By Prof. Avinash
Static characteristics
Range and Span Accuracy and Precision Reliability Calibration Hysteresis and Dead zone Drift Sensitivity Threshold and Resolution Repeatability and Reproducibility Linearity
Dynamic characteristics Speed of response Fidelity
Dynamic errors Overshoot
Errors in measurements
Human error • operational • personal
Systematic error • instrumental • environmental
Random error
Systematic errors ď‚— Instrumental error 1.
Due to inherent shortcomings in the instrument
2. Due to misuse of the instruments 3. Due to loading effects of instruments
Contd. ď‚— Environmental errors due to changes 1.
2. 3.
4. 5. 6.
Temperature Pressure Humidity Dust Vibrations External stray fields
Transducers ď‚— It is a measurement device which processes a
given input signal into acceptable output. ď‚— Consists of 3 elements
Input device (sensing element) 2. Signal conditional or processing (transducer) 3. Output device 1.
Classification of transducers Types: 1.Mechanical transducer 2.Electrical transducer a. Active transducer b. Passive transducer
Mechanical transducers Bourdon tube Springs
Manometric liquid Thermocouple
Thermistor /resistance thermometer Float Mass damper system
Electrical transducers Active
It is self –energized transducer which does not require external energy supply E.g. Piezoelectric transducers, thermoelectric transducers Passive
Reverse of active E.g. resistive, inductive, capacitive transducers.
Resistive transducer Passive transducer R= ρL/A Applications
Resistance thermometer 2. Strain gauge 1.
Inductive transducer Passive transducer Based on change of magnetic characteristic of
electrical ckt due to measurement Problems:
Change in self inductance(N,G,μ) 2. Change of mutual inductance(L1,L2,K) 3. Production of eddy currents (dL) 1.
Capacitive transducer ď‚— Passive transducer ď‚— Change in capacitance due to variation in distance
between plates
Piezoelectric transducer Active transducer The piezoelectric material produces change in voltage
when subject to mechanical vibrations/deformations This principle is used to measure acceleration, velocity,
dynamic pressure
Control Systems It is a system by means of which, any
parameter in a machine/process can be varied in a desired manner Improves quality
Improves uniformity and gives better
production capability
Types of control systems ď‚— Open loop control system: It does not check the value
of the O/P
ď‚— Closed loop control system: It checks the value of O/P
Feedback control system 2. Feed forward control system 1.
Open loop control system
ď‚— Examples: 1. Sprinkler used in lawn 2. Stepper motor positioning system 3. Automatic toaster 4. Traffic light controller 5. Automatic door closing and opening
Contd. ď‚— Advantages: 1.
2. 3.
4. • 1. 2. 3.
Simple in construction Very convenient when o/p difficult to measure Easy maintenance Economical Disadvantages: Inaccurate and unreliable Insensitive to environment changes and disturbances Recalibration necessary
Closed loop control system
ď‚— Examples: 1. Human being 2. Home heating system 3. Temperature control system 4. Missile launching system
Temperature control system
Feedback control system ď‚— Error in the system is fed back to the controller
Feed forward control system ď‚— The error is estimated before the o/p and fed back to
the actuator ď‚— Used to avoid undesirable o/p