounded load V I o I R but V - V Vin R V I o in R Note: RL < R for practical circuit operation. OP AM output voltage determines magnitude of RL for constant current 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 50 Signal Conditioning Example • Convert a 0-5 V dc voltage signal to a 4-20 mA current signal using OP AMP circuits. 19.22mA DC A R6 Vbias 1V R2 470k Vin 5V R1 470k R5 100k R3 470k -12V U1A LM324 R4 100k -12V -12V U1B LM324 U1C LM324 RLoad 100 + 12V + + 12V 12V • Determine the ratio of spans Vin Vin (max) Vin (min) 5V-0V 5V R6 312.5 I0 I 0 max I 0 min 20 mA - 4 mA 16 mA 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 51 Signal Conditioning Example… • Compute the value of Vbias to give the value of minimum output current Check the output What in max value of RL? with Vin = 5 V I 0 min 4 mA Vin I 0 R Vin R Vbias I 0 min R I0 Assume Vsat = 10.5 V Vin Vsat Vin (max) R L (max) R I o max Vmax Vin(max) Vbias V 10.5 V sat Vmax 5 V 1.25 V I o max 0.020 A 20 mA I0 R 312.5 Vin (max) 6.25 V 6.25 V I Vbias (4 mA) (312.5 ) 0 312.5 10.5 V 6.25 V R L (max) 212.5 0.02 A Vbias 1.25 V I 0 0.02 A 20 mA Assumes OP AMP has sufficient current output 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 52 Signal Conditioning-Bridge Circuits Dc bridges (Wheatstone bridges) Used to detect small resistance changes in sensors. Typically used with sensors that measure force, temperature, and pressure. Rs = sensor R R3= variable resistor used to balance bridge When bridge is balanced: Iab = 0 since Va = Vb so I1Rs = I2R3 and I1R2 = I2R4 Rs R 3 R2 R4 Solve for Rs 06/21/2022 Rs R3 R2 R4 DMU, ECEG-5323: Instrumentation Engineering Adjust R3 until Iab = 0 compute Rs 53 Bridge Use Methods • There are two operating modes for a dc bridge: balanced (null) and unbalanced Null Mode - adjust R3 variable resistor until Iab = 0. Need automatic nulling circuit for automatic operation. Unbalanced Mode - insert sensor and null bridge for sensor measurement. When initial value of sensor changes measure difference in voltage. Bridge only balanced at one point 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 54 Unbalanced Bridge Operation • Note the unbalanced bridge shown below. U1 and U2 provide high-Z input. Circuit gain provided by U3 using the following formula. Rs U1 Rf Ra U3 Vo Rf (Vb Va ) Ra Ra Rf U2 Find expression for bridge equation. in terms of the change in sensor resistance Rs. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 55 Unbalanced Bridge Analysis • Bridge Circuit Find the voltage V ba in terms of the resistors R2 Va Vdc Rs R2 R4 Vb Vdc R3 R4 R4 R2 Vb Va Vdc R R R R 3 4 s 2 Rbal R2R3 R4 Normalize Vba by dividing by supply V. Find common denominator Vb Va R4 R2 R4 (Rs R2 ) R2 (R3 R4 ) Vdc (R3 R4 )(Rs R2 ) R3 R4 R s R2 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 56 Unbalanced Bridge Analysis(...) • Expand terms and simplify Vb Va R4Rs R4R2 R2R3 R2R4 R4Rs R2R3 Vdc (R3 R4 )(Rs R2 ) (R3 R4 )(Rs R2 ) From the previous balance equation RbalR4 R2 R3 Substitute these equations into the above relationship Vb Va R4Rs RbalR4 Vdc (R3 R4 )(Rs RbalR4 / R3 ) 2 (R3 R4 )(Rs RbalR4 / R3 ) R3Rs R4Rs RbalR4 RbalR4 / R3 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 57 Unbalanced Bridge Analysis(...) • Combine the past two equations and clear R3 from the denominator. Vb Va R 3 R 4 (R s R bal ) 2 2 Vdc R 3 R s R 3 R 4 R s R 3 R 4 R bal R bal R 4 Now let R4=R3 and simplify further 2 2 Vb Va R (R s R bal ) R 3 (R s R bal ) 23 2 2 2 2 Vdc R 3 (2(R s R bal ) R 3 R s R 3 R s R 3 R bal R bal R 3 2 2 Vb Va R 3 (R s R bal ) R 3 (R s R bal ) 2 2 2 Vdc 2R 3 R s 2R 3 R bal R 3 (2(R s R bal )) Vb Va (R R bal ) s Vdc 2( R s R bal ) 06/21/2022 Desired Relationship DMU, ECEG-5323: Instrumentation Engineering 58 Unbalanced Bridge Analysis(...) • Equation below relates the output voltage change (per volt of supply V) to the change in sensor resistance. Original Bridge Vb Va (R R bal ) s Vdc 2( R s R bal ) When R3=R4 Make Equal Determine the output voltage linearity compared to the sensor resistance change. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 59 Instrumentation Amps with High Impedance Input • Two-stage circuit E2 E1 2 R1 A v1 1 V 2 V1 R2 V0 R4 A v2 E2 E1 R3 V1 U1 U1 and U2 Noninvertin g Hi-Z input R5 = R1 V2 R1 E 1 R4 R4 * R3 R3 R2 U3 R5 U2 R6 E2 R7 Overall circuit gain 06/21/2022 2 R1 A v 1( A v 2 ) 1 R2 U3 = basic differential amp configuration with R3=R6 and R4=R7 2 R1 R4 V0 1 * * V 2 V1 R2 R3 DMU, ECEG-5323: Instrumentation Engineering 60 Unbalanced Bridge Analysis Example • ex 2.) Plot the output voltage over the range of operation at 5 ohm increments for both designs a.) R3=R4 = 1000 ohms b.) R3=120 and R4 = 1000 ohms 3.) Find the zero based linear approximation of the bridge output responses. 4.) Determine the maximum non-linearity for each case 5.) Determine the gain required for the instrumentation amplifier gain if a span of 15 V dc is required. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 61 Unbalanced Bridge Analysis Example(…) xample • eSolution (Part 1a) Balance bridge with R3=R4=1000 ohms Balance Equation 120 10 Vdc 1000 ohms ??? Rs R3 R2 R 4 Plug in known values 120 1000 R2 1000 R2 120 Ans xample Solution (Part 1b) Balance bridge with R3= 120 ohms R4=100 Rs R 3 R2 R4 06/21/2022 120 120 R2 1000 R2 1000 DMU, ECEG-5323: Instrumentation Engineering Ans 62 Unbalanced Bridge Analysis Example(…) xample • eSolution (Part 2a) plot output V with R3=R4=1000 ohms Vb Va R bal R s R bal Vdc 2 R s R bal R 2R 4 120(1000) 120 R3 1000 Rbal = 120 Plot values of Rs from 90 to 150 with Excel or MathCAD using the equation below Vd Vb Va Sample calculation Rs =90 ohms 06/21/2022 R s 120 10 2 R s 120 90 120 2 90 120 Vd 10V Non-linear output Vd 0.714V DMU, ECEG-5323: Instrumentation Engineering 63 Unbalanced Bridge Analysis Example(…) xample • eSolution (Part 2b) plot output V with R3= 120 R4=1000 ohm Use the alternative formula written as function of rs, sensor resistance R4 rs Rbal V Vd1 rs RbalR4 dc R3 R4 rs R 3 Less output voltage than first case R R (1000 )(120 ) Wher R bal 2 3 120 R 1000 4 e 1000 r s 120 10V Substitute V d r s Values 1120 r 1000 Sample Calculatio n 06/21/2022 Vd1 1000 90 1120 90 s 120 10V 1000 Vd1 0.246V DMU, ECEG-5323: Instrumentation Engineering 64 Unbalanced Bridge Analysis Example(…) xample • e Solution (Part 3) Find zero based linear approximation. Assum asses through zero and the average of the end points For bridge with R3=R4=1000 ohms at Rs = 90 Vd = -0.714 at Rs =150 Vd = 0.556 Average max value (|-0.714|+|0.556|)/2 = 0.635 For bridge with Use two data points: Rs1=120 Vd1 = 0 R3=R4=1000 ohms Rs2=150 Vd2=Vave =0.635 Use twopoint form of line to Non-linearity find ( Vd 2 Vd1 ) (Vd Vd1 ) equation(R s 2 R s1 ) (rs R s1 ) Increase as (0.635 0) ( Vd 0) ( rs 120) (150 120) Vd 0.021167 rs 2.54 Rs moves from balance point Equation 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 65 Unbalanced Bridge Analysis Example(…) at Rs1 = 90 Vd1 = -0.246 e with R3= 120, R4=1000 ohms For•bridge at Rs2 =150 Vd2 = 0.233 Average max value (|-0.246|+| 0.233|)/2 = 0.2395 (V V ) ( Vd Vd1 ) d2 d1 (rs R1 ) (R2 R1 ) (0.2395 0) ( Vd 0) (rs 120) (150 120) Vd 0.007983rs 0.958 Equation Less output voltage but greater linearity Dc bridge approximately linear for small deviations around balance point 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 66 Unbalanced Bridge Analysis Example(…) Example Solution (Part 4) Determine the maximum non• e linearity for each bridge. Take difference between actual bridge output and the zero based lines. Maximum occurs at either end of the graph. termine the percent non-linearity using the following formula Output % Non-linearity 8 % Non-linearity 6 %error 4 | Vdl Vd | 100% | 2 Vdl (max) | Where Vdl(max) max linear output 2 0 90 06/21/2022 Non-linearity greatly 100 110 120 130 140 150 improved by making R3 Sensor Resistance (Ohms) R3=R4 equal to nominal R3=120, R4=1000 resistive sensor value DMU, ECEG-5323: Instrumentation Engineering 67 Unbalanced Bridge Analysis Example(…) xample • eSolution (Part 5) Determine the gain necessary for a span of Use average value of Vd to compute gain For R3 =R4=1000 ohms Average max value Rf Vb-Va = Vd Vo Ra Rf (Vb Va ) Ra (|-0.714|+|0.556|)/2 = 0.635 V For R3=120 and R4=1000 ohms Average max value Instrumentation Amplifier Gains (|-0.246|+|0.233|)/2 = 0.2395 V 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 68 DAC for Signal Conditioning • A DAC, inputs a binary number and outputs an analog voltage or current signal. In block diagram form, it looks like this: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 69 DAC for Signal Conditioning(…) • is a variation on the inverting summer op-amp circuit. If you recall, the classic inverting summer circuit is an operational amplifier using negative feedback for controlled gain, with several voltage inputs and one voltage output. The output voltage is the inverted (opposite polarity) sum of all input voltages: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 70 DAC for Signal Conditioning(…) • The R/2R DAC An alternative to the binary-weighted- input DAC is the so-called R/2R DAC, which uses fewer unique resistor values. A disadvantage of the former DAC design was its requirement of several different precise input resistor values: one unique value per binary input bit. 6 bit binary weighted DAC Simulation result using SPICE 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 71 ADC for Signal Conditioning • An ADC inputs an analog electrical signal such as voltage or current and outputs a binary number. In block diagram form, it can be represented as such: • Flash ADC Also called the parallel A/D converter, this circuit is the simplest to understand. It is formed of a series of comparators, each one comparing the input signal to a unique reference voltage. The comparator outputs connect to the inputs of a priority encoder circuit, which then produces a binary output. The following illustration shows a 3-bit flash ADC circuit: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 72 ADC for Signal Conditioning(…) • V i ref is a stable reference voltage provided by a precision voltage regulator as part of the converter circuit, not shown in the schematic. As the analog input voltage exceeds the reference voltage at each comparator, the comparator outputs will sequentially saturate to a high state. The priority encoder generates a binary number based on the highest-order active input, ignoring all other active inputs. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 73 ADC for Signal Conditioning(…) • Digital ramp ADC Also known as the stairstep-ramp, or simply counter A/D converter, this is also fairly easy to understand but unfortunately suffers from several limitations. The basic idea is to connect the output of a freerunning binary counter to the input of a DAC, then compare the analog output of the DAC with the analog input signal to be digitized and use the comparator's output to tell the counter when to stop counting and reset. The following schematic shows the basic idea: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 74 ADC for Signal Conditioning(…) • Slope (integrating) ADC It is possible to avoid using a DAC if we substitute an analog ramping circuit and a digital counter with precise timing. The is the basic idea behind the so-called single-slope, or integrating ADC. Instead of using a DAC with a ramped output, we use an op-amp circuit called an integrator to generate a sawtooth waveform which is then compared against the analog input by a comparator. The time it takes for the sawtooth waveform to exceed the input signal voltage level is measured by means of a digital counter clocked with a precise- frequency square wave (usually from a crystal oscillator). The basic schematic diagram is shown here: 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 75 Practical considerations of ADC circuits . 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 76 Practical considerations of ADC circuits(…) . Because the ADC is fixed at 10 bits of binary data output, it will interpret any given tank level as one out of 1024 possible states. To determine how much physical water level will be represented in each step of the ADC, we need to divide the 40 feet of measurement span by the number of steps in the 0-to1024 range of possibilities, which is 1023 (one less than 1024). Doing this, we obtain a figure of 0.039101 feet per step. This equates to 0.46921 inches per step, a little less than half an inch of water level represented for every binary count of the ADC 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 77 Practical considerations of ADC circuits(…) . 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 78 Practical considerations of ADC circuits(…) . Another important consideration of ADC circuitry is its sample frequency, or conversion rate. This is simply the speed at which the converter outputs a new binary number. Like resolution, this consideration is linked to the specific application of the ADC. If the converter is being used to measure slow-changing signals such as level in a water storage tank, it could probably have a very slow sample frequency and still perform adequately. Conversely, if it is being used to digitize an audio frequency signal cycling at several thousand times per second, the converter needs to be considerably faster. 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 79 Q? Thank you!!! 06/21/2022 DMU, ECEG-5323: Instrumentation Engineering 80