On test day our scale came through
with flying colors. It not only amazed
our peers, but also fullfilled its task of weighing measures precisely. The scale hardly had any errors. We also met the cost specifications. Even though most of out technology had been
donated to us, we calculated that our scale would have been ten dollars below
the $150 maximum, even if we had bought all our own supplies. Due to our scales excellence in our class we
were chosen to represent our class in the overall engineering school scale
competition. In the overall competition
our scale came in second place total, one tenth of a point behind the first
place winner. Our scale won first place
for aesthetics.
I regret to add that
the toughest obstacle the team reached, by far, was the conflict of finding
anyone with the proper knowledge to help us.
We never did find out why the amplifier refused to amplify beyond 1.38
volts (when supplied +5V), nor did we find an answer to as to why switching
from breadboard to PC board would make the AD input oscillate. This absolutely, without a doubt, caused us
to come in second place. I can only
hope, the reason we did not have access to adequate university staff, was that
because we are in a low-level course.
2. Analysis
2.1 Design
Our scale
will not have a target customer since it will be donated to schools ranging
from kindergarten to grade 12. They
will use the scale to better understand how scales work. The scale that our group made had several
restrictions that had to be met. First,
the scale had to weigh up to 2000g, with an accuracy of +/- .5g. Secondly, the scale has to fit in a size 15-shoe
box. Finally, the cost of the scale has
to be under $150.
The scale
will be an enclosed devise, with an aluminum pan to hold the weight of the
sample. A thin aluminum bar, 1/4” in
width and 1/8” in height, will support this pan by a vertical rod. Attached to this beam will be a 120 strain
gauge. This strain gauge will be
mounted directly under the pan’s support piece for maximum efficiency. The gauge will be connected within a
wheatstone bridge, which will deliver its output to an amplifier. The voltage of the amplifier’s output (0 to
+5V) would be directed into a 12Bit A/D converter. This data will be sent via digital I/O lines to Parallax’s Stamp
II microprocessor. This information
will be converted to a weight in grams, and a formula will be made to calculate
the correct postage and display this on a back-lit LCD screen. This screen will be mounted on an angle
towards the user for maximum readability.
This robust design should have no problems due to its simplicity, yet
the power of the embedded system will provide the user with a state-of-the-art
full-featured scale. After pushing the
zero button, the scale will zero itself.
There is an on/off button that is used to turn the scale on and off.
2.2 Specifications
q
Max weight: 1600g
q
Temperature range: 0º to +50º C
q
Operating Relative humidity: 90% max non-condensing
q
Power: Unregulated 9 Volts DC dry-cell
q
Power Consumption: 108mA
q
Power Capacity: 2000mAh
q
LCD Color: Yellow green (Easy to read, large display)
q
“Hi Tech” scale appeals to young students.
q
Back panel hinged for students to learn from inside
component arrangement.
2.3 Constraints
q
The LCD’s backlight consumes 90mA, accounting for most
of the power consumed by the unit.
q
The fluid in the LCD display is the limiting component
for the operative temperature range and operating relative humidity.
q
The maximum weight was limited by the amplification of
the amplifer.
3. Assembly
3.1 Bottom, Front, Sides, and Back Panels
- Apply
glue to all four edges of the bottom panel
- Place
front, sides, and back panel against the corresponding sides of the back
panel(refer to 3D drawing)
- Clamp
all opposite sides in order to secure the glue
- Drill
1/8" holes through the side panels into the bottom, front, and back
panels. Also drill holes along top edge for the top panel that will be
placed later. Note: Holes are represented in picture
- Fasten
panels together with 3/4" brass screws using screwdriver.
3.2 Top and Display Panel
- Place
Angled sides of the top and display panels adjacent to one another
- Apply
the two hinges over the two panels 1/4" from the edges parallel with
the gap between them
- Fasten
the hinges to the panels with the 1/4" brass screws supplied with the
hinges
- This
assembly is now ready to be joined with the other part
- Apply
glue to sides of the top panel and assemble into the place where the top
panel is to go.
- Fasten
together the top and side panels with 3/4" screws
3.3 Internal Assembly
- Along
the inside of the front panel, place the two small blocks against the
front panels in the corners with glue
- The
round AL bar comes down from the pan to a horizontal bar.
- On
this bar is a strain gauge, which is connected to the processor board.
- A
speaker mounted on the right is also connected to this board.
- The
LCD and power/zero buttons are also connected.
- A
Micro switch & light bulb combo provide Refrigerator door technology.
3.4 Buttons and LCD Screen
- Slide
button boxes into slots Note: No glue is needed
- Apply
button covers to the boxes
- Apply
LCD screen into slot from the back side of the display panel
- Fasten
four corners of the screen to the display panel with the supplied screws
3.5 Program
Below is the source code to the BS2-IC.
'PREPARE VARIABLES
butt var byte
' Variable for Zero button
zero var word
' Offset for zeroing
weight var word
' Holds weight (after zeroing)
misc var word
' Hold misc calculations (weight/postage..)
AD var word '
Variable to hold 12-bit AD result.
' INITIALIZE DISPLAY, AD, SOFTWARE
butt=0
pause 450
serout 3,16416,[$FE]
serout 3,16416,["C"]
serout 3,16416,[$FE]
serout 3,16416,["C"]
serout 3,16416,[$FE]
serout 3,16416,["F"]
serout 3,16416,[$FE]
serout 3,16416,["X"]
serout 3,16416,[$FE]
serout 3,16416,["B1"]
high 0
high 2
'SHOW LOGO
'gosub logo
'ZERO SCALE
:zer
misc=0
serout 3,16416,[$FE]
serout 3,16416,["X"]
serout 3,16416,[$FE]
serout 3,16416,["G"]
serout 3,16416,[$04]
serout 3,16416,[$02]
serout 3,16416,["Z e r o i n g" ]
FREQOUT 4,50,200:pause 50:gosub
inzero:zero= misc:pause 50
FREQOUT 4,50,200:pause 50:gosub
inzero:zero=zero+misc:pause 50
FREQOUT 4,50,200:pause 50:gosub
inzero:zero=zero+misc:pause 50
FREQOUT 4,50,200:pause 50:gosub
inzero:zero=zero+misc:pause 50
zero=zero/4
'MAIN LOOP
again:
pause 5
button 5,1,255,255,butt,1,zer
'GET INPUT
low 0
shiftout 2,1,lsbfirst,[%1101\4]
shiftin 2,1,msbpost,[AD\12]
high 0
if AD>1050 then oops
weight=(AD-Zero)
if weight>4096 then lw
if weight<0 then lw
:lwb
if weight>1024 then hw
:hwb
:backc
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$09]:serout 3,16416,[$03]
serout 3,16416,["debug "]
serout 3,16416,[DEC AD]
serout 3,16416,[" "]
'SHOW WEIGHT
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$01]
serout 3,16416,[DEC Weight * 20 / 13]
serout 3,16416,["g / "]
serout 3,16416,[DEC Weight * ((20 / 13) * 3)
/ 85]
serout 3,16416,["."]
serout 3,16416,[DEC Weight * ((20 / 13) * 3)
// 85 * 10 / 85]
serout 3,16416,[ "oz "]
'FREQOUT 4,8,AD*4,AD*3
' BAR GRAPH
serout 3,16416,[$FE]:
serout 3,16416,["h"]:
serout 3,16416,[$FE]:
serout 3,16416,[$7C]:
serout 3,16416,[$01]:
serout 3,16416,[$04]:
serout 3,16416,[$00]:
serout 3,16416,[weight / 10]:
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$03]
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$03]
misc=Weight * 20 / 13
if misc > 10 and misc < 35 then g1
if misc > 80 and misc < 120 then g2
if misc > 180 and misc < 220 then g3
if misc > 370 and misc < 430 then g4
if misc > 470 and misc < 530 then g5
if misc > 960 and misc < 1040 then g6
backguess
serout 3,16416,[" "]
'SHOW POSTAGE
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$02]
if (Weight * ((20 / 13) * 3) / 85) > 31
then pri2
if (Weight * ((20 / 13) * 3) / 85) > 14
then pri
serout 3,16416,["First-Class $"]
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$11]:serout 3,16416,[$02]
serout 3,16416,["0 "]
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$0E]:serout 3,16416,[$02]
misc=33 + (22 * (Weight * ((20 / 13) * 3) /
85))
serout 3,16416,[DEC misc/100]
serout 3,16416,["."]
serout 3,16416,[DEC misc-(misc/100*100)]
goto again
pri:
serout 3,16416,["Priority Mail
$3.20"]
goto again
pri2:
serout 3,16416,["Priority Mail
$4.30"]
goto again
'WARNING
oops:
weight=1050
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$03]:serout 3,16416,[$04]
serout 3,16416,["Too much
weight!"]
FREQOUT 4,100,600
FREQOUT 4,200,300
goto backc
'ERROR CHECKING ON INPUT
lw:
weight=0
goto lwb
hw:
weight=1024
goto hwb
'LOGO
logo:
serout 3,16416,[$FE]:serout
3,16416,["X"] ' Clear screen
' Large Digits
serout 3,16416,[$FE]
serout 3,16416,["n"]
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$06]:serout 3,16416,[$02]
serout 3,16416,["Tech"]
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$08]:serout 3,16416,[$03]
serout 3,16416,["Team"]
serout 3,16416,[$FE]:serout
3,16416,[$23]:serout 3,16416,[$0D]:serout 3,16416,[$02]
gosub beeps
pause 2000
serout 3,16416,[$FE]
serout 3,16416,["X"]
serout 3,16416,["***Tech Team
Two****"]
serout 3,16416,[$FE]:serout 3,16416,["G"]:serout
3,16416,[$01]:serout 3,16416,[$03]
serout 3,16416,[" Our Postal Scale "]
serout 3,16416,[" Welcomes You :) "]
gosub beeps
pause 2000
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$02]
serout 3,16416,[" Brian Klug "]
serout 3,16416,[" John Sergeant "]
serout 3,16416,[" Garret Skinner "]
gosub beeps
pause 2500
serout 3,16416,[$FE]:serout
3,16416,["G"]:serout 3,16416,[$01]:serout 3,16416,[$02]
serout 3,16416,[" Mike Tran "]
serout 3,16416,[" Justin Bell "]
serout 3,16416,[" Nabeel Kibria "]
gosub beeps
pause 2500
serout 3,16416,[$FE]:serout
3,16416,["X"] ' Clear screen
return
'AUDIO OUTPUT
beeps:
FREQOUT 4,300,1000
PAUSE 70
FREQOUT 4,200,2000
PAUSE 70
FREQOUT 4,100,7000
PAUSE 70
FREQOUT 4,40,2000
PAUSE 70
FREQOUT 4,40,2000
PAUSE 70
FREQOUT 4,40,1000
return
inzero:
low 0
shiftout 2,1,lsbfirst,[%1101\4]
shiftin 2,1,msbpost,[misc\12]
high 0
return
‘Insti-Guess technology
:g1
serout 3,16416,["20g"]
goto backguess
:g2
serout 3,16416,["100g"]
goto backguess
:g3
serout 3,16416,["200g"]
goto backguess
:g4
serout 3,16416,["300g"]
goto backguess
:g5
serout 3,16416,["500g"]
goto backguess
:g6
serout 3,16416,["1000g"]
goto backguess
5. Conclusion
5.1 Parts List
|
Part
|
Description
|
Price
|
|
Battery
|
9 Volts DC
|
$5
|
|
|
11Mhz
Microprocessor
|
$41
|
|
A/D
Converter
|
12 Bit
|
$26
|
|
LCD Module
|
20x4
Backlit
|
$40
|
|
Quad Op Amp
|
1/4 used
|
$1
|
|
Assorted
Resistors
|
|
$1.95
|
|
Wire/Solder/Misc.
|
|
$1
|
|
PC Board
|
Standard
Radioshack
|
$5
|
|
Metal Pan
|
4x5”
|
$3.48
|
|
Aluminum
bar
|
5 ½”
|
$0.30
|
|
Strain
Guage
|
120 Ohm
|
$1
|
|
Paint
|
Red and
Silver
|
$2
|
|
Silver Knob
|
Round
|
$1
|
|
SPST Switch
|
Left
|
$0.89
|
|
SPST Button
|
Right
|
$0.89
|
|
Hinge &
Screws
|
Brass
|
$2
|
|
Pine Wood
|
3 piece
6" * 2 ft
|
$12.60
|
|
|
Total
|
~$146
|
5.2 Cost
The final cost of the scale was $146 (including donated
parts).
5.3 Special Thanks
q
Thanks to Parallax Inc for their generous donation of
the BS2-IC chip and related programming tools.
q
Thanks to Matrix Orbital for the generous donation of
their top-of-the-line 20x4 Backlit LCD module with serial interface.
5.4 Conclusion
We are very proud of our scale. I have
never learned so much about electronics or engineering in my life. We enjoyed the challenge. I’ve never seen a scale like it before; we
appreciate all the comments from the staff “This is the best design I have ever
seen..”, etc. Thanks
6. References
"Postal Accounting Makes Money--Legally." Modern_Office
Technology, 33(1), 24(1998). Page 24.
“Interactive Guide To Strain Measurement Technology” http://www.measurementsgroup.com/guide/index.htm
Online. Internet. Measurements Group, Inc. January
1999 Edition
Burns, Robert W. “On-board truck scale--Digital Strain Gage
Conditioner Philips” Microcontroller Electronic NewsLetter. ISSUE 34 - APRIL 1998
James W. Dally Introduction to Engineering Design Book 2
Knoxville TN: College House Enterprises, LLC. 1997
