Accessible Wheelchair Scale

Accessible Wheelchair Scale

By

Eric Bernstein

Matt Veilleux

Julie Rosario

Maria Elescano

Contact:

RehabilitationEngineeringResearchCenter on Accessible Medical Instrumentation

National Student Design Competition

John Enderle

860-486-5521

Executive Summary

An accessible weight scale that can be used either at home by handicapped persons or at a health care facility would be of great use to many patients. The weight scale is accessible to patients who suffer from a variety of disabilities, such as patients with limited movement of the extremities, general frailty, paraplegics, wheelchair users, or those in need of a cane or a walker.Patients suffering from renal failure, heart failure, diabetes, multiple sclerosis, or others who need to monitor their weight regularly can also benefit from this device.

The weight scale will have a ramp to make it wheelchair accessible and will have removable side support bars for those patients in need of additional support to stand. In order to avoid rolling accidents, the weight scale will have stops maintaining the wheelchair and other walking instruments in place. To accommodate patients with poor eyesight the output will be easy to read, and available in several formats.

Statement of need

Many clients suffer from paralysis, renal failure, heart failure, multiple sclerosis, stroke, diabetes, and heart attack, and need to monitor their weight regularly. The following is a description of these conditions and why weight monitoring is important:

• Renal failure is a loss of the ability of the kidneys to excrete wastes, concentrate urine, and conserve electrolytes. Those suffering from kidney failure need to measure their weight often because their body may be retaining excess fluid.
• Heart failure is a condition where the heart cannot pump enough blood throughout the body. The weakening of the heart’s pumping ability causes buildup of fluid in the feet, ankles, and legs. Weighing oneself is necessary because a sudden weight gain could mean extra fluid building up in the body.
• Multiple sclerosis (MS) is a chronic, energy-taxing, debilitating disease that affects the brain and spinal cord. Those with this illness weigh themselves often since weight gain is common in people who are less active, since fewer calories are burned.
• A stroke is a complication that affects the blood vessels that supply blood to the brain. Being overweight increases the chance of developing high blood pressure, heart disease, atherosclerosis and diabetes — all of which increase stroke risk.
• Diabetes is an illness where either the body does not produce enough insulin or the cells ignore the insulin. Insulin is necessary for the body to be able to use sugar. Being overweight or obese is a leading risk factor for developing type 2 diabetes.
• A heart attack occurs when the blood supply to part of the heart muscle itself is severely reduced or stopped. Maintaining appropriate body weight improves cardiovascular health.

Market Research:

Market research shows the price range for similar weight scale designs to be between $1,500 and $3,000.[1] Many of the scales found were offered from Detecto Medical Supplies. Some of the common features are AC or battery power, a LCD, and portability. To compete with the other products on the market, our scale should accommodate as much of these features as possible.

Brand / Model / Cost / Features / Max Weight / Platform Size
Detecto / 6550 / 2860.99 / LCD display, Keypad Tare, Battery Powered / 800lb / 28”Wx28”L
Detecto / 25495 / 1254.93 / Mechanical, dual sided balance, removable ramp / 350lb / 30”Wx 26”L
Tanita / PWC 620 / Not listed / Battery or AC power source, foldable, wheels for easy mobility, tare, adjustable leveling feet / 440lb / 40”Wx36.25”Dx4”H
Health O Meter / 2450KL / 1495 / Rail, motion sensing weighing technology, foldable, / 600lb / Small
Detecto / 475 / 865.49 / Lift-away arms and footrest, portability, transport. / 350lb / Chair Scale: N/A

Similar Projects:

Two similar products were found in the NSF Engineering Senior Design Projects to Aid Persons with Disabilities. The first is “A Scale for Weighing a Client While in the Wheelchair” (1999). This design is that of a portable scale, with a cost of only $300. It has two LED displays that come from two bathroom scales that are mechanically attached to the platform. For this scale, assistance is needed because a calculator is used to compute the users weight, and the weight of the wheelchair must the known.

The second design is “Visual Output Weight Scale for Wheelchair Users” (2004). This device uses load cells, which allows for more accurate measurements. It too, is a portable weight scale, with an estimated cost of $1,155. It allows the user to calibrate the weight of the wheelchair, or manually enter its weight. No assistance is needed to use this scale.

Patent Opportunities:

There is great opportunity for obtaining a patent for our design, since only one patent for a similar design was found after searching the United States Patent and Trademark Office. The patent for this device, Wheelchair Portable Scale Apparatus, was filed on March 19, 2003, and the patent number is D489,279.

Project Description:

Objective:

To calculate a person’s weight, the scale must have several critical components. First, a ramp allows for easy access to weighting platform for a wheelchair. Next, the platform rests on either one or several devices designed to convert mechanical force into an electrical signal, such as load cells. The electronic signal from the measuring device is then interpreted by a microcontroller along with decoding the desired user input from a keypad. Finally, the weight value is output to either a computer or an LCD for interpretation by the user.

This basic structure for a weight scale is used in the majority of the scales available on the market today. During the design phase we hope to investigate alternative methods of measurement that may decrease the cost of the final project.

Methods:

Mechanical:

Figure 1: Basic load cell positioning and scale design

To measure the weight of the person and wheelchair load cells will be distributed in either a triangular or square formation at the base of the scale. The base will be either a flat square plate with a ramp or two ramped rails (this will conserve materials). Three to four cells will be required because the load will not always be in the center of the plate. In order to determine the weight only two cells are required but three would provide better accuracy and more equally distributed support. The type of load cell is yet to be determined, however, the thin film strain gauge and button load cell look promising. The button cell turns force directly into linear voltage while the strain gauge measures deformation under loading and converts the magnitude into signal. Button cells will save on space and measure more weight but are a good deal more expensive than strain gauges.

Electrical Systems:

Figure 2: Preliminary electric circuit design

One of the most important aspects of designing the electrical systems of the weight scale will be power consumption issues. Selection of appropriate devices can minimize energy usage and increase the operational battery life of the project. To accomplish this task, the PIC16F737 was selected as the core processor for the system. This processor has the minimum amount of pins required to be programmed by the MPLAB ICD 2 programming device. The processor has over ten 10-bit A/D converters onboard to accommodate however many analog inputs are required. The integrated USART module will allow the processor to communicate with a personal computer through the serial port and the MSSP module allows for expansion of additional peripheral devices if necessary. Finally, the remainder of the I/O pins are available to decode a keyboard interface or communicate with an LCD display board. The 16F737 is also equipped with a variety of integrated power management solutions. An internal oscillator block allows for a selectable system clock. Therefore, when the device is in use, a faster clock will facilitate faster response times. However, when not in use the clock speed can be significantly decreased to allow for minimal power consumption. Also, the processor is design with Microchip’s nanoWatt technology, which is intended to greatly decrease current draw.

In order to interface the microcontroller with a computer’s RS-232 port, an extra chip is required to pump the voltage signals up to the necessary levels. The MAX220 chip produced by Maxim Integrated Circuits is ideal for this application because it is intended for ultra-low power consumption. At an operating voltage of 5V, the supply current swings from .5 microamps at no load to 12 microamps at a 3 kilohm load at both inputs. The MAX220 is also pin-for-pin compatible with the MAX 232 chip which is a standard chip used in serial port communications between a microcontroller and a computer.

While the LCD and keypad have not been selected yet, low power consumption is still a major consideration. The keypad tactile switches must be large enough to accommodate any normal size finger as well as have a large enough impedance value to adequately limit the current. The LCD will most likely be one of the most power draining electronic components. Selection of either a character or graphic module LCD display has yet to be determined since the final output features have not been finalized. The characters must be large enough for people with poor eyesight, as well as have a backlight to increase contrast in low light environments.

Software design:

Figure 3: Program flow graph

The program for the microcontroller will simply consist of an infinite loop that switches through various subroutines. First, the A/D converters digitize the signals from the four load cells and convert the signal into a weight. Second, a keypad algorithm decodes the user input from the keypad and translates that input into the proper action. Finally, the desired output in pounds or kilograms is sent to the LCD display and the serial port if those functions are enabled. After a given time interval of inactivity, the entire system should switch to a low power standby mode in which the contents of the system memory are saved, but power consumption is minimized. An interrupt will most likely be tied to one of the A/D converter ports connected to a load cell. When the value of the conversion switches beyond some threshold point the system will exit standby and resume normal operation.

Professional Component:

In the process of designing and constructing the weight scale a myriad of engineering standards and realistic constraints must be considered. The scale should be designed with a factor of safety in mind. This means that the side bars and weighing plate must be able to support well above the published maximum measurable weight to prevent injuries. The display needs to be visible from several feet away and accommodating to those with slight visual impairments. Pounds and kilograms should be the standard options for measurement. Electrical equipment that drives the display and load cells will require a 5 volt power source and the chips will be prefabricated. Designing a microprocessor from scratch would be too costly and time consuming. In addition to these general standards, the weight scale will adhere to the following specifications.

Mechanical:

Maximum measurable weight500 lbs

Weight resolution.5 lbs

Weighing time30 seconds

Dimensions

Lengthless than 36 “

Widthless than 36 ”

Heightless than 48” (with support bars)

Less than 5” (platform only)

Scale weightless than 100 lbs

Environmental:

Operating temp 40 to 100 F

Storage temp0 to 120 F

Display:

LCD

Character size at least 1”

Number of Characters16

Input:

Tare button

Button size.5” x .5 “ minimum

Power:

Both AC and DC

DC5-12 Volts

AC 120 V 60 Hz

The constraints fall into several categories: economic, environmental, sustainability, manufacturability, ethical, health and safety, social, and political. There are several scales on the market already, so economic concerns will play a large role in the design. The scale must stand out either because it has superior parts and design, or because it is less costly than the current production models. To accomplish this, the power consumption should low to reduce operating costs and the product itself should cost no more than $1500. To minimize the harmful environmental effects that are inherent in any manufacturing process, the amount of silicon and batteries used should be small. Silicon waste is quickly becoming a concern as computers are replaced every few years and typically are not recycled. Battery disposal has long been detrimental to the environment because the materials are toxic, so rechargeable batteries may be a better option. Power consumption also becomes an issue if the scale does not turn off automatically, and therefore the electronic components should be selected for minimum supply current. Scale lifetime is not crucial but the parts should be as durable as possible. With this in mind, the weight and cost of the materials have to be taken into consideration. Weight should be sacrificed to cost up until the point where the scale becomes unmovable by a single, able-bodied individual. The most suitable materials to fabricate the scale are metals and plastics. Selection of recyclable materials limits the impact the scale will have on the environment at the end of its operational lifetime. The timeline for this project is relatively short and manufacturing the designed parts will no doubt take a good portion of the time. The parts therefore must be designed with a consideration for fabrication time. Handicapped individuals necessitate a design in which there are two safe holds: a stop for the wheelchair and sturdy hand rails. Social and political factors do not pose any foreseeable design constraints or issues and this time.

Budget (rough estimate):

Load Cells or alternative measuring device: $800

Materials (metals, plastic etc): $500

Electronics (display, microprocessor, batteries etc): $200

Timeline:

Conclusion

This weight scale is intended to improve on other similar scales currently on the market. Our design can be easily operated without any assistance required. The device will have a ramp for wheelchair access, and removable support bars to stand. The display will be clear, easy to read, and available in different formats (kg, lb, etc.). Our goal is to accomplish these features and remain within a cost of about $1,500; comparable scales presently on sale, cost up to $3,000.

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