Use of a zero-gravity suspension system for testing a vibration isolation system
Idle, M K
Current and next-generation, satellite-based scientific and surveillance systems require platforms with extreme inertial stability to meet their mission objectives. For these same spacecraft, mechanical disturbances result from mission critical devices such as reaction wheels, solar-array drive mechanisms, and specialized devices with moving or rotating mass, such as cryocoolers. Present methods of eliminating this jitter demand the use of a low-noise bus at a considerable cost. The requirement for higher-quality space sensors, communication devices, and astronomical telescopes is incompatible with the desire to move toward lighter and cheaper buses. An alternative solution to the highprecision bus is the isolation of the precision sensor from the rest of the satellite. This paper describes testing the gravity off-load devices used in experiments on the Vibration Isolation and Suppression System (VISS). The VISS system will isolate an optical system from spacecraft disturbances. As a quiet platform, VISS uses six hybrid isolation struts in a Stewart configuration.l The six struts provide the capability to control the payload in all degrees of freedom. The system uses a hybrid actuation concept to provide passive isolation and active control. Figure 1 shows the VISS hexapod supporting a medium-wavelength infrared (MWIR) sensor.
The six VISS struts are grouped in pairs about three launch lock towers. Isolation system performance requires very compliant struts; consequently, they must be protected during launch. The launch lock system consists of a frangibolt and heater combination at each launch lock tower. The frangibolts and heaters provide a method to control release of the payload when on-station. These space-qualified fasteners are designed to fracture when the heater encasing the bolt shank reaches a well-defined temperature.
Each frangibolt pulls a payload bracket into a seat in a launch lock tower. The payload is mechanically fastened to the three payload brackets. Two struts are attached to a common payload bracket through compliant flexures. The testing of VISS was required to validate three performance goals: 1. Providing 20 dB or more isolation above 5 Hz. 2. Suppressing the MWIR-mounted cryocooler fundamental
frequency and its first two harmonics. 3. Actively steering the payload through a predetermined profile. Each of these goals was met during system development, and the ground-based testing discussed here provided a means to measure this success. On-board instrumentation includes 12 servo-type accelerometers: six mounted at the strut bases and six on the payload brackets. Each accelerometer sensitivity axis is collinear with one of the strut lines of action in the hexapod. The payload accelerometers constitute the feedback sensor suite for the isolation, suppression, and steering control loops. In addition, this sensor suite will provide an accurate measure of the system’s performance when on-station.
Strut compliance is extremely high, as determined by the passive isolation performance requirement. Each strut has a bounce frequency of approximately 5 Hz. Consequently, the hexapod has insufficient stiffness to support the MWIR payload in a 1-g field. In the laboratory, a gravity off-load system was needed to conduct these experiments. This experiment is somewhat different than the typical space simulation because the test article is not flexible in the standard sense: the struts form a load path to ground. In addition, VISS strut stroke limitations required that the off-load device provide a method to fine-tune the “ride height” with an accuracy of several mils and a high repeatability.
The use of a zero-gravity suspension device in the VISS performance testing is described here. Control system development and testing is not the subject of this document. The Air Force Phillips Laboratory was responsible for VISS performance and flight qualification tests. Honeywell Satellite Systems and their subcontractors designed and fabricated the VISS hardware, software, and support electronics. The Defence Research Agency is responsible for design and construction of the infrared imaging telescope using composite techniques. Boeing has developed an all-composite structure that will house VISS and several experiments on the Space Technology Research Vehicle (STRV-2) mission. The Jet Propulsion Laboratory is the system integrator and developed the VISS control algorithms. TEST METHODS The VISS performance test objectives require simulating the boundary conditions that the structure will experience when on-orbit. These include approximating the boundary conditions at the base of the VISS hexapod and the free boundary condition of the MWIR payload. VISS base boundary conditions consist of the STRV-2 experimental module compliance, the satellite inertia against which VISS reacts, and the free boundary conditions of the entire satellite assembly. Each of these conditions was addressed in the test-bed design and integration. In the flight configuration, VISS is mounted on the All Composite Experimental Spacecraft Structure (ACESS), which is attached to the host satellite. This composite deck will be flexible; the fully populated deck has its first mode at about 80 Hz. The flexible base will influence the VISS mechanical performance, as the deck dynamics will be observable by the control system.
VISS performance test fixturing was originally designed to accommodate the Boeing ACESS module experimental model (EM) if it became available for use by the VISS test team. This would provide the best representation of the actual flight configuration, but delivery cannot be ensured. Thus, an aluminum plate was fabricated having a first membrane mode similar to the first mode of the ACESS module. This plate is mounted to a support ring that sits on a small optics table, which rests on four pneumatic legs. The air springbased support simulates the free boundary conditions that the satellite will experience when on-orbit. The VISS base support structure has mass and inertia properties 10 times greater than the VISS payload. One of the VISS performance test goals was to exercise the flight instrumentation whenever possible to verify ruggedness and become familiar with its limitations and strengths in the laboratory before the mission. The accelerometers integrated in the hexapod are a servo-type that provide a true DC response, a requirement driven by the hybrid strut control loops. Additionally, the transduction factors associated with these flight sensors are quite high because of the low acceleration amplitudes expected on-orbit. Because of these two factors, these sensors saturate in the 1-g field of the laboratory. A set of custom breakout boards were fabricated to AC couple the signals with very low break frequencies for groundbased testing. However, the flight data acquisition and control computer was not used extensively in this test series due to its limitations and requirement for custom software development. A large channel count modal analysis system was used instead that provided the flexibility for the variety of measurement types and ease of archiving an experimental database. Early on in the VISS test setup, it was determined that acoustic coupling to the payload would pose a limitation to the high-gain control loops. To circumvent this, a test area was constructed to acoustically isolate the VISS experiment from the rest of the laboratory. The contamination control requirement also demanded that the experiment be performed in a Class 100,000-or-better cleanroom. HEPA filtration and humidity controls were implemented in the VISS test area to adhere to the contamination control requirements.
To operate VISS in a laboratory environment, free-free boundary conditions must be simulated, as the VISS dynamics are extremely sensitive to constraint stiffness. A major challenge to the test setup design was to implement a gravity off-load system that did not significantly alter the lowfrequency operation of the VISS.
The gravity off-load system has several requirements: it must have a minimal influence of the system on VISS operation; influence of suspension-system modes must not corrupt the measured VISS dynamics or interfere with the closed-loop control; and the ride height of the system must be extremely accurate and repeatable. The suspension modal frequencies must provide good separation between the fixturing and payload suspension dynamics to limit the influence of the gravity off-load system on the VISS control loops.
The gravity off-load requirement of fine-ride height adjustment and repeatability is due to the small VISS strut stroke. The nominal payload position must be 50 mils above the “down hard” position to provide full range of motion without hitting the mechanical stops. This elevation is determined by the strut design that incorporates highly compliant bellows-based springs to passively support the MWIR payload. In addition, multiple gravity off-load systems are required because the payload center of gravity cannot be accessed directly. The light MWIR payload created a challenge to the off-load system. The suspended load per device is quite low (roughly 18 lb). Thus, any suspension system moving mass could be a proportionally high percentage of the payload mass. There are several commonly used options for off-loading test articles, including bungee (shock) cords, steel coil or leaf springs, elastomeric springs, and pneumatic springs. Each of these devices, however, has suspension stiffnesses that would adversely influence the operation of VISS. Additional disadvantages include poor static stability (i.e., creep) and a multitude of fixture modes with relatively high modal mass that would be introduced. An additional purely passive approach considered is a zero spring rate mechanism (ZSRM), which has been successfully used on another Air Force program, but with a much more massive test article. One of the most critical problems with these devices is the difficulty in maintaining the ride height of the suspension system, which is crucial to VISS operation. It is also difficult to achieve a low spring rate and to reduce friction to acceptable levels with the light VISS payload.
An active/passive gravity off-load solution has the ability to address these issues. CSA Engineering, Inc., Palo Alto, California, has developed such a system, called a zero-g suspension device (ZGS). The ZGS has been used in other laboratory experiments and is ideal for this situation because it has a low moving mass, it operates virtually friction free, and the ride height can be remotely adjusted. Typically, three or more suspension systems are used to off-load a space structure for such testing. Due to the low payload mass and small footprint, only two suspension systems were used in the VISS performance test series. Figure 2 shows an assembly drawing of one of the suspension systems utilized in the VISS performance test program. The passive portion of the suspension device consists of a frictionless piston ported to an external air tank. The active system, which does not have to be used at all times, has a voice coil actuator and two feedback sensors that are fed through an analog controller and power amplifier.
The pneumatic subsystem provides the so-called brute force to off-load the MWIR. The air piston is specially designed to be almost frictionless, with friction being less than 0.005 percent of the payload. The large external air tank provides a soft spring rate of about 0.15 Hz and thus dictates low suspension modes in the vertical direction. The payload is connected through a long load rod that reduces the horizontal suspension modes by acting as a long pendulous payload suspension. An electromagnetic actuator is used with custom-designed electronics to fine-tune the system’s characteristics. Both the linear actuator and the sensors are noncontacting, ensuring that no additional friction is introduced. A precision pneumatic regulator is used to adjust the payload ride height, which can be remotely fine-tuned through an electronic offset trim. Altering the loop gain on the control panel can control the centering stiffness, and an acceleration feedback loop, using positive feedback, can be used to reduce the suspension system apparent mass, which can be critical when suspending light payloads. So-called cable stretch modes (where the payload and zero-g suspension system moving masses are out of phase) limit the maximum gain in the mass cancellation loop. Any such dynamics will also be observed on the payload and affect the VISS closed-loop performance. For this reason, a cable with high stiffness is required to drive the cable stretch mode as high in frequency as possible. Additional damping is required to reduce the cable stretch mode quality factor (Q), thereby allowing a higher mass cancellation loop gain. In an effort to increase the modal damping in the cable stretch mode, high-loss elastomeric grommets are placed in the load path at the upper end of the load rod (i.e., where it attaches to the suspension system). While the load rod provides high axial stiffness, it must also possess a low flexural stiffness so as to decouple its bending modes from the payload and be lightweight to reduce modal mass in the rod-bending modes. Small-diameter, 3.175-cm (0.125-in.), graphite/epoxy composite rods have been used in experiments like VISS to offload flexible and lightweight space structures.
The connection of the load rods to the payloads is performed by using an extremely fine thread, 1/4-80, fastener to allow fine height adjustments of 12.5 mils per revolution. Universal joints relieve the moments reacted into the MWIR from the load rods. Two pick-off point adjustments are required, pick-off point elevation and its location along optical axis. The fine-pitch mechanical fastener ensures that the universal joint is slightly above the payload center of mass (consequently, the suspension is statically stable), and a slotted fixture allowed the pick-off point to be moved along the optical axis to statically balance the payload. This balance procedure included monitoring outputs from four noncontacting displacement sensors configured to measure the MWIR interface plate ride height at its four corners. TEST RESULTS VISS Satellite Inertia Simulator Modal Tests Structural dynamic measurements were made on two candidate setups for the satellite inertia simulator studied in the laboratory. Of particular interest was the suspension dynamics and the optical “breadboard” flexible body modes. The VISS suspension modes are approximately 5 Hz, so it was uncertain if bench suspension modes would couple into the MWIR suspension dynamics. The first approach consisted of the breadboard on four pneumatic air springs; the second configuration consisted of the bare breadboard on air isolation legs intended for machine vibration isolation. In addition, high modal damping was desired in these test fixture modes to ensure that the VISS closed-loop control loops would be immune from these dynamics.
Figure 3 shows a measured frequency response function with the breadboard supported on the four “air bags.” Both low rigid and flexible body resonant behaviors are apparent in this measurement. The flexible body dynamics observable in this measurement consist of the breadboard, low-order bending, and torsion modes.
Damping in the breadboard suspension modes was significantly higher when using the air legs than the air bags. Thus, the air legs were implemented in the actual test-bed setup. Table 1 summarizes the modal parameters of the air leg-breadboard suspension system. Suspension System Characterization Tests Prior to the flight hardware performance tests, full and half-mass mockups of the payload were constructed and tested using a single suspension system and a dual suspension system setup. The single suspension system test familiarized the test team with the device and served as a test setup debugging aid. Dual suspension system testing characterized the cable stretch and string modes, determined the zero-g suspension control loop performance, measured the effects of cable dynamics on the payload, and quantified the suspension payload dynamics. One of the first tests conducted with the dual ZGS setup is the measurement of the test-bed dynamics. Each load rod is instrumented with four uniaxial flyweight accelerometers, and triaxial accelerometers are mounted on the MWIR approximation, the satellite mockup, and the floor beneath the test bed. The measurements show high density of the load rod bending modes; however, these modes are not seen on the payload. This lack of coupling can be attributed to the low modal mass of the load rods and the use of universal joints to release the moments at the payload hang points.
The initial testing demonstrated the influence of MWIR motion on the ZGS control loops to measure the dynamics of the ZGS devices and to study their influence on the test bed. One of the VISS goals is to suppress the disturbance of a cryocooler mounted to the MWIR. It is critical to understand how this force will influence the ZGS system and, ultimately, VISS performance. A small, suspended shaker serves to simulate the cryocooler disturbance. Stationary sinusoidal shaker inputs at three frequencies (the cryocooler fundamental and its first and second harmonics) establish the ZGS displacement and mass cancellation loop stability. These measurements verified that the suspension system control loops remain stable with payload disturbance traceable to the flight cryocooler. Bandlimited random input characterizes the ZGS and payload dynamics across the 100-Hz frequency range of interest. Figure 4 shows the driving point frequency response function on the MWIR along the camera line of sight. The low-frequency resonances are those associated with the “rigid body” modes of the payload on the compliant struts. The resonance near 50 Hz is the cable stretch mode The payload and the ZGS moving mass are traveling out of phase. Mass cancellation loop stability margins are determined by the gain associated with this mode. Specially designed fixtures on the ZGS load rod attachment increased the cable stretch modal damping, thereby allowing for high gain in the mass cancellation loop. Additional measurements, using the voice coil actuator in the ZGS as the disturbance source, quantified the influence of the ZGS dynamics on the payload. Minimal interaction is required for the VISS control system development. Figure 5 shows a measured inertance function, the ratio of output acceleration to input force, between payload vertical response and the controlled disturbance at the suspension system measured in the vertical direction. This measurement was made with the previously discussed VISS mockup prior to receipt of the actual hardware. Again, the low-frequency modes are the payload suspension modes, and the highly damped resonance near 35 Hz is the cable stretch mode. This mode differs from the 50-Hz mode seen with the actual flight hardware due to inertia differences in the mockup. Short of this artifact of the test fixturing, the dynamics are those expected for a payload under true free boundary conditions.
Noise-floor measurements formed a basis for VISS closedloop vibration isolation performance. The motion of the ZGS carriages, the MWIR, the satellite simulator, and the laboratory floor were sampled and processed to determine the rms acceleration below 200 Hz. The test series included three ZGS configurations: completely passive (both the displacement and mass cancellation loops open), displacement loop closed, and both loops closed. Floor motion was approximately 40 pg rms, while the satellite simulator was about 150 tg rms. Both were measured in the vertical direction. Payload response remained consistent for each ZGS configuration, with accelerations between 20 jg rms in two directions, and 135 jig rms in the remaining axis. Performance System Testing During the course of the VISS program, several measurements were required. These include plant transfer functions, open and closed-loop system transfer functions, and transmissibility measurements. VISS closed-loop control algorithms are based on a database of accurate plant transfer functions measured throughout the performance test series. The success of the closed-loop control schemes is directly attributed to the unobtrusive behavior of the ZGS devices. Lessons Learned Once identified, the cable stretch modes were not difficult to pick out of the payload accelerometer signals. Cable stretch mode dampers attached on the ZGS lower carriage produced dynamics that were benign enough to close highperformance control loops.
Additionally, a 12-Hz signal appeared intermittently during testing. Subsequent investigation determined that this signal was due to the building air handling system. With the requirement to measure low-amplitude payload responses, the test team found that turning off the air handling unit was the only viable approach to obtain uncorrupted measurements. The ZGS devices are mechanical isolation systems between the payload and their supports, but the combination of high amplitude and a low-frequency ceiling response due to the air handlers produced measurable payload response. CONCLUSIONS The choice of these ZGS devices was the correct one for the VISS performance test series. The unique capabilities of remote (and stable) ride height adjustment, for example, proved to be a hard-and-fast requirement for this extended test series. The minimal influence of the suspension devices on VISS operation, another experimental specification, was evident with the success of the closed-loop control development. Minimal interaction of the ZGS with the VISS payload dynamics allowed the test team to concentrate on the VISS performance, and not on debugging fixture modes.
Copyright Institute of Environmental Sciences Nov/Dec 1998
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