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Sensors Flashcards

1
Q

Difference between sensors and transducer !

A

Sensors convert any kind of stimulus (any form of energy) into electrical current (sensor signal). (ENERGY CONVERTERS)

Transducer convert any kind of stimulus/energy into any other kind of energy.

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2
Q

Distinguishing sensors by need for auxiliary energy !

A

Active sensors - consume auxiliary power (e.g. strain gauges, flux gates …)
Passive sensors - do not need auxiliary power (thermocouples, photodiode, piezoelectric sensors)

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3
Q

Auxiliary energy !

A

energy required to operate the device

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4
Q

Auxiliary power !

A

Electric power that is provided by an alternate source and that serves as backup for the primary power source at the station main bus or prescribed sub-bus.

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5
Q

Distinguishing sensors signals by their need for reference (definition)

A
  • Absolute signal - Signal does not need a reference to be interpreted; directly refer to a commonly used scale ( temperature in Celsius, position in mm)
  • Relative (incremental) signals - related to a reference value that is not commonly known. Reference value for incremental systems is specific to an application or even the current power cycle, e.g. incremental encoders: shaft rotated 10 degrees clockwise from last position (Which has to be known to superordinate control)
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6
Q

Smart (intelligent) sensors (definition)

A

Perform more tasks than only the conversion of the physical sensor signal in an electrical signal; typically use microcontrollers, often FPGAs (field programmable gate array) to perform such tasks:
- Compensation of disturbances (e.g. temperature, humidity…)

  • Self-check capabilities;
  • Bus communication;
  • WiFi communication;
  • Switching of measuring range;
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7
Q

Transfer function - definition

A

Describes the relationship of stimulus (s) to the corresponding sensor signal (E).

Transfer functions are obtained by measuring
the sensors signal in operating conditions of
which the stimulus is precisely known by:
• Using measurement standards as stimulus
(e.g. weight, length,…)
• Using Reference sensors with higher (and
known) precision to characterise the
stimulus (e.g. speed, temperature,…)

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8
Q

Transfer function various types:

A 
- Linear: E= A+Bs (A: intercept or offset,
B: slope or sensitivity)
- Logarithmic
- Exponential
- Power Functions: E= A+Bs ^ k
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9
Q

What defines sensors behaviour? (transfer function)

A
  • High sensitivity (B) - significant change of
    sensor signal also at only small changes in
    stimulus - good sensor resolution

• Offset (A) describes sensors behaviour without
stimulus (e.g. no speed)

Sensitivity is constant only for linear transfer
functions

In case of non-linear transfer functions,
sensitivity depends on the stimulus; it
becomes the first derivative of the stimulus
function at the particular stimulus:

𝐵(s) (sensitivity) = dE(s)/ds (first derivative of stimulus)

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10
Q

(Re-) Calibration !!!

A 
The process of aligning the sensors transfer
function with the real and specific conditions.
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11
Q

Various ways of calibration

A
  1. Modifying the currently used transfer function based on actual measurements
    with known conditions (e.g. using precise reference sensors)
  2. Modifying the sensor itself in order to change its transfer function, often referred
    to as trimming (e.g. trimming the resistor layer of a thermistor with a grinder)
  3. Modifying the electronic circuit that the sensor is operating in (e.g. laser trimming of a matching resistor)
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12
Q

Parameters to characterise a sensor (1)

A
  1. Sensitivity - Ratio of change in sensor signal related to a change in the sensor stimulus
  2. Stability - Change of sensor signal over time with no
    change of the stimulus (drift). Should ideally
    be as small as possible.
  3. Accuracy:
  • (Absolute) accuracy: Maximally expectable
    error of the measurement compared to the
    real (precise) value of the stimulus
  • Repeatability: Range of sensor readings
    when one and the same stimulus is
    measured multiple times
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13
Q

Parameters to characterise a sensor (2)

A
  1. Speed of response - Time required until the sensor recognizes (and displays) an instant change of the stimulus;
  2. Sensor Bandwidth - Maximum frequency of an oscillating stimulus,
    at which the sensor signal still displays the stimulus curve correctly;
  3. Overload characteristic - Sensor behaviour once the stimulus exceeds a specified measurement range. Could range from signal saturation until sensor destruction.
  4. Hysteresis - Difference in sensor signal, if one and the same stimulus is approached from two sides (e.g. T =
    70°C when cooling down or heating up);

Graphs : Speed of response, Sensor bandwith, Hysteresis curve

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14
Q

Parameters to characterise a sensor (3)

A
  1. Linearity
    Deviation of a sensors transfer function from
    ideal linear behaviour
  2. Operating life:
    Time span or operating cycle that the a sensor
    can operate within its specification
  3. Size & weight
  4. Sensor (Stimulus) range
    Range between maximally and minimally
    detectable stimulus. Sensor might probably
    also work outside range, but accuracy,
    hystesresis and other specifications might
    probably not be kept
    12.efines the upper end of the sensor range.
    Sometimes, accuracy and linearity values are
    given in % FS

Graphs - Linearity and Full scale

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15
Q

Parameters to characterize a sensor (4):

A
  1. Resolution
    Smallest possible change of the stimulus that
    still causes a (noticeable) change in sensor
    signal. Mainly relevant for sensors with
    analog/digital (A/D) conversion, e.g. 12 bit
    resolution = 212 = 4096 possible subdivisions of
    the whole sensor range
  2. Selectivity
    Range limitation of parameters describing the
    object to be measured other than the stimulus
    (e.g. detection of radiation power in a limited
    wavelength interval, measuring of mass flow
    only of certain substances,…)\

Resolution graph and calculations!!!!

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16
Q

Parameters to characterise a sensor (5):

A
  1. Environmental conditions
    Definition of the environmental conditions that a
    sensor can operate in (within its specifications).
    Most commonly temperature, humidity,
    vibrations, electromagnetic compatibility (EMC),
    …
  2. Output signal
    Definition of an electric signal that represents
    the measuring range.
    Main distinction between
    - Analog signals: 0-5 V, 0-10 V, 4-20 mA,…
    - Digital signals: digital outputs 24V, TTL,
    open collector, CAN-bus, Profibus,…
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17
Q

Physical conversion phenomena

A

• Thermoelectric
Mutual influence of temperature and electrical
parameters, e.g. Seebeck-effect, Peltier-effect
• Photoelectric
(Visible) radiation effects electrical parameters
and vice versa, e.g. Photodiode, LED
• Magnetoelectric / Electromagnetic
Magnetic parameters effect electrical
parameters and vice versa. Very common
conversion principle, e.g. hall effect
• Thermoelastic
Temperature influences affect elastic properties
of a body (inversion practically not used), e.g.
thermostat

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18
Q

Manual Switches

A

Manual switches are the simplest way of
human-machine-interface (normally not
referred to as sensor)

Various variants are used
• Pushbuttons or turn knobs
• Latching / non-latching
• Various numbers of switching positions
• With / without safety level

Their electrical behaviour can differ
• Normally open (NO): Electrical contact is
open until button is pushed
• Normally closed (NC): Electrical contact is
closed until button is pushed
• Multiple switching positions (latching)

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19
Q

Proximity sensors

A

• Are used to detect the position (presence) of
an object in a defined area
• Their signal output is digital: object is present or not

Commonly used physical principles:
• Mechanical switches
• Capacitive switches
• Inductive switches

20
Q

Main applications for proximity sensors

A

• Machine controls give commands to actuators (e.g. pneumatic cylinders).
Confirmation on the actuator reaching a
desired position is required before executing
subsequent commands
• Machine controls might execute specific actions depending on the presence of an object (e.g. workpiece)
• Proximity sensors can be used to detect rotational speed by arranging the projections and recesses of an encoder wheel in a way
that projections are detected (high state) and recesses not (low state)
• Fill level sensors for tanks

21
Q

Mechanical switches (PS)

A
  • Are used to detect the position (presence) of an object
  • Common application: final position switch for actuators and drives (e.g. positioning axis, electric garage door drive)
Advantages:
• Cheap
• Flexible application Disadvantages
• Limited accuracy (repeatability)
• Mechanical contact actuation is subject to wear and fatigue - amount of switching cycles is limited
22
Q

Reed switches (PS)

A

Application example: final position switch in pneumatic cylinders;

• NO version (=„normally opened“) closes
circuit in presence of magnetic field
• NC version (=„normally closed“) opens
closes circuit in presence of magnetic field

Advantages
• Cheap
• Switching contacts encapsulated in
protective atmosphere
Disadvantages
• Limited amount of switching cycles
• Limited switching frequency
23
Q

Inductive switches (PS) !!!

A

Advantages of inductive switches
• Contactless sensing is not afflicted with
contact wear / fatigue  high amount of
switching cycles
• High switching frequencies possible ( can
be used for speed sensing)
• Hermetically sealed encapsulation gives
high robustness
• High accuracy
Disadvantages
• Only metallic objects can be detected
• More expensive compared to e.g. reed
switches due to need of oscillating circuit

24
Q

Capacitive switches (PS)

A

Advantages of capacitive switches
• Contactless sensing is not afflicted with
contact wear / fatigue  high amount of
switching cycles
• High switching frequencies possible ( can
be used for speed sensing)
• Hermetically sealed encapsulation gives
high robustness
• Many materials can be detected
• Switching sensitivity can be easily adjusted
by altering the oscillation circuit (
switching distance can be set, fluids can be
detected behind plastic tank walls)
Disadvantages
• More expensive compared to e.g. reed
switches due to need of oscillating circuit
• Accuracy lower compared to inductive
switches due to more disturbing effects (e.g.
humidity, …)

25
Q

Rotary encoders - speed sensors

A

• 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)

26
Q

Inductive vs Capacitive switches

A

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

27
Q

Optical encoders

A

high to low, formula!

28
Q

Incremental measurement with internal

reference mark

A
  • 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
29
Q

Absolute measurement with optical encoders

A

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°

30
Q

Absolute multi-turn measurement with optical

encoders

A

• 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

31
Q

Absolute vs. relative encoders

A

• 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

32
Q

Many superordinated machine controls require

information on the spatial orientation of the machine. How it can be achieved?

A

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

33
Q

Absolute Encoders: (+/-)

A

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

34
Q

Incremental Encoders with external reference switch: (+/-)

A

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

35
Q

Linear vs. Rotary encoders

A

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

36
Q

Torque and force sensors - Measuring and application Principles

A

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

37
Q

Metal strain gages, foil-based

A 
• 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
38
Q

DMS based sensors

A 
• 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
39
Q

Piezoelectric sensors

A 
• 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)
40
Q

Strain gage based torque

measurement

A 
• 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
41
Q

Telemetry

A 
• 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

42
Q

Signal transmission with slewing ring

A
  • Sliding contacts
  • Multi-channel transmission possible

Advantages
• Cheap

Disadvantages
• Contact wear
• Limited speed
• Dragging torque

43
Q

Torque measurement based on

twisting angle

A

• 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
44
Q

Magnetoelastic torque measurement

A

• 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

45
Q

Application example for magentoelastic

torque sensors

A 
• 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
46
Q

Pressure measurement (Pressure sensors)

A 
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

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