A DIGITAL CONTROL SYSTEM FOR THE TRITON UNDERSEA ROBOT

Oli Francis, Graeme Coakley, Christopher Kitts

Robotic Systems Laboratory, Santa Clara University, Santa Clara CA 95053

Abstract: Santa Clara Universitys Triton undersea robot is a small shallow-water vehicle used to support a variety of marine science, technology, and education objectives. With a human-in-the-loop, analog-based control system at its core, a microcontroller-based digital control layer has been developed as a class exercise in order to augment piloting with automatic control and to support networked operation of the vehicle. This paper presents the Triton system, describes the design of the digital control layer, and presents experimental results of the system during operation. Copyright 2002 IFAC. Keywords: Engineering Education, Robot Teleoperation, Microcomputer-based control.

1. INTRODUCTION Since 1998, the Santa Clara University Robotic Systems Laboratory has engaged in the aggressive development of low-cost robotic devices in order to support scientific exploration, technology validation, and engineering education. These projects are typically developed in less than a year as part of the Universitys senior undergraduate design program. To date, the Laboratory has been involved in the design of more than twenty highly capable robotic systems to include spacecraft, airships, land rovers, undersea vehicles, and telescopic observatories. These projects have supported a wide range of field operations and have been financed through more than $1,250,000 of external funding from government agencies, industrial partners, and university collaborators. The Triton undersea robot, shown in Figure 1, was developed in 1999 by a group of seven mechanical and electrical engineering undergraduates. Developed for shallow water (less than 1000 feet) tethered operations, Triton is a 270 pound vehicle powered by two hp horizontal thrusters (for horizontal plane motion) and two hp vertrans thrusters (for vertical and lateral motion). The

vehicle includes a camera and lights to support both piloting and video-based science operations; in addition, the vehicle is able to support modular instrumentation for specific scientific objectives [Weast, et al., 1999].

Fig. 1. The Triton undersea robot during a pool test. A 750 foot tether provides power, command, and telemetry connectivity between Triton and a surface control console. The pilot controls the vehicle through the use of a video monitor displaying the realtime on-board video as well as a pilot box consisting of two joysticks (for maneuvering) and an

array of switches (for light and camera control). As the pilot provides control inputs, analog control signals and power modulated by analog circuitry is transmitted through the tether to the vehicle; the overall component control architecture is depicted in Figure 2. This analog approach was originally adopted for several reasons to include its common use in the marine industry, the expertise of design mentors from local industry, and the resources and knowledge available to the student team. To date, Triton has been used to support geologic studies of Lake Tahoe by the Scripps Institution of Oceanography, ecological surveys of the Channel Islands region for the National Marine Sanctuary, and marine biology studies for the National Undersea Research Program. Triton has also been used as a technology testbed for undersea manipulators and stereo vision applications [Yoshida, et al., 2000; Weast, et al., 1999]. Fig. 2. Simplified Triton system block diagram

showing analog, digital, and extended control layers.

2. DIGITAL CONTROL SYSTEM As part of a class project, the authors developed a digital control system as an extension to the Triton undersea robot. The team had several specific objectives. First and foremost, this experience was designed to provide hands-on exposure to issues in teleoperated systems. Second, the team wished to develop experience with microcontroller systems and digital control techniques that were being considered for use as the primary control system for Mantaris, a second generation undersea robot being developed within the Laboratory. Third, the resulting system was intended to improve piloting by implementing automatic depth and heading control. Fourth, the digital control layer was intended to provide a natural interface to a robotic teleoperation architecture being developed as a means of supporting distributed research and education. In its original form, Triton had proven to be a highly capable and robust system. Because of the experimental nature of the project, the predominant constraint for the team was to maintain the fidelity of the original human-in-the-loop analog control system. For this reason, the digital control system was implemented as a layer above the core analog system; accordingly, existing interfaces and design implementations were not open for redesign, and all digital control elements were required to be located within the surface control console. 2.1 Architecture of the Digital Control Layer As shown in Figure 2, the Digital Layer interfaces with the existing Triton control console via the pilot box interface for control inputs and existing audio/video and auxiliary port for telemetry. This connectivity allows the digital control console to perform the following functions: Provide a direct pilot interface at the digital layer

via a digital pilot box and video monitor with digitally imposed telemetry data.

Implement simple but effective automated depth and heading control.

Provide an interface to additional command and telemetry processing/distribution layers via a serial port interface.

The Digital Layer is implemented with a network of Ubicom SX and PICMicro PIC microcontrollers and support circuitry, as shown in Figure 3. Three control modes are supported for the Triton vehicle: Pilot Only Control: Complete human-in-the-loop

control is achieved by having the Joystick subsystem monitor the pilot inputs and compose the control directives.

Pilot Assist Control: Human piloting is selectively combined with automated depth and/or heading control. Within the Hub, control

Video Monitor

Pilot Box

Triton Vehicle

Horiz. Thruster

Vert. Thruster

Horiz. Thruster

Vert. ThrusterSensors

Lights

Camera

Routing &

Filtering

Digital Console

Video Monitor

Digital Pilot Box

DIGITAL LAYER ANALOG LAYER

EXTENDED LAYER DIGITAL LAYER

Analog Console

Power System

Sensor Display

750 ft 30 conductor tether

directives from the Joystick are merged with computed directives from the Sensor subsystem.

Extended Layer Control: Control and monitoring is implemented by human and/or automated systems at a higher layer in the control hierarchy. In this mode, the digital layer serves as a relay for the necessary data flows. Extended layers can be used to enable distributed pilot/observer connectivity, support more advanced controls/interfaces, and provide enhanced data processing.

Hub Processing. Given the control mode options, the Hub SX microcontroller is capable of receiving commands from three sources: the Joystick SX, the Sensor PIC, and processors in the Extended Layers. The Hub selects, occasionally merges (for Pilot Assist Control), and routes the control directives to the Control microcontroller based on the selected control. Fig. 3. The Digital Layers Microcontroller

Architecture

Sensor Processing. The Sensor microcontroller accepts depth and heading telemetry from the Triton Analog Layer. Depth data is provided as a 0-10V analog signal, which must be attenuated and digitized. Heading data is provided in a pulse width format, which must be timed for interpretation. Using a text overlay system, the Sensor microcontroller superimposes depth and heading data onto the realtime video, and the result is displayed on a monitor for use by the pilot. Control Processing. With the appropriate commands routed to it by the Hub microcontroller, the Control microcontroller operates an interface circuit that buffers, filters and differentially amplifies microcontroller outputs in order to provide a +/-10V motor control signal to the Analog Layer. Joystick (Digital Pilot Box) Processing. Similar to the analog pilot box, the digital pilot box consists of a joystick and camera control switches. The Joystick microcontroller continuously monitors these input devices in order to compose its command directive to the Hub microcontroller. It is interesting to note that, in contrast to the analog pilot box, the development team chose a single low-cost joystick common in the personal computing industry; this component provided the necessary functionality and robustness for approximately 1/30th of the price. Furthermore, a flight joystick configuration was selected which fully supported pilot control with a single stick. The flexible nature of the digital configuration allows this joystick to be reconfigured with ease in order to support individual pilot preferences. 2.2 Depth and Heading Control Implementation In the Pilot Assist Control mode, the Sensor PIC exploits its direct access to vehicle telemetry and it underutilized computational ability in order to compute motor commands capable of implementing closed loop depth and/or heading control. To enter this mode, the pilot simply presses the appropriate button in order to lock the current depth and/or heading. A proportional control strategy with an empirically tuned gain is used in order to achieve simple but effective control (see Section 3). This strategy is executed by determining the error in depth and/or heading, computing the proportional motor command, and possibly applying deadbands or safety limits. 2.3 Data Handling Network communication among the SX and PIC microcontrollers relies on a standardized serial data protocol as summarized in Table 1. This protocol

ANALOG LAYER joystick auxiliary video port port port

serial serial port 1 port 2 EXTENDED LAYER DIGITAL LAYER

Digital Console

Hub SX

Sensor PIC

Joystick SX

Control SX

Control Drivers

Sensor Filters

Pilot Inputs

Text Overlay

Video Monitor

represents the range of all Triton commands supported by the Digital Layer. Flow and control between the microcontrollers is executed by virtue of a native communication technique implemented within the SX and PIC chips. Flow and control between the Hub microcontroller and any PC in the Extended Layer is implemented through a send/receive strategy in which the Hub microcontroller serves as the master. The first character in the protocol designates the source of the command. The second character nominally specifies normal operation; sensed emergency conditions cause a control override, which leads to thruster shut-down. Characters 3-18 designate four vehicle-level motion behaviors: forward/backward motion, left/right turning, vertical motion, and crabbing motion (a slide slip motion implemented via the vertrans thrusters). Each of these behaviors is specified with a set of four characters; the first designates direction while the remaining three explicitly represent thruster power over a 0 - 100% range. The final character specifies control of the on-board camera. Values of this character allow adjustment of camera zoom and focus.

Table 1 Serial Communication Protocol

Char # Data Notes 1 Source Identifies processor sending string 2 Mode Standard or Emergency 3 Main Dir. Forward or Backward 4-6 Main % 0-100% Thruster Power 7 Turn Dir Left or Right 8-10 Turn % 0-100% Thruster Power 11 Depth

Dir. Surface or Dive

12-14 Depth % 0-100% Thruster Power 15 Crab Dir. Left or Right 16-18 Crab % 0-100% Thruster Power 19 Camera Zoom +/-, Focus +/-, Autofocus

There are two interesting issues worth pointing out with respect to the aforementioned representation of motion control. First, the protocol specifies four vehicle-level behaviors each of which involves the coordinated operation of two thrusters. For example, if the turn motion is commanded left, then the port and starboard horizontal thrusters are commanded to thrust backward and forward, respectively in order to produce this motion. A more natural manner of specifying vehicle control would be to explicitly represent and command each thruster separately within the communication protocol. For this project, however, the vehicle-level behaviors were used by virtue of the Analog Layer control interface, which uses the vehicle-level behaviors.

Second, the number of overall bits used in the command string has far more information capacity than is used. The inefficient data representation was used to improve the interpretation of the string by human developers and operators. Given that the control performance does not suffer from communication bandwidth issues, this representation was judged to be appropriate for this project. 2.4 Extended Command & Telemetry Layers In addition to providing a direct pilot interface, the Digital Layer provides connectivity to additional, extended layers of control and data distribution. Although discussion of these layers is largely beyond the scope of this paper, several have been implemented. First, a PC has been connected to the digital layer in order to provide a computerized pilot interface. This consists of a graphical user interface and a flexible combination of control inputs to include both the keyboard and joysticks. This computer interface has also been used to route commands and data to remote pilots and observers via combinations of radio frequency and internet-based communications systems. This capability is part of a significant SCU effort to develop a robotic control architecture for distributed educators and researchers. 2.5 Safety and Implementation Issues With high voltage circuitry and rapidly rotating propellers, Triton poses significant risks to students in both the development and operational environments. The digital control layer was developed in order to implement a variety of features to improve safety. First, the overall design of the Triton console was reorganized into physically distinct high and low voltage circuitry. Second, an emergency stop function directly disables the motor driver circuitry when the emergency button is manually pressed or automatically if the pilot box becomes disconnected. Third, in the event of microcontroller failure, the motor drive circuitry slowly discharges thereby stopping the thrusters. 3. EXPERIMENTAL TESTING & RESULTS To support iterative development, verify end-to-end functionality, and characterize system performance, the Triton digital control layer was tested in a variety of settings. For laboratory development, a simple hardware-in-the-loop simulator was developed to support component-level functional verification and testing. Several days of testing in two SCU pools provided the opportunity to refine digital control procedures, to

observe system operation, and to empirically tune the control loop gains. These pool tests also allowed the team to demonstrate the digital layers ability to support remote, internet-based piloting of the vehicle via an Extended Layer control system. To begin a quantitative assessment of the performance of the Triton digital control system, the vehicle was evaluated in the test tank at the Monterey Bay Aquarium Research Institute (MBARI) in Monterey, California. The results presented here, while not exhaustive, provide an initial indication of the quality of the resulting system. Figures 4 and 5 show the dynamic response of the vehicles depth and heading control loops for step inputs. A more applied indication of performance involves how these control loops directly enhance robot piloting. For example, one essential piloting task is to keep an object fairly stationary within the on-board cameras field of view. In quantifying this objective, the development team noted that the target a) should move slowly enough to permit human scrutiny, and b) should be far enough away from the edge of the field of view to prevent its moving out of sight due to environmental disturbances. In a comparative sense, any proposed improvement should lead to tighter target paths over time and to slower target velocities across the screen. To observe the Triton Digital Layers ability to improve these two criteria, a pilot attempted to keep a light on the side of the MBARI test tank in the cameras field of view; this was done in both the Pilot Only and the Pilot Assist (with both depth and heading lock enabled) control modes. Figure 6 compares the target paths across the camera image plane over a 30 second period; this comparison clearly shows that the Pilot Assist mode enabled a tighter target path. Figure 7 charts the horizontal velocity of the target across the image plane during this same period; this comparison highlights a dramatic improvement in overall task performance. Although this is only an initial experiment with several uncontrolled parameters (such as distance from the target), it suggests that Tritons Pilot Assist capability will be beneficial during field operations. Apart from these quantitative results, the development team also relied on qualitative feedback from experienced pilots in order to improve and ultimately characterize the performance of the system. During the teams visit to the MBARI test tank, MBARI pilots, a group that includes some of the most experienced and skilled operators of undersea robots in the world, also tested the system. All of the pilots provided positive comments regarding the stability of the vehicle under pilot control; one specifically characterized the robot and the Digital Layer as being quick and nimble.

Fig. 4. Step response for automated depth control

Fig. 5. Step response for automated heading control

Fig. 6. Comparison of a 30 sec target path across

image plane for Pilot Only vs. Pilot Assist (with depth and heading lock) control modes.

Fig. 7. Comparison of horizontal target velocity for

Pilot Only vs. Pilot Assist (with depth and heading lock) control modes

0 10 20 30 40 50

Additional use of Triton during a science deployment in Lake Tahoe, California provided valuable opportunities to gain feedback from the ships captain and a science team. The two SCU pilots for this mission had previous experience controlling Triton via its standard Analog Layer. These pilots were very impressed with the usefulness of the heading lock function during terrain-following tasks and in configurations where large tether disturbances would have otherwise spun the vehicle in an unwanted manner. In addition, both pilots praised the single joystick interface and its ability to be easily reconfigured for individual pilot preferences. These pilots were unable to notice any loss of responsiveness due to the Digital Layers operation. 4. FUTURE WORK The flexible nature of Tritons digital control interface will enable a variety of undergraduate and graduate-level engineering studies. As an engineering system, the technical performance of the depth and heading control loops will be improved. While the presented work is an outstanding first step, its ad-hoc and empirically-tuned nature can be improved through formal analysis and improved testing. Future work in this area will include adjustment of the control gains, implementation of depth and heading control with arbitrary set-points (rather than locking only at the current value), the use of position sensors for automated navigation control, and more extensive system verification and validation. The digital control layers ability to support internet-based piloting and realtime distribution of science data will be used to provide science and technology validation services to a wide range of distributed collaborators. This capability has been demonstrated in test environments with acceptable performance; future work in this area will focus on quantitatively characterizing and validating its benefits as well as maturing the capability to support marine operations. Finally, the project has provided significant insight into the design and implementation of a distributed control architecture currently being developed for the Mantaris vehicle. For Mantaris, this system is the native system with several microcontrollers deployed throughout the undersea and surface components and with combined Ethernet / RS485 networks supporting command and data handling. 5. CONCLUSION Development of the Triton digital control layer has proved to be valuable technical exercise.

Educationally, this class exercise provided unprecedented opportunities for experiencing engineering design, implementing control theory, understanding the challenges of marine technology, and conducting peer-reviewed engineering research. The digital control layer has improved the ability to conduct Triton undersea operations by enhancing pilot operations via the introduction of autopilot functions, by enabling a flexible, customizable control interface, by incorporating diagnostics, and by providing an interface for distributed control. Finally, the use of a high-quality yet simple robotic vehicle provides an ideal opportunity for experiencing the benefits of project-based education, for building collaborations with local engineering organizations such as MBARI, and for establishing highly motivating learning opportunities. ACKNOWLEDGEMENTS The work described in this paper has been supported by the Santa Clara University Technology Steering Committee, the NOAA National Undersea Research Program. In addition, this material is based upon work supported by the National Science Foundation under Grant NO. EIA-0079875; any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors thank the dozens of students who have participated in the development and operation of the Triton system. For their mentoring, special recognition is given to Chad Bulich, Jeff Ota, Dr. Geoff Wheat, and members of the SCU Engineering Alumni Board. Finally, appreciation is extended to MBARI and Deep Ocean Engineering for their engineering assistance and use of facilities. This work was performed in partial satisfaction of undergraduate and graduate studies at Santa Clara University. REFERENCES Weast, A., et al., (1999a). Triton: Design of an Underwater Remotely Operated Vehicle (ROV). Undergraduate Senior Thesis, Santa Clara University. Yoshida, K., et al., (2000). Development of an adaptive multi-finger gripper system for an underwater ROV. Proceedings of the Advanced Robotics Conference. Weast, A., et al., (1999b). Integrating Digital Stereo Cameras with Mars Pathfinder Technology for 3D Regional Mapping Underwater. Proceedings of the 1999 IEEE Aerospace Conference, Snowmass, CO.