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Transcript
Additional information on Passive Probes (10x) Take note that the scope’s input capacitance, Cin must falls within the probe’s compensation range For example, Agilent’s 8000 Series oscilloscope’s input characteristics show that at 1 M input it exhibits 13 pF of typical input capacitance. And the probe you’re going to use with this scope has 6-15 pF of compensation range. Active oscilloscope probes: • For connecting fast rising and high frequency signal, active probe is used - range of frequency up to 2 GHz • As indicated by the name, this type of scope probe has active components incorporated within the probe itself. • This enables greater levels of functionality and higher levels of performance to be attained. • However they are much more expensive and normally reserved for more exacting or specialist requirements. Probe tip Block Diagram of an Active Probe FET acts as a source follower. The stage that follows the FET provides bipolar transistors wired as emitter follower. The FET provides a high-input resistance for low frequency signals and a low capacitance for high frequency signals to the circuit being probed, and the bipolar transistors produce a current gain. Normally active probe is used for i. high input signal impedance – normally around 1 M ii. high frequencies signals iii. low signal level At low frequency 1 M + + 10 M Vin High Impedance – no current flow - Vout - Circuit under test At high frequency 1 M + Vin 10 M Low capacitance (2 pF) - small reactance - Circuit under test The common collector provides a current source with small output impedance to reduce the loading effect BASIC ARCHITECTURE OF ELECTRONICS INSTRUMENTATION MEASUREMENT SYSTEM MEASURAND TRANSDUCER SIGNAL CONDITIONING DISPLAY RECORD SENSORS AND TRANSDUCERS TRANSDUCERS • a device that converts a primary form of energy into a corresponding signal • take form of a sensor or an actuator SENSORS a device that detects/measures a signal ACTUATOR a device that generates a signal Example, a Heater is an actuator while a Thermometer is the sensor Electronic sensors • Generally electronic sensor consists of a primary transducer: changes “real world” parameter into electrical signal for example, heat, sound, etc • for example, a microphone (input device) converts sound waves into electrical signals for the amplifier to amplify (a process), and a loudspeaker (output device) converts these electrical signals back into sound waves Quantity being Measured Light Level Temperature Input Device (Sensor) Light Dependant Resistor (LDR) Photodiode Photo-transistor Solar Cell Thermocouple Thermistor Thermostat Resistive Temperature Detectors Force/Pressure Strain Gauge Pressure Switch Load Cells Position Potentiometer Encoders Reflective/Slotted Opto-switch LVDT Speed Tacho-generator Reflective/Slotted Opto-coupler Doppler Effect Sensors Sound Carbon Microphone Piezo-electric Crystal • Input type transducers or sensors, produce a voltage or signal output response which is proportional to the change in the quantity that they are measuring . • The type or amount of the output signal depends upon the type of sensor being used. • The types of sensors can be classed as two kinds, either Passive Sensors or Active Sensors. Active Sensors Generally, active sensors require an external power supply to operate, called an excitation signal which is used by the sensor to produce the output signal. Active sensors are self-generating devices because their own properties change in response to an external effect producing for example, an output voltage of 1 to 10V DC or an output current such as 4 to 20mA DC. EXAMPLE 1 – STRAIN GAUGE It does not generate an electrical signal itself, but by passing a current through it (excitation signal), its electrical resistance can be measured by detecting variations in the current and/or voltage across it. Force The Wheatstone bridge provides a way to convert these changes in resistance to changes in voltage, which are easy to work with. • R2 in the diagram is set at a value equal to the strain gauge resistance with no force applied. • R1 and R3 are set equal to each other. • Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge. where R4 is the resistance of the strain gauge So let say R1 = R3 = R2 = R Which means that before any force is applied R4 also equals to R Hence, any changes in R4 can be denoted as (R + R) No. of coins Output Voltage (mV) 0 0 1 0.769 2 1.389 3 2.108 4 2.76 5 3.38 6 3.98 7 4.64 8 5.32 9 6.35 10 7 REF: http://www.slideshare.net/umangIITD/transducer-andinstrumentation Consider that the excitation voltage applied is 10 V and the value of R is 120 . i. ii. What is the new resistance value of the strain gauge for Vo = 5.32 mV? Calculate the percentage of increment of the resistance Answers: 120.26 , 0.22 % Relationship with Change of resistance, R = RoG Where Ro = initial resistance when there is no applied stress, G = gauge factor and is the strain unit deformation and = / E Where = the mechanical stress (N/m2) E = Young’s Modulus which is specific for each type of material Continue from previous example: Given G = 2.12, and the Young’s Modulus of aluminium is 72 GPa. Calculate the mechanical stress. Answers: 74.3 MPa Derive the output voltage equation Answers: Vo = (R / 2R) Vi Passive Sensors Unlike an active sensor, a passive sensor does not need any additional energy source and directly generates an electric signal in response to an external stimulus. For example, a thermocouple or photo-diode. Passive sensors are direct sensors which change their physical properties, such as resistance, capacitance or inductance etc. EXAMPLE 1 – THERMOCOUPLE Operation: • a conductor generates a voltage when subjected to a temperature gradient. • a second conductor material will also generates a different voltage under the same temperature gradient. • The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. • thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings. Where, SA and SB are referred to as Seebeck Coefficients (unit is V/K) and with the assumption that the coefficient remains constant through out the metal Ttip Tref Material Selenium Tellurium Silicon Germanium Antimony Nichrome Iron Molybdenum Cadmium, tungsten Gold, silver, copper Rhodium Tantalum Lead Aluminium Carbon Mercury Platinum Sodium Potassium Nickel Constantan Bismuth Seebeck coefficient relative to platinum (μV/K) 900 500 440 330 47 25 19 10 7.5 6.5 6.0 4.5 4.0 3.5 3.0 0.6 0 (definition) -2.0 -9.0 -15 -35 -72 The thermocouple below is using Metal A as copper and Metal B as constantan. If the measured voltage in the following circuit is 3.53 mV, what is the temperature of the hot junction (in °C), if the cold junction is at 0°C? Ttip Answer: 85°C Tref