can receive four or more signals, it can provide altitude as well. Some services are even available that will download maps for the area in which a receiver is located.
The biggest disadvantage to GPS is the low resolution. A straight GPS receiver can only provide a position fix to within about 15 meters (50 feet). In the past, this was improved upon by placing a receiver in a known location and sending position cor- rections to the mobile unit. This technique was called differential GPS. Today there is a service called WAAS or Wide Area Augmentation System. A WAAS equipped receiver automatically receives corrections for various areas from a geostationary satellite. Even so, the accuracy of differential GPS or a WAAS equipped GPS is 1 to 3 meters. At this writing, WAAS is only available in North America.
The best accuracy of a WAAS GPS is not adequate to produce acceptable lane tracking on a highway. For this reason, it is best used with other techniques that can mini- mize the error locally.
Thus, the advantages of the GPS include its low cost, and its output of absolute posi- tion. The biggest disadvantages are its accuracy and the fact that it will not work in most indoor environments. In fact, overhead structures, tunnels, weather, and even heavy overhead vegetation can interfere with GPS signals.
Guidelines for selecting and deploying navigation and
4. Deploy sensors to face in the directions where useful data is richest. Gener- ally, this is from the sides forward.
5. Allow the maximum flexibility possible over the data acquisition process and signal-processing process. Avoid fixed thresholds and filter parameters.
6. Never discard useful information. For example, if the data from a sonar transducer is turned into a range reading in hardware, it will be difficult or impossible to apply additional data filters or adaptive behaviors later.
7. Consider how sensor systems will interact with those of other robots.
8. Sensor data processing should include fail-safe tests that assure that the robot will not rely upon the system if it is not operating properly.
9. Read sensor specifications very carefully before committing to one. A sensor’s manufacturer may state it has resolution up to x and ranges out to y with update rates as fast as z, but it may not be possible to have any two of these at the same time.
Since Polaroid transducers have been the most popular sonar sensors for mobile ro- bots, it is instructive to look at some of the configurations into which they have been deployed. Figure 12.6 shows a configuration that resulted from the recognition of the importance of focusing on the forward-direction. This configuration has sacrificed the rear-looking elements of the ring configuration in favor of a second-level oriented forward.
While this configuration provides better coverage in a collision avoidance role, it still suffers from gaps in coverage. The most glaring gap is that between the bottom beam and ground level. There is also a gap between the beams as seen in the top view.
Since the beams are conical in shape, the gaps shown between beams is only repre- sentative of coverage at the level of the center of the transducers. The gap is worse at every other level.
While it would appear that the gaps in the top view are minimal, it should be remem- bered that a smooth surface like a wall will only show up at its perpendicularity to the beams. In the patterns of Figure 12.6, there are 12 opportunities for a wall to be within the range of the transducers and not be seen. For this reason, when two half- rings are used, the lower set of transducers is usually staggered in azimuth by one-half the beam-width angle of the devices.
Notice that this configuration has 26 transducers to fire and read. If the range of the transducers was set to 3 meters (10 feet), then the round-trip ping-time for each
transducer would be about 20 ms. If no time gap was used between transducer firings, then the time to fire the whole ring would be just over one half a second. This slow acquisition rate would severely limit the fastest speed at which the robot could safely operate. For this reason, ring configurations are usually setup so that beam firings overlap in time. This is done by overlapping the firing of beams that are at a signifi- cant angle away from each other. Unfortunately, even with this precaution the process greatly increases the number of false range readings that result from cross beam interference.
Figure 12.6. Dual half-ring configuration
Figure 12.7 demonstrates yet another variation that attempts to improve on the weak- nesses of the ring. In this staggered ring configuration, transducers are mounted in a zigzag pattern so that their beam patterns close the gaps that plague the earlier examples.
This same pattern can be duplicated at two levels as was done with the straight rings in the previous configuration. This configuration provides better coverage in the plan view, but at the expense of having even more transducers to fire.
Staggered configurations may increase the number of transducers per a half-ring from 13 to 16 (as in Figure 12.7) or even to 25, depending on the overlap desired. By this point, we can see that getting good collision avoidance coverage with a ring configu- ration is a bit like trying to nail jelly to a tree. Everything we do makes the problem worse somewhere else.
Figure 12.7. Staggered-beam configuration
Several commercial robots have achieved reasonable results with narrow-beam trans- ducers by abandoning the ring configuration altogether. Figure 12.8 is representative of these configurations, which concentrated the coverage in the forward path of the robot. This concentration makes sense since this is the zone from which most obstacles will be approached.
The weakness of the forward concentration pattern is that a robot can maneuver in such a way as to approach an obstacle without seeing it. The solid arrow in Figure 12.8 indicates one type of approach, while the dashed arrow shows the correspond- ing apparent approach of the obstacle from the robot’s perspective.
To prevent this scenario, the robot must be restricted from making arcing move- ments or other sensors can be deployed to cover these zones. The commercial robots that have used this approach have typically added additional sensors to avoid the problem.
At Cybermotion, we developed our strategy in much the same way others did. We took our best guess at a good configuration and then let it teach us what we needed to change or add. Unlike most of the industry, we decided to use wide-beam Piezo- electric transducers, and to use extensive signal processing. We never had reason to regret this decision.
All sonar transducers have a minimum range they can measure. While this minimum range is usually less than .15 meters (half a foot) for electrostatic transducers, it is typically as much as .3 meters (1 foot) for Piezo-electric transducers. In order for the SR-3 to navigate from walls close to the sides of the robot, the side transducers were recessed in slots in the sides of the robot so that it could measure distances as low as .15 meters.
Early configurations had two forward transducers and two side transducers only. The forward transducers were used for collision avoidance and to correct longitudinal posi- tion. The beam patterns of these transducers were set to be cross-eyed in order to provide double coverage in front of the robot. Because most objects in front of the robot can be detected by both beams, the robot can triangulate on objects. In fact, the SR-3 uses this technique to find and precisely mate with its charging plug.