Patent Description:
Cryogenic refrigeration systems are typically used to cool other devices to low temperatures between around <NUM> and around <NUM>, for example, depending on an overall heat load presented by a particular device. Cryogenic refrigeration systems may be, or may be referred to as, cryocoolers. Such cooled devices are often one of a variety of different types of sensor systems that operate better (e.g., produce measurements with less noise, higher sensitivity, higher accuracy, higher responsiveness, and/or with other generally more desirable performance metrics) when cooled and/or otherwise unable to operate without being cooled. For example, one such category of sensor systems that can benefit from being cooled includes infrared cameras (e.g., including a focal plane array (FPA) of individual infrared sensors), which measure or capture infrared (e.g., thermal) emissions from objects as infrared/thermal images and/or video. Cooling such infrared cameras generally increases detector sensitivity (e.g., by decreasing thermal noise intrinsic to the individual infrared sensors), which can result in overall more accurate and reliable infrared imagery.

In some cases, cryocoolers for use with infrared cameras can be quite small (e.g., designed to fit within a volume of approximately <NUM>×<NUM>×<NUM> inches, or less), yet be able to provide sufficient cooling power (e.g., a measure, typically in Watts, of a refrigerator's ability to extract heat from a coupled device) to cool at least portions of an infrared camera to the range of temperatures desired for, for example, relatively low noise thermal imagery, while experiencing the thermal load typical of an operating infrared camera. Reductions in system size and weight can be helpful to facilitate various compact system applications, including integration with a flight platform, an unmanned aerial vehicle (UAV), as a handheld weapon sight, and as a handheld camera, for example. <CIT> discloses monitoring the health of a cryocooler including monitoring physical properties of the cryocooler to obtain failure precursor parameters that indicate cryocooler health. A health fingerprint of the cryocooler is accessed. The health fingerprint associates the failure precursor parameters with a health level of the cryocooler. The health of the cryocooler is estimated in accordance with the health level.

According to a first aspect of the present invention, there is provided a method as defined in claim <NUM>. It includes determining, for each setpoint temperature of a plurality of setpoint temperatures, a respective power applied to a cryocooler to set a cold tip of the cryocooler to the setpoint temperature. The method further includes determining a first load line associated with the cold tip based on the plurality of setpoint temperatures and the respective powers applied to the cryocooler. The method further includes determining a health metric associated with the cold tip based on the first load line and a reference load line associated with the cryocooler.

According to a second aspect, there is provided a refrigeration system as defined in claim <NUM>. It includes a cryocooler including a cold tip. The refrigeration system further includes a processing circuit configured to determine, for each setpoint temperature of a plurality of setpoint temperatures, a respective power applied to the cryocooler to set the cold tip to the setpoint temperature. The processing circuit is further configured to determine a first load line associated with the cold tip based on the plurality of setpoint temperatures and the respective powers applied to the cryocooler. The processing circuit is further configured to determine a health metric associated with the cold tip based on the first load line and a reference load line associated with the cryocooler.

It is noted that sizes of various components and distances between these components are not drawn to scale in the figures.

However, it will be clear and apparent to those skilled in the art that the subject technology is not limited to the specific details set forth herein and may be practiced using one or more embodiments. In one or more instances, structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology. One or more embodiments of the subject invention are illustrated by and/or described in connection with one or more figures and are set forth in the claims.

Various systems and methods are provided for monitoring/assessing health of cryocoolers using variable cold tip temperatures. The cold tip of a cryocooler interfaces with and cools an electronic device, such as an FPA in a cooled camera. In some embodiments, a health assessment of a cryocooler may be based on load lines that relate applied powers (e.g., applied compressor input powers) to cold tip temperatures. The load lines may include reference load lines of the cryocooler and load lines of the cryocooler measured in the field (e.g., by a user). In an aspect, a reference load line may be a beginning of life load line generated/determined in the factory for the cryocooler. In some cases, the health assessment may further be based on measured life test data (e.g., determined in the factory for a given cryocooler design/model/implementation). The health assessment may be user-initiated and/or may be a built-in test provided by a refrigeration system that includes the cryocooler. In some cases, techniques utilized for health monitoring may also be used for performance screening of the cryocoolers.

Such a health assessment may be used at a beginning of life (e.g., in production, during manufacturing) to verify cooler performance and/or used later to determine a remaining useful life of the cryocooler. In this regard, the health of a given cryocooler may be characterized using a remaining cryocooler life. The remaining cryocooler life may be provided as an estimated amount of remaining cryocooler life (e.g., in hours), an estimated maximum power dissipation, an expected percent of life remaining bar (e.g., based on a nominal life of the cryocooler), and/or other health metrics. An indication of a result of the health assessment, such as an indication of the remaining cryocooler life, may be provided to the end users (e.g., audibly and/or visually). The end users may determine whether to perform an action (e.g., perform maintenance on a cryocooler, replace a cryocooler) based on the health assessment.

In one example, cryocoolers may be used to cool infrared sensors. Cooled infrared sensors (e.g., infrared sensors cooled using cryocoolers) may be deployed in applications in which predicting an end of life prior to failure is of particular interest to end users. For a cooled camera that includes cooled infrared sensors, the cryocooler lifetime generally determines a reliability and time before service of the camera. Capability to predict an end of life of the cryocooler prior to failure of the cryocooler may allow users to avoid mission failures, sensor downtime (e.g., due to cryocooler failure), and/or unplanned maintenance by scheduling sensor replacement and/or cryocooler replacement prior to failure.

In various embodiments, features/components to monitor the cryocooler health, such as setpoint temperature control and power measurements, are generally already included in a refrigeration system (e.g., a cryocooler controller and/or camera electronics) that includes the cryocooler. In this regard, such sensing capability is generally available to cryocoolers, such as power measurement features of cryocooler drive electronics, and are thus applicable, for example, to almost any tactical Stirling cryocooler. In some cases, baseline load line characteristics may be stored in persistent memory (e.g., flash memory) on-board of a cooled system (e.g., infrared camera core, cooler controller). The baseline load line characteristics may be stored at the beginning of the cooler lifetime in the factory. A user accessible feature, such as a user interface, may be provided to initiate an automated routine or built-in test (BIT) to recharacterize and store a load line of the cooler at any point when fielded. In an aspect, a programmable reference lookup table that correlates estimated remaining cooler hours and a difference between load lines in the field relative to baseline data may allow health metrics to be output to a user. By way of non-limiting examples, the health metric may be provided as an estimated amount of remaining cryocooler life (e.g., in hours), an estimated maximum power dissipation, an expected percent of life remaining bar (e.g., based on a nominal life of the cryocooler), and/or other health metrics.

Referring now to the drawings, <FIG> illustrates a block diagram of a refrigeration system <NUM> in accordance with one or more embodiments of the present invention. The refrigeration system <NUM> includes a power supply <NUM> providing an input power signal over power leads <NUM> to a cryocooler controller <NUM>, which then provides motor drive signals and/or other system drive signals over power leads <NUM> to drive a compressor/motor <NUM>. and/or other elements of a cryocooler <NUM>. In general, the cryocooler <NUM> operates to cool a cold finger <NUM>, which is thermally coupled to and configured to cool/extract heat from at least a portion (e.g., an FPA <NUM>) of a camera <NUM><NUM>. The cryocooler controller <NUM> may be configured to receive various sensor signals (e.g., corresponding to an input voltage of the input power signal provided by the power supply <NUM>, an output voltage of motor drive signals generated by a motor driver <NUM>/cryocooler controller <NUM>, temperatures of various components of the refrigeration system <NUM> measured by temperature sensors <NUM>, and/or other sensor signals corresponding to operation of the cryocooler <NUM>, the compressor <NUM>, and/or other elements of the refrigeration system <NUM>) as feedback of operation of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM>, and to adjust drive signals provided to the compressor <NUM> and/or other elements of the cryocooler <NUM> accordingly (e.g., so as to provide a stable and/or desired temperature and/or cooling power with relatively little mechanical vibration at the cold finger <NUM>). As one example, the cryocooler <NUM> may be implemented using a split design with separate compressor and expander modules. An example temperature range to operate cold tip temperatures may be approximately <NUM> to <NUM>.

It is noted that although the cryocooler <NUM> of the refrigeration system <NUM> of <FIG> cools the camera <NUM>, the cryocooler <NUM> of <FIG> may be used to cool other electronic devices. In this regard, the camera <NUM> may instead be any device (e.g., sensor, imaging device, or other device type) that operates better (e.g., with higher signal to noise operational characteristics and/or with higher performance according to other performance metrics) when cooled, for example, or that is otherwise unable to operate without cooling.

A user interface <NUM> may be implemented as a personal computer, a tablet, a smart phone, a mobile computing device and/or vehicle interface, and/or one or more of a display, a touch screen, a keyboard, a mouse, a joystick, a knob, a button, a switch, and/or any other device capable of accepting user input and/or providing feedback to a user. More generally, the user interface <NUM> may be configured to provide user-level control of the refrigeration system <NUM> and to provide operational feedback to a user of the refrigeration system <NUM>. In an embodiment, the operational feedback may include an indication of a health metric. In an aspect, a user may provide an input via the user interface <NUM> to initiate a test (e.g., built-in test) of the cryocooler <NUM> (e.g., to determine a health metric of the cryocooler <NUM>) and/or other components of the refrigeration system <NUM>,.

The user interface <NUM> may be integrated with any appropriate logic device (e.g., processing device, microcontroller, processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), memory storage device, memory reader, or other device or combinations of devices) that may be adapted to execute, store, and/or receive appropriate instructions, such as software instructions implementing a control loop for controlling various operations of the refrigeration system <NUM>. In addition, the user interface <NUM> may include a machine readable medium provided for storing non-transitory instructions for loading into and execution by the user interface <NUM>. In these and other embodiments, the user interface <NUM> may be implemented with other components where appropriate, such as volatile memory, non-volatile memory, and/or various analog and/or digital components for interfacing with devices of the refrigeration system <NUM>.

In various embodiments, the user interface <NUM> may be configured to provide an initialization signal to the cryocooler controller <NUM> to begin operation of the cryocooler <NUM>, for example, or to provide a temperature setpoint and/or other operational parameters (e.g., corresponding to a desired operational state of the cryocooler <NUM>) to the cryocooler controller <NUM>. In specific embodiments, the user interface <NUM> may be configured to provide and/or update configuration data, including logic-level configuration data, to the cryocooler controller <NUM> to facilitate control of operation of the cryocooler <NUM>. The user interface <NUM> may also be configured to receive an operating temperature, a power draw, an efficiency, and/or other operating characteristic and/or measured feedback of operation of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM> (e.g., from the cryocooler controller <NUM> and/or other elements of the refrigeration system <NUM>) and provide such information for display or indication to a user. In some embodiments, the user interface <NUM> may be configured to receive infrared images captured by the camera <NUM> (e.g., over data leads <NUM>) and provide the infrared images for display to a user.

The power supply <NUM> may be implemented as a battery, a solar cell, a mechanical generator, and/or other power generator, and/or a delivery device, which may be provided specifically to power the refrigeration system <NUM>, for example, and/or be coupled to, integrated with, or generated as part of the operation of a separate platform, such as a sensor, a vehicle, an aircraft, a watercraft, or other fixed or mobile platform. In some embodiments, the power supply <NUM> may be configured to provide an input direct current (DC) power signal over the power leads <NUM>, such as a <NUM> V, <NUM> V, <NUM> V, or other voltage level DC power signal. More generally, the power supply <NUM> may be configured to provide any type of input power signal over the power leads <NUM> that can be converted by the cryocooler controller <NUM> into motor drive signals and/or other drive signals appropriate to drive the compressor <NUM> and/or other elements of the cryocooler <NUM>.

The cryocooler controller <NUM> includes a motor driver controller <NUM>, a feedback interface <NUM>, a motor driver <NUM>, and optional other modules <NUM>. In additional embodiments, such as where the cryocooler <NUM> includes multiple motors and/or compressors, the cryocooler controller <NUM> may be implemented with multiple motor drivers, for example, that may each be controlled independently by motor driver control signals generated by the motor driver controller <NUM>. In some cases, the cryocooler controller <NUM> may include a temperature control system to regulate a temperature (e.g., the cold tip temperature) of the cryocooler <NUM>. In an aspect, the temperature control system may include and/or have access to memory that stores setpoint temperatures of the cryocooler <NUM>.

The motor driver controller <NUM> may be implemented as any appropriate logic device (e.g., processing device, microcontroller, processor, ASIC, FPGA, memory storage device, memory reader, or other device or combination of devices) that may be adapted to execute, store, and/or receive appropriate instructions, such as software instructions implementing a control loop for controlling various operations of the cryocooler <NUM> and/or other components of the refrigeration system <NUM>. For example, the motor driver controller <NUM> may be configured to receive operational parameters corresponding to operation of the cryocooler <NUM> and generate motor driver control signals configured to control operation of the motor driver <NUM> based, at least in part, on the received operational parameters.

in addition, the motor driver controller <NUM> may include a machine readable medium provided for storing data and/or non-transitory instructions for loading into and execution by the motor driver controller <NUM>. In these and other embodiments, the motor driver controller <NUM> may be implemented with other components where appropriate, such as volatile memory, non-volatile memory, and/or various analog and/or digital components for interfacing with devices of the refrigeration system <NUM>. In a particular embodiment, the motor driver controller <NUM> may be implemented substantially entirely by a programmable logic device (PLD), such as an FPGA, which may be configured to implement (e.g., using programmable resources) and perform any of the methods described herein. In such embodiments, the user interface <NUM> may be configured to provide/update configuration data over the data leads <NUM> to the motor driver controller <NUM> that is configured to implement/update/modify such methods in programmable resources and/or other elements of the motor driver controller <NUM>.

The motor driver <NUM> may be implemented by one or more electrical components, such as various electrically controllable switches/transistors, an inductor, and a capacitor, that are configured to receive motor drive control signals and/or other drive signals from the motor driver controller <NUM> and to generate drive signals based, at least in part, on the motor driver control signals and/or the other drive signals, to drive the compressor <NUM><NUM> and/or other elements of the cryocooler <NUM>.

The feedback interface <NUM> may be implemented by one or more of a multichannel analog to digital converter, a temperature sensor, a digital communication interface, and/or other electrical or electronic components configured to receive and/or measure sensor signals corresponding to operation of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM> (e.g., over sensor leads <NUM>) and convert such sensor signals into corresponding feedback data indicative of an operational state of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM>. The feedback interface <NUM> may be configured to provide such feedback data to the motor driver controller <NUM> to help adjust operation of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM> according to various desired operational characteristics or states of the cryocooler <NUM> and/or other elements of the refrigeration system <NUM>.

For example, the feedback interface <NUM> may be configured to receive one or more sensor signals (e.g., from the temperature sensor <NUM>) and generate feedback data corresponding to operation of the cryocooler <NUM>, and the motor driver controller <NUM> may be configured to receive the feedback data from the feedback interface <NUM> and generate motor driver control signals and/or other drive signals based, at least in part, on the feedback data. In some embodiments, one or more of the temperature sensors <NUM> may be implemented as diodes with characteristic voltage/temperature responses. The feedback interface <NUM> may be configured to provide a reference current to a diode and to measure/digitize the resulting voltage developed across the diode, which is proportional to the temperature of the temperature sensor <NUM>. In some cases, such diodes may be integrated with the FPA <NUM> of the camera <NUM>, for example, allowing direct and precise measurement and feedback of a temperature of the FPA <NUM>.

In some embodiments, the one or more sensor signals received by the feedback interface <NUM> may include a measured temperature of the cold finger <NUM> of the cryocooler <NUM> and/or the camera <NUM> thermally coupled to the cryocooler <NUM> (e.g., via a thermal interface <NUM>). Corresponding feedback data may be provided to the motor driver controller <NUM>, which may be configured to determine a feedback error based, at least in part, on a set point corresponding to a desired temperature for the cold finger <NUM> and/or the camera <NUM> and the received feedback data. In such embodiments, the motor driver controller <NUM> may be configured to generate motor driver control signals based, at least in part, on the determined feedback error.

In other embodiments, the one or more sensor signals received by the feedback interface <NUM> may include a measured vibration amplitude of the cold finger <NUM> of the cryocooler <NUM> and/or the camera <NUM> thermally coupled to the cryocooler <NUM> (e.g., via the thermal interface <NUM>). Corresponding feedback data may be provided to the motor driver controller <NUM>, which may be configured to determine a constant or time varying amplitude, phase, and/or other drive signal characteristic based, at least in part, on a desired maximum vibration amplitude for the cold finger <NUM> and/or the camera <NUM> and the received feedback data. In such embodiments, the motor driver controller <NUM> may be configured to generate driver control signals based, at least in part, on the determined feedback error. Optional other modules <NUM> may include various power, digital, and/or analog signal interfaces, sensors, and/or additional circuitry configured to facilitate operation of any element of the cryocooler controller <NUM>.

The cryocooler <NUM> may be implemented as any cooler or refrigeration system configured to convert electrical power delivered over the power leads <NUM> to the compressor <NUM> into cooling power generated by an expander/refrigerator <NUM> at the cold finger <NUM>. In some embodiments, the cryocooler <NUM> may be implemented as a Stirling refrigerator. As shown in <FIG>, the cryocooler <NUM> may include one or more temperature sensors <NUM> configured to provide sensor signals indicative of a measured temperature of a corresponding element of the cryocooler <NUM> (e.g., of the compressor <NUM>, for fault detection, or of the cold finger <NUM>, for operating temperature feedback) to the feedback interface <NUM> of the cryocooler controller <NUM>. Optional other modules <NUM> may include additional temperature or electrical signal sensors, vibration sensors, various mechanical or thermal linkages, dewar cavities, working gas reservoirs, and/or other mechanical or electrical components or sensors configured to facilitate operation of any element of the cryocooler <NUM> and/or provide additional operational feedback to the cryocooler controller <NUM>.

The cryocooler <NUM> may be thermally coupled to the camera <NUM> via the thermal interface <NUM>. For example, the thermal interface <NUM> may be implemented by thermal grease, thermal tape, copper or aluminum plate or film, and/or other materials and/or structures configured to provide a reliable and highly thermally conductive link between the cryocooler <NUM> and at least a portion of the camera <NUM>.

For example, the camera <NUM> may include an infrared imaging sensor implemented as an FPA <NUM>, which may be coupled to optics <NUM> and be configured to image infrared radiation (e.g., including thermal radiation) emitted from a scene in view of the optics <NUM>. In some embodiments, the cryocooler <NUM> may be directly coupled (e.g., via the thermal interface <NUM>) to a sensor (e.g., the FPA <NUM>) of the camera <NUM> and primarily be configured to cool such a sensor. In other embodiments, the cryocooler <NUM> may be coupled to various elements of the camera <NUM> (e.g., the optics <NUM>, camera body <NUM>, and/or other modules <NUM>) and be configured to cool such various elements to help increase performance of the camera <NUM>.

The camera <NUM> may include one or more temperature sensors <NUM> configured to provide sensor signals indicative of a measured temperature of a corresponding element of the camera <NUM> (e.g., of the FPA <NUM>, for operating temperature feedback) to the feedback interface <NUM> of the cryocooler controller <NUM>. Optional other modules <NUM> may include additional temperature or electrical signal sensors, FPAs of sensors sensitive to different spectra (e.g., visible light), other optical elements, and/or other mechanical or electrical components or sensors configured to facilitate operation of any element of the camera <NUM> and/or provide additional operational feedback to the cryocooler controller <NUM>.

Also shown in <FIG> is optional other modules <NUM> of the refrigeration system <NUM> coupled to the user interface <NUM> over the data leads <NUM> and to other elements of the refrigeration system <NUM> over leads <NUM>. Other modules <NUM> may include additional sensors, additional temperature or electrical signal sensors, an actuated gimbal and associated control subsystem to aim the camera <NUM> according to a desired direction, an accelerometer, a gyroscope, a global navigation satellite system receiver, a compass, other orientation and/or position sensors, vibration sensors, thermal management subsystems, structural support, thermal and/or electrical shielding, and/or other mechanical or electrical components or sensors configured to facilitate operation of any element of the refrigeration system <NUM> and/or provide additional operational feedback to the cryocooler controller <NUM>.

<FIG> illustrates a block diagram of a split-pair Stirling refrigerator/cryocooler <NUM> including a non-integrated cylindrical linear compressor/motor 172B in accordance with one or more embodiments of the present disclosure. <FIG> illustrates a perspective view of the split-pair Stirling refrigerator/cryocooler <NUM>. In the embodiment shown in <FIG>, the cryocooler <NUM> includes the non-integrated cylindrical linear compressor/motor 172B adjacent to and in fluid communication with a refrigerator/expander <NUM> via a gas transfer line/tube <NUM>. In general operation, the compressor/motor 172B may be energized by the motor driver <NUM> to compress working gas within a compression space (e.g., between pistons <NUM>) and deliver a compression wave/mass flow of working gas through the gas transfer line <NUM> to the expander/refrigerator <NUM>. Heat in the working gas generated at least in part by the compression is extracted at the motor/compressor 172B and dissipated into the environment, rather than injected into the expander <NUM>.

The compression wave/mass flow causes a regenerator/displacer <NUM> to move towards the cold finger <NUM> and through inductive windings <NUM> within an expander cylinder head <NUM>, and at least a portion of the working gas travels through the regenerator/displacer <NUM> (e.g., a porous regenerator/displacer) and into an expansion space <NUM>. A restoring force provided by a transducer/balancer system <NUM> and the inductive windings <NUM>, and the drawback of the pistons <NUM> (e.g., as controlled by drive signals provided by the motor driver <NUM>) in between compression strokes draws the regenerator/displacer <NUM> back towards the expander cylinder head <NUM> and expands the working gas within the expansion space <NUM>, thereby extracting heat from the environment through the cold finger <NUM> and embedding it within the expanded working gas. Repeated operation of such cycle moves heat extracted from the cold finger <NUM> (e.g., and anything thermally coupled to the cold finger <NUM>) to the motor/compressor 172B, and the transferred heat is dissipated into the environment (e.g., using various heat exchangers and thermal management coupled to the motor/compressor 172B), as is common with various Stirling cycle refrigeration systems.

As shown in <FIG>, the motor/compressor 172B may be implemented with inductive windings <NUM> configured to cause the pistons <NUM> to move towards each other to compress gas within the compression space therebetween. In some embodiments, the motor driver <NUM> of the cryocooler controller <NUM> may be electrically coupled to the windings <NUM> of the motor/compressor 172B (e.g., over the power leads <NUM>) and the motor drive signals generated by the motor driver <NUM> may be used to drive the pistons <NUM> to generate the compression wave/mass flow, as in a linear motor/compression arrangement. Other motor/compressor arrangements are contemplated, including various linear motor arrangements, other compressor arrangements, and/or cyclical motor and/or motor/compressor arrangements.

As also shown in <FIG>, the expander <NUM> may be implemented with the inductive windings <NUM> configured to limit the stroke of the displacer <NUM> (e.g., so as not to impact the cold finger <NUM> or the expander cylinder head <NUM>) and to help balance motion of the displacer <NUM> and/or compensate for the mechanical vibrations caused by reciprocation of the displacer <NUM> within the expander <NUM>. In some embodiments, the motor driver <NUM> of the cryocooler controller <NUM> (e.g., or an additional motor driver of the cryocooler controller <NUM>) may be electrically coupled to windings/coil <NUM> of the expander <NUM> (e.g., over the power leads <NUM>) and balancer system drive signals generated by the motor driver <NUM> may be used to drive the displacer <NUM> and/or motion of the windings/coil <NUM> as in a linear motor arrangement, similar in some aspects to operation of the motor/compressor 172B described herein. In alternative embodiments, the transducer/balancer system <NUM> and/or the inductive windings <NUM> may be replaced and/or supplemented with a mechanical spring or spring system coupled to the displacer <NUM> within the expander cylinder head <NUM> and configured to provide such restoring forces.

<FIG> illustrates an example system <NUM> and associated flow for facilitating cryocooler health monitoring in accordance with one or more embodiments of the present disclosure. In an embodiment, the system <NUM> may facilitate health monitoring of the cryocooler <NUM> and <NUM> of <FIG> and <FIG>, respectively. The system <NUM> includes test logic <NUM>, health assessment logic <NUM>, and flash memory <NUM>. In <FIG>, various examples of inputs and outputs, data stored to power cycle persistent memory, and operations are provided. The test logic <NUM> and the health assessment logic <NUM> may be implemented by a processor(s), such as an FPGA, system-on chip, etc. In an embodiment, the test logic <NUM> and the health assessment logic <NUM> may be implemented by one or more processing circuits of the cryocooler controller <NUM> of <FIG>. Communication between the test logic <NUM>, the health assessment logic <NUM>, and the flash memory <NUM> may be hardware-based and/or software-based. As an example of hardware-based communication, an input general purpose input/output (GPIO) may be used to initiate a BIT and an output GPIO may provide outputs indicative of a health metric of the cryocooler, such as varying output electrical signal levels (e.g., voltage levels) proportional to the remaining lifetime.

The flash memory <NUM> includes a factory flash space <NUM> to store baseline load lines (e.g., also referred to as reference load lines) and relationships (e.g., equations, correlation/lookup tables) to correspond parameters to a health metric (e.g., remaining cryocooler lifetime in hours). In <FIG>, the factory flash space <NUM> stores baseline load lines associated with different ambient temperatures (e.g., also referred to as environmental temperature) and a correlation table(s) to map a slope and a power increase determined using the test logic <NUM> and the health assessment logic <NUM> to a remaining cryocooler lifetime. In one case, the relationships may be, may include, or may be based on, lifetime data collected using a Standard Advanced Dewar Assembly (SADA) test protocol to correlate measured parameters with a remaining lifetime of the cryocooler. The flash memory <NUM> also includes a user flash space <NUM> to store various data/parameters measured and/or determined by the test logic <NUM> and the health assessment logic <NUM>, as further described herein.

A user may initiate the flow of <FIG> by providing an input to initiate the built-in test. Such a built-in test may be performed at the factory (e.g., to test the cryocooler) and/or in die field. A counter value M may be initiated to a <NUM> value. The flow associated with the test logic <NUM> may be performed for each of N setpoints. Each setpoint may be considered or referred to as a heat load of the cryocooler. At block <NUM>, an ambient temperature is measured (e.g., by a temperature sensor of or otherwise coupled to the refrigeration system <NUM>). At block <NUM>, a cold tip is set to a temperature of an Mth setpoint temperature (e.g., <NUM>th setpoint temperature for an initial iteration of the flow). The cold tip may be set to the temperature by controlling an operational parameter input, such as a power input, of the cryocooler. At block <NUM>, the temperature of the cold tip is measured (e.g., by a temperature sensor of the refrigeration system <NUM>). At block <NUM>, the measured temperature of the cold tip is compared to the desired setpoint temperature to determine whether the desired setpoint temperature has been reached. If the setpoint temperature is determined to not have been reached, the flow proceeds from block <NUM> back to block <NUM>.

If the setpoint temperature is determined to have been reached, the flow proceeds from block <NUM> to block <NUM>. At block <NUM>, a delay before proceeding to block <NUM> is implemented to allow the cold tip temperature to settle and ensure that the cold tip temperature is stable (e.g., and to ensure that subsequent power measurements are also stable). Different coolers may be associated with different amounts of delay. As non-limiting examples, a cooler may be allowed to settle for around <NUM> minutes, <NUM> minutes, <NUM> minutes, <NUM> minutes, <NUM> minutes, <NUM> minutes, any duration of time between these time durations, or other time durations dependent on die cooler. If during settling the cold tip temperature changes from the desired setpoint temperature, the flow may proceed back to block <NUM> to cause appropriate control of the operational input to be performed to adjust the cold tip temperature to the desired setpoint temperature.

After the cold tip settles at the setpoint temperature, a power is measured at block <NUM>. In some cases, the power may be a compressor input power applied to the cryocooler to set and maintain the cold tip temperature at the setpoint temperature. In some cases, the power measurements may be based on a voltage output to the cryocooler and/or a current output to the cryocooler. At block <NUM>, the ambient temperature (e.g., measured at block <NUM>), the cold tip temperature (e.g., measured at block <NUM>), and the power (e.g., measured at block <NUM>) is stored in the user flash space <NUM> of the flash memory <NUM>. At block <NUM>, a determination is made as to whether the counter value M is greater than a value N (e.g., to determine whether the ambient temperature, cold tip temperature, and power has been measured for each of the N setpoint temperatures). If the counter value M is not greater than the value N, the counter value M is incremented at block <NUM> and the flow proceeds back to block <NUM> (e.g., to obtain a power measurement for a next setpoint temperature). If the counter value M is greater than the value N, the flow proceeds to block <NUM> implemented by the health assessment logic <NUM>.

At block <NUM>, factory baseline load lines are read from the factory flash space <NUM> and the ambient temperature (e.g., measured at block <NUM>), the cold tip temperature (e.g., measured at block <NUM>), and the power (e.g., measured at block <NUM>) are read from the user flash space <NUM>. In some cases, the ambient temperature measured at block <NUM> when setting each temperature setpoint may be averaged. At block <NUM>, the cold tip temperature (e.g., measured at block <NUM>) and the power (e.g., measured at block <NUM>) are scaled to an ambient temperature associated with the factory baseline load lines to provide a cryocooler-test load line (e.g., also referred to as a BIT load line). For example, in <FIG>, the factory baseline load lines may include baseline load lines associated with an ambient temperature of -<NUM>, <NUM>, and <NUM>. In this example, if the measured ambient temperature (e.g., average ambient temperature of ambient temperatures measured at block <NUM>) is around <NUM>, the cold tip temperature and the power may be scaled to a corresponding cold tip temperature and power for an ambient temperature for <NUM>, since <NUM> is the closest ambient temperature for which a factory baseline load line is available. At block <NUM>, a power increase for each setpoint temperature is determined. The power increase may be determined by computing a difference between the factory baseline load line and the cryocooler-test load line for each setpoint temperature. At block <NUM>, a slope and a maximum power increase are determined based on the factory baseline load line and the cryocooler load line, and a remaining lifetime (e.g., in hours) of the cryocooler is determined based on the slope and the maximum power increase. The estimated remaining lifetime of the cryocooler is provided as an output of the health assessment logic <NUM>. In some cases, an indication of the estimated remaining lifetime may be provided to the end users (e.g., audibly and/or visually). The end users may determine whether to perform an action (e.g., perform maintenance on a cryocooler, replace a cryocooler) based on the health assessment.

As an example, <FIG> illustrates a graph showing a load line performance metric at different cold tip setpoint temperatures for a given ambient temperature in accordance with one or more embodiments of the present disclosure. In the graph, the load line performance metric is an input compressor power. The graph includes a baseline load line <NUM> for the ambient temperature and a cryocooler-test load line <NUM> measured in the field (e.g., via a user-initiated built-in-test). It is noted that different implementations of cryocoolers and/or different applications of the cryocoolers are associated with different load lines and/or different temperature setpoints. In some cases, the baseline load line <NUM> may be one of multiple baseline load lines stored in the factory flash space <NUM>. Each of the baseline load lines may be associated with a respective ambient temperature. The baseline load line <NUM> may be selected based on its associated ambient temperature in relation to an ambient temperature during which the input power is measured. For example, the baseline load line <NUM> may be associated with an ambient temperature closest to the ambient temperature during which load line performance metrics are measured to obtain the cryocooler-test load line <NUM>. The cryocooler-test load line <NUM> may include the input power measured at the various setpoint temperatures for an ambient temperature. As an example, the cryocooler-test load line <NUM> may be a result of a built-in test performed after around a runtime of <NUM>,<NUM> hours of the cryocooler. In some cases, the cryocooler load line <NUM> may be a measured load line that is scaled (e.g., at block <NUM>) to the ambient temperature associated with the baseline load line <NUM>. In <FIG>, the cold tip temperature setpoints are T<NUM>, T<NUM>, Ti, T<NUM>, T<NUM>, and T<NUM>. As one example, the temperature setpoints T<NUM> and T<NUM> may be <NUM> and <NUM>, respectively. As another example, the temperature setpoints T<NUM> and T<NUM> may be <NUM> and <NUM>, respectively. Other temperature setpoints may be used based on cryocooler design and/or application. An example range along the power axis may be <NUM> W to <NUM> W.

As shown by the baseline load line <NUM> and the cryocooler-test load line <NUM>, for a certain cold tip temperature, a higher input power is needed as the cryocooler wears down to set the cold tip of the cryocooler to the cold tip temperature relative to the baseline. <FIG> illustrates a graph with a curve <NUM> showing a power increase from the baseline load line <NUM> to the cryocooler-test load line <NUM> in accordance with one or more embodiments of the present disclosure. In this regard, the curve <NUM> may be computed (e.g., at block <NUM>) by subtracting the input power at each setpoint temperature for the baseline load line <NUM> from the input power at each corresponding setpoint temperature for the cryocooler-test load line <NUM>. For example, at the setpoint temperature of T<NUM>, the baseline load line <NUM> is at a power PB5 and the cryocooler-test load line <NUM> is at a power PT5, such that the difference leads to the difference line <NUM> being at a power change (e.g., a power difference/increase) PΔS = PT5 - PB5 at the T<NUM> setpoint temperature. A best fit line <NUM> (e.g., an equation thereof) may be determined (e.g., at block <NUM>) based on the curve <NUM>. The best fit line <NUM> may be characterized at least by a slope and a maximum power increase (e.g., the power increase PΔ5 at the T<NUM> setpoint temperature).

Based on the slope and the maximum power increase, an assessment of the cryocooler's remaining lifetime (e.g., in estimated hours remaining) may be determined (e.g., at block <NUM>). The assessment may be provided to the user and/or used (e.g., by the user) to determine if the cryocooler should remain in service or should be taken out of service for preventative maintenance and/or replaced. For example, the assessment may be used to reduce or avoid any loss of mission capability. In an aspect, such an assessment may be performed as part of a scheduled maintenance and/or prior to scheduling a maintenance to track cryocooler health (e.g., using other tests and/or based on other performance metrics). In one case, to estimate the hours remaining, lifetime data collected with the SADA profile may be assessed to correlate the remaining hours with the maximum power increase (e.g., at the maximum heat load or equivalently at the lowest setpoint temperature) and the slope (e.g., ratio of power increase and temperature setpoint delta) for a given ambient temperature. In some cases, an associated correlation table and resulting remaining hours estimation may be stored on-board (e.g., in the flash memory <NUM>) along with baseline load lines (e.g., collected in the factory) and any subsequent user-initiated load lines (e.g., BIT user-initiated load lines collected in the field).

It is noted that <FIG> provides a non-limiting example system and flow. As another example, although <FIG> illustrate an example with six setpoint temperatures, other systems and flows may use more than six setpoint temperatures, fewer than six setpoint temperatures, and/or different setpoint temperatures from those shown in <FIG> may be used. It is further noted that adjacent setpoint temperatures need not be substantially equally spaced from each other as shown in <FIG>. As an example, at block <NUM>, alternatively or in addition to an ambient temperature, other ambient conditions (e.g., air flow, conduction) and/or operational conditions (e.g., warm up time) may be measured. As an example, at block <NUM>, rather than measuring power directly, one or more other parameters may be measured and these other parameters) may correlate to / be indicative of a power. In the foregoing, an implicit load is associated with each setpoint. In some aspects, to oscillate the load, the FPA or other device being cooled by the cryocooler may be turned on and off.

In one or more embodiments, a cryocooler may be integrated with a thermal test dewar or an imaging dewar. The thermal test dewar may include a temperature measurement diode and a load resistor. The load resistor may provide a variable heat load on a cold tip of the cryocooler. Using the load resistor, the thermal test dewar may generate load lines of input power versus applied heat load. For example, the thermal test dewar may apply load to a resistor and create load lines at a constant cold tip temperature. As an example, these load lines may be used to characterize performance of the cryocooler by driving the cryocooler harder and looking at the input power. As another example, similar characterization may be achieved by lowering a setpoint (e.g., of the FPA coupled to the cryocooler) and looking at the input power needed to maintain each setpoint.

As an example, <FIG> illustrates a temperature-based load line associated with a thermal dewar in accordance with one or more embodiments of the present disclosure. The load line is generated at a certain operating temperature of the cryocooler (e.g., a cold tip temperature). As one example, the cold tip temperature may be a temperature within a range from <NUM> to <NUM>. Additional heater power may be applied in <NUM> mW increments from <NUM> mW to <NUM> mW while the input power (e.g., input compressor power) to the cryocooler is measured. The <NUM> mW case may be referred to as a "no load" case and may represent a parasitic conduction and radiation load of the dewar.

In various aspects, the imaging dewar does not have an adjustable load heater incorporated into it and thus does not generate a temperature-based load line in the same manner as the temperature-based load line (e.g., such as the load line in <FIG>) for the thermal dewar. As an example, <FIG> illustrates a temperature-based load line associated with an imaging dewar in accordance with one or more embodiments of the present disclosure. For the imaging dewar, power may be measured while a cold tip temperature is varied. This has an effect of increasing parasitic loads on the cold tip as its temperature decreases, increasing the input power and generating a load line similar to that generated by the applied head loads of the thermal dewar.

In this regard, a load line of a cryocooler may be generated for the imaging dewar without needing to add an additional heat load and using various embodiments described herein, such as with respect to <FIG>, with such a load line used as the basis for a health determination of the cryocooler. For initial production, load lines generated in this manner may be compared with a standard and a performance of the cooler may be verified. This may save time and cost in the production process by removing a test with a thermal dewar.

In one or more embodiments, life test results may be used to facilitate cryocooler health monitoring. As an example, <FIG> illustrates a graph with life test results for various coolers. As an example, the life test results shown in the graph may be for the coolers at a nominal ambient temperature (e.g., <NUM>, <NUM>, or other temperature). The life test results may be based on the SADA test protocol. SADA testing is the industry standard for testing and reporting cryocooler lifetime (e.g., tactical cryocooler lifetime). The SADA testing may utilize a certain cycle duration, around a portion of the cycle duration being operational at each of three external temperatures (e.g., -<NUM>, <NUM>, and <NUM>), and a cycle repeated until the coolers fail. As an example, a cryocooler may have a cold tip at <NUM> with a <NUM> mW applied load. A cooler power may generally creep up slowly during a life test (and during the cryocooler lifetime). A failure may be indicated by an elevated power draw (e.g., an input power crossing a threshold), an inability of the cooler to maintain a temperature, and/or a long cooldown time (e.g., a cooldown time exceeding a threshold).

In some aspects, alternatively or in addition to the life test (e.g., shown in <FIG>), an accelerated life test may be performed. In some cases, accelerated life testing may be performed to reduce a time for providing design changes. <NUM> illustrate graphs with accelerated life test results for various coolers. In such tests, accelerating stresses may be applied to the coolers while they are run at a constant cold tip temperature (e.g., <NUM>, <NUM>, <NUM>). The accelerating stresses in the test data shown are an applied heat load (e.g., an elevated cold tip heat load) and a heat rejection temperature. Results may be translated to SADA equivalent hours. Elevated power data may provide additional data points and better signal-to-noise ratio for determining cooler performance degradation. Although in <FIG> the accelerated life test is performed on a linear Stirling cryocooler, the accelerated life test may generally be performed on any cryocooler.

In one or more embodiments, monitoring of cryocooler health and predicting remaining useful life may be based on a combination of a cold tip temperature-based load line with life test results (e.g., SADA life test results and/or accelerated life test results). By reducing the setpoint of the cooler and measuring the input power, a load line can be generated and compared with values from the cooler's beginning of life. The use of multiple points and the increased input power of the lower temperature points may help to improve an accuracy of determining a magnitude of performance degradation. Furthermore, coolers may exhibit performance degradation at high input power before the degradation is evident at their typical operating condition. This may be observed as an increase in cooldown time due to the cooler running at a maximum power during cooldown. A correlation of the measured load line degradation to the increase in power measured during the SADA and accelerated life tests can be used to estimate the cooler's remaining useful life.

Where applicable, various embodiments provided by the present disclosure can be implemented using hardware, software, or combinations of hardware and software which are not part of the present invention. Also where applicable, the various hardware components and/or software components set forth herein can be combined into composite components comprising software, hardware, and/or both. Where applicable, the various hardware components and/or software components set forth herein can be separated into sub-components comprising software, hardware, or both. In addition, where applicable, it is contemplated that software components can be implemented as hardware components, and vice versa.

Claim 1:
A method comprising:
determining (<NUM>), for each setpoint temperature of a plurality of setpoint temperatures, a respective power applied to a cryocooler (<NUM>) to set a cold tip of the cryocooler to the setpoint temperature;
determining (<NUM>) a first load line associated with the cold tip based on the plurality of setpoint temperatures and the respective powers applied to the cryocooler; and
determining (<NUM>) a health metric associated with the cold tip based on the first load line and a reference load line associated with the cryocooler.