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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 return tubes are carried to the cell via the right candy-cane. The supply (pressure) tube is 3/8 braid-reinforced vinyl about long. The return (vacuum) tube is 3/8 vinyl, also about 60 feet long. All fittings are nylon fittings, except the connectors at the pistons and the manifolds.

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2.6 Pumping Station The Pumping Station provides pressurized air to feed the valves and vacuum to exhaust them. It is located on a balcony in the mid-level under the left candy-cane. This location minimizes the length of the tubing runs into the mirror cell. Figure 2-4 shows a schematic diagram of the pumping station plumbing. The pumps and solenoid valves in the pumping station are operated by a controller mounted in the relay rack below the balcony. The controller provides both manual and automatic control of the pumps and the valves by way of a bank of solid state relays housed in a cabinet above the pumps. A schematic diagram of the relay panel is shown in Figure 2-5. Two electric rocking piston pumps (Gast model 71R647-P112-D500X) plumbed in parallel provide both pressure to the supply line and vacuum on the return line. Since the pumps are not capable of starting under load (i.e., with a differential pressure more than a few psi), a shunt valve (Parker Series 30) has been provided between the pumps to allow starting even when the system is pressurized. At the outlet of the pumps a three-way vent valve selects between exhausting to atmosphere for pumping water vapor out of the system, or discharging into the pressure receiver for normal operation. The pressure receiver smooths out pump pulsations and provides any surge air requirements (such as during response to a wind gust). The relief valve (Plasti-Valve model xxxxx) on the pressure receiver is set for 60 psig to allow exhaust of air buildup in case of a vacuum leak. The supply line is equipped with a desiccating filter and a pressure regulator. The pressure regulator sets the actual working pressure of 40 psig, a figure that provides about 10 psi of overhead above the maximum steady state pressure requirement of the transverse axis. Pressure hose from the pumps is 3/8 inch braided vinyl. The vacuum side of the pumps is connected through a vacuum receiver to the return line. About 5 inches of vacuum is required for proper operation. More vacuum will enhance the dynamic performance of the system. Air is supplied to the system from a facility air connection, regulated down to the minimum operating pressure of 45 psig (providing 5 psi of overhead for the supply line regulator). This connection provides makeup air for any leakage on the pressure side. It also charges the system correctly after initial startup.

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Figure 2-4. Pumping Station Schematic.

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2.7 Control Electronics The Mirror Controller electronics resides in a cabinet on the front of the mirror cell. The cabinet contains four controller cards (one for each sector), a power supply, and the monitor patch panel. Circuits are given in the appendix. Each controller board implements the control elements described in section 4 (and shown in the block diagram in Figure 4-2), along with the signal processing and monitoring functions described in section 3. The electronics cabinet is cooled by airflow through the cable access hole into the mirror cell. Although the power supply (Power One Model HBB-15-1.5-A) is rated for 40 W, actual total power dissipation is less than 10 W. An input fuse (3/8 AGC) is mounted next to the transformer on the power supply. 3.0 METROLOGY AND MONITORING 3.1 General Description 3.2 LVDTs Each hardpoint includes an LVDT that senses displacements of the mirror at the position of the hard point, thus providing an indication of proper positioning and of dynamic response. The LVDTs are Sensotec Model S2C, DC-DC units with spring return plungers. The axial LVDTs bear directly on the back plate of the mirror by way of a plunger mounted in the front face of the mirror cell. The transverse LVDT bears on the edge of the Cassegrain hole in the mirror back plate. The LVDTs are aligned with the centers of the each sector (with the load cells slightly off center). Excitation for the LVDTs is ±5 VDC for a total span of 10 VDC. With this excitation, the sensitivity of the LVDT is nominally 6.6 V/inch. [Accurate calibration data for the LVDTs is not available.] The first stage of amplification has a fixed gain of ten, with an adjustable zero offset to allow for very sensitive displacement measurements. The second stage of LVDT amplification has a switchsettable gain of either unity or ten. Consequently the LVDT Gain Switch (S3, located near the center of the circuit board) sets the overall gain at either X10 (low gain) or X100 (high gain). In order to reduce noise and to eliminate residual chopping frequency feedthrough, the bandwidth of the LVDT amplifier is limited to 160 Hz with a single-pole filter. To reduce common-mode noise pickup, the LVDT is excited with a bipolar excitation supply, and the first amplifier couples differentially to the LVDT signal leads. Normally the LVDT Gain should be set at X10 (low) and the LVDT Zero pot should be set for zero reference (as measured at U8-14). This setting can always be reproduced and allows monitoring of

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the nominal LVDT position. For sensitive dynamic measurements, the LVDT Gain can be set to X100 (high). In this condition, the nominal LVDT output may saturate the amplifier, so the LVDT Zero pot must be readjusted for zero output. 3.3 Proximity Sensors Axial rotation of the mirror in the mirror cell is prevented by two struts that attach to the mirror near the front and rear edges of its back face. Adjustment of the struts is used to position the mirror laterally and rotationally. Lateral and transverse positioning can be measured conveniently using a micrometer depth gauge in the gauging ports provided in the Cassegrain hole for this purpose, but accurate measurement of rotational positioning is impossible by manual means. Consequently a pair of inductive proximity sensors has been provided sensing of the mirror axial rotation. The sensors are mounted inside diametrically opposite honeycomb cells on the lateral axis of the mirror. The difference between the left and right prox sensors is an indication of mirror rotational displacement. [No technical information available on the sensors or their mounting.] The controller cards do not include support circuitry for the proximity sensors. 3.4 Gauging Ports The inside of the Cassegrain ring on the mirror cell is fitted with four ports that provide a firm gauging surface and an access hole to the inside edge of the mirror backplate. A micrometer depth gauge can be placed on each port to measure the position of the two transverse directions, thus indicating its centering relative to the mirror cell. The transverse hard point equipment is normally mounted in the top Gauging Port. 3.5 The Monitor Connectors The main connector panel in the controller cabinet includes four DB-25S connectors that provide access to the eight monitor signals from each of the
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PMSS Monitor Jack. DB-25S, Front View.
1 2 3 4 5 6 7 8 9 0 1 2 3 14 15 16 17 18 19 20 21 22 23 24 25 DGND

Press Err Corr Press Pressure Press Cmd Position Err Position LVDT Posn Valve Curr

VPE VPD VPX VPC VE VX VL IV Bit 0

1 1 1 1

AGND AGND AGND AGND AGND AGND AGND AGND Bit 1

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controller cards. Figure 3-1 shows the pinout for these connectors. Table 3-1 summarizes the characteristics of each monitor channel. The connector panel is electrically connected to the frame of the telescope, but signal ground is isolated from the frame. Therefore, do not use the shell of the DB-25 connector as a ground, and do not carry shield ground to the connector shell. The signal outputs on the Monitor connector are all driven from low impedance op amp outputs. Although maximum output voltage is ±11 V, the signals are scaled for a ±5 V input data acquisition system. Table 3-1. Monitor Channels.
Monitor Signal Location (DB-25) 5/17 6/18 7/19 Scaling Nominal Value

VPC VE VX VL

Pressure Command Position Error Load Cell Position LVDT Position

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19.6 mA/V

4.0 THEORY OF OPERATION 4.1 Plant Dynamics 4.1.1 Mirror Dynamics Figure 4-1 shows schematically the arrangement of a single piston and the pneumatic system supplying air to it. Each piston supports a mass m corresponding to its share of the total mass of the mirror. For the axial pistons m is approximately 1/78th of 1800 kg, or 23 kg. Although the inertia of the mirror segment supported remains constant, the weight supported scales as the cosine of the telescope elevation angle . At any given moment, the cylinder contains n moles of air and develops a pressure P which, acting on the cylinder area A = 32 cm2, produces a force which counters the supported weight and provides any acceleration required. In addition, it must also counter the

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Pressure Error Corrected Pressure Absolute Pressure

+2.0 psi/V -2.0 psi/V (VPX-1) 10.0 psi/V

0 V (0 psi) Approx 0 V at 45 deg Axial: 3.04 - 2.15 V 20.4 - 11.5 psia Xvrse: 2.04 - 4.92 V 10.4 - 39.2 psia Approx 0 V at 45 deg 0 V (0 u m ) 0.52 V (7.9 um) A: B: C: T: 0 - 5 V (0 - 100 mA)

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atmospheric pressure, PA, operating on the opposite side of the piston. At the observatory's elevation of 9,200 feet, PA = 71.86 kPa. Nominally, the cylinder contains n0 moles of air and the piston floats a distance l0 from the bottom of the cylinder. The position x of the mirror is thus measured relative to the nominal cylinder depth. The mirror position is transduced by a load cell with a position sensitivity kx. When x is zero (actual cylinder depth is l0), the load cell