«An Autonomous Vision-Guided Helicopter Omead Amidi August 1996 Department of Electrical and Computer Engineering Carnegie Mellon University ...»
On the average, a five minute initialization time was required for receiver phase ambiguity resolution down to the 1 cm positioning accuracy whenever satellite signals were interrupted. Therefore, to be effective, the GPS receiver must have an uninterrupted view of satellites during test flights away from large satellite-occluding objects such as buildings or hills.
In summary, the calibrated tests with the instrumented table proved that the GPS performance is adequate as a backup positioning system and for evaluating the vision-based positioning using a nearby ground station. To investigate GPS performance in flight, additional tests were performed under real flight conditions to determine if the main rotor blades occlude satellite signals. The GPS receiver and antenna were mounted on the helicopter as shown in Figure 5-4. Repeated human piloted flight tests proved that the R50 fiberglass and metal rotor blades did not occlude satellite signals. In addition, the receiver maintained satellite tracking even with the quick and significant attitude variations commonly exhibited by the helicopter during flight maneuvers.
Figure 5-5 shows a block diagram of the on-board integrated system built for outdoor autonomous helicopter flight experiments. The system is an extension of the indoor prototype visual odometer machine and retains the same modular point-to-point comm-port architecture.
The vision system is augmented by a GPS receiver, a flux-gate magnetic compass, and a laser rangefinder for redundant helicopter position estimation. Comm-port interfaces were developed for each of these sensors for easy integration and compatibility with all system components. The system’s real-time controller implements the PD servo loops, and commands the helicopter actuators through a safety circuit. Human pilot interfaces are incorporated into the safety circuitry for computer augmented helicopter control. This section describes the operation of the on-board integrated system components.
5.2.1 Vision Processor The visual odometer machine, calibrated and tested using the indoor testbeds, is integrated on-board with few small modifications. Taking advantage of the modular system architecture, more DSP elements are incorporated into the system for additional sensor data acquisition and integration with vision. The tasks of attitude compensation and coordinate transforms are divided into two DSP C40s.
One simply performs synchronized filtering of attitude data while the other receives position data from the remaining sensors, including a GPS receiver, a laser rangefinder, and a magnetic compass. In addition, a video transmitter is integrated on-board to send processed images to the ground for monitoring.
5.2.2 GPS Receiver As described in Section 5.1, experiments verified that the carrier phase differential GPS receiver, NovAtel RT20, is an accurate and reliable source of positioning for free flight experiments. The GPS receiver performs satellite tracking and carrier phase ambiguity resolution using transputers which provide high-speed external links for system expansion. With cooperation from NovAtel Communication, Ltd, these links are internally accessed to develop a low-latency comm-port interface to the C40s and the remainder of the system. For synchronized operation, the sync generator triggers the GPS receiver every 12 images or 5 Hz. For performance evaluation, on-board wireless modems receive differential corrections and transmit position logs during helicopter flight to the ground station.
To remedy the accumulating drift of the directional gyroscope, a North seeker digital magnetic compass is integrated into the system. The compass uses a toroidal flux-gate sensing element free floating in an inert fluid to keep the sensing element horizontal. The compass manufacturer, KVH Industries, provided a more viscous fluid filled sensing element to help reduce vibration effects on heading measurements. In spite of this precaution, the compass heading data is quite noisy and requires extensive filtering.
The digital output from the compass is sampled with the same external trigger as the GPS receiver, and a hardware interface is developed to transmit the heading data to other system components via a comm-port.
The difference between the compass heading and the yaw gyroscope is filtered to correct the drift of the yaw gyroscope. The filter is a low-pass with time constant of 5 seconds. A C40 acquires compass data, performs low-pass filtering, and transmits corrected heading data to other system modules.
5.2.4 Laser Rangefinder For redundant height measurement, a laser rangefinder is integrated on-board. The laser rangefinder, manufactured by Yamaha Motor Company, has a 20 meter range with a 20 Hz measurement frequency. Preliminary experiments demonstrated small (1-296) range variations with reflective surface color which are not explicitly modeled for helicopter height estimation. Data are clocked out of the sensor serially and a comm-port bridge that includes shift registers is developed to transmit range data to other system modules.
5.2.5 Real-Time Controller
The real-time controller, integrating an MC68040 microprocessor, implements the helicopter control system. Composed of several PD servo loops, the controller receives position and attitude estimates from the integrated vision system at field rate (60 Hz) and controls the helicopter by transmitting commands to the on-board actuators and augmentation systems.
The real-time controller also bootstraps and configures the C40 network as well as performs several other functions during system operation. The network interface of the controller provides access to mass storage for data logs and initialization. The controller provides user interfaces for run-time system configuration and supports 128 digital I/O lines for interfaces to safety circuits, the actuation system, and two system comm-ports.
5.2.6 Actuator Control The Yamaha R50 is designed to be remotely controlled with an RF transmitter during flight experiments indoors. The transmitter provides control sticks and trims for remote control by human pilots and incorporates decoupling curves to reduce cross coupling of helicopter control inputs. Off-board computing generated analog signals in place of the stick potentiometers of the transmitter to remotely control the R50 for the indoor test flights.
To control the R50, the four stick inputs of the transmitter are sampled and converted to five actuator positions. Three actuators control the main rotor collective and cyclic pitch as shown in Figure 5Two side actuators move in opposite directions to produce lateral pitch, while the middle actuator controls longitudinal pitch. All three actuators move in parallel for regulating the collective pitch. The remaining two actuators (not shown) control tail rotor pitch and engine throttle.
Figure 5-6. Main Rotor Actuators
The transmitter digitally encodes the desired actuator locations which it sends to an on-board receiver. The receiver produces pulse width modulated signals at 50 Hz to motor controllers which move each actuator. For autonomous operation, the integrated system must directly generate pulse width modulated signals to control the five actuators. In addition, the actuator coupling terms must be resolved on-board.
98 Chapter 5. Outdoor Autonomous FlightOn-board Integrated System Pulse width decoders and encoders were developed using AMD Mach 435 complex PLDs for onboard helicopter actuator control. To command helicopter actuators, the real-time controller was interfaced to the PLDs. The controller internally stores the actuator mixing tables to convert desired collective and cyclic pitch terms to the three main rotor actuation positions. Changes in throttle setting and tail rotor are also made by the controller using an internally stored piece-wise linear tables.
For augmented control, the five actuator positions sent from the remote transmitter are decoded using inverted actuator mixing tables to determine human collective, cyclic, and tail rotor commanded positions. These inputs are then normalized and used as augmented control inputs to regulate helicopter position or velocity using the on-board control system.
5.2.7 Safety Circuit and Human Interfaces
The indoor testbeds provide protection and safety by limiting helicopter travel and measuring ground truth helicopter position for stable recovery from out-of-control flight. The absence of this protection for outdoor free flight requires on-board safety circuits and human interfaces to minimize accidents.
A “heartbeat” mechanism is developed to detect system failures during autonomous operation.
The heartbeat is a periodic signal that indicates system health. The real-time controller is configured to periodically monitor all on-board systems, including the vision system, GPS, actuator locations, and battery voltage, and to generate a heartbeat by outputting a pulse on one of its external digital lines. In case of a malfunction, the heartbeat signal remains unchanged for an extended period. During normal system operation, the heartbeat signal reports system processing duration and frequency.
The safety circuit times the heartbeat intervals; if the heartbeat signal is stuck in one state, the circuit detects a system fault. In addition, the safety circuit includes multiplexors for switching actuator control between the on-board RF receiver, carrying out human pilot’s actions, and the on-board computer. An extra channel of the receiver, set by the remote transmitter, switches between the control modes.
For outdoor flight, the chassis for supporting the cameras and attitude sensors for the indoor R50 is integrated on-board another R50 helicopter. Figure 5-7 shows the R50 on-board system components.
The on-board visual odometer machine and real-time controller are housed in the computing cage which is mounted below the helicopter fuselage. To the immediate right of the cage are the laser rangefinder and bridge assembly, and the video transmitter. Small hardware circuits implementing the sync generator and sensor bridge modules are mounted in the front of the cage. On either rear skid, there is a wireless modem for GPS differential correction and ground telemetry. The main system battery is mounted in the front of the helicopter to balance out the weight of the GPS receiver mounted
on the tail. The two ground pointing cameras are mounted on the main system chassis as in the indoor R50 helicopter. Critical components such as the attitude sensors and the safety circuit are well protected inside the helicopter frame and are not visible in the figure.
This following subsections describe the major components of the on-board implemented visionbased helicopter control system.
5.3.1 Weight and Power The weight of all on-board equipment, about 18 Kg, is less than the 20 Kg payload of the R50. The power dissipated by the system is on the order of 180 W and the computer operation is possible for 12-14 minutes using inexpensive ($20) 7 Ah lead-acid batteries. Silver-zinc batteries of the same weight could power the system for 1 hour but are not used because they are an order of magnitude more expensive. For redundancy, a separate lower capacity battery powers the receiver, actuators, and the safety circuit for reliable remote piloting in case the computing battery power is drained in free flight.
5.3.2 On-board Computing
Figure 5-8 shows the on-board VME computing cage and the hardware modules. The ribbon cable connections are identical comm-port links between different components. Image A/D, D/A, and convolution modules are merged into one image capture and preprocessor printed circuit board, shown in Figure 5-9. Using comm-port connections, the board can transmit digitized images and receive processed images for display. Image synchronization is performed with intelligent state machines to eliminate additional external connections. The comm-port interfaces provide internal buffering and termination to guard against data corruption from signal reflection or other noise sources; transfers are rated at 10 Megabytes per second. Images can be digitized from up to 4 multiplexed NTSCPAL, camera inputs by the integrated A/D module. The module incorporates all programmable image sampling and blanking features described in Chapter 3. In addition, the A/D module provides access to an internal synchronized video data bus for external synchronization and can pass images through an 8x8
image convolver for image preprocessing. In addition, processed images can be displayed through an RGB pseudo-color display driver. An input comm-port interface transfers the images to the display driver and provides the proper synchronization for video screen refreshing. Processed images can be overlayed on top of captured images from the A/D module. A VME bus bridge module is also incorporated for system initialization and external communication.
The next step following the development and integration of system components was outdoor autonomous flight experiments. The experiments were conducted in an isolated flight site with grassy terrain and an open view of GPS satellites.' The site's terrain provided enough features for establishing visual lock by the visual odometer, but was not locally flat. The helicopter tests were conducted at the summit of gently sloping hill.
5.4.1 Experimental Setup The Navlab I autonomous land vehicle was modified to house the helicopter for transportation and to provide a mobile ground computing platform for system development and evaluation. The interior of the vehicle is shown below in Figure 5-10.
1. The flight site is the property of William Wittey who kindly permitted the use of his farm in Zelienople, PA for helicopter flight tests.