Alexander Veprik

Vibration control of linear split Stirling cryogenic cooler for airborne infrared application

Modern infrared imagers often rely on the split Stirling cryogenic coolers the linear compressors of which are the well-known sources of harmonic disturbance. The traditional method of their passive isolation fails to meet the restraints on the static and dynamic deflections which are originated by the combined action of the airborne g-loading and harsh random vibration.

Vibration protection of sensitive components of infrared equipment in harsh environments

This article addresses the principles of optimal vibration protection of the internal sensitive components of infrared equipment from harsh environmental vibration. The authors have developed an approach to the design of external vibration isolators with properties to minimise the vibration-induced line-of-sight jitter which is caused by the relative deflection of the infrared sensor and the optic system, subject to strict constraints on the allowable sway space of the entire electro-optic package. In this approach, the package itself is used as the first-level vibration isolation stage relative to the internal highly responsive components.

Split Stirling linear cryogenic cooler for a new generation of high temperature infrared imagers

Split linear cryocoolers find use in a variety of infrared equipment installed in airborne, heliborne, marine and vehicular platforms along with hand held and ground fixed applications. An upcoming generation of portable, high-definition night vision imagers will rely on the high-temperature infrared detectors, operating at elevated temperatures, ranging from 95K to 200K, while being able to show the performance indices comparable with these of their traditional 77K competitors. Recent technological advances in industrial development of such high-temperature detectors initialized attempts for developing compact split Stirling linear cryogenic coolers. Their known advantages, as compared to the rotary integral coolers, are superior flexibility in the system packaging, constant and relatively high driving frequency, lower wideband vibration export, unsurpassed reliability and aural stealth. Unfortunately, such off-the-shelf available linear cryogenic coolers still cannot compete with rotary integral rivals in terms of size, weight and power consumption. Ricor developed the smallest in the range, 1W@95K, linear split Stirling cryogenic cooler for demanding infrared applications, where power consumption, compactness, vibration, aural noise and ownership costs are of concern.

Low vibration microminiature split Stirling cryogenic cooler for infrared aerospace applications

The operation of the thermo-mechanical unit of a cryogenic cooler may originate a resonant excitation of the spacecraft frame, optical bench or components of the optical train. This may result in degraded functionality of the inherently vibration sensitive space-borne infrared imager directly associated with the cooler or neighboring instrumentation typically requiring a quiet micro-g environment. The best practice for controlling cooler induced vibration relies on the principle of active momentum cancellation. In particular, the pressure wave generator typically contains two oppositely actuated piston compressors, while the single piston expander is counterbalanced by an auxiliary active counter-balancer. Active vibration cancellation is supervised by a dedicated DSP feed-forward controller, where the error signals are delivered by the vibration sensors (accelerometers or load cells). This can result in oversized, overweight and overpriced cryogenic coolers with degraded electromechanical performance and impaired reliability. The authors are advocating a reliable, compact, cost and power saving approach capitalizing on the combined application of a passive tuned dynamic absorber and a low frequency vibration isolator. This concept appears to be especially suitable for low budget missions involving mini and micro satellites, where price, size, weight and power consumption are of concern. The authors reveal the results of theoretical study and experimentation on the attainable performance using a fullscale technology demonstrator relying on a Ricor model K527 tactical split Stirling cryogenic cooler. The theoretical predictions are in fair agreement with the experimental data. From experimentation, the residual vibration export is quite suitable for demanding wide range of aerospace applications. The authors give practical recommendations on heatsinking and further maximizing performance.

Distributed Absorber for Noise and Vibration Control

An approach to a wide-band frequency passive vibration attenuation is introduced in this paper. This aims to suppress noise and vibration of extended multimode objects like plates, panels and shells. The absorber is arranged in the form of a single-layer assembly of small inertial bodies (balls) being distributed and moulded within the light visco-elastic media (e.g. silicone resin). The absorber as a whole is embedded into object face covering the critical patches of the system surface. For the purpose of characterization, the authors introduced the complex frequency response function relating the volume velocity produced by the vibrating object surface (response) stimulated by a point-wise force (stimulus) applied to a particular point. The simulation and optimization of the main frequency characteristics has been performed using a full scale 3-dimensional Finite Element model. These revealed some new dynamic features of absorbers structures, which can contribute to vibration attenuation. A full-scale physical experimentation with synthesised absorbers structures confirmed the main results of simulation and has shown significant noise reduction over a staggering 0–20 kHz frequency band. This was achieved with a negligible weight and volume penalty due to the addition of the absorber. The results can find multiple applications in noise and vibration control of different structures. Some examples of such applications are presented.

Life test result of Ricor K529N 1watt linear cryocooler

The authors summarize the results of the accelerated life testing of the Ricor type K529N 1 Watt linear split Stirling cooler. The test was conducted in the period 2003-2006, during which the cooler accumulated in excess of 27,500 working hours at an elevated ambient temperature, which is equivalent to 45,000 hours at normal ambient conditions, and performed about 7,500 operational cycles including cooldown and steady-state phases. The cryocooler performances were assessed through the cooldown time and power consumption; no visible degradation in performances was observed. After the cooler failure and the compressor disassembling, an electrical short was discovered in the driving coil. The analysis has shown that the wire insulating varnish was not suitable for such elevated temperatures. It is important to note that the cooler under test was taken from the earliest engineering series; in the later manufacturing line military grade wire with high temperature insulation was used, no customer complaints have been recorded in this instance Special attention was paid to the thorough examination of the technical condition of the critical components of the cooler interior. In particular, dynamic piston-cylinder seal, flying leads, internal O-rings and driving coil were examined in the compressor. As to the cold head, we focused on studying the conditions of the dynamic bushing-plunger seal, O-rings and displacer-regenerator. In addition, a leak test was performed to assess the condition of the metallic crushed seals. From the analysis, the authors draw the conclusion that the cooler design is adequate for long life performance (in excess of 20,000 working hours) applications.

Virtual accelerometer for sensorless overstroking control in a linear compressor of a cryogenic cooler

Overstroking in a linearly driven piston compressor of a cryogenic cooler may occur as a result of numerous factors, among which are: uncontrollable leaking of the working fluid from the compression into the bounce space, magnitude and asymmetry of the driving voltage, action of the gravity and g-forces, etc. The known solutions to a passive and active overstroking control are rather expensive, bulky and not always reliable. The authors report on the development of the novel approach to an overstroking protection, where instead of the piston position, the piston acceleration is evaluated using the analytical model of the linear actuator and a real-time measurement of the motor voltage and current, thus enabling for a convenient sensorless estimation of a direction and magnitude of the internal collisions. This eliminates the need in the dedicated position sensor and allows obtaining an observer which is insensitive to the environmental conditions (shock, vibration and temperature) and to the inaccuracies inherent in the regular sensorless observers. Based on this approach a full-scale controller was developed using the National Instrument real-time engine PCI 7030/6040E operating under the LabView software. From experimentation, the overstroking was practically eliminated in the wide range of the operational conditions without affecting the overall power consumption and cooling performance.

Dynamically counterbalanced pneumatically driven expander of a split Stirling cryogenic cooler

Low vibration Stirling cryocoolers, which find use in numerous vibration sensitive electronic and electro-optic applications, typically comprise a dual-piston linear compressor and a pneumatically driven expander. While such compressors have inherently low level of vibration export, the unbalanced motion of the displacer-regenerator of the traditional expander inevitably leads to an essential vibration export into the supporting structure to which the cryogenic cooler is normally rigidly attached. The authors report on the novel approach to a passive cancellation of vibration export from a pneumatically driven displacer of a split Stirling cryogenic cooler. This patent pending technique relies on the principle of dynamic counterbalancing, where an auxiliary movable mass is flexibly attached to a hot part of the movable displacer-regenerator assembly and to the stationary expander casing using two auxiliary mechanical low-damped springs. A theoretical analysis yields the simple condition of canceling the fundamental component of vibration export at the same power consumption and cooling performance. The authors successfully attempted to redesign the existing expander of the Stirling Ricor model K535 cryocooler, where the vibration export at the driving frequency was reduced 150-fold under typical thermal loading at the same power consumption.

Technical diagnostics of linear split Stirling cryocooler through the analysis of self-induced forces

The effective operation of a linear split Stirling cryogenic cooler with pneumatically driven expander relies primarily on the matched sinusoidal motion of the compressor piston and displacer. The measurement of magnitudes and relative phase shift is required not only at cryocooler development and refinement, but also throughout the manufacturing process for the purposes of on-line diagnostics. Presently practiced methods of motion measurement are based on using different sensors, which are typically mounted directly on the moving parts inside the hermetic gas space. The technical problems associated with the application of these methods are evident. The authors present a novel approach to the indirect measurement of the motion of the internal components of the linear split Stirling cryocoolers through the measurement of their self-induced forces. This approach is based on the simple fact that self-induced forces developed both by the linear compressor and expander are proportional to the accelerations of...

Suppression of cryocooler induced microphonics in infrared imagers

Commercially available inexpensive and rugged miniature Stirling cryocoolers are frequently used in sophisticated infrared imagers. Such coolers are well known sources of high-frequency interference contaminating the image signal and causing essential degradation in the overall imager performance. This phenomenon is generally referred to as microphonics, which mostly manifests itself at the resonant frequencies of sensitive mechanical components of infrared packages. The ruggedizing of these components involves stiffening and dampening which are, evidently, not the best solutions for implementation inside, say, an evacuated envelope, where the issues of added heat load, thermal mass, ageing, vacuum contamination due to outgassing are of the primary concern. The paper examines the idea of using externally mounted wideband dynamic absorbers for suppressing the above resonant responses and microphonic noise. The analytical model suggested relies on a set of complex frequency response functions experimentally measured on the original infrared package. The optimised dynamic absorber yields about a tenfold reduction in the amplification ratio at typical resonant frequencies and a threefold reduction in the overall rms level of microphonic noise. The results of the analytical calculation are in fair agreement with experimental data.