Positioner for optical element

An optical element positioner in which a carriage holding the optical element is driven in response to electrical control signals to selected locations along an optical path by a moving-iron, limited-rotation actuator, with the reflected load inertia substantially matched to the actuator rotor and crank arm inertia.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for positioning an optical element at 
selected locations along an optical path. 
In typical positioners for optical elements, the optical element rides on a 
carriage which moves along an optical axis and a motor is connected to 
drive the carriage between selectable positions along this axis. Prior 
positioners have had disadvantages such as relatively large mass, long 
response time and high cost that have made them disadvantageous or 
entirely impractical for various applications. 
SUMMARY OF THE INVENTION 
The invention features a carriage for holding the optical element, driven 
through a crank arm by a moving-iron, limited rotation actuator (or 
galvanometer), with the length of the crank arm connecting the actuator 
shaft to the carriage being arranged to cause the reflected inertia of the 
carriage to substantially match the inertia of the actuator rotor. 
In preferred embodiments, a taut, flexible metal band translates the rotary 
motion of the actuator to linear motion of the carriage without permitting 
any free play, the axis of the shaft is spaced from and parallel to a 
plane containing the optical path, and the crank arm and taut band lie in 
a plane perpendicular to the axis of the rotor. 
The positioner can operate quickly and accurately to precisely position the 
optical element at selected locations and can be made at low cost. 
Other features and advantages will be apparent from the following 
description of the preferred embodiment and from the claims. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
We first briefly describe the drawings.

STRUCTURE AND OPERATION 
Referring to the FIGS. 1, 2 and 3, carriage 110 (i.e., a shuttle assembly) 
holds lens assembly 112 and defines a straight optical axis 114 oriented 
along the length of the carriage with the optical axis passing through 
lens assembly 112. Carriage 110 rides back and forth along axis 114 on two 
guide rods 115, 116 which are held parallel to and on opposite sides of 
the optical axis. The carriage moves on three pairs of wheels 117, 118, 
119 held in ball bearings mounted on the carriage. One pair 118 is mounted 
at one point along the axis and ride (at right angles to one another, see 
FIG. 3) on one of the guide rods 115. The other two pairs of wheels 117, 
119 are mounted at two other points along the optical axis and the two 
wheels of each pair ride (at right angles to one another) on the second 
guide rod 116. Each of the guide rods is supported at both ends by mounts 
120 which are attached to base 122. Slot 124 in base 122 permits a small 
amount of relative movement between the two rods to permit precise 
alignment of the rods, using adjusting screws 126 (shown in FIG. 2). 
Rotary actuator 128 is mounted on base 122 with the axis of the rotor shaft 
130 perpendicular to the plane of the optical axis and offsets to one side 
of the axis by a small distance (e.g., 1 inch). Shaft 130 is linked to the 
carriage by a taut-band drive mechanism consisting of crank arm 132 having 
on one end a compressible mounting ring 134 which is held tightly on the 
shaft by screw 136. The other end of the crank arm has a curved surface 
138 over which a pair of taut bands 140, 142 lie. Taut band 142 is a 
single thin flexible sheet metal element. It is attached to one end of the 
carriage, passes about the curved end surface of the crank arm, and is 
attached to the opposite side of the crank arm. Taut band 140 comprises a 
pair of parallel flexible sheet metal band elements and is similarly 
attached to the other end of the carriage and passes about the curved end 
surface of the crank arm to a point of attachment on the other side of the 
crank arm. The space between the parallel band elements of band 140 is 
sufficiently wide to accommodate band 142 and the two bands are so mounted 
that as the crank arm rotates on the motor shaft, the bands do not 
obstruct each other. Bands 140, 142 are attached to the crank arm by means 
of tension control strips 144, 146 having tension adjusting screws 148, 
150 for adjusting the tension on band 140. 
Actuator 128 is connected to a controlled source of power 70 by wires 152. 
The length of the crank arm and the maximum excursion angle of the motor 
are chosen to generally match the inertia of the rotor shaft to the 
inertia of the translating carriage. 
Referring to FIGS. 3, 4, actuator 128 is a moving-iron galvanometer 
characterized by a moving-iron rotor 80, a pair of pole pieces 82, 84 each 
defining a pair of pole faces A, B, a pair of drive coils 86, 88 
associated respectively with the pole pieces for generating a control 
flux, and a pair of permanent magnets, 90, 92 connected between the pole 
pieces to produce a bias flux. Further details of such a galvanometer are 
set forth in the U.S. Pat. No. 3,624,574, issued Nov. 30, 1971 to the same 
assignee as this application and incorporated herein by reference. A 
suitable galvanometer is General Scanning Incorporated Model G360PD or 
G300PD having a torque of 2000 gram--centimeter, an inertia of 6 and 4 
gram--cm.sup.2, respectively, an angular excursion of 60 degrees, 
peak-to-peak, and dimensions of 4.6.times.4.6.times.5.0 cm. The crank arm 
has a radius of 1 inch and an inertia of 5 gram--cm.sup.2. The linear 
travel of the carriage is 0.5 cm. peak-to-peak, and the clear opening 95 
in the carriage is 1 cm. in diameter for mounting the lens. The 
approximate linear travel time, peak-to-peak, is 5 milliseconds. 
Referring to FIG. 5, controlled current source 70 typically comprises a 
source of position command signals 72, a rotor position sensor 74 
(connected to the shaft of the actuator), and a suitable feedback circuit 
76. The feedback circuit, in a manner known in general for galvanometers, 
regulates the control current 78 in accordance with an error signal based 
upon the difference between actual and desired lens position, thus to 
enable the rotor to accurately follow the command signals in a suitably 
damped manner. This enables rapid and accurate positioning of the carriage 
at any desired position on the optical axis. (Details of one useful 
control circuit are set forth in U.S. Pat. No. 4,142,144, Rohr, Feb. 27, 
1979, assigned to the same assignee as this application and hereby 
incorporated by reference.) 
Other embodiments are within the claims. E.g., the lens assembly can be a 
mirror or other optical element.