Download Sensor concepts

Basic Sensor Concepts
and experimenting with a compound pendulum
Prof. R.G. Longoria
Department of Mechanical Engineering
The University of Texas at Austin
June 2015
ME 144L Dynamic Systems and Controls Lab (Longoria)
Lab Setup
In the laboratory, you will experiment with a compound pendulum setup
equipped with a potentiometric sensor to measure rotational displacement
about a fixed support shaft.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Most modern sensors are electromechanical
We can classify by the sensing
mechanism.
Resistive (potentiometers,
strain gauges, thermistors,
light, etc.)
Capacitive (Very common in
MEMS; accelerometers, stud
sensors, etc.)
Inductive and Magnetic
(proximity, distance, ...)
Piezolelectric (force, ...)
ME 144L Dynamic Systems and Controls Lab (Longoria)
Resistive sensors rely on changes in resistance
The resistance of a uniform conductor is given by, R = ρL/A, with ρ
the resistivity, L the length and A the constant cross-sectional area
through which current flows.
Resistance changes either by a geometric (A, L) or material change
(ρ) in the resistive element.
Resistance can be directly measured (by an ohmmeter) or inferred
through a signal conditioning circuit (e.g., a voltage-divider)
ME 144L Dynamic Systems and Controls Lab (Longoria)
Signal conditioning for resistive sensors converts resistance
change to voltage change
Signal conditioning refers to the
devices and processes we use to
modify and/or improve the nature
of a sensor signal. Examples
include filters, amplifiers, etc.
Consider a basic voltage divider,
where
R2
vin
vout =
R1 + R2
ME 144L Dynamic Systems and Controls Lab (Longoria)
By using a voltage divider, we can
transform the resistance change
into a voltage change which is
more readily measured.
Calibration of the potentiometric sensor
Effectively, the potentiometric sensor is configured like a voltage divider where the
output voltage is related to the change in shaft position.
Calibration builds a relation between the output voltage and angular position.
We seek relation θ = f (vout ), where vout is the measurable output voltage. It is
desirable to have a sensor that has a linear relation between the measurand (here
θ) and the measured voltage.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Why we like linear sensor models
Linear model relations between measured voltages, say vm , and a
measurand of interest, ym , make it easy to represent calibration with a
single constant or line (e.g., from regression),
ym = K · vm
Another advantage is that if the relation between a measured voltage
signal and the measurand is linear then when you look at the temporal
trends in the measured signals these are the same for the actual physical
variable(s) of interest.
Having a nonlinear sensor is tolerable, especially since modern computing
can easily represent the model.
NOTE: It is expected that when calibrations are conducted, the regression may introduce a ‘y-intercept’ (i.e.,
ym = K · vm + b). This model is more generally called affine, meaning there is a linear relation with some translation (or
rotation).
ME 144L Dynamic Systems and Controls Lab (Longoria)
Suggestions for lab practice
Make notes on how to connect power, sensors, and measured signals
properly. Simple circuit knowledge is all that is needed, and it can
help you make sure you collect the signals correctly and don’t damage
equipment.
Use this lab to begin building experience using simple sensors
ME 144L Dynamic Systems and Controls Lab (Longoria)