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New feedback control for a scanning tunneling microscope
Author(s)
Bredekamp, Adriaan Hendrik
Date Issued
1999
Type
Thesis
Publisher
Cape Technikon
Abstract
This thesis describes the design and implementation of a new feedback controller for a
scanning tunneling microscope or STM. The previous controller had several shortcomings
when it came to the data throughput rate of the data acquisition system, the scan rate, and the
way the data was stored and displayed.
The initial investigation was done to determine the most cost effective way to implement the
data acquisition system. Various approaches such as DSP systems, analogue systems and
microcontroller systems were looked at. The investigation also looked at the best way to get
the data from the Z directional control loop to the PC for displaying the data. The final choice
was to use an ultra fast microcontroller for the control loop implementation and to change the
DOS based software for Windows based software.
The embedded system was divided into two parts. The first was the controller for the X and
Y scan directions, and the second was for the Z scan direction. A digital PI control loop was
implemented on the Z controller to control the height of the scan tip above the specimen
surface. The microcontroller that was chosen for this was the Microchip PIC17c43. The data
transfer to the PC was done with a PC-14 programmable digital input/output card. Two
options for the implementation of the PC-14 software were considered. The first option was
the software that was bundled with the card. This software proved to be very slow, so special
device-driver-based software was developed to control the PC-14 card and the data transfer to
and from the Pc. The PC software was implemented using Visual C++.
Both the XY and the Z controllers proved to be working satisfactorily in the existing STM
arrangement. It was discovered that the XY controller was overloaded with the many tasks
that it has to perform, and a suitable alternative system to replace the XY controller is
proposed. The selection of the PC that will be used for the data acquisition system is also
discussed. It was found that this choice had a very big influence on the design of the final
system because of the difference in PC bus design. Several proposals to increase the
functionality of the PC software are also made.
scanning tunneling microscope or STM. The previous controller had several shortcomings
when it came to the data throughput rate of the data acquisition system, the scan rate, and the
way the data was stored and displayed.
The initial investigation was done to determine the most cost effective way to implement the
data acquisition system. Various approaches such as DSP systems, analogue systems and
microcontroller systems were looked at. The investigation also looked at the best way to get
the data from the Z directional control loop to the PC for displaying the data. The final choice
was to use an ultra fast microcontroller for the control loop implementation and to change the
DOS based software for Windows based software.
The embedded system was divided into two parts. The first was the controller for the X and
Y scan directions, and the second was for the Z scan direction. A digital PI control loop was
implemented on the Z controller to control the height of the scan tip above the specimen
surface. The microcontroller that was chosen for this was the Microchip PIC17c43. The data
transfer to the PC was done with a PC-14 programmable digital input/output card. Two
options for the implementation of the PC-14 software were considered. The first option was
the software that was bundled with the card. This software proved to be very slow, so special
device-driver-based software was developed to control the PC-14 card and the data transfer to
and from the Pc. The PC software was implemented using Visual C++.
Both the XY and the Z controllers proved to be working satisfactorily in the existing STM
arrangement. It was discovered that the XY controller was overloaded with the many tasks
that it has to perform, and a suitable alternative system to replace the XY controller is
proposed. The selection of the PC that will be used for the data acquisition system is also
discussed. It was found that this choice had a very big influence on the design of the final
system because of the difference in PC bus design. Several proposals to increase the
functionality of the PC software are also made.
Additional information
Thesis (MTech(Electrical Engineering))--Cape Technikon, Cape Town,1999
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