Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 7

Project Number: 06205

Design and Manufacture of a Medical Standing Table

Matthew Bell/Mechanical Engineering / Craig Hudson/Mechanical Engineering
Jeffrey Matusik/Industrial Engineering / Kahamala Morgan/Mechanical Engineering
Maria Spagnola/Mechanical Engineering / Aditya Srinivas/Electrical Engineering

Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 7

Abstract

The following provides a concise design overview of a medical standing table, a device used to lift disabled individuals out of a wheelchair and support them in an upright standing position. Funded by a National Science Foundation grant, the table was tailored for use in a residential group home for disabled people run by the ARC of Monroe County in New York State.

Background ergonomics research conducted by the team as well as the final design solution employed for safe and effective therapeutic uses is discussed. An emphasis has been placed on the structural analysis discussion.

introduction

Standing, in general, has a great deal of health benefits proven over the years by a number of physicians and therapists. Standing facilitates the improvement of balance, upper body strength, improves the range of motion of the spine, hips, knees, and ankles, and aids in good blood circulation throughout the body. Standing is also able to reduce pressure sore issues through changing positions, improves systemic functions (digestive, bladder, respiratory, etc.), prevents bone density loss, alleviates pain caused by unnatural sitting positions, and develops tolerance and endurance in the body. Lastly, for wheelchair bound individuals, standing is essential to boosting self esteem.

The ARC of Monroe County is a non-profit organization dedicated to providing services to mentally or physically disabled individuals. Currently, the ARC does not posses the necessary funds to procure a commercially available standing table, hence the initiation of a team to design a custom table, cost free, to the ARC.

Objective Statement

Design and build for less than $1,500, in a period of twenty weeks, a fully functioning medical standing table capable of lifting up to a 275 pound individual. The table should comfortably support the individual in a standing position while providing a stable work surface for tabletop activities.

Nomenclature

α: Knee angle relative to ground

θ: Strap angle relative to leg

FBD: Free body diagram

FE(A): Finite element (analysis)

HDPE: High density polyethylene

L: Total leg length

L1: Length to middle of leg

UHMWPE:Ultra high molecular weight polyethylene

Wb: Lumped upper body weight

Wl: Lumped upper leg weight

SPDT: Single pull double throw

SPST: Single pull single throw

Terminology

8020: Structural T-slot aluminum extrusions manufactured by 8020 Inc.

Actuator: DC motor actuator used to generate lifting force

Arms: Portion of the table upon which the harness is attached (Also: Lift Arms)

Arm Nub: A perpendicular piece attached to the drive side lift arm to relieve toggle points

Base: 2.5” Square steel tubing forming the center of the unit, houses the linear bearings

Drive Cylinder: 2 inch bore air cylinder that receives user input to the fluid drive. Consists of both “expand” and “retract” cylinders

Drive Side: The arm and corresponding frame side to which the actuator is attached

Fluid Drive: Term given to water filled pneumatic cylinders used to extend/retract legs (Also: Cylinders)

Foot Plate: Mat covered plate supporting patient’s feet

Frame: Combination of the base, supports, and lifting mechanism brackets

Harness: Strapped textile device that contacts the patient and attaches to the table

Lift Time: Time required to put on the harness and lift patient to a full standing position.

Lift Phase: The actual time when the patient is being lifted and is neither fully seated or standing.

Main Supports: Steel tubing welded to the base that supports the actuator, lift arms, and the knee pad.

Secondary Supports: Steel tubing that nest within the main supports.

Stall Point: The load which forces the actuator clutch to disengage

Standing Table: General name for the entire unit. (Also: Table, Lift)

T-Slot Nut: A specialized nut designed to fit inside the 8020 T grooves for quick fastening

Torsion Bar: A hollow shaft connecting the drive side lift arm to the non-drive side arm

Final Design specifications

Max User Weight: 275 lbs (300lb stall point)

Table Height (max/min): 52”/32”

Leg Width (max/min): 54”/34”

Lift Time: ~30 seconds

Unit Weight: ~80 pounds

Unit Cost: <$1400

Initial Design Process

While the complete set and corresponding descriptions, of the ARC’s needs is quite extensive, a simplified chart shown in figure 1 has been provided which groups the needs into one of four categories, including ergonomics, safety, adjustability, and usability/capacity.

Figure 1: Condensed Customer Needs

Translating these needs into design constraints resulted in a set of 11 generalized parameters: Power, force generation, foldability, frame material, frame geometry, ergonomics, adaptability, safety, user interface, mobility, and table top (type). A morphological chart consisting of these parameters outlined available solutions regardless of practicality, cost, or ease of implementation. Through a set of weighted and unweighted Pugh analyses, the chart of more than 42 different options was solidified into the final design choices shown below in figure 2. Final design parameters where chosen to accommodate the ARC’s needs, time and cost constraints, team skill and ability, as well as RIT’s available manufacturing facilities.

Figure 2: Final Design Features

A CAD assembly model of the final proposed design, with a modeled individual using the table, is shown below in figure 3. A few design changes, including the removal of one of the original two actuators and the addition of the torsion bar are not shown. These were post-initial design phase changes made that were not reflected in an updated assembly.

Figure 3: Completed CAD Design

Ergonomic Considerations

Solid patient support, along with comfort, was the main ergonomic project goals. To create a nearly universal design, critical frame dimensions were selected to accommodate 5th percentile female and a 95th percentile male body sizes. Table 1 lists the maximum and minimum dimensions determined from the link-length approximation developed by Drillis and Contini. The link length approximation theorizes that the length of human segments (legs, feet, lower and upper arms, head, etc) are a ratio to the total body height, with the ratios being consistent among the human population. From these determined dimensions, pad height, pad size, table height, and lift arm lengths could rather easily be determined. Using the above named percentiles, 95% of all potential users will be able to use the standing table within the as designed dimensions.

Relative Dimensions of Avg Human Body (in inches)
Ratio / 5th female / 95th male / Avg / Std Dev
Ground to knee / 0.285 / 17.04 / 20.98 / 19.01 / 1.97
Ground to hips / 0.53 / 31.69 / 39.01 / 35.35 / 3.66
Ground to elbow / 0.63 / 37.67 / 46.37 / 42.02 / 4.35
Ground to chest / 0.72 / 43.06 / 52.99 / 48.02 / 4.97
Ground to shoulder / 0.818 / 48.92 / 60.20 / 54.56 / 5.64
Width of hips / 0.191 / 11.42 / 14.06 / 12.74 / 1.32
Chest to grip reach / 0.293 / 17.5 / 21.9 / 19.70 / 2.20

Table 1: Link Length Approximations

Ergonomics design

The first ergonomic design concern taken into consideration was support of a patient, weighing up to 275 pounds, during the actual lift phase. While there are a few different methods currently employed to lift patients, including overhead slings and collapsible chairs (similar to “stand easy” recliners), a simple lift arm and harness method was chosen. As the name implies, a custom sewn harness wraps behind the user’s midsection and attaches to a set of lift arms via steel rings hooked over a set of retention pins to provide the necessary moment about the knees. A knee pad is located in front of the user’s knees, and extends part way down their shins, to prevent the patient from merely being pulled out of their chair, but rather, create a natural pivot point about their knees. The selected foam has an indentation load deflection (ILD) of 80 pounds. Figure 4 details the design sketch for pad placement.

Along with support during the lift phase, long term comfort was an extremely important feature that was given a good deal of attention. Starting from the bottom and working, up, the standing table utilizes a self supported footplate covered in a high density antifatigue mat. Similar to mats found in businesses where workers are often required to stand for a complete 8 hour shift, such as cashiers at a grocery store, the mat helps to evenly spread loads across the patient’s feet, absorb external vibrations, and decreases soreness of the feet, ankles, and calves. The mat, along with a back stop, also prevent slipping during the lift phase or while the table is being moved.

Figure 4: Pad Placement Sketch

A six inch thick knee pad, briefly mentioned before, reduces knee loading during the lift phase, and helps to stabilize and maintain patient comfort during therapy. Rather than developing an adjustable knee pad, it was determined possible to size the knee pad such that, without modification, the height from the footplate and the width would accommodate 95% of all users.

The harness is a large high strength nylon web that wraps beneath, around, and above the buttocks to provide maximum surface contact for load distribution. Metal rings have been sewn into the far left and right of the harness to facilitate quick and easy attachment to the lift arms. A strap adjuster located between the metal rings and the bulk of the harness allows for fine tuning of the strap lengths for maximum safety and comfort. Lining the webbed harness is a layer of synthetic batting also intended to reduce pressure points and distribute loads. Finally, the entire harness was covered in a soft layer of cloth sewn together with Kevlar thread to increase the strength at the sewn seams. An eighth-inch thick HDPE plastic piece provides a firm, yet flexible, lumbar support.

The final piece of ergonomic equipment is a chest pad located on the front of the table top surface. Consisting of a 6 inch wide foam covered section of poplar wood with the top edge beveled at 45 degrees, the pad stabilizes the upper portion of the body while allowing free access to the table top. The bevel allows for a flat contact surface for the patients chest even while the table top is tilted.

The work surface itself is mounted on top of the secondary supports and is height adjustable. A combination of a quick release pin and a spring loaded pin allow for quick and easy height adjustments. In the event that the quick release pin is removed and the table is left unsupported, the spring loaded pin will prevent the table from falling more than 2 inches, the step increment for height adjustments The height range itself was determined from the elbow height of a 5th percentile female and a 95th percentile male. When the table height is adjusted, the attached chest pad is automatically placed in the proper location without a need for additional adjustments.

Structural Design

Withholding the aforementioned ergonomic features, the standing table can be broken into the following subsystems: Table & supports, lifting mechanism, legs, and the base and fluid drive system. Design of the subsystems centered, first, about the ergonomic needs and, secondly, the lifting mechanism. These two subsystems formed the dimension constraints and loading criteria for the entire system. To minimize the variances in loading conditions, a weak link was designed into the lifting mechanism as a passive patient weight monitor. Following the desired maximum user weight of 275 pounds, a 300 pound stall point was designed into the system, meaning that if a person weighing 300 pounds attempted to use the table, a force just over 1000 pounds would be required by the actuator. At this point, the actuator clutch will slip, preventing any extension of the rod. The entire table was then designed for 300 pounds plus a factor of safety of at least 1.5.

Lifting Mechanism

Using the average of the 5th percentile female and 95th percentile male link length models, a model for the harness strap tension in terms of the knee angle alpha and strap angle theta was formulated. Figure 5 details the free body diagram as well as the simplified tension equation.

Figure 5: Strap Tension FBD and Equation

Given the tension equation, the following parameters were chosen or solved for:

· Actuator stroke and force

· Actuator mounting point (frame and arm)

· Lift Arm dimensions (overall and nub)

· Harness strap length

· Harness mounting point

Budgetary constraints limited the actuator force to 1000 pounds, available in a variety of stroke lengths up to 12 inches. The chosen actuator, manufactured by Danaher Motion Controls, has a 1000 pound limit, 12 inches of travel, a 15 percent duty cycle, a ball screw for quiet operation, and a clutch to prevent overloading. A simple 2D model of the frame and a human leg determined the overall arm length and harness strap lengths required. A horizontal arm orientation was chosen for simplicity and practicality. The frame mounting point was chosen to place the actuator in a near vertical initial position to maximize available torque in the beginning of the lift phase. A second equation relating arm, strap, and actuator positions was generated and, in conjunction with the tension equation, was solved to give the dimensions of the lift arm and arm nub. To simplify some of the involved math and reduce time between different iterations, a 2D cad model was developed to help solve for the angles developed in the mechanism. Chart 2 maps strap tension, actuator force, and the moments generated about the torsion bar while lifting a 300 pound individual as a function of the knee angle. In the standing position, strap tension tends to near zero while the actuator has a static load limit of 3000 pounds.