Displacement and Motion Flashcards
Linear variable resistors
e.g. rheostat.
Changing length of resistor used changes
resistance, can attach moving object to slider.
§ Use as a voltage divider
§ Has high sensitivity, moderate precision
and large range (up to 100mm)
o Strain gauge
G= change in R/ R
over change in L/ L
Made using constantan conductors laminated between polymide film
and can be uniaxial, biaxial or triaxial. Resistivity increases as tensilenstress is applied to a wire.
§ Often used in a Wheatstone bridge with an instrumentation amplifier
§ Foil/silicon strain gauge has low/high sensitivity, moderate precision and small range
§ Be able to show how to use strain gauges in Wheatstone Bridge on cantilever beam
o Liquid metal strain gauge
Elastic resistors are constructed by filling a
compliant elastic tube with a conductive
fluid (e.g. aqueous electrolytes)
§ Gauge produces a force which depends upon
the elastic properties and geometry of the
elastic tubing. Must be kept in tension to
avoid buckling the tube.
§ Are inexpensive and simple to construct.
o Piezo-electric strain sensors
§ Charges displaced in response to mechanical stress.
§ Mostly used for measuring alternating strain in vibrating structures.
§ Inherently very stiff and are typically used as force sensors and pressure sensors.
Eddy-Current Sensors
§ Uses Lenz’s law
§ A high frequency current is
generated in a coil.
§ The magnetic field created by the coil passes through a
conducting target attached to
the moving surface.
§ The changing magnetic field generates eddy currents in the target.
• Eddy currents = a localized electric current induced in a
conductor by a varying magnetic field
§ The eddy currents alter the impedance of the coil.
§ The change in impedance can be measured as a change in the
amplitude and/or phase of the current through the coil
§ Are simple, low-cost and light weight, rugged, unaffected by dirt,
moisture, high bandwidth
§ Need to be linearised
o LVDT
Consists of a primary coil and
two secondary coils
symmetrically spaced from the
primary. The ferromagnetic
core can move axially within
the coils. The primary coil is
excited by an AC which
produces a magnetic field that
induces voltages in the two
secondary coils.
§ The secondary coils are connected in series opposition so that, when
the moveable ferromagnetic core displacement is in the middle, the
voltage generated in one secondary coil cancels the other.
§ When the ferromagnetic coil is displaced, the voltages induced in the
secondary cores are no longer equal, resulting in a nonzero output
voltage
o LVDT§ Properties:
• Made for a wide variety of length scales
• Relatively insensitive to temperature and aging
• Very sensitive
• No contact between the moving part of the sensor and the
coils
• Maximum frequency response is limited to well below the
frequency of the excitation voltage
• The moving core can add significant inertia to the item under
test
o Capacitive Modes
• Variable Spacing: Capacitance decreases in inverse proportion to electrode spacing, while impedance increases linearly with electrode spacing • Variable area: Capacitance decreases linearly with change in area, while impedance decreases in inverse proportion to the change in area
o Capacitive Properties
- Highest displacement resolution (~0.01 nm)
- High linearity
- High stability
- Limited range (~100 �m to 1mm)
- Moderate bandwidth
Hall-Effect:
• Charges flowing (in a current I with velocity v) through a conductor that is placed in a magnetic field will experience a Lorentz Force, given by: • F = qV × B • The force on the charges moves them in a direction orthogonal to the magnetic field, creating a voltage, V.
o Hall-Effect Sensor
Using Hall-Effect to transduce a magnetic field into a voltage:
Mount a magnet near a Hall Effect sensor, can create a
displacement transducer (the movement of the magnet will
determine the force experienced by the charges and therefore
determine the voltage induced).
§ Relatively poor temperature stability, are inexpensive, good for
displacements < 25mm, non-linear
§ Michelson Interferometer
• Uses interference of monochromatic light to measure displacement • As the moving mirror moves toward or away from the beamsplitter, the interference pattern cycles from constructive to destructive every �/2 • The direction of movement can be obtained by using two interferometers in a quadrature configuration • Ideal for measuring large displacements with very high accuracy and high speed
Heterodyne Interferometer
•
Light beam consists of two frequencies of similar wavelength light to produce beats at the optical receiver. • Change in beat frequency is directly proportional to the velocity of the moving target. The measured velocity can then be integrated to provide a measured change of position.
Optical Cross Correlation
• Motion detect between two images by performing crosscorrelation between subimages of each image
§ Optical Stereography
• Uses the information obtained from multiple cameras to
determine the position of points in space.
• Using Euclidean geometry, the position of a point can be
calculated from the location of the image of the point in the
cameras and a knowledge of the camera positions
• Accuracy of position estimation is dependent on the
resolution of the cameras, geometric distortion of the imaging
devices, knowledge of camera positions
• For dynamic imaging there is often a trade off between
spatial, temporal and image resolutions because of the rate
at which information can be moved from the camera to
memory
