Static and Dynamic Stability 

Performance Measurements of the HLPR 

Chair/Forklift

Edited by: 

 Joshua Johnson 

Florida Gulf Coast University 

Ft. Myers, FL 

and 

Roger Bostelman

U.S DEPARTMENT OF COMMERCE

National Institute of Standards and Technology 

Intelligent Systems Division 

Gaithersburg, MD 20899-8230 

NISTIR 7667
 

NISTIR 7667

Static and Dynamic Stability Performance

Measurements of the HLPR 

Chair/Forklift

Edited by: 

 Joshua Johnson 

Florida Gulf Coast University 

Ft. Myers, FL 

and 

Roger Bostelman

U.S DEPARTMENT OF COMMERCE

National Institute of Standards and Technology 

Intelligent Systems Division 

Gaithersburg, MD 20899-8230 

January 2010

U.S. DEPARTMENT OF COMMERCE 

Gary Locke, Secretary 

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY 

Patrick Gallagher, Director
ABSTRACT 

The Home Lift, Position and Rehabilitation (HLPR) Chair, developed at the National 

Institute of Standards and Technology (NIST), has unique capabilities and a unique shape 

as compared to conventional wheelchairs and powered chairs. The evaluation of its static 

and dynamic stability is necessary to ensure rider safety. Recently, the HLPR Chair was 

converted into an autonomous forklift for the manufacturing industry. The HLPR forklift 

includes articulated forks that replace the chair from the original HLPR Chair design. 

Both the HLPR Chair and HLPR forklift prototypes can carry approximately 114 kg (250 

lb) beneath an upper rotation joint (i.e., rotates the seat or the forks). HLPR stability was 

evaluated based on the American National Standard (ANSI) and the International 

Organization of Standardization (ISO) wheelchair and forklift standards. Also, new test 

methods were created and tested that are currently not provided in these standards 

because of HLPR’s unique lift and rotation capabilities. The following report details the 

standard and new test methods and test results. 

2
TABLE OF CONTENTS

1. INTRODUCTION.......................................................................................................... 4 

2. STATIC STABILITY .................................................................................................... 6 

2.1 Background ........................................................................................................... 6 

2.2 Experimental setup................................................................................................ 6 

2.3 Methodology ......................................................................................................... 7 

2.3.1 Testing forward stability............................................................................. 7 

2.3.2 Testing rearward stability ........................................................................... 8 

2.3.3 Testing sideways stability........................................................................... 9 

2.4 Results ................................................................................................................. 11 

3. DYNAMIC STABILITY ............................................................................................. 13 

3.1 Background ......................................................................................................... 13 

3.2 Experimental Setup ............................................................................................. 13 

3.3 Methodology ....................................................................................................... 14 

3.3.1 Rearward Dynamic Stability..................................................................... 14 

3.3.2 Forward Dynamic Stability....................................................................... 15 

3.3.3 Lateral Dynamic Stability......................................................................... 15 

3.3.4 Dynamic Stability while Traversing a Step .............................................. 15 

3.4 Dynamic Test Results.......................................................................................... 16 

4. DISCUSSION .............................................................................................................. 17 

5. SUGGESTED CHANGES TO CURRENT STANDARDS........................................ 18 

6. ACKNOWLEDGEMENTS ......................................................................................... 21 

7. REFERENCES............................................................................................................. 22 

8. APPENDIX: HLPR CHAIR TEST ORIENTATIONS................................................ 23 

8.1 Forward Static Stability....................................................................................... 23 

8.2 Rearward Static Stability..................................................................................... 27 

8.3 Lateral Static Stability......................................................................................... 28 

LIST OF TABLES

Table 1. Configurations for Forward Stability ................................................................. 7 

Table 2. Configurations for Rearward Stability ................................................................ 9 

Table 3. Configurations for Sideways Stability .............................................................. 10 

Table 4. Static Forward Stability Test Results ................................................................ 11 

Table 5. Static Rearward Stability Test Results .............................................................. 12 

Table 6. Static Lateral Stability Test Results .................................................................. 12 

Table 7. ANSI/RESNA Scoring System for Dynamic Stability ..................................... 15 

Table 8. Dynamic Stability Test Results ......................................................................... 16 

Table 9. Dynamic Stability while Traversing a Step Test Results ................................. 17 

Table 10. Mean Static Tipping Angles ........................................................................... 17 

3
1. INTRODUCTION 

A novel Home Lift, Position, and Rehabilitation (HLPR) Chair has been designed and 

built at the National Institute of Standards and Technology (NIST) [1, 2, 3] to provide 

independent patient mobility for indoor tasks, such as moving to and placing a person on 

a toilet or bed, and lift assistance for tasks, such as accessing kitchen or other tall shelves. 

These functionalities are currently out of reach of most wheelchair users. One of the 

design motivations of the HLPR Chair is to reduce back injury, typically, an important 

issue in the care of this group. The HLPR Chair was also tested as an autonomous 

mobility device to assist cognition by route and trajectory planning. 

To date, two HLPR Chair (1 and 2) prototypes have been built. HLPR Chair 1, used in 

these stability experiments, was also developed into an autonomous wheelchair. HLPR 

Chair 2 was modified from the first design to develop a more cost effective 

manufacturing design, a more ergonomic and standard seat design and a larger seat 

rotation system for higher side-to-side stability, as well as other smaller modifications. 

The unique shape (see Fig. 1) and capabilities of the HLPR as compared to typical wheeland powered chairs requires examination of its stability. Existing standards do not 

adequately cover this design. Unique characteristics of the HLPR chair that are not 

included in current standards are: 1 m lift, seat rotation, and two casters with single wheel 

drive-and-steer wheel design with stabilizers. 

Figure 1: Typical powered chair (left) [courtesy Jazzy1

] and the HLPR Chair 1 prototype 

(right three photos). HLPR Chair is shown in the mobility (center photos) and the lift 

(left-most) configurations. Note the unique shape of HLPR Chair as compared to typical 

powered chairs. 

 

1

 Certain commercial equipment, instruments, or materials are identified in this report in order to specify 

the experimental procedure adequately. Such identification is not intended to imply recommendation or 

endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the 

materials or equipment identified are necessarily the best available for the purpose. 

4
As a load-carrying device, the frame and wheelbase closely resemble a walk-behind 

forklift. For this reason, ANSI/RESNA and ISO standards for wheelchairs and forklifts 

were adapted to assess the stability of the HLPR under both static and dynamic 

conditions. The standards referenced for these stability tests were: 

 International Standard IS0 7176-1:1999 (E) Wheelchairs – Part 1: Determination 

of static stability [4] 

 International Standard IS0 7176-2: 2001 (E) Wheelchairs – Part 2: Determination 

of dynamic stability of electric wheelchairs [5] 

 International Standard IS0 7176-7:1998 (E) Wheelchairs – Part 7: Measurement 

of seating and wheel dimensions [6] 

 International Standard IS0 7176-10:1998 (E) Wheelchairs – Part 10: 

Determination of obstacle-climbing ability of electric wheelchairs [7] 

 International Standard IS0 7176-11:1992 (E) Wheelchairs – Part 10: Test 

dummies, first edition 1992-05-01. [8] 

 International Standard IS0 7176-22:2000 (E) Wheelchairs – Part 22: Set-up 

Procedures, first edition 2000-05-15 [9] 

 International Standard IS0 1074: 1991 (E) Counterbalanced fork-lift trucks – 

Stability tests [10] 

 Australian Standard AS 3581-1988, Mechanical Aids for Patient Lifting and 

Moving – Safety Requirements [11] 

 American National Standard ANSI for Wheelchairs – Volume 1: Requirements 

and Test Methods for Wheelchairs (Including Scooters), [12] 

 American National Standard ANSI for Wheelchairs – Volume 2: Additional 

Requirements for Wheelchairs (Including Scooters) with Electrical Systems, [13] 

Non-existent stability test methods were developed with consideration of HLPRs 

transition from an intelligent powered and lift wheelchair to a load-carrying and lift 

device for the manufacturing industry. Figure 2 shows HLPR in the articulated forklift 

configuration. By utilizing HLPR rotation joint with an articulated forklift, a user can 

manually load and secure the load by placing it onto the lower frame. A user can then 

either manually push or autonomously send the load to the target location. In the case of 

existing and similar standards between wheelchairs and forklifts, the most rigorous test 

was adapted. The purposes of the stability tests were to determine the maximum safe load 

capabilities, the maximum safe angle of operation, and the braking capabilities of the 

HLPR. The stability tests were performed between May and August 2008. 

This paper will describe the testing procedures used for static and dynamic stability 

testing of the HLPR, as well as standard and new suggested standard methods of stability 

testing for this unique mobile/lift wheelchair and forklift. The paper begins with static 

stability background, methodology, tests and results followed by dynamic stability 

background, methodology, tests and results. Following are a discussion section which 

summarizes the results, an acknowledgement section and a references section. An 

appendix section then follows which includes several photos of the experimental setup. 

5
 

Rear stabilizer 

casters 

Figure 2. HLPR 1 prototype transitioned into an articulated forklift and shown in the 

(left) loading and (right) loaded-mobility configurations. 

2. STATIC STABILITY 

2.1 Background 

International Standard IS0 7176-1 for Wheelchairs deals with the determination of static 

stability. This standard parallels Section 1 of the ANSI/RESNA Wheelchair standards. 

The intention of the tests is to determine basic information about the angle of tip of the 

HLPR in its most and least stable configuration. The HLPR was tested in three 

orientations: uphill, downhill, and sideways. 

Depending on the direction of tip, the HLPR can tip about the front wheel axle or about 

any of the three wheel contact points with the ground. The footprint of the vehicle has a 

large bearing on stability. When the HLPR is on a level surface, the two front wheels and 

the singular rear wheel touch the ground. If imaginary horizontal lines connect the three 

wheels in a triangle, the result would be the footprint. As long as the center-of-gravity 

(COG) of the HLPR/load remains within the footprint, the HLPR is statically stable. The 

HLPR has been modified with rear stabilizers as a safety feature. If tip occurs, 

particularly when the device is orientated sideways (see Fig. 2), the stabilizers provide a 

very small, adjustable HLPR tip angle (e.g., less than 5°). 

The HLPR design enables a rider or load to be positioned at a height above that of a 

traditional wheelchair. The COG of the HLPR/load system is greatly affected by the 

height. The HLPR seat can be raised or lowered so that a person can transfer to and from 

a chair, toilet, or bed. Also, the seat frame rotation that enables patient transfer shifts the 

COG of the HLPR/load system and affects static stability. Due to the flexibility of the 

HLPR these configurations must be tested individually: load at lowest height, load raised 

1 m to great height, seat frame rotated to 4 different positions. 

2.2 Experimental setup 

A tilting platform was designed and built to support the 113 kg HLPR Chair and its 113 

kg payload. The 2.44 m x 11.22 m (8 ft x 4 ft) platform included a hand crank winch to 

lift one end of the platform to various angles from 0o through 25o as shown in Figure 3. 

6
The platform friction was tested using the standard method of a weight placed on a piece 

of rubber while tilting the platform prior to beginning actual HLPR tests. Angles were 

measured using a digital angle measurement device with 25o maximum angle. 

Figure 3. Stability test platform and safety devices. 

hand crank winch 

safety strap 

platform

tip restraint bar 

slip prevention bar 

During the test, a piece of paper, attached to the platform edge by a rubber band, was 

placed beneath the most raised HLPR wheel(s) to alert the platform winch operator as to 

the exact moment the HLPR wheels tip off the platform. At this point, the platform angle 

is measured and recorded. Figure 8 in the Appendix section shows this test setup. 

2.3 Methodology 

2.3.1 Testing forward stability 

1. The HLPR was tested in all configurations listed in Table 1. Figure 4 shows the 

HLPR Chair in the forward static stability test configuration. Photographs of all 

test configurations can be found in the Appendix. 

Table 1. Configurations for Forward Stability 

Chair Height Seat Orientation to Frame 

Medium Forward 

Medium Side (90°) 

Medium Rearward 

High Forward 

High Side (90°) 

High Rearward 

7
sandbags 

Figure 4. The HLPR being tested for forward static stability 

2. Five sandbags, totaling105 kg, were strapped onto the HLPR seat with weight 

distributed similar to a human body sitting in the seat. 

3. The HLPR was placed facing downhill with the front wheels obstructed by a 

crossbar to prevent it from rolling or sliding. A piece of high-gloss paper (0.08 

mm thick) was placed under the rear wheel and affixed to the platform 

perpendicularly with a rubber band. The tipping angle was reached when the 

piece of paper slid from beneath the uphill rear wheel. 

4. The sandbags were adjusted to be in similar weight distribution for each test, step 

3 was repeated, and tipping angle was recorded for each combination. 

2.3.2 Testing rearward stability 

1. The HLPR was tested in all configurations listed in Table 2. Figure 5 shows the 

HLPR Chair in the rearward static stability test configuration. Photographs of all 

the various configurations can be found in the Appendix. 

2. Five sandbags, totaling105 kg, were placed in the HLPR seat with weight 

distributed similar to a human body sitting in the seat. 

3. The HLPR was placed facing uphill with the rear wheel obstructed by a crossbar 

to prevent it from rolling or sliding. A piece of high-gloss paper (0.08 mm thick) 

was placed under the front wheel and affixed to the platform perpendicularly with 

a rubber band. The tipping angle was reached when the piece of paper was slid 

from under the uphill front wheel. 

8
4. The sandbags were adjusted to be in similar weight distribution for each test, step 

3 was repeated, and tipping angle was recorded for each configuration. 

Table 2. Configurations for Rearward Stability 

Chair Height Seat Orientation to Frame 

Medium Forward 

Medium Side (90°) 

Medium Rearward 

High Forward 

High Side (90°) 

High Rearward 

Figure 5. The HLPR being tested for rearward static stability 

2.3.3 Testing sideways stability 

The HLPR was tested in all configurations listed in Table 3. Figure 6 shows the HLPR 

Chair in the sideways static stability test configuration. Photographs of all configurations 

can be found in the Appendix. 

9
Table 3. Configurations for Sideways Stability 

Chair Height Seat Orientation to Frame 

Medium Forward 

Medium Side (90°) 

Medium Rearward 

High Forward 

High Side (90°) 

High Rearward 

1. Five sandbags, totaling105 kg, were placed in the HLPR seat with weight 

distributed similar to a human body sitting in the seat. 

2. The HLPR was placed sideways with the most uphill wheel (a front caster) 

obstructed by a crossbar to prevent it from rolling or sliding. A piece of highgloss paper (0.08 mm thick) was placed under the most uphill wheel and affixed 

to the platform perpendicularly with a rubber band. The tipping angle was reached 

when the piece of paper was slid from under the uphill wheel. 

3. The sandbags were adjusted to be in similar weight distribution for each test, step 

3 was repeated, and tipping angle was recorded for each combination. 

Figure 6. The HLPR being tested for sideways static stability 

10
11

2.4 Results 

Static stability tests were performed a total of five times per a given configuration. In 

Tables 4-6, μ is the mean value of the five tests and σ is standard deviation of those 

values. Data listed in the Platform and HLPR columns are slightly different from each 

other suggesting that deformation of the tires and/or frame may have occurred. This is an 

important point as the flexibility of the system being measured may alter the expected 

COG. For these experiments, maximum tip angle was set to the digital measurement device 

maximum of 25° since this angle is well beyond what the test operators expect HLPR to 

encounter in normal use.

Table 4. Static Forward Stability Test Results 

Medium Chair Height 

(1.28m) 

High Chair Height 

(1.78m) 

Test #, μ, σ Platform HLPR Platform HLPR 

1 18.5° 19.5° 13.7° 14.4° 

2 15.8°1 16.3°1 12.6° 13.8° 

3 18.6° 19.3° 13.0° 13.7° 

4 18.5° 19.4° 12.9° 13.0° 

5 17.8° 18.6° 12.4° 13.0° 

μ 18.4° 19.2° 12.9° 13.6° 

Forward 

σ 0.32 0.35 0.44 0.53 

1 18.3° 19.1° 12.9° 13.7° 

2 17.5° 18.1° 12.7° 13.4° 

3 18.3° 19.3° 12.8° 13.6° 

4 17.4° 18.1° 12.7° 13.4° 

5 17.5° 18.3° 12.3° 13.1° 

μ 17.8° 18.6° 12.7° 13.4° 

Side (90°)

σ 0.41 0.51 0.20 0.21 

1 17.4° 18.2° 11.8° 12.5° 

2 16.8° 17.5° 12.1° 12.8° 

3 17.1° 17.7° 12.1° 12.6° 

4 17.2° 18.0° 12.1° 13.0° 

5 18.0° 19.0° 12.7° 13.4° 

μ 17.3° 18.1° 12.2° 12.4° 

Seat Orientation 

Rear 

σ 0.40 0.52 0.29 0.32 

[1] The values for seat orientation forward test #2 were not included in the mean 

or standard deviation, as they may have been impacted by the orientation of the 

safety catch device. 

Table 5 shows every tip angle to be >25° and is therefore, the most stable of the three static 

configurations tested. The researchers concluded that it was best to show the actual data 

tabulated for each test in a format consistent with Tables 4 and 6.
Table 5. Static Rearward Stability Test Results 

Medium Chair Height 

(1.28m) 

High Chair Height 

(1.78m) 

Test #, μ, σ Platform HLPR Platform HLPR 

1 >25° >25° >25° >25° 

2 >25° >25° >25° >25° 

3 >25° >25° >25° >25° 

4 >25° >25° >25° >25° 

5 >25° >25° >25° >25° 

Forward 

μ >25° >25° >25° >25° 

1 >25° >25° >25° >25° 

2 >25° >25° >25° >25° 

3 >25° >25° >25° >25° 

4 >25° >25° >25° >25° 

Side (90°)

5 >25° >25° >25° >25° 

μ >25° >25° >25° >25° 

1 >25° >25° >25° >25° 

2 >25° >25° >25° >25° 

3 >25° >25° >25° >25° 

4 >25° >25° >25° >25° 

5 >25° >25° >25° >25° 

Seat Orientation 

Rear 

μ >25° >25° >25° >25° 

Table 6. Static Lateral Stability Test Results 

Medium (1.28m) High (1.78m) 

Test #, μ, σ Platform HLPR Platform HLPR 

1 7.9 9.8 5.5 6.9 

2 8.2 10.1 5.0 6.5 

3 7.8 9.3 4.4 6.0 

4 7.9 9.5 4.5 6.3 

5 8.3 10.1 5.0 6.7 

μ 8.0 9.8 4.9 6.5 

Forward 

σ 0.19 0.32 0.40 0.31 

1 8.2 10.1 5.4 7.2 

2 7.9 9.5 4.8 6.3 

3 7.9 9.9 4.5 6.1 

4 8.2 10.1 4.3 5.9 

5 8.1 10.4 4.8 6.4 

μ 8.1. 10.0 4.8 6.4 

Side (90°)

σ 0.14 0.30 0.37 0.44 

1 8.4 10.4 5.0 7.0 

2 8.7 10.5 5.2 7.2 

3 8.7 10.5 5.1 7.0 

4 8.7 10.6 5.0 7.0 

5 9.1 10.8 5.4 7.3 

μ 8.7 10.6 5.1 7.1 

Seat Orientation 

Rear 

σ 0.22 0.14 0.15 0.13 

12
3. DYNAMIC STABILITY 

3.1 Background 

International Standard IS0 7176-2 for Wheelchairs details the determination of dynamic 

stability of electric wheelchairs. This standard parallels Volume 2 of the ANSI/RESNA 

Wheelchair standards. This test is intended to determine the stability of a powered 

wheelchair when it is driven up and down inclined planes of varying degrees. The 

wheelchair is driven at maximum speed on slopes of 0o, 3o, 6o, and 10o. Forward downhill 

braking, backward downhill braking, and forward uphill braking are performed on the 

slopes. The stability of the wheelchair during these maneuvers is then rated. 

3.2 Experimental Setup 

A different platform was designed and built to support the HLPR Chair dynamic stability 

measurements. This platform measured approximately 2.4 m wide x 3.7 m long (8 ft x 

12 ft) and was built as a modular platform to disassemble into three approximately 1.2 m 

x 2.4 m (4 ft x 8 ft) sections while built to ANSI specifications to allow sufficient 

wheelchair travel. The platform was capable of tilting, using hand crank winches, to 0°, 

3°, 6°, and 9° angles. 

As shown in Figure 7, a passenger is being used as the payload (approximately 93 kg) for 

forward tests. Not shown are tests that used sand bags as payload for all rear and lateral 

tests. In the figure, the passenger is ready to drive forward at full speed and turn left. 

Extreme caution was exercised to prevent passenger or HLPR Chair harm by using straps 

to facility hard points (fixtures to walls and superstructures) and a second test 

operator/observer. 

HLPR dynamic stability test start positions were chosen to ensure that the HLPR would 

not traverse off the platform. HLPR was estimated by an observer to accelerate on a 

level surface from a stopped position to a maximum 0.7 m/s in approximately 1 m when 

fully loaded with approximately 93 kg. 

The list of tests performed were as follows: 

1. Rearward dynamic stability on ramp 

2. Forward dynamic stability on ramp 

3. Lateral dynamic stability on ramp 

4. Lateral dynamic stability while turning in circles 

5. Lateral dynamic stability while turning suddenly 

6. Dynamic stability while traversing a step 

HLPR tip was measured during each test by an observer. Each test from 1 through 5 was 

performed with the HLPR at a slope of 0°, 3°, 6°, and 10° with results shown in Table 8. 

For test 6, Dynamic stability while traversing a step, only a slope angle of 10° was used 

as suggested by ISO 7176-2 and results of this test are shown in Table 9. 

13
operator on 

HLPR Chair 

ready to test 

platform 

center platform 

support leg 

Figure 7. HLPR chair dynamic stability test setup. 

3.3 Methodology 

3.3.1 Rearward Dynamic Stability 

1. The HLPR chair height was set at the medium height (1.28m), as marked on the 

HLPR. 

2. Six sandbags totaling 105 kg were strapped into the HLPR seat. 

3. The performance of the HLPR was rated according to the scale shown in Table 7. 

4. The HLPR was then placed on a level test plane. 

5. From a stationary start, the controls were operated to give maximum acceleration 

in the forward direction. The response was scored according to Table 7. 

6. Step 5 was then repeated on slopes of 3°, 6°, and 10° with the HLPR facing 

uphill. 

7. The HLPR was then run at maximum speed on the level test plane. 

8. Braking was initiated by releasing the joystick and the dynamic response of the 

HLPR was scored according to Table 7. 

9. Steps 7-8 were repeated, but braking was initiated by putting the joystick in 

reverse. 

10. Steps 7-8 were repeated again, but braking was initiated by turning while pressing 

the rear emergency stop pushbutton (located with the caregiver controls). 

11. Steps 7-10 were repeated on slopes of 3°, 6°, and 10° with the HLPR traveling 

forward on the uphill slope. 

12. Steps 7-11 were then repeated with the HLPR traveling at maximum speed 

backwards down the slope. 

14
Table 7. ANSI r Dynamic Stability 

O Score 

/RESNA/ISO Scoring System fo

bserved Dynamic Response 

No tip At least one lifting wheel remains on the test plane. 3 

Transient tip o 

or not any anti-tip devices 

All lifting wheels 2 2 lose contact, then drop back ont

the test plane, whether 

contact the test plane. 

Stuck on anti-tip 

device , and the HLPR remains stuck 

All lifting wheels lift off, the HLPR anti-tip device 1 

contacts the test plane 

on the anti-tip device 

Full tip re from 

 a 

restraining device or testing personnel. 

The HLPR tips completely over (90° or mo 0 

its original orientation) unless caught by

[2] Lifting wheels are the wheel(s) that lose contact with the test plane which, for these 

experiments, is the sloped platform. 

LPR chair. 

sting. 

dynamic response of the 

 were repeated, but braking was initiated by putting the joystick in 

 plane. 

9. The response of the HLPR was scored and recorded according to Table 7. 

PR chair. 

PR was turned to the left with maximum acceleration until it was facing 

orded according to Table 7.

7. Steps 4-6 were repeated on 6° and 10° slopes

transitioning to/from a 25 mm horizontal step. HLPR was controlled at a relatively slow 

3.3.2 Forward Dynamic Stability 

1. The HLPR was set with the seat orientation facing forward. 

2. The seat height was set to medium (1.28m) as marked on the H

3. A human driver of a 97 kg mass was used to perform te

4. The HLPR was run at maximum speed down 3° slope. 

5. Braking was initiated by releasing the joystick and the 

HLPR was scored and recorded according to Table 7. 

6. Steps 4-5

reverse 

7. Steps 4-5 were repeated again, but braking was initiated by activating the 

emergency stop located with the caregiver controls at the rear of the HLPR. 

8. The HLPR is run at maximum speed down a 3° slope onto a horizontal test

3.3.3 Lateral Dynamic Stability 

1. The HLPR was set with the seat orientation facing forward. 

2. The seat height was set to medium (1.28 m) as marked on the HL

3. Six sandbags at a total of 105 kg were placed in the HLPR seat. 

4. The HLPR was positioned facing downhill on a 3° test plane 

5. The HL

uphill.

6. The response of the HLPR was scored and rec

3.3.4 Dynamic Stability while Traversing a Step 

Similar to section 3.3.2 Forward Dynamic Stability, forward tests were run for the 

Dynamic Stability while Traversing a Step test with a ramp slope height set at 10° 

15
HLPR speed until it transitioned to/from the slope to/from the step. Each test was 

repeated three times and no other step heights were tested. Results are shown in Table 9. 

3.4 Dynamic Test Results 

Table 8 shows the Dynamic Stability Test Results for the HLPR Chair when following 

ISO 7176-2 Wheelchairs – Part 2: Determination of dynamic stability of electric 

wheelchairs, Annex B. Each column in Table 8 is described as follows: 

Test: lists the type of test and how it was performed. For example, the Rearward 

Dynamic Stability test was performed by HLPR starting forward motion, by stopping 

after traveling forwards and by braking when traveling backwards. 

Method of Retardation: is the method used to cause HLPR to decelerate from maximum 

speed to a complete stop. 

Release: the operator lets go of the joystick used to command HLPR acceleration. 

Power off: the operator turns off power to HLPR 

Applying reverse: the operator rapidly moves the joystick from full commanded 

HLPR acceleration in the direction of travel to full commanded HLPR 

acceleration in the exact opposite direction 

Stability Score at various ramp angles: at each ramp angle HLPR was tested, an 

associated score of 0, 1, 2, or 3 as described in Table 7 was listed. 

Comments: comments for each test were listed during the test by the researcher. Slipping 

was noted when HLPR wheels slid on the platform. 

Table 8. Dynamic Stability Test Results 

Stability Score 

at various ramp angles 

Test Method of 

Retardation 

0° 3° 6° 10° 

Comments 

Rearward Dynamic Stability 

Starting Forward 3 3 3 3 Slipping occurs at 3° 

R Release 3 3 3 3 Slipping occurs at 3° 

P Power off 3 3 3 3 Slipping occurs at 3° 

Stopping after 

traveling forward 

A Applying reverse 3 3 3 3 Slipping occurs at 3° 

R Release 3 3 3 3 Slipping occurs at 3° 

P Power off 3 3 3 3 Slipping occurs at 3° 

Braking when 

traveling backward 

A Applying reverse 3 3 3 3 Slipping occurs at 3° 

Forward Dynamic Stability 

R Release 3 3 3 3 Slipping occurs at 3° 

P Power off 3 3 3 3 Slipping occurs at 3° 

Braking when 

traveling forward 

A Applying reverse 3 3 3 3 Slipping occurs at 3° 

Traveling forward 

down a slope onto a 

horizontal surface 

N/A N/A 3 3 0 

2 

 Complete tip 

 Transient tip 

Dynamic Stability in Lateral Directions 

Turning on a slope N/A 3 2 2 0 Transient tip on 

highest positioned 

front wheel 

Turning suddenly at 

maximum speed 

N/A 3 N/A N/A N/A

16
HLPR Chair is not designed to travel up and down steep ramps and transition steps from

these ramps. When following ISO 7176-2 Wheelchairs – Part 2: Determination of 

dynamic stability of electric wheelchairs, Annex B, the steep 10° ramp angle to/from a 

step caused transient tipping or complete tipping. The test was determined unsafe by the 

researchers and halted. Therefore, Table 9 shows the Dynamic Stability while Traversing 

a Step Test Results for the HLPR Chair when HLPR traveled from a horizontal surface 

to/from a step transition. This test better fit the HLPR design as it may encounter 

doorway thresholds. Each column in Table 9 is described as follows: 

Test: lists the type of test and how it was performed. 

Stability Score: 25 mm step height: at a 25 mm step height that HLPR traversed, an 

associated score of 0, 1, 2, or 3 as described in Table 7 was listed. The researchers only 

tested one step height three times each to understand the test methodology although ISO 

7176-2 suggests testing at several step heights. 

Table 9. Dynamic Stability while Traversing a Step Test Results using a horizontal test 

surface instead of a 10° ramp as suggested by the standard. 

Stability Score 

25 mm step height 

Test 

1 2 3 

Comments 

Rearward Dynamic Stability 

Travelling forward up a step transition from a 

standing start 

3 3 3 Horizontal test surface 

up a step 

Travelling backward down a step transition from 

a standing start 

3 3 3 Horizontal test surface 

down a step 

Forward Dynamic Stability 

Travelling forward up a step transition at 

maximum speed 

3 3 3 Horizontal test surface 

down a step 

Traveling forward down a step transition from a 

standing start 

3 3 3 Horizontal test surface 

down a step 

Dynamic Stability in Lateral Direction 

One side of HLPR drops down a step transition 3 3 3 Horizontal test surface 

down a step 

4. DISCUSSION 

While undergoing static stability testing in the forward and rear directions, the HLPR 

demonstrated favorable results as shown by the mean tipping angles in Table 10. The 

tipping angle in the rear direction exceeded 25° for both the medium and high seat 

heights. Conversely, the tipping angle recorded was very low for static testing in the 

lateral direction. Of particular interest is the mean tip angles when the HLPR is tested 

laterally with the seat in the high position; the tip angle for all seat orientations is under 

6°. This is due to the narrow width of the HLPR. At the high configuration the COG 

easily leaves the footprint of the device. 

During dynamic stability testing, the HLPR frequently slipped before tipping occurred. 

As to be expected based on static performance, the HLPR experienced transient tipping 

or complete tipping when undergoing dynamic lateral stability testing. The Dynamic 

17
Stability while Traversing a Step tests and results shown in Table 9 were slightly 

different tests from those suggested in standards where HLPR transitioned from a 

horizontal surface to a step. For these tests, results were excellent. However, HLPR 

traveling forward down an incline set at 10° onto a horizontal surface, as suggested by the 

standards, caused either transient or complete tipping. 

Table 10. Mean Static Tipping Angles 

HLPR Frame HLPR Height 

Orientation 

Seat 

Orientation Medium (1.28m) High (1.78m) 

Forward 18.4° 12.9° 

Side 17.5° 12.7° 

Forward 

Rear 17.3° 12.2° 

Forward >25° >25° 

Side >25° >25° 

Rear 

Rear >25° >25° 

Forward 8.0° 4.9° 

Side 8.1° 4.8° 

Lateral 

Rear 8.7° 5.1° 

Suggested design changes to prevent HLPR Chair slipping and tipping might be to 

lighten the weight of the upper HLPR Chair frame and/or add weight to its bottom frame. 

HLPR Chair weighs approximately 113 kg without payload and totals 226 kg with a 113 

kg rider or test dummy. To prevent tipping, a wider base or caster design is suggested 

which can also be changeable for narrow doorway entrance as needed. 

5. SUGGESTED CHANGES TO CURRENT STANDARDS 

The modifications listed below would enhance current standards to support devices built 

now and in the future that have similar characteristics as the HLPR Chair. 

1. Seat Rotation:

a. The ISO 7176-1 Wheelchairs – Part 1 Determination of Static Stability standard states: 

9.2 Wheels unlocked and the wheelchair in the least stable configuration 

9.2.1 Set adjustable parts of the wheelchair in the least stable configuration for 

forward stability. Table 1(in the standard) illustrates the effect of typical 

adjustments. 

The standard refers to a forward stability table where there is currently no listing for seat 

rotation as HLPR Chair can provide. Therefore, we suggest the following additions to 

ISO 7176-1 Table 1-Forward Stability: 

Adjustable wheelchair 

component Least stable Most stable 

Seat position, rotation Reverse Forward 

The reason for the suggested addition to the ISO 7176-1 standard is because when tilted 

forward, the loaded seat can cause the HLPR center-of-gravity (CG) to be forward of the 

18
casters and cause increased tip potential dependent upon the angle and HLPR Chair 

design and construction. 

b. Furthermore, a similar description is shown in ISO 7176-1 standard section 10, Table 

2-Rearward Stability and in section 12, Table 3-Sideways Stability. We therefore suggest 

the additions as follows: 

Addition to ISO 7176-1 standard Table 2-Rearward Stability 

Adjustable wheelchair 

component Least stable Most stable 

Seat position, rotation Forward Reverse 

Addition to Table 3-Sideways Stability 

Adjustable wheelchair 

component Least stable Most stable 

Seat position, rotation Down-slope Up-slope 

c. Seat rotation is also not listed in ISO 7176 – 22 Wheelchairs – Set-up procedures and 

specifically within section 6.3.4 which discusses the Seat and backrest set-up for testing. 

Also, Annex A should include an entry in the “record of measurements and settings for 

set-up procedures in accordance with ISO 7176-22” table for Seat rotation. 

2. Test apparatus and method 1

ISO 7176 Part 1, section 3.2 tipping angle states that the tipping angle occurs when the 

uphill-wheels forces on the platform become zero. Below this statement in the standard 

is a note stating there are a number of methods to determine when this occurs. One 

method occurs when someone is able to pull a piece of paper from beneath the uphill 

wheel and the test plane. This is difficult for a single test operator using a manual test 

plane lift mechanism such as a hand winch or hydraulic piston. The operator may be 

required to stop platform tilting to do the paper-pull test. Similarly, if the operator or 

other test helper is available to do the paper-pull test, it places them at some risk near the 

test. 

Alternatively, a method used to test the HLPR Chair was a simple rubber band, (see 

Appendix Figure 9) attached between the paper and the test plane so that when tip occurs, 

the paper springs out by itself from beneath the wheel. At that moment, the test is 

stopped and the tilt angle can be recorded. This allows continuous platform tilting with 

no safety risk to the test operators. Similarly, remote reading of electronic force-sensing 

instrumentation as pointed out in the standard could be used. 

3. Test apparatus and method 2

ANSI/RESNA and ISO wheelchair standards show a horizontal restraint bar to support 

the wheelchair and payload (rider or test dummy) from tipping too far during static 

stability tests. The horizontal restraint bar is very useful but provides only one axis of 

restraint and at only one level which requires rigid adjustment for tall patient mobility 

devices. The single axis restraint may be all this is currently necessary for existing 

devices. However, this may not be sufficient for future lift-wheelchairs with HLPR Chair 

19
capabilities. Instead, NIST researchers testing HLPR stability used an adjustable safety 

harness (rope or strap) (see Figure 8) attached between a facility hard-point and the top of 

the HLPR to prevent it from tipping too far. The overhead harness is very useful in that it 

provides a safe, flexible, yet loose restraint attached to the HLPR Chair top to prevent 

collapse from any angle or height being tested. Also, a sliding crossbar with a flexible 

link was attached between the platform and the base of HLPR that prevented the HLPR 

from raising more than 50 mm off of the test surface. ANSI/RESNA WC/Vol. 1-1998 

and ISO 7176-1 shows a roll restraint fixed to the platform only at the wheel and 

downhill point of the device being tested and/or a flexible restraint both combined with 

the tip restraint horizontal bar. We used both the upper harness restraint and a sliding 

crossbar for maximum safety during tests because there were no facility hard-points 

beside the test platform that supported the use of a heavy duty horizontal restraint bar. 

Use of an upper harness and sliding crossbar improved the test safety especially for tall 

devices like HLPR Chair. 

4. Test apparatus and method 3

HLPR Chair not only provides patient mobility and transfer capabilities, but also 

provides for rehabilitation. Current off-the-shelf (OTS) stand/mobile powered chair 

technology (see Figure 8 (left)) can typically raise a rider to a standing position. 

However, this technology does not allow the rider to walk nor stand on the floor nor to 

move or exercise their legs in normal walking fashion. As shown, the OTS and HLPR 

Chair supports the patient as they command the powered chair to move. 

 

Figure 8. (left) Off-the-shelf standing type powered chair (courtesy Permobil), (center) 

HLPR Chair 1 in the patient rehabilitation configuration. The prototype is supporting a 

patient while they or a caregiver (behind HLPR) commands HLPR to move. (right) 

HLPR Chair 2 supports a patient with a sling ready for transfer to a seat, toilet or bed. 

HLPR Chair provides for mobility and also provides a rehabilitation and exercise 

configuration, as shown in Figure 8 (center), allowing the patient to stand directly on the 

floor while supported by HLPR. 

20
The patient or a caregiver can also command the HLPR to drive in this configuration 

allowing the patient to exercise their legs or for leg rehabilitation. The figure shows the 

two devices, OTS powered chair (left) and HLPR Chair 1 (center) and 2 (right), 

supporting the patient by arm-rests in the OTS powered chair and with crutches-type 

underarm torso lifts or a sling on the HLPR Chair. These configurations also need to be 

considered in the ANSI/RESNA and ISO stability standards. Standing (and sitting) 

slings can be attached to these mobile/lift devices adding further complexity to stability 

measurements. For example, Figure 8 (right) shows a patient ready to be transferred to 

another seat where the HLPR Chair seat has been rotated behind the patient and the 

patient is suspended by a sitting sling. 

Similarly, a standing (parachute-type) harness, which can prevent under arm stress, could 

be attached to the HLPR Chair to allow patients to walk while HLPR Chair moves. 

These relatively new configuration types and slings could be added to stability test 

method standards for mobile patient transport devices. Some industry standards, among 

others not listed here, that may have text to add to current wheelchair standards are: 

 ASME / ANSI Standards. B30.2. Overhead Cranes 

 ANSI/ASME B30.9 – 1984 Slings 

 ASME B30.21 – 1982 Manual Lever Operated Hoists 

 ANSI/ITSDF B56.10 - 2006, Safety Standard for Manually Propelled High Lift 

Industrial Trucks 

 Occupational Safety and Health Administration (OSHA) 1910.67 or ANSI A92.2-

1969 Vehicle Mounted Elevating and Rotating Work Platforms. 

5. Other suggested standard tests to consider

These are standard tests that the researchers could not locate in the current standards but 

may be useful towards safety of wheelchair operators: 

 need for seatbelts when a human rider is used in place of a test dummy. This is 

especially true of ISO 7176-2 dynamic stability testing where the device is restrained, 

but the rider is not. 

 test dummy leaning forward. This changes the CG for the rider and device perhaps 

also changing stability test results. 

 autonomously-controlled wheelchairs that could reference ANSI/ITSDF B56.5 Safety 

Standard for Driverless, Automatic Guided Vehicles and Automated Functions of 

Manned Vehicles. 

6. ACKNOWLEDGEMENTS 

The authors would like to thank the following people for their support of these stability 

experiments: Alex Page, Richard Norcross, Alexsandr Gorbashev and the NIST SURF 

Student directors. 

21
7. REFERENCES 

[1] Bostelman, R., Albus, J., HLPR Chair – A Service Robot for the Healthcare Industry, 

3rd International Workshop on Advances in Service Robotics, Vienna, Austria, July 

7, 2006 

[2] Bostelman, R., Albus, J., Sensor Experiments to Facilitate Robot Use in Assistive 

Environments, Proc. of the 1st International Conference on Pervasive Technologies 

Related to Assistive Environments (PETRA), Athens, Greece, July 15-19, 2008. 

[3] Bostelman, R., Albus, J., Robotic Patient Transfer and Rehabilitation Device for 

Patient Care Facilities or the Home, Journal of Advanced Robotics, 22 (2008) 1287–

1307. 

[4] International Standard IS0 7176-1:1999(E) Wheelchairs – Part 1: Determination of 

static stability, second edition 1999-10-01. 

[5] International Standard IS0 7176-2: 2001 (E) Wheelchairs – Part 2: Determination of 

dynamic stability of electric wheelchairs, second edition 2001-06-15. 

[6] International Standard IS0 7176-7:1998 (E) Wheelchairs – Part 7: Measurement of 

seating and wheel dimensions. 

 

[7] International Standard IS0 7176-10:1998 (E) Wheelchairs – Part 10: Determination of 

obstacle-climbing ability of electric wheelchairs 

[8] International Standard IS0 7176-11:1992 (E) Wheelchairs – Part 10: Test dummies, 

first edition 1992-05-01. 

[9] International Standard IS0 7176-22:2000 (E) Wheelchairs – Part 22: Set-up 

Procedures, first edition 2000-05-15. 

[10] International Standard IS0 1074: 1991 (E) Counterbalanced fork-lift trucks – 

Stability tests, second edition 1991-03-15. 

[11] Australian Standard AS 3581-1988, Mechanical Aids for Patient Lifting and Moving 

– Safety Requirements. 

[12] American National Standard ANSI for wheelchairs – Volume 1: Requirements and 

Test Methods for Wheelchairs (including Scooters), April 2000 Supplement to 

ANSI/RESNA WC/Vol. 1-1998. 

[13] American National Standard ANSI for Wheelchairs – Volume 2: Additional 

Requirements for Wheelchairs (Including Scooters) with Electrical Systems, 

ANSI/RESNA WC/Vol. 2-1998. 

22
8. APPENDIX: HLPR CHAIR TEST ORIENTATIONS 

8.1 Forward Static Stability 

Safety rope attached to 

facility. 

Sand bags (test 

dummy) strapped to 

HLPR seat. 

Sliding crossbar safety 

support with flexible 

link to HLPR 

Paper attached to 

rubber band to test 

when rear wheel 

separates from the 

platform during tip 

tests. 

Figure 9. Seat Height Medium Seat Forward Orientation and safety devices used. 

23
Figure 10. Seat Height Medium Seat Side Orientation 

Figure 11. Seat Height Medium Rear Chair Orientation 

24
C-clamp 

used to 

prevent seat 

from freely 

rotating. 

Figure 12. Seat Height High Seat Forward Orientation 

25
Figure 13. Seat Height High Seat Side Orientation 

Figure 14. Seat Height High Seat Rear Orientation 

26
8.2 Rearward Static Stability 

Figure 15. Seat Height Medium Forward Chair Orientation 

27
8.3 Lateral Static Stability 

 Figure 16. Seat Height Medium Forward Figure 17. Seat Height Medium Side Chair 

Chair Orientation Orientation 

28
 Figure 18. Seat Height Medium Rear Figure 19. Seat Height High Forward 

Chair Orientation Chair Orientation 

29
Figure 20. Seat Height High Side Chair Orientation 

30