S.M. Smith

Design of sliding mode fuzzy controllers for an autonomous underwater vehicle without system model

For 2 dimensional systems, the time optimal control law with physically limited control command follows Pontryagins maximum principle. The control switches its sign on both sides of a nonlinear switching curve. The key factor in robust time optimal controller design is to approximate that nonlinear curve while maintaining robustness. Time optimality and robustness are contradictory to each other. This paper describes the development of a sliding mode fuzzy pitch controller and a sliding mode fuzzy heading controller for Ocean Explorer (OEX) series autonomous underwater vehicles (AUVs) developed in Florida Atlantic University. A sliding mode fuzzy controller (SMFC) inherits the robustness property of sliding mode control and interpolation property of fuzzy logic control such that the nonlinear switching curve can be approximated and the robustness can be maintained. Moreover, since the physical meaning of the parameters in a SMFC is straightforward, the on-line tuning with an SMFC is much easier than with the old fuzzy logic controllers used on the OEX series AUVs. The at-sea experimental results show the effectiveness of the design philosophy.

Experimental results of an inexpensive short baseline acoustic positioning system for AUV navigation

The Ocean Engineering Department at Florida Atlantic University (FAU) and the Naval Postgraduate have implemented AUV acoustic positioning systems based on the Desert Star Dive Tracker short base line (SBL) system. The motivation for these implementation efforts was to develop a short range, inexpensive, but relatively high precision navigation system for each institutions autonomous underwater vehicles (AUVs). In the NPS case the SBL system is to support navigation of their Phoenix vehicle in mine reconnaissance type missions where the vehicle needs to more precisely locate obstacles and mine like objects. In the FAU case the SBL system is to support guidance of the Ocean Explorer (OEX) vehicle in rendezvous and docking maneuvers. The Dive Tracker SBL can produce supposed positioning accuracies on the order of 6 rms. FAU has build a portable easily deployed collapsible frame for supporting 3 SBL transducers or beacons. The frame is arranged in star that is anchored to the bottom. The baseline between transducers is approximately 20 ft. The OEX carries a transducer and electronics that compute its position relative to the baseline. A docking target placed near the baseline and anchored to the bottom. The location of the target is then determined by manually positioning the AUV transducer at the target and taking SBL fixes, A series of at-sea tests of this system have been conducted. This paper presents results of the at sea tests of this positioning system off the coast of Boca Raton Florida. The analysis will comment on the efficacy of this type of system for guidance of small AUVs in docking maneuvers.

A peer-to-peer communication protocol for underwater acoustic communication

The Advanced Marine Systems (AMS) group at Florida Atlantic University has developed a peer-to-peer communication protocol for use with acoustic communication systems interfaced with the Ocean Explorer (OEX) autonomous underwater vehicles (AUVs). The protocol does not require a master-slave control structure between telemetry units and is applicable to arbitrary signaling formats. This architecture allows the communication channel to remain open such that any telemetry unit in the network can have channel access, and therefore insures the opportunity to transmit important data that may become available. The protocol has been implemented in an acoustic modem (AM) capable of transmitting binary data through the shallow-water acoustic waveguide. Data transmitted from an OEX AUV to a surface ship using this protocol in the ocean are presented.

Coastal Oceanography Using a Small AUV

Bathymetry, current, temperature, and depth (CTD) measurements using a small, mobile, autonomous underwater vehicle (AUV) platform are described. Autonomous surveys of a shallow water column off the east coast of Florida during December 1997 were carried out using a 2.13-m long, 0.53-m maximum diameter Ocean Explorer series AUV, equipped with a 1200-kHz acoustic Doppler current profiler (ADCP) and a CDT package. At a speed of 1‐2 m s21, this AUV can perform preprogrammed missions over a period of several hours, collecting in situ oceanographic data and storing it on an onboard datalogger. The vehicle may also carry sidescan sonar or a custom small-scale turbulence measurement package or other instruments for subsidiary measurements. The versatility of the AUV allows measurement of oceanographic data over a substantial region, the motion of the platform being largely decoupled from that of any surface mother ship. In the missions of 5 and 11 December 1997, ‘‘lawn mower pattern’’ AUV surveys were conducted over 1 km2 regions on the east coast of Florida, north of Fort Lauderdale, at depths of 7 and 3 m, respectively, in a water column where the depth ranged from 10 to 32 m. During 5 December, the region was subjected to a cold front from the northwest. Local wind measurements show presence of up to 10 m s21 winds at temperatures of up to 108‐158C below normal for the time of the year. The fixed ADCP indicates occurrence of significant internal wave activity in the region. The data collected using the mobile AUV are utilized to develop a map of the bottom topography and examine current, temperature, and density variations in the context of the background information from a fixed bottom‐mounted ADCP and Coastal-Marine Automated Network buoys. The work described here is a significant step in the development of an autonomous oceanographic sampling network, illustrating the versatility of an AUV platform. The data collected during the missions described will form part of a bank for information on the impact of a cold front on shallow subtropical waters. The authors expect to repeat the missions during other such fronts.

A bidirectional coherent acoustic communication system for underwater vehicles

An acoustic modem for bidirectional communication with an unmanned underwater vehicle has been developed and installed on the Florida Atlantic University Ocean Explorer. The modem is used to test and demonstrate two-way phase-coherent communications between surface platforms and AUVs. However, in addition to serving as a testbed for investigating specific issues associated with high-rate vehicle communications, the modem has been used in actual practice to transmit vehicle status information, CTD data and compressed images to observers on the surface in near real-time. The system includes both medium frequency (2-4 kHz) and high frequency (20-30 kHz) transmission capability for short and medium range data uplink, as well as a towed array for reception at the medium frequency (MF) and a small vertical array for use at the high frequency (HF). The communication system has been tested in Florida, New England and the Bahamas where ranges of 2 km at HF and 4 km at MF have been achieved at burst rates of 6700 bps and 1670 bps respectively in water depths of 10-30 m.

Performance results of a fuzzy behavioral altitude flight controller and rendezvous and docking of an autonomous underwater vehicles with fuzzy control

This paper describes the performance of the fuzzy behavioral decision based controller (FBDC) approach used to control the bottom altitude flight controller of the Ocean Voyager II (OVII) and Ocean Explorer (OEX) autonomous underwater vehicles. This paper also describes the approach planned for implementing rendezvous and docking capability on the OEXs. This approach is based on an inexpensive short base-line navigation system and a fuzzy docking controller.

A simple weight based fuzzy logic controller rule base reduction method

This paper proposes a new rule base reduction method for Takagi-Sugeno type fuzzy logic controller. This method is cell state space based. First, the controller inputs are fuzzified and a generic rule base is built. This rule base includes all the possible combinations of input values. A search algorithm called Incremental Best Estimate Directed Search (IBEDS) is invoked to find the parameters in rule output functions. IBEDS starts with an initial training set. Each point inside the training set represents a currently best estimate control command for a cell center. Then another random FLC is trained in an iterative procedure by a Least Mean Square (LMS) algorithm. In each iteration, the cell state space based global and local performance of the trained FLC are evaluated, the training set is then updated based on the evaluation. When IBEDS converges, the final training set contains the maximal number of cells that a single FLC can control. At this stage, for each rule in the rule base, the firing strength or weight of the rule is calculated with every point from the training set. All the weights are added up to get a final Importance Index for that rule. A designer can cut off the rules with smallest importance indexes. A designer can cut as many rules as wanted according to the importance indexes. An FLC with reduced rule base is optimized by IBEDS again to achieve optimal performance. A 4D inverted pendulum is tested to justify the method. Each of the four inputs is fuzzified into 3 fuzzy values. A generic controller with 81 rules is built. After the optimization, the rule base is reduced to 40, 28, 17, and 5 respectively. The performances of the controllers with different rule bases are compared. It is shown that a controller with only 5 rules can perform comparably well with a controller of 81 rules.

A comparison of sliding mode fuzzy controller and fuzzy sliding mode controller

Sliding mode control and fuzzy logic control have been combined together in a variety of ways. One approach taken by many researchers is to use fuzzy logic to smooth the chattering in sliding mode control. Another unusual approach taken by few researchers is to use sliding mode control in a fuzzy logic controller. This paper presents the details of both approaches and compares their respective properties. The paper concludes that the first approach has little technical value, if not totally useless, and the second approach is technically applicable.

Automatic design of nonlinear controllers with optimal global performance using best estimate directed search and continued propagation cell mapping

The paper describes an optimal nonlinear control design method which consists of using an analysis tool, continued propagation cell mapping (CPCM), as feedback to the synthesis tool, best estimate directed search (BEDS). The method is successfully applied to the design of a TSK-type fuzzy logic (FL) controller and a sliding mode-type (SM) controller for the uncertain nonlinear system of an inverted pendulum on a cart for large pole angles (+/-86 degrees). The resulting controllers performance compares favorably to others designed with cell mapping (CM) and a genetic algorithm (GA).

Ocean flow measurement using an autonomous underwater vehicle

In this paper, we discuss making flow measurements in a shallow-water ocean environment using Florida Atlantic Universitys 2.2 m long autonomous underwater vehicle (AUV). An AUV has the advantage over tethered vehicles in that its self motion is not coupled to the motion of the sea surface and that of a mother ship, and it can potentially operate in stormy conditions. Two high-wavenumber shear probes and a Pitot tube, housed in a pressure-vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Salinity, temperature, mean velocity gradients and possible body vibrations are monitored using auxiliary local probes. The AUV allows surveying 3-5 mile square regions at a speed of 3-4 knots. The influence of the self motion of the AUV on the flow measurements is discussed.