«An Autonomous Vision-Guided Helicopter Omead Amidi August 1996 Department of Electrical and Computer Engineering Carnegie Mellon University ...»
On-board integration for autonomous outdoor flight raised a number of important concerns. It was not clear that the visual odometer machine and other control and power systems could, in fact, be integrated on-board a small helicopter capable of performing a useful mission. Furthermore, since the indoor flight tests employed off-board computing and power, vibration and noise effects of the helicopter power plant on the on-board systems were never investigated. Of major concern were the effects of engine noise and vibration on camera image quality, high bandwidth image transfer integrity, and on-board navigational sensor data.
This chapter describes the experimental approach to develop and verify an integrated visionbased helicopter control system on-board a mid-sized model craft, the Yamaha R50. The chapter presents the integration of different system components such as vision computing, attitude sensing, lowlevel control, and power systems. Each component was individually tested using indoor testbeds, and was designed to physically fit and weigh within the small available space and payload of the helicopter.
Shown in Figure 4-1 under human pilot remote control, the R50 is a commercial product of Yamaha Motor Company for aerial agricultural pest control. Designed for spraying fields in hard to access areas, the R50,unlike airplane crop dusters, can fly close to the ground at slow speeds. This ability significantly improves pesticide effectiveness and reduces the undesirable overspray and dispersion of chemicals in the atmosphere.
Powered by a 98 cc water cooled 2-stroke engine which produces 12 HP, the R50 has a payload of 20 Kg with a maximum takeoff weight of 67 Kg and can continuously operate for 60 minutes. Its overall length is 3.5 meters, with a main rotor diameter of 3.1 meters and body length of 2.7 meters.
4.7 Indoor R50 Testbed Several factors prompted the development of a tethered testbed for the R50 as an intermediate step towards autonomous operation. Initial experiments with the R50 would be significantly safer with protective tethers since the R50 is quite massive and can accelerate to dangerous speeds rather quickly. In addition, it is easier to investigate and resolve different system integration issues indoors using proper test equipment. In particular, effects of vibration on sensors and computing enclosures can be quite significant and requires careful investigation.
4.7.1 Testbed Design As shown in Figure 4-2, the testbed design allows relatively large (1.5 meter) longitudinal travel while severely limiting helicopter travel laterally and vertically. Yet, despite this limitation, the one-axis travel arrangement can provide great insight regarding the behavior of the R50 under total computer control outdoors, and can test the integrity of the vision-based positioning and control systems.
An off-the-ground, heavily reinforced platform is built to provide a level area for helicopter longitudinal movement. To prevent tail collisions during large helicopter pitching which may occur during undesirable oscillations, the platform is designed to be shorter than the R50.
The R50 is tethered with ropes which are fastened to the ground and two poles positioned on either side of the platform as shown in Figure 4-3. A steel rod with hooks on either end connects the ropes to the R50. The rod is secured to the helicopter’s center of gravity to eliminate any torques from restraining forces which could cause dangerous rotations.
Different kinds of rope restrain the lateral and vertical axes of the R50. High strength and rigid Kevlar rope is used for rigid vertical restraint and limit the possibility of large rotations. High strength flexible nylon rope is used for lateral restraint to ease impacts to the helicopter when it reaches the travel extremes. This flexibility significantly reduces the magnitude of impacts experienced by the main rotor hub which absorbs forces generated by changes in the momentum of the spinning rotor blades as the helicopter hits travel extremes. To further limit potentially dangerous helicopter rotation, longer cylindrical skids with smooth plastic ends to reduce friction are installed on the helicopter.
4.7.2 Testbed R50 Helicopter The testbed R50 helicopter is used to design and evaluate the power system, sensing, and computing for on-board integration. An on-board chassis supports all on-board systems for indoor flight tests.
The chassis aligns cameras and attitude sensors, as well as isolating harsh vibration from system components. Figure 4-4 shows the different components of the testbed R50 helicopter.
126.96.36.199 On-board Power A 12 V (7 AH) battery supplies all the power for on-board computing and sensors. The power dissipated is about 150W. DC-DC regulated converters and custom-made filtering circuits provide clean +/-12 and +5 V power signals from the 12V battery even when it is drained to as low as 9 Volts. The on-board computer monitors the battery voltage by an A/Dconverter and produces warnings if the voltage drops below 10.5 Volts.
188.8.131.52 Attitude Sensors Two light-weight (40 g) and low cost gyroscopes, made by Gyration Inc., are mounted close to the center of gravity for attitude measurement. One gyroscope is directional for measuring helicopter heading and the other is vertical to measure roll and pitch. Both gyroscopes have two nested, optically encoded gimbals with 0.2 degree angular resolution. The gyroscopes are mechanical and incorporate a mass spun at high speeds by a DC motor. The motor speed is regulated by the input voltage and is set at its maximum (15,000 RPM) for best performance.
The directional gyroscope is quite drift prone and a few (2-4) degree per minute accumulating drift rates are not uncommon. The drift rate of the vertical gyroscope’s roll and pitch angles is similar but does not accumulate over time since it incorporates a pendulous inner gimbal with a 10 minute time constant. Using the gravity vector to eliminate long-term drift, the pendulum levels the gimbals.
This leveling scheme assumes zero lateral acceleration over long time periods the order of 15-20 minu tes.
Both gyroscopes provide relative angular measurement by generating digital pulses as the helicopter changes its attitude. The pulses are integrated to estimate attitude relative to starting gimbal positions. The vertical gyroscope levels itself in a 20 minute period and the roll and pitch values are initialized using the gravity vector measured by three accelerometers. The heading from the directional gyro is simply initialized to zero in the forward direction of the testbed platform.
An accurate navigational quality vertical gyroscope, on loan from Humphrey Gyroscopes, is used to evaluate the performance of the small inexpensive vertical gyroscope during indoor flight experiments.
184.108.40.206 Testbed Cameras, Scenery, and Lighting As shown in Figure 4-5, two ground pointing CCD cameras (Sony XC-75) are mounted on the side of the R50. The cameras are fitted with wide angle (6 mm focal length) lenses to provide the large view angle necessary for low altitude image processing.
A non-repeating stone pattern is printed on the testbed platform to provide a feature rich scene for the visual odometer to lock on to. The pattern is covered with plexiglass for protection from the helicopter skids. The plexiglass is matted with sandpaper to reduce undesirable reflections.
The cameras are synchronized by the sync generator to produce non-interlaced fresh image fields at approximately 60 Hz. The shutter speed is set at open at 1 millisecond intervals to provide clear images under the harsh vibration as well as proper synchronization with attitude sensors.
The testbed is lit by multiple light sources from different out-of-phase circuits to reduce image flickering due to 60 Hz AC power, exaggerated by the short shutter opening interval. To further reduce
the flickering effects caused by the indoor light sources, the cameras are synchronized with 59 Hz vertical frequency, governed by a synchronization generator, instead of exactly 60 Hz.
220.127.116.11 On-board Computing A seven slot VME cage houses all computing hardware (only six slots are required). The cage is not used on-board for indoor experiments and is removed during actual flight experiments to protect it from the unknown vibration characteristics of the R50. A tether transmits camera signals and sensor data to the cage and an RF transmitter sends control signals to the helicopter actuators.
18.104.22.168 On-board Chassis Mounting For easy integration on multiple helicopters, the sensors and the computing enclosure are integrated into one detachable package using a chassis shown in Figure 4-5. During the indoor experiments, all the sensors are used during indoor experiments except the laser rangefinder which is mounted on the chassis for future outdoor experiments.
The chassis could not be mounted on the helicopter until vibration characteristics of the R50 are determined. Since the chassis itself can affect the vibration, all chassis components are weighed and replaced by one large metallic plate of equivalent weight to approximate the inertial characteristics without using actual components. For vibration analysis, three accelerometers, aligned with helicopter axes, are mounted on the metal plate.
A variety of mounts are tested, starting with solid rubber at the chassis’s four mounting points.
With rubber mounts, the accelerometers sensed a 15 g wave at frequencies as high as 167 Hz. To reduce this vibration, cylindrical rubber mounts with varying density are tested under real flight conditions. Commercial mounting materials could not withstand the vibration amplitudes and caused large chassis oscillations which are undesirable for image processing. Different rubber mounts are manufactured by drilling holes in different densities in the rubber and individually tested until the maximum observed vibration of each axis is within 0.5 g.
4.7.3 Testbed Safety
Careful tethering of the helicopter does not guarantee the safety of nearby individuals. The rotor blades may shatter or tear off the helicopter, thereby causing major injury. For safety, a control room is constructed for indoor flight experiments. To stop flying debris, two small windows are covered with wooden barricades supporting multiple layers of bullet-proof Lexan and steel mesh reinforcements.
To prevent violent helicopter oscillations, a safety control system is developed as a backup for vision-based flight experiments. In case of vision system failure, the backup control system measures helicopter position using string potentiometers. The back up system automatically takes over helicopter control if large longitudinal velocities are observed. In case of backup control system failure, an experienced safety pilot is also present in the control room to take over control using an RF transmitter.
4.8 Helicopter Controller An effective helicopter control strategy is as important as the positioning estimation method. Helicopter control is inherently challenging due to dynamic coupling, and nonlinearities. Despite this, classical PD control can be quite effective for stable hovering and low-speed point-to-point maneuvering.
Employing indoor testbeds, a helicopter control system composed of a number of PD servo loops was developed. The system is capable of both hovering and slow (e 10 m/s or 20 mph) speed helicopter flight. The success of the simple PD control approach is attributed to the high quality of visionbased positioning feedback.
78 Chapter 4. Design and Evaluation of an On-Board Vision-Based Helicopter Control System Helicopter Controller 4.8.1 Helicopter Control Inputs Similar to full-sized helicopters, the remotely piloted models have four primary control inputs which are: the collective pitch angle, the lateral and longitudinal cyclic pitch inputs, and the tail rotor pitch angle. The collective pitch angle changes the angle of attack of the main rotor blades producing vertical lift which regulates vertical ascending or descending. The two cyclic pitch inputs, lateral and longitudinal, vary the rotor blade pitch sinusoidally within each revolution thus causing the rotor plane attitude changes which produce lateral and longitudinal forces to accelerate the helicopter back and forth or from side to side. The tail rotor input controls helicopter yaw angle. An additional control input to the helicopter is the engine throttle position which regulates rotor speed. This input is typically controlled by a governor or is varied with the collective pitch input to keep the rotor RPM as constant as possible.
4.8.2 PD Servo Control Loops Figure 4-7 shows the block diagram of the control system implemented for helicopter stabilization.
copter control inputs and throttle position. The controller is composed of several PD servo control loops for controlling helicopter attitude, 1ateralAongitudinal position and velocity, and height. The blocks labeled PD symbolize a linear combination of measured position and measured velocity.
A PD controller is biased and tuned for height control. The output of the controller is limited to an operating range determined by actual flight experiments. The limit also prevents sudden loss of altitude should the height sensors malfunction. In addition, the collective pitch input is used to set the engine throttle opening to maintain constant rotor RPM. A piece-wise linear function of collective pitch, recommended by the helicopter manufacturer, consisting of three operating points is used to bias the throttle input based on the collective pitch. Similarly, the tail rotor input is offset based on the collective pitch input to decouple helicopter yawing with varying loads on the main rotor. This offset is added to the output of the PD yaw control loop.