Instrumentation: Transducers

Environmental

Changes e.g.

Heat, pressure,

Light etc.

Changes in

Resistance,

Inductance,

Capacitance,

Shape, colour

Etc.

Conditioning

system

Available output:

4 – 20 mA or

1 – 5 volt electrical

and/ or

0.2 – 1.0 kg/cm2

pneumatic

Sensor (primary)

Transfer characteristics:

1. Transfer function, f = ������������� �

������������� �

Sensitivity, G = �������������������� �

�������������������� �= Gain

2. Error

- Scale error, e.g. zero (offset), sensitivity, non-conformity, hysteresis.

- Dynamic error, e.g. due to capacitor

- Due to noise, e.g. DC & AC , voltage or current, signal level & range

(4 – 20) mA ≡ (0.25 – 1.25) V across 62.5Ω, output impedance

- Noises, e.g. pickup, crosstalk, interference.

- Frequency change

3. Effect due to environmental changes, e.g. temperature, pressure,

humidity,

4. Linearity

(Secondary)

(a) Offset (zero) (b) Sensitivity (c) Combined

(d) Non comformity (e) Hysterisis (f) Dead space

Types of error

Sensor time responses:

a. Zero order b. First order

c. 2nd order oscillatory

b(t) = bi + (bf – bi)[1 – e-t/T]

R(t) = Roe-atsin(2πfnt)

4. Linearity

Analog signal conditioning

- Convert the signal from sensor, etc. , suitable for process control.

- Sensor produces change in voltage/ current/ pulse (frequency).

(electrical signals)

Types:

1. Offset (bias), also called zero error.

E.g.

Both potentiometers are used for full scale

deflection (FSD) adjustment on the

Ammeter.

So that, 0.1 V ≈ 1 mA range.

2. Linearization – provides output that varies linearly with physical

Parameter.

3. Conversion

e.g. sensitivity 20 mV/oC

i) mV to V or current [4, 20] mA or [0.25, 1.25] V via 62.5 Ω.

ii) freq. to V/I

iii) Digital interface (ADC & DAC).

d. Loading effect

- Open circuit – no load, Vy = Vc

- With load, Vy = ��

�����Vc

e. Noises – unwanted signal

- Eliminate using - a discriminator or comparator.

- filters

f. Wheatstone bridge

Va = ��

�����Vs

Vb= ��

�����Vs

∆V = Va – Vb= = ���������

��������������Vs

Eg. R1 is RTD from Pt100, with conversion range

[95, 105]Ω ≡ [-100, 100]oC

Then, choose R2, R3 and R4 = 100Ω, Vs = 5V.

Using MATLAB, R1= [95:0.1:105]

dv=5*(100*100-R1.*100)./(R1+100)/(100+100);

plot(R1,dv)

If R3 is Pt100 and other R’s = 100Ω

Band reject RC filter (notch filter):

fL= 0.187fc and fH = 4.57fc, fc = 1/(2πRC)

Important features:

- Open loop gain

- Freq. responses

- Offset

- Slew rate

- CMRR

Id = Is exp(Vd/Vt)

Id

Power, PL = i2ZL

Matched: no reflection, λ & Amplitude

not changed.

Amplifier with impedance of 500 Ω drives 8 Ω speaker

70V line matching transformer:

Insert a buffer here!