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Figure 4.13: SICK LMS200 laser range finder
In conclusion, this sensor lived up to its promise of fast and accurate data and is deemed to be a very good addition to the Meercat platform. The solution is not low cost as required, however it is only a matter of time before it becomes available at a lower cost. Refer to the discussion at the end of paragraph 2.4.4 for additional details.
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Fortunately, besides providing header information, the IC also provides access to its primary magnetometer sensors. Each magnetometer reports the sensed field strength in micro Teslas (µT) and a resultant total vector can then be computed. A calibration scheme is now used where the sensor’s X and Y magnetometer data is taken, normalised and interpreted into a heading.
The raw data obtained from the calibration process is shown in Figure 4.14 where the vehicle was slowly rotated around its axis and the data was sampled at an interval of 5 Hz.
Figure 4.14: Magnetometers raw values
As these are raw values from uncalibrated magnetometer sensors, the values must be normalised to each other so that they can be used in further calculations. This is done by finding the extreme values in both sensors and adjusting the readings such that the data fits symmetrically around the zero-axis as shown in Figure 4.15.
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Figure 4.15: Normalised magnetometer values
After further calculations, the final heading was computed and is shown in Figure 4.16. The data is shown as radians ranging from -π to +π as this is the platform orientation standard used in the higher levels in the software.
Figure 4.16: Compass heading
The Z-axis data only plays a role when the vehicle should experience tilt and this is not the case in an indoor environment where the tests took place.
4.3.2 GPS
The EVK-5H evaluation kit was chosen from the manufacturer u-blox for the GPS sensor, as shown in Figure 4.17. The kit is equipped with a USB link for direct interface to a computer.
The alternative RS-232 interface was used for the Meercat as the GPS was interfaced to the AVR32 embedded platform.
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Figure 4.17: u-blox EVK-5H GPS evaluation kit
The kit comes with an active GPS antenna, which means that the weak GPS signal is additionally amplified before it enters the receiver. The manufacturer claims that its SuperSense feature allows for indoor GPS as the module can receive signals down to -161 dBm. However, based on the tests performed, this seemed to be a little optimistic as acquisition of satellite signals in the office’s corridor was never successful.
By moving close to a window or open door, a satellite lock was obtained and subsequently the module was very successful at maintaining at least a time signal indoors, although position information was not reliable. Generally, the solution worked well in outdoor situations, with a very good positional accuracy. As expected, some problems were experienced when GPS signals were obstructed by trees and buildings. The application of a metal base plate underneath the GPS antenna made a consierable difference ito the sensitivity.
4.3.3 Differential GPS
Traditionally, a specialised receiver is required for differential or augmented GPS, which handles both the GPS signals and the augmentation service’s signal. Several routes were explored, but the solutions were all very expensive. The lowest quoted price started at R70K, which is clearly not a feasible solution for a low-cost service robot.
Two solutions were subsequently considered:
• RTKLIB by the Tokyo University of Marine Science and Technology [104].
• The Trignet service by the South African Department of Land Affairs [105].
RTKLIB is an open source software package, which can be used to calculate precise positions using a variety of Global Navigation Satelite Systems (GNSS) sources. In a recent paper [106]
the authors of this software describe an implementation of a full working real-time positioning
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system using RTKLIB. Trignet is a service recently launched by the South African Department of Land Affairs. This service provides data which is compatible with the RTKLIB software package.
The crux of any augmentation service is that it needs to be physically close to the point of interest, as the atmospheric corrections are only relevant within a limited range. As shown in Error! Reference source not found. the Trignet stations are spread over the entire country, with a higher density around Cape Town, Durban and Gauteng. One of the Trignet reference stations is located in the Pretoria suburb of Silverton, only a few kilometres from the CSIR, making the available correction data highly relevant. With an internet connection through a standard GPRS mobile phone, information was obtained from the reference network.
Figure 4.18: Trignet stations map
Unfortunately, the EVL-5H GPS module that was used for the GPS experiments was not suitable for the envisaged solution as it does not generate the correct raw data stream. A new evaluation kit, UBlox LEA 4L, was purchased at a cost of less than R3 000, making this the total cost of the implemented solution as the other elements were free.
With this solution, according to the Division of Geomatics at the University of Cape Town, it is generally possible to obtain RMS accuracies of 35cm in a horizontal position and 1 m accuracy in height [107].
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