Hello, I am Dr.
Ali Mohammadi, a lecturer in the Department of Electronic and
Electrical Engineering, University of Bath.
I teach the first half of sensors and instrumentation unit over
the last few years, which is specifically developed for our
master's mechatronics students.
This is one of the five units in the second semester of this one
year program. And here I have provided a brief outline about the
unit content and its learning outcomes.
I hope that you find it useful.
In a typical mechatronic system, we use sensors and controllers
in a closed loop feedback system to improve the precision and
accuracy of the system output.
For example, a car crash system consists of accelerometers that
detect sudden changes in acceleration caused by a crash in a
very short time, and that activates the vehicle airbags, which
saves the passengers life in disastrous car accidents.
Our unit will focus on the transducers that are used as sensors
and actuators, particularly in micrometer dimensions such as
accelerometers in this example.
Therefore, we will start by introducing the micro fabrication
processes used to make different transducers.
We will discuss different transduction mechanisms such as
piezoelectric, electrostatic or capacitive and electro thermal
sensors and actuators with real world applications in micro
electromechanical systems or MEMS.
We will also investigate cutting edge research, being carried
out on these transducers.
For example, we discuss atomic force microscopy on a chip,
programmable magnetic transducers and micro-energy harvesting
applications. In the next few slides I will show microscopic
images and videos recorded from our ongoing research in MEMS.
The signals recorded from micro transducers are very small in the
range of microvolts and nanoamps, in the second part of this
unit. We will learn the signal conditioning interface electronic
circuits such as amplifiers, filters and analog to digital
converters required to process the output of these sensors.
Therefore, the unit lectures are designed in two parts.
And the first part that we introduce the transducers, we discuss
the microfabrication processes used for making these sensors and
micro mechanical rules that govern the operation of the
transducers. We also study the most popular transducers,
including piezoelectric, piezoresistive, capacitive and electro
thermal devices.
In all mechatronics systems interface electronic circuits are
required to amplify and enhance the recorded signal from sensors
and make it available for the following signal processing and
control steps.
So in the second part of this unit the readout and conditioning
circuits, including amplifiers, filters and A to D converters
and PLL's are discussed.
Micro fabrication skills and equipment required for making
integrated electronic components and micro electromechanical
devices in clean rooms are usually expensive processes for
prototyping, though there is a cheaper way of running the
research and development in micro fabrication and that is
Fabless approach. So in Fabless approach we design and simulate
the micro devices using commercially available process
technology files in CAD tools.
So we introduce basically CAD tools here in this unit.
We outsource the micro fabrication phases of this of these
projects, and we run in-house measurement and characterisation
steps. So example, micro fabrication process or MUMPS,
InvenSense and Tronics HARM.
We start by reviewing the photolithography process, which is the
common approach to make micro devices on semiconductor
substrates. This is followed by introducing some commercially
available micro fabrication process technology such as MUMP and
XFAB technologies, for example, this image is at the cross
section of Piezo-MUMP process where piezoelectric layer is
sandwiched between two conductive electrodes that by designing a
two dimensional layout of the device, we make 3D microstructures
to be used as sensors and actuators, all integrated on a silicon
substrate as shown here with the blue colour.
Although countless sensors are developed and used over the
years. But only a few of these have higher chances of being
integrated in micro fabrication processes.
For example, in this slide on the right, the operation principle
of electro thermal actuator is demonstrated, where applying
electric power to a resistive silicon beam cause longitudinal
expansion, which is extremely small.
Displacement amplification by chevron shaped transducer amplifies
the displacement of shuttle at the middle, the CAD simulations
show temperature distribution all over the actuator in response
to applying electric power to the pads.
This video here shows a chevron actuator that is used to
generate unchip mechanical impact.
So you can see the moving shuttle at the middle in response to
an applied electric sinusoidal voltage.
Atomic force microscopy is another example which was patented in
1986 by physics Nobel winner Gerd Binnig, still, is a still
enabling microscopic technique that offers 1000 times higher
resolution than strongest optical microscope.
So the operation principle is absolutely simple here.
So if we put the sample on a scan table, that can move in X and
Y directions as shown in this image.
So in a plan our directions and allow microcantilever beam scan
the surface of sample based on the reaction force of the beam we
can find the Z coordinates of every point.
The processor or computer can sort these x, y, z, coordinates and
generate a surface image of the sample.
The imaging speed of existing devices are limited by the
mechanical resonance frequency of mesoscale actuators.
So you can see the very large dimensions of this commercially
available tabletop AFM, which is limited in scanning speed and
it's also very costly.
So using a microscale electrostatic actuator that could move
much faster is the solution to this bottleneck.
In addition, by integrating electro thermal displacement micro
sensors, enclose new control system we improve the displacement
measurement accuracy up to subnanometres.
So you can see a closed loop control system here where there is
a stage, a scan cable which can move in one direction only here
by electrostatic actuators and displacement can be measured by
electro thermal sensors, which are extremely small.
This slide shows the magnified image of electrostatically
actuated XY scan table and integrated electro thermal sensors,
all built on silicon and using commercially available silicon on
insulator micro fabrication processes.
Here we use the cantilever beam of a commercial tabletop AFM to
tap on the surface of the samples, which is the gold features we
initially built on the scan table to test our design.
Our X Y nanopositioner scan table is located underneath the beam
and the images on the right are generated on the computer
screen using this scan table.
The closed loop system achieved by integrated electro thermal
sensors improve the quality of images especially at high scan
speeds. So when we increase the scan speed from 10 hertz to 50
hertz and 100 hertz in an open loop system, we lose the quality
of image, whereas is in closed loop system, using the integrated
sensors, we can improve and enhance the quality of images.
Here is a video of one degree of freedom electrostatic actuated
scan table and its microchip packaged and soldered on the PCB on
the right side of the image.
So you can see the Left-Right movements of the stage.
And this is a magnified image, electro thermal sensor is there
to measure the displacements of this stage.
Further research has been progressed on this prototype by
integrating cantilever beam in the same desired beam in the same
micro fabrication process by my colleagues at University of
Texas, Dallas.
So that's that was one practical example of advanced
mechatronics system.