Sensors Flashcards
Difference between sensors and transducer !
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.
Distinguishing sensors by need for auxiliary energy !
Active sensors - consume auxiliary power (e.g. strain gauges, flux gates …)
Passive sensors - do not need auxiliary power (thermocouples, photodiode, piezoelectric sensors)
Auxiliary energy !
energy required to operate the device
Auxiliary power !
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.
Distinguishing sensors signals by their need for reference (definition)
- 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)
Smart (intelligent) sensors (definition)
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;
Transfer function - definition
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,…)
Transfer function various types:
- Linear: E= A+Bs (A: intercept or offset, B: slope or sensitivity) - Logarithmic - Exponential - Power Functions: E= A+Bs ^ k
What defines sensors behaviour? (transfer function)
- 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)
(Re-) Calibration !!!
The process of aligning the sensors transfer function with the real and specific conditions.
Various ways of calibration
- Modifying the currently used transfer function based on actual measurements
with known conditions (e.g. using precise reference sensors) - 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) - Modifying the electronic circuit that the sensor is operating in (e.g. laser trimming of a matching resistor)
Parameters to characterise a sensor (1)
- Sensitivity - Ratio of change in sensor signal related to a change in the sensor stimulus
- Stability - Change of sensor signal over time with no
change of the stimulus (drift). Should ideally
be as small as possible. - 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
Parameters to characterise a sensor (2)
- Speed of response - Time required until the sensor recognizes (and displays) an instant change of the stimulus;
- Sensor Bandwidth - Maximum frequency of an oscillating stimulus,
at which the sensor signal still displays the stimulus curve correctly; - Overload characteristic - Sensor behaviour once the stimulus exceeds a specified measurement range. Could range from signal saturation until sensor destruction.
- 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
Parameters to characterise a sensor (3)
- Linearity
Deviation of a sensors transfer function from
ideal linear behaviour - Operating life:
Time span or operating cycle that the a sensor
can operate within its specification - Size & weight
- 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
Parameters to characterize a sensor (4):
- 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 - 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!!!!
Parameters to characterise a sensor (5):
- Environmental conditions
Definition of the environmental conditions that a
sensor can operate in (within its specifications).
Most commonly temperature, humidity,
vibrations, electromagnetic compatibility (EMC),
… - 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,…
Physical conversion phenomena
• 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
Manual Switches
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)
Proximity sensors
• 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
Main applications for proximity sensors
• 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
Mechanical switches (PS)
- 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
Reed switches (PS)
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
Inductive switches (PS) !!!
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
Capacitive switches (PS)
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, …)
Rotary encoders - speed sensors
• 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)
Inductive vs Capacitive switches
Inductive
- Detects only conductive materials (metals)
- Low susceptibility to deposition of (unwanted) materials on sensor surface
- High accuracy (repeatability of switching distance)
- Robust with regard to disturbing effects
- 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
Optical encoders
high to low, formula!
Incremental measurement with internal
reference mark
- 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
Absolute measurement with optical encoders
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°
Absolute multi-turn measurement with optical
encoders
• 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
Absolute vs. relative encoders
• 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
Many superordinated machine controls require
information on the spatial orientation of the machine. How it can be achieved?
• Using absolute encoders
• Using incremental encoders with reference mark
• Using incremental encoders with external reference
switch
Absolute Encoders: (+/-)
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
Incremental Encoders with external reference switch: (+/-)
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
Linear vs. Rotary encoders
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
Torque and force sensors - Measuring and application Principles
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
Metal strain gages, foil-based
• 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
DMS based sensors
• 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
Piezoelectric sensors
• 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)
Strain gage based torque
measurement
• 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
Telemetry
• 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
Signal transmission with slewing ring
- Sliding contacts
- Multi-channel transmission possible
Advantages
• Cheap
Disadvantages
• Contact wear
• Limited speed
• Dragging torque
Torque measurement based on
twisting angle
• 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
Magnetoelastic torque measurement
• 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
Application example for magentoelastic
torque sensors
• 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
Pressure measurement (Pressure sensors)
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
