Sensors

1. Sensors IIT, 10th Nov. 2010

2. Contents • Sensors • Definitions • Main Properties • Angular/Linear displacement sensors • Accelerometers • Force sensors • Fluid Pressure sensors • Laser range sensors

3. Sensor Definition a device for sensing a physical variable/property of a physical system or an environment

4. Sensor Signal Generation • Passive • Directly generate an electrical signal in response to an external stimuli without the need for an external power supply • Active • Require external power supply or an excitation signal for their operation

5. The need for Sensors • provide knowledge of the surrounding environment • allow system to determine its physical state • allow interaction with environment/perform the appropriate action • help in Protection & Self-Preservation • give the automation equipment the capability to seek and achieve a target Sensing/measurement is a fundamental process of any mechatronic/automation system

6. Classification of Sensors • Mechanical: • displacement • velocity • acceleration • Strain, • pressure, force/torque • flow • Thermal: • temperature • heat. • Electromagnetic: • voltage • current • magnetism • Optical • visual/images • light • Chemical • Humidity • pH value

7. Sensor Specifications • Range • Nominal / Maximum limits that the sensor will respond to within specified performance tolerances. • Accuracy • Describes the error /uncertainty between the measurement and the real value • Repeatability • the ability of the sensor to output the same value for the same input over a number of trials • Resolution • the smallest increment of measure that the sensor can make. 7

8. True value measurement Accuracy and Resolution

9. Accuracy and Repeatability Repeatability without accuracy Accuracy without Repeatability Repeatability and accuracy

10. Sensor Specifications • Linearity • The measure of closeness of a calibration curve to a specified straight line • Sensitivity • How output reflects input? the change in the output signal to a small change in input physical signal

11. Sensor Specifications • Response time • How fast the sensor gives an output of a certain level at the application of the stimulus • Drift and stability • How the output signal varies slowly with the time • Offset • The level of the output when there is no input • Hysteresis • The maximum difference in output at any measurand value when the value is approached first with increasing measurement and then with decreasing measurement

12. Linear/Angular Displacement Sensing • Linear/Angular Potentiometer • Linear/Angular Encoder • LVDT (Linear/Angular Variable Differential Transformer)

13. Linear Displacement Sensing Linear Potentiometer • Resolution (infinite), depends on the A/D • Velocity (up to 2.5 m/s) • Low cost • Finite operating life (2 million cycles) due to contact wear • Accuracy: +/- 0.01 %

14. Linear Displacement Sensing • Linear Encoder • Absolute • uses a line with a special black/white code • Returns the absolute position • Incremental • uses a single line that alternates black/white two slightly offset sensors produce outputs as shown below • detects motion in either direction, pulses are counted to determine absolute position (which must be initially reset)

15. Linear Displacement Sensing LVDT (Linear Variable Differential Transformer) • Inductance-based sensor • Very high resolution • Depends on the A/D resolution • No contact between the moving core and coil structure • no friction, no wear, very long operating lifetime • Accuracy limited mostly by linearity • 0.1%-1% typical • Models with strokes from mm’s to 1 m available Photo courtesy of MSI

16. Vcc R1 V = Vcc R R2 R1 0 V Angular Displacement Sensing Angular Potentiometer R

17. Angular Displacement Sensing • Made of glass or plastic with transparent and opaque areas. • A light source and photo detector array reads the optical pattern that results from the disc's position at any one time. • The absolute type produces a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm). Optical Encoder

18. Non-contact potentiometers • Rotating magnet facing the sensor generates output signals • Magneto Resistors • Resistance changes as the magnetic field varies

19. Angular Displacement Sensing Relative Optical Encoder Signal obtained after displacing the sensor over a coded disc 1 2 3 4 5 6 7 8 9 10 11 12 Detectors

20. Angular Displacement Sensing Absolute Optical Encoder Detector Gray code Standard binary code

21. Ambiguity when reading the natural binary code Angular Displacement Sensing 10 bits 1024 div.  Resol. 0.35º 12 bits 4096 div.  Resol. 0.088º 14 bits 16384 div.  Resol. 0.022º Optical Encoder Elimination of the reading ambiguity using the Gray code

22. Angular Displacement Sensing Rotary Variable Differential Transformer (RVDT) • an electromechanical transducer that provides a variable alternating current (AC) output voltage that is linearly proportional to the angular displacement of its input shaft. • RVDT’s utilize brushless, non-contacting technology • long-life, reliable and repeatable position sensing • infinite resolution. • RVDT’s utilize brushless, non-contacting technology • a wound, stator and • a two-pole rotor. • The stator contains both the primary winding and the two secondary windings

23. Magnetic Encoders Principles of Operation • magnetic encoders utilize a magnetoresistive (MR) sensing element • or Hall effect • and a magnetic code wheel

24. Magnetic Encoders • Lower power requirements than optical encoders • a simple and robust structure • excellent resistance to humid and dirty environments. • Disadvantages are • sensitivity to temperature effects • and generally lower resolution capabilities.

25. Velocity/Acceleration Sensors • Displacement sensors can be used to obtained indirectly velocity and acceleration • by differentiating displacement • differentiation tends to amplify noise • Direct measurement of velocity/acceleration • by measuring the frequency of pulses speed can be measured • some sensors give acceleration directly

26. Accelerometer Used to measure • acceleration of moving objects • machine vibrations • inclination • dynamic distance and speed • collisions between moving objects • user interface controllers (game controllers, mobile phones)

27. Principle of Accelerometer Conceptually, an accelerometer behaves as a mass attached on a spring

28. Photo courtesy of PCB Piezotronics Acceleration Sensing • Capacitive accelerometer • Good performance over low frequency range, can measure gravity! • Heavier (~ 100 g) and bigger size than piezoelectric accelerometer • Measurement range up to +/- 200 g • More expensive than piezoelectric accelerometer • Sensitivity typically from 10 – 1000 mV/g • Frequency bandwidth typically from 0 to 800 Hz • Operating temperature: -65 – 120 C 28

29. Photo courtesy of PCB Piezotronics Acceleration Sensing • Piezoelectric accelerometer • Nonzero lower cutoff frequency (0.1 – 1 Hz for 5%) • Light, compact size (miniature accelerometer weighing 0.7 g is available) • Measurement range up to +/- 500 g • Less expensive than capacitive accelerometer • Sensitivity typically from 5 – 100 mv/g • Broad frequency bandwidth (typically 0.2 – 5 kHz) • Operating temperature: -70 – 150 C

30. MEMS Accelerometer • Single monolithic IC • Integrated micromachined sensor and signal conditioning circuitry • Outputs are analog voltages proportional to acceleration • Can also be used as a tilt sensor since it measures gravity • Polysilicon mass in center is held by polysilicon springs that create the resistance against acceleration forces 30

31. ADXL320 continued • Deflection of the mid-structure is measured using a differential capacitor. • Acceleration deflects and unbalances the differential capacitor, resulting in an output signal whose amplitude is proportional to acceleration. 31

32. ADXL320 – typical MEMS accelerometer • Dual axis ±5g accelerometer on a single chip • Ultra small 4 mm x 4 mm x 1.45 mm LFCSP package • Low power: 350 µA at VS = 2.4 V (typical) • Output Type: Analog • Supply Current: 0.5mA • Typical Band Width: 2.5kHz • Voltage Supply: 2.4 to 6 • Range: +/- 5g • Sensitivity174 mV/g • # of Axes: 2 • Temp Range: -20 to 70°C 32

33. Elastic Sensing: Strain Sensing: Pressure Sensing: Acceleration Sensing: Force Sensing

34. Sensing Elements for Force Sensors • There are many types of sensors can be used to measure force • Strain gages, optoelectronics, CCD cameras or LVDT • The use of resistive type force sensors, such as strain gages is the most common used for force measurement in Robotics

35. Strain Gages

36. Metal Strain Gages • The able to measureup to 30000μεfor gage lengths under 3mm, up to 50000 μεfor gage lengths of 3mm and over • Nonlinearity = 0.02% • Small and light • Able to response to high frequency signals • Gage factor of 2-4

37. Metal Strain Gages • Low in cost • Low resistance and gage factor to temperature coefficients • Easy compensation • Temperature Range -80 oC to +200 oC • Fatigue Life of 104 cycles at 2000με • Come in different patterns

38. Metal Strain gage patterns • The gage pattern refers to • shape of the grid, • the number and orientation of the grids in a multiple-grid (rosette) gage, • Arrangement types include • uniaxial, dual linear,90 degrees patterns, tee rosette, rectangular rosette, and delta rosette. 

39. Semiconductor gages • As opposed to other types of strain gages, semiconductor strain gages depend on the piezoresistive effects of silicon or germanium and measure the change in resistivity with stress as opposed to strain. • The semiconductor bonded strain gage is a thin slice of silicon substrate with the resistance element diffused into a substrate of silicon. • The wafer element usually is not provided with a backing, and bonding it to the strained surface requires great care as only a thin layer of epoxy is used to attach it

40. Semiconductor Strain Gages • Operating strain  2000με • Maximum strain up to  5000με • Subminiature size • Wide range of frequency response • Linearity • Better than 0.25% for up to  600με • Better than 1.5% for up to  1000 με • Gage factor of 100-180 • High resistance/temperature and gage factor/temperature coefficients • More difficult temperature compensation

41. Semiconductor vs Metal • The larger signal output from silicon gages allows measurement of very low tensions. • Transducers with metal gages have difficulty with low tensions because the nominal output is so low that it can be lost in the ambient electronic noise present in the electrical system Low tension output and extended range

42. Semiconductor vs Metal • Silicon gages have an extremely long fatigue life as compared to metal gages. • As a result of operating under higher strains metal gages suffer from shortened life spans and fatigue life when compared to silicon gages Fatigue Life

43. Semiconductor vs Metal • The greater sensitivity of silicon gages means that less stress is needed to produce a useable amount of signal and therefore a transducer's bending beam can be built more robustly and better protected from overload than beams with metal gages Overload

44. Semiconductor vs Metal • The low output of the metal-gage transducers requires associated amplifier electronics to have a much higher gain for the amplified signal to be useful • The high gain makes the system much more susceptible to EMI and RF noise Electronic noise and interference

45. Semiconductor vs Metal • Foil gages are cheap. Semiconductor are delicate and expensive • Foil gages are fixed easily to the sensing geometry • Semiconductor gages require an elaborate process for attachment which must be done by a specialist. Practicalities.

46. Strain Gauges Principle of Operation • Consider an electrical wire… • When stretched along its length, it elongates and reduces its cross-section. Therefore its resistance will change. If this wire is glued to the surface of an object, it may be utilized to measure the strain in the object.

47. Strain Gauges Principle of Operation • Piezoresistive effect in metal strain gages The resistance change effect of metal gage sensors is only due to the change of the sensor geometry resulting from applied mechanical stress. • Piezoresistive effect in emiconductor strain gagesThe resistance of silicon gages changes not only due to the stress dependent change of geometry, but also due to the stress dependent resistivity of the material. This results in larger gauge factors than those observed in metals.

48. Poisson's ratio • Poisson's ratio is a measure of the simultaneous change in elongation and in cross-sectional area within the elastic range during a tensile or compressive test. • During a tensile test, the reduction in cross-sectional area is proportional to the increasein length in the elastic range by a dimensionless factor, the Poisson's ratio Definition

49. P D2 L2 D1 L1 P Strain Gauges Principle of Operation

50. Strain Gauges Principle of Operation • GF and R are known, by measuring dR the strain ε can be determined • By knowing the sensing geometry and material properties the force can be obtained