Flying is a movement that human beings cannot do under natural conditions, but they can do with the tools they have invented. Vehicles such as helicopters, and airplanes, are produced for such situations. Although it may seem easy to move at first glance, there are too many factors to be controlled by the pilot to operate/fly these vehicles.
In its infancy, the air vehicles were more mechanical systems. Pilots were operating the vehicles mostly with their skills and with basic and limited functionality instruments such as speed meters, altimeters, etc.
Over the years the technology evolved and much more sophisticated instruments are integrated into the cockpit. With the invention of the autopilots, the importance of flight instruments has also increased. Also, the aviation authorities have placed many different rules and regulations for the quantity and quality of the flight instruments. However there was a limited market need as the number of flying machines built, was relatively small.
With the proliferation of UAVs, drones, and small-scale/mode-made rockets the need for flight instruments increased tremendously. Similar to major aircraft manufacturers - if not all - most of the users need ways of testing their flight instruments that are installed on the vehicle. In this article, we will explain a few methods of executing flight instrument tests using a robotics platform called Hexapod.
Flight instruments are the instruments in the cockpit of an aircraft or attached to the autopilot module of a UAV/drone that provides data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading, and much more other crucial information of the flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon.
We will focus on four basic flight instruments, which can be tested with a moving laboratory platform such as the hexapod:
Also known as, the artificial horizon, the AI uses a rigidly mounted internal gyro to display the aircraft’s attitude relative to the horizon. The display consists of a miniature aircraft aligned to the horizon in straight-and-level flight, with a blue sky above and the brown or black ground below.
The TC uses a canted internal gyro to display both the initial roll rate and stabilized rate of turn. An aircraft silhouette mimics the aircraft to show the direction of turn, and will align with a mark on the display if the aircraft is turning at a standard rate of three degrees per second.
The HI uses a rotating gyro to display the current compass rose direction (otherwise known as the heading) that the aircraft is flying. Using a 360-degree compass card with north as zero or “N,” the HI shows headings in 5-degree increments with every 30 degrees enumerated. To reduce crowding, the last “zero” of the heading is omitted—”3” is 30 degrees, “12” is 120, etc.
The Inertial Measurement Unit (IMU in short) measures the inertial quantities, such as accelerations and angular velocities. Modern, low-cost and low-speed aircraft use those quantities for automatic feedback control loop (as, for example, the gyro to stabilize) and/or process the data to estimate the Attitude (roll, pitch, yaw or quaternion) as well. IMU can be considered as the digital-only version of the AI, TC and HI instruments.
The Stewart platform (also known as Hexapod) is a special mechatronics system used for precision position and motion control, originally proposed in 1965 as a flight simulator. Since then, a wide range of applications has benefited from the Stewart platform. By definition, a Stewart Platform is also a parallel robot, but with limited reach and mobility.
There are variants of the platform, but most of them have six linearly actuated legs with varying combinations of leg-platform connections. The full assembly is a parallel robotic system consisting of a rigid body top or mobile plate connected to an immobile base plate. At least three coordinates define the center of the top plate, and the aim is to move this center point in 3D space.
Devices placed on the top plate can be moved in the six degrees of freedom in which it is possible for a freely suspended body to move. These are the three linear movements x, y, z (lateral, longitudinal, and vertical), and the three rotations (pitch, roll, and yaw). These movements make the Stewart Platform a good candidate for testing various flight instruments or other motion-related parts of the aircraft.
Testing flight instruments can be done manually. For instance, if you apply pressure slowly and steadily you can test the Airspeed Indicator or Altimeters. These items require testing and adjustment to ensure that the altitude displayed is within 25 feet of the altitude indicated by calibrated test equipment.
To test/calibrate the Turn Coordinator, Attitude Indicator a known and precise angular displacement should be applied to the sensor.. This method will give certain and repetitive displacement to the device under test, ie. the flight instrument. Test operators or maybe the test software then observe the measured signal of the flight instrument and compare its measurement against the input value. If the measurement is within the error margin, then the instrument will be accepted as an okay product. In case it is out of the error limits, then it is a Not Ok product and needs further analysis. As an example drone manufacturers like Fotokite use Acrome’s Stewart Platforms to simulate and test the sensors of the drones that they are manufacturing.
ACROME’s Stewart Platform also provides an example application to connect to flight simulation software. The Platform can exchange data over the TCP port of the simulation software. This gives a way to integrate a virtual reality component into the hardware tests. In this case, users or test operators can also analyze the measurements together with the visual markers available in the scene.
With the open source software option of ACROME’s Stewart Platforms -apart from the provided ready-to-use software- users can expand their test scenarios, and change and improve the Platform’s software based on the new requirements. This makes it a very powerful feature for long-term hardware investment.