Mashavu Adult Weighing Scale Project

[Laura Arnold]

Contact Information

University and Department: Penn State, Department of Bioengineering

Team Members: Charu Dhavalikar, Greg Feldman, Sourav Kole

Faculty Advisors:Dr. Peter Butler, Khanjan Mehta

Primary Email Address:czd5023@psu.edu

Project Information

List all parts (hardware, software, etc.) you used to design and complete your project:

[List Parts Here]

Describe the challenge your project is trying to solve.

Our goal is to create a weighing scale with a digital output that can be used with the Mashavu kiosk. The scale is designed to be used for medical weighing of adults and small children. These patients do not have easy access to doctors, and also struggle to pay their medical costs. They need a scale that has a digital output that can be transmitted over the Internet, is easily maintainable, inexpensive, and easy to use. Therefore, the scale needed to be made of relatively inexpensive materials, use a sensor that had an electrical or digital output, and that to be user-friendly. The design used was a simple spring/plate design with an IR distance sensor. The output from the IR sensor is a current that is dependant on the intensity of the light that is sensed by the sensor. The current then runs through a circuit, such that we can read the resulting voltage output across a resister, which is then sent to the NI-DAC board, and then is converted to a weight using a Labview program. The prototype has been built, and initial testing shows that the concept is sound. However, the Labview has been difficult to program, and should be completed this week. The testing will be performed with known weights, as well as with several people in the class, and a linear regression test will be performed to verify our results.

The customers of Mashavu are mostly situated in Nyeri, Kenya and include the caregivers who run the kiosk (such as caregivers of orphanages, youth centers, etc.) and the actual people who will have their vitals taken with these devices. The caregivers are relying on the engineers to come up with a simple, easy to use scale that will not take much training to learn how to use the scale. The people using the scale to obtain their weight include children, men, and women of different ages and sizes. The only individuals who will not be able to use the scale are infants, who will most likely get their weight from a separate baby weighing scale device.

Other customers include local and international doctors, who rely on the accuracy and precision of the scale to diagnose and check up on their patients. For now, the information will be sent to local doctors, but it is hoped that eventually doctors from around the world who wish to use their services to help patients in other countries will be able to use the Mashavu website and cut down on travelling costs. This benefits both the customer and doctor; instead of having the customers travel five hours to visit a local doctor, now they can travel 30 minutes to the closest Mashavu kiosk and have their vitals taken, which will cut down on both time and cost.

The adult weighing scale is only one medical device among eight other medical devices, which together will make up a kiosk that will provide basic healthcare information services. This kiosk will be marketed as part of Mashavu, which means “chubby cheeks” in Swahili. This saying represents Mashavu’s desire to make sure every individual is healthy. The overall goal of Mashavu is to provide quality healthcare service and an interface between doctors overseas and customers in Kenya. Basic vital signs such as weight will be measured and the results will be sent to either a local or international doctor, who can provide healthcare advice based on these results.

Describe how you addressed the challenge through your project.

Design Specifications

After much consideration of the customer needs, these are the design specifications for the adult weighing scale. Some of the specifications conflict with each other, but every specification was carefully considered in the final design.

• Accuracy
  
  After some research it was decided that the adult weighing scale needed to have an accuracy of 0.5 kg. This was determined to be accurate enough to diagnose diseases such as malaria and malnutrition with weight loss as a possible symptom.
• Range
  
  It was decided that the range of the sensor detecting the weight needed to be at the very least 10 – 100 kg. This covers the range from the average 2 year old girl to the average weight of the man.
• Cost
  
  Since not much is known about the cost of certain materials in Kenya, it is very difficult to determine what the final cost of the design will be for Kenya. For now, the scale was built using materials found in the United States, and the materials were carefully selected for their cost. For now, the budget limit is about $20, although this is fairly expensive and the team is hoping to bring the cost down to $10 by using recycled materials rather than store bought materials.
• Ease of Use/Repair
  
  This includes ease of use for the caregiver and ease of use for the customer. The customer should be able to stand on the scale and have it successfully record a weight. The caregiver should be able to calibrate the scale easily (see below) and be able to fix the scale if something goes wrong. Also, if there are any individuals who are incapable of standing on the scale, the scale should have wide enough dimensions to allow the person to sit on the scale.
• Ease of Calibration
  
  It is inevitable that some calibration will need to be done to provide an accurate weight of the customer. In addition to having the scale be easily repairable for the caregiver, the scale should also have very easy instructions for calibration using a known unit measurement, such as a bag of sugar.
• Durability
  
  The scale should be able to last a long time so that the whole scale or parts have to be replaced. Extensive research of possible materials suggests that durability may come into conflict with cost.
• Interactive and Fun
  
  The customer and caregiver should have fun using the scale so that they will want to continue using the scale some more. Since the adult weighing scale should be able to accommodate sitting or standing, the children can have fun sitting instead of standing on the scale and the scale can paint footprints on the boards to show where the children should place their feet.

Detailed Project Design

The goal of this project was to create a weighing scale that could be used for adults and small children as part of the Mashavu project in Kenya. Therefore, the scale had to be simple, easily buildable and maintainable, and have a digital output.

The design is a simple spring based scale. Two 16” by 16” plywood boards spaced 1.5 inches apart will have dowels attached 2” from each edge. These dowels will serve as guides and holders for four springs. The springs will act as dampers and will compress a certain distance when a force is applied to the top plate. An IR sensor will be placed under the center of the top plate, and a reflective surface will be placed on the bottom of the top plate. The sensor emits an IR beam, and converts the amount of IR light that is sensed into a current. When a patient steps onto the top plate, the springs will compress, and the top plate will move closer to the sensor, thus altering the current that will be output by the sensor. The current will then travel to the DAC, where it will be measured and digitized, and passed on to the Labview program. The program will then convert the current into a weight, using a pre-programmed formula relating the current to a distance, and then the distance to a weight by multiplying by the spring constants. Since the current scale of the sensor depends on the external temperature, an initial, zero-weight calibration must be performed before every measurement. Since the initial distance between the top board and the sensor is known, a single measurement will allow scaling of the original formula, thus giving the appropriate formula for weight at the ambient temperature.

The Chosen Parts:

The Sensor:

The Springs:

In order to convert our signal into a weight, a program called Labview was used. The program takes in the input from the DAQ, or data acquisition, device. This input is then converted to a distance through a power law model. Due to the fact that our sensor is an IR sensor, and thus is affected by heat, an extra heat based initial calibration constant is used. This constant is used to scale the measurements appropriately, and thus account for the ambient temperature. The calibration constant is found by using the inverse of the formula used in the main program. Multiple readings of distance are obtained, and then an average is taken. This average is subtracted from the initial distance to get the compression distance, which is multiplied by the spring constant to get the patient’s weight.

Block Diagram Of Labview Program

View of Front Panel

The data that is collected from the DAQ device is in the form of a voltage. From our circuit and calibration curve, the formula that was calculated relating distance and voltage is:

5-V = C/r^3.2854

Thus, the formula for distance dependant on voltage is:

r = (C/(5-V))^-1/3.2854

This is the formula that is used in the program. The data is collected using the DAQ Assistant, and inserted in to this formula. The constant, C is from the calibration, which is run before every reading. The formula for the calibration constant is the initial formula evaluated at 0 kg.

C = (5-V0)*r^03.2854

The data, after being put through the formula, has been converted to a distance. This distance is then appended onto a matrix, and the process is repeated fifty times, once every twenty milliseconds. The matrix that is generated is then put into a statistics function which then outputs the average of the distance measurements. This average is then subtracted from the initial distance, giving the compression distance. The compression distance is multiplied by the spring constant of our spring system, thus converting our data into a weight. This value is then rounded to the nearest multiple of .5 kg, by multiplying it by 2, rounding to the nearest integer, and then dividing by 2. This is then output to the indicator and meter.

The program is initiated by clicking run. As soon as the program is initiated, the calibration VI is called. This is then passed into a while loop containing the weighing formula, as well as the reset statement. The weigh button initializes the measurement array, and starts the measurement loop, detailed above. After the loop has run, the calibration is called again, while the indicators still show the weight. This can be reset by clicking the reset button, which will make the indicators show a weight of 0 kg. The entire program can be stopped by pressing the stop button.

Construction

Our goal is to measure the weight of the user. We accomplish this by using two parallel wooden plates (16” x 16” x 5/8”) with springs (25/64” radius, 1-5/8” height) placed between them. The wooden plates each have wooden circular dowels (1/4” radius, 3/8” height) which act to guide the springs. Each dowel is attached to the plate by gluing the dowel into a small circular indentation (1/16” deep, 1/4” radius) made on the plate (four indentations on each plate) with maximum strength wood glue. The springs (and also the dowels) are equally spaced from the center of the plates to maximize stability when the user steps on the plates.

When the user steps on the wooden plates, the distance between the plates will decrease a distance that is directly proportional to the weight of the user. We use an IR sensor to measure the change in distance. The IR sensor is glued to the center of the bottom plate and emits IR light towards the top plate, where it reflects off a mirror attached to the center of the top plate, and then the IR light travels back to the sensor. The sensitivity of the IR light returned to the sensor correlates to the distance traveled by the IR light and ultimately the distance between the plates. The sensor converts the sensitivity of the IR light and creates a current that can be converted into the distance the top plate fell when the user stood on the plate, which can then indicate the weight of the user standing on the plate.

Testing

The primary goal of the experiment is to test for precision, accuracy, and range. In order to accomplish this task, there will be two types of experiments run that will test for all three design specifications:

Human error:The adult weight scale team will test the prototype with 15 individuals on a standard bathroom weight scale to for correct human error. In this case, the tilting and stability aspect of the weight scale will be taken in account in testing. The stress distribution diagram on COMSOL will accurately show the amount of stress our weight scale will be experiencing and using this we will adjust to minimize any stability issues. This will mitigate in finding the optimal position for the footprints (which will guide the user to put his/her feet on) and hence make a better final product.

This experiment will also have individuals both sitting down and standing up on the scale to test if there is a difference between standing up and sitting down on the scale. This is to simulate customers that are disabled and are unable to stand on the scale.

This experiment will also test for the accuracy of the scale itself. The human subjects will both have their weight taken using a normal, standard bathroom scale first and then step on the prototype, or they will ideally know their weight beforehand. The data measured will reflect on how accurate the scale can output a weight versus a standard bathroom scale.

Instrumental error: Various standard weights (10-20 kg) will be taken and will be weighed on our prototype weight scale. The difference in error will be measured and a linear progression analysis will be done on the values. This standard of testing will take into account the technical and instrumental (wood, springs, sensor, etc) errors and help us make the final product more accurate as well as test the range of weights that the scale can accurately measure. This will ideally simulate customers with smaller weights, such as children, since the experiment will not be allowed to test actual children subjects.

For both these types of errors we will calculate the percentage of error and compare it via linear progression analysis on Microsoft Excel. This is the formula that will be used to measure the error from the correct weight:

% Error = |prototype weight – correct weight| x 100
correct weight

When intital calibration was done using the circuit on the breadboard, we tested with weights ranging from 0 to 30 kilograms. First we took the weight on an accurate weight scale and then put the standard weight on our weight scale and analyzed the output in the form of volts, distance and weight (guaging sensitivity). We also took data from five human subjects weighing from 55 to 115 kilograms, to see actual effect of unequal mass distruibution and standing position on the scale.

After putting the circuit of our sensor on the final circuit board, we again calibrated and tested it. Standard weights ranging from 0 to 160 lbs were taken and weighed in increments of 2.5 lbs to get an accurate calibration curve and consequently an accuarte multiplication factor for our LABVIEW program. This was done two times as after the first calibration we saw the wood was warping and this was effecting the distance from the sensor and hence our output weight. The calibration for these 57 data set was done again to see if the warping remained constant. But after testing we found that the warping is not constant (as seen in figure) with weight and varies over time and there is no potential way to take that into account in the calibration.

The following are the data we collected and calibration graphs plotted using Microsoft EXCEL:

Our team came to the conclusion that the way this problem can be solved is by choosing a better wood in terms of being resistant to warping from factors such as force, uneven pressure, temperature, etc. Otherwise, we can use low grade plastic that are used in the weighing scales found in cheap and standard weighing scales which range between $2 to $3 but which we cannot get as they only sell that in bulk.