Dead reckoning methods in positioning systems for moving objects..
Petrov Nikolay Nikolaevich, Candidate of Technical Sciences
DAY REPORTING METHODS IN POSITIONING SYSTEMS FOR MOBILE OBJECTS
This article is a continuation of a series of articles on vehicle positioning systems (AVL systems) and is devoted to the so-called dead reckoning methods (inertial navigation). The author does not claim to provide a complete overview of all possible systems of this kind. The article attempts to describe some of the most common methods as applied to vehicle positioning systems.
According to the classification given in the CCIR recommendation (Report 904-1, Dubrovnik, 1986), one of the methods for determining the location of moving objects is the dead reckoning method, also called the inertial navigation method. This method involves equipping a vehicle with direction (course) and distance sensors, the readings of which are used to determine the location of an object relative to fixed benchmarks, which can be specific points on the ground, directions to objects, etc. Depending on the purpose and structure of the AVL system, the location can be calculated either directly on the vehicle itself using an on-board navigation computer or in the control computer of the data processing subsystem. When constructing dispatch systems, information from the on-board device is transmitted to the dispatch center (or control center) via a data transmission subsystem, which can be any communication systems (cellular, trunking, satellite, etc.). There are various methods for determining the direction of movement and the distance traveled. Determining the distance traveled is a simpler task compared to calculating the course. In most vehicle location systems, the navigation computer is connected to the vehicle speedometer. Modern electronic speedometers can generate so-called wheel pulses every 20 cm of travel, which ensures high accuracy in measuring the distance traveled. Other methods of measuring distances are currently being developed, such as applying optical tapes to tires and placing magnetic tapes on car wheels. They can be used in cases where connection to the speedometer is not entirely convenient in terms of layout and placement of equipment. A more complex task is determining the course. The simplest and cheapest method for determining the direction of a vehicle is to use a magnetic compass. The main disadvantages of such a device include low accuracy, the need to introduce a correction for magnetic declination and, most importantly, the need to take into account the magnetic fields of the car itself and other factors distorting the Earth's magnetic field. The use of more accurate geomagnetic devices based on magnetic sensors (ferroprobes) and powerful on-board computers that take into account the correction of the directional angle makes it possible to get rid of some of these disadvantages. However, the main disadvantage of such devices, associated with magnetic field distortions, is not eliminated. Therefore, in a number of vehicle location systems, magnetic direction sensors, which are usually three-component meters of the Earth's magnetic field, are supplemented by other devices that compensate for magnetic field distortions that arise due to various factors. Acceleration sensors — accelerometers — are most often used as such devices. The combination of magnetic direction sensors with an accelerometer (using an on-board computer) is sometimes called a strapless magnetic compass.
The principle of operation of this device is as follows. Magnetic sensors measure the full vector of the Earth's magnetic field. However, to calculate the course, it is necessary to know not the full vector, but only its horizontal component. To do this, using a three-component accelerometer, the direction of the vertical in the instrument coordinate system is determined, after which the magnitude and direction of the horizontal component of the Earth's magnetic field in relation to the vehicle, i.e. its course, are calculated. Errors associated with distortions of the Earth's magnetic field can be eliminated by pre-calibrating the device, for which it is enough to take readings from the magnetic sensors in four positions obtained by turning the vehicle by 90° in the horizontal plane. Subsequently, as the car moves, the computing device constantly calculates the parameters of the parasitic magnetic field and determines the corrections that are used to calculate the course.
When implementing accelerometers in inertial navigation systems for cars, an important problem is their miniaturization. Currently, accelerometers of the piezoresistive, piezoresistive and piezoelectric types are known. However, so far they have significant dimensions and weight, as well as energy consumption. A more promising direction can be considered the creation of sensitive elements (SE) of acceleration sensors based on the capacitive principle of conversion using electrostatic compensation on silicon materials. SE developed on the basis of this technology are called micromechanical. The design of the capacitive type SE is a flat differential capacitor with two fixed plates and an internal movable electrode. Such sensitive elements are characterized by potentially high thermal stability, stability of metrological characteristics over time, the absence of noise and self-heating. The principle of operation of the capacitive accelerometer is based on measuring the difference in capacitance between the movable electrode and the fixed plates. In the absence of acceleration, the air gaps between the movable electrode and the fixed plates are the same, and accordingly the capacitance values are maintained equal. When acceleration is applied in any direction, the air gap values change, resulting in a difference in capacitance and currents flowing through these capacitances. Using a differential amplifier, this difference is amplified and converted into an output voltage proportional to the acceleration value. Accelerometers based on capacitive sensitive elements allow measuring accelerations of up to several tens of m/s2, have a consumption current within units of mA, and can be made in the form of integrated circuits. Examples of such accelerometers are integrated circuits from Analog Devices ADXL150, ADXL250, ADXL202, ADXL202, which are one- and two-component acceleration meters. In addition to accelerometers, angular velocity sensors based on gyroscopes can be used as correctors for geomagnetic devices.. Mechanical gyroscopes are practically not used in vehicle positioning systems due to their significant dimensions and power consumption. In AVL systems, it is possible to use laser fiber-optic gyroscopes. The operating principle of which is based on the Sagnac effect. Along a circular optical path, due to beam splitting, light propagates in two opposite directions. If the system is at rest relative to the inertial space, both light beams propagate oppositely along an optical path of the same length, so when the beams are added in the splitter, there is no phase shift. However, when the optical system rotates in the inertial space, a phase difference arises between the light waves, proportional to the angular velocity of rotation. There is also information on the creation of gyroscopes based on electrochemical converters, gyrosensitive piezoresonant sensors, and capacitive converters.
As an example of a complete and functioning inertial navigation device, we can consider the autonomous navigation device of the geomagnetic type (ANPGT), presented by ZAO Avtonavigator (Moscow). The ANPGT is based on mathematical modeling of the current coordinates of a moving object based on signals coming from a path sensor, direction sensors, and acceleration sensors. The distance sensor is connected to the speedometer and measures the distance traveled in meters.
The direction sensor (ferroprobe) is three orthogonally located magnetic field sensors. At the output of the ferroprobe there is an analog signal, the value of which is proportional to the angle of rotation relative to the Earth's magnetic meridian. The acceleration sensor (accelerometer) is three orthogonally located capacitive acceleration sensors. At the output there is a signal proportional to the measured acceleration. The sensor is used to eliminate the ferroprobe error that occurs due to the non-horizontal location of the object relative to the Earth's surface.
Analog signals from the sensors are converted into digital ones and sent to the coordinate calculation processor (CCP), based on the INTEL 296 series microcontroller. The CCP calculates the autonomous coordinates of the object. To eliminate the accumulating error of autonomous coordinates, the ANPGT implements a passive correction method based on a digital vector map of road network polylines. Digital cartographic information is stored in a reprogrammable memory device. Precise coordinates can be transmitted via the RS-232 interface to the on-board navigation computer or to the vehicle location display device. Precise coordinates are updated once per second.
The advertising materials for the ANPGT indicate that the error in determining autonomous coordinates is 1.2% of the distance traveled, and the error in determining coordinates with internal correction using digital cartographic information does not exceed 5 m. Despite the significantly rarer use of dead reckoning methods in vehicle positioning systems compared to other positioning methods, these methods have a number of advantages.
For example, compared to satellite radio navigation system (SRNS) receivers, inertial navigation devices are not susceptible to radio interference. They begin to work immediately after switching on (they do not require 1 — 2 minutes to download information from the satellite, as in SRNS), their coverage area is practically unlimited (line of sight of several satellites is not required), they provide heading indication, determine the distance to landmarks, and measure the azimuth angle. It is obvious that in the near future, inertial navigation devices will find the greatest application for vehicle location systems not as stand-alone devices, but as an addition to satellite radio navigation devices. Integration of SRNS receivers and direction and distance sensors will increase the accuracy of location, eliminate “dead zones”, and eliminate the loss of initial sections of vehicle routes.