Ele

ctro

mo

tiv

e fo

rce

[V]

2.2.3 Induction transducers

The induction transducers operates on the electromagnetic induction principle, which are in the form of embodiment of electrical generators or alternating with rotation or linear movement. The induction transducers whose input size is the rotation angle are called Tahogenerators.

Fig. 2.2. DCt tahogenerator, operating scheme.

7,0

6,0

5,0

4,0

3,0

2,0

1,0

0,0

0 1,57 3,14 4,71 6,28

Rotation angle [rad]

Fig. 2.3. DC tahogenerators, caracteristica de tensiune le mersul în gol.

( 2.1)

The output value of the transducer – electromotive force collected from the brushes– has a pulsating character, figure 2.3. Pulse amplitude is reduced by increasing the number of parts of the rotor winding and the number of pole pairs.

The tahogenerators are built for nominal speeds between750 r / min and 2000 rev / min, have a rated power of 75-1000 mW and a linearity better than 1%. Emf is usually 5- 30 V.

The main drawback of the dc tahogenerator is the collector, which restricts operation at low speeds and is the main element of the transducer wear. For this reason the scope of use of this type of sensor is limited.Another alternative of the induction dc current sensor is linear speed sensor, Figure 2.4. At this,the idle running emf is proportional to the speed of movement to the mobile solenoid 2,

that is driven and moved through the connecting rod 3 in the magnetic field of the permanent magnet 1, the graph in Figure 2.5. Mechanically coupling the connecting rod by a mechanical system which is the subject of the measurement,it is measured the linear velocity at the coupling point.

AC tahogenerators, figure2.6, presents the advantage that windings are fixed and the moving part consists of the permanent magnets,where brushes and collectors are not needed.

Fig. 2.4. Linear speed transducer, functioning scheme.

Ten

siu

ne

a la

bo

rne

le t

rad

uct

oru

lui

[V]

Te

ns

iun

ea

ele

ctr

om

oto

are

[V

]

For this reason the reliabilityis improved,we can rise the measurement speed and reduce the power required to drive. The impeller is made as a permanent magnet with several pairs of poles, and the stator is made of sheets provided with notches in which the winding is inserted.Increasing the number of poles reduces the minimum speed when the output voltage measurement is made with a moving coil device. The minimum frequency value must be 4 to 10 Hz, depending on the degree of damping of the measuring device.

Viteza [m/s]

Fig. 2.5. Traductorul de viteză liniară, caracterstică de funcţiune în gol.

The linearity of the transducer’s drive speed and the value of the output signal is obtained only sitting idle and gets worse with decreasing load resistance Figure 2.7. The phenomenon is explained by emphasizing the voltage drop across the inductive component and the internal impedance transducer current with increasing frequency (speed). The three curves correspond to the following values of load resistance 10 kΩ curve RA1, 2 kΩ Ra2 curve;0.5 kΩ RA3 curve.

Another constructive way is the alternating current tahogenerator with separate arousel, schematically shown in Figure 2.8, the (working schedule), and 2.8, bequivalent circuit. In the case of this embodiment stator 1 has two windings

Fig. 2.6.Alternating current tahogenerator

A coil of excitation, supplied by an audiofrequency oscillator with excitation voltage Uex and a secondary winding at the terminals from which is obtained the output signal UO, in proportion to the angular speed measured on the shaft 3 ω and a core 5. The rotor 4, in the form of a glass moves in electromagnetic field winding excitation. Induced electromotive voltage UO will have secondary frequency equal to that of the excitation voltage Uex and amplitude proportional to the angular speed of the drive shaft ω. Tahogenerators which operate after this principle have the advantage of the expension of the measuring zone in the low rotation zone.

70

60

50

40

30

20

10

0

0 50 100 150 200 250 300

Viteza unghiulară [rad/s]

Ra1 Ra2 Ra3

Fig. 2.7. Variation of emf on AC tachometer.

.

Fig. 2.8. AC tachogenerator, with separate excitation: a – operating scheme; b –Equivalent circuit.

The calibration of the tachogenerators is done by measuring the angular velocity ω and electromotive Uo in parallel . It thus determines the calibration curve of the transducer and its transformation’s constant . Transducer is shown as generally the angular velocity in particular be calibrated measuring chain.To measure the angular velocity it must be selected the method and the equipment that ensures the accuracy required for calibration . A precise and relatively simple method of calibration is to use a strobe. It drives shaft angular rate sensor with a facility that allows you to change the drive speed throughout the range of use of the transducer . Maintaining a constant drive speed , will illuminate with stroboscopic lamp the tachogenerator shaft, or another piece that rotates with the same angular velocity and changes the frequency of the strobe lamp until the image appears linear. Between the frequency of the strobe lamp and driving angular velocity of the transducer is the following dependency :

ω = 2 ⋅ π ⋅ν

str

( 2.2)

where: ω is the angular velocity of the driving transducer ; vstr is the lighting stroboscope frequency.Simultaneously measures the output quantity value Uo of the transducer and it will be determined the transducer’s constant calibrarion.In the case of the induction transducers for the linear speed the calibration will be made with the help of a vibration excitator who moves after a sinusoidal known law.The velocity determined after an analitical way v , will be compared with with the output voltage Uo and will determine de transducer’s constant calibration.

δ

⎝

f⎜ ⋅ ⎟f A ⎠

2.2.4 Inductive transducers

The inductive transducers are based on their functioning on their own inductive variation of an iron core coil or on their mutual inductive variation,under the action of the size to measure.On the constructive point of view, the following inductive transducers are known: Inductive transducers with variable air gap,transformer type inductive transducers, mobile core inductive transducers

Inductive transducers with variable air gap, schematically presented in figure 2.9,a,have a fix core 1 in their componence on which we find a coil 2 and a mobile core 3 which it moves in fron if the fix core under the action of the non-electrical measuring size.

Fig. 2.9. Inductive transducer with simple core: a) Operating scheme; b) Conversion characteristic

The inductance L of the coil changes depending on the value of the air gap δ, giving a hyperbolic-shaped conversion characteristic as in Figure 2.9; b In this case the value of inductance is given by:N 2

L = [H],Rm

( 2.3)

Where: N is the no.of turns; Rm reluctance of the magnetic circuit.Reluctance can be expressed by the relationship:

R = 1

⎛ l+ =

2 ⎞[1/H] , ( 2.4)

m4 ⋅ π ⋅10−9 ⎜

μ⋅ Af a

⎟

in which: lf is the length of the iron circuit, in cm; δ-is the value of the air gap, in cm; Af- sectional area of the iron

core, in cm2; Aa- area of the air gap, in cm2; μf-the iron core’s relative permeability. For small displacement of the air gap ∆δ=(0,1.....0,2)δ0 the transducer’s characteristic is sufficient, deviation from linearity being between 1% and 5%.

Fig. 2.10. Traductorul inductiv, diferenţial.

To improve the linearity and to improve the measuring domain it is used transducers realized from the differential scheme, figure 2.10, which can be achieved by connecting in bridge of two inductances.The differential assembly coils have equal number of turns N1 = N2 = N, they are placed in a deck loaded with an alternating voltage Uafrequency f, typically 5 kHz. Ud voltage imbalance is obtained by the relationship:

U d ≈ k p ⋅ U a ⋅ R ⋅ 2 ⋅ ΔL ⋅ ω

( 2.5)

where: kp proportionality factor which depends on the measuring deck; Ua vaoltage amplitude; R the resistance of the two arms of the bridge; ∆L inductance variation L, which is due to changes in air gap δ; ω is the angular frequency of the supply voltage of the deck.

From the relation analisys (2.5) shows that in the case of the differential inductive transducers the sensivity doubles and the linearity domain increases. În figure 2.11 it is presented graphically the simple active transducer’s characteristics along with the static characteristic of the air gap inductive transducer in the differential mount..

For obtaining an optimum between good sensibility an linearity,the variation domain of the air gap has to be limited on the domain

Fig. 2.11. The differential transducer’s characteristics

δ∈[δo +/-(0,3.........0,4) δo].

Transformer type inductive transducer,have as sensible elements two wrappings mounted on an iron core,which have a variable inductance under the action of a non

electrical force to be measured. The principle scheme of a such transducer is presented in figure 2.12. Wrapping N1 is driven by an alternative voltage source Ua, and by the

displacement of the mobile armature of the transducer modifies the air gap δ, determining the apearance of a tension Ue due to the magnetic induction phenomenon:

U e = k x ⋅ n ⋅ U a ( 2.6)

Fig. 2.12. Transformer type inductive transducer.

where: n=N1/N2 is the ratio of the no. of turns of the 2 windings kx coupling coefficient whose value depends on the coeficientul m a g n e t i c c i r c u i t c o n s t r u c t i o n o f t h e s y s t e m a n d t h e s i z e o f m e a s u r e The main advantage of the transformer type induction transducers relies in the fact that it supplies directly on the output an alternative voltage,without being necessary to introduce the transducers in a measuring circuit separately.

Mobile core inductive transducers: in a simple way, figure 2.13, it is made from a coil 1 in which interior is moving a ferromagnetic core 2 under the measuring size action. Variation of the inductance L displacement x is represented by the graph in Figure 2.14. It appears that it is nonlinear. Also at this type of transducer an attraction force is exerted on the core..

Fig. 2.13. Mobile core inductive transducers

Fig. 2.14. mobile core inductive transducer’s characteristics

The differential inductive mobile cell core variant, Figure 2.15, significantly improves the performance with respect to sensitivity and linearity and eliminates the parasitic magnetic forces drawback.

Fig. 2.15. Traductorul inductiv cu miez mobil şi bobine diferenţiale.

The sensitive element of the transducer is composed of two coils L1 and L2, with different winding directions and are connected to a bridge supply high frequency current. As a result of the input size variation it produces x displacementof x-input of the mobile core 1, resulting in changes of the two inductors in the opposite direction, in accordance with the curves L1 and L2 in the diagram of figure 2.15. The transducer’s inductance L results as the algebraic sum of the two inductances. It notes that the linearity is considerably increased, occupying a central symmetric position with respect to the mobile core. Depending on the characteristics of the inductive transducers and the mobile core coil may be used for measuring the differential displacement between tenths of a mm up to hundreds of mm. The inductive transducer with mobile core differential transformer type, Figure 2.16, to be used as linear or angular displacement transducer. By changing the mobile core position changes the coupling factor of the transformer primary and

its secondary. Supplying the mayor AC pulsation ω inwill be induced secondary voltages U01 and U02, which can be represented by

rotating vectors as shown in Figure 2.16, b Electomotive voltage across the secondary U0 is calculated

Fig. 2.16. Differential transformer.

sum voltages U01 and U02 in both the secondary’s windings. At their turn this will depend on factors coupler M1 and M2, corresponding to a certain position of the mobile core, and is expressed by a relationship of the form:

U 01 = M1 ⋅di p

;dt

U 02 = M 2 ⋅di p

.dt

( 2.7)

The output voltage of the transducer

depends on the mobile core’s position, due to coupling factor changing, the middle position corresponding a

zero voltage.

U 0 = U 01 − U 02

= (M 1− M 2)⋅ di p

. ( 2.8)dt

Fig.2.17.Differential transformer feature.

According to the conversion characteristic of the differential transformer, Figure 2.17, the linearity of this is included between xmin and xmax values of the measured quantity being symmetrically in relation to the median position. For the race transducers, the usual values of the measuring range with an error

Maximum non-linearityof 5% is ± (from 0.10 to 80 mm) and for the angular displacement of ± 60 °. Excitation winding of the transducer is supplied with voltages 3-15 V at frequencies from 50-20000 Hz. Low frequencies are used for measuring linear and angular displacements with slow variation in time, and the highest for those with rapid variation in time because with increasing power frequencyimproves the frequency response of the transducer.

Fig. 2.18. The chain measuring with differential transformer scheme

The differential transformer 1 , works on the principle of the carrier wave modulating equipment and it connects in the measuring scheme similar with that in Figure 2.18, which include: AC amplifier 2.demodulator 3; filter 4 and voltmeter 5.The output size form the filter is the wrapping of the modulated carrier wave In Table 2.5 are presented the main characteristics of inductive transducers manufactured by Hottinger Baldwin FR Germany.

Tabelul 2.5

Tipul Modelul Domeniu de măsurare [mm]

Tipul legăturii

Sensibilitatea[mV/V]

Abaterea de la liniaritate [%]

Clasa de protecţie

WAT cu palpator 0...2 – 0...100 jumătate depunte şi punte întreagă

+ 80 +/-0,2 la +/-0,1 IP67(IP54)

WAL - 0...2 – 0...500 jumătate depunte şi punte întreagă

+ 80 +/-0,2 la +/-0,1 IP67(IP54)

WET cu palpator +/-1; +/-5 jumătate depunte

+/-10 <0,3/0,9 IP64

W1EL/0 - +/-1 jumătate depunte

+/-80 +/-0,2 IP20

WI cu palpator 2; 5; 10; jumătate depunte

+/-40 +/-0,2 IP67

WE - 10; 20; 50; 100 jumătate depunte şi punte întreagă

+ 80 +/-0,2 IP67(IP54)

Inductive transducer calibration is done on special devices for measuring linear or angular displacements with the precision required. . Engaging the inductive transducer to measure the size of the device output power corresponding to each value of the displacement, point by point, the entire calibration curve. Adjusting the conversion constant can be done by changing the supply voltage of the transmitter without exceeding the limit of the impure heating condition.

2.2.5 Potentiometric transducers

Potentiometric sensor consists of an electrical resistance uniformly distributed on a support along which one cursor can slide in perfect contact with the resistance.The transducer’s cursor is mechanically coupled in the measuring object, which by it’s action , modifies it’s position. The transducer input value can be a linear or an angle displacement.

The necessary conditions for a potentiometer to be used as a transducer are its resistivity to be as evenly distributed along the resistant element, resistance contact between cursor and resistant elements has to have values as low as possible and to be constant during operation, the resistivity variation with temperature are minimized.

Fig. 2.19. Schema de funcţionare a traductoarelor potenţiometrice: a) liniare; b) de rotaţie.

the impossibility of adjusting the zero level.

Potentiometers which best meet these requirements are wired potentiometers with metal film and carbon film.The principle schemes of the potentiometric transducers are shown in Figure 2.19, respectively in Figure 2.19, a linear potentiometer transducer is shown and in Figure2.19, b rotary potentiometric trandsucer.In parallel are presented the output voltage characteristics according to the Uo linear x and angle α. This transducer bonding scheme presents the following drawbacks: sensitivity to temperature variations,

To eliminate these drawbacks in practice recommends the use of potentiometric transducers bridge connected, as shown in Figure 2.20. Re potentiometer is connected In parallel, with which equilibrate the bridge. Rezistenţa de sarcină Rs are rolul de a limita curentul care trece prin contactele cursorului

t

potentiometric sensor and the resistance Rn allows the adjustment voltage deck. Based on the equivalent circuit, Figure 2.21 can be calculated the potentiometric transducer’s impedance. Impedance value varies depending on the position of the potentiometer’s cursors Rt and Re. According to the scheme in Figure 2.21, the cursor’s middle position achieve maximum impedance:

1Fig. 2.20. Circuit de măsurare cu traductor

Z max = Rmax

=⋅ (Re + Rt ) + Rs

,4

( 2.9)

potenţiometric.

Extreme cursor position:

and the minimum impedance for the scheme in Figure 2.21, b, is obtained for

Z min = Rmin = Rs . ( 2.10)

Fig. 2.21Potentiometric transducer equivalent circuit: a) maximum impedance b)minimum impedance

.

Potentiometric transducers must be connected to the measuring circuit with bigger impedances to reduce measurement errors due to variation of the load impedance.

The calibration of the potentiometric transducer is the same as the calibration of the inductive transducer, using special devices for measuring linear or angular displacement with desired accuracy. Coupling the sensor to these devices and measuring the output voltage corresponding to each value of the displacement,it outlines, point by point the calibration feature. If the calibration feature is assumed to be linear, it is sufficient to draw through two points.

To calculate the transducer conversion constant kt in [V / A] or [V / m] we use the

following equation:k = U o1 − U o 2 . ( 2.11)

α1 − α 2

where: Uo1, Uo2 are the output voltages; α1, α2 are the input corresponding measures (angular or linear displacements).

Potentiometric transducers are simple, robust and reliable in operation. Due to the output signal which has a sufficiently large value, it is not necessary to use the amplifier and are not affected by electric noise. Due to these advantages the potentiometric transducers are widely used to measure linear and angular displacements. To improve the accuracy of the results obtained with this type of transducers must take into account the following features:

¾ wirewounded potentiometric transducers can not be used to measure small displacements, due to the fact that the step of coils can not be reduced under 0,02-0,05 mm;¾ mechanical contact between the slider and the mechanical strength negatively affects the transducer’s reliability;¾ durability decreases considerably when the rotation isalternative;¾ transducers wear will increase the electrical noise produced by it;¾ frequency response of the sensor is limited due to inertia forces andfriction in the mechanical system, and ectrical noise generated by the mobile contact increases with increasing displacementspeed. It is recommended to limit the linear speed of the cursor and the electrical resistance within 3 m / s.

Connecting two or more potentiometric transducers in a single measuring deck can be made complex measurements by adding or subtracting signals. Potentiometric transducer characteristics are that they are very often used in the vehicle testing to measure linear or angular displacements of controlling elements: steering wheel, pedals, etc..2.2.6 Impulse transducers

In the case of impulse transducer, the input value, linear and angular displacement,it is transformed in electric impulses.Rectangulary shaped impuses are mostly used due to high precision. If the output signal of the sensor is different from the shape, it can be made in a rectangular shape using special electronic circuits, known as training circuits. In some cases, to notify immediately when the encoder fails, their training is done so that both branches of the constant voltage wave signal have voltages different than zero. In this way the zero voltage info means a faulty transducer and can not be confused with lack of movement. Proportionality between the number of pulses and angular or linear displacements ensures linearity conversion characteristic. By the way impulses are obtained, the transducers can be divided into two

groups:¾ Transducers with pulse contact¾ Transdurcers without pulse contact.

Contact pulse transducers produce electrical impulses by closing and opening some electric switches controlled by a driven mechanical system due to displacement to be measured. Figure 2.22 presents a transducer

Fig. 2.22. Impule transducers without contact.

Contact pulse for measuring angular displacements. Non-electrical control cursor size 1, moving on the disk contacts2, closing the circuits pulse counterers A, B, C, D, E, F, G and H. Each electrical pulse has a corresponding angular displacement Δa. Pulse counters provide information on the class distribution of the movements in measuring domain of the transducer + /-amax. Due to mechanical contacts such transducers have low reliability compared with non-contact transducers and is only used for measuring displacements at low speeds, such as moving the pedals, throttle movement, movement volanului, ş.a.

To eliminate the disadvantages of contact pulse transducers are used contactless induction transducers, Figure 2.23, or photoelectric Figure 2.24.

Fig. 2.23. Contactless impulse transducers

In the case of the contactless pulse induction transducers, Figure 2.23, generating pulses is done by the variation of the magnetic field of coil 1 ,by moving to the right of magnetic core of a disc 2. The shape of the teeth made in disc 2, inf luence the shape of the output transducer’s signal, and the angular speed w is proport ional with i ts amplitude.The electr ic forming system 3, modif ies signal 1 into a rectangular signal.Step type signal can be processed much easier with the help of a counting block 4, or electr ical ly, obtaining derivated sizes speed,acceleration,etc.The minimum inferior frequency is l imited by the electr ic pulse amplitude decrease, drawback which can be part ly corrected by increasing the no. of grooves. The maximum frequency is also l imited by the characterist ics of the meter, the main sampling t ime in which he works because the sampling frequency must be at least two t imes the frequency of the signal processed. Therefore in addit ion to the geometric l imitation concerning the possibi l i ty to increase the no.of grooves interpheres a l imitation due to the measuring chain and measuring size characterist ic.

Fig. 2.24. Contactless optical impulse transducer.

In the case of the photoelectric pulse transducer, figure 2.24, to obtain pulses is performed using a photo-transistor 3, the light source 1 and the disc with slots2, by means of which the flow of light is interrupted. The output signal of the transducer , which has a trapezoidal shape can be used directly to control the pulse counter 4. Phototransistor is characterized by a very low response time1.5 mS - 8μs, so that the measurement range can be greatly expanded in comparison with the non-contact encoder induction. If we take into consideration the modern possibility of processing disc 2, the measurement is limited only by the range is only

limited by the sampling frequency of the counter, the range of variation of the quantity measured and the conversion feature of the transducer. . By using the photoresist other photovoltaic elements in place of the photo-transistor, one can obtain other pulse transmitters, which because of the higher response time of the control device lowers the work significantly higher frequency of the transducer.

Due to the fact that the amplitude of the signal at the output of the transmitter remains substantially constant (about equal to the voltage of the sensor), this type of transducer can be used and when frequency of the signal is very small.

As main disadvantage of the photoelectric impulse transducers shoulb be mentioned the need to isolate from the daylight influence, which could negatively influence the test results.

Pulse transducers can be easily used for measuring linear position, or other quantities that can be converted mechanicaly In the case of vehicles, they are used for measuring the area, speed and acceleration, as well as for measuring the angular velocities of the different components in rotation. In conclusion we can say that pulse transducers, especially those without contact, ensure high measurement accuracy, and allow easy signal processing directly and electrically.