Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.4 Reciprocating Chemical Muscle
providing low friction by spreading the wear over the bearing contact surfaces with narrow
clearances which facilitated good conditions for lubrication and tight constraint of the motion.
As stated above, initial efforts under the present grant focused on the development of a scaled up
model manufactured with transparent polycarbonate and larger tolerances to intentionally magnify
effects of manufacturing out-of-tolerance, large temperature changes, and prolonged wear.
The clear polycarbonate body allowed for quick and precise understanding of performance since
the movements of the internal components could be monitored visually in real time.
The original third generation configuration had only two lower pair joints (piston and valve
spool) without fixed joints in between, and allowed for a minimal control loop. One of the major
improvements over the previous third generation design is the fourth generation RCM valving
control having two separate pistons for the extend and retract motions. This was a large departure
from the original generations of RCM, and it was done in order to minimize or almost
totally eliminate the moments of inertia of action and reaction between the RCM body and the
rest of the RCM actuator system. In the third generation RCM the valve actuation was directly
(and externally) coupled to the piston pair and cycle speed was directly proportional to gas volume/pressure
and the natural harmonics of the kinematic system.
To assure positive action that has timing independent of piston friction, a pneumatic control
valve actuation gated by a pilot valve directly machined on the pistons shaft was used. In this
manner the actual shifting of the control (spool) valve is directly linked to piston shaft position,
however (unlike the third generation devices) no mechanical linkages exist between the piston
shaft and control valve, other than pneumatic pilot pressure. The result is positive valve control
with no external parts or physical internal linkages. Vibration is eliminated and the resulting
RCM is a single piece unit with equal and opposite-acting piston actuators, a single input pressure
port, and a single waste gas exhaust port.
In order to avoid out-of-timing scenarios, the two piston shafts were internally mechanically
linked in their extend/retract trajectories by a spur gear mounted in a rack and pinion fashion
between both (rack portions of the) shafts as shown in Figure 3-142. This new design architecture
controls speed by modulating inlet pressure or exhaust volume. Regardless of the speed of
reciprocation (even when transitioning between different reciprocation speeds) the actuator
extension is equal and opposite from each end of the RCM. Forces are balanced between the two
actuator pistons because they share a common pressure source. Axial vibration is eliminated for
all operating conditions.
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