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Apache Point Observatory 3.5-Meter Telescope Primary Mirror Support System

Pneumatic Servo Upgrade Hardware Design Report

Prepared for

The University of Washington Apache Point Observatory

by

Yorke J. Brown, PhD
96 King Road Etna, NH 03750 (603)643-0518 yjb@valley.net

In compliance with Purchase Order 449937

Rev 0 10Jun98 Rev 1 6Feb00


EXECUTIVE SUMMARY The Primary Mirror Support System of the APO 3.5-meter telescope utilizes an array of small pneumatic pistons (often called "Belloframs") which are servoed to provide an appropriate distribution of force over the back of the mirror and on the upper inside surfaces of the honeycomb cells. The forces provided by the pistons is intended to maintain both the position and the figure of the mirror as the telescope changes elevation angle, and as it responds to transient loads caused by wind gusts or other disturbances. The Primary Mirror Support System has been upgraded by installing a new pneumatic servo system incorporating improved, high bandwidth servo valves, new pressure sensors for more precise pressure control, and more elaborate electronic servo controllers. The new system is supported by modifications to the air supply system incorporating a new recirculating pump system which simultaneously provides a pressurized air supply and a subatmospheric return circuit.

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1.0 INTRODUCTION The primary mirror of a large telescope is heavy enough that it cannot support its own weight without distortions that alter its figure significantly. Even if the mirror is uniformly supported across the mirror cell, the cell itself cannot be made sufficiently rigid to support the mirror without distortion throughout its entire range of motion. One solution to this problem is to provide a support system in the mirror cell which actively synthesizes a perfectly rigid mounting structure supporting the mirror uniformly over its entire back face. Such a system must respond to the changes in the alignment of the gravity vector due to telescope tracking motions. In addition to simply providing an even force distribution that exactly compensates each component of gravity, the system must also maintain the correct position and orientation of the mirror, responding to transient disturbances such as wind gusts and telescope tracking accelerations. The Primary Mirror Support System (PMSS) of the 3.5 Meter Telescope at Apache Point Observatory (APO) uses an array of pneumatic pistons to provide active support of the telescope's primary mirror. Pistons distributed across the back of the mirror support it axially, while pistons placed on arms extending inside the honeycomb structure and bearing on the upper surfaces of the cells support the mirror transversely. Three axial position sensors and one transverse position sensor detect the position and orientation of the mirror relative to the cell. An electronic servo system controls the air pressure in the pistons so as to maintain the position and attitude correctly as the cell moves and as the mirror is subjected to wind loading. 2.0 SYSTEM DESCRIPTION 2.1 Support Actuator Arrangement Since the 3.5 Meter telescope uses an altitude-azimuth mounting system, the primary mirror moves only in those two degrees of freedom. The mirror support system therefore needs only to support the mirror actively in two directions: axially (perpendicular to the face of the mirror), and transversely (parallel to the face of the mirror and in a vertical plane). The mirror is positively constrained against rotation in the cell and against horizontal transverse motion. Vertical transverse displacement is controlled by the transverse support system; axial translation, tip, and tilt are controlled by the axial support system. With the telescope pointed at the zenith, the axial support system bears the entire weight of the mirror while the transverse supports bear none; with the telescope pointed at the horizon, the axial supports bear nothing while the transverse supports bear the entire mirror. The axial support system comprises an array of 78 air pistons distributed over the back face of the mirror and resting on the bottom of the mirror cell. In order to control tip and tilt as well as axial translation, the axial piston array is divided into three radial sectors, each with its own independent

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control system. Position feedback for each sector is provided by a strain-gage load cell captured between the mirror back plate and an adjustable post mounted to the mirror cell surface below the center of each sector. The control system adjusts the quantity of air in each cylinder array to keep the load cell compressed to a fixed setpoint. The transverse support system employs 38 air cylinders mounted inside the honeycomb sections of the mirror. Each piston is mounted on a post anchored to the bottom of the mirror cell and extending through a hole in the mirror back plate into one of the honeycomb cells that make up the mirror body. The piston acts on the "top" flat surface of the honeycomb cell, thus providing a force transverse to the optical axis. The entire transverse support system includes a total of 38 pistons distributed over the entire mirror. The transverse position sensor is a load cell facing on the "top" of the inside of the Cassegrain hole in the center of the mirror. The load cell actually bears against the edge of the mirror backplate inside the Cassegrain hole. The three axial sectors are referred to as sectors A, B, and C, starting at the back of the mirror and proceeding clockwise as viewed from above. The A sector is on the plane of symmetry as the telescope moves in elevation; the B sector is on the left, and the C sector is on the right. The transverse system is referred to as sector T. Figure 2-1 shows the arrangement of the axial sectors, the locations of the valves and manifolds, the locations of the hard points, and the layout of the major plumbing runs. The actuators for each axial sector are plumbed with 1/16 inch tubing from single manifolds denoted MA, MB, and MC. The valves for each axial sector are denoted VA, VB, and VC. The transverse valve is denoted VT. It feeds two manifolds, MT1 and MT2. MT1 (collocated with VT) serves the back half of the mirror; MT2 serves the front half of the mirror. 2.2 Hard Points The three axial hard points are located in the triangular bays of the mirror cell, as shown in Figure 2-1. The transverse hard point is on the back of the Cassegrain hole. Each hard point assembly consists of a load cell and a Linear Variable Differential Transformer (LVDT) position sensor. The load cell is the actual position sensing device used to control the mirror; the LVDT is provided for diagnostic and monitoring purposes only. The load cells are designated XA, XB, XC, and XT; the LVDTs are designated LA, LB, LC, and LT. The load cells are strain-gage tension devices (Sensotec Model 41/5741-04-04 equipped with Sensotec Model VPV amplifiers). The output span is 0-5 volts for a load span of 0-50 psi. The mounting of each load cells to its steel post includes a spring-loaded plunger with a breakaway force of 7 lb. The plunger is normally bottomed out, but in the event of a force exceeding 7 lb it begins to compress, thus protecting the load cells from overstress. The controller establishes the nominal setpoint of the load cell at 0.5 volt--corresponding to 5 pounds of force on the hard point.





























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Figure 2-1. Layout of primary mirror cell, plan view, zenith position. Not to scale.



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Next to each load cell, an LVDT position detector (Sensotec Model S2C) is also provided for diagnostic purposes. Each LVDT has a nominal sensitivity of 6.6 V/in using the 10 V excitation provided. It is essential to understand that although the load cells are made for the purpose of transducing force, they in fact transduce displacement over a span of 0.003 inch. The forces involved in deflecting the load cells are only a few pounds--negligible compared to the weight of the mirror. Although the load cell mounting posts are referred to as the mirror mount "hard points," they offer negligible stiffness to the mounting system. On a macroscopic scale, the "hard points" determine grossly where the mirror will rest; on the scale of interest, however, they are merely very sensitive position sensors. The stiffness of the mounting system is derived from the closed-loop control of the air pistons. Open-loop, the pneumatic system is quite soft, due to the compliance of the air; closed-loop, it is infinitely stiff except under transient loads--and even then, the stiffness is such as to permit only fractions of a micron of displacement before recovery. The stiffness of the load cells is essentially irrelevant to the dynamics of the system. It may be tempting to think of the mounting system as a force control mechanism which uses the pressure in the air cylinders to relieve all but a constant small force on the load cells, which themselves provide hard position references for the mirror. This picture, however, neglects the very considerable inertia of the mirror. A disturbance will set the mirror in motion, and a compensating force must be generated to overcome that inertia. A constant force implies constant acceleration, not constant position. 2.3 Pneumatic Cylinders The air cylinders are designed for a maximum piston area with minimum volume so as to minimize the effect of air compliance. The assemblies are made with rolling membrane seals: the piston is essentially a puck sitting on top of a bladder captured inside the cylinder. This design effectively eliminates the "stiction" effect common to conventional sliding seals. This type of air piston is manufactured by the Bellofram Corporation and is often called a "Bellofram." There are three sizes of piston, each type being anodized a characteristic color. The piston dimensions and numbers used in each sector are shown in Table 2-1. All three types have approximately the same stroke (0.254 in). As shown in Table 2-1, the pressure requirement to support the mirror on the transverse axis is nearly three times that required on the axial axes. The cylinders are fed by 1/16 in Tygon tubing connecting to each cylinder by means of a 0.048-inch ID barbed fitting. Each group of axial support cylinders is fed from a single manifold close to its associated valve. The length of tubing from the manifold to each cylinder varies from a foot or so to more than 3 feet depending upon the distance of the actuator from the manifold. The T sector

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valve is mounted right next to the A sector valve and feeds two manifolds, one mounted on and serving the top half of the mirror (T1), and the other mounted on and serving the bottom half of the mirror (T2). The two transverse manifolds are connected by way of 1/8 inch tubing. Table 2-1. Pneumatic Actuator characteristics and distribution.
Piston Type Radius (i n ) Area (i n 2 ) 6.07 4.99 3.60 Axial per Sector Number Total Area (i n 2 ) 4 18 4 24.28 89.82 14.40 128.5 10.4 Transverse Number Total Area (i n 2 ) 0 0 38 0 0 136.8 136.8 29.2

Large (Black) 1.39 Medium (Blue) 1.26 Small (Red) 1.07

Total Area (in2) Max Pressure Required (psig)

2.4 Control Valves The air charge in the pneumatic cylinders of each sector is controlled by a high-bandwidth proportional valve (Dynamic Valves model PC-2). The PC-2 allows continuous air flow from its supply (pressure) port to its return (exhaust) port. A flapper between these ports controls the pressure in the valve chamber and thus the pressure to the control port. (See the spec sheet in the Appendix for a diagram.) Because the flapper has low inertia, there are no sliding surfaces, and there are no seals, the valve can achieve very high bandwidth with essentially no hysteresis or stiction. The disadvantage of the design is the requirement for a continuous flow of air through the valve--which amounts to something on the order of a half CFM for each sector. Each valve mounts to a manifold supplied by the manufacturer (Dynamic Valves P/N 55-0700-1; see the valve spec sheet in the appendix) which provides 1/8 inch NPT ports for the nylon tube fittings which connect to the supply, return, and pressure tubes. The valve and manifold assembly mounts directly to the manifold by way of a bracket which fits over the manifold mounting stud as shown in Figure 2-2. This arrangement minimizes the length of 1/8 inch
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tubing required to connect the valve to the manifold. 2.5 Pneumatic Plumbing Figure mirror 60 feet barbed 2-3 shows a schematic of the in-cell plumbing. The supply and r