Smart Wheelchair Scale

Smart Wheelchair Scale

Design #3: Strain gages

By:

Roee Ramot

Ahmad Paintdakhi

Beth Showers

April 19th, 2004

Introduction:

The device to be designed is a smart weighing scale that wheelchair-bound persons can use to weigh themselves with daily frequency while inside a wheelchair and that will store previous weights and other information. The need for this device is exhibited in patients with COPD (Chronic Obstructive Pulmonary Disorder) and other lung disorders such as emphysema. The individuals suffering from such diseases may have to monitor their weight frequently to avoid medical complications. Since long term monitoring of a patient’s weight may have to occur at home, it should not be assumed that assistance is always available and the device must be designed for a home environment.

The scale must be durable enough to provide long-term accurate weighing of the user and should keep the user as stable during the weighing operation as he or she is on solid ground. No assistance should be required for any level of operation for the device except installation. Taring (the process of weighing the individual by subtracting the wheelchair weight from the weight of wheelchair plus occupant) must be a process the user can execute unassisted. The system should have a simple, easy to use interface that can remember previous weightings of a user and also the weight of the wheelchair for taring. Interface components such as buttons and displays should be large and accessible. The cost of the device should be affordable compared to other marketed models filling the same need, and should create a pleasant weighing experience to help motivate individuals with weight control.

Design:

A set of four force transducer units (FTUs) will be used to determine the weight resting on the platform. The units will be positioned near the four corners of the platform, supporting the platform statically and inhibiting movement. Since all downward force exerted on the platform must be sensed by the FTUs, they must be the only support for the platform; however, there cannot be a significant horizontal component to the forces exerted on the cells. Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees will be level with the platform, which will have height no greater than 2 inches. The FTUs and ramp will rest on or be supported by a lower platform or crosspiece assembly under the platform. The signal from the FTUs will travel through wiring to processing circuitry and a computing / display unit.

The processing and display unit will have a memory capable of storing 11 numerical values with a precision of four decimal places (10 historical weights, 1 wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen sufficiently large enough for low-vision accessibility. Large buttons and audible cues through a speaker will also make the device more accessible to handicapped individuals.

The processor will hold a written software program containing:

• an interface through which users can weigh themselves, store weight, key in wheelchair weight and cycle through previous weightings
• an algorithm to calculate actual weight based on stored or inputted wheelchair weight and signals from force transducers

This processor and other components mentioned will be in a circuit powered by an AC adapter or standard batteries and will turn on and off by a switch or by a threshold load placed on the platform.

Four Strain Gage (FSG)

MS Paintbrush: top view minus supporting platform and ramps:

Static Analysis:

At each corner of the weighing platform, a beam of steel with assumed uniform properties undergoes 3-point bending. The bending moment of the bottom of the beam (tension) directly under the loading sleeve is proportional to the transverse load on the beam, and the bending strain produced on the outer surface of the beam is in turn proportional to this moment. A strain gage is attached to this area of the bending beam and records the strain produced. Four strain readings are each read using a quarter-bridge Wheatstone bridge circuit, filtered, converted into load readings, and summed to calculate the total downward force on the platform.

Bending bar material in Force Transducer Unit (FTU): A36 hot-rolled (structural) steel

Given (to 3 significant digits):

Length of each side of weighing platform: 0.889 m (35.0 in.)

Total length of bending bar in FTU:0.180 m (7.09 in.)

Length of bar experiencing free bending moment: 0.150 m (5.91 in.)

Width of bar: 0.02 m (0.787 in.)

Thickness of bar: 0.003 m (0.118 in)

Maximum measuring load on center of bar:200 lbs (889 N)

Maximum load on center of bar:400 lbs (1780 N)

Yield strength of A36 hot-rolled structural steel:150 Mapcompressions

250 Map tension

Elastic modulus of A36 hot-rolled structural steel:200 Gpa

Calculations:

1) Calculate forces F1, F2, F3, and F4 for wheelchair entrance to ramp (loading on edge of platform, see Diagram 3 in notes):

Edge-loading, 800 lbs - each wheel assumed to transfer 400 lbs:

Fw1 = Fw2 = 400 lbs

Center-edge loading assumed: A = (0.889 m – C) / 2, B = (0.889m + C) / 2

Assume wheelchair wheel width of 0.7 m, A = .095 m, B = 0 m

Symmetry: F1 = F2, F3 = F4

Sum of moments about axis through F1, F2 = (400 + 400lbs)*(0.0635m) + (F3 + F4)*(0.762m) = 0

F3 = F4 = -67 lbs, downward force (tension)

Sum of vertical forces = -400 - 400 – 67 – 67 + F1 + F2 = 0

F1 = F2 = 467 lbs, upward force

2) Calculate forces F1, F2, F3, and F4 for wheelchair in center of ramp:

Fw1 and Fw2 are centered: A = 0.095, B = 0.381 m

Four-way symmetry: F1, F2, F3, F4 all equal

Sum of vertical forces = -400 – 400 + F1 + F2 + F3 + F4 = 0

F1 = F2 = F3 = F4 = 200 lbs

3) Calculate maximum shearing stress in bending beam:

Factor of safety (Nfs) = 1.3:467 * 1.3 = 600 lbs

Reactions at either end of beam: R1 = R2 = 600 / 2 = 300 lbs

Max. shear in beam, Vmax = 300 lbs / cross sectional area

Convert to Newton’s: 300 lbs * 0.4536 kg/lb * 9.8 m/s2 = 1334 N

Shear: 1334 N / (0.02*0.003m) = 22.2 Map

4) Calculate maximum bending strain on underside of beam:

Factor of safety (Nfs) of 1.3:467 * 1.3 = 600 lbs

Convert to Newton’s: 600 lbs * 0.4536 kg/lb * 9.8 m/s2 = 2670 N

Here, b = width of beam, h = thickness, c = h / 2, L is length of beam, F is the vertical load in the center of the beam, E is the elastic modulus, and M is the bending moment in the center of the beam. The bending moment equation is a generalized equation assuming ideal beam behavior, and yields the maximum bending moment in the beam, happening in the center. The maximum strain as produced under a 600 lb load is given as:

where L is the bending length of the beam, a is the width of the beam, and w is the thickness.

For a load of 600 lbs (2670 N), a free-bending length of 0.150 m, a width of 0.020 m, and a thickness of 0.003 m (see given specifications above),

Maximum bending strain = (2670 N) (0.150) / (16*200e9*0.02*0.003^2)

= 6.953e-4 = 695 micro strain. = (delta L / L)

Analysis:

1. None of the stresses calculated exceeded the minimum yield strength for A36 structural steel, 150 MPa in compression. It is assumed that any stress exceeding this value will permanently deform the FTU, leading to failure or inaccurate readings.
2. Strain gages typically measure 1-1000 or 1-1500 micro strain. The amount of micro strain calculated for maximum loading conditions with a safety factor of 1.3 was 695 micro strain. Since the maximum loading for a centered wheelchair would be one-third this value, the maximum strain to be measured is 232 micro strain. This is very much within the operating conditions of a strain gage.

Tentative Parts List:

Part / Qty
Fasteners, 1/4" bolts with nuts / 36
beams / 4
support squares / 8
sleeves / 4
weigh platform / 1
support platform / 1
ramps / 2
LCD display / 1
Strain gages / 5
A/D converter / 4
Filters / 4

Wheatstone bridge Circuit:

The strain gages are measured using the Wheatstone bridge circuit analysis, where four resistors are placed of equal value, if the fourth resistor which is the strain gage itself doesn’t experience any changes then the circuit will generate no voltage, on the other hand if the circuit experience a slight resistance change in the strain gage resistor then the circuit will generate a voltage in milivolts. Since we are using five strain gages all their outputs are being added together and then send to the A/D converter of the microprocessor. The fifth strain gage is placed in the circuit so the issue of temperature affecting the circuit is dealt with, the fifth strain gage is placed on the piece of metal similar to the one used for the other four strain gages, and one the temperature changes the resistance of the strain gage then we know how much calibration value we should add to the main circuit. The would be circuit of the strain gage is displayed below using only one strain gage;

The above circuit shows only one part of the Wheatstone bridge analysis, the way it is designed, resistors R1, R2, R3, and strain gage resistor are all equal in value, which means no voltage generated to the output but if there is a slight resistance change in strain gage resistor due to strain then the output terminal gets a voltage value.

Circuit Analysis:

The above circuit is a generic view of a real circuit that is going to be mounted on our design. The design might have changes in terms of resistors values and the type of MosFets we are going to use in our optimal design. Also the real design might have a less inputs in order to make the design more accurate and affordable. The basic approach to the circuit is very simple, the four main outputs from the load cells are going through filters so the voltage values entering the microprocessor are without any noise and interference, also these voltage values go through some resistance in order to make them suitable for the requirements of the microprocessor, different microprocessors have varying input voltage acceptance values. The microprocessor also gets input values from the User Interface, all these inputs of the microprocessor go through calculations and memory storage sequences and then the result is displayed on the LCD Display.

Block Diagram of System:

The above block diagram is the electrical part of the design; the power supply is shown supplying a voltage to the load cell (force transducer) and at the same time, the load cell’s resistance changes in proportion to the load applied axially. A Wheatstone bridge circuit is used to detect the extremely small changes in resistance, and this constitutes the output signal from the cell. Once the load cell signal reaches the linear or steady state, then the analog signal is sent to the Signal filter where it is transformed from a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for possibly voltage or current adjustments before the signal goes to the Microprocessor. The Microprocessor gets two inputs one from the user and the other from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all the memory storing and sorting, then the final weight is displayed onto the LCD display. The scale turns on as the user moves onto the platform with some minimum weight or when the power switch is toggled.

Flow chart of the Microprocessor:

The computation and user interface for the weighing system is accomplished by a microprocessor with user input switches and an LCD monitor, as well as possibly a piezoelectric speaker. The following is a microprocessor flow chart, which shows the different functions performed by the user and the microprocessor while computing the weight of the person (see Fig. 4).

Figure 4: Flowchart of processing and display unit

User interface flowchart:

Figure 5: User interface flowchart

Figure 5 above shows the user interface flowchart enabling the user to navigate through four options – turn the scale on/off, weigh, check weight history, and change wheelchair weight. There are few menu levels, making the interface easy to use for most people. The memory is accessed by the processor whenever the user selects the option to actually store their weight, and the oldest entry for weight is deleted and all entries are moved back to make room for the new one. The user must know their wheelchair weight and type it on only once as it is stored in the memory. The machine memory must be able to handle repeated and constant use, and must not fail in a power outage. A battery can be provided to ensure that stored data is not lost.

Conclusion:

This design offers many beneficial features that make this product highly accurate. This design includes a higher cost aspect but involves a more accurate approach to weight measurements with the addition of the strain gage analysis. The combination of five strain gages provides an in-depth average of the individual’s weight, because it will not matter where exactly the individual is placed on the scale providing easier access the weight of the individual will be calculated very accurately. This design also does not include springs to act as supports, instead the five strain gages will act as supports for the platform with their placement in different part of the platform.
The display unit will be large enough for an individual with moderately low vision to see and operate. The operation of the scale and display unit will also be user friendly and allow enough memory to store collected weightings for up to 10 times. The circuit analysis in this design is more accurate and affordable to build. The usage of fewer resistors and capacitors is taken into consideration.