Tech Team Two

Team Members

Brian Klug Mike Tran

Nabeel Kibria Garret Skinner

Justin Bell John Sargent

[email protected]

(Sends E-mail to all members)

ii. Table of Contents

i. Title Page

1. Abstract

2. Design

2.1 Description

2.2 Specifications

2.3 Constraints

3. Analysis

3.1 Configuration

3.2 Analysis of Parts

3.3 Materials

3.4 Program

3.5 Program Output

4. Drawings

4.1 Concept Drawing

4.2 Assembly

4.2.1 Sub Assembly

4.3 Detail Drawings

4.4 Electrical Schematic

5. Conclusion

5.1 Parts List

5.2 Cost Estimate

5.3 Special Thanks

5.4 Conclusion

6. References

1. Abstract

Our goal as a team is to assemble a highly accurate electronic scale with an efficient digital display of weight and postage. We have been given approximately three months to reach this ultimate goal. The three months will serve as time in which we will consult within our group to construct initial design proposals, final design proposals, initial and final design specifications and drawings. Finally, we will assemble, test and present our final product to the class. Our ultimate goal can only be achieved if all these sequential goals are achieved one by one.

Throughout the first month and a half the team consulted each other and multiple references in order to propose different designs that met and exceeded the specifications for our electronic scale. After searching various Internet sites and magazines the team finally decided on what we wanted our scale design to be. We settled on a simple yet effective design. The scale design we chose is that of a simple lightweight box with a slanted face and backlit LCD screen. The design also calls for the back panel to open on a hinged side, which will allow us to access the inner constituents. The opening will also serve as a window for those who wish to examine the inner components of the scale in an educational setting.

Once our design proposition was finalized, the team began to master ProEngineer, a computerized 3-D drawing and assembly board. Within two weeks the final designs, 3-D drawings, and 3-D assembly designs had been completed. Once these steps were completed the team began analyzing the mathematics behind building the actual processor and electronics of the scale. The team also began to analyze how the strain gauge and wheatstone bridge setup could most effectively be utilized for integration with the A/D converter. This is the point at which the team is positioned and working on to this date.

The team will take recess for one week for spring break and then resume work on the scale. The individual parts will be acquired, or manufactured. The team will mount the strain gauge on the bar, and build the electronic system. The processor will be programmed at this point, and we will begin modifications.

2.1 Description

The scale will be an enclosed device, with an aluminum pan to hold the weight of the sample. A thin aluminum bar, ¹/₄" inch in width and ¹/₈" in height, will support this pan by a vertical rod. Attached to this beam will be a 120W 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 amplifiers 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 then be will be converted to a weight in grams, and a formula will then calculate the correct postage and display this on the backlit LCD screen. This screen will be mounted angled 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. The scale will automatically zero itself during initial power-up, and enter low-power consumption sleep mode. The scale leaves sleep mode when an object is placed on the pan and the backlight on the LCD turns on while the display shows the correct output. The scale turns “off” after approximately 35 seconds of inactivity.

2.2 Specifications

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.

3.1 Configuration

The electronic microprocessor-controlled scale was chosen because of its high level of accuracy, professional user interface, ease of use, and powerful features offered. Strain gages are excellent tools to determine accuracy in pressure. One advantage of choosing an electronic strain gauge is that it is accurate and fast in determining the weight of an object. Compared to other scales such as mechanical or hydraulics scale, electronic scales are smaller and more compact. To measure the weight of bigger objects also, using a strain gage is more accurate compared to mechanical scales in determining the weight of the object. While mechanical and hydraulics scales are often cheaper, electronic scales seem to be more appealing because the processor does the calculation in determining the weight of an object. Although the cost of electronic components are higher than mechanical components, using today's technology such as in strain gages and processors has its advantages such as in determining the precise weighing of an object.

The basic designs of our electronic scale is similar to the design of a typical digital scale, however with added power of our feature-packed processor. The reason why we choose this design is that it is simple to read the scale compared to the spring scale or mechanical scale. The capability we have with a processor controlled scale is almost limitless. In our electronic scale, the weight is and postage is simply displayed on the LCD screen. The object to be weighed is put on top of our electronic scale. The mechanism that zeros our electronic scale is through the processor. It is a software routine that sets the digital value to zero when there is no pressure on the strain gage. The software approach in a scale is important, as it allows us greatest flexibility. We can program postal rates easily, and add features without changing hardware.

3.2 Analysis of parts

Ø The pan will hold the items to be weighed. We have chosen a pan that will provide the user with a reasonably large weight surface, for ease of use.

Ø The large easy-to-read LCD screen is on an angle and is backlit for even easier viewing.

Ø The batteries will provide an unregulated power source for the LCD, backlight and the
processing chip, each with built in regulators. We choose this method so that inexpensive standard 9V cells may be used.

Ø A strain gage will be mounted under the center of the beam, which will
support the pan. This will provide best input to the bridge.

Ø A wheat stone bridge will be connected to the string gage to amplify from approx. 1.3mv max to 5V max. This is designed so that the A/D converter will be supplied 5V at max weight, and 0v at 0 weight.

Ø The signal from the strain gauge will be amplified and then sent to the
12 bit analog digital. This will provide 4096 unique values of possible measurement from the gauge. A typical A/D circuit provides the computer standard 8 bits of data, which provides for only 256 values of resolution.

Ø The A/D converter will send a digital signal to the processor. From here, anything we want can be done with the input, and be presented to the user in any format.

3.3 Materials

The pan we use is made of wood and metal. We basically decided to make our scale from wood because it was cost effective and easily build where metal structure of an electronic scale are harder to build. Even though the look of our scale is not as swift and elegant as if it would if made from metal. It brings an old American Style of look along with the advance technical use of its features. The slant use of the LCD display is a nicer to be read. Our source of energy comes from 5 volts of power to our electronic scale and we used battery as our energy source because it is the best method with the compliment of the strain gage.

3.4 Program

Below is a QBASIC Program and its output. This program was used to generate and test values to give us the correct values. We feel this program has enabled us to perform a large number of experimental values, which allowed us to quickly create accurate figures.

PRINT "----------------------------------------------------------------------------"

PRINT "Data for a "; LTRIM$(RTRIM$(STR$(r0))); "ê strain guage on a "; LTRIM$(RTRIM$(STR$(b))); "x"; LTRIM$(RTRIM$(STR$(h))); "x"; LTRIM$(RTRIM$(STR$(s))); " inch bar."

PRINT "with a guage factor of "; LTRIM$(RTRIM$(STR$(gf))); ", max weight"; W; "pounds."

PRINT "----------------------------------------------------------------------------"

3.5 Program Output

5.1 Parts List

5.2 Cost Estimate

5.3 Special Thanks

5.4 Conclusion

Our team is working together to construct a fully operational postal scale. Our efforts will now be centered at creating the scale itself, and redefining the design to favor improvement. We hope our desired goal is achieved properly and fashionably by May 12, 1999.

"Postal Accounting Makes Money--Legally." Modern_Office Technology, 33(1), 24(1998). Page 24.

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

Pollock, John L. "Cognitive carpentry: a blue print for how to build
a person" Cambridge, Mass: MIT Press, 1995. Page 377

Part	Description	Price
Battery	9 Volts DC	$5
BS2-IC Basic Stamp Module	11Mhz Microprocessor	$49
A/D Converter	12 Bit	$26
LCD Module	20x4 Backlit	$45
Wood	Total cost of wood / Wood used in project	$1.70
Aluminum	For pan, and bars	$8
Quad Op Amp		$1
Resistors	5 120W, 1 172KW	$1.95
Wire/Solder/Misc.		$1
Breadboard		$5

Team Members

Brian Klug Mike Tran

Nabeel Kibria Garret Skinner

Justin Bell John Sargent

ii. Table of Contents

1. Abstract

2. Design

2.1 Description

2.2 Specifications

2.3 Constraints

3. Analysis

3.1 Configuration

3.2 Analysis of parts

3.3 Materials

3.4 Program

3.5 Program Output

5. Conclusion

5.1 Parts List

Price

5.2 Cost Estimate

5.3 Special Thanks

5.4 Conclusion

6. References