1. The first task was to construct a printed
circuit board that connects the board
containing the PIC microcontroller and
the board containing the analog-to-
digital converter (ADC).
The board contains the mbed
microcontroller and other electric
components. The constructed board
(Figure 1) is 3.20 by 2.25 inches. This
was done using the Eagle software.
The Automated Safe-To-Mate Tester
(ASTM) is essential to make sure
flight equipment is secure and
functioning properly. It measures
impedance between any two pins to
verify connectivity and isolation. This
is done in a fast and reliable manner
and is useful when testing
electronics with up to 632 pins.
The ASTM is important because it
allows checking of spaceflight
instruments and hardware in an
automatic way. In the past, in order
to make sure spaceflight instruments
were safe to operate and not
damaged, hardware was usually
checked manually. As a result of this
project, checking instruments can be
done much quicker, much more
accurately, and with more precision.
Currently, the ASTM is being modified
to operate faster and more efficiently.
As a result, the mbed microcontroller
is being incorporated. This embedded
processor requires a firmware to
execute all operations. In addition, the
embedded microcontroller need
additional commands in order to
communicate with the PC effectively.
Finally, a new printed circuit board is
required to incorporate all new
components, in particular the mbed.
By the end of the internship, a fully-
constructed and fully-working
ASTM Tester will be produced.
Modifying the Automated Safe-To-Mate (ASTM) Tester
Tony Zhang1; Thomas Capon2; Umeshkumar Patel2
1University of Maryland, College Park
2Electromechanical Systems Branch, NASA Goddard Space Flight Center, Code 544
Introduction Process Process (continued)
Conclusion
Tony Zhang
Rising Electrical Engineering Junior
University of Maryland, College Park
Email: tonyzhang2013@yahoo.com
Phone: (410) 299-0832
Contact
The next task is to write the firmware for
the mbed. The first step for the firmware
is to have the mbed communicate with
the ADC and read what the ADC is
outputting. This is accomplished via the
serial peripheral interface (SPI) module.
After that, the firmware has the mbed
read ADC values at a certain frequency.
Figure 2 shows the mbed connected
with the ADC board.
Process (continued)
Then the firmware has the mbed respond
to PC commands. This was accomplished
by writing a program with all possible
commands and corresponding actions.
Finally, an additional serial communication
path for the mbed was added in the
firmware of the PIC microcontroller. In
addition, multiplexer settings in the PIC
firmware have been updated to reflect the
addition of the mbed.
Schematic
The mbed Microcontroller
The mbed NXP LPC1768 (Figure 4)
microcontroller is used. It is being added
to the ASTM because the PIC
microcontroller proved too slow to run
the algorithm fast enough.
This microcontroller produces high
speed, high performance, and strong
connectivity. Moreover, there are
numerous pins, which are used for
digital input and output, serial peripheral
interface (SPI) communication, and
serial communication.
Future Work
Based on the most up-to-date schematic
design (Figure 3), the ASTM system
begins at the PC, which communicates
with the PIC microcontroller. Next, the
PIC microcontroller selects one pair of
the 632 input channels to measure the
impedance between.
Then the PIC communicates with the
mbed microcontroller, which selects the
resistance range, picks the best circuit
to measure the impedance, retrieves
measurements from the ADC, calculates
the impedance. Finally, the mbed sends
the impedance reading to the PIC
microcontroller, which then sends it to
the PC.
For the rest of the internship, I will be
working on updating the mbed firmware
to incorporate the updated algorithm,
which selects the resistance range,
picks the best circuit to measure the
impedance, retrieves measurements
from the analog-to-digital converter, and
calculates impedance.
Additionally, I will be updating the PIC
firmware in order to reflect the updated
algorithm and reflect the addition of the
mbed.The mbed Firmware
The mbed firmware is written using the
mbed online compiler. All the programs
are written in C++. Currently, the
firmware has the ability to read and
respond to commands. The list of
commands are:
• VERSION – returns the version string
• SETGAIN <gain> - set the multiplexers
according to the gain
• SAMPLE <n> - takes n samples from
the ADC and returns them all
• SMUXBUF <n>,<gain> - sets gain,
takes samples from the ADC, and then
returns them all
• SBUF <n> - take samples from the
ADC and returns them all
• SAVG <n> - take samples from the
ADC and returns the average of the
samples
• SRATE <f> - sets the sample
frequency for ADC readings
Figure 1: ARM circuit board Figure 2: mbed and ADC board
connected together
Figure 3: Updated ASTM Schematic Figure 4: mbed Schematic