Position, Velocity, and Acceleration

Sensing Principles

Electrical resistance
Electrical capacitance
Electromagnetism
Photoelectric effect
Piezoelectric effect
Hall effect
Doppler effect
Interferometry

Measurement Devices

Potentiometers
LVDT (Linear Variable Differential Transformer)
Optical Encoders
Tachometers
Accelerators
Interferometers
Capacitance Sensors

potentiometer

a measuring instrument for measuring direct current electromotive forces
a variable voltage divider

LVDT

Linear Variable Differential Transformer
a linear displacement transducer
a type of electrical transformer used for measuring linear displacement
smaller displacement measurements
linear only over a certain range

LVDT Signal Output

Capacitive Displacement Sensing

C = ε_o*ε_r*(A/d)
C: capacitance
ε: permittivity
A: plate area
d: distance between plates

Capacitance Sensor

non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target". They are also able to measure the thickness or density of non-conductive materials.

Encoder

a sensor that detects rotation angle or linear displacement and provides feedback in an electrical signal that can be read by some type of control device in a motion control system

Position Encoders

Used to determine the mechanical position of an object
Can be arranged in a linear or rotary configuration
Encoders senses: position, direction, speed, or counts
Different encoder detections devices: magnetic, mechanical, resistive, inductive, capacitive, laser, and optical (common)

Absolute and Incremental Encoder

Two types of encoders

Linear Encoder

It uses a linear transducer to measure the distance between two points
ex: in CNC milling machines
it can be either absolute or incremental

Rotary Encoder/Shaft Encoder

Collects data and provides feedback from the rotating device

Absolute Rotary Encoder

Measures angular position

Incremental Rotary Encoder

Measures speed, distance, and linear position

Absolute Position Encoder

the mechanical position of the object x = 2 in

Incremental Position Encoder

the change in position in relation to the object and encoder

Optical Encoder

interpret data in pulses of light to measure the position, direction, and velocity of the object
ex: printers, CNC machines, robotics

Quadrature

arranging in _________ provides finer resolution (by monitoring transitions rather than states) as well as direction.
A quadrature encoder is an incremental encoder with 2 out-of-phase output channels used in many general automation applications, where sensing the direction of movement is required.

Photoelectric Effect

a phenomenon in which photon energy (from light) releases electrons from their otherwise more stable state on a surface

photodiode

photon energy produces free charge carriers (electron-hole pairs) to overcome the otherwise current-impeding junction between p-doped and n-doped semiconductor layers.
- emission/rejection of electrons from the surface of, generally, a metal in response to incident light.

Laser Interferometer

measures the resulting distance by splitting a laser beam into two, sending each of the two beams along different directions in space (each along one of the two arms of the detector), and then recombining the beams

Hall Effect

disruption to the "ordinary" flow of current and charge distribution across a thin sheet (typically a semiconductor) by the presence of a magnetic field oriented perpendicular to it.
- a wide variety of hall effect sensors can measure magnetic field strength or subtle changes in the field by outputting a voltage that is proportional to it.

Lorentz force

the force experienced by a point charge moving along a wire that is in a magnetic field
F = q(E + v x B)

Tachometer

device that measures angular velocity
- can be implemented in:
electromagnetic generators
magnetic pickups
photo-detectors

Accelerometer

senses acceleration (changes in velocity) by measuring position as a function of time.
a mechanical vibration system having spring-mass-damper elements
commonly expressed in multiplies or fractions of g (9.81 m/s^2)

Piezoelectric effect

the ability of certain materials to generate an electric charge in response to applied mechanical stress.
A piezoelectric crystal is placed between two metal plates. At this point the material is in perfect balance and does not conduct an electric current. Mechanical pressure is then applied to the material by the metal plates, which forces the electric charges within the crystal out of balance.

piezoelectricity

the two-way relationship between mechanical stress and electric potential exhibited by some materials, based on changes in the crystalline ionic structure along internal asymmetries

piezoresistive

______ materials exhibit a change in electrical resistance when experiencing mechanical strain

MEMS Accelerometers
(Analog Devices ADXL)

Features:
- 2-axis acceleration sensor
- ~ 5mm x 5mm chip
- 2-100g operating ranges
- mili-g resolution
- mA power consumption
- 1000g Shock Survival
Applications:
- Inertial Navigation
- Portable Device Security
- Motion Detection
- Camera Image Stabilization
- Game controller feedback

Criteria for Position/Motion Sensors

Contact vs Non-contact
Range
Resolution
Accuracy
Repeatability
Frequency Response
Signal Conditioning Requirements
Environmental Robustness
Cost