Lecture 8 Flashcards
What are the different sensing principles?
(7 answers)
- Resistive
- Capacitive
- Inductive
- Piezoelectric
- Photoelectric
- Optical
- Electromyography
What is a potentiometer/how does it work?
- Resistance changes with the contact length at the resistor
- Change of length is acheived by a mechanical slider on resistive path
What are examples of resistive sensing?
Potentiometers
Strain gauges
Wheatstone bridge
What is a strain gauge/how does it work?
- Resistance changes with the length of the resistor
- Change of length is achieved by a mechanical stress and elongation of the material with resistivity
- Temp. sensitive
- Direction sensistive
R = rho(resistivity) x length/area
What is a wheatstone bridge/how does it work?
Sensitive to small changes in resistance
Measure strain in different directions
Application in force torque sensors
What is an example of a 6DOF force/torque sensor?
- Resistors are printed on a steel substrate
- 3 Wheatstone bridges (1 per force axis)
What is capacitive sensing/how does it work?
Uses a capacitor:
- Charge permittivity (epsilon), size of the gap between the plates d, or the cross-sectional area of the plates A
C = epsilon x A/d
What is inductive sensing/how does it work?
Measure position of ferromagnetic materials by interacting with the induced magnetic field
What is Magnetic sensing/how does it work?
Hall effect sensor:
- When magnetic field is applied to a conductor with current flowing through it, a voltage drop across the conductor is generated
- Measures magnetic field
- No physical contact required
What is Piezoelectric sensing/how does it work?
Measures voltage across crystal structure to sense applied force
What are examples of inertial sensing?
Accelerometers
Gyroscopes
How does inertial sensing with accelerometers work?
- Based on accelerated mass & measured force (a = F/m
- Many different ranges and resolutions
How does inertial sensing with gyroscopes work?
- Angular rate sensor
- Based on rotating or vibrating mass
- Many different sizes, ranges, and resolutions
What is Photoelectric sensing/how does it work?
- Object is detected through the presence/absence of light
- Light is interrupted by the object’s presence & the sensor detects the changes
How does a (absolute) position encoder work? (Principle)
- Pattern of light and dark lines are printed onto a strip, which is detected by an optical sensor
- Lines are arranged in a way that the combination is unique at each point
- Sensor is an array of diodes
- Sensor OR strip is moving
What are the characteristics of absolute position encoders?
- Linear or angular encoders exist
- Can have very good resolution, up to 25 bits (which corresponds to 0.00001 degrees)
- Exist in single/multi-turn versions
- Disks are fragile and expensive
What are examples of position encoders?
- Absolute position encoder
- Incremental position encoder
- Resolvers
How does an incremental position encoder work?
- Converts motion into a sequence of digital pulses
- Uses a single line or disk that alternates black & white
- Channel A & B (either 2 light sensors or 2 tracks)
- Allows to determine rotation direction
How does a Resolver (position encoder) work?
- Rotation encoding is done with inductive coils
- Angle is found from sin and cos coils
- There is NO CIRCUITRY so it’s robust to extreme environments
What are some features of “markerless tracking”?
- Uses image analysis to identify the skeleton with PATTERN RECOGNITION & SILHOUETTE TRACKING
- Skeleton is improved with: FEATURE EXTRACTION, MODEL-BASED FITTING, USING MULTIPLE CAMERAS
- Fast set up
- Offline image analysis (not real time)
What are some features of “marker based tracking”?
Camera Based:
- Active markers
- Passive markers
- Mixed systems
Based on rigid body landmarks (e.g. bones and joints)
Long set-up time
Online (real time)
What are the two types of electrodes?
Surface and needle electrodes
What are some characteristics of Static sensors?
- Requires a transfer function (input-output)
- There may be on offset (output value under zero-input condition)
- Span (max. input minus min. input)
- Overload is the max. input before breaking sensor
- Resolution (smallest detectable change)
What is a dynamic properties of dynamic sensors?
- Sensor’s response to time-variable input is different in steady state conditions
