Sensing for Robotics and Control

1. Sensing for Robotics and Control ME 4135 R. R. Lindeke

2. General Ideas about Sensors • Sensor are truly systems! • Sensors systems consist of three separable ideas: • Informational sources: physically measurable data sources (light beams, audio beams, electrical fields, etc) • Detector areas: Devices that react to changes in the informational sources • Data Interpreters: devices (hard or soft based) that convert informational changes into useful information

3. Sensor Topics • Positional Control Sensing • Kinesethic Sensing • Resolvers • Absolute Encoders • Incremental Encoders • Environmental Sensors • Contact • Point • Field Sensors • Proximity – typically single point • Remote • Single Point • Field Sensors

4. Kinesethic Sensing • These sensors provide feedback information to the joint/link controllers (servo information) • They use analog or digital informational responses • We will explore 3 generally used types: • Resolvers • Absolute Encoders • Incremental Encoders

5. Resolvers • Operating principle is that a charged rotating shaft will induce voltage on stationary coils • Secondary Voltages are related to Primary voltage as Sin and Cos ratios of the primary field voltage

6. Resolver Ideas: Typically we use 2 stators one (not shown) mounted normal to an axis that is 90 away from the one thru Winding A

7. Resolvers, cont.  of interest • Position is determined for computing stator ratio • Winding A carries Sin signal • Winding B carries Cos signal • A/B = tan so • Shaft position =Atan2(B_Reading, A_Reading)

8. Resolver Issues • These devices are susceptible to Electrical Noise – must be highly shielded • Usually use gearing to improve resolution • Typically are expensive but very rugged for use in harsh “shock motion” environments

9. Optical Encoder Positional Sensors • Based on Photoelectric source/receiver pairs • Looks for change of state as changing receiver signal level (binary switching) • Uses a carefully designed disk with clear and opaque patches to control light falling on a fixed sensor as disk rotates • Can be made ‘absolute’ with several pairs of emitters/receivers or Incremental with 2 ‘out of phase’ photosensors

10. Optical Servo Measurement Systems • Absolute Encoders • Use Glass Disk marked for positional resolution • Read digital words (0010111011) at receiver to represent shaft position • Commonly Available with up to 16 bits of information (216) to convert into positional resolution

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12. Absolute Encoders – optical disks

13. Absolute Encoder Variations – 8bit

14. Comparing Natural Binary to Gray Code • Natural Binary give actual position when read • Actual position is known w/o analysis • Gray code is designed so only one bit changes “at a time” • Where Bit change is subject to positional errors as light “bleeds” around patch edges • Gray codes are, therefore, less error prone, but require an ‘intelligent converter’ to give actual shaft position

15. Using Absolute Encoders Resolution: ABS for ‘5 liner’ = 360/25 = 360/32 = 11.25

16. To Improve Resolution: • Add Gearing to shaft/encoder coupling • New Resolution is: • Increase # of Lines – this increases complexity and cost of the encoder (can be a significant cost increase)

17. Absolute Encoder for 0.18 Resolution

18. Incremental Encoders • This devices use 3 pairs of Emitter/receivers • Two are for positional resolution, the third is a ‘calibrator’ marking rotational start point Sine wave is observed due to leakage (light bleeding) around opaque patches!

19. Incremental Encoders • The positional detector uses what is called “Quadrature” techniques to look at the changing state of the 2-bits reporting position for each opaque/clear patch on the optical disk

20. Incremental Encoders • Notice the “square wave” quadrature signals • they are offset by “½ phase” • Each patch resolves into 22 or 4 positions! • Without hardware change, resolution is a function of the number of patches – or lines

21. Incremental Encoders Consider a 500 ‘Line’ incremental encoder? For 500 line Inc. encoders, resolution = .18 (w/o gearing)

22. Comparing Absolute and Incremental Encorders: • Incremental are usually cheaper for same level of resolution • Absolute are able to provide positional information at any time under power • Incremental must be homed after power loss to recalibrate count numbers • Compared to resolvers, encoders are fragile so must be shock protected during operation

23. Environmental Sensors • These sensors provide ‘code decision making’ power to the Manipulator • These sensors can be simple • Single point devices, • Simple devices typically trigger yes/no decisions with switch changes • These sensors can be complex 2-D array (or even 3-D field) devices • Typically the receivers are complex arrays • The data interpreters are sophisticated software and hardware devices • They can add “intelligence” for decision-making by the manipulator

24. Contact Sensors – Force and Deflection Sensing • Force Sensors: • Measure pressure for gripping – direct or indirect • Measure deflection during contact – typical of indirect contact sensing

25. Contact Sensing • Indirect contact sensors use Strain Gages (and Hooke’s Law: Stress = E*Strain) • The strain gage is a resistive device that exhibits a change in resistance due to changes in shape (length or width) • The Strain Gage is mounted into a carefully built (and calibrated) Wheatstone bridge • small changes to the strain gages resistance, observed while using a highly linear voltage source, are calibrated against observed deflection • This ‘bar’ deflection is strain and multiplying the strain times the bar’s modulus of elasticity yields stress and hence applied force! • Stress = Force/Areabar

26. Contact Sensing • Other contact sensor are “Direct Reading” • These devices use the piezoelectric principle (effect) of the sensor material • Piezoelectric effect states that in certain material (quartz and some silicates) applied forces (dynamically) will cause a minute – but measurable – flow of electrons along the surface of the crystal based on di-polar disruption due to shape change • This flow is measure as a “Nano-current” • The Current is linearized, amplified and measured against a calibrated force

27. Contact Sensing • A second general type would be the class of “Micro-Switches” • Like at the end of the Conveyor in the S100 cell • Typically, applied forces directly move a common contact between NC and NO contact points

28. Examples of Micro-Switches: • One Directional Reed Switch: • Omni-Directional Reed Switch: • Roller Contact Switch: • Etc., etc., etc.!!!

29. Tactile Sensors – “feeler arrays” • Potential Advantages of Tactile Sensors: • They generate far fewer data bits (compared to visual arrays) leading to simpler interpretation analysis • Collection is more readily controlled – we completely control background and contrast • The properties we measure are very close to (exactly?!?) the properties we desire

30. Defining the “Ideal” Tactile Sensor • They must be rugged and compliant to faults in the manufacturing (operating) environment • They should be “Smart” – That is able to process most of the data into information for decision making locally • they send only results to the main controller • Resolution should be on the order of about 100 mils (about 10-4 inch) • Sensors should respond to forces on the order of about 5 -10 gmforce (0.1 N or 0.022 lbf)

31. Tactile Arrays: • Machine Equivalent of Human Skins • Use arrays of micro-sized switches or other methods to detect shapes and sizes due to contact images of “made” Switches

32. Tactile Arrays • This device “measures” shapes and sizes by determining which of an array of target points have been charged • Targets are “charged” through contact with the conductive Elastomer skin and the PC ‘board’ targets

33. Tactile Arrays • In this device, a series of thin rods are pushed into an object • A “positive” image of the object is produced by the displaced rods • In modern sensors, displacement of each rod is measured by the detector/interpreter system – this might be a vision system located normal to the direction of contact application or an LVDT unit at each ‘rod’

34. Tactile Arrays • The Anisotropic conductive rubber sensor • The ACR and gold contact surface is separated when unloaded • As load is applied contact patches grow indicating shape and size of external object and force being applied

35. Proximity Sensors: • Devices, including Photocells, Capacitance sensors and Inductive sensors, that can be used in areas that are near to but not directly contacting an object to be sensed • Like all sensors they use structured signal sources, receive changes of state in their energy (sensing) fields and interpret these changes with signal changes to the “outside”

36. Photo Sensors • The modern photosensor (in the proximity range) emits modulated light (at infrared or near-infrared wavelengths). The emitters are LED. • The receivers (phototransistors) are ‘tuned’ to be sensitive to the wavelength of the source emitter during the ‘on’ steps in the modulated output stream • The interpreters are (typically) transistors that switch the power (or ground) source on to the output lead

37. Diffuse Mode Photosensor • In proximity mode, the device is looking for its own emitted beam reflected back to its paired receiver • The level of light falling on the receiver to trigger positive response can be ‘tuned’ to the task • The sensors can be tuned to “Light-Operate” or “Dark-Operate” • Light operate means positive output when reflective light is sensed • Dark operate means positive output when NO reflective light is sensed

38. Retro-Reflective Photosensors • These devices rely on “broken beams” to detect • They are “typically” dark operate – that is waiting for the object to interrupt the light path to the reflector

39. Thru-beam or Separated Systems • The Emitter and Receiver are separate devices • These again rely on dark operate mode (typically) – that is a broken beam indicates objective present

40. Inductive Sensors They typically oscillate In ranges: 3 KHz – 1MHz

41. Inductive Sensors Shielded types have slightly longer range but smaller field of view

42. Uses: • Inductive Sensors can (only) detect metals as they draw power by induced surface currents (eddy currents) • The more magnetic the metal the greater the sensor’s range

43. Principles of Capacitive Sensing

44. Uses And Capabilities • Capacitive Sensors are able to detect any material that raises the field dielectric in the vicinity of the sensor • In air this is nearly any other material!

45. Uses of Capacitive Sensors: When properly calibrated, the sensor can detect any higher Dielectric Material thru any lower Dielectric Material Typical Application of Capacitive Sensor: Detecting Liquid (H2O) levels in bottles

46. Dielectric Values of Various Materials: