Sensors Flashcards

Question

Rotary encoders - speed sensors

Answer 1

• Are detecting the angular position of a shaft Main applications • Defining a position in processing / machining industries • Attached to a motor used for closed loop control: servomotor • Speed sensors in mobile applications (e.g. ABS)

Answer 2

Inductive 1. Detects only conductive materials (metals) 2. Low susceptibility to deposition of (unwanted) materials on sensor surface 3. High accuracy (repeatability of switching distance) 4. Robust with regard to disturbing effects 5. Limited adjustment of switching point Capacitive switch 1. Detects many materials (metal, glass, most polymers, grease and oil, ceramics, solvants and alcohol) 2. Unwanted deposition of material on the sensor surface can cause erroneous switching 3. Low accuracy (repeatability of switching distance) 4. Any changes in dielectric constant can cause disturbances (e.g. variation in water content of fruits). Also temperature fluctuation has an effect. 5. Switching point can easily be adjusted by altering oscillation circuit

Answer 3

high to low, formula!

Answer 4

* Are operating like incremental encoders * Have a reference mark indicating a unique reference position integrated in the same scale * Used in linear and rotative form * Single and multiple reference marks possible

Answer 5

Using a single encoder disc while using only two digital signal levels would theoretically give only 2 shaft positions per revoulution • Higher resolution of shaft positions requires more than 1 signal channel • Gray code: multi-channel binary encoding disc (or linear scale) with n channels • Gray code enables a resolution of 2n positions per revolution • Measuring range 0 – 360°

Answer 6

• If not only the angular position of a shaft is required to be detected, but also the amount of shaft revolutions, multi turn encoders are required • One encoder disc is defining the angular position of the shaft with regard to a fixed coordinate system • Additional means required to detect the amount of turning cycles, e.g. reduction gears with second encoder • Measuring range e.g. 0- 720° • Position (including turning cycle) instantly available after power on • Important application: automotive steering wheel sensor

Answer 7

• Some applications require not only speed measurements, but also the current position • The current machine position is determined by the kinematic geometry (robot arms, linear spindle drives,…) and the current shaft position of the motor • Depending on the application, position must be available instantly after start of power cycle or after a refernce run • Absolute position can only be calculated starting from a reference position when using incremental encoders • Some applications store the last position during power down mode to be reused in next power cycle. This holds the risk of undetected movement during power-off mode

Answer 8

• Using absolute encoders • Using incremental encoders with reference mark • Using incremental encoders with external reference switch

Answer 9

Advantages • Position instantly available after start of power cycle • Easy recapturing of position after signal disturbance during power cycle • No position drift due to skipping increments Disadvantages • Limited dimensional range and/or limited resolution • Expensive

Answer 10

Advantages • Very flexible arrangements possible • Simple and cheap • Resolution and dimensional range not limited Disadvantages • Reference run required • Position drift possible when failing to read single increments due to disturbances • Control must count increments: Signal input‘s speed capability of control must match to frequency of increments sent by encoder

Answer 11

Rotary encoders can easily be used to detect also linear speeds/ displacements • The inversion does not apply: hardly any common application uses linear encoders to detect rotary motion

Answer 12

Measuring principles • There is hardly any physical principle to usefully directly detect force and torque • Only photoelasticity can directly show internal stresses based on a stress dependent refractive index, but is only suitable in laboratory environment • Industrial torque and force sensors are detecting strains or displacements caused by the applying force / torque  indirect measurement • Torque measurement is typically achieved by measuring strains under 45° inclination to the shafts main axis Application examples • Processing force detection in tool machines • Mass flow detection in fertilizer spreaders • Power measurement in gear boxe

Answer 13

``` • Very common and versatile sensor • Meander-shaped resistor made of metal alloy (containing Nickel) placed on a carrier foil • Foil is fixed to a structure to be measured that is subject to stress and strain Advantages • Cheap • Easily applied to many structures • Precise Disadvantages • Limited robustness • Susceptibility to humidity and environmental influences • Electronic contacting difficult ```

Answer 14

``` • Nominal measuring range is determined by the mechanical structure • Designs encapsulating the strain gage improve robustness Wheatstone bridge • Supply voltage UB • Measuring signal UA • Typical output signal 1-4 mV/V • More complex (than the one shown) circuits are used including temperature, drift and offset compensation ```

Answer 15

``` • Piezeoelectric materials show a load displacement under mechanical stress • Typical piezoelectric active element is silica quartz • Electrical charge is measured with charge amplifier (causing the charge to disappear) Advantages • Robust device • Highly dynamic measurement • Multiaxial measurement possible Disadvantages • No static measurement possible (requires charge integration) ```

Answer 16

``` • Strain gages aligned under 45° or 135° to shaft axis • Transmission of supply and signal voltage with telemetry or slewing ring between rotating shaft and standstill structure ``` Advantages • Precise Disadvantages • Requires Telemetry / slewing rings • Electrical contacting under high dynamic mechanical load is critical - Hollow shaft - Solid shaft - Cage design

Answer 17

``` • Supply voltage has to be transferred contactless with antenna onto rotating shaft • Signal output voltage has to be transferred back to standstill structure via antenna • Signal transferred with carrier frequency modulation ``` Advantages • Contactless transmission of signals Disadvantages • Limited robustness • Susceptibility to EMC influences • Expensive

Answer 18

* Sliding contacts * Multi-channel transmission possible Advantages • Cheap Disadvantages • Contact wear • Limited speed • Dragging torque

Answer 19

• Angular displacement of two distant shaft cross section Advantages • No telemetry required ``` Disadvantages • Good sensor sensitivity requires high shaft torsion  unfavorable with regard to drivetrain dynamics • High packaging space • Limited precision ```

Answer 20

• Principle of inverse magnetostriction (Wiedemann-effect) • Magnetostriction: Ferromagnetic bodies change their dimensions once they are subject to an external magnetic field • Inverse magnetostrictive effect: magnetic properties (permeability, anisotropy,…) of a magnetized ferromagnetic body change once they are subject to mechanical stress/strain • Circular magnetization of shaft, no external magnetic field in idle condition • Applied torque causes magentic field to exit the shaft, detected by field sensors (e.g. fluxgates) • External magnetic fiedl corresponds to applied torque

Answer 21

``` • Fertilizer spreader uses rotating discs to spread fertilizer on the field • Driving torque of spreading discs corresponds to th emass flow of fertilizer being spread Advantages • Contactless and robust (no telemetry required) • Low influences of temperature on measuring results • Direct measurement near the process reduces disturbing effects Disadvantages • Dedicated sensor required ```

Answer 22

``` Pressure is typically measured indirectly by measuring the deformation of a diaphragm For deformation (displacement) measurement, commonly used principles are employed: • Strain gages • Piezoelectric strain sensors • Inductive sensors • Optoelectronic (interferometric) sensors ```