Sensors, Optical Encoders, LVDTs
How can you determine the direction of rotation with dual channel?
sign of phase shift between A and B, +90 between A and B vs. -90 degree
What does LVDT stand for and what is it?
Linear Variable Differential Transformer Electromechanical sensor for measuring linear position
Name 3 alternatives to an optical encoder give a pro and con of each
Potentiometer, Magnetic Encoder, Resolver Potentiometer (pro:Inexpensive, Con: finite # of rotations) Magnetic encoder (pro:more robust, con: reduced accuracy) Resolver (pro:high accuracy, con: more expensive)
What are pros and cons of AC operated LVDT's vs. DC operated LVDTs
Pro AC: Increased device life, functions in more extreme enviornments, smaller package size Con AC: Requires external signal conditioning electronics, more expensive Pro DC: Reduced system cost and complexity Con: doesnt work in as many places and is bigger
What are pros and cons of LVDT
Pros: Infinite resolution Repeatability Frictionless measurements Cross-axis rejection Rugged design Cons: Cost Size Power consumption Limited linear range Limited frequency response
How do absolute encoders work?
The combined output of multiple tracks each represent a unique position. Output in gray code (modified binary)
What specifications should you consider when choosing an LVDT?
Stroke range Excitation voltage Excitation Frequency Linearity Accuracy Stroke to length ratio
How does incremental dual channel work?
- Two tracks/two detectors offset half a segment from each other result in two square waveforms phase shifted 90⁰ from each other.
What is the resolution of a single channel with N windows?
360/(2*N), also can use this to get angular displacement
What is the resolution of a dual channel with N windows on a single track?
360/(4*N)
How does a LVDT work? Name the 5 steps
1. AC to primary coil 2. Ferromagnetic core fluctuates magnetic polarity Note: Core increases coupling between primary and secondary coils 3. Polarized core induces voltages in secondary coils 4. Secondary coils in series opposition connection, allowing differential voltage measurement 5. Core movement changes induced voltages at each secondary coil, changing the differential voltage
What are the 3 types of armature mechanisms? When should each be used
1. unguided armature 2. Captive (guided) armature 3. Force-extended armature Unguided armature LVDTs should be used when external guidance is possible Guided armature LVDTs should be used when: External guidance is not possible High risk of misalignment Operating over a long range Force-extended LVDTs should be used when the shaft cannot be attached or should not be attached to the component being measured
What is the resolution for absolute encoders?
Determined by number of encoder bits which is equal to the number of tracks... 360/2^n where N is the number of bits (or number of tracks)
When would one need an absolute encoder?
For applications that require knowledge of absolute position, retained even after power loss. E.g. robot joint positions.
What is grey code?
For every sector transition of an optical encoder, only one bit at most changes
What are 3 optical encoder components
Housing assembly, shaft, light source, code disk, mask, photodetector assembly, electronics board
What is the difference between Incremental Encoders and Absolute Encoders?
Incremental encoders Measure relative position (absolute with homing) Output data in the form of pulses Less expensive than absolute encoders Absolute encoders Measure absolute position without homing Output data in gray code note: Homing - initialization of the encoder to zero position
What is an optical encoder
Optical encoders are electromechanical devices that utilize light detection to measure angular or linear position.
What are 3 things to keep in mind when selecting an encoder?
Output: incremental or absolute Resolution: Noise + Cable length: longer cable=more noise Index channel: is homing necessary Cover/base: How will the encoder be secured? In what environment?
What are the components of a LVDT?
Sheild, Primary coil, secondary coil, ferromagnetic core, shaft
What are the characteristics of a Captive armature? and what is an example application
Shield mechanically fixed in place Bearings within shield interior guide shaft Bearings provide low friction operation Shaft may or my not be attached to component being measured ex. Measuring the liquid height / volume in a tank by measuring the position of a float
What are the characteristics of a Force-extended armature? And what is an example application
Shield mechanically fixed in place Bearings within shield interior guide shaft Bearings provide low friction operation Shaft not attached to component being measured Shaft continually pushed to fullest extension by force-extension mechanism: ex. Measuring surface profile and/or surface roughness for product inspection
What are the characteristics of an unguided armature? and what is an example application
Shield mechanically fixed in place Shaft attached to component being measured Shaft requires external guidance through shield interior ex. Feedback control for control valve actuator
What are the 3 incremental types? and what do they detect?
Single channel: detects motion Dual Channel: Detects motion and direction Dual Channel with Zero Index: Detects motion, direction, and has a reference marker for home position
How does single channel incrementing work?
Single code strip blocks/passes light from a source to a photodetector. Resultant "sine" wave voltage filtered to form a square wave signal for digital processing (pulse counting).
What are the two types of encoders (based on light detection technique) and how do they work?
Tranmissive encoders and reflective encoders. Transmissive encoders have emitter and detector on opposite sides of the codewheel (windows of code wheel are holes). Reflective encoders have emitter and detector on same side of the codewheel (windows of code wheel are mirrors).
Name 3 applications of optical encoders?
robotic arms, coveyors, turbines, actuators