Beginners Robotics Guide : Mobile Robot Navigation
by, 03-06-2010 at 02:46 AM (13582 Views)
"Mobile Robot Navigation" covers a large spectrum of different systems. requirements and solutions. The aim of this article is to study this range of requirements, identify the major areas within the scale which are most commonly used, and to discuss appropriate systems for meeting these requirements.
The key difference between robotic and human navigation is the quantum difference in perceptual capabilities. Humans can detect, classify, and identify environmental features under widely varying environmental conditions, independent of relative orientation and distance. Current robots, while being able to detect stationary obstacles before they run into them, have very limited perceptual and decisional capabilities. Although much research is being done to improve robotic navigational performance through enhanced perception, decisions to utilize these emerging technologies must be based on a critical analysis of considerations of technical risk and cost.
The physical scale of a device's navigation requirements can be measured by the accuracy to which the mobile robot needs to navigate - this is the resolution of navigation. These requirements vary greatly with application, however a first order approximation of the accuracy required can by taken from the dimensions of the vehicle it self. Any autonomous device must be able to determine its position to a resolution within at least its own dimensions, in order to be able to navigate and interact with its environment correctly.
At the small end of the scale there are robots just a few centimeters in size, which will require high precision navigation over a small range (due to energy supply constraints), while operating in a relatively tame environment. At the other end of the scale there are Jumbo jet aircraft and ocean going liners, each with some sort of auto-pilot navigation, which requires accuracy to a number of meters (or tens of meters), over a huge (i.e. global) range, in somewhat more rugged conditions.
To help in categorizing this scale of requirements, we use three terms:-
* Global navigation , which is the ability to determine one's position in absolute or map-referenced terms, and to move to a desired destination point.
* Local navigation , the ability to determine one's position relative to objects (stationary or moving) in the environment, and to interact with them correctly.
* Personal navigation , which involves being aware of the positioning of the various parts that make up oneself, in relation to each other and in handling objects.
With the jet auto-pilot example Global navigation is the major requirement, for cruising between continents. Local navigation becomes necessary were the aircraft is expected to fly autonomously in crowded airways, or on approach to the runway on landing. Personal navigation is not an issue, as the vehicle is, fundamentally, one object, and should (hopefully) never come into contact with any other significant objects while under autonomous control.
The "micro" robot on the other hand, is almost exclusively interested in Personal and Local navigation. Such devices are rarely concerned with their position globally, on any traditional geographic scale. Instead their requirements are far more task based - the are concerned with their immediate environment, in particular relative to any objects relevant in the successful completion of their task. This involves Personal navigation, when it is in contact with other objects, and Local navigation for actual movement.
Following on, and going in hand with, the scale of navigation requirements, is the frame in which position fixing is performed relative to. The two terms used here are, understandably, Absolute and Relative. What isn't so obvious, however, is what Relative is relative to, and where Absolute is absolute from; this is because these terms are somewhat context sensitive.
In terms of position fixing, absolute implies finding ones position relative to an absolute origin; a fixed stationary point common to all position fixes across the range of navigation. Hence in Global navigation, there should be one such point on the planet which all fixes are relative to. In Local navigation the absolute origin is some fixed point in the robot's environment, and in Personal navigation the origin can be viewed as the centre of the robot itself.
A Relative position fix when navigating Globally, taken relative to some other reference point (environment-relative), is analogous to the absolute position fix in Local navigation. Likewise, a position fix taken relative to the same robot's own position at some other point in time (self-relative), is like the personal absolute position fix. Through knowledge of the absolute reference frame (typically using a map), absolute position fixes in one navigation domain can be transformed into position fixes in another. Indeed, almost all global absolute position fixing is carried out by finding either an environment- or a self- relative position fix, and then converting this into a global position (see Beacon Navigation and Dead-Reckoning respectively).
Two Contemporary Systems
The technology employed in mobile robot navigation is rapidally developing. Here two relatively modern systems are studied, satellite based Global Positioning Systems and image based Vision Positioning Systems, which have the common feature of being under continual development. Between the two, a large scale of naivgational requirements can be met.
* Global Positioning System : GPS provides an accuracy of 100 m (95 % of the time) to Standard Positioning Service (SPS) users, due to the Selective Availability (S/A) errors introduced intentionally by the US military, for defence reasons. This can be improved to about 15 m (95 %) for authorised Precision Positioning Service (PPS) users [Kaplan, 1996]. The SPS accuracy is not good enough to be individually useful for mobile robot navigation. However, when augmented by the benefits of Differential techniques, GPS does become a viable method for global reference navigation.
* Vision Based techniques : Vision based positioning or localisation uses the same basic principles of landmark-based and map-based positioning but relies on optical sensors rather than ultrasound, dead-reckoning and inertial sensors.
Fixed position and mobile robots
A fixed industrial robot essentially consists of a mechanical structure. One end is firmly fixed to the floor while the other end (the end-effector) is free to move under programme control. Sensors are attached to the moving parts of the Robot so that the position of the end-effector can be calculated mathematically (since the lengths of the links are known), relative to a fixed frame of reference originated at the base. A mobile robot however, is in a moving frame of reference, thus its position must be determined relative to a fixed frame of reference somewhere in the surroundings.
Robots guided by off-board fixed paths
This is one of the most popular guidance techniques for industrial robot's. It uses buried cables arranged in complex closed loops, each closed loop carries a different frequency a.c. signal. Small magnetic plates are fixed to the ground at junctions and before and after sharp bends to allow detection of these potential danger points and for appropriate speed reduction. The system also has communication points along the paths where the robot can report its status to the main computer which co-ordinates all the robot's and plans and blocks routes to avoid collisions.
These systems are popular in industries because they are fairly reliable and simple. However, they suffer from the following drawbacks:
* The paths are not easily alterable because the cables are buried in channels about 1 cm deep
* The cost of laying cables into the ground is quite high.
Painted-line guided robot's
These are popular in light engineering or office environments. The system used is very similar to wire-guided robot's, except that their guidance technique is different. They follow lines on the floor, which have been painted using visible or invisible fluorescent dye (which are usually caused to fluoresce by shining UV light on them). The advantage of this guidance technique over wire-guidance is that, paths can be fixed quickly and are easy to alter.
The disadvantages are:
* Networks must be kept fairly simple, since junctions are not as easy to manage as in the wire-guided case.
* Through wear and tear the dye gets erased and so the lines must be repainted from time to time.
* Lines can be obscured by objects thus disabling the robot guidance.