机械二极管 Precision Linear Actuator for Space Interferometry Mission (SIM)

Precision Linear Actuator for Space Interferometry Mission (SIM) 
Siderostat Pointing
Brant Cook*
, David Braun*, Steve Hankins*, John Koenig* and Don Moore*
Abstract
“SIM PlanetQuest will exploit the classical measuring tool of astrometry (interferometry) with
unprecedented precision to make dramatic advances in many areas of astronomy and astrophysics” (1).
In order to obtain interferometric data two large steerable mirrors, or Siderostats, are used to direct
starlight into the interferometer. A gimbaled mechanism actuated by linear actuators is chosen to meet
the unprecedented pointing and angle tracking requirements of SIM. A group of JPL engineers designed,
built, and tested a linear ballscrew actuator capable of performing submicron incremental steps for 10
years of continuous operation. Precise, zero backlash, closed loop pointing control requirements, lead the
team to implement a ballscrew actuator with a direct drive DC motor and a precision piezo brake. Motor
control commutation using feedback from a precision linear encoder on the ballscrew output produced an
unexpected incremental step size of 20 nm over a range of 120 mm, yielding a dynamic range of
6,000,000:1. The results prove linear nanometer positioning requires no gears, levers, or hydraulic
converters. Along the way many lessons have been learned and will subsequently be shared.
Introduction
SIM will improve “our understanding of the physical properties of stars, determining the mass, including
the dark matter component, and its distribution in our Galaxy, observing the motions of the Milky Way’s
companions in the Local Group, and probing the behavior of supermassive black holes in other galaxies”
(1). Using three interferometers, 1 science and 2 guides, SIM will deliver a dramatically more accurate
mapping of our universe as well as a better understanding of the formation and evolution of other
planetary systems outside our own. Accurate mapping using interferometers requires high precision
actuation, pushing the limit of both the mechanical positioning realm and the electronics/control realms.
The required lifetime of SIM is also extremely challenging with a need to meet performance requirements
for no less than 5.5 (with a goal of 10) years of continuous science observation. This is approximately 3
million large angle gimbal moves.
Figure 1:  SIM PlanetQuest
While many precision mechanisms are required for SIM to deliver its science data, the Siderostat is the
initial pointing mechanism used to direct starlight into the instrument. The Siderostat tips and tilts a 304.5-
mm clear aperture optic across a 15-degree Field of Regard (on the sky) via a two-axis hexfoil flexured
gimbal mechanism, with a required coarse accuracy of 1arc-second (as) and a fine accuracy of 
                                                         
*
 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
Proceedings of the 39th
 Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008
3735 milli-arc-second (mas) on the sky. Implementing a pair of linear actuators approximately 300 mm from
the gimbal axis yields a required linear coarse accuracy of approximately 1 micron and a fine linear
accuracy of approximately 5 nanometers. See Figure 2, Potential Siderostat Configurations.
Figure 2:  Potential Siderostat Configurations
This paper will discuss the design, build, and test of a linear actuator capable of repeatedly, over long
periods of operation, performing submicron positioning maneuvers. The design successfully pushes the
limits of mechanical positioning while remaining true to the JPL principles of heritage, simplicity, and
robustness. Along the way interesting lessons were learned and will be put forth for the reader’s benefit.
Mechanical Design
Generally speaking, design is a continuous balancing act to meet competing requirements. It requires the
designer/engineer to carefully balance the design process between many conflicting requirements. In
actuator design, if mutually exclusive positioning requirements are equally important a two-stage
mechanism is often the solution. Unfortunately, multiple stages add to complexity, mass and cost. In our
design, the need for large fast moves, trump the needs for fine positioning, and vice versa. It was
originally thought that no single actuator could meet the large stroke, high speed, small incremental step
size (actuator resolution), required by the Siderostat. The original intention of our Precision Linear
Actuator, Direct Drive (PLADD) was to fill the role of a coarse actuator.
The design approach taken with PLADD is one of simplicity, heritage, and robustness. The initial design
required at least 2000 incremental positions per revolution of the nut/motor in order to obtain a linear
incremental step size of 1 micron. Figure 3 shows a cross section of the actuator.
Figure 3: SID PLADD Cross Section View
374The bearing housing, as shown in Figure 4, is the center of the actuator, housing the back-to-back duplex
angular contact bearings, brake, and motor. Both sets of bellows are suspended from the bearing
housing, while the housing bolts directly to the carbon fiber outer tube for thermal stability.  
Figure 4: Bearing Housing
The angular contact bearings are assembled into the bearing housing from opposing ends of the housing.
The housing is stepped, creating a bearing spacer against which the bearings can be preloaded. The
bearings are thin section MPB (Miniature Precision Bearings) angular contact ball bearings with a
phenolic retainer ball spacer. The bearings have 440C balls and races. The bearings are shown in Figure
5.
Figure 5: Timken MPB Angular Contact Ball Bearing
The bearing inner races are mated to the motor drive link. As depicted in Figure 6, the inner race of the
inner bearing seats on a non-sliding preload flexure. The DC brushless motor rotor is assembled to the
rear of the drive link via set screws. The ballnut is mounted to the front of the drivelink via bolts.
Figure 6: Bearing Preload Flexure and Rotor mounted to Drive Link
The motor is coupled to the NSK ballnut by way of an in-house designed and built drive link. The drive
link is shown in Figure 7.
The complexity of the mechanism is reduced and backlash of a gear train is eliminated via a direct drive
approach. By eliminating mechanical backlash in the actuator the control system is able to actively
eliminate any mechanical imperfections in the actuator using feedback from the linear glass scale
375encoder mounted to the actuators’ output. The direct drive approach also greatly simplifies manufacturing
and assembly of the actuator by lowering part count and complexity.
Figure 7: Motor Drive Link
The heart of the actuator is a 12-mm diameter, 2-mm lead, NSK ballscrew mounted onto an NSK Double
nut utilizing a spring preload of 130 newtons. The combination can be seen in Figure 8.
Figure 8: NSK Ballnut
The ballnut is connected directly to the drive link via 8 bolts.
Figure 9: Ballnut bolted to Drive Link
The ballnut drive link combination is assembled into the bearing housing with the angular contact
bearings, preload flexure, and finally the piezo brake as pictured in Figure 10.
376Figure 10: Housing Assembly
The piezo brake consists of a set of flexured levers, preload springs, and guide flexures machined out of
a single piece of titanium. The drive-link is radially compressed by the machined springs upon power-off.
This constrains the drive link, and thus the ballnut, in rotation. The power must be applied in order to
release the brake. The braking force is applied by the compressed preload springs via a wedged preload
shim. The preload force is eliminated by the application of power to the piezo stacks.
Figure 11: Piezo Brake
The actuators’ driving torque is delivered via an off the shelf DC brushless motor from BEI/Kimco
Magnetics as pictured below.
Figure 12: BEI DC Brushless Motor
Long life requirements along with a high correlation between lubricant consumption and life limitation led
the team to immerse the mechanical drivetrain of PLADD in oil via a set of hermetically sealed bellows
mounted to each end of the ballscrew. The bellows create a constant volume to house Brayco 815Z oil,
and subjects the mechanical system to a continuous oil flush. The bellows additionally are used to hold
the screw in rotation, thus allowing the rotating ballnut to produce linear motion at the screw. The
continuous motion of oil through the mechanical system is conjectured to eliminate many failure modes.
This hypothesis is being tested via life tests that are in progress.
The motor windings, piezo brake, and inner bellows are bolted to the outer housing via thru bolts.
377Figure 13: Inner Titanium Bellows
Figure 14: Motor Windings Mounted
A MicroE linear glass scale is connected to the end screw via a set of parallel motion flexures. These
flexures provide a thermally stable mount for the glass scale via a low CTE metering rod. The scale is
allowed to float relative to the inner tip of the ballscrew, minimizing the coupling of thermal drifts in the
parts of the actuator not in the position path. The use of metering rods helps minimize false delta
readings at the sensor head. The glass scale is mounted on a carbon fiber rectangular rod. A dummy
glass scale is mounted opposite the operating scale to help eliminate bending in the scale due to
changes in bulk temperature.
Figure 15: Glass Scale Encoder Mount via Flexures
The sensor head is mounted via a set of parallel motion flexures and the composite tube that acts as a
metering rod. This completes the thermally stable sensor mount.2560