Patent Application
20220242548
Telecommunications connectivity via the Internet, cellular data networks and other systems is available in many parts of the world. However, there are locations where such connectivity is unavailable, unreliable or subject to outages from natural disasters. Some systems may provide network access to remote locations or to locations with limited networking infrastructure via satellites or high altitude platforms. In the latter case, due to environmental conditions and other limitations, it is challenging to keep the platforms aloft and operational over a desired service area for long durations, such as weeks, months or more. Mechanical issues, electrical faults and other adverse conditions may cause a platform to become inoperable or otherwise adversely affect its operational lifespan.
Aspects of the technology relate to a high altitude platform (HAP) that is able to remain on station or move in a particular direction toward a desired location, for instance to provide telecommunication services, video streaming or other services. The high altitude platform may be a lighter-than-air (LTA) platform such as a balloon, dirigible/airship or other LTA platform configured to operate in the stratosphere. For instance, the LTA platform may include an envelope filled with lift gas and a payload for providing telecommunication or video services, with a connection member coupling the payload with the envelope. The envelope may be a superpressure envelope, with or without a ballonet that can be used to aid in altitude control as part of an altitude control system. The payload may be configured to rotate relative to the envelope, such as to align solar panels with the sun or to improve coverage in an area of interest. A lateral propulsion system may provide directional thrust for moving the LTA platform toward a destination or remaining on station over a location of interest (e.g., a city or regional service area). This can include a pointing mechanism that aligns a propeller assembly of the lateral propulsion system along a certain heading. By way of example, the propeller assembly may be able to rotate up to 360° or more around the connection member in order to adjust the balloon's heading. Onboard systems are configured to handle adverse conditions, such as a fault or failure of the envelope, an ACS component, or the lateral propulsion system. This may be done according to a ranked list of adverse operational conditions.
According to one aspect of the technology, a method of operating a lighter-than-air high altitude platform (HAP) in the stratosphere is provided. The method comprises receiving, by one or more processors of the HAP, status information regarding at least one of an altitude control system (ACS) of the HAP or a lateral propulsion system of the HAP; determining, by one or more processors, whether the status information indicates an adverse operational condition associated with either the ACS or the lateral propulsion system; upon determining that the status information indicates the adverse operational condition, evaluating, by the one or more processors, a ranking of the adverse operational condition, the ranking being in relation to one or more other adverse operational conditions; and causing, by one or more processors, either the ACS or the lateral propulsion system to take a corrective action for the adverse operational condition.
In one example, evaluating the ranking of the adverse operational condition includes retrieving a ranked list of conditions from a memory module onboard the HAP. In another example, receiving the status information includes obtaining the status information from one or more sensors of the HAP. Here, the method may further comprise monitoring, by the one or more sensors, components of the ACS or the lateral propulsion system. The monitoring may be performed continuously.
The adverse operational condition and the one or more other adverse operational conditions may comprise a set of conditions stored in memory as a prioritized list. When the adverse operational condition is associated with the ACS, the corrective action may include halting all altitude control maneuvers. When the adverse operational condition is associated with the ACS, the corrective action may include ending a current descend maneuver. In this case, the corrective action may further include waiting for a temperature value of a component of the ACS to drop below a threshold before resuming the descend maneuver.
In another example, when the adverse operational condition is associated with the ACS, the corrective action includes immediately venting air from an envelope of the HAP. Alternatively, when the adverse operational condition is associated with the ACS, the corrective action may include closing a valve of the ACS and retrying a descend maneuver. In another example, when the adverse operational condition is associated with the ACS, the corrective action includes halting a descend maneuver while permitting an ascend maneuver.
In a further example, when the adverse operational condition is associated with the ACS, the corrective action includes halting a descend maneuver until either a threshold value is met, the threshold value being associated with one of (i) an ACS compressor temperature, (ii) a battery charge value, or (iii) a lift gas pressure value.
When the adverse operational condition is associated with the lateral propulsion system, the corrective action may include immediately applying a brake to a propeller assembly of the lateral propulsion system. Alternatively, when the adverse operational condition is associated with the lateral propulsion system, the corrective action may include immediately applying a brake to a propeller assembly of the lateral propulsion system. In another alternative, when the adverse operational condition is associated with the lateral propulsion system, the corrective action includes ending a current lateral propulsion operation. Here, the corrective action may further include applying a brake to a propeller assembly of the lateral propulsion system when a propeller of the propeller assembly has stopped rotating. And in another example, when the adverse operational condition is associated with the lateral propulsion system, the corrective action includes pausing a current lateral propulsion operation until a battery charge exceeds a threshold value.
According to another aspect of the technology, a lighter-than-air high altitude platform (HAP) is configured for operation in the stratosphere. The HAP comprises an envelope configured to hold lift gas, an altitude control system (ACS) operatively coupled to the envelope, a connecting member, a payload, and a lateral propulsion system. The connecting member is operatively coupled at a first end thereof to the envelope. The connecting member has a connecting axis along a length thereof. The payload is coupled to a second end of the connecting member. The lateral propulsion system is rotatably engaged with the connecting member. The lateral propulsion system includes a propeller assembly having a propeller and a control assembly operatively coupled to the propeller assembly and configured to rotate the propeller in a clockwise or counterclockwise direction about a propeller axis. The control assembly is being configured to rotate the lateral propulsion system along the connecting axis of the connecting member. The HAP also includes one or more processors configured to control operation of the ACS and the lateral propulsion system. The one or more processors are configured to receiving status information regarding at least one of the ACS or the lateral propulsion system; determine whether the status information indicates an adverse operational condition associated with either the ACS or the lateral propulsion system; upon determination that the status information indicates the adverse operational condition, evaluate a ranking of the adverse operational condition, the ranking being in relation to one or more other adverse operational conditions; and cause either the ACS or the lateral propulsion system to take a corrective action for the adverse operational condition.
In one example, the lighter-than-air HAP further comprises a ballonet, and the ACS comprises an air intake assembly and a vent assembly. The air intake assembly includes a compressor or an impeller configured to flow ambient air into the ballonet, and the vent assembly includes one or more valves to vent air from within the ballonet to an external environment of the HAP.
The technology relates to automatic operational control for LTA high altitude platforms configured to operate in the stratosphere. The control may include implementation of automated rules that restrict operation of one or more components of the HAP when conditions do not support such operation. Stratospheric HAPs, such as LTA platforms, may have a float altitude of between about 50,000-120,000 feet above sea level. The ambient temperature may be on the order of −10° C. to −90° C. or colder, depending on the altitude and weather conditions. These and other environmental factors in the stratosphere can be challenging for lateral propulsion systems and altitude control systems. The architectures discussed herein are designed to effectively operate in such conditions, although they may also be used in other environments with different types of systems besides LTA-type platforms.
As explained below, an example HAP may include one or both of altitude control and/or a lateral propulsion system. Altitude control may be employed using an active altitude control system (ACS) with a pump and valve assembly coupled with a onboard ballonet. The lateral propulsion system may employ a propeller arrangement to provide directional adjustments to the HAP, for instance to counteract movement due to the wind, or to otherwise cause the HAP to move along a selected heading. Such altitude and lateral adjustments can enhance operation across a fleet of HAPs. For instance, by employing a small amount of lateral propulsion and/or vertical adjustment at particular times, a given platform may stay on station over a desired service area for a longer period, or change direction to move towards a particular place of interest. The platform may also be able to return to the desired service area more quickly using lateral propulsion and/or altitude adjustments to compensate against undesired wind effects. Applying this approach for some or all of the platforms in the fleet may mean that the total number of platforms required to provide a given level of service (e.g., telecommunications coverage for a service area) may be significantly reduced as compared to a fleet that does not employ lateral propulsion.
The ACS may include a pump and valve arrangement as part of a vent and air intake assembly for a ballonet, which may be received within an outer compartment of the balloon envelope. One or more motors can be used to actuate a lateral propulsion system of the HAP to affect the directional changes. This can include a pointing axis motor for rotating the lateral propulsion system to a particular heading, and a drive motor for causing a propeller assembly or other propulsion mechanism to turn on and off In one example, a controller of the lateral propulsion system is configured to cause the pointing axis motor to rotate the propeller assembly about a connection member of the HAP by 360° or more. A rotational control mechanism may be part of the pointing axis motor arrangement. A fault or failure condition with any of these components, or with the envelope itself, can prevent the HAP from operating as intended, or otherwise reduce its useful life.
Automated operational control using, e.g., onboard firmware for the ACS and/or the lateral propulsion system may enable corrective measures to be taken immediately to ensure proper HAP performance. Various conditions regarding the HAP and its environment may be continuously or regularly monitored. Different conditions may map to a corrective action to take, which may be prioritized by importance.
The devices in system 100 are configured to communicate with one another. As an example, the HAPs may include communication links 104 and/or 114 in order to facilitate intra-balloon communications. By way of example, links 114 may employ radio frequency (RF) signals (e.g., millimeter wave transmissions) while links 104 employ free-space optical transmission. Alternatively, all links may be RF, optical, or a hybrid that employs both RF and optical transmission. In this way balloons 102A-F may collectively function as a mesh network for data communications. At least some of the HAPs may be configured for communications with ground-based stations 106 and 112 via respective links 108 and 110, which may be RF and/or optical links.
In one scenario, a given HAP 102 may be configured to transmit an optical signal via an optical link 104. Here, the given HAP 102 may use one or more high-power light-emitting diodes (LEDs) to transmit an optical signal. Alternatively, some or all of the HAP 102 may include laser systems for free-space optical communications over the optical links 104. Other types of free-space communication are possible. Further, in order to receive an optical signal from another HAP via an optical link 104, the HAP may include one or more optical receivers.
The HAPs may also utilize one or more of various RF air-interface protocols for communication with ground-based stations via respective communication links. For instance, some or all of the HAPs 102A-F may be configured to communicate with ground-based stations 106 and 112 via RF links 108 using various protocols described in IEEE 802.11 (including any of the IEEE 802.11 revisions), cellular protocols such as GSM, CDMA, UMTS, EV-DO, WiMAX, and/or LTE, 5G and/or one or more proprietary protocols developed for long distance communication, among other possibilities.
In some examples, the links may not provide a desired link capacity for HAP-to-ground communications. For instance, increased capacity may be desirable to provide backhaul links from a ground-based gateway. Accordingly, an example network may also include downlink HAPs, which could provide a high-capacity air-ground link between the various HAPs of the network and the ground-base stations. For example, in network 100, dirigible 102A or balloon 102B operating in the stratosphere may be configured as a downlink HAP that directly communicates with station 106.
Like other HAPs in network 100, downlink HAP 102F may be operable for communication (e.g., RF or optical) with one or more other HAPs via link(s) 104. Downlink HAP 102F may also be configured for free-space optical communication with ground-based station 112 via an optical link 110. Optical link 110 may therefore serve as a high-capacity link (as compared to an RF link 108) between the network 100 and the ground-based station 112. Downlink HAP 102F may additionally be operable for RF communication with ground-based stations 106. In other cases, downlink HAP 102F may only use an optical link for balloon-to-ground communications. Further, while the arrangement shown in
A downlink HAP may be equipped with a specialized, high bandwidth RF communication system for balloon-to-ground communications, instead of, or in addition to, a free-space optical communication system. The high bandwidth RF communication system may take the form of an ultra-wideband system, which may provide an RF link with substantially the same capacity as one of the optical links 104.
In a further example, some or all of HAPs 102A-F could be configured to establish a communication link with space-based satellites and/or other types of non-LTA craft (e.g., drones, airplanes, gliders, etc.) in addition to, or as an alternative to, a ground based communication link. In some embodiments, a stratospheric HAP may communicate with a satellite or other high altitude platform via an optical or RF link. However, other types of communication arrangements are possible.
As noted above, the HAPs 102A-F may collectively function as a mesh network. More specifically, since HAPs 102A-F may communicate with one another using free-space optical links, the HAPs may collectively function as a free-space optical mesh network. In a mesh-network configuration, each HAP may function as a node of the mesh network, which is operable to receive data directed to it and to route data to other HAPs. As such, data may be routed from a source HAP to a destination HAP by determining an appropriate sequence of links between the source HAP and the destination HAP.
The network topology may change as the HAPs move relative to one another and/or relative to the ground. Accordingly, the network 100 may apply a mesh protocol to update the state of the network as the topology of the network changes. For example, to address the mobility of the HAPs 102A to 102F, the balloon network 100 may employ and/or adapt various techniques that are employed in mobile ad hoc networks (MANETs). Other examples are possible as well.
Network 100 may also implement station-keeping functions using winds and altitude control and/or lateral propulsion to help provide a desired network topology, particularly for LTA platforms. For example, station-keeping may involve some or all of HAPs 102A-F maintaining and/or moving into a certain position relative to one or more other HAPs in the network (and possibly in a certain position relative to a ground-based station or service area). As part of this process, each HAP may implement station-keeping functions to determine its desired positioning within the desired topology, and if necessary, to determine how to move to and/or maintain the desired position. Alternatively, the platforms may be moved without regard to the position of their neighbors, for instance to enhance or otherwise adjust communication coverage at a particular geographic location.
The desired topology may thus vary depending upon the particular implementation and whether or not the HAPs are continuously moving. In some cases, HAPs may implement station-keeping to provide a substantially uniform topology where the HAPs function to position themselves at substantially the same distance (or within a certain range of distances) from adjacent balloons in the network 100. Alternatively, the network 100 may have a non-uniform topology where HAPs are distributed more or less densely in certain areas, for various reasons. As an example, to help meet the higher bandwidth demands, HAPs may be clustered more densely over areas with greater demand (such as urban areas) and less densely over areas with lesser demand (such as over large bodies of water). In addition, the topology of an example HAP network may be adaptable allowing HAPs to adjust their respective positioning in accordance with a change in the desired topology of the network.
The HAPs of
In an example configuration, the HAPs include an envelope and a payload, along with various other components.
The envelope 202 or 252 may take various shapes and forms. For instance, the envelope may be made of materials such as polyethylene, mylar, FEP, rubber, latex, fabrics or other thin film materials or composite laminates of those materials with fiber reinforcements embedded inside or outside. Other materials or combinations thereof or laminations may also be employed to deliver required strength, gas barrier, RF and thermal properties. Furthermore, the shape and size of the envelope may vary depending upon the particular implementation. Additionally, the envelope may be filled with different types of gases, such as air, helium and/or hydrogen. Other types of gases, and combinations thereof, are possible as well. In some examples, an outer envelope may be filled with lift gas(es), while an inner ballonet may be configured to have ambient air pumped into and out of it for altitude control. Other ballonet configurations are possible, for instance with the ballonet forming an outer envelope, while an inner envelope holds lift gas(es).
Envelope shapes for LTA platforms may include typical balloon shapes like spheres and “pumpkins”, or aerodynamic shapes that are at least partly symmetric (e.g., teardrop-shaped), provide shaped lift, or are changeable in shape. Lift may come from lift gasses (e.g., helium or hydrogen), electrostatic charging of conductive surfaces, aerodynamic lift (wing shapes), air moving devices (propellers, flapping wings, electrostatic propulsion, etc.) or any hybrid combination of lifting techniques.
According to one example 300 shown in
The data can be retrieved, stored or modified by the one or more processors 306 in accordance with the instructions. For instance, although the subject matter described herein is not limited by any particular data structure, the data can be stored in computer registers, in a relational database as a table having many different fields and records, or XML documents. The data can also be formatted in any computing device-readable format such as, but not limited to, binary values, ASCII or Unicode. Moreover, the data can comprise any information sufficient to identify the relevant information, such as numbers, descriptive text, proprietary codes, pointers, references to data stored in other memories such as at other network locations, or information that is used by a function to calculate the relevant data.
The one or more processors 306 can include any conventional processors, such as a commercially available CPU. Alternatively, each processor can be a dedicated component such as an ASIC, controller, or other hardware-based processor. Although
The payload 302 may also include various other types of equipment and systems to provide a number of different functions. For example, as shown the payload 302 includes one or more communication systems 310, which may transmit signals via RF and/or optical links as discussed above. The communication system(s) 310 include communication components such as one or more transmitters and receivers (or transceivers), one or more antennae, and a baseband processing subsystem. (not shown). In one scenario, a given communication module of the communication system operates in a directional manner. For instance, one or more high gain directional antennas may be mechanically or functionally pointed (e.g., via beamforming) in a selected direction(s) to enable uplink and/or downlink connectivity with other communications devices (e.g., other LTA platforms, ground stations or personal communication devices). In this case, it may be particularly beneficial to ensure that the given communication module is pointed at a target heading to ensure the communication link(s) (e.g., according to a determined communication bit error rate, signal-to-noise ratio, etc.).
The payload 302 is illustrated as also including a power supply 312 to supply power to the various components of the balloon. The power supply 312 could include one or more rechargeable batteries or other energy storage systems like capacitors or regenerative fuel cells. In addition, the payload 302 may include a power generation system 312 in addition to or as part of the power supply. The power generation system 314 may include solar panels, stored energy (e.g., hot air relative to ambient air), relative wind power generation, or differential atmospheric charging (not shown), or any combination thereof, and could be used to generate power that charges and/or is distributed by the power supply 312.
The payload 300 may additionally include a positioning system 316. The positioning system 316 could include, for example, a global positioning system (GPS) such as differential GPS (D-GPS), an inertial navigation system, and/or a star-tracking system. The positioning system 316 may additionally or alternatively include various motion sensors (e.g., accelerometers, magnetometers, gyroscopes, and/or compasses). The positioning system 316 may additionally or alternatively include one or more video and/or still cameras, and/or various sensors for capturing environmental data. Some or all of the components and systems within payload 302 may be implemented in a radiosonde or other probe, which may be operable to measure, e.g., pressure, altitude, geographical position (latitude and longitude), temperature, relative humidity, and/or wind speed and/or wind direction, among other information. Wind sensors may include different types of components like pitot tubes, hot wire or ultrasonic anemometers or similar, windmill or other aerodynamic pressure sensors, laser/lidar, or other methods of measuring relative velocities or distant winds.
Payload 302 may include a navigation system 318 separate from, or partially or fully incorporated into control system 304. The navigation system 318 may implement station-keeping functions to maintain position within and/or move to a position in accordance with a desired topology or other service requirement. In particular, the navigation system 318 may use wind data (e.g., from onboard and/or remote sensors) to determine altitudinal and/or lateral positional adjustments that result in the wind carrying the balloon in a desired direction and/or to a desired location. Lateral positional adjustments may also be handled directly by a lateral positioning system that is separate from the payload, which is discussed further below. Alternatively, the altitudinal and/or lateral adjustments may be computed by a central control location and transmitted by a ground based, air based, or satellite based system and communicated to the HAP. In other embodiments, specific HAPs may be configured to compute altitudinal and/or lateral adjustments for other HAPs and transmit the adjustment commands to those other HAPs.
As illustrated in
In order to affect lateral positions or velocities, the platform includes a lateral propulsion system.
Example 400 illustrates a lateral propulsion system 410 using, for instance, one or more propeller assemblies. While this example of the lateral propulsion system 410 is one possibility, the location could also be fore and/or aft of the payload section 404, or fore and/or aft of the envelope section 402, or any other location that provides the desired thrust vector. Details of the lateral propulsion system 410 are discussed below. This example also includes an ACS 412, which is coupled to an interior ballonet 414 disposed within the envelope 402. The ACS 412 is configured to draw ambient air into the ballonet 414 and to expel air therefrom.
While this configuration or other similar configurations gives the lateral propulsion system 410 two degrees of operational freedom, additional degrees of freedom are possible with other pointing mechanisms or air-ducting mechanisms. This flexible thrusting approach may be used to help counteract continually changing wind effects. Rotation of the control assembly 504 and propeller assembly 502 about the down connect member 406 is desirably independent of rotation of the solar panel assemblies (and/or payload) about the down connect member 406. Furthermore, such rotation is also independent of operation of the despin mechanism 424 that may be used to adjust the relative rotation of the envelope and the payload.
The payload or the lateral propulsion system (or both) may have on-board sensors (e.g., differential GPS or D-GPS) to provide accurate attitude and/or position and velocity measurements, enabling highly accurate pointing of the propeller in an absolute direction as well as relative to the payload direction. These sensors are also able to provide measurement of the balloon platform's lateral speed. The propeller motor assembly 606 is configured to rotate the propeller in a clockwise or counterclockwise direction, with or without additional gearing. The propeller motor assembly 606 may be brushless, which can generate more torque than a brush-type motor. By way of example, the brushless motor may be a 300 W-1000 W motor, which is capable of rotating the propeller between 900-2500 rpm or more. The motor may employ a cooling system, for instance using cooling fins or air ducts (not shown) to remove excess heat from the motor or electronics. The system may only need to drive the propeller to achieve a balloon lateral speed of between 1-15 m/s relative to the ground in order to significantly increase the ability of the balloon to stay on or return to station. The speed may be dependent on the location of interest, wind currents at a particular location or altitude, season/time of year, time of day, and/or other factors.
As shown in
As shown in
A block diagram of an exemplary electronics module 700 is illustrated in
The control subsystem may include a navigation controller 716 that is configured to employ data obtained from onboard navigation sensors 718, including an inertial measurement unit (IMU) and/or differential GPS, received data (e.g., weather information), and/or other sensors such as health and performance sensors 720 (e.g., a force torque sensor) to manage operation of the balloon's systems. The navigation controller 716 may be separate from or part of the processor(s) 702. The navigation controller works with system software, ground controller commands, and health & safety objectives of the system (e.g., battery power, temperature management, electrical activity, etc.) and helps decide courses of action. The decisions based on the sensors and software may be to save power, improve system safety (e.g., increase heater power to avoid systems from getting too cold during stratospheric operation) or divert power to altitude controls or divert power to lateral propulsion systems.
When decisions are made to activate the lateral propulsion system, the navigation controller then leverages sensors for position, wind direction, altitude and power availability to properly point the propeller and to provide a specific thrust condition for a specific duration or until a specific condition is reached (a specific velocity or position is reached, while monitoring and reporting overall system health, temperature, vibration, and other performance parameters). In this way, the navigation controller can continually optimize the use of the lateral propulsion systems for performance, safety and system health. Upon termination of a flight, the navigation controller can engage the safety systems (for example the propeller braking mechanism) to prepare the system to descend, land, and be recovered safely.
Similarly, the ACS may be controlled to start or increase airflow into a ballonet or to pump air out from the ballonet. This can include actuating a compressor, pump or other mechanism to effect the desired amount of airflow or otherwise adjust the vertical position of the HAP in the stratosphere.
Lateral propulsion controller 722 is configured to continuously control the propeller's pointing direction (e.g., via worm gear mechanism 624), manage speed of rotation, power levels, and determine when to turn on the propeller or off, and for how long. The lateral propulsion controller 722 thus oversees thruster pointing direction 724, thruster power level 726 and thruster on-time 728 modules. The lateral propulsion controller 722 may be separate from or part of the processor(s) 702. Processor software or received human controller decisions may set priorities on what power is available for lateral propulsion functions (e.g., using lateral propulsion power 714). The navigation controller then decides how much of that power to apply to the lateral propulsion motors and when (e.g., using thruster power level 726). In this way, power optimizations occur at the overall system level as well as at the lateral propulsion subsystem level. This optimization may occur in a datacenter on the ground or locally onboard the balloon platform.
The lateral propulsion controller 722 is able to control the drive motor of the propeller motor assembly so that the propeller assembly may operate in different modes. Two example operational modes are: constant power control or constant rotational velocity control. The electronics module may store data for both modes and the processor(s) of the control assembly may manage operation of the drive motor in accordance with such data. For instance, the processor(s) may use the stored data to calculate or control the amount of power or the rotational propeller velocity needed to achieve a given lateral speed. The electronics module may store data for the operational modes and the processor(s) of the control assembly may manage operation of the drive motor in accordance with such data. For instance, the processor(s) may use the stored data to calculate the amount of current needed to achieve a given lateral speed. The processor(s) may also correlate the amount of torque required to yield a particular speed in view of the altitude of the balloon platform.
The processor(s) may control the drive motor continuously for a certain period of time, or may cycle the drive motor on and off for selected periods of time. This latter approach may be done for thermal regulation of the drive motor. For instance, the propeller may be actuated for anywhere from 1 second to 5 minutes (or more), and then turned off to allow for motor cooling. This may be dependent on the thermal mass available to dissipate heat from the motor.
As noted above, the lateral propulsion controller 722 regulates the thruster pointing direction 724, such as by causing the pointing motor assembly to drive the worm gear mechanism in a first direction to rotate clockwise about the down connect longitudinal axis or in a second direction to rotate counterclockwise about the longitudinal axis.
For example,
As discussed above, the lateral propulsion controller may be configured to control the propeller's pointing direction via the worm gear mechanism. This controller or another processor of the electronics module or the control system of the payload may also manage operation of the despin mechanism, which addresses relative rotation between the envelope and payload.
In one configuration, the controller(s) employs a stepper motor driver operatively coupled to a stepper motor having a rotary encoder disposed on the stepper motor shaft. For instance, the stepper motor driver may be an actuator that is able to convert a pulse signal into angular displacement signal, and actuate the stepper motor accordingly. Using the encoder, the controller is able to measure the load on the stepper motor and increase or reduce the power sent to the motor accordingly, which can substantially reduce power consumption.
By way of example, on the despin system (e.g., despin mechanism 424), power consumption may be reduced from 8 W on average to 1.5 W, or by at least 80%. While a brushless DC motor may be employed for despin, using a stepper motor allows for better operational control if the encoder fails, since a stepper motor is designed for low speed no-feedback usage (e.g., open loop operation).
The controller works by monitoring the difference between the rotor position as measured by the encoder and the position set by the stepper driver. The stepper driver's set position can be determined by either directly querying the drive on its position (if available) or by counting the step pulses sent to the drive. In an ideal situation these two positions will be identical, but due to a load on the motor the two positions may differ.
When the difference between the stepper driver position and rotor position is large, it indicates a high load on the stepper motor. In response, power to the motor is increased, which reduces the position difference. Accordingly, if the difference between the stepper driver position and rotor position is too small, the load on the stepper motor is low, and power to the motor is reduced to increase the difference and save power.
Maintaining a set difference between the stepper driver position and rotor position improves efficiency not only by adapting the motor power to match the load, but also due to the fact that motors most efficiently convert electricity to motion when the rotor position lags behind the driver position by a specific amount. It is a similar approach to field-oriented control for three phase brushless DC motors, but adapted for low speed, stepper motors, and existing stepper drivers, which can reduce hardware development time.
However, this approach requires high encoder accuracy and can be disrupted by manufacturing and assembly variation. By way of example, a positional error of at most 0.3 degrees may be needed for the stepper motor. The required accuracy is a function of the steps per revolution of the stepper motor. Thus, according to one aspect of the technology, a calibration routine may be employed to compensate for the error. The calibration routine drives the motor to several locations along its rotation. At each location, random noise is injected into the motor, and averages the position read by the encoder. This provides sub-count resolution on the position, and a greater resistance to calibration error caused by friction and external loads, which can be especially beneficial in the stratosphere, where extreme temperature and other environmental factors can affect operation of an LTA craft. Random noise may be generated by commanding the stepper driver to make partial steps forward and backwards around a reference position, such that the average of the positions is the reference position. This may be done via a predetermined operating mode to facilitate this, but it may also be implemented via the pulse signal driving forwards and backwards.
This calibration approach creates a high efficiency stepper drive system that can compensate for encoder error and can fall back to encoder-less operation more effectively than a brushless DC motor. Calibration can be employed both for despin between the envelope and payload. Calibration may also be employed with the pointing actuator, such as via a calibration routine implemented by the lateral propulsion controller 722.
In one scenario, on despin, when motor power is commanded to increase in response to the load, the controller increases the voltage applied to the stepper motor. The current is monitored but is not acted upon until it passes a limit, such as a predetermined threshold. Here, if the load continues to increase, the rotational speed is reduced while current is maintained at a maximum selected value. By way of example, despin may have a current threshold on the order of 1.5 A-1.7 A. Once the motor current reaches this threshold, the output speed is reduced while the motor current is maintained. If the load remains excessive the output may even reverse direction.
This scenario allows for extremely smooth motion of the despin mechanism, since the torque ripple in voltage control mode is allowed to converge to a constant value, while in current controlled mode the current is always oscillating around a setpoint, which causes vibration. However, this drive method imposes limits on the maximum motor winding inductance and limits the motor's maximum speed, resulting in restricting this to slow, low power applications such as despin. Note that a voltage setpoint may be set as a function of a mechanical load associated with the despin mechanism in the voltage control mode. And a current setpoint may be set as a function of a mechanical load associated with the stepper motor in the current control mode.
In contrast, lateral propulsion control may employ higher power, higher speeds and/or higher torque than what is used during despin. For instance, the higher torque is due to having to counteract gyroscopic precession from the propeller. Additionally, a larger motor is used in lateral propulsion control than for despin. Such a larger motor has a correspondingly higher inductance, which can also make the voltage control mode a less beneficial approach. Thus, in a scenario involving lateral propulsion control, such as for managing the thruster pointing direction, a different drive method can be used. In particular, instead of setting a motor voltage in response to load, the controller is configured to set a motor current limit instead. In this case, the drive circuit (e.g., stepper motor driver) then adjusts the motor voltage until the measured motor current reaches the commanded motor current. By way of example, the motor current may range from 0.1 to 1.5 A. This enables adjustment of the pointing direction of the lateral propulsion system (thruster pointing direction) with reference to another portion of the LTA platform. This approach allows for power savings and feedback on the stepper loading over traditional open loop stepper control
This control scheme for a lateral propulsion assembly is much more tolerant of higher inductance motors, allowing the use of larger motors at higher speeds. A possible downside would be a higher torque ripple due to coarser resolution of control as compared to voltage control, with slightly less responsiveness to load due to lag from the current control loop, and less optimal performance when running at a very low motor current which could reduce efficiency. However, the LTA platform would readily tolerate such situations without impacting operational performance.
As shown in the example illustrated in
As can be seen from the above, both the ACS and the lateral propulsion system play an important role in operation of the LTA platform. Unfortunately, a fault or failure in a component of either system or a problem with the envelope itself can have minor or serious implications for the HAP. According to one aspect of the technology, one or more sets of rules may be applied to different conditions. These rules may restrict operation of the ACS or lateral propulsion system when the condition(s) does not support such operation. In one scenario, each condition maps to an action to take, prioritized by importance. Two tables are presented below that identify exemplary conditions and the corresponding action by either the ACS or the lateral propulsion system. Table I relates to ACS-related situations, and Table II relates to lateral propulsion system-related situations.
In these examples for rule-based operation, the conditions in each table are ranked based on severity, with the first (“Internal sensor stopped reporting” in Table I and “Ambient pressure rate of change too high” in Table II) being considered the most severe or highest priority to address, while the last (“Lift gas superpressure high but not critically high above a threshold” in Table I and “Battery charge low” in Table II) being considered the least severe or lowest priority to address. With regard to the ACS, certain conditions detected by sensors (e.g., temperature sensing module 322, pressure sensing module 324 or altimeter 325) or determined by the processor(s) (e.g., processor 306) may result in the control system (e.g., 304 in
It is possible for multiple conditions to occur simultaneously or otherwise overlap in time. For instance, with regard to Table I, an internal sensor may stop reporting at the same time an ACS valve jams. Or the ACS compressor temperature may be too high (e.g., exceeding an operational threshold for a given set of conditions) at the same time that the compressor stalls and the ACS overcurrent trips. Here, the processor may cause the actions to be implemented sequentially according to the severity of the individual conditions. Alternatively, multiple actions may be taken at the same time to address the various conditions, so long as an action associated with a lower severity condition does not conflict with an action associated with a higher severity condition. For conditions where the temperature or other measurement is too high or too low, these relative situations may have one or more set thresholds or may be variable depending on current HAP operating conditions (e.g., altitude, ambient temperature, ambient pressure, location, etc.)
One example of ACS-related prioritized situations is as follows. In this scenario, an ACS maneuver controller may operate by periodically checking a variety of runtime conditions, such as the target pressure, the envelope pressure, and the ballonet pressure, among other things. The ACS system is controlled depending on these conditions. For instance, the system may first determine whether a maneuver logic error has occurred. If so, the system may exit automated control, instead relying on a remote system to make any necessary maneuver changes. If not, any error condition may be cleared. Should an ACS-related error occur, such as a sensor that has stopped reporting, the system may issue a stop command, halting all maneuvers and raising an error. If a vent lift gas superpressure condition is detected, the system may issue an ACS command to ascend. If such a condition is not detected, a flight mode condition may then be evaluated. Here, if the condition is true, the system may issue a stop command, halting all maneuvers and raising an error. A next prioritized condition may be a low lift gas superpressure. If this condition is true, then the system may issue a stop command, halting all maneuvers and raising an error. Next, the system may first evaluate an ascend situation and then a descend situation if there is no ascent. If there is no descent, the system may then evaluate whether the ambient pressure is above a setpoint. If so, or if there is an ascend situation, then the system may evaluate a ballonet occlusion safety condition. If the condition exists (a “true” condition), then the system may issue a stop command, halting all maneuvers and raising an error. Otherwise, the system may allow the HAP to ascend. If the ambient pressure is not above the setpoint, the system evaluates whether it is below the setpoint. If not, then an error may be flagged. If the ambient pressure is below the setpoint, then the system may next evaluate whether the ACS compressor temperature is too low. If so, all descend maneuvers may be halted until the compressor temperature rises. Here, ascend maneuvers may still be permitted. If the ACS compressor temperature is not too low, the system may next evaluate whether there is a low battery charge. The low battery charge condition may also be evaluated when descent indicates that a condition has been met. If there is a low battery charge condition, all descend maneuvers may be halted until the compressor temperature rises although ascend maneuvers may still be permitted. If there is no low battery charge condition, the next prioritized condition may be whether the lift gas superpressure is high, but not critically high above a threshold. If it is above the threshold, the system may halt descend maneuvers until the superpressure drops, while ascend maneuvers are still permitted. If it is below the threshold, then descend maneuvers are permitted.
The information about the conditions may be stored in memory, e.g., memory 308 and/or memory 704, for instance in a lookup table or other relational database. The processor(s), e.g., processor 306 and/or processor 702 may receive information from one or more sensors. The processors may evaluate the conditions sequentially as they are received, or they may evaluate HAP operational characteristics over a period of time, such as 10-50 milliseconds or 1-2 seconds. In one scenario, the ACS and lateral propulsion systems may be operated entirely independently via control system 304 and electronics module 700. In this case, the responses to conditions for the ACS may be done without considering any conditions associated with the lateral propulsion system. Similarly, the responses to conditions for the lateral propulsion system may be done without considering any conditions associated with the ACS. Alternatively, the ACS and lateral propulsion systems may be operated in conjunction with one another, either using a single control system or by different control systems that are able to share information with one another. In these cases, the system(s) may evaluate the conditions associated with both the ACS and lateral propulsion system before deciding on what actions or corrective operations to take.
As shown above, the HAP's systems can implement propulsion and/or altitude control operation inhibitors, such as with a rule-based approach according to a hierarchy of conditions. Thus, faults, failures or other adverse conditions that relate to altitude control and/or lateral propulsion can be addressed systematically according to priorities for different conditions.
The foregoing examples are not mutually exclusive and may be implemented in various combinations to achieve unique advantages. As these and other variations and combinations of the features discussed above can be utilized without departing from the subject matter defined by the claims, the foregoing description of the embodiments should be taken by way of illustration rather than by way of limitation of the subject matter defined by the claims. In addition, the provision of the examples described herein, as well as clauses phrased as “such as,” “including” and the like, should not be interpreted as limiting the subject matter of the claims to the specific examples; rather, the examples are intended to illustrate only one of many possible embodiments. Further, the same reference numbers in different drawings can identify the same or similar elements.
