Transducers

1. Transducers Measurement/Information Processing System or Nano/Microelectromechanical System (N/MEMS)

2. N/MEMS: Basic Components • Measurand: input signal which is the physical or chemical quantity to be measured. • Sensor (input transducer): it is a device that converts a non-electrical physical or chemical quantity into an electrical signal. Sensor detects the measurand. • Preprocessor/processor: is a device(s) that modifies the signal from the sensor without changing the form of energy that describes the signal. • Actuator (output transducer): is a device that converts an electrical signal into a physical or chemical quantity.

3. Classification of Sensors by Signal Form

4. Classification of Common Actuators

5. Ideal Transducer Characteristics • Definitions • Self-exciting transducer: is one which does not need an external poser supply to work (e. g., thermocouple). • Modulating transducer: is one which needs an external modulating source (e. g., photodiode is a radiant sensor whose forward current is modulated by photo-induced electrons) • A transducer may be regarded as a system with an input x(t) and output y(t). • In case of a modulating transducer xo(t) is the external supply signal which should ideally be stationary and noise free. yo(t) is zero-signal output.

6. Self-exciting: Modulating: Ideal input-output relationships for linear transducers

7. Non-Ideal Linear Transducer Characteristics • In general for a time-dependent linear transducer we have: • For a first order case: • Where for a self-exciting transducer y(t)=0 and x(t)=0 at all t, and y(t)=yo(t) at x(t)=0 for all to for a modulating transducer. ao relates to the transducer gain, and a1 relates to its characteristic time response. (a)

8. In the case of instantaneous change in the measurand from zero to xo: i. e., a step change in input signal: • Taking the Laplace transform of both sides of (a) y(0) is zero for a self exciting transducer and is yo for a modulating transducer. • The transfer function for self-exciting transducer G(s)=Y(s)/X(s) is • 1/ao= Gain and t=a1/aois the characteristic response time.

9. The output of the transducer Y(s) in Laplace space to the step input of height xo is • Taking the inverse Laplace transform, one obtains for self-exciting transducer • For a modulating transducer we have;

10. Important Parameters Response: Gain (A): Sensitivity Band-width 1/t Baseline Signal (yb) is y(t) at x(t)=0

11. Transient response of linear first-order Transducer

12. Desirable Transducer Characteristics

13. Undesirable Transducer Characteristics • Non-linearity: response not proportional to input signal. • Slow response: output is slow to reach a steady state value. • Small working range: operating range is narrow. • Low sensitivity: can only respond to high input signal. • Sensitivity and baseline drifts: output varies with time. • Aging: output varies with age. • Noise: output contains unwanted random signal. • Hysteresis: non-reproducible readings.

14. Input-sensitivity relationship for ideal and real transducers

15. System Control: Open-Loop Process • Xin(s) = input signal to system • Ys(s) = Gs(s)Xin(s) = sensor’s output • Yp(s) = Gp(s)Ys(s) = processor’s output • Ya(s) = Ga(s)Yp(s) = actuator’s output = system output = Yout • Gs, Gp, and Ga are sensor’s, processor’s, and actuator’s transfer functions, respectively. • Overall • Go(s) = Gs(s)Gp(s)Ga(s), which is system’s transfer function, Yout(s) = Go(s)Xin(s)

16. System Control: Closed-Loop Process • The overall transfer function Gc(s): • H(s) is the transfer function for the control sensor