Smart Wheelchair Scale

Design #1: Single Load Cell (SLC)

By:

Roee Ramot

Ahmad Paintdakhi

Beth Showers

March 29, 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 single load cell will be used to determine the weight resting on the platform, since this component is particularly expensive, although a design featuring four load cells at the corners would be more stable and accurate. The corners of the platform will be supported by springs or another elastic element such that the platform must be depressed by some threshold weight before contacting the top of the cell under it. 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 springs, load cell, and ramp will rest on or be supported by a crosspiece-assembly under the platform. The signal from the load cell will travel through a cable to a processing and 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 signal from load cell

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.

Figure 1: Side view of scale unit

Figure 2: Another side view of scale unit - entrance ramps visible

Equipment

Load cells are the primary measurement instruments in weighing devices. Load cells consist of multiple strain gages (see figure 3), which measure resistance when it is put under a compression or tension force. Strain gages are made up of thin metallic foils called a “Gage Patch.” The multiple gages patches are connected tighter and attached inside of the load cell. When the surface of the cell experiences a force the metallic wires experience the same force. The multiple strain gages that make up a load cell are wired together to form the legs of a Wheatstone bridge (see figure 3). The Wheatstone bridge creates an electric current that results from the response of a load that is applied to the cell in the form of an analog signal. A microprocessor is then used to convert the analog signal to a digital signal, so the output from the load cell can be determined and read on an electronic LCD display.

Figure 4: Beam load cell, model LCEC – 1k

“Model LCEC load cells are designed to operate in all weather or washdown environments. Their low profile and high side load capability simplify mechanical installation considerations. Their weather sealing, high precision and repeatability make them ideally suited for rugged industrial applications such as testing, batching, weigh pits and other applications exposed to the elements…”

SPECIFICATIONS

Excitation: 10 Vdc (15 V max)

Output: 3 mV/V nominal

Calibration: NIST Traceable

Linearity: ±0.03% FS

Hysteresis: ±0.02% FS

Repeatability: ±0.01% FS

Creep (after 20 minutes): ±0.03%

Zero Balance: ±1% FS

Operating TempRange:

–55 to 90°C (–65 to 200°F)

Compensated TempRange:

–15 to 65°C (0 to 150°F)

Thermal Effects:

Zero: ±0.0015% Rdg/°F max

Span: ±0.0008% FS/°F max

Safe Overload: ±150% of Capacity

Ultimate Overload: ±300% of Capacity

Input Resistance: 350 +50/–3.5 

Allowable Side Load at Rated Load:

50% Rated Capacity

Output Resistance: 350 ±3.5 

Construction: High Carbon Steel

Electrical: 5 ft. (1.5 m) insulated

4-conductor shielded color coded cable

MetricRanges Available - Consult Engineering *See Section D For Compatible Meters

Ordering Examples: 1) LCEC-250 is a 250 lb capacity load cell, $265.

2) LCEC-1K is a 1000 lb capacity load cell, $285.

(*Information taken from Omega Engineering, 2004)

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. 5).

Figure 5: Flowchart of processing and display unit

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 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.

Figure 7: User interface flowchart

Figure 7 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.

Static Analysis:

A static’s analysis is a valid method of calculating the forces acting on the weighing platform because we can assume there is no significant dynamic behavior. The scale unit is a passive weighing device, and it is assumed the wheelchair user moves onto the platform relatively slowly. Two static’s properties are used here – the sum of moments around a point and the sum of forces acting on the platform, both equaling zero.

Assumptions:

1)Platform of equal length and width greater than 30 in.

2)platform is supported at five points – at four corners by springs, and at center by load cell

3)axial centers of springs make 30 in. * 30 in. square representing platform modeled here

4)load cell is contacted by platform only if load is greater than some threshold, i.e. springs must undergo some displacement before load cell begins supporting

Figure 7: FW = 800 lbs at maximum loading

A)Sum of moments about one corner, F1 :

B)Sum of vertical forces

* Load cell must be able to measure maximum weight of 800 lbs, as per design specifications. In both equations, A) and B), FW = 800 lbs

* Assume when midpoint between wheels is over center of load cell, most accurate reading is obtained.

Since the platform is suspended in part by elastic elements and only supported occasionally by a stable fulcrum (the load cell), the tilting of the platform must be accounted for.

C) Spring Constant:

Assume maximum acceptable platform tilt angle = 5 deg.
Side view of platform: worst case scenario is 800 lb load applied to middle of one edge of platform. At this point, platform must have angle of no more than 5 deg.
Assume that two-spring system with springs supporting one-dimensional lever with load cell in middle as fulcrum approximates actual scenario – from symmetry, load reduces to 400 lbs
Assume that lever with one end as fulcrum, spring on other end; load applied at spring is an approximation of the one-dimensional lever.
Assume Hooke’s law applies to any displacement of spring:

Max angle = 5 deg.

Tan (5 deg) = 0.08749 = (spring displacement) / (Lever length of 30 in.)

= (400 lbs / k) / (Lever length of 30 in)

Solving for k, the spring constant:

k = 150 lbs/in., approximately

This constant provides a good indication of what springs will be suitable for supporting the platform. Any elastic element that is short and wide enough to allow movement only in a vertical direction (with tall springs, torsional and shear forces may cause significant movement to the sides) and will furnish approximately 150 lbs of force per inch of compression or tension from equilibrium is suitable to install at the four corners of the platform and scale base support so that an 800 lb force at the middle of one edge of the platform will not tilt the platform more than five degrees. If the platform is allowed to tilt, the wheelchair may roll to the side or off the scale, or the resulting gap between the platform and ramp could damage the scale or tip the wheelchair.