Skylab Experiment M131 Rotating Litter Chair 

JAMES S. EVANS, DENNIS L. ZITTERKOPF, ROBEBT L. KONIGSBERG, AND CHARLES M. BLACKBURN 

Physiological considerations suggest that the response of the vestibular system can be substantially modified during weightlessness and that such modifications affect susceptibility to motion sickness and to judgment of spatial localization. Evaluation of such effects requires measurement of responses to rotational accelerations before, during, and after exposure to conditions of prolonged zero-gravity. For this purpose, a precisely controlled rotating chair (fig. A.I.j.-1) was designed, constructed, tested, and installed in the Skylab Orbital Workshop. Their chair was used in three test modes to measure changes in the vestibular (balance) organs of the astronauts. 

Experiment Description 

To evaluate the subject’s ability to detect small angular accelerations, the chair is rotated through any of 24 acceleration profiles, which range between 0.020°/S² and 6.0°/S². To evaluate the changes in the subject’s susceptibility to motion sickness, the chair is rotated at any of 10 constant velocities while the seated subject executes certain prescribed head and upper-body movements. The purpose of the test is to establish the degree to which motion sensitivity of a subject changes as a result of exposure to zero-g. To evaluate the ability of a blindfolded subject to retain spatial orientation in the absence of customary cues, the chair can be extended into a litter or reclining position. In addition, it can be tilted into a combined pitch-roll attitude in either the chair or litter mode. The major feature of the chair is that it imparts very smooth, tightly controlled, rotational accelerations and velocities to the vestibular organs of a seated test subject. The chair and its servo control system can effectively perform at levels below the threshold of sensitivity of the vestibular organs of the normal test subject.

A direct-drive brush-type d.c. torque motor is used as the prime mover to impart these closely controlled angular accelerations and velocities to a seated subject. The angular velocity of the chair is sensed by a precision grade, brush-type d.c. tachometer that forms the feedback element in a closed-loop, angular-velocity servo. This combination, and the appropriate electronics, control the oculogyral illusion test acceleration rates to within 1 percent + 0.0015°/S ²of the selected value (fig. A.I.j.-2). This same system maintains the motion sensitivity test angular velocity rates to within 1 percent + 0.05 r/min of the selected value (fig. A.I.j.-3) while the chair undergoes variations in torque loading as the test subject changes his body position during head movements. During the oculogyral illusion threshold test, the subject is seated in the rotating chair and wears a pair of test goggles that contain a visual target, i.e., a luminous arrow. The chair is then accelerated and decelerated at specific rates. The subject verbally reports the direction of apparent (hence the term illusion) lateral motion, if any, of a visual target in the test goggles. This illusion results from stimulating the fluid in the semicircular canals in the inner ear. This is done by accelerating and decelerating the chair. The term oculogyral is related to the words ocular (of the eyes) and gyrate (to move in a circular course).

The electrical control system (fig. A.I.j.4 and fig. A.I.j.-5) consists of an angular velocity profile generator, to provide a desired command angular velocity-time profile, and a servo loop. The profile generator is contained in the Control Console (fig. A.I.j.-6). Positive and negative reference voltages from the Zener diode power supply, combined with precision resistive dividers, provide d.c. voltages that are proportional to a desired motion sensitivity or oculogyral illusion velocity-time profile. The appropriate voltages for a particular program are selected by the motion sensitivity or oculogyral illusion Program Select switches. When a profile is to be generated for a test, these voltages are switched by the control switches to the input of the command integrator, a chopper stabilized d.c. operational amplifier. The control switches are actuated by the control programmer by various electrical timing circuits. If a constant d.c. voltage is fed into the command integrator, its output will be the integral of the constant input voltage and therefore a linear ramp function (steadily increasing or decreasing voltage). Since a velocity ramp represents a constant acceleration or deceleration, it follows that a particular constant input voltage to the integrator represents a particular constant acceleration or deceleration. If a constant voltage is applied to the integrator input and then removed (i.e., set to zero), the integrator output will be the expected ramp function until the input is set to zero.

The integrator will "hold" the last value on the ramp function until the input is again changed from the zero value. The analog voltage being "held" represents a constant angular velocity. When deceleration from this constant velocity is desired, the integrator is fed a constant voltage of opposite polarity to the one applied during the acceleration period. The output of the integrator will then, in a descending ramp, approach zero voltage, and the chair will decelerate under the control of the servo linearly to zero velocity. When this occurs, the chair remains at zero velocity until another test is started.

The output of the profile generator is the command input to the servo loop. The integration in the servo loop, which makes this a type I servo system, guarantees zero angular acceleration and deceleration errors in the chair shaft motion in the steady state. The output of the servo integrator is fed to a solid-state linear power amplifier. The output of the amplifier connects to a direct drive d.c. torque motor, which drives the chair. A precision d.c. tachometer mounted on the shaft of the chair measures the actual chair angular velocity in terms of a voltage analog. The actual-chair angular velocity is then compared to the command angular velocity, in terms of analog voltages, at the input of the servo integrator thus completing the servo loop.

Several devices such as an oculogyral illusion threshold computer, an oculogyral illusion and motion sensitivity response keyboard for recording the subject’s responses to these tests, and an audio cadence signal are included in the electrical system to assist the operator and subject in performing the experiments. Additionally, proximity detectors are provided to signal the subject and notify the ground of the completion of a head movement, and an emergency switch is provided on the chair to permit the subject to remove power from the chair.

During the nonrotational portion of the tests, the chair shaft is locked and the electrical system disabled. The observer can slowly tilt the chair from the horizontal to a ±20 degrees position with an accuracy of 0.50. When a preselected position is reached, visual orientation is measured by having the subject attempt to align the visual target in the test goggles (fig. A.I.j.-7) with an unseen reference axis within the Orbital Workshop. The result is recorded from pitch and roll scales provided.

Kinesthetic orientation is measured by having a blindfolded subject make similar judgments using a metal rod instead of the visual target. Only the rod is used in the litter position. The subject’s accuracy in making these alignments is an indication of the degree to which he depends upon various clues from spatial orientation. 

Conclusion 

Flight data indicated that the system performed its designed functions without anomaly. A more convenient seat belt restraint was supplied for the last two manned missions.

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