Signals and Systems

Sensor Characteristics: Input and output Characteristics

G R Sinha

Lecture Outline

 Input output Characteristics

2 Lecture 03: Input and Output Characteristics

G R Sinha

Sensors Characteristics Characterized by few properties  Input and Output;

 Accuracy and Calibration;  Hysteresis and Non-linearity;  Saturation, Repeatability and Resolution;

 Output Impedance and Reliability;  Environmental Factors; and  Dynamic and Application Characteristics

3 Lecture 03: Input and Output Characteristics

G R Sinha

Input Output Characteristics  An ideal (theoretical) output–stimulus relationship is characterized by transfer function. This function establishes dependence between the electrical signal ‘S’ produced by the sensor and the stimulus s : S =f (s).  That function may be a simple linear connection or a nonlinear dependence, (e.g., logarithmic, exponential, or power function).  In many cases, the relationship is unidimensional (i.e., the output versus one input stimulus).  A unidimensional linear relationship is represented by the equation

S =a +bs where a is the intercept (i.e., the output signal at zero input signal) and b is the slope, which is sometimes called sensitivity, S is one of the characteristics of the output electric signal used by the data acquisition devices as the sensor’s output.

4 Lecture 03: Input and Output Characteristics

G R Sinha

Different Transfer Functions Characterized by few properties

 Logarithmic function: S =a +b ln s  Exponential function: S =aeks  Power function: S =a0 +a1sk where k is a constant number.

5 Lecture 03: Input and Output Characteristics

G R Sinha

Multidimensional Transfer Function  A transfer function may have more than one dimension when the sensor’s output is influenced by more than one input stimuli, e.g. thermal radiation (infrared) sensor.

 It connects two temperatures (Tb, the absolute temperature of an object of measurement, and Ts , the absolute temperature of the sensor’s surface) and the output voltage V : V =G(Tb 4 −T s4) where G is a constant

6 Lecture 03: Input and Output Characteristics

G R Sinha

Full Scale Input (Span)  A dynamic range of stimuli which may be converted by a sensor is called a span or an input full scale (FS).  It represents the highest possible input value that can be applied to the sensor without causing an unacceptably large inaccuracy.  For the sensors with a very broad and nonlinear response characteristic, a dynamic range of the input stimuli is often expressed in decibels, which is a logarithmic measure of ratios of either power (P) or voltage (V).

1 dB=10 log (P2/P1) 1 dB = 20 log (V2/V1)  It should be emphasized that decibels do not measure absolute values, but a ratio of values only.  Being a nonlinear scale, it may represent low-level signals with high resolution while compressing the high-level numbers.  The logarithmic scale for small objects works as a microscope, and for the large objects, it works as a telescope.

7 Lecture 03: Input and Output Characteristics

G R Sinha

Full Scale Output (FSO)  Full-scale output (FSO) is the algebraic difference between the electrical output signals measured with maximum input stimulus and the lowest input stimulus applied.

8 Lecture 03: Input and Output Characteristics

G R Sinha

Thanks with Happy Learning

E= mc2

9 Lecture 03: Input and Output Characteristics

G R Sinha