Robotic Platform Hardware / / Project Deliverable

PROJECT DELIVERABLE D6

Robotic Platform Hardware

Shared-cost RTD

Project acronym: TOURBOT

Project full title: Interactive Museum Tele-presence Through Robotic Avatars

Contract Number:IST-1999-12643

Key Action: 3

Action Line: 3-2-3








TOURBOT: Interactive Museum Tele-presence Through Robotic Avatars

Project Deliverable D6: Robotic Platform Hardware

Date Produced: March 19, 2001

Authors:Dimitris Papanikolaou, Vassilis Savvaides and Antonis Argyros

Contents

1Introduction......

2Hardware Configuration......

2.1Basic robotic platform......

2.1.1Robot base......

2.1.2Robot enclosure......

2.2Sensory System of the platform......

2.2.1Sonar sensors......

2.2.2Infrared sensors......

2.2.3Tactile Sensors......

2.2.4Laser Scanner......

2.2.5Position (odometry) sensors......

2.2.6Vision system......

2.3Communication system......

2.4Hardware for TOURBOT/on-site user interaction......

2.4.1Mechanical head......

2.4.2LCD touch screen......

2.4.3Speakers......

2.5On-board processing power......

3Safety......

4Conclusions......

1Introduction

The current document describes the hardware configuration of the robotic platform of the TOURBOT system. The related developments took place within WP5, which has been completed according to schedule.

The complete TOURBOT hardware consists of a mobile robotic platform and off-board workstations. The configuration of the mobile platform has been designed to meet TOURBOT’s operational requirements and consists of:

The basic robotic platform, including:

  • The robot base (containing motors and batteries).
  • The robot enclosure (containing processing power and hosting most of the sensors and robot electronics).

A sensory system that includes

  • Two rings of twenty-four sonars, each.
  • One ring of 24 infrared (IR) sensors.
  • Full-body coverage tactile sensors.
  • One horizontal laser scanner.
  • An odometry sensor.
  • A high-performance Stereo Vision System (HPSVS) for visualization, which includes
  • Two high resolution, color cameras.
  • A pan-tilt head.
  • Two frame grabbers for image digitisation.
  • Synchronization electronics.

A communication system that includes wireless communication between the robotic platform and the base station.

Specific hardware for TOURBOT/on-site user interaction which includes:

  • A mechanical (anthropomorphic) head that it is interfaced to the pan-tilt unit and the vision system. The head consists of eyes (the cameras of the vision system) as well as of a mouth and two eyebrows that can be moved by software in order to control the facial characteristics of the head. This way, the robot can communicate its internal status (e.g. inability to move due to a large number of obstacles etc) to the on-site visitors in a very natural way.
  • An LCD touch screen display, through which the on-site user can (a) control the robot and (b) visualize information regarding the exhibits.
  • A speaker that is used to convey sound information about the exhibits of the museum.

On-board processing power (two PCs) to handle user interaction, processing of sensory information, control of the platform and the communication with the base station.

The following sections describe in detail the hardware issues related to the TOURBOT system. Moreover, it is described how the hardware design deals with important safety issues.

2Hardware Configuration

The TOURBOT system is designed according to the user requirements in order to fulfill its tasks, which are:

(a)safe navigation through a museum,

(b)interaction with people in a museum,

(c)interaction with web visitors of a system.

2.1Basic robotic platform

2.1.1Robot base

The robot base contains the motors that drive the robot and the batteries that are needed for supporting the power requirements of the system. The robot base has a cylindrical shape with a radius of 26.3 cm and a height of 31 cm. The cylindrical shape of the base (as well as of the enclosure) is an important design aspect of the whole system because it greatly simplifies robot motion and control. The basic property of this design is that it guaranties that a purely rotational robot motion (i.e. in-place turn) will never result in a collision of the robot with obstacles.

The base contains four motors. Robot motion relies on differential drive, which enables the system to turn in-place. This is very important aspect of the hardware design because it facilitates a lot the task of controlling the robot. The motors have the necessary power to enable TOURBOT system to move at a speed of 90 cm/sec (3.24 Km/h). This speed corresponds to the average speed of a walking person within an indoor environment and it is in full agreement with the user requirements: the robot should never move much faster than people do in a museum.

The four batteries included in the base are rechargeable and enable the system to operate autonomously for approximately 6 hours. After this period of time, batteries need to be recharged. In order to avoid long breaks between TOURBOTs’ operation due to battery recharging, two sets of batteries are going to be used, alternatively.

The batteries and the motors of the robot are essentially the heaviest components of the robots. Their placement in the lower part of the robot guaranties:

  • A center of mass close to the ground, which enables stability even in slanted terrains
  • Good contact of the wheels to the ground, which minimizes slippage, especially in cases of accelerated motion.

The cylindrical surface of the base consists of 8 panels, which host some of the sonar and tactile sensors of the system. These panels can be opened in a way that facilitates easy access to the batteries and inner electronics of the base.

2.1.2Robot enclosure

The robot enclosure is placed on top of the robot base and includes

the on-board computers,

most of the sensory apparatus of the robot,

most of the electronics of the system.

The shape of the enclosure is cylindrical, with the same radius as in the case of the base and with a height of 66 cm. The system is designed in such a way that a rotational motion of the robot corresponds to a rotation of the enclosure with respect to the base. The cylindrical surface of the enclosure consists of 6 panels, which host most of the infrared, sonar and tactile sensors of the system. These panels can be opened in a way that facilitates easy access to the inner electronics and subsystems.

Besides containing most of the sensors and the electronics of the robot, the enclosure hosts two important subsystems:

An LCD screen display that displays information about the status of the system (motors, sensors etc).

Emergency buttons which, when pressed, immobilize the robot. This is very important for safety reasons.

2.2Sensory System of the platform

In order to create an autonomous robotic system it is necessary to provide a means of acquiring information about the environment. This ability is given to a robot by its sensors.

The TOURBOT system is equipped with six sensory modalities:

  • Two rings of twenty-four sonars, each.
  • One ring of 24 infrared (IR) sensors.
  • Full-body coverage tactile sensors.
  • One horizontal laser scanner.
  • An odometry sensor
  • A high-performance Stereo Vision System (HPSVS) for visualization, which includes
  • Two high resolution, color cameras.
  • A pan-tilt head.
  • Two frame grabbers for image digitisation.
  • Synchronization electronics.

2.2.1Sonar sensors

SONAR, an acronym for “SOund Navigation And Ranging”, models the contours of the environment based on the emission of sound waves. The sender generates a sonic (or sound) wave that travels outwards in an expanding cone, and listens for an echo. The characteristics of that echo facilitate the listener to estimate distances from surrounding objects.

The TOURBOT system has two rings of twenty-four sonars, each. Each of the sonar rings is located at appropriate height that corresponds to positions where obstacles usually appear in indoor, man made environments. Sonar readings become available to the system at about three times per second. For each reading, the total time between the generation of the ping and the receipt of the echo, coupled with the speed of sound in the robot’s environment, generates an estimate of the distance to the object that bounced back the echo.

Each sonar sensor detects obstacles in a cone-shaped range that starts out, close in to the robot, with a half-angle of about 15 degrees, and spreads outwards. The obstacle’s surface characteristics (smooth or textured, for example), as well as the angle at which an obstacle is placed relative to the robot, significantly affect if and how that obstacle will be detected. The sonar sensors can sometimes be fooled because of the following reasons:

  • The sonar sensor has no way of knowing exactly where, in its fifteen-degree and wider cone of attention, an obstacle actually is.
  • The sonar sensor has no way of knowing the relative angle of an obstacle. Obstacles at steep angles might bounce their echoes off in a completely different direction, leaving the sonar sensor ignorant of their existence, as it never receives an echo.
  • The sonar sensor can be fooled if its ping bounces off an oblique-angled object onto another object in the environment, which then, in turn, returns an echo to the sonar sensor. This effect, called specular reflection, can cause errors; the sonar sensors overestimate the distance between the robot and the nearest obstacle.
  • Extremely smooth walls presented at steep angles, and glass walls, can seriously mislead the sonar sensors.

The robot has multiple sonar sensors in order to provide sensory redundancy and to enable cross checking.

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2.2.2Infrared sensors

TOURBOT system uses infrared ranging sensors for obstacle detection. These sensors provide an indication of the proximity of an obstacle by emitting light and measuring the intensity of the reflection bounced back off the obstacle’s surface. Infrared light, the part of the electromagnetic spectrum of radiation above 0.75 millimeter in wavelength, is not visible to humans. Infrared sensors provide the robot information it can use for understanding its immediate surroundings.

2.2.3Tactile Sensors

The TOURBOT system is equipped with bump sensors, constituting the simplest and lowest level sensor compared to the rest, which is only able to detect the presence or absence of collision.

2.2.4Laser Scanner

In mobile robotics, two problems of great importance are the problems of

  • Map building: The capability of a robot to build a map as an internal representation of its environment.
  • Localization: The capability of a robot to maintain a position estimate within a map.

For a robot to be able to solve both problems, accurate range information should be provided. The TOURBOT system acquires such information by its SICK PLS Scanner (see Figure1below). The advantage of this laser scanner is that it has the potential to deliver data at high rate and that its spatial resolution typically varies between 2.5 cm and 5 cm. The angular resolution of the laser scanner is 0.5°. Thus, the laser scanner provides very accurate distance measurements, which can be utilized both for map building and robot localization.

Figure 1: PLS Proximity Scanner

The PLS is a configurable, single point safety sensor able to match its protective field to irregularly shaped areas. This self-contained proximity sensor monitors up to 180 degrees of its surroundings.

Using an infrared laser beam and time-of-flight measurement, the PLS can create three independent, sensing fields: two configurable protection zones (Safety Zone and Early Warning Zone) and a “surveyed area” (see Figure 2 below). The protection zones work together to protect personnel and prevent inadvertent machine/vehicle stops. If the control reliable Safety Zone is entered, hazardous machine motion is signalled to stop. Entrance into the Early Warning Zone initiates a warning signal. The “surveyed area” can be used for navigational support on TOURBOT system.

PLS Zone Configuration

Figure 2: The three zones that can be defined based on the laser scanner readings

The long sensing range of the laser scanner allows higher travel speeds. Objects are detected early and the PLS refreshes its readings to reflect the current environment status. If objects are sensed too close, the robot may be stopped or re-routed before making contact with persons or objects inside the defined Safety Zone, protecting personnel and the robot.

With its ability to continuously survey its surroundings, the PLS also provides navigational and positioning information. It transmits real time data from its surroundings to the on-board robot processor. This data can be used to supplement a navigational system or to guide the robot into the desired position.

2.2.5Position (odometry) sensors

Position sensors are situated on the drive shaft. Their purpose is to give information to the control system about the position and the orientation of the wheels. The encoder used for the TOURBOT system is an optical incremental encoder, which detects the quantum of the change of the angular position.Traditionally, odometry is used to provide position feedback. However, because of slippage etc., a drift is introduced. This drift is, qualitatively speaking, proportional to the complexity of the maneuvers performed by the robot. To circumvent this problem it is necessary to complement the odometry based estimation with input from other sensory modalities.

2.2.6Vision system

Figure 3: Components of the vision system

TOURBOT is equipped with a High-Performance Stereo Vision System (HPSVS), which can send video images from the robot to the external world. The HPSVS consists of two XC999 Color CCD cameras with 6mm lenses (PAL), adjustable stereo camera mounting bar, Pan-Tilt Head, 2 PCI Frame Grabbers and Synchronization electronics (Figure 3).

The frame grabbers can digitize up to 25 fps (frames per second). The cameras are mounted on a pan-tilt device. Both degrees of freedom (pan/tilt) are controlled through a standard RS-232 interface. The maximum rotational velocity of both pan and tilt is 300 degrees per second. The pan-tilt unit can carry a maximum payload of 4 lbs.

The two cameras have different functionality within TOURBOT. The one of the two cameras is used to deliver a continuous video stream to the WEB user. The other camera is used to deliver high quality still images, again to the web user, whenever he/she asks for such an image.

2.3Communication system

The TOURBOT system includes two types of communication links: one between the on-board computers of the robotic platform and the off-board workstations of the system and one for the communication of the whole system with the World Wide Web (WWW). In order to achieve the communication of the robotic platform and the workstations, a wireless communication unit is used.

To communicate with the server a 2.4Ghz four (4) port Ethernet radio pair with antennas (100mW) has been installed. Client adapters and access points are up to three times faster than other wireless network devices. The data rate that is offered by this wireless communication link is 3 Mbits/sec. In typical indoor environments the supported range is in the order of 150m. This figure is substantially improved wherever direct visual contact between the transmitter and the receiver is guaranteed.

2.4Hardware for TOURBOT/on-site user interaction

TOURBOT contains specific hardware aiming at facilitating the interaction between the system and the on-site users. This hardware includes a mechanical (anthropomorphic) head, an LCD touch screen display and a speaker.

2.4.1Mechanical head

Two different mechanical heads have been designed and developed within TOURBOT. Both designs aim at providing anthropomorphic characteristics to the TOURBOT system, a fact that greatly facilitates acceptance of the robot by the on-site visitors and the interaction between robot and visitors.

In the first design (Figure 5(a)) the mechanical head has been developed around the pan-tilt unit of the vision system. In the second design (Figure 5(b)), a new pan device has been built. In both cases, the head consists of eyes (the cameras of the vision system) as well as of a mouth and two eyebrows that can be controlled by software changing the facial expressions of the head. This way, the robot can communicate its internal status (inability to move due to a large number of obstacles, etc) to the on-site visitors in a very natural way.

Figure 5(a): Initial design of the TOURBOT mechanical head / Figure 5(b): An alternative design of the TOURBOT mechanical head

2.4.2LCD touch screen

At the top of the robot enclosure, an LCD touch screen display has been added. This touch screen display serves as a bi-directional communication medium between the robot and the on-site visitors. More specifically, through an appropriately designed user interface, the on-site user can (a) control the robot and (b) visualize information regarding the exhibits.

2.4.3Speakers

A speaker is used to deliver sound information about the exhibits of the museum. A pre-recorded sound message related to a particular exhibit, is automatically played-back whenever TOURBOT arrives in front of this exhibit.

2.5On-board processing power

In order to support system/user interaction, processing of sensory information, control of the platform and communication with the base station, the TOURBOT system is equipped with two powerful on-board computers. Both computers are based on PENTIUM III processors running at a clock speed of 800MHz. Each of these computers is equipped with 256 Mbytes of RAM. The individual characteristics of these computers and the tasks allocated to each of them is described below:

Computer 1: Runs LINUX operating system. It is responsible for

  • The acquisition of all sensory information, except from visual information (video, still images)
  • The processing of sensory information for map construction and robot reactive navigation.
  • The control of all robot actuators (robot motion, control of the pan-tilt device, control of robot’s mouth and eyebrows)

Computer 2: Runs under Microsoft Windows 2000 operating system. It is responsible for:

  • Acquisition and broadcasting of video to the off-board workstation.
  • Acquisition and broadcasting of still images to the off-board workstation.
  • Handling the commands issued by on-site users to the on-board LCD touch screen display.

3Safety

TOURBOT is meant to operate in museum environments and exhibitions. Such environments are typically crowded and the audience consists of people that are often not acquainted with technological apparatuses, such as robots. Moreover, many of the visitors can be either elderly persons or young children. On the other hand, in museums and exhibitions there are a lot of items of great value in many respects. Taking these facts into consideration it becomes evident that TOURBOT should meet a number of safety requirements. Both software and hardware design and implementation took seriously into account all these safety requirements. From the hardware’s point of view, which is relevant to this document, the following characteristics guarantee normal and safe operation of TOURBOT in its environment: