In this project, the students will be required to develop, using LEGO block, a device capable of determining the mass of an arbitrary object. Throughout the development of the solution, the students will be exposed to concepts from several engineering disciplines. Teamwork is also a fundamental part of this project, as little engineering in the "real world" is done individually.

Specifications and Requirements

The scale to be developed must conform to the following restrictions:

1. The scale must be nearly fully automated. The only interaction that the students are allowed to make is the placing of the unknown mass on the scale, the placing of one counterbalance mass (if necessary), and the setup of any controls that the user interface (described below) requires. Once the "go" command is given from the interface, the scale must determine the mass of the object completely on its own.
   


2. The scale must be accurate to within 5 grams. Points will be subtracted for scales that do not achieve the required accuracy.
   


3. The scale must use no more parts than are available in one LEGO Dacta kit. Using more Lego parts will impose a point penalty. If parts outside the Dacta kit (such as string or elastic materials) are required, a judgment by the course instructor as to the need for such parts will be made.
   


4. The scale must provide some estimate of the error in the measurement. This may be done in several ways, as described below.
   


5. A graphical user interface must be provided. The minimum interface will have controls for calibrating the scale, a display of the measured mass and error estimate, and a button to start the measuring process. Additional controls will be determined by the particular design that is implemented. Some of these might include controls for setting a counterbalance mass, graphical displays of measured data, and so forth.
   

Appendix A of this handout covers some basic physics that is applicable to scale design. In addition, some rough sketches of possible designs are given.

The parts requirement stated in (3) will be strictly enforced when LEGO blocks and parts are considered. There is a provision for obtaining parts not included in the kits, as stated. Most scale designs will requires some simple extras such as springs. Use of necessary extra parts is allowed and encouraged, but only after the design team consults with the instructor and explains the need for the parts. This need must also be documented as described below.

The error estimate in (4) can be obtained in several different ways. If a particular design requires the motion of some measuring device, then the distance the device can move without changing the measurement can be used to obtain an error which will then be propagated through the necessary calculations. A sheet of error propagation formulas is included in Appendix B. Another possibility is to make many independent measurements of the mass of a particular object. These measurements can then be analyzed using statistical methods to obtain an estimate of the error. A page of useful statistical formulas is also provided in Appendix C. Note that this method requires the scale to perform the independent measurements. No user interaction is allowed. A third option is to measure the errors associated with the operation of the scale, and include these factors in the mass calculation routines in the control program. For example, if some distance measurement is off by some amount, this error can be propagated through the calculations.

The user interface should be functional as well as user-friendly. Calibration controls, measurement displays, and a start button are only the bare requirements. More features and options are welcome, but the group must be careful not to make the interface too complicated and unwieldy. Extra features will earn extra points, but only if they provide useful functions or displays. An example would be a graphical chart of the independent measurements made if the error is determined by statistical means. Another might be a graph of error estimates versus the mass of the object. This would show how the accuracy of the scale is related to the mass being measured. A chart of masses measured does not provide much useful information not already given by other displays and would earn no extra points.

In addition, the user interface should contain some sort of diagnostic readout. For example, teams may want to include a log of actions taken by the control program, and provide printouts of intermediate variable values. This will not only provide useful information when the design is finished, but it will also be a significant aid in the creation of the design.

Economic Considerations

With any engineering project, the use of raw materials is an important consideration. One of the main goals of an engineer is to provide the best solution at the minimum cost. The scale design will be evaluated using the following economic model:

Since each design will be likely be quite different, teams may use as many "regular" LEGO blocks from the Dacta kit as necessary with no penalty. However, the use of the following parts will be measured and used to evaluate the cost of the solution:

• Each LEGO motor used will cost 100 points.
• Each sensor used will cost 50 points.
• Each gear used will cost 30 points.
• Each light or sound element used will cost 25 points.
• Each connecting wire used will cost 20 points.
• Each axle used will cost 10 points.
• Parts used that are not included in the Dacta kit and not previously approved by the instructor will cost 5 points. This includes things like extra LEGO blocks from other kits.

The design will be evaluated as follows:

• 600 points or lower - 10% score bonus.
• 1000 points or lower - no score penalty.

For each extra 100 points, 10% will be deducted from the score. So a design that costs 1025 points will result in a 10% score reduction. A 1150 point design results in a 20% reduction in score.

Groups must be careful not to become too entangled by the economic considerations. The engineering performance metrics described below give substantial bonuses to designs that meet or exceed the requirements or provide more functionality. There is a tradeoff, and part of the design process is determining where the happy medium is.

Engineering Performance Metrics

Each project will be evaluated to measure its performance. A design that meets all of the requirements stated under "Specifications and Requirements" will earn full credit. Failure to meet the requirements will result in a penalty as described below:

• Scales that require user intervention after the "go" command is given will result in a 20% score penalty.
• Scales that fail the accuracy requirement will be given a .5% penalty for every gram over the accuracy requirement.
• Points will be given for each "stage" of the project that is completed:
  • 50% for a design whose mechanical components are built, but are not controlled through the computer.
  • 60% for a design controlled through the computer.
  • 75% for a design that includes an automated control program.
  • 90% for a design that includes error estimates.
  • 100% for a design that includes a well-designed user interface.

Extra credit will be given to those groups who exceed the design requirements:

• A 1% bonus for every gram under the accuracy requirement.
• Useful extra features included in the user interface will earn points as determined by the instructor, but will generally result in a 5% bonus.

In addition, the time to determine a mass should be minimized. Of course, the time required will vary widely from design to design, but it should be within a reasonable limit.

Provided Extra Equipment

A laboratory scale will be provided so that groups may evaluate the performance of their design. Also, tools such as scissors, soldering irons and so forth will be provided. Required equipment beyond that provided will be considered on an individual basis.

Project Validation and Demonstration

Each team will be required to demonstrate the design to the course instructor. The demonstration must clearly show the accuracy of the design and the associated error estimates. The function of the user interface must be demonstrated, and any additional features or enhancements will be explained and demonstrated. Teams should have a demonstration plan put together in advance to show the function of their design in a comprehensive and organized manner.

Project Documentation

Each team will document their design thoroughly. The documentation will include, but is not limited to:

1. Introduction explaining the nature of the project.
2. Explanation of the design and its functionality. This should be a general overview of the design suitable for presentation to someone with no engineering background. The operation of the scale and user interface should be explained, and general estimates of its accuracy must be given.
3. Technical overview. This section should include greater detail about the design. The specific engineering concepts involved will be explained. All calculations used in the design will be covered. In particular, teams should specify any other design ideas that were explored, and why one design was chosen over the others. Schematics and program listings should be used to help clarif specific points. In addition, extra features of the design should be explained in detail, including the reasons they were developed.
4. Development process. This section should describe the steps taken to develop the proposed design. Teams should explain how work was partitioned among team members. Were certain individuals responsible for specific areas, such as programming or documentation, or did team members participate in all areas? This section should include a description of the specific "large-scale" goals that were set for the development of the project, and should also cover any unforeseen difficulties that were encountered.
5. Performance assessment. This section will cover both the economic and engineering evaluations of the design. The economic and engineering evaluations should be clearly distinguished, but an overview of the tradeoffs involved will be very informative. In particular, teams should cover why certain decisions were made and how the performance/cost ratio might be improved upon.

   Items may be interchanged among sections. For example, you might want to discuss the extra design features in the engineering performance assessment.

   For the economic portion of the discussion, the ability to mass produce the final design should be discussed. In particular, mention of unique features that may be difficult to manufacture should be highlighted. Of course, this will require that the team pretend that the Lego parts are not already manufactured. In other words, what would happen if a manufacturing plant had to machine all the parts?

   There is no set length requirement for the project documentation. Often, shorter, more concise documentation is preferable to a lot of reading that includes little information. Teams should concentrate on covering all of the above points and writing in a clear and understandable style rather than looking for the most bulk. Technical appendices including schematics and tables are a welcome addition to the above requirements, but all of the flashy charts in the world will not rescue poorly written documentation. The level of professionalism and clarity of the documentation will be a major factor in the evaluation of each project.

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   APPENDIX A

   
   

   Possible Scale Design Outlines

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   Pivot Scale

   One possible scale design uses the familiar lever arm from physics. The design looks something like this:

   

   The equation governing this relationship is

   M1L1 = M2L2

   Spring Scale

   Another possibility is to use a spring or other elastic material. The mass is obtained using Hooke's Law:

   F = kx

   In this equation, k is the spring constant (obtained from measurement of known masses) and x is the distance a mass stretches the spring when it is allowed to hang freely.

   "Weightless" Balance

   A third scale design does not even requires the presence of gravity to function! If a mass is connected to a spring or other elastic device, and the mass is moved from the equilibrium position and released, the mass will exhibit simple harmonic motion with a period determined by

   

   Where K is the spring constant, and T is the period of motion. M, of course, is the mass being measured.These are only three rough sketches of possible designs. Students may want to explore other possibilities.

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   APPENDIX B

   
   

   Useful Statistical Formulas

   
   

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