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$: \documentclass{article}rendas/sumare/IntegrationTechnicalReportApril… · Web viewNavigation of Autonomous Robots via Active Environmental Perception. Workpackage: WP4 - [TMS]$
ESPRIT-LTR Project 30185 (NARVAL)

Navigation of Autonomous Robots via Active Environmental Perception

Workpackage: WP4 - [TMS] Subsystem Integration

Task: T4.3 to T4.4

Title, Author(s), Date, Doc. No., Version:

Subsystem Integration

Hervé Goutelard, Christophe Robert and João Rendas

TMS/CNRS

5 November 2000

Interim report

Pages: 10 (Document)

Filename: Interim technical report on sensor integration.

Keywords: Integration, subsystem, validation

Abstract: Describes the progress on the integration of the sensors mounted on the ROV Phantom. For each sensor, the law for conversion form raw measurements to physical units is given, and plots are shown that allow evaluation of the quality of the data.

Distribution: all partners

Format MS-Word 7

Interim Integration Report Version 1.0

1. Introduction.................................................................................................................................2

2. Overview of the system...............................................................................................................2

3. Acquisition system overview.......................................................................................................2

4. Sensors integration......................................................................................................................5

4.1 Proprioceptive sensors..............................................................................................................................54.1.1 Watson Rate Gyro...........................................................................................................................54.1.2 TCM2 sensors.................................................................................................................................54.1.3 Propeller encoders...........................................................................................................................74.1.4 Pressure sensor................................................................................................................................8

4.2 Acoustic sensors.........................................................................................................................................94.2.1 Altimeter sensor..............................................................................................................................94.2.2 Profiler sonar...................................................................................................................................94.2.3 Tilt paltform..................................................................................Error! Bookmark not defined.

4.3 Video sensor.............................................................................................................................................10

5. Principle of measurement. Major results....................................Error! Bookmark not defined.

5.1 Cycle principle and accuracy.....................................................................Error! Bookmark not defined.

5.2 Data results..................................................................................................Error! Bookmark not defined.

5.3 Proprioceptive sensor..................................................................................Error! Bookmark not defined.

6. Conclusion..................................................................................................................................10

7. GRAPHS.........................................................................................Error! Bookmark not defined.

7.1 Graph 1: Time duration of acquisition data.............................................Error! Bookmark not defined.

7.2 Graph 2: RPM measurement.....................................................................Error! Bookmark not defined.

7.3 Graph 3: Altimeter and immersion measurement...................................Error! Bookmark not defined.

7.4 Graph 4: Heading sensor............................................................................Error! Bookmark not defined.

7.5 Graphs 5 and 6: Pitch, roll and gyro rate.................................................Error! Bookmark not defined.

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1. Introduction

This document describes the integration of the sensors in the ROV Phantom performed in the framework of the NARVAL LTR Esprit Project 30185.

The document is organised in the following manner. First, a rapid presentation of the system is done. The subsequent section presents the general organisation of the acquisition system. Then, in section 4, we present for all sensors, the expressions that must be used to convert the acquired data to standard units, and show plots representative of the data acquired. In a separate section we summarise the problems that still need more study, and present a plan for these developments. Finally we conclude by summarising the results of this second year of activity of the project.

2. Overview of the system.The complete system is based on a robotic platform (the Phantom 500 XL) connected to an external

computer system able to drive the vehicle (the brain of the vehicle).The design of the system as changed from what has been described in previous documentation. In the

up-link direction, the ROV is now directly connected to the PC without going through the surface TDS, which is still in charge of the on screen display and of the control of the pan and tilt of the video camera.

3. Overview of the Acquisition system.State of the art:

The acquisition system has been simplified for a better control of the data acquisition rates. The previous three-step transmission link has been changed in the upward direction to a two stage one (see figure below).

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Right Thruster Left Thruster

Vertical Thruster

Right Speed Encoder

Junction Box

Left Speedencoder

Vertical Speed encoder

Lamp

Profiling sonar

Pan & tilt camera

Tilt actuator

Position encoder

Lamp

Umbilical

Angular rate sensor

Tilt platform

Multiplex card

Altimeter sonar

Depth sensor

Power control

Compass andpitch & roll

ROV crash frame

Leak detector

TDSss

Sonar Line conditioner

Sea level …

Overview of the ROV equipment


The system has been totally redesigned to transfer control of the acquisition loop to the controller performing the acquisition (the subsea TDS).

The final version.In the final version of the system, the RS 232 link between the Subsea TDS and the PC will also perform downward transmission for piloting the video camera.The surface TDS is used only for on-screen display.

Principle.The subsea TDS performs data acquisition and control of the loop duration (cycle). Data is sent to the PC through an RS 232 full duplex line. The PC is not yet sending data to the TDS (the downward link is not implemented).

The frame send by the subsea TDS to the PC is formatted in the following way: "& EMIT Header of the frame #HEADING @ . Heading in tenths of degrees #RATE_GYRO @ . Raw value of the gyro #PITCH @ . Value in tenths of degrees #ROLL @ . Value in tenths of degrees #DEPTH @ . Raw value of the sensor #TRAY_TILT_ANGLE @ . Value of the tilt of the sonar platform #RPM1 @ . Raw value of the sensor #RPM2 @ . Raw value of the sensor #RPM3 @ . Raw value of the sensor #TEMPS0_RPMS @ U. End of the cycle#TEMPS1_RPMS @ U. Beginning of the cycle #WATER_LEAK @ . Emergency #DUREE_RPMS @ . Cycle duration CR End of the frame

Any device with an RS 232 port is able to read this frame (all values are sent in ASCII format).

The measurement loop is implemented in the subsea TDS and executes the following sequence of basic actions:

a) Reset counter for RMP (zero value)b) Get the value of the free run counter (TEMPS1_RPMS or T1)c) Read the A/D channels (proprioceptive sensor value)d) Wait for timer event (controlling the 100 ms cycle)

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Summary of the connections for the ROV system.


e) Read counter of the RPMf) Get the value of the free run counter (TEMPS0_RPMS or T0)g) Interrogate and read the TCM 2 valueh) Send message to the PCi) Read data from PC (not yet implemented).

The timer control follows the following scheme:

We initialise a counter limit. Each time the free run counter reaches this limit, it is re-initialised and its

status byte is set to one. After reading the status byte (when set to one), it is set to zero automatically. So we get the 10 Hz control loop. The measurement of the RPM is performed between T1 and T2.

The previous plots represent the duration of the acquisition cycles (for proprioceptive measures in the pink curve and for altimeter data in the yellow curve). The time is computed using the PC clock. For the

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Principle of the cycle measurement.


RMP (blue curve), the time used for RPM computation is smaller than the NAS cycle (about the half, see previous figure) and the value is extracted from the TDS clock.

The fluctuations shown in the previous plot are due to the fact that the time slice of the PC clock, under Windows NT, is 10 ms. Consequently, the 100 ms cycle appears as 90, 100 or 110 ms due to problem of round-off.

4. Sensor integration.In this section, we describe, for each sensor, the law that must be used to convert the raw measures into

standard units We have grouped the sensors by their main functionality. Using this choice, we could identify three categories:

Proprioceptive sensors: used for assessing the dynamic behaviour of the ROV Acoustic sensors: used for getting a image of the environment of the (distance information). Video sensor: typically used for video mosaicing, station keeping and inspection…

4.1 Proprioceptive sensors

4.1.1 Tilt platformThe tilt platform is equipped with a single encoder that gives us an absolute angular position between

–90° to 90°, 0° corresponding to the horizontal plane for the sonar plan. The data are not yet up-link (due to an error in the TDS software).

4.1.2 Watson Rate GyroThis sensor is used for getting the rotation speed value of the ROV in the horizontal plan. The law for

converting the raw measures to degrees per second is the following:

The offset must be evaluated at each new mission. By experimentation, we found that 200 is the average of the observed offset when the ROV is on a stable plan. The ratio used for the computation is derived from the data sheet of the sensor.

Currently the data given by the gyroscope is not satisfactory. More investigation is needed for calibrating this sensor. A set of experiments with calibrated equipment will be held in May 2001.

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4.1.3 TCM2 sensorsThe 3-axis compass TCM2 is used for determining the attitude of the ROV with respect to the

orientation of the magnetic field and the vertical direction.The compass is connected to the subsea TDS by a serial link (RS 232), which forwards the measures to

the surface PC.

The measures of pitch and roll returned by the TMC2 compass are given with respect to the earth reference frame. Their signification is illustrated in the above figure. These measures do not correspond to the pitch and roll of the ROV (rotation around the y-axis followed by a rotation around the x-axis of the ROV coordinate frame, see figure below). A transformation of the raw measures is done by the following equations:

,

where (the vector perpendicular to the compass platform, corresponding to the z-axis of the ROV’s coordinate frame)), can be obtained from the compass measures and as (from TMC2 specifications):

, , .

The tilt, the roll and the heading are correctly measured in terms of accuracy and value. The TMS facilities do not allow full test of this sensor because the local magnetic field is highly perturbed by ferro-magnetic equipment in the vicinity of the robot. Only sea trials will give the right calibration.

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In spite of this, the curves below show that the acquisition presents no longer the blocking behaviour observed previously and the values are consistent with the observed robot motion. All the experiments done during the last months yield the same kind of result.

The next plots show measures of the pitch and roll of the platform. The non-zero values are due to the fact that the vehicle is not perfectly balanced and have an initial roll and tilt.

4.1.4 Propeller encodersThere are three encoders in the ROV, each one measuring the rotation of one of the motors. They are

identical and give the number of impulses between reset and the time we read the buffer of the sensor Their sensitivity is 1000 impulses per turn.

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We compute the cycle used for the measurement of the RPM by the formula: Cycle Time = T0-T1+FreeCounter limit (see chapter 3 for definition). Using this value, we get value of rotation speed in rotations per minute using the following expression:

The first factor yields the absolute number of turns (resolution of 1000 impulse per turn), then the cycle ratio converts this into the number of turns in a tenth of a second, and this value is finally multiplied by 600 to yield a measure in rotations per minute.

The curves below represent a simple test. The ROV has been driven for turning then going ahead, turning again, going ahead and then surfacing. When turning, both ellors turn in the same direction as can be seen on the figure (yellow and purple curves, for left and right propellor ectively), and are of opposite site when going in straight line. The blue curve represents the rotation speed of the vertical propeller.

The RMPs measured vary smoothly and agree with the motor specification. The measurements have been performed using the maximum speed of the thruster in the water (in the air, the value is about 3 times larger). The results are highly reproducible.

4.1.5 Pressure sensorThe pressure sensor is used for measuring the depth of the ROV. We

used the fact that 10 m depth is equivalent to 1 Atm. For the measurement, the conversion between the raw data and the depth of the vehicle is given by:

.

The result is in cm from the position of the sensor (see picture on the right).

The offset value of 32612 is implemented in the software. This value will be dynamically defined at each new mission (a set zero immersion function). The ratio corresponds to the conversion from Volts to centimetres.

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Pressure sensor Altimeter


4.2 Acoustic sensors

4.2.1 Altimeter Tritech manufactures the altimeter which runs as a completely autonomous system. It is linked to the

surface PC through its own RS 232 link. It performs acquisition at the rate of 10 Hz. Its range is 50.0 m. When the distance to the closest obstacle is larger than this value, the altimeter returns a value of 0.0 m (no detection).

The value is directly send in ASCII and the accuracy is about one centimetre.The plot below is dedicated to the measurement of the immersion (blue curve), the altimeter (purple

curve) and the correlation between them (sum of the previous two curves, yellow curve). It can be noticed that the value of the total height of water is quite stable expect during rapid variations of depth.

A possible explanation of the variation observed is that the altimeter is able to answer faster than the pressure sensor. Another one is the fact that we must use the tilt and the roll of the ROV to correct the measurements of the altitude (angle deviation). At the same time, the pressure sensor seems to have an offset and an inertial motion. This system need to be more investigated.

4.2.2 Profiler sonarTritech is the manufacturer of the profiler sonar. It is a dual frequency sonar that can be used in two

modes: low frequency (580 kHz) : for “long range” acquisition with low level of detail high frequency (1260 kHz) : for “short range” with a high level of detail.

.The major limitation of this kind of sensors is its low speed of acquisition (time of propagation times

number of profiles, e.g. 360 ° scan at 50 m with 1° resolution takes around 24 seconds)This sonar is driven through an RS 232 link (bi-directional). The complete protocol for configuring and

using this sonar is really complex, especially when the sub-contractor delivers not-clear documentation. The current implementation satisfies the basic requirement for this project and its development has required a large effort from the involved partners. The choices are , in this implementation:

Frequency: low or high (default value high) Range: from 1 to 100 m (default value 10 m)

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Number of point in one scan: from 10 to 800 (default value: 800) Angle for profile: start and end angle from –180 to +180, start angle lower than end one.

With these parameters, the sonar is able to execute all the measurements we are expected to perform during the NARVAL project. Some other parameters can be calculated from these variables such as the sampling rate (time step for digitalisation).

The picture above shows one sonar scan of the vertical plan of the TMS facility tank taken at the highest sonar frequency.

We can see the walls of the tank (5 meters high, 5 meters large, 2.5 meters from each side of the robot).

With low losses (transmission loss are low in this tank), we get a mirage of the boundaries. This is due to the fact that the acoustic wave reflects between the tank walls. In some cases, the surface of the water is also detected (a low level echo is sometimes detected).

4.3 Video sensorThe video sensor installed in the robot is a Sony PAL colour camera. The RS 232 link for control of the

camera is not already software implemented. A modification of the TDS subsea software will be performed before the end of May.

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Shadow due to steel detail

Position of the sonar

2nd measurement of the length of the size of the tank (5+2.5 m)


5. Future planFor establishing the final report of the integration of the ROV, we need to perform the following test and

improvement:1. Install the download link from the PC to the subsea TDS to command the video camera2. Validate the Gyro rate scale with calibrated facilities. We will use some turrets in TMS for

calibration. Those data will be correlated to the heading variation (integration or derivative).3. Calibration of the pressure sensor with the altimeter data. This will be held in the TMS facilities.4. Upgrade the TDS software for up-load the tilt platform information (needed for obstacle

avoidance and mosaicing with the sonar).

The task scheduling is the following:

According to this plan all sensors will be fully exploitable by the end of May 2001.

For each task, a set of tests will be performed to fully guaranty the behaviour of the sensor (specific internal documentation is under redaction for agreement between the involved partners).

6. ConclusionDuring the second year, TMS has put the major part of its effort on the integration and validation of the acquiring system. A great support has been provided by CNRS/I3S for identifying disruptive behaviours of sensor, and to understand the software and hardware architectures that have been delivered by Deep Ocean Engineering. An acquisition system with good performance is needed for a good characterisation of the vehicle (WP 3). The present system allows an initial characterisation of the platform dynamic behaviour, although the availability of the gyro sensor would obviously lad to better performance (better models, better controllers).

For the acoustic sensor, all problems seem to have been solved. The complete acoustic acquisition is set on work. The only restriction, observed with the altimeter, is a limitation of the altitude of the ROV from the seabed. In fact, if the sensor is too close to the seabed, the altimeter is not able to perform the right data acquisition because the altimeter has a shadow in the time measurement and can’t perform an acquisition at less than 20 cm. In the TMS facilities, we get in fact the first reflected signal as measure so we get 10 m for an expected 20 cm…

Some improvements still need to be done and will be performed before end of May 2001. The next point is to have sea trails for the final test of calibration and a improving the TDS software for piloting the video camera.

At the end of the integration, a synthesis document (Integration Final Report) will be delivered with all the tests and qualifications performed.

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May 2001April 2001

Gyro calibration

Altimeter and depth qualification

Upgrade of TDS software

Video control link


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