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GROUP MEMBERS:
Links: NASA Micro-gravity Test Flights
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Space flight osteoporosis is a common problem experienced by astronauts. To reduce the bone decalcification that occurs in micro-gravity, astronauts engage in exercise while in space. Unfortunately, the effectiveness of this exercise is limited partly because the passive control system of the exercise harness allows variation in the loading force and this restricts the allowable range of motion by the subject. The goal of this project is to improve the subject load device (SLD) used by astronauts to load their bodies during exercise in micro-gravity by implementing an active control system. This system will maintain a constant tension on the exercise harness in an attempt to simulate gravity's constant force on our bodies on earth.
The NASA jet (shown left) does parabolic dives at 45 degree angles to simulate micro-gravity at several minute intervals. This is why the Flyin' Lions have dubbed the plane the "vomit comet."
Plan of Action:
In order to monitor the encoder signals and determine which direction to turn the motor, we decided to use two HC11 boards (one for each side). Channel A of the top encoder (which goes high for every one degree of rotation) is tied to the STRA input on the HC11, thus causing interrupts for every degree of motion. Our code is designed to wait until an interrupt causes a change in CNTA, a variable set up to monitor changes in user motion. When CNTA changes, the program enters one of two subroutines designed to turn on the motor either clockwise or counterclockwise, and then wait for the spring shaft to turn as much as the spring itself has turned. The motor then shuts off and the program returns to waiting for interrupts. This design is intended to change the spring shaft exactly the same number of degrees as the spring has turned due to the user. The second part of the project required us to design a frequency ramping circuit to control the motor. The frequency needed to be ramped from 4kHz to 11kHz very fast so that it could reach top speed and thus become the most effective. To accomplish this, we used a 555 timer chip and two cascaded 100kohm digital potentiometers in place of one of the resistors to generate the necessary ramping frequency. The timer chip, digital pots, and a pulse train to change the wiper position in the pots are all controlled by the HC11 programming.
Shown above is the block diagram of our final design. Channel A of the encoders go high for every one degree of rotation, while Channel B is used as a comparative signal to determine which direction the motion occurred in. Of course, there will be duplicates of this system for the left and right sides, which operate independently. Within the 555 timer block, the two cascaded digital potentiometers play a vital role in ramping the frequency of the motor in order to achieve high speeds. |
The pre-existing system that we were presented with contained a passive
spring-based tension workout machine that became increasingly more
difficult to pull out in proportion to the length of displacement.
Attached to the base of the spring shaft was a motor that could turn the
shaft either direction to release or increase tension on the spring.
Two digital optic encoders measure both the displacement of the spring
caused by user motion and rotation of the shaft due to the motor.
The idea is that the motor will compensate user induced displacement of
the spring by rotating the shaft an equal number of degrees in the same
direction, thus decreasing spring tension during outward motions and
increasing spring tension during inward motions. It was our job to
design circuitry that could monitor these encoders and initiate the
stepper motor accordingly. Also, we needed to design a manual
adjustment mechanism so that the tension in the springs could be adjusted
before the active system is engaged while in flight. There will be a
switch that controls whether the operator initiates the "smart" circuit,
or whether the individual sides can be adjusted manually.
