Student Projects
Contact Information
University and Department: Penn State, Department of Bioengineering
Team Members: Becca Moretz, Lauren Sawarynski, Heather Weber, Kasey Catt, Dylan Frank
Primary Email Address: ram5109@psu.edu
Project Information
Describe the challenge your project is trying to solve.
Our mission is to create a baby weighing scale for early detection of some prevalent diseases which affect young children in Kenya and Tanzania. The scale will be designed so that the values it obtains can easily be input into a computer so that clinicians around the world can easily access the data via internet. We hope that with our scale, the overall health of the children in these countries will be improved due to more frequent monitoring of their weight. As part of the Mashavu organization, it is our responsibility to provide a safe, durable, economically feasible, user friendly, and accurate baby weighing scale to the people of Kenya and Tanzania.
The importance of frequently monitoring a baby’s weight is of high priority, as it can help in the early diagnosis, and hopefully treatment, of the diseases which plague the children of communities in Kenya and Tanzania (malnutrition, HIV/AIDS, diarrheal diseases, malaria, and pneumonia). Improvements in diagnosing these diseases using our baby weighing scale will benefit the overall health of the children in these communities. Our customers are the babies which will be weighed by the scale, their parents, and finally the Mashavu kiosk operators. With these customers in mind, we must be mindful of contextual and cultural concerns which may affect the success of our device. Some of the cultural and practical aspects which have been incorporated into our design are closely related to the design specifications listed in the "Design Criteria and Specifications" section. These design criteria include making our scale safe, durable, easy to use, accurate, cost efficient, and easy to repair and calibrate. These design criteria, and their relationship to contextual issues related to our project, are elaborated upon below:
Describe how you addressed the challenge through your project.
Detailed Description of Design Concept
A SolidWorks model of our design
For our design, we set out to create a cheap, durable, easily repaired, culturally relevant, sensitive, accurate, easy to use baby weighing scale that was able to input its data into a computer. Upon evaluation of prospective designs, it was determined that a strain gauge will used as the sensor. This led to the simple design of a metal bar in which the weight will be focused on one end causing the bar to bend. The bending of the bar will create a strain which, through the strain gage and LabVIEW can be effectively translated into a weight.
Our design will utilize two strain gauges that are placed on the underside underside and on top of a steel bar. The bar will be stationary at one end, and will be bolted to a sturdy frame. The other end of the bar will have a basket suspended from it in which the baby will be placed causing all of the baby’s weight to apply a force at the end of the bar. The strain that results in the metal bar from the placement of the bay in the basket will be translated into a change in resistance by the stain gages. Using a wheatstone bridge, these changes of resistance will be converted into a voltage change. Through an amplifier system, these voltage changes will be fed into the LabVIEW program. Once in the program, these voltages will be related to the weight based on calibrations performed earlier.
The one major part of the design that we would still like to modify is the frame. This part of the design adds coast and bulk to the design. We are seeking a way to eliminate the needs for a frame so that the bar can be attached to large stationary objects such as trees.
A Table of Design Needs and How they Translated into Specifications:
List of Design Goals | How They Were Met |
Cheap/Cost effective | Materials that could be easily found locally were used and the cost of our design was kept under $20 to ensure a reasonable price in Africa. |
Easily Repaired | Aside from the strain gage, the device consists of simple attachments through nuts and bolts. Replacement parts could be easily found in Africa and implemented into the device. |
Sensitive | The half wheatstone bridge allows for the detection of even small changes in the resistance of the strain gages so that minute changes in weight can easily be detected. |
Accurate | A calibration curve was initially created and the equation was implemented in LabVIEW. The accuracy of the device was then tested using numerous test trials of known weights. |
Interfaced with computer | Through the DAQ device and LabVIEW the voltage differences observed from the wheatstone bridge were translated into weights and displayed on the screen. |
Ease of use | The user interface was designed with three simple buttons that allow the user to obtain the weight of the baby quickly. This results in simple training to use the device. |
Culturally relevant | Through the Mashavu classes, the context manager guided the design process so that the use of the device will be acceptable in Africa. |
Durable | The materials used were of industrial grade to ensure durability. The strain gages were protected with a coating so as to reduce the chance of them being affected by conditions during use. The frame designed was sturdy, so much so that it could support a group member’s weight. |
Safety |
LabVIEW Programming
The LabVIEW Virtual Instrument was created as a simple program that converts a voltage to a weight. The computer first sends a 5V source through the DAQ device to the circuit, which contains a half Wheatstone bridge with our two strain gauges connected to it. When a weight is applied to the bar containing the strain gauges, it causes the Wheatstone bridge to become “unbalanced”, thus producing an output voltage. This output voltage is first sent through a differential op amp with a gain of 100, and is then sent back to the DAQ device through two differential analog input slots, back into the computer. LabVIEW samples this output voltage at a set interval, averages it, and then multiplies the averaged value by a conversion factor which was found from a calibration curve obtained from testing results. This new value is then output to the Front Panel as a weight when one pushes the READ button on the Front Panel.
The program also has a TARE button, which is the calibration mechanism for the program. When someone presses this button, the program subtracts whatever it’s currently reading the weight to be from itself so that it now reads the weight to be zero. From this, one can then get an accurate measurement. If the one wants to terminate the program, all they would need to do is press the STOP button.
Block Diagram of the LabVIEW Program:
Experimental Testing and Evaluation
Method
The voltages of weights ranging from 0 to 13 kilograms were measured in increments of .50 kilograms using LabVIEW. Three trials were performed to test the precision of the scale. The standard deviation was taken between the trials with an average standard deviation of .00164 which means that the scale is precise to .00164 volts which relates to ± 70.7 grams. This meets our design specifications of ± 86 grams. The average voltages of the trials were used to make a calibration curve that relates the changes in voltage to weight (Figure 1, below). An equation for the line was found using the trend-line function in Microsoft Excel. The equation was then implemented into LabVIEW by adding a scaling factor so that a weight reading could be output from the system.
The final step was to test the scale with a control. Weights from 0 to 13 kilograms were again measured to verify that the weight reading output from LabVIEW was accurate. The percent error between the measured value and the known value ranged from .04 to 26.16% with an average percent error of 2.97% as can be seen in the attached file under column H. The average percent error was very close, about 1% more, than our desired percent error of less than 2%. According to the average percent error, the scale has an accuracy of ± 253 grams. The accuracy found meets our design specifications of ±300 grams.
Results
The testing results produced a calibration curve that showed a linear relationship between weight and measured voltage. The scale met the design criteria for precision and accuracy. In the future, testing the scale with smaller increments of weight and making a calibration curve with the results could possibly improve the accuracy of our design.
Summary of Final Design
In Kenya and Tanzania, medical conditions such as pneumonia, malaria, tuberculosis, malnutrition, and HIV/AIDS affect many young children. One of the symptoms of all of these diseases is rapid weight loss or change. Our goal was to create a baby weighing scale which could be integrated into the Mashavu system, that with frequent use would help doctors detect minute, but meaningful, weight changes in children in order to diagnose these diseases. In order to implement our design in these developing countries, we developed a baby weighing scale which will meet the needs of our customers. In order to do so, the scale has been made out of primarily locally available materials, and will thus be economically feasible and easily repaired. In addition, our design is safe, user-friendly, durable, and accurate. Special care was taken throughout our design process to make sure that every aspect of our design meets our customer's needs and the design criteria mentioned above. Our final design thus consists of a sturdy wooden frame that holds a steel bar from which a locally-made baby basket is suspended. Strain gages bonded to the top and the bottom of the steel bar will measure small deformations in the bar when a baby is placed in the basket. Changes in voltage measured by the strain gages are converted to a weight measurement using LabVIEW. Our final design met all of the aforementioned criteria, and was especially accurate with an overall error less than 3% for any given weight measurement.
