1. Field
Advancements in mechanical isolation are needed to provide improvements in performance, efficiency, and utility of use.
2. Related Art
Unless expressly identified as being publicly or well known, mention herein of techniques and concepts, including for context, definitions, or comparison purposes, should not be construed as an admission that such techniques and concepts are previously publicly known or otherwise part of the prior art. All references cited herein (if any), including patents, patent applications, and publications, are hereby incorporated by reference in their entireties, whether specifically incorporated or not, for all purposes.
An example of a camera is an image capturing system that captures imagery using a lens that focuses light on at least one Petzval surface (e.g. a focal plane), and captures an image with at least one image sensor on the Petzval surface. A focal plane is an example of a planar Petzval surface. In general, Petzval surfaces are not required to be planar and may be curved due to the design of the lens. Examples of image sensors include film and electronic image sensors. Examples of electronic image sensors include Charge Coupled Device (CCD) sensors and Complementary Metal-Oxide Semiconductor (CMOS) sensors. An example of an emerging optical axis of a camera is the path along which light travels from the ground at the center of the lens field of view to arrive at the entrance to the camera. The light path inside the camera may be folded with reflecting surfaces, but eventually light arriving along the emerging optical axis will converge at the center of the Petzval surface(s).
Some maps assume a camera perspective looking straight down, called an orthographic (or nadir) perspective. In some scenarios, this is also the perspective of the captured images used to make these maps (e.g. orthographic imagery). However, orthographic imagery eliminates all information about the relative heights of objects, and information about some surfaces (e.g. the vertical face of a building).
Other maps assume a camera perspective looking down at an angle below the horizon but not straight down, called an oblique perspective. An example of a down angle of a camera is the angle of the emerging optical axis of the camera above or below the horizon; down angles for nadir perspectives are thus 90 degrees; down angles for oblique perspectives are usually 20 to 70 degrees. Sometimes, the camera used to capture an oblique perspective is referred to as an oblique camera and the resulting images are referred to as oblique imagery. In some scenarios, oblique imagery is beneficial because it presents information that is useful to easily recognize objects and/or locations (e.g. height and vertical surfaces); information that is typically missing from orthographic imagery.
An example of ground-centric oblique image collection is capturing the same point on the ground in multiple oblique images from multiple perspectives (e.g., 4 perspectives looking at a building, one from each cardinal direction: North, South, East, and West). Ground-centric collection yields ground-centric oblique imagery. In various scenarios, ground-centric aerial oblique imagery is useful, e.g. for assessing the value of or damage to property, particularly over large geographic areas. It is usually a priority in a ground-centric collection program to collect an image of every point in some defined target area for each of the cardinal directions. The capture resolution is measured in distance units on the ground (e.g., 4 inch per pixel) and usually does not vary much between different points in the target area.
An example of a strip of oblique and/or nadir imagery is a sequence of individual oblique and/or nadir images. In some scenarios, sequential images overlap (e.g., by 50-60%) to ensure that each point on the ground is captured at least twice (e.g. for stereopsis). To capture an entire region, multiples strips are collected and then stitched together. Typically, portions of the strips are discarded (e.g. jagged edges) to ensure a smooth fit.
An example of sky-centric collection is capturing multiple oblique images from a single point, with multiple perspectives (e.g., 4 perspectives looking from a building in each cardinal direction), also known as sky-centric collection. In some scenarios, sky-centric imagery is commonly used to form a panoramic view from a single point. It is usually a priority in a sky-centric collection program to collect a continuous panorama from each viewpoint. Capture resolution is usually measured in angular units at the viewpoint (e.g., 20,000 pixels across a 360 degree panorama).
An example of a camera-group is a system of one or more cameras that approximately capture the same image (e.g. the optical axes are aligned within 5 degrees of a common reference axis). For example, an ordinary pair of human eyes acts as a 2 camera-group, focusing on a single image. Generally, a camera-group can have an arbitrary number of cameras.
An example of a camera-set is a system of one or more cameras and/or camera-groups that capture different images. One example of a 2 camera-set is a nadir camera and an oblique camera. Another example of a 4 camera-set is 4 oblique cameras, each pointing in a different cardinal direction. Generally, a camera-set can have an arbitrary number of cameras and/or camera-groups.
An example of the nominal heading of a vehicle is the overall direction of travel of the vehicle. In many scenarios, the instantaneous direction of travel deviates from the nominal heading. For example, an airplane may be flying along a flightpath heading due north, so that the nominal heading is north, while experiencing a wind blowing from west to east. To keep the plane on the flight path, the pilot will point the plane into the wind, so that the instantaneous heading is many degrees west of north. As another example, a car is driving down a straight road that runs from south to north and has several lanes. The nominal heading is north. However, to avoid hitting an obstacle, the car may changes lanes, instantaneously moving northwest, rather than strictly north. Despite this instantaneous adjustment, the nominal heading is still north. In contrast, when the car turns 90 degrees from north to travel west, the nominal heading is now west.
The motion of a vehicle collecting oblique and/or nadir imagery poses significant challenges to collecting high quality imagery; in some scenarios motion or vibrations from the vehicle couple to the camera (or collectively the camera-set) while it captures images, causing blurring or other artifacts. Some vehicles, such as planes, move freely throughout three dimensions of space (sometimes referred to as linear motion or translation, e.g. forward/back, up/down, and left/right) as well as three dimensions of rotation (sometimes referred to as angular motion or rotation, e.g. yaw, roll, and pitch).
In some scenarios, crosswinds cause significant yaw, which alters the angles of the emerging optical axis of the camera and changes the captured image. Sufficiently large crosswinds could cause the camera to miss portions of the intended target (e.g. a portion of a strip on the ground). In other scenarios, pitch and roll cause motion blur, which forces shorter exposure times and decreases the sensitivity of the camera. For cameras that use an array of multiple sensors, roll can cause the camera to miss portions of the ground.
In various scenarios, linear motion along the nominal heading causes motion blur. In other scenarios, linear acceleration cause one or more cameras to flex, which disturbs the relative emerging optical axes of the different cameras in a camera-set and in extreme cases could cause the mirrors to sag and change focus.
In some scenarios, vibration is problematic because it represents a combination of linear and angular motion. Vehicles have a number of sources of vibration, which can couple into the camera and cause various problems as outlined above.
An example of a stabilizer is a device that isolates a payload (e.g., a camera, a robot, a drill, etc.) from linear and/or angular motion (e.g., shock and vibration). One type of stabilizer is a passive stabilizer (e.g., a spring and/or dashpot), where the behavior of the stabilizer is primarily determined by the materials and structure. Some passive stabilizers are critically damped; in response to a shock, the stabilizer will converge to the original position with a single overshoot in the shortest amount of time possible. Other passive stabilizers are overdamped; in response to a shock, the stabilizer will return the payload to the original position without overshoot over a relatively longer period of time. Yet other passive stabilizers are underdamped; in response to a shock, the stabilizer will oscillate the payload around the original position with exponentially decaying amplitude and eventually converging to the original point. This oscillation will happen at the resonant frequency of the isolated payload. An underdamped stabilizer will more strongly attenuate high frequency vibrations, but certain low frequency vibrations (e.g., at the resonant frequency of the system) are transmitted more strongly. All passively stabilized systems have a resonant frequency (even critically damped and overdamped systems), which can typically be tuned by varying the spring rate, mass, or damping factor.
Another type of stabilizer is an active stabilizer (e.g., an actuator, a sensor, and a control system), where behavior of the stabilizer is primarily determined by an electronic control system and limited by the measurement and actuation systems. The electronic control system measures variables of the stabilizer and payload via sensors (e.g. accelerometers), and based upon these measurements (and potentially other factors) decides the motion of the stabilizer. The frequency of decision-making by the electronic control system is sometimes known as the control loop frequency. Different sensors are sampled at different rates, some are sampled faster than the control loop frequency (e.g., for variables that change rapidly), while others are sampled slower than the control loop frequency (e.g., for variables that change slowly, such as the temperature of the sensors).
One of the limitations of an active stabilizer is the control loop frequency, which governs the ability of the active stabilizer to respond to external stimuli. Specifically, an active stabilizer cannot effectively isolate vibrations that are faster (e.g., higher frequency) than half the control loop frequency (sometimes called the Nyquist frequency). For example, an active stabilizer with a 100 Hz control loop frequency will perform best at isolating vibrations below 5 Hz. However, at frequencies substantially lower than the Nyquist frequency, active stabilizers are highly effective because of the adaptive and non-linear nature of the system.
An example of transmissibility of a stabilizer is the ratio between the amplitude of an input vibration and the amplitude of the resulting output vibration. Conceptually, transmissibility is a measure of the attenuation for a given frequency of vibration. For example, when a stabilizer receives a 10 Hz vibration with amplitude 2 mm, but the payload only receives a vibration with amplitude 0.2 mm, the transmissibility is 0.1. Generally, the lower the transmissibility, the better a stabilizer is at isolating the payload.
In some scenarios, stabilizers are used to isolate cameras from linear and angular motion, thereby improving image quality. For example, consumers and professional photographers commonly use Steadicams (a type of passive stabilizer) to reduce motion (e.g. from the unstable photographer's hands) and produce higher quality photos and videos; the same principal applies to cameras mounted on vehicles. In other scenarios, cameras are stabilized with active stabilizers, for example the Skycam used to record and televise many sporting events.
An example of an Inertial Measurement Unit (IMU) is an electronic device that measures characteristics of an object such as linear acceleration, angular velocity, and magnetic flux field.
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