Actuator Components 2

Transcription

1 Actuator Components 2 Term project midterm review Bearings Seals Sensors 1 Actuator Components

2 Term Project Midterm Review Details of term project are contained in first lecture of the term Should be using the techniques covered in class to perform design trades Goal is a complete design for the rover mobility systems by the end of the term 2 Actuator Components

3 Objectives of Mid-Term Review Make sure that progress is being made on the project Verify results to date to ensure that there are no misunderstandings of fundamental principles Get an initial look at the concepts under consideration Due Tuesday, Nov. 1 3 Actuator Components

4 Mid-Term Review Expectations Notional/ strawman design of rover Solid models of design Details of mass, power, etc. Trade studies (NOT an exhaustive list!) Number, size, configuration of wheels Diameter and width of wheels Size and number of grousers Suspension design Steering design Alternate design approaches (e.g., tracks, legs, hybrid) 4 Actuator Components

5 Mid-Term Review Expectations (2) Vehicle stability Slope (up, down, cross) Acceleration/deceleration Turning Combinations of above Terrain ability ( terrainability ) Weight transfer over obstacles Climbing/descending vertical or inclined planes Hang-up limit (e.g., high-centering, wheel capture) 5 Actuator Components

6 Mid-Term Review Reach Goals Suspension dynamics Development of drive actuator requirements Detailed wheel-motor design Development of steering actuator requirements Detailed steering mechanism design Mass budget (with margin) Power budget (with margin) 6 Actuator Components

7 Mechatronics Smart components Combination of (or technologies for) Sensors Actuators Structures and mechanisms Processors and control systems Subsumes microelectromechanical systems (MEMS) 7

8 Ball Bearings 8 Actuator Components

9 Types of Bearings 9 Actuator Components

10 Angular Contact Bearing 10 Actuator Components

11 Ideal Bearing Applications 11 Actuator Components

12 Dual-Row Angular Contact Bearing 12 Actuator Components

13 Thrust Bearing 13 Actuator Components

14 Tapered Roller Bearings 14 Actuator Components

15 Wheel Hub Cutaway 15 Actuator Components

16 O-Ring and Groove 16 Actuator Components

17 Lip Seals 17 Actuator Components

18 Spring Energized Seals 18 Actuator Components

19 Labyrinth Seals 19 Actuator Components

20 Sensor Components An overview of robotic operations Generic discussion of sensor issues Sensor types Proprioceptive (measures robotic interaction with environment) Exteroceptive (measures environment directly, usually remotely) Interoceptive (internal data - engineering quantities) 20

21 Sensing Definitions Resolution Accuracy Precision Repeatability 21

22 Some Notes on Data and Noise Noise is inherent in all data Sampling errors Sensor error Interference and cross-talk For zero-mean noise, Integration reduces noise Differentiation increases noise Use the appropriate sensor for the measurement Don t try to differentiate position for velocity, velocity for acceleration 22

23 Shannon Sampling Limit For discrete measurements, can t reconstruct frequency greater than 1/2 the sampling rate Discretization error creates aliasing errors (frequencies that aren t really there) Signal frequency ƒ signal Sampling frequency ƒ sample Alias frequencies ƒ sample ± ƒ signal 23

24 Analog and Digital Data 1" 0.9" 0.8" 0.7" 0.6" 0.5" 0.4" 0.3" 0.2" 0.1" 0" 0" 20" 40" 60" 80" 100" 120" 24 Analog" Digital"

25 Analog and Digital Data with Noise 1.2% 1% 0.8% 0.6% 0.4% 0.2% 0% 0% 20% 40% 60% 80% 100% 120%!0.2% 25 Analog% Digital%

26 Some Notes on Analog Sensors Analog sensors encode information in voltage (or sometimes current) Intrinsically can have infinite precision on signal measurement Practically limited by noise on line, precision of analog/digital encoder Differentiation between high level (signal variance~volts) and low level (signal variance~millivolts) sensors Advice: never do analog what you can do digitally 26

27 Proprioceptive Sensors Measure internal state of system in the environment Rotary position Linear position Velocity Accelerations Temperature 27

28 Proprioceptive Sensors Position and velocity (encoders, etc.) Location (GPS) Attitude Inertial measurement units (IMU) Accelerometers Horizon sensors Force sensors 28

29 Representative Sensors 29

30 Absolute Encoders Measure absolute rotational position of shaft Should produce unambiguous position even immediately following power-up Rovers typically require continuous rotation sensors General rule of thumb: never do in analog what you can do digitally (due to noise, RF interference, cross-talk, etc.) 30

31 Potentiometers 31

32 Potentiometers Advantages Very simple (three wires) Unambiguous absolute position readout Generally easy to integrate Low cost Disadvantages Analog signal Data gap at transition every revolution Accuracy limited to precision of resistive element Wear on rotating contactor Liable to contamination damage 32

33 Resolvers 33

34 Resolvers 34

35 Resolvers Advantages Non-contact (inductively coupled) Unambiguous absolute position reading Similar technology to synchros Disadvantages AC signal Analog Requires dedicated decoding circuitry Expensive 35

36 Rotary Binary Encoder 36

37 Binary Absolute Position Encoders 37

38 Gray Code Absolute Position Encoders 38

39 Absolute Encoder Gray Codes 39

40 Optical Absolute Encoders Advantages No contact (low/no friction) Absolute angular position to limits of resolution 8 bit = 256 positions/rev = 1.4 resolution 16 bit = 65,536 positions = resolution Require decoding (look-up table) of Gray codes Number of wires ~ number of bits plus two 40

41 Magnetic Absolute Encoders Advantages No contact (low/no friction) Absolute angular position to limits of resolution 8 bit = 256 positions/rev = 1.4 resolution 16 bit = 65,536 positions = resolution Robust to launch loads Require decoding (frequently on chip) Choice of output reading formats (analog, serial, parallel) 41

42 Incremental Encoders Measure change in position, not position directly Have to be integrated to produce position Require absolute reference (index pulse) to calibrate Can be used to calculate velocities Generally optical or magnetic (no contact) 42

43 Incremental Encoder Principles 43

44 Quadrature Incremental Encoder 44

45 Incremental Encoder Interpretation Position Count up/down based on quadrature (finite state machine) Resolution based on location, gearing, speed 256 pulse encoder (1024 with quadrature) Output side 0.35 deg Input side 160:1 gearing deg = 7.9 arcsec Velocity Pulses/time period High precision for large number of pulses (high speed) 90 deg/sec, input side 41 pulses/msec (2.5% error) Time/counts High precision for long time between pulses (low speed) 1 deg/sec, output side 350 msec/pulse 45

46 Quadrature Direction Sensing 46

47 Velocity Measurement Number of bits/unit time High precision for rapid rotation Low resolution at slow rotation For n bit encoder reading k bits/interval Amount of time between encoder bits High precision for rapid rotation Low resolution for slow rotation! = k 2 n 2 47 t CLK h rad sec i! = n h rad t pulses sec i

48 Linear Variable Displacement Transformer 48

49 LRV Wheel Motor and Gearing 49 Case Study Lunar Roving Vehicle