MEASUREMENT SYSTEM PERFORMANCE Flashcards
Generic structure of a measurement system:
Sensor -> Signal Conditioning -> Signal Processing -> Data presentation
What is a “sensor”?
Sensor is in some sort of contact with the process being measured, and yields a signal which depends on the measurand, e.g. :
- Strain gauge, where resistance varies with strain;
- The moving coil in a PMMC meter, deflection that responds to current flow through
the coil.
What is “signal conditioning”?
Signal Conditioning is necessary for many reasons, e.g. :
- amplification of a voltage signal from the sensor to be in the correct range for
subsequent processing (e.g. in a DVM, where the input amplifier presents a voltage to
the ADC that is within the ADC’s input range).
- conversion of the signal value from one electrical form to another, such as current to
voltage;
- High-pass filtering to remove low-frequency interference.
What is “signal processing”?
is the term given to any manipulation of the conditioned signal, e.g. :
- Analogue-to-Digital Conversion (ADC) to yield the signal in appropriate digital form for digital display (DMM), or graphical display (digital storage scope), or for storage
and further manipulation on a computing system;
- integration of a current signal over defined time periods to measure charge;
- digital signal processing to calculate a parameter from stored signal values.
What is “Data Presentation”?
is the step in which the measurement result is presented to the user, in the form which is required, e.g. :
- alpha-numeric display, such as the display on a DVM;
conversion to the units which the user requires, for example to show a measured
resistance value on a DMM in units of ;
recording the variation of a voltage signal with time on a pen-chart plotter;
display of variation of voltage with time on a scope.
Active Sensors
require an external power supply in order that the sensor can give a voltage or current output signal.
e.g. resistive sensor element in a Wheatstone Bridge,
such as a strain gauge
Passive Sensors
require no external power supply
e.g. the thermocouple, which we will cover later
SEEPAGE 35-41, 42-45 AGAIN!!
OK
Define accuracy
The quality or state of being correct or precise. (The degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard.)
Define precision
The quality, condition, or fact of being exact and accurate. (Refinement in a measurement, calculation, or specification, especially as represented by the number of digits given.)
