Underwater Planar Vertical Take-Off and Landing vehicle design
Keywords: Vertical Take-Off and Landing (VTOL) vehicles, advanced non-linear control
The Underwater Planar Vertical Take-Off and Landing (UWPVTOL) vehicle
is a control rig under construction for the demonstration of advanced
non-linear control theory. The problem of building an airborne VTOL vehicle is being tackled at many universities across the world, consuming large amounts of money and research time. The UWPVTOL was devised to avoid these problems and concentrate on the underlying control theory. By operating in a long and skinny water tank the
problem is simplified to three degrees of freedom, and the buoyancy of the vehicle reduces the time-constants of the platform, making it easier to control. In addition operating in water allows the effects of hydrodynamic drag to be simulated at low speeds.
Planar Vertical Take-Off and Landing (PVTOL) systems are a group of platforms operating in three degrees of freedom, namely two translational degrees of freedom (say x and y) and a rotational degree of freedom (say roll). The problem of non-linear stabilisation and control of such systems has been the subject of much research over the past 10 years. This quantity of research may be attributed to the systems interesting dynamics. Namely, it is underactuated, non-linear, unstable and non-minimum phase.
More information about the current state of the UPVTOL project can be found in the report listed below.
The following tasks still remain before the platform may be used to validate the many PVTOL control laws:
- Waterproofing: There were some issues with water leakage (both into the tadpole and from the tank). It is believed that the leaks from the tank have been addressed, but the seal on the tadpole needs improvement. Once the new seal is implemented it needs to be pressure tested.
- RF Communications: The existing RF modems suffer from several problems. New modems have been selected and purchased. Several tasks are still required for these to work with the system, namely:
- (1) Installation of the new RF modems in the tadpole and the host computer
- (2) Coding the MiniDragon microprocessor to work with the new modems
- State Estimation: In order to conduct control it is necessary to know the plant states. This requires the successful completion of the following subtasks:
- (1) Inertial Measurement Unit (IMU): Currently the platform uses a Microstrain 3DMG IMU to provide inertial estimates of the plant states. It is very likely that this will not work that well being so close to the motor, since the EMI from the motor will impact on the magnetometer data. If this is the case, then a cheaper and smaller solution is to mount some (two) accelerometers and a gyroscopes such as those sold by Sparkfun.
- (2) Pressure sensor: A set of two pressure sensors at the extremes of the craft could be used to determine the approximate depth of the platform. This will of course be impacted by dynamic pressure from flow.
- (3) Absolute positioning system: It is expected that the above two positioning solutions will not be adequate and a low bandwidth absolute positioning system will be necessary to augment the other measurements. An external vision system or perhaps an onboard sonar could be used for this purpose.
- (4) Kalman Filter: All the position data will need to be fused by a complementary filter or a Kalman filter.
- System Identification: In order to control the plant it will be necessary to conduct several forms of system identification.
- (1) Measurement of mass distribution
- (2) Measurement of thrust
- (3) Measurement of drag coefficients in translation and rotation
- Control system:... either onboard the MiniDragon, using the RF modems as diagnostics/parameter updates, or externally.
Relevant papers by us on VTOL control
- UWPVTOL (4 Images)
The Under Water Planar Vertical Take-Off and Landing project has been launched as a platform for exeriments with advanced non-linear control strategies.