Robot design

To perform the R1 mission, the robot must swim freely for many hours liberated from a troublesome tether cable. Accordingly, high autonomy and a high density power source are essential for the robot.

To answer these requirements, a CCDE for the power system was introduced in place of the traditional battery. The R1 robot for the first stage was designed, therefore as a testbed to demonstrate the robot and the CCED as an AIP for the unmanned vehicle, and will be used to examine and establish highly autonomous control technology in shallow water.

1. Configuration

The general arrangement of the R1 robot is designed as shown in Fig.3. A torpedo-shaped hull, which consists of a main pressure vessel, a fore payload space, and a propulsion system, is introduced based on consideration of power consumption, control, handling, maintenance, cost, etc.


Fig.3

The principal particulars of the robot are as follows;
Overall Length 8.2m
Hull Diameter 1.1m
Weight in Air 4.0ton
Operating Depth 400m
Max. Crusing Speed 3.6knots

The main pressure vessel houses a power system and a robot control system, and provides the major buoyancy for the robot. The fore and aft frames, which support all external sensors, research payload, thrusters and actuators, are attached to each end of the vessel. These components are developed in flooded fairings made of FRP, to provide a torpedo-shaped hull. The forward fairing is hemispherical in shape to provide minimum drag.The aft fairing has a smooth transition to a spindle shape for good hydrodynamic performance. The dynamics of the R1 robot have been examined through numerical analysis and scale model testing. A CTDO data logger, a color video camera and a recorder, two halogen lamps for the video, and a still camera with a strobe light will be arranged in the forward fairing as research payload.

2. Pressure vessel structure

The central structural element of the robot is a cylindrical main pressure vessel with ellipsoidal heads made of A5083 aluminum. Ordinary support procedures on the support ship, such as replenishing fuel and liquid oxygen for the CCDE and data transfer between the robot and an onboard support computer, are carried out by using small access holes and bulkhead connectors located at appropriate positions in the wall of the pressure vessel. The pressure vessel can also be opened at every flange connection and be separated into 6 parts for maintenance of power system and controller.

3. Propulsion system

The propulsion system consists of a main thruster and two tunnel vertical thrusters which are arranged forward and aft of the main pressure vessel. Compact high power DC brushless geared motors have been newly developed for the R1 based on the results of small ROVs. The motors are housed in pressure housings with integral motor controllers, and drive propellers through magnet couplings. The thrusters are equipped with angular speed sensors for an inner feedback loop and are controlled by means of PWM (pulse width modulation). They can be used in full power reverse action. The main propeller is a four-blade type of a modified NACA 66 series shown in Fig.4. The specification of the thrusters are given in Table.1.

Supply power Main thruster Vertical thruster
280 VDC 280VDC
Shaft output Max. power 1.5kw 0.75kw
Max. speed +/-280rpm +/-1000rpm
Thrust 50kgf(@3.6kts) 28kgf(bollard)
Propeller diameter 609mm 256mm(tunnel)
Number of blades 4 4
Table 1


Fig.4 Main thruster

4. Steering gears

Steering the robot is accomplished with a pair of control fins with elevators attached on the aft fairing and by the main thruster by changing its horizontal position. Size of each control fin is about 0.35m including 30% of movable control surface area. Moving parts of the control fins are driven independently by two DC brushless motors and high reduction ratio gear actuators. The actuator motors equipped with angle sensors for inner loop feedback, are provided with 120 VDC 200W and have the same basic structure as the thruster motors. At lower speeds where the elevators are less effective, pitch and depth control is accomplished with the vertical thrusters.

5. Navigation system

For autonomous free swimming, the robot must be able to determine its own geographical position and avoid collision with obstacles.

An Inertial Navigation System (INS) is employed for positioning, which is associated with a doppler sonar by velocity feedback of the robot relative to the sea water and seabed. The INS is a strap-down type with a ring laser gyro based on an airplane model associated with a doppler sonar.

The specifications of the doppler sonar is as follows;
Frequency 1000kHz
Velocity range 0 to +/-3.75m/s (accuracy +/-0.25m/s)
Altitude range 2 to 30m

Obstacle avoidance control will be carried out using data from a forward looking sonar and an altitude sonar, whose transducers are housed in the forward fairing. The drivers of the sonars are housed in the fore part of the main pressure vessel. These scanning profiler sonars are multibeam type of 53 beams across a 108 degree sector, 300kHz operating frequency, 5 degree beam width, and over 100m range.

Although the robot swims autonomously, it has a transponder to tell its position to the support ship by a supper short baseline acoustic system. The support ship can let the vehicle know its position with 20kHz band half duplex pulse width modulation two-way link with 64 bit data transmission. The link allows supervisory control by the operator onboard the support ship, for example in the occasion when the emergency shut down and/or deballast command must be sent.

6. Control system

Fig.5 shows a block diagram of the control system of the robot. The main computer is a PEP9000 with VM40 using Motorola 68040 processor which runs under the VxWorks real time operating system. The processor, memory and I/O boards are single-height versions of the VME bus circuit boards and are installed in a standard VME chassis in the fore part of the main pressure vessel.

The main computer gathers and logs the data from navigational and other sensors, and controls the total robot system in accordance with a mission plan, such as the motion and activity control of the robot including obstacle avoidance judgment, research payload control, watchdog and emergency control, and so on.

The software for the R1 robot, including control logic and algorithms etc., have been under research and development.(Fuji and Ura, 1990)(Ura and Suto, 1991).

When the robot is on the surface, it will be controlled by the operator on the support ship through the computer system onboard the robot using a radio half duplex communication system. The radio link is 400MHz band, 4800bps with a serial RS232C interface.

7. Power system

7.1 Requirements for underwater power system

General requirements for AIP as an underwater power generation system are as follows:
  1. High energy density (per unit weight and volume)
  2. Reliability
  3. Cost-efficiency (Initial costs and running costs)
  4. Simple system management
Since no power system can satisfy these requirements perfectly, it is necessary to find a suitable system of compromise according to the purpose of application which involves mission plan, dimensions of the vehicle and energy capacity. The resultant options may, therefore, vary according to the terms which are emphasized.


Fig.5 R1 robot's control system

Considering one-day mission, it is concluded that the Closed Cycle Diesel Engine(CCDE) is the most suitable at the present technology level on the advantage of its high energy density, high reliability and low running costs.

Although AIPs, based on a diesel engine or an external combustion engines, are not suitable for short term operation due to bulky and weighty peripheral devices such as an exhaust gas processing unit, they become superior to batteries as ahown in Fig.6 in case of long term operation, for example over 24 hours.


Fig.6 System weight and volume for batteries and CCDE

7.2 CCDE system for the R1 robot

A prototype CCDE system for the R1 robot has been constructed based on the test results with a test plant (Obara, Ura< 1991).

Fig.7 shows the schematic diagram of the CCDE system for the robot. The exhaust outlet is connected to the inlet of the engine passing through exhaust gas processing units, in which exhaust gas is cooled down and combustion producs are removed. After mixing with replenishment oxygen, the remainder is recirculated to the engine as synthetic atmosphere.


Fig.7 CO2 absorbing unit

The combustion products of hydrocarbon fuel are mainly carbon dioxide (CO2), water and soot. The water vapor in the exhaust can be removed easily by condensing in a cooler, and the soot also can be considerably reduced by slight increase of the oxygen concentration in recirculated gas at the engine inlet. Since disposal of the remaining carbon dioxide is more complicated, the CCDE method is mainly characterized by the method adopted for the disposal of carbon dioxide . Accordingly various types of CCDEs have been proposed and developed.

The "CO2 absorption by chemical solution" exhaust gas processing method was selected with non-regenerative absorbent, i.e. potassium hydroxide(KOH). This method was choosen on the basis that the R1 robot aims for deep diving and long endurance, and requires depth independence, and no buoyancy control, i.e. no weight chage. Since the system can be operated fully independently from its surroundings, the diving depth is determined by the strength of the pressure hull. It should be emphasized that the commercially available engine runs in the original design conditions and maintains its performance such as high thermal efficiency and high durability.

The CCDE system for the R1 robot consists of four major units, namely the power generation unit, CO2 absorbing unit, liquid oxygen (LOX) tank unit and CCDE control unit. A fuel tank is installed in the CCDE control unit section, and a drain tank for the condensed water from the exhaust gas is placed under the LOX tank. Among these units, the power generation unit shown in Fig.8 and the CO2 absorbing unit have been examined for submerged performance in a fresh water pool.


Fig.Fig.8 Power generation unit of the CCDE

The specifications of the CCDE system are as follows;

The capacity of 60 kWh means 12 hours endurance of the robot operation at maximum power of the CCDE system. However, not so much power is required for robot cruising, including the main thruster, navigation system and control system. Accordingly, in the low power consumption mode of the research payload, for example CTDO measurement and still camera recording, the robot can cruise for 20 hours at 3.6 knots or 25 hours at 2.0 knots.



Last modified: Tue Jun 6 17:03:22 1995