Patent Application
20160229533
Fuel is a major expense for most aircraft, so there has been a great deal of effort expended in developing energy efficient flight plans. A power plant large enough for take-off and climbing flight, is two to four times larger than needed for level flight. Most engines run much less efficiently when operated at a fraction of their maximum power. Transmissions are used to correct this mismatch in terrestrial vehicles.
Soaring gliders provide the best energy efficiency for experienced pilots since they use energy in thermals to stay aloft. Touring motor gliders provide energy efficient cross-country flight without reliance on thermals. However, they must plan for a landing site just in case the engine does not restart. They have a minimum safe height above ground.
Unmanned aerial vehicles (UAV) are an excellent vehicle for close-in inspection of utility corridors. They do not require the same safety margin as manned flight, so they can be flown at much lower heights above ground. Regulatory agencies limit their weight, speed, and/or the height above ground under which they may be used. For example, in the United States they must currently stay under 400 feet above ground level. New regulations are forthcoming, but if prior regulations are a guide, then there will be different requirements based on total weight and speed. Previous regulations had breakpoints at 2, 9, and 25 kg. In the UK a 20 kg airframe flown below 400 feet operates under BNUC, but a higher flight or larger vehicle is treated as a manned airframe. In Germany a 5 kg UAV may be flown up to 300 m. A light UAV has fewer regulatory constraints. In a UAV with an electric power plant, the battery is a large fraction of the weight of the airframe. Efficient flight reduces the energy required to complete the mission and thus the battery weight.
Utility and transportation corridors are long linear rights of way with occasional point features or points of interest that have to be inspected in detail. For example overhead electrical lines include transmission, distribution, telephone, cable TV, and electric railway lines plus the towers to support these lines. They need the right of way inspected for vegetation incursions; and the towers inspected for damage and deterioration. Oil, gas, and water lines need the right of way inspected for incursions or leaks; and the pumping stations and valves inspected in much greater detail. Transportation corridors include roads, railways, and canals. They need a general inspection of the road, rail, and canal right of way with much more detailed inspection of bridges, signs, signals, and locks.
Although energy efficient flight paths are well known in the prior art, various aspects of the embodiments of my corridor inspection flight path are advantageous because:
The power plant is run at optimal efficiency for the climbing segments,
The power plant consumes no energy during the gliding segments.
The power plant and fuel or battery can be sized correctly for the mission thereby minimizing airframe weight,
Further unexpected advantages include:
When the power plant is off, the vibrations in the airframe are much less, eliminating one source of blur in the photographic exposures,
The arcs around the towers reduce blur due to forward motion,
The drift in the inertial measurement unit can be reset in the loops around the towers,
Excessive corona discharge, possibly indicating insulator failure at a tower, is much easier to detect with microphones (audible and ultrasound) and radio frequency meters when the power plant is off,
Altitudes measured on the outbound right of way inspection can be used for precision flying during the return detailed inspection.
Other advantages of one or more aspects will be apparent from a consideration of the drawings and ensuing description.
1. Perspective view of utility corridor inspection flight path for transmission lines.
2. Cross-path section of utility corridor inspection flight path for transmission lines.
3. Perspective view of transport corridor inspection flight path for highway.
4. Cross-path section of transport corridor inspection flight path for highway.
5. Flight planning flowchart for determining flight parameters.
6. Autopilot flowchart for flying an efficient corridor inspection.
This section describes several embodiments of the efficient corridor inspection flight path with reference to
This example is for a single circuit 500 kV transmission line where towers 40, 42, and 44 are ˜40 m high, ˜25 m wide, and spaced 200-600 m apart. Phase conductors 46, 48, and 50 are aluminum strands over a steel core with a total diameter of 3-6 cm. Shield wire 36 and 38 are 1-2 cm in diameter.
Transmission line inspection involves three different mission profiles, each with angle of view and resolution tradeoffs. These will be illustrated in the following paragraphs with a sample camera that has a full-frame sensor (24×36 mm), 5 micron pixel spacing, and a 28 mm focal length lens.
It is desirable to capture the entire field of interest in one flight, rather than repeatedly flying the corridor. It is also desirable to use a fixed focal length lens since zoom lenses have changing interior orientations that make them difficult to calibrate for photogrammetry. Fixed prime lenses generally have better optical characteristics than zoom lenses of equivalent focal lengths. A wide angle lens with a 24 to 28 mm focal length (35 mm equivalent) provides a wide angle of view without excessive distortion and aberration. For transmission line inspection, cameras are usually oriented with the long sensor dimension along the flight path. This allows faster flight speeds for a given image overlap and the camera framing rate.
The angle of view for a given lens and sensor combination equals two times the arctangent of half the sensor dimension divided by the focal length
AOV=2*atan(sensor dimension/(2*focal length))
For a full-frame sensor aligned with the flight path (24 mm dimension across the path) with a 28 mm focal length lens, the angle of view is 46 degrees, as shown in
The resolution in aerial photography is estimated by the ground sample distance (GSD). The GSD is the separation between camera pixels as projected on the ground. The GSD equals the object distance (flying height above ground here) times the pixel separation divided by the lens focal length. So for 5 micron pixels and a lens focal length of 28 mm, at a flying height of 400 feet (˜120 m), the GSD˜2.1 cm. The sample distance requirement sets the maximum distance from camera 16 to the object of interest.
The first type of mission in transmission line inspection is checking the right of way for vegetation incursion, man-made incursions, and wire clearances. A ground sample distance (GSD) of about 5 cm and an angle of view (AOV) encompassing the entire right of way 58 would allow these checks. For a 28 mm equivalent lens, a flying height of about 75 m as shown in angle of view 52 in
If towers 40 and 42 are 500 m apart and airframe 10 has a 20:1 glide ratio, then starting with a flying height of 75+25=100 m at tower 40 allows linear right of way glide 18 to be done with power plant 11 off. At the start of linear right of way glide 18, the field of view is larger than required, but the GSD˜1.8 cm is still fine enough to meet the mission objectives. By turning power plant 11 off, linear right of way glide 18 does not consume energy and it reduces vibration in airframe 10, thereby allowing much sharper images.
For transmission lines in hilly terrain, the towers may be at different altitudes. If tower 42 is uphill from tower 40, then the start of linear glide 18 at tower 40 will need to be higher. If the required start is so high the GSD distance gets too big, then the end of linear flight path 18 may need an additional power climb.
A second type of mission in transmission line inspection is checking towers for insulator damage (gun shots and contamination), bird nests, tripped surge arrestors, loose nuts, corrosion, etc. These can be detected with a sample distance at the tower of about ½ cm. At the 74 m flying height at the end of linear right of way glide 18, the object distance to the center conductor 48 is about 55 m to give a tower sample distance of about 1 cm. To get a smaller tower sample distance, airframe 10 enters spiral tower glide 20, again with power plant 11 off. This spiral provides higher resolution inspection images from all angles around the tower and still does not use any energy for flight.
Several more advantages accrue from turning power plant 11 off for tower inspection. First, it is much more effective to use audible and ultrasound microphones to detect corona and partial discharge. Partial discharge can be a predictor of insulator failure. Second, radio and television frequency interference is much easier to detect. Thirdly, vibration from power plant 11 is stopped, reducing blur due to camera shaking. Finally flying an arc pivoting around the point of interest reduces forward motion blur.
When the tower inspection is complete, power plant 11 is turned on at maximum efficiency in power climb 22 to increase the altitude of airframe 10 quickly, ready for next linear right of way glide 24.
The third type of mission in power line inspection is checking the wires. Phase conductors are checked for broken strands or corrosion; deteriorating splices, spacers, or dampers; and missing marker balls. Shield wires are checked for lightning damage. If a shield wire is 1.5 cm in diameter and a lightning strike melts a third of the wire, then for reliable detection the wire sample distance should be less than ¼ cm. For the example camera with 5 micron pixels and a 28 mm focal length lens, airframe 10 with camera 16 should be within 14 m of the shield wire. At that distance the angle of view is not large enough to image all the wires in the configuration of
Photogrammetry for mapping and measuring areas of the earth's surface emphasizes vertical photographs taken from a uniform flying height. Vertical photographs (tilt<3 degrees) have the same scale at all parts of the image. A uniform flying height makes it easier to stitch adjacent photographs together into a mosaic. The mosaic is then orthorectified to produce a digital elevation model and orthophoto with accurate results for an entire area of the earth's surface.
Oblique photographs are more useful in corridor inspection because they use the available angle of view on the objects of most interest. The scale does change from large at nadir to small in the distance. For corridor inspection the goal is detailed images of the right of way and points of interest in the corridor. Since resampling blurs detail, it may be better to present the viewer with the original images and calculate distances as needed. If the camera interior and exterior orientation are known at the time of exposure, then the scale in different parts of the image can be calculated.
Safety regulations require unmanned aerial vehicles to be able to sense and avoid other aircraft and obstacles. This is difficult to do with current sensor technology, so a human observer with radio control over the UAV acts to sense and avoid problems. For corridor inspection, a useful configuration is two UAVs flying on port and starboard of the right of way with a general aviation plane flying above them carrying an observer to sense and avoid problems. The UAVs each image a quadrant of the towers and wires that is difficult to see from the ground. If the interior and exterior orientations are known at each camera exposure, then the images can be used as stereo pairs to measure ground elevations and wire clearances.
In the configuration of two UAVs with an observer trailing in a general aviation plane, an out-and-back flight is preferable to a one-way flight. The general aviation plane has to return to its home airport and it is much more convenient to launch and land the UAVs in one location. The relative tower elevations can be measured on the way out during the right-of-way 18, 24 and tower 20, 26 inspections. Then the wire inspection 28, 30, 32, 34, which is flown much closer, can be flown precisely and safely.
Most transmission lines have the same tower configuration along the entire line to keep the wire clearances uniform. However, for impedance balancing, they will likely have transposition towers at one third and two thirds of the line length. The flight path described above is typically loosened up at transposition towers.
The above example is for a transmission line, but other overhead lines such as distribution, telephone, cable TV, and electric railway lines can be inspected with this efficient flight path.
Other utilities in corridors such as oil, gas, and water pipelines can be inspected with a similar flight path. The right of way is inspected for encroachment, signs of leaks (dead vegetation, discoloration, Airborne Laser Methane Assessment), sunken backfill, erosion, and evidence of heavy traffic. The pumping or compressor stations and valves are inspected for leaks, corrosion, and deterioration in much more detail.
Similar to the transmission line utility corridor inspection example above, the transportation corridor inspection illustrated in
The detailed point inspection of a bridge with spiral glide 102 needs to capture the bridge in the angle of view so it can be flown lower and closer with a better GSD. The right of way is larger at an interchange, so power climb 104 flies a larger circle inspecting the right of way while climbing. At the appropriate altitude and lateral offset, airframe 10 starts the next linear glide 106.
A detailed roadway inspection with linear glide 108 and power climb 110 is flown much lower and closer with a field of view just encompassing the roadway to get a GSD of about ½ cm. Power climb 110 can be done between taking inspection photographs to maintain the benefit of low-vibration images.
The inspection objectives 130 include the minimum field of view, maximum sample distance, and photo overlap for each mission profile. For the transmission line example illustrated in
The corridor configuration 132 includes the width of the right of way as a minimum. The operator may specify the flight path lateral offsets from the right of way centerline, both for asymmetric corridors, as well as for those situations where adjacent landowners may not agree to overflights for their property. The vertical elements in the right of way, e.g. towers 40, 42, 44; fences 120 and 122; bridges, signs, signals, pump houses, etc. have a big impact on the field of view and flight height above ground. For the transmission line example of
A very flexible approach is to provide a small computer aided design (CAD) interface to allow the operator to manipulate the corridor geometry and the angles of view for different missions to produce figures much like
The corridor location is defined by a set of waypoints (latitude, longitude, and altitude) for both the corridor centerline and the points of interest. If readily available, these may be entered by the operator. Easier for the operator is to ask for starting and ending points and then to trace the corridor on maps or satellite images. For example, in satellite imagery with ˜0.5-1 m resolution (e.g. Google and Bing), power transmission towers can be clearly identified, and depending on the light and contrast with the background, the phase conductors may be visible. While mapping the corridor location it is helpful to define a number of accessible planned and emergency landing sites.
After the mission objectives 130, corridor configuration 132, and location 134 are known the flight path has to be checked 136 against regulatory, airframe, and camera constraints. UAV operation is limited within a certain radius of airports (e.g. 3 miles in US), over military installations, or in other restricted airspace. Given the waypoints for the corridor location 134 and the flight height above ground from the corridor configuration 132, the flight path can be checked against government maps published for pilots. If problems are detected, the system can allow the operator to split the inspection into multiple segments, or suggest they ask an exemption for the flight.
The airframe will have a minimum flight speed of about 20% more than the stall speed, a maximum climb rate, specific glide slope, and fuel or battery capacity. The camera will have a maximum framing rate, a limit on picture storage, and a limit on battery life. The flight paths have to be checked against each of these limits with some reasonable estimates of headwinds to avoid problems in the field. If satellite imagery is available, then a simulated flight over the terrain (e.g. Google Earth) makes a quick check for errors or slipups in altitude or location.
After the flight path check 136, the flight plan is saved 138 for current and future inspections of the corridor. They are communicated to autopilot 14 to fly the efficient flight path, as described with reference to
Right of Way Inspection: Both port and starboard flight paths, including lateral offsets from corridor centerline, maximum height above ground for desired sample distance, minimum height above ground for desired field of view, camera declination from horizontal at both maximum and minimum heights, and image overlaps.
Point Inspection: Object location and height above ground relative to corridor centerline waypoint, maximum distance from object for desired sample distance, minimum object distance for desired field of view, camera declination from horizontal for maximum and minimum distances, and number of images spaced at angles around object.
Detailed Inspection: Camera offset lateral and vertical to get field of view; follow catenary (overhead electrical lines), terrain (pipelines), or road/railbed (transport); and image overlaps.
Corridor: Waypoint latitude, longitude, and altitude for the corridor and each point of interest, including planned and emergency landing sites and transposition towers.
To calculate glide start altitude 144, in this example for linear glide 24, autopilot 14 determines the distance and altitude of the next point of interest, for example tower 44. Given the minimum height above ground for the right of way inspection at the end of linear glide 18; the airframe glide ratio; the airframe penetrating glide speed; and the current head or tailwinds, it is straightforward to calculate the speed over ground and thus the altitude required at the start of linear glide 18 to get to tower 44 with power plant 11 off. If this is less than the maximum height above ground for the desired sample distance, then power climb 146 proceeds directly to this altitude. If it is more than the maximum, then the autopilot plans for an intermediate power climb before the point of interest.
To fly power climb 146, autopilot 14 turns on power plant 11 and adjusts control surfaces 12 for maximum climb to the altitude calculated in step 144 and the lateral position defined by the flight plan from
To fly linear glide 148, autopilot 14 turns off power plant 11 and adjusts control surfaces 12 to get a glide speed that allows camera 16 to meet the image overlap requirements at its framing rate. If it comes close to the minimum height above ground for the desired field of view, then it plans for another power climb by going directly to calculate glide start altitude 144.
If there is an object of interest at the end of linear glide 148, then autopilot 14 adjusts control surfaces 12 to spiral glide 150 around the point of interest. Using the waypoint location, the object height above ground, the minimum distance for desired field of view, and the maximum distance for the desired sample distance at the point, airframe 10 spirals down and typically inward, taking photographs at the orientations specified in the flight plan.
If this is not the last waypoint 152, then the loop starts again with calculate glide start altitude 144. Otherwise, if there is no return flight 154, airframe 10 lands 156.
If there is a return flight, either with a different mission profile or because one UAV is flown out and back, then the climb, glide loop begins again (158, 160, 162, 164, 166). If the return mission is a more detailed inspection for a smaller field of view, then more precise flying is required in following the catenary arcs for overhead lines (28, 30, 32, 34), the terrain for pipelines, and the road/railbed for transportation corridors (108, 110, 112).
This section illustrated details of specific embodiments, but persons skilled in the art can readily make modifications and changes that are still within the scope. For example, the figures have illustrated a fixed wing UAV, but a rotary wing with a shallow descent path could also use this energy efficient flight path. The embodiments described have focused on inspection with a camera, but other inspection sensors such as UV cameras, IR cameras, LiDAR, RADAR, audible and ultrasound noise, or RF noise could also be used.
This application claims the benefit of provisional patent application Ser. No. 61/937,048 filed 2014 Feb. 7 by the present inventor.
