TRANSDUCERS What’s the purpose of this presentation? • To learn a technical subject ( the content) in English (the language) • Because there is no other way to learn a language than listening and talking • Because 90% of technical literature is in English • Because we’d like you to be competitive with European students of your age

Some friendly reminders • From now till the end of this presentation we will talk (possibly) in English only! • I will speak in plain English and slowly but I won’t speak, preferably, in Italian/English and neither in English/Italian! • Questions are most welcome … if they are simple!

What is a transducer ? (a fairly good definition)

A transducer is a device which transforms a non-electrical physical quantity (i.e. temperature, sound or light) into an electrical signal (i.e. voltage, current, capacity…)

Where are they used and what for? • Antenna: is the most basic transducer and can be made from a simple piece of wire. It converts electromagnetic energy into electricity when it receives signals and does the opposite when it transmits

• Strain gauges: have a long thin wire attached to a foil backing which is glued to an object. When the object changes shape, the strain gauge also changes shape and its resistance changes. The amount of stress or strain in the object is calculated from this change in resistance • Accelerometer: which converts the change in position of mass into an electrical signal. Accelerometers measure the force of acceleration and deceleration. They are used in car airbags, stability control, hard drives, and many electronic gadgets.

…more • Geiger counter: detects radiation levels by using a

transducer called a Geiger-Muller tube

• Microphone and Speaker. Microphones convert sound

pressure waves into electrical current, while speaker convert electrical current into sound pressure waves. • And many many others.

non-electrical physical quantity

electrical signal

What is its structure A transducer is made of three blocks: o Input I/F o Sensor o Output I/F

• Another definition (American National Standards

Institute) – A device which provides a usable output in response to a

specified measurand

• A sensor acquires a physical quantity and

converts it into a signal suitable for processing

(e.g. optical, electrical, mechanical)

• Nowadays common sensors convert

measurement of physical phenomena into an

electrical signal

• Active element of a sensor is called a transducer

What does Transducer

mean? A device which converts one form of energy to

another

When input is a physical quantity and output

electrical → Sensor

When input is electrical and output a physical

quantity → Actuator

e.g. Piezoelectric: Force -> voltage Voltage->

Force

=> Ultrasound! Microphone, Loud Speaker

Where does it fit in the DAQ (Data

AcQuisition)

Important parameters of a transducer • Static response: how does it respond to slowly variable signals, is it precise and accurate

• Dynamic response: how does it respond to quickly variable signals (bandwidth of control system, tr, ts !!!) • Environmental factors: how these factors are affecting transducer performance • Reliability: MTBF (MTBF. Mean time between failures is

calculated in hours and is a prediction of a power supply’s reliability. MTBF = 1/λ,

failure rate. MTTF, mean time to failure, may be substituted in some datasheets for

units that will not be repaired. These definitions are purely statistical parameters and

are used to represent the reliability of (electronic) components.

Commonly Detectable Phenomena •Biological

•Chemical

•Electric

•Electromagnetic

•Heat/Temperature

•Magnetic

•Mechanical motion (displacement, velocity,

acceleration, etc.)

•Optical

•Radioactivity

Common Conversion Methods •Physical

–thermo-electric, thermo-elastic, thermo-

magnetic, thermo-optic

–photo-electric, photo-elastic, photo-magnetic,

–electro-elastic, electro-magnetic

–magneto-electric

•Chemical

–chemical transport, physical transformation,

electro-chemical

•Biological

–biological transformation, physical

transformation

Choosing a Sensor

Need for Sensors

• Sensors are pervasive. They are embedded

in our bodies, automobiles, airplanes, cellular

telephones, radios, chemical plants, industrial

plants and countless other applications.

• Without the use of sensors, there would be

no automation !!

– Imagine having to manually fill Poland

Spring bottles

TRANSDUCERS • Temperature transducers ▫ Thermocouples

What is a Thermocouple It’s a temperature sensitive device which works thanks to the Seebeck effect: “a voltage is generated in a circuit containing two different metals by keeping the junctions between them at different temperatures” Estonian physician Thomas Seebeck (1770–1831)

Pros and Cons in temperature measuring using Thermocouples • Pros ▫ They are inexpensive. ▫ They are rugged and reliable. ▫ They can be used over a wide temperature range. • Cons ▫ low output voltage ▫ low sensitivity ▫ non-linearity ▫ electrical connections.

How does a thermocouple look like ? Here it is! please note the two wires (of two different metals) joined in the junction.

How does a thermocouple work ?

High impedance voltmeter !

In normal operation, cold junction is placed in an ice bath

What types of thermocouples can we have ? temp. range (°C)

• Type K : Chromel - Alumel -270 / 1370 • Type J : Iron-Constantan -210 / 1050 • Type E : Chromel -Constantan -270 / 790 • Type N : Nicros -Nisil -260 / 1300 • Type T : Copper-Constantan -270 / 400 It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature

More features: • Type K ‘General Purpose' and low cost thermocouple, very popular • Type J Limited range (-40 to +750°C), less popular than type K. • Type E High output (68 mV/°C) well suited to low temperature (cryogenic) use • Type N High stability and resistance to high temperature oxidation, designed as an 'improved' type K, it’s becoming more popular. • Type T They are used for moist or sub-zero temperature monitoring applications because of superior corrosion resistance

Alloys used: • Constantan: 55% Copper and 45% Nickel • Cromel: 90% Ni + 10% Cr • Alumel: 95% Ni + 2% Mn + 2% Al + 1% Silicon • Nicrosil: 14.4%Cr +1.4 Silicon + 0.1% Mn + Ni • Nisil: same as Nicrosil but different %

How much are thermocouples ?

• Type K Thermocouple (Exposed wire, fiberglass insulated) Tip Diameter: 1.5 mm Tip Temperature: -60 to +350 °C Price $9.90 • Type K Thermocouple (Insertion Probe) Tip Diameter: 3.3 mm Tip Temperature: -50 to +250 °C Price $39.60

Magnitude of thermal EMF Thermal electromotive force. An electromotive force arising from a temperature difference at two points along a circuit, across a junction, or within an object, as observed, for instance, in the Seebeck effect. The temperature is usually expressed as a polynomial function of the measured voltage. Sometimes it is possible to get a decent linear approximation over a limited temperature range. where c and k = constants of the thermocouple materials T1 = the temperature of the ‘hot’ junction T2 = the temperature of the ‘cold’ or ‘reference’ junction

Accurate conversion of the output voltage V, to T1-T2 is achieved either by using calibration (lookup) tables or by using a higher order

polynomial

T1 −T2= a + aV + a V 2 + + a n V

Where a0, a1, ···, an are coefficients specified for each pair of thermocouple materials, and T1-T2 is the difference temperature in oC

Which one is the best? Measurement circuit: issues We would like DVM to read only V1, but the voltmeter created two more metallic junctions: J2 and J3 → voltmeter reading V will be proportional to the temperature difference between J1 and J2 → we cannot find the temperature at J1 unless we first find the temperature of J2.

The circuit will provide accurate readings, but it is desirable to eliminate the ice bath One way to do this is to replace the ice bath with another isothermal block directly measures the temperature of the isothermal block (the reference junction) and use that information to compute the unknown temperature

Last but not least: reference junction compensation circuit

Thermocouple - applications • Thermocouples are most suitable for measuring over a large temperature range, up to 1800 K. • They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such

applications, Thermistors and RTD’s are more suitable (Resistance -

Temperature Detectors).

Key features • the change in electrical resistance when subjected to a corresponding change in body temperature is ▫ Predictable ▫ Precise ▫ Stable • extremely high temperature coefficient of resistance • typical temperature range of -100° to over +600° F • Thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement and control.

Inductive transducers (IT) • Inductive Transducers may be either ▫ Self - generating type transducers ▫ Passive type transducers.

Accelerometer–I • Accelerometers are used to measure acceleration along

one or more axis and are relatively insensitive to orthogonal

directions

• Applications – Motion, vibration, blast, impact, shock wave

• Mathematical description is beyond the scope of this

presentation.

Accelerometer–II • Electromechanical device to measure acceleration forces –

Static forces like gravity pulling at an object lying at a table –

Dynamic forces caused by motion or vibration

• How they work – Seismic mass accelerometer: a seismic mass is connected to the

object undergoing acceleration through a spring and a damper;

– Piezoelectric accelerometers: a microscopic crystal structure is

mounted on a mass undergoing acceleration; the piezo crystal is

stressed by acceleration forces thus producing a voltage

– Capacitive accelerometer: consists of two microstructures

(micromachined features) forming a capacitor; acceleration forces

move one of the structure causing a capacitance changes.

– Piezoresistive accelerometer: consists of a beam or

micromachined feature whose resistance changes with

acceleration

– Thermal accelerometer: tracks location of a heated mass

during acceleration by temperature sensing

Accelerometer Applications • Automotive: monitor vehicle tilt, roll, skid, impact,

vibration, etc., to

deploy safety devices (stability control, anti-lock breaking

system,

airbags, etc.) and to ensure comfortable ride (active

suspension)

• Aerospace: inertial navigation, smart munitions,

unmanned vehicles

• Sports/Gaming: monitor athlete performance and injury,

joystick, tilt

• Personal electronics: cell phones, digital devices

• Security: motion and vibration detection

• Industrial: machinery health monitoring

• Robotics: self-balancing

The Accelerometer One important mechanical transducer, already mentioned

above, is the accelerometer. It provides a measure of

acceleration in form of an electrical output signal. One

reason this sensor is of special significance is the fact that

by integrating the output signal, an accelerometer can also

provide a measure of velocity and position, figure.

Accelerometers also provide a measure for velocity and position.

Additionally to the direct digital output signal the

accelerometer should employ some sort of feedback for

closed loop operation which yields the well known

advantages over open loop devices such as an increase in

bandwidth, dynamic range and linearity. Quite often

accelerometers and other sensors are closed loop devices

having an analogue output signal which is then subjected to

an analogue to digital converter (ADC). From the systems

engineering point of view the entire transducer is a chain of

the closed loop sensor and an ADC, consequently many of

the advantages of having a closed loop structure are lost.

The figure illustrates this point. In this work a closed loop

accelerometer is described which incorporates the analogue

to digital conversion within the loop, to produce a true

digital transducer. However due to the complexity of this

design procedure the research programme was divided in

three stages:

. design of an open loop accelerometer,

. design of a closed loop, analogue accelerometer,

. and finally the design of an inherently digital, closed loop

accelerometer.

For the design of these devices the following key factors

could be identified:

. choice of the sensing element,

. method of signal pick-off,

. reset mechanism (for the closed loop devices),

. suitable form of compensation (for the closed loop

devices),

. system stability, static and dynamic performance.

Each stage comprises the development of a mathematical

model, simulation of the accelerometer,

implementation in hardware, measurement and testing.

The design approach used in this research work was

attempted mainly from the control engineering’s point of

view as these devices have a closed loop structure. The last

stage of the project, the design of a digital accelerometer,

was an especially challenging task because the

device is a highly nonlinear, discrete system.

A chain of a closed loop sensor and an ADC results in an

open loop system.

When a coil of wire is moved through a magnetic field, a voltage is induced across the

end wires of the coil. The induced voltage is

caused by the transferring of energy from the flux field of the magnet to the wire coil. As the

coil is forced through the magnetic field by vibratory motion, a voltage signal representing

the vibration is produced. The velocity probe was one of the first

vibration transducers to be built. It consists of a coil of wire and a magnet so arranged that if

the housing is moved, the magnet tends to remain stationary due to its inertia. The

relative motion between the magnetic field and the coil induces a current that is proportional to

the velocity of motion. The unit thus produces a signal directly proportional to vibration

velocity. It is self-generating and needs no

conditioning electronics in order to operate, and it has a relatively low electrical output

impedance making it fairly insensitive to noise induction.

For example, sensitivity, frequency range, residual noise

level in the measuring range, temperature range, maximum

operational and shock levels, weight, connectors,

mountings, type of out put (charge /voltages), etc.

Accelerometers are available based on applications, e.g.,

the general purpose, high sensitivity, high temperature, high

frequency (very small size), shock, human vibration, under

water, modal analysis, industrial, aerospace and flight test,

special purpose like the tri-axial and rotational

measurements, etc. Figure

The seismic instrument is a device that has the functional

form of the system shown in Figure

which is a single-DOF spring-mass-damper system with the

support motion. A schematic of a typical

instrument is shown in the next Figure. The mass is

connected through the parallel spring and damper

arrangement to the housing frame. This frame is than

connected to the vibration source (e.g., bearing

housing) whose characteristics are to be measured. From

figure (spring-mass damper) using Newton’s law of motion,

we have

where 1 y and 2 y are the absolute displacements of the

housing and the suspended mass, respectively.

It is assumed that the damping force is proportional to the

velocity. We assume that a harmonic motion is applied on

the instrument such that

y1 Y1 cost

where Y is the displacement amplitude. The aim is to obtain

an expression for the relative displacement (y 1y2 in

terms of this base motion.

With mathematical passages we found the relation with the

fondamental terms natural frequency nf and critical

damping coefficient c are given by:

A plot of equation is given in Figure with Displacement

response of a seismic instrument as given by equation

Frequency ratio

The acceleration amplitude of the input vibration is

The design of a transducer for particular response

characteristics must involve a compromise between these

two effects, combined with a consideration of the

sensitivity of the displacement sensing transducer and its

transient response characteristics.

• Signal Conditioning & Analysis Equipments: The raw

signal from the vibration transducer may need to be

transformed into the right form, e.g., signals from

accelerometers may need to be integrated to provide a

velocity or displacement signal. Furthermore, signals may

need to be amplified before being fed to the metering and

alarm circuits, or in some cases passed through a filter

system to eliminate unwanted portions of the frequency

spectrum, and finally the system impedance may to be

reduced. All of these processes are known as signal

conditioning and this can be defined as the transformation

of the transducer signal into a form, which is suitable for

the analysis, metering, or feeding into an alarm or advance

signal processing system.

• Oscilloscope, Spectrum analyzer and Data Acquisition

System

An oscilloscope (commonly abbreviated to scope or O-

scope) is a type of electronic test equipment that allows

signal voltages to be viewed, usually as a two-dimensional

graph of one or more electrical potential differences

(vertical axis) plotted as a function of time or of some other

voltage (horizontal axis). Oscilloscope can have several

functions that helps in capturing and analysis the vibration

signal.

Depending upon the level of the signal can be amplified or

reduced. The time base can also be varied to have better

visulaising of the signal on the screen before capturing.

A spectrum analyzer is an instrument that displays signal

amplitude (strength) as it varies by signal frequency. The

frequency appears on the horizontal axis, and the amplitude

is displayed on the vertical axis. A spectrum analyzer looks

like an oscilloscope and, in fact, some instruments can

function either as oscilloscopes or/and as spectrum

analyzers. In spectrum analyzer various in-built functions

for statistical processing of periodic or random signals are

available. It includes FFT, power spectrum, autocorrelation,

cross-correlations, spectral density, probability density

function, ensemble or temporal averages, etc. Multi-channel

spectrum analyzers are very expensive and that has led to

the development of various software for the analysis of the

vibration signal.

Example:

Determining Natural Frequencies of the Rotor Bearing

System Using Impact Hammer Test

Natural frequencies of the rotor bearing system are

important parameters to be determined prior to any

investigation. For a two rotor system two natural

frequencies are obtained by using the impact test.

Impact is applied at one of the rigid disks while the rotor is

stationary (non-rotating). Displacement to impulse force is

measured at the bearing end both in the horizontal and

vertical directions using proximity probe transducer. The

FFT of the measured impulse response then gives

frequency domain impulse response. In the frequency

domain response natural frequencies appear as higher

amplitude peaks. Figures show the absolute value of the

FFT of the measured impulse response in the horizontal and

vertical directions, respectively. These plots indicate the

first and second natural frequencies, and these are equal to

38 Hz and 125 Hz, for the present configuration of the rotor

bearing system.

• Other applications are the Sound Measurements

Sound waves are a vibratory phenomenon. Acoustic effects

also give rise to “harmonic pressure fluctuations” that they

produce in a liquid or gaseous medium. They also

characterized by an energy flux per unit area and per unit

time as the acoustic waves moves through the medium. A

mathematical description of different acoustic will be given

in this section. It is standard practice in acoustic

measurements to relate the sound intensity and the sound

pressure to certain reference values 0 I0 and p0 , which

correspond to the intensity and mean pressure fluctuations

of the faintest audible sound at a frequency of 1000 Hz.

The magnitudes of the particle velocity and pressure

fluctuations created by a sound wave are small.

For example, a plane sound wave having an intensity of 90

dB is considered the maximum permissible level for

extended human exposure. In many circumstances we shall

be interested in the sound intensity that results from several

sound sources.