Transcript Chapter 12
Electronic Troubleshooting Chapter 12 Sensors and Transducers Sensors and Transducers • Characteristic • Transducers converts the form of energy • A microphone coverts sound energy into electrical energy • A speaker converts electrical energy into sound • Sensors are transducers that used to detect and/or measure something • Used to convert mechanical, thermal, magnetic, chemical, or etc variations into electrical voltages and currents Sensors and Transducers • Temperature Sensors • Types of Temperature Sensors • • • • Thermocouple Resistance temperature device(RTD) Thermistor Monolithic IC Sensors NOTE: See Chart on next slide or page 344 • Thermocouple • Characteristics • Most common sensor • A pair of dissimilar wires welded together at the sensing location • A temperature difference from the welded end and the other end causes a DC voltage at the non welded end • Can be used under extreme conditions » Ovens, Furnaces, Nuclear tests Sensors and Transducers • Temperature Sensors Sensors and Transducers • Temperature Sensors • Thermocouple • Operation • When wires made of dissimilar metals are » Welded together at both ends » With different temperatures at both ends Current flows Sensors and Transducers • Temperature Sensors • Thermocouple • Operation • Open the pair of wires in between the two ends a voltage develops » Called Seebeck Voltage » Proportional to temperature difference V s T Where V OpenCircui tVolts s SeebeckTempCoefficient T TempDifferenceBetweenEnds Chart on page 344 Example 12-1 page 345 to 12-4 on page 365 Sensors and Transducers • Temperature Sensors • Thermocouple • Operation • Equation is linear over only a small range of temperatures • Tables of corrected voltages in 10 increments is available from the NBS for each type • Reference Junction • Voltage developed is dependent upon the temperature difference between ends – NOT Absolute Temperature of the welded end » Where’s the cold junction • It’s at room temp • Voltage will be wrong • Need a 00C ref Sensors and Transducers • Temperature Sensors • Thermocouple • Reference Junction • Lab Set up » Not practical for most situations • Practical Reference Junction solutions • Electronic Ice points » Available for All types of thermocouples » Encased electronic device that balances an internal bridge circuit which generates a voltage to cancel out effect that the measurement end isn’t at OOC Sensors and Transducers • Temperature Sensors • Thermocouple • Practical Reference Junction solutions • Isothermal block » Usually used with computerized (also microcontrollers) data collection systems » The isothermal block is a good conductor of heat not electrical current » However it’s resistance is effected measurably by changes in temperature » Block is always near the point were the voltages are measured » Computerized measuring system calculates cool end temperature based on the block resistance and corrects the voltage reading Sensors and Transducers • Temperature Sensors • Thermocouple • Typical Problems • A short of the two wires » Junction then will be at the point of the short » Temperatures readings will be incorrect • No Reference Junction Compensation » Temperatures readings will be incorrect » Test – Short the inputs to the compensator and room temperature should be the new reading • If extensions of the thermal couple wires are used they should be of a larger size and material » Different materials create Incorrect readings since the connection of dissimilar materials creates a new junction » Larger size is needed for IR drops Sensors and Transducers • Temperature Sensors • Thermocouple • Typical Problems • Noise pick-up » Long leads form an antenna – uses shielding. e.g., grounded over braiding of copper • Extreme Temperature Gradient » Can damage the thermocouple should have protection • Environment can change the metal and it’s thermal characteristics » Chenicals » molten metals – new alloys Commercial thermocouple assemblies – see page 348 Sensors and Transducers • Temperature Sensors • Resistance Temperature Device • Key principle • As the temperature of a resistor increases so does its resistance • Measure the change in the resistance of a known resistor – calculate the temperature change » Linear relation ship for smaller changes – more linear than thermocouples – NBS has correction tables for the typical types of measurement resistors • Typical construction • Wire wound resistor in on a ceramic core using platinum wire » Stable (linear) over a wide range of temperatures » Temperature coefficient = 0.00385/0C • Typical Values: 10 – several kilo-ohms • Most common value 100Ω Sensors and Transducers • Temperature Sensors • Resistance Temperature Device • Measuring Circuit Types • RTD Bridge circuit • Constant Current Source • RTD Bridge circuit • Platinum resistor is remote from the bridge circuit which is isolated from the sensing point • Bridge is balanced at a known temperature » Eliminates consideration of the connecting leads • Voltage developed is proportional to the temperature change Sensors and Transducers • Temperature Sensors • Resistance Temperature Device • Constant Current through RTD • Voltage across the RTD rises and the resistance increase with the rise in temperature • The constant current also increases the temperature of the resistance and effects the temperature reading » The correction factor for common platinum RTDs has been determined TC 0.50C / mW » Example Problems on page s 349 & 350 Sensors and Transducers • Temperature Sensors • Thermistor • Resistors with high negative temperature coefficients • Resistance decreases with an increase in temperature • High temperature coefficients means that there is a significant change in resistance for a small temperature change • Construction • Semiconductor material » In either tube or bead shapes • Can be used as a plain resistor in circuits such as a bridge or voltage divider » Come in a Wide range of values » Also come with manufacturer provided resistance vs temperature curves Sensors and Transducers • Temperature Sensors • Thermistor • Construction • Also come manufacturer provided resistance vs temperature curves • Sample for thermistor with nominal value of 5kΩ at 00C Sensors and Transducers • Temperature Sensors • Monolithic IC Sensors • Current or voltage types are available • They have linear output voltages or currents with temperature changes • Typical values: 1µA/0K; 10mV/0K » 1 0K = 1 0C Sensors and Transducers • Light Sensors • Typical uses of the sensors • Measure intensity of the light • Detect the presence or absence of light • Types of Light Sensors • • • • Photovoltaic Cells Photoconductive Cells Photo Diodes Phototransistors • Photovoltaic Cells • aka, Solar Cells • Semiconductor material that generates a voltage when light shines on it • 2.5 by 5 cm cell can produce 0.4 V with 180mA of current Sensors and Transducers • Light Sensors • Photovoltaic Cells • Sometimes used to detect the presence of light • Photoconductive Cells • aka, photoresistors • Characteristics of Photoresisters • Uses bulk resistivity which decreases with increasing illumination, allowing more photocurrent to flow. • Signal current from the detector can be varied over a wide range by adjusting the applied voltage. • Thin film devices made by depositing a layer of a photoconductive material on a ceramic substrate. Sensors and Transducers • Light Sensors • Photoconductive Cells • Characteristics of Photoresisters • Metal contacts with external connection. These thin films have a high sheet resistance. Therefore, the space between the two contacts is made narrow and long for low cell resistance at moderate light levels. Sensors and Transducers • Light Sensors • Photoconductive Cells • Light Intensity Application • With little or no light the voltage at point X is low • As the intensity of the light on the sensor increases the voltage at X will increase • By adjusting Rf,a usable output range of voltages that the is proportional to the light intensity can be obtained • Presence or Absence of Light application • Activates a electromechanical counter when the light is blocked Sensors and Transducers • Light Sensors • Photoconductive Cells With a Microcontroller • Critical aspect of this application a BASIC command for measuring the RC decay time on a connected circuit • RCTIME command is designed to measure RC decay time on a circuit like the one below. The lower the count recorded the brighter the light measured • RCTIME Pin, State, Duration » Pin argument is the number of the I/O pin that you want to measure » State argument - 1 if the voltage across the capacitor starts above 1.4 V and decays downward. 0 if the voltage across the capacitor starts below 1.4 V and grows upward » Duration argument has to be a variable that stores the time measurement, which is in 2 μs units • Very simple circuit – range of measured light is limited only by the size of the variable used to store the count. Sensors and Transducers • Light Sensors • Photodiodes • • • • A diode that is forward biased by light Very fast reactions to changing light levels Same physical size as LEDs Have small windows through which light is sensed • Testing is simple • When the window is blocked » High resistance is read • Shine a bright light (several footcandles) on it while still connected to an ohmmeter » The resistance will drop significantly • Phototransistors • Usually used instead of photodiodes when low light levels are measured Sensors and Transducers • Light Sensors • Phototransistors • Usually used instead of photo resistors when low light levels are broken at high rates • Typical ratings • Like low power transistors » 30-50V maximum collector to emitter voltages » Max collector currents of 25mA • Typical application • See slide on the next page or the bottom of page 355 • Monitors droplets falling through an IV administration set » Drip rate is set by nurses with a small valve not shown • IR LED is the Light beam sourse • The drops block enough light to turn off the phototransistor » Positive spikes on it’s collector feed an inverter that squares off and amplifies the spikes » Sent to a counter, alarm, or monitoring equip Sensors and Transducers • Light Sensors • Replacement Considerations • Best option is an exact replacement • If not possible match the following characteristics : • Voltage, current, & power ratings; physical size • Light sensitivity » Can be specified nm (human sight 400 -700) nm (700nm – red light) » Called spectral response » Can also be specified in angstroms Å. 10 Å = 1nm • Light Insensitivity » For photoresistors – X-kΩ at Y-footcandles » 1 Foot candle = light falling on 1 square foot – one foot from a standardcandle » For phototransistors: Collector current at a specified light level Sensors and Transducers • Light Sensors • Other Problems with light sensing systems • Burned out, weak, or obstructed light sources • Can be a simple problem of dirty light filters or lens • Light shields may have been misaligned by a bump • Mechanical Sensors • Characteristics • Used to measure: • Force • motion • position • The chapter covers Strain gages • They measure Forces • Weight is a common force Sensors and Transducers • Strain gages • Characteristics • Sensors used to measure change in the dimensions of solid objects caused by forces • Information is critical to designs of mechanical systems • Used in load cells which are used to measure weights of objects • Measurements can range from a few pounds to the weight of a fully loaded tractor trailer rig • Strain and Stress • Strain = ΔL/L0 , where ΔL = change in length due to a force and L0 = the original length before the force was applied • Can be caused by tension or compression forces Sensors and Transducers • Strain gages • Strain and Stress • Strain = ΔL/L0 , where ΔL = change in length due to a force and = the original length before the force was applied • Can be caused by tension or compression forces • Stress is a measure of the force applied that has been normalized to a unit area • Stress = F/A , where F= the total force applied and A= crosssectional area • The ratio of Stress/Strain is a constant value for each material • Called Young’s Modulus and has been tabulated for many material • Most metals won’t stretch beyond 0.5% without deforming Sensors and Transducers • Strain gages • Strain and Stress • Resistor conductance can be determined from: R=ρL/A • Where R= resistance in ohms, ρ (rho) is the resistivity of the material, L= length of the material, & A is the cross-sectional area of the material • If the gage material under stress increases it length by0.4% - it’s resistance will increase by 0.4% » Some commercial gages have been designed to yield multiples of the change in length in change of resistance – A Gage Factor • Construction • Metal or semiconductor foil woven back and forth to increase the length • Range of common values 30 -3000 Ω • Most common sizes 120 Ω and 350 Ω Sensors and Transducers • Strain gages • Strain and Stress • Calculations L R R0 (1 GF ) L • Where R =resistance of the gage under stress, R0 = Original resistance of the gage, ΔL = change in length of the gage, L = original length of the gage, GF = gage factor • Example Problems 12-4 and 12-5 on page 359 • Typical Bridge configurations Sensors and Transducers • Strain gages • Typical Bridge configurations • The 1/4 bridge has a gain factor of 1 • Change of resistance causes the bridge to unbalance • The ½ bridge has two strain gages • One in tension mode and one in compression mode, like in the metal beam drawing –bottom right of previous slide » rg1 is stretched and rg2 is compressed » Changes double the resistance change • GF = 2 • The full bridge has four gages and a GF of four • Problems with Strain Gages • Temperature changes • If outside the circuitry must have temperature compensation » e.g., Las Vegas temperatures range from the 20’s – 115+ Sensors and Transducers • Strain gages • Typical