Functional Characteristics of Final Design

 

Mechanical System Description:

The current prototype is composed of two ¼ inch thick aluminum plates that will eventually have sensors between them to measure a subject’s weight. There are two arm rests, also made of ¼ inch thick aluminum, that use a four bar mechanism attached to the top plate. There is cushioning where the subject’s arm will come in contact with the mechanism. There is also a leg rest, made of ¼ inch thick hollow aluminum bar, attached to the top plate with cushioning where the subject’s feet will be placed.

First Electrical System Description:

The logical process of incorporating the strain gauges into our system is shown below as a step process of the system that we wanted to use to handle our weights.

The electrical system is composed of four strain gage sensors taken from a “Biggest Loser” scale. The sensors are set up as a half Wheatstone bridge with a 1 kΩ resistance across each strain gage and a 2 kΩ resistance across both strain gages. The strain gages are connected to a circuit which contains the other half of the Wheatstone bridge. The strain gauges, when put into a Wheatstone bridge, will change the output voltage across the bridge when a force is applied to them. We needed to take that voltage and change the small signal, on the mV level, into a cleaner larger signal that we could adjust easier. To do this an instrumentation amplifier would be the best choice for amplification and signal conditioning. After the signal would be amplified we would use an A/D converter to change the analog voltage in to a digital signal so that we can use digital logic circuits to then display our final weight. Instead of using our own resistors and potentiometers to create the bridge circuits needed for summing we decided to use the API SUM-000 summing board which sums the voltages from each of our four strain gauges and is easy calibrate with the provided potentiometers on the board.

From this summed signal we then used the AD625JN from Analog Devices. This chip is a Programmable gain Instrumentation Amplifier with a 16 DIP interface. We needed to have an amplifier that we could program our own gain into so that we could change our voltage to a weight. We would find out the slope of our Weight to Voltage curve after testing and use that to calibrate the voltage. This amplifier also has a very good noise protection to condition the signal into a more crisp and precise signal. From here we needed to convert the voltage into a digital signal and send that to a digital display driver. We decided to do this in one step with the ICL7129 from Maxim Electronics, which is a analog to digital converter and a 4-1/2 digit display driver. This micro controller will convert the voltage to digital and then send it through a decoder and a multiplexer to properly drive our LCD display. For our external excitation in the circuit we used two sets of four battery holders. Each battery is 1.5 V and they are all connected in series giving is a total of 6 V for each pack. This voltage will be applied to the summing board, the display and chips as the power source.

Final Electrical System Description:

Some of the integral parts for our first electrical system were held up during shipment and held in customs for a little longer then a week, which put us behind schedule. After finally receiving our parts we spent a large amount of time wiring the components together and incorporating them into our summing system only to find that we could not get it to read properly. With only a couple of weeks left until the project deadline, our team decided to implemented our backup plan. We used the entire electrical system from one of the scales that we had purchased. We had already done testing on all of the sensors so that we knew they would be linear. Instead of creating our own circuit board, we used the circuit board and display that came with the scale which consisted of an external 3.5 V lithium battery connected to the board. The board also has an operational amplifier along with numerous resistors and capacitors which make up the bridges and regulators for the voltages in the system. These are sent to a central microcontroller which drives the LCD display attached to the board. We then implemented this electrical system into our mechanical system.

 

Custmer Needs Addressed by Final Design:

The following are the most important customer needs (in bold), according to interviewees and the Hypothetical Client List, along with how they were addressed by our final design:

Scale is able to be used while sitting with grab bars for balancing

Scale is safely secured with grab bars and hand rails

Scale has adjustable hand rails for loading and unloading users

• The final prototype incorporates grab bars that operate as a four-link mechanism so that they can be easily adjusted while sitting on the seat.

Toilet seat is raised for easier access on and off of scale

• The entire sensing mechanism raises the toilet seat ___ in. from its original position.

Scale is safe with no potential hazards existing

• Foam was placed over the aluminum in places where the subject will come in contact with the mechanism.

The scale is sanitary

The scale is easy to clean

• The aluminum on the scale allows for easy cleaning. However, the electrical components are not encased in a water-tight unit. If the prototype was mass produced, this issue would need to be resolved. This is not major for our prototype since it is not connected to any plumbing lines and will not actually be used as a toilet.

Scale is accessible to all types of patients

Scale is easy to maneuver on and off of

• The arm rests can be adjusted individually to allow for height adjustability as well as easier transfer on and off the scale. The leg rest is close to the scale so that it will not hinder transfer on and off the scale.

Scale is affordable to all users

• The scale cost $600.00 to produce a working prototype. If it were mass produced and put on the market, it would cost much less. Hopefully, it would be under $200.00 and insurance would cover some of the cost.

Scale has large numbers for easy reading

The scale has a bright display

The scale has a visible, fixed display

The scale displays weight in 3 to 4 seconds

• The display is a 2” LCD display with extra large number readout. The weight reading is displayed a few seconds after weight is placed on the scale.

The scale is accurate

• According to testing done on the scale using known weights, there is no statistically significant difference between the weight readout and the actual weight of the object. The scale is accurate to 0.2 lbs.

Scale is small enough to fit into a normal size bathroom

The scale is accessible in size

The scale can be used as a normal toilet seat

• The scale mechanism is easily attached to a common bathroom toilet in between the toilet rim and toilet seat. A toilet seat of the user’s preference can be placed above the sensing mechanism. The arm and leg rests are compact and should easily fit within the space allotted in a normal bathroom.