. 

r 

NASA Technical Memorandum 100 474 

A Method of Isolating Treadmill Shock 

and Vibration on Spacecraft 

William E. Thornton 

~NBSA-TB-loO474) B HETHOD OF ISOLATING N89-24790 

TREAD8ILL SmCK AND VZBRATION ON SPACECRAFT 

{SASA. 18 P CSCL 06s llnclas Lyndon B. I Johnson Space Center) 

G3/52 0216853 

April 1989 

National Aeronautics and 

Space Administration 

-. 

i 
NASA Technical Memorandum 100 474 

A Method of Isolating Treadmill Shock 

and Vibration on Spacecraft . 

William E. Thornton 

Lyndon B. Johnson Space Center 

Houston, Texas 

, 
A Method of Isolating Treadmill Shock and Vibration on Spacecraft’ 

A major problem is currently felt to exist in the 

implementation of materials processing on a spacecraft. 

Crystal growers and others have placed requirements of 

one micro-g or less on the vehicle. Simple math produces 

startling figures for such a restriction e.g., for each ton 

of vehicle mass with 10-6 g acceleration limit; Perturbing 

Force limit, F = .002 lb. For each 105 lbs F = 0.1 lb. 

For each 106 lbs F = 1.0 lb. 

Forces generated by normal human movement on 

spacecraft of 5x105 pounds weight are on an order-ofmagnitude greater than allowed by this specification and 

forces generated by locomotion on a treadmill are more 

than two orders-of-magnitude greater. Other exercises and 

normal onboard functions generate forces in between. See 

figure 1. To accommodate many essential functions it 

is obvious that even on a vehicle as large as Space Station, 

a reduction of more than two orders of magnitude in 

force is required. Commonly used passive shock and 

vibration isolation devices cannot meet such demands. 

Active isolation devices are complex, heavy and also 

would have difficulty meeting the requirements. However, 

by a new arrangement, adequate isolation can be obtained. 

Isolation of the treadmill will be treated here since it is 

considered the most significant disturbance at this time. 

There are many other mechanical events not treated here 

which must also be considered. 

c 

by acceleration/ deceleration from muscle forces, producing 

the typical biphasic pattern for one foot shown in figure 

3. Since ground contact of the steps overlap, a composite 

FGF pattern is produced as in figure 4. In jogging, running, 

and jumping the body is actually thrown clear of ground 

contact and as would be expected there is a sharp increase 

in vertical forces (figures 3 and 5). For walking, the basic 

vertical foot ground force patterns are cyclic, relatively 

complex waveforms which vary from approximately 1 to 

1.5 BW with frequency variations from <l. to, say, 2.5 

Hz. Jogging/ running produces a series of zero based pulses 

which approximate a half sine waveform with amplitudes 

from 1.5 to 3.0 + BW and frequencies from 1 to 4 Hz 

and periods of -500 to <100 msec. 

There are some individual variations including 

assymetries between right and left legs which can reach 

25% difference. See figure 5. Chief of the individual 

variations are ‘heel’ and ‘forefoot’ runners with the heel 

runners producing a brief spike on each step (figure 6). 

Horizontal forces are much less with a typical example 

of fore aft forces shown in figure 7. These forces increase 

with increasing velocities. Lateral forces are smaller still 

as shown in figure 8. Again, note that the period of ground 

contact and step frequency are a function of speed. Figure 

9 is a comparison of FGF in 3 axes at a slow walk and 

fast run. 

There are no significant differences between ordinary 

FGF and those from treadmill locomotion in one-g. 

Treadmill shock and vibration characteristics 

Forces generated by human locomotion, walking, jogging, 

running, and jumping, have been studied since the last 

century and are well known and reasonably constant 

between individuals and circumstances (1 -6). 

Biodynamics of locomotion 

In walking on Earth, one foot is always in contact with 

the ground, but force is cyclically shifted from foot to 

foot. The movement of each foot is a step and a step 

by each foot completes a cycle or stride (7). In both 

walking and running step rate and stride increase with 

velocity (figure 2). In walking, magnitude of the vertical 

foot groundforces (FGF) vary about the individual’s body 

weight (BW) as the center of mass is raised and lowered 

Treadmill in weightlessness 

The basic scheme of a treadmill for weightlessness is shown 

in figure 10 with the major difference from Earth 

treadmills being a force system which replaces gravity. 

While FGF have never been measured in weightlessness 

there is little reason to expect major waveform differences 

at one-g equivalent force levels. Conversely, there will 

be a difference of 1 BW constant force2 transmitted to 

the spacecraft since the subject weight equivalent force 

system is attached to the treadmill frame and only inertial 

forces will be transmitted to ship’s structure, e.g., the 

typical waveforms and amplitudes shown in figures 4 and 

5 will have 1 BW as their zero reference. 

1 

r ‘Only crew related activities are addressed here. 

*This assumes that there is no change in bungee force with their slight 

extension on each step. 

1 
In summary, then, inflight treadmill locomotion will 

produce cyclic and pulsatile forces ranging from a fraction 

of BW to possibly three BW in the Z axis with lesser 

forces in other axes. The major, or Z axis force during 

jogginglrunning will be an almost half sinusoidal pulse 

with a base width of <0.1 to -0.5 sec at a frequency 

of -1 to 4 Hz. 

Isolation means 

Passive 

A wide variety of shock and vibration isolation systems 

using springs and mechanical resistance are available. They 

all function in a manner equivalent to either a resonant 

circuit, whose frequency is below that of the vibration, 

or else as a low pass filter. In either case, a compliant 

connection is involved (figure 11); and in the case of the 

treadmill this would allow excessive motion without a 

counterpoise mass since the treadmill weighs less than 100 

pounds and would undergo excessively large excursions 

under the force of walking and especially running. However, 

even adding counterpoise mass to the TM will not solve 

the problem with passive shock mounting for the force 

attenuation will still be inadequate to meet the requirements. 

Active attenuation 

A relatively recent development is the use of forces and 

displacements to offset those of shock and vibration. A 

single-axis schematic is shown in figure 12 for such a 

system. M1 is the mass to be stabilized, and is also the 

support mass. G1 is the disturbing force generator, 

and MZ the unbalanced mass driven by G1. Any accelerated motion along fX axis will transmit a force Fl to 

the mounting of M1, which also contains a sensitive force 

transducer T, (alternatively, the mounting could be 

compliant and an accelerometer mounted on M1). 

Any force sensed would generate an (almost) equal 

and opposite force Fz in Gz, M3. The mass M3 could 

be smaller than MZ but larger accelerations and resulting 

displacements would be required. 

While practical in many situations, in this case, six 

degrees of freedom would require compensation; the 

forces/ displacements are large and compensating forces 

would consume large amounts of power. To achieve good 

compensation in such a system, extreme care would also 

have to be taken in amplifier and generator characteristics 

and even then compensation would never be perfect. 

Proposed Solution 

the subject to run on the TM and produce acceptably 

small oscillations of it. 

In practical terms, this mass would not simply be 

ballast but relatively fixed portions of the vehicle (not 

primary structure) such as storage lockers, etc., which will 

provide the mass for free. This mass will be calculated 

to limit TM movement to say, a few tenths of an inch. 

The combined TM and counterpoise mass will be floated 

in the vehicle and provides an acceptable, totally isolated 

system; however, air currents and other small pertubations 

will cause it to ultimately drift into contact with structure. 

To avoid this, small appropriate counter forces must be 

applied. There are many ways to do this, ranging from 

non-contact sensors controlling airjets to magnetic or other 

fields. The simplest is a combined sensor/force generator 

(SFG) attached to a filament. In the example shown, two 

such SFGs (G/S,,J are used. They operate as follows. A 

small drift may be detected and a force F1 or F2 will 

be developed to offset it. This force will have two 

characteristics: it is constant regardless of short-term 

displacements of Mz, and it is limited such that it can never 

exceed the allowable spacecraft shock and vibration g limits. 

There are several simple approaches to such an 

arrangement, and two are shown in figures 14 and 15. Both 

methods use a filament and a reel which contains an optical 

position encoder to detect drift. When a drift1 is detected 

by the OE (which could be analog instead of digital as 

shown) it will cause an increase in DC motor current I 

which produces a torque increase by the motor and reel 

and an applied force F in a direction to correct the drift 

and restore the mass to its usual neutral position. The small 

forces required are compatible with small available DC 

motors. An alternative is shown in figure 15 in which a 

small brushless AC motor runs continuously, and after 

torque multiplication by a gear train, is coupled to the 

reel and filament through a variable torque clutch. In the 

same way as above, errors in position produce changes 

in DC current to the clutch and corrective increases in 

torque and force F. The error detector/current generator 

will contain frequency selective components such that only 

slow drifts are responded to and short term position changes 

or oscillations ignored. If desired or necessary error rate 

damping may be incorporated. If multiple units are 

interconnected as below it may be desirable to coordinate 

their outputs through the computers shown. An alternative 

is shown in figure 15 where a small motor runs continuously 

and varying amounts of torque are applied through an 

electromagnetic clutch which transmits torque in proportion to the input current. 

A member of these active tethers will be required to 

maintain position and the number and arrangement will 

be a function of the designer's ingenuity. My own minimum - - -- A practical alternative is shown for one axis in figure 

13- Mass M2 is made large enough to be an effective 

counterpoise to MI, the moving mass (TM). This allows 

'Drift is defined as a contained displacement of the mean distance X. 

Short term cyclic variations will be ignored by the system. 

2 
scheme, to date, which requires four and eight, would 

probably be more reasonable. 

A conceptual arrangement of this system is shown 

in figures 16 and 17. In figure 16, the treadmill is attached 

to storage containers to provide adequate counterpoise 

mass. For launch or other activities, links will connect this 

assembly to secondary structure and during use these links 

may be easily removable. In figure 17, the subject is shown 

running on such a TM. Incidentally, such a unit can be 

self-powered through a battery and generator driven by 

the TM. In any event, power required will be a few watts. 

Implementation 

There are several approaches to solution of this problem. 

One could vigorously analyze all aspects of it, study inflight TM forces, etc., and then try to extend the capabilities 

of existing resources, time and money. 

A more efficient alternative is to mock up the proposed 

system in one axis on Earth using an air bearing. Next, 

a three-dimensional scale mock-up could (and should) be 

flown on the Orbiter. It is possible to do a full scale mockup on Spacelab. 

This effort could be started immediately with 

development of a key item, the dynamic tether, for a few 

thousand dollars. A brief review of hardware available 

shows that suitable components are readily available. 

This technique may also be applicable to other 

problems which have not been considered here. 

A patent application is in progress and details may 

be obtained from JSC Patent Counsel. The author will 

be happy to provide any additional details as desired and 

to continue to provide detailed recording of foot ground 

forces from a variety of locomotor activities and provide 

other sources for additional data. 

! 

References 

1. Cavanagh, P.; and LaFortune, M. A.: Ground Reaction 

Forces in Distance Running, Journal of Biomechanics, 

13(5): 397-406 (1980). 

2. Okumura, S.; et al.: Analysis of the Resultant Floor 

Reaction Forces in Normal Gait in Biomechanics VIIIB, 

Proc. of Eighth Int. Congr. of Biomechanics, Human 

Kinetics Publishers, 1198-1204. 

3. Lanshammar, H.; and Strandberg, L.: Horizontal Floor 

Reaction Forces and Heel Movements During the Initial 

Stance Phase, Ibid., 1123-1 128. 

4. Payne, A. H.: Foot to Ground Contact Forces of Elite 

Runners, Ibid., 746-753. 

5. Bates, B. T.; et al.: Identification of Critical Variables 

Describing Ground Reaction Forces During Running, 

Ibid., 635-640. 

6. Adrian, M. J.; and Laughlin, C. K.: Magnitude of 

Ground Reaction Forces While Performing Basketbail 

Skills, Ibid., 903-914. 

7. Braune, W.; and Fischer, 0.: The Human Gait (Transl.) 

Springer Verlag, Berlin. 

8. Taves, C. L.; Charteris, J.; and Wall, J. C.: A SpeedRelated Kinematic Analysis of Overground and 

Treadmill Walking, Biomechanics VIII B, 423426. 

9. Matsusaka, N.; et al.: Relationship Between Right and 

Left Legs in Human Gait, From a Viewpoint of Balance 

Control, Ibid., 427430. 

3 

t 
VEHICLE WEIGHT, LBS. 

I.V. ACTIVITIES I 

JOGGING 

WALKING 

I 

OTHER 

EXERCISE 

ACTIVITIES 

ANIMALS 

MOVING 

1 o-' 1 o-6 1 o-~ 1 o-4 1 o-~ 1 o-2 I G's PRODUCED 

LIMIT 

Figure 1.- Plot of relation between peak shock and vibration and disturbance produced for various vehicle weights. A single 

axis force on a solid mass was assumed. Values shown are typical for various activities. 

1.8 r 

z 

W 

1.6 - 

1.4 - 

OVERGROUND 

90 110 130 150 

STEPS MIN-' 

Figure 2.- Average variation of stride and step rate. Velocity 

is product of steps per min X stride length. The slight difference 

is found between ground and T.M. locomotion. From Taves, 

et al. 

I 4 
BODY 

-~~ 

LBS. N WT 

630 

540 

450 

360 

270 

180 

2800 

2400 

2000 

1600 

1200 

800 

3.5 

3.0 

2.5 

2.0 

1.5 

1 .o 

.5 

0 

-.5 

-1.0 

RUN 

0 .1 .2 .3 .4 .5 .6 -7 .8 .9 

From HAMlL 

and PAYNE 

TIME (SEC) 

Figure 3.- Vertical foot-ground forces from one foot illustrating range from slow walk to rapid running. 

The waveforms of walking and running are each characteristic but change period and amplitude with 

velocity. 

5 
2BWFOOT GROUND 

FORCES 

4 +1.0 BW 

FORCE 

TRANSMITTED 

TO SPACECRAFT 

0- - -1.0 BW 

TREADMILL WALKING 3.5 mph 

Figure 4.- Vertical foot-ground forces while walking at 3.5 mph. Overlap of foot ground contact produces the complex waveform. 

In weightlessness, with the treadmill arrangement shown in figure 10, forces transmitted to the spacecraft are given by the scale 

on the right. 

6 
2BW - 

FOOT GROUND 

FORCES 

1 BW * 

0- 

1 SEC r 

i 

t 

I 

4 +1.0 BW 

FORCE 

TRANSMITTED 

TO SPACECRAFT A 

L 

TREADMILL JOGGtNG 6.0 mph 

'I 1 I 

0 

-1.0 BW 

Figure 5.- Vertical foot-ground forces from jogging at 6 mph. Period of zero force occurs when both feet are clear of the ground. 

Note marked asymmetry between right and left legs. In weightlessness, using the treadmill in figure 10, the forces transmitted 

to the spacecraft correspond to the scale shown on the right. 

7 
(4 

Figure 6.- Comparison of vertical foot-ground forces from 

each axis. 

‘heel’, (a), and ‘forefoot’, (b), runners. Force scales are different for 

8 
BODY 

-- 

LBS. N WT 

180 800 1.00 

.80 

0 

-.20- 

-.40- 

- 

- 

- 

-.GO - 

- 

-.80 - 

-1.00 

.40 1 

i RUN .............. ***e *. ..... WALK 

..yi* . ..- ... ; .. ............... ... .. ..... : P. ....... ...... .... :a ............... 

i :. : .. 

. :: . : .... .

. .:: i ::; 

i i :j .. 

.. 

:. 

.. .. 

: : 

.. .. .. .. 

:: 

ii 

-. 

* I I I I I 1 I I I I I I I I I I 1 

4 36 160 .20 

.. .. 

-. 

.. 

a. 

.. 

:. 

:' 

.. 

.. .. *. 

a. .. 

.. 

*. 

.. 

:* 

.. 

e. 

.. 

.. 

.. 

.. 

*I 

REARWARD 

FORWARD 

TIME (SEC) From HAMIL 

and PAYNE 

Figure 7.- Fore-aft horizontal, foot-ground forces in walking and running. During steady state conditions, the integral of these 

forces will be zero, except for wind and other resistance losses. 

9 
BODY 

LBS. N WT 

90 400 50 

--~ 

.10 

0 

-.lo 

- 

-.so 

-1 .oo 

MEDIAL 

'1'1'1'1'1'1'1'1'1 

.1 .2 .3 .4 .5 .6 .7 .8 .9 

TIME (SEC) From HAMIL 

and PAYNE 

Figure 8.- Lateral horizontal foot-ground forces in walking and running. Note change from a slow walk to a rapid run. 

10 
BODY LBS-N WT630 2800 3.5 7 

540 2400 3 0- 

1 

i 450 2000 25- t 

I 

I 

360 1600 20- 

270 1200 15- 

0- 

180 800 10- 

5- 

-5- 

-1 0 , 

0 

HORIZONTAL 

’ 

VERTICAL H 0 R I20 NTAL 

BODY 

LES N WT 

180 800 100 7 

BODY 

--~ 

LES N WT 

90 400 50 ~ 

REARWARD 

LATERAL 36 160 /7--;i L - ~~ .--. MEDIAL - -20 - 

-40 ~ FORWARD 

-60 ~ 

- 80 

, , , , , , , , I , , , , I , , , -1 00, , , , , , , , , , , , , , , , , , , -100, I. I, I. I. I., , , . , 12 3 4 5 6 7 8.9 0 12 3 4 5 6 7 8 .9 012345678 

Figure 9.- Horizontal, lateral, foot-ground forces in walking and running plotted to a common scale. This was simultaneous 

measurement shown individually in figures 3, 7, and 8. 
1 Tread 5 Speed control 

2 Pulleys 6 Speedometer 

3 Flywheel 7 Control 

4 Brake 8 Generator 

Figure 10.- Diagram of a passive treadmill for space flight. Bungees and harness replace weight 

on Earth. 

w 

RUBBER 

Figure 11.- Cross section of traditional passive shock mounting and its response curve. Modern devices are more sophisticated 

and efficient but suffer the same basic limitations. 

I 12 
-x 4 - +X 

POWER 

Tr 

F1 0 

F2 0 

FR 0 

4- 

I 

Figure 12.- Schematic of active compensation system for a single axis and response curves. See text. 

Figure 13.- Schematic of improved isolation system for one axis. MI is subject equivalent mass, G is the equivalent force 

generator, X, is motion of center of mass, M, is treadmill mass plus counterpoise, X, is motion of M2, G/S,,z are tether force 

generators which sense changes in distances of treadmill assembly from structure X3,4 which are processed by computer 

elements A to produce constant limited correcting forces Fl,*. 

13 
T 

* 

Position 

t 

-- i Computer 

5 sw 

v-- 

OE 

t n 

Power P 

Figure 14.- Diagram of combined sensor and constant force generator. The X is a 

tether, SW a swivel, T is corrective torque provided by the pulley (P) which contains 

an optical position encoder whose output is fed to the computer. Torque for correction 

of position is generated by a motor (M) whose torque is proportional to its drive 

current (I). G is a large ratio gear reduction and T, is a tachometer to produce a signal 

for rate damping. 

T 

.- i 

- T + 

I 

sw 

v- - 

Power 

Computer 

Velocity 

Position 

Figure 15.- Alternative form of combined sensor and constant force generator. This is 

the same as figure 14 except the motor runs continuously and torque from it is applied 

to the tachometer (T,) and gear train through a clutch (C) whose transmitted torque 

is proportional to a control current (I). 

14 
Figure 16.- Conceptual sketches of an isolated treadmill with storage lockers for a counterpoise. 

Links shown in dotted outline would be attached as required. 

Figure 17.- Conceptual sketch of an isolated treadmill in use. 

15 I 
REPORT DOCUMENTATION PAGE 

1. ReportNo. 

TM 100474 

2. Government Accession No. 

I 

4. Title and Subtitle 

A METHOD OF ISOLATING TREADMILL SHOCK AND VIBRATION 

ON SPACECRAFT 

19. Security Classification (of this report) 20. Security Classification (of this page) 

Unclassified Unclassified 

7. Author@) 

William E. Thornton 

21. No. of pages 22. Price 

18 

9. Performing Organization Name and Address 

Lyndon B. Johnson Space Center 

Houston, Texas 77058 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546 

15. Supplementary Notes 

3. Recipient's Catalog No 

5. Report Date 

April 1989 

6. Performing Organization Code 

8. Performing Organization Report No. 

S-59 1 

10. Work Unit No. 

073-36-00-00-72 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

TM April 1989 

14. Sponsoring Agency Code 

16. Abstract 

Stringent limits (one micro-g) for shock and vibration have been placed on Space Station. There are many planned 

operations which will exceed this limit by one to two orders-of-magnitude or more. One of the major shock and 

vibration sources is the treadmill (TM) exercise which produces cyclic forces of one to five Hz and up to three times 

body weight of the person using it. Current isolation means, neither active nor passive, can attenuate such forces 

to the desired level, hence some alternative must be used. One method for meeting such levels is to provide adequate 

counterpoise mass to the TM to allow it to be used while floating freely. To prevent drift and structural contact, 

a dynamic tethering system, whose forces will never exceed allowable limits, yet will allow appreciable motion will 

maintain the assembly's position. Concept and design of this system is described. 

17. Key Words (Suggested by Author(s)) 

Isolation Treadmill 

Micro-g Weightlessness 

Shock and vibration 

Spacecraft 

18, Distribution Statement 

Unclassified - Unlimited 

Subject Category: 52 

For sale by the National Technical Information Service, Springfield, VA 22161-2171