Measurement Flashcards
what is the general structure of a measurement system?
6 elements
input (true value )
sensing element
signal conditioning element
signal processing element
data presentation element
output (measured value)
What does the sensing element of a measurement system do? And what are 2 examples?
In contact with he physical process (being measured) and gives an output.
- Strain gauges
- Thermocouple
What does the signal conditioning element of a measurement system do? And what are 2 examples?
Takes the output of the sensing element and converts it into a form more suitable for processing - usually dc voltage or current.
- Bridge circuit
- Amplifier
What are 4 things the signal processing element of a measurement system do?
Further amplification of signal
linearisation
error compensation
data conversion (analogue to digital)
What are the 2 fundamental characteristics of a measurement system?
Static characteristic (system response to a constant or slowly changing input)
Dynamic characteristic (system response to a sudden or periodic change in input)
Name 9 characteristics considered ‘static’
Range
Resolution
Accuracy
Precision
Repeatability
Linearity
Sensitivity
Hysteresis
Thermal drift
what is resolution?
what limits resolution?
the smallest mount of input signal change that an instrument or sensor can detect.
nosie
The least significant bit
How to you calculate resolution with ref. to the leats significant bit.
12-bit A/D converter has input range of 10 V
Resolution given by:
1/2^bits x input range =
1/2^(12) x 10 (V) =
2.44 e-3 (V)
if input range is +/- :
1/2^(bits-1) x input range
Static characteristics:
Linearity, explain.
Relationship between output and input is expressed in a straight line equation.
y = Kx + a
K = sensitivity =
output range / input range
Static characteristics
Hysteresis, explain.
For a given input x, the output y may be different depending on whether the input is increasing or decreasing. Hysteresis is the difference.
static characteristics
Thermal drift, explain.
zero drift, when x=0 but y not equal to 0…. the reading has drifted due to temp change
sensitivity drift: the sensitivity of an instrument varies with the ambient temp
what do dynamic characteristics of a measurment system describe ????
behaviour between the time when the input changes and the time when the output attains a steady value
what are the two typical output responses for dynamic characterstics
STEP RESPONSE
FREQ. RESPONSE
most measurements can be classified into two types…. of systems
first order systems
second order systems
how to identify whether a system is first order?
if there is no mechanical movement
a movement of mass
detection part there is a sensor or an instrument
E.g temperature sensors, and RC circuit, RL circuit
Characteristic of signlas
3 things)(brief exp
Deterministic signals - values at any time can be exactly predicted by mathematical models.
Random signals - not predictable treated as noise - but can be estimated statistically
Real.Measured signals - combination of the above two
Variance intergral ?
equation
σ^2 =
1/T integral( y(t) - ymean )^2 dt
(between To and 0)
Any signal can be expressed by Fourier series - what domain is it in?
frequency breddah
Internal noise sources (2)
Thermal or Johnson noise
- random temp-induced motion of electrons and other charge carries
Noise sources
- shot noise
caused by random charge fluctuations across a junction in diodes and transistors
-1/f flicker noise
1/f spectrum
external noise sources (2)
-interference
electromagnetic interference from power source, motors
radio and TV
-vibration
ways to reduce external noise
electromagnetic shielding to reduce inductive coupling by twisted pairs to reduce the loop area, as the magnitude of the induced voltage is proportional to the loop area
electrostatic screening and shielding to reduce captive coupling by proper grounding
ways to reduce internal noise
irreducible in general
can be bypassed through physical decoupling mechanisms
Four types of filters
low pass
band pass
high pass
band stop
Filters can be implanted in two forms ..
analogue
digital
Analogue first order lop pass
2 kinds? examples of each
Passive
- RC circuit
- RL circuit
Active
- RC + op-amp
digital filters :
and ideal filter should have :
2
- high attenuation rate, ideally straight vertical line
- very small phase shift, ideally zero
what is meant by attenuation ?
reduction of the amplitude of a signal
digital filters:
the higher the order, the sharper the ….?
DROP EDGE
what are signal conditioning circuits used for ? (4)
Amplify weak signals
filter out noise
convert voltage to current or current to voltage
implement special functions (linearisation , log an antilog, integration, differential etc)
over what frequency will the first order frequency response no longer give a good response
1/tau
how do you get steady state sensitivity for a second order transfer function
make substitutions to get transfer function into standard form (and the factor is the your sensitivity) KG(s)
Laplace to complex domain by….
s = jw
At low frequencies w/wn what is the value of the magnitude of the transfer function G(jw) for second and first order systems
(can be seen in tables)
1
how do you calculate the displacement from a step input to a second order response?
y(t) = K x fo(t)
the resulting change in displacement =
steady-state sensitivity × unit step response
what is bandwidth
it is the range of signals for which the magnitude of the transfer function (GAIN) is greater than 1 / root(2) …. or 0.707
Three components of two port network measurement system
sensing source
amplifier
load as indicator/ display unit
