POSSIBILITIES OF OBD DATA USAGE FOR
THE RETRIEVAL OF VEHICLE TRAJECTORY
Merkisz J., Wrona A.
Poznań University of Technology, Institute of Internal Combustion Engines and Transport, Automex, Gdańsk
Abstract: The paper presents a recording device fitted in vehicles accumulating data from on-board diagnostic devices such as OBDII or EOBD (On Board Diagnostic/European OBD) and, supposedly in the future, from other on board devices. The recording device also registers linear and angular acceleration of the vehicle. The paper also describes the algorithms used to describe and reconstructs the actual movement of the vehicle. The device was designed as an integrated module fitted in a solid metal casing to be used prospectively as a “black box”. The module enables a measurement of a multitude of parameters of a vehicle in motion. The here applied integrated accelerometers ensure the measurement of acceleration in three mutually perpendicular axes. Two circuits of integrated gyroscopes– fitted in optimal locations – enable the measurement of angular velocity of a vehicle. Such an array of sensors and the information retrieved from the OBD systems facilitate the determining of the motion parameters and vehicle trajectory including vehicle skids or rollovers.
1. Introduction
On-board recording devices commonly go by the name of black boxes. The name has its roots in the fact that the device itself is placed in a solid smash resistant casing. The color of the casing is not in fact black at all. The colors of the casing are mostly bright, denoting attention - yellow and orange prevail, so that they could be easily found when necessary. Even though the term black box denotes an obsolete thing the name stayed in the language signifying an on-board recording device.
New technologies entered into every sphere of the automotive industry beginning from the engine control unit through electronic car keys to vehicle diagnostic and supervision. In recent years we may see a growing role and number of applications of devices registering various vehicle parameters. They go by the name of ORD (On Board Recording Devices) or EDR (Event Data Recorder). The direct purpose of such devices is to provide a tool for a detailed quantitative and qualitative analysis of an event that took place on the road.
These Systems may compare to an objective, disinterested observer having an outstanding memory of unlimited size that may, at any time, describe what has just happened. A post collision opportunity to quickly and accurately reconstruct the trajectory of a vehicle and the behavior of the driver will inevitably lead to better conclusions. The legal consequences may bear fruit in many aspects. It enables to prove people innocent of a road collision or accident particularly if there is not enough substantial evidence. Situation where only material damage results will also be quickly and effectively analyzed – until now such events have not been analyzed at all. Besides, a quick and precise determination of the course of accident will result in ascertaining the perpetrators and the level of guilt and will only add to a better and faster settlement with the insurance companies. In case of a serious road accident there would be no disputes whether the direct cause of the accident was excessive speed or, immediately prior to the accident, any faults or malfunctions occurred that may have had impact on the driving stability or braking system failure. An important aspect seems the scientific research approach as well. The recorded data enable the engineers to improve their products. So far, such knowledge has been derived exclusively from crash tests. Currently we can learn from actual road situations which might not be entirely ethical but will surely save lives in the future. On-board parameter recording may be useful in developing the training methodology and teaching how to drive cars in a smooth and economical way. This is clearly reflected in the operating costs of a vehicle let alone the driving comfort of the passengers. The latter is particularly important in urban driving.
We cannot neglect the psychological impact of the recording device on the driver. A driver who is aware of the presence of such a device in his vehicle will naturally make smooth his/her driving style. The sense of a responsibility for the passengers and the vehicle increases, providing an effective means of the safety improvement on the road.
2. Recording device – the design
Prior to the designing process of a device one must decide what purpose it is to serve or what functions it is to realize. After an in-depth analysis of the idea of a recording device the designers came to a conclusion that its primary purpose is to record static and dynamic driving parameters. The device must provide the possibility to examine and determine and ensure the following:
vehicle trajectory in a moment before, during and immediately after the collision, conditions in which the goods and/or people were transported,
supervision of vehicle fleet,
driver’s techniques,
In order to provide the above, a group of parameters needs to be selected for recording and the source of these parameters must be specified. The number of parameters will have a direct influence on the size of the memory that is necessary for a flawless operation of the device.
All of the above tasks have one thing in common: in order for those tasks to be successfully performed the parameters of vehicle in motion must first be recorded. The only difference is that in the case of vehicle trajectory before during and after the collision the data are recorded in a specified time frame. In the case of comfort and driving technique analysis the recording is continuous. In both cases this has a large influence on the size of the recorded data, thus, on the memory space that a given device should have. As for the recording of dangerous road events much smaller memory is needed as opposed to continuous recordings that could last for several hours. Depending on the function of a device in question a varied sampling frequency is necessary. Dangerous event recording devices will have a larger sampling frequency than in other cases because of the unusual speed of events. In such devices the frequency should be on the level of 1 kHz i.e. in one second the device should store approximately 1000 information clusters and a single cluster may contain several bytes of data. Hence, the whole record lasting 35 seconds will require memory from several KB up to 1MB. One memory card of 256MB can thus store over 100 records of dangerous events. To recapitulate, a dangerous event recording device requires higher sampling frequencies, yet it needs smaller memory whereas the “driving comfort” recording device does not need high sampling frequencies (below 50Hz is sufficient ) but larger memory.
The engineers are capable of designing a device that will perform all of the above tasks. The difference in the realization of these tasks will consist in selecting appropriate memory resources and developing specialized software.
3. Sources of the data acquisition
The device has at its disposal the following data sources:
internal sensors – these are the sensors located directly in the device casing
permanently dedicated to a particular device,
peripherals – this group includes other devices that enable the scanning of the parameters,
diagnostic systems – these are on-board data buses applied in modern vehicles.
The internal sensors are a basic source of data. These sensors allow basic parameters of the surrounding environment and the vehicle in motion to be determined. These sensors are as follows:
linear accelerometers,
angular accelerometers (gyroscopes), real time clock,
temperature sensors, humidity sensors etc.
The advantage of such a source of data is that, irrespective of the vehicle where the device is fitted, the above group of parameters is always available for acquisition by the device. Other data sources are a bit more complex.
Out of the most significant sources of external data Geopositional Systems (GPS) deserve attention here. Currently GPS systems are available to almost everyone. Their mass production and a multitude of available solutions resulted in a sizeable price reduction. In the case of the GPS systems, their low pricing does not necessarily mean degradation in quality. They become faster and more accurate. In most solutions they allow the acquisition of positioning data via standard bus such as RS232, USB or Bluetooth. The GPS systems may be a valuable source of information for the on-board recording devices. Another group of external devices are wireless communication systems such as GSM. After hooking up to the on-board device via a GSM network a fleet manager can get access to the data in the on-board device. This allows a continuous recording of a task currently performed by a vehicle equipped with the recording device.
The last but not least of the data sources is the vehicle itself. All modern vehicles cannot operate without electronics. This includes the control of such a complex vehicle component as the engine (ECU or on-board computer). The computer controls engine operation based on the data received from a variety of sensors fitted in the vehicle. Additionally, the recent EU legislation has unified the diagnostic procedures required for the vehicles. In the EU countries and the U.S., vehicle electronics has been unified to a large extent. The EU standards are known as the EOBD ( European On Board Diagnostic) and in the U.S. as the OBDII (On Board Diagnostic – second generation). The following elements underwent the unification process:
diagnostic procedures, error codes,
the shape and location of the diagnostic port,
communication protocol between the vehicle and the scanning tool.
Due to such demanding requirements regarding the vehicles the first idea of the recording device was to build it exclusively on the basis of the sensors already existing in the vehicle. This, however, led to a conclusion that such a device would serve a limited purpose. A serious flaw of such a concept is the fact that the on-board computer was designed to control the engine and, thus some limitations as to the data retrieval speed from OBDII/EOBD were introduced. The on-board computer can receive only ten commands. Sending one command will result in a single response from the computer. A single response is a piece of information regarding the current, momentary value of one
of the engine’s parameters. Hence, if we want to record one parameter, the maximum frequency may be approximately 10 Hz, if we want two parameters, the frequency drops to approximately 5 Hz, if three parameters need to be recorded, we arrive at the value 2.5 Hz only. The above frequencies are definitely too small for the precise reconstruction of a given parameter of a vehicle in motion.
Another issue is the set of sensors fitted in the vehicle. Only speed, engine revolution and throttle position sensors may be useful for the analysis. The remaining sensors merely inform us about the proper engine operation. Hence, it is impossible to determine the vehicle transverse acceleration based on these sensors. Yet, the data retrieved from the OBDII/EOBD may constitute perfect complementary data for a full report of the vehicle behavior on the road. We should not forget that the OBDII/EOBD systems present error codes in a unified form i.e. data regarding the technical condition of a vehicle which in many cases turns very useful.
4. Data recording
Automatic static and dynamic parameter recording device has a very sophisticated design and functionality and is based on a complex analysis. Despite the technology advancement developing a particular structure is not an easy task. The functions and applications that the device should cover require leading edge technology such as signal processors, data transfer systems (FLASH memory), specialized measurement-processing units.
The basic problem that arises during the design of any recording device is the development of the assumptions as regards the parameters it is to record. These assumptions should utilize the scope of the work of the device to the maximum but the very scope cannot be too large for technical limitations.
In the case of a universal recording device its operation should entail the following functions:
register longitudinal and angular accelerations in three perpendicular axes;
register the vehicle speed;
read data from diagnostic ports;
analyze the surrounding environment (temperature, condition/state of some of the
vehicle’s components: brakes, direction indicators etc.);
store data in the RAM memory (enough memory necessary);
enable to transfer the data from the device to an external computer easily and quickly.
have a real time clock;
enable to connect such systems as GPS and GSM modems.
Fig. 1. System schematics
5. Vehicle trajectory reconstruction
Having a given vector of acceleration a affecting a vehicle in time we can calculate the speed and the distance of a vehicle by the integration of the equation of motion:
– the speed is related to acceleration, and the distance is related to speed: dt ) t ( v d ) t ( a dt ) t ( s d ) t ( v (1)
– the speed and the distance can be calculated from the following relations:
v
(
t
)
a
(
t
)
dt
v
0
0s
dt
)
t
(
v
)
t
(
s
(2)where s0, v0are the vectors of the initial values of distance and speed respectively. Assuming s0 0 i v0 0 we arrive at:
v
(
t
)
a
(
t
)
dt
s
(
t
)
v
(
t
)
dt
(3)In the domain of numerical calculations we should convert the vector of the analog signal from the accelerometers into its digital form. It is done by sampling its particular constituents with a known frequency that we call the sampling frequency. As a result of this operation we will obtain a data set a(i), (i = 0, 1, 2, 3, …, n) determining the accelerations affecting on the vehicle. The timing between the subsequent samples is called the sampling period. Based on signal a(i) and sampling period T the above formulas can have the following notation (assuming a two dimensional acceleration vector
)] t ( a ), t ( a [ ) t ( a x y ):
– momentary change of speed and distance in particular axes:
T ) i ( a ) i ( vx x vy(i)ay(i)T x(i)vx(i)T y(i)vy(i)T (4)
– the speed and distance can be ascertained based on the formulas below:
T ) i ( a ) 1 i ( v ) i ( v ) 1 i ( v ) i ( vx x x x x T ) i ( a ) 1 i ( v ) i ( v ) 1 i ( v ) i ( vy y y y y T ) i ( v ) 1 i ( x ) i ( x ) 1 i ( x ) i ( x x y(i) y(i1)y(i) y(i1)vy(i)T (5)
Fig. 2. Speed signal on the vehicle longitudinal axis (thick line – OBDII readout, thin line – value calculated based on the signal from longitudinal acceleration sensor)
Fig.2 shows the vehicle longitudinal speed graphs obtained from two sources. The thick line represents the speed values obtained from the OBDII system and the thin line represents the speed values obtained from the performed calculations. The „stepping” shape of the speed course scanned from the on-board diagnostic system results from the specificity of the system (only one parameter can be scanned with the frequency of max. 10 Hz). In the performed tests, three parameters were recorded simultaneously (vehicle speed, engine speed and throttle position), hence the variation of parameter value occurred with the frequency of approximately 3Hz.
From the graph presented in fig. 2 we can see that the value obtained in the calculations is in-line with the value scanned from the on-board computer. The conclusion is that the acceleration measurement is appropriate. We can also see that the values scanned from OBDII are averaged, which decreases the dynamics of the signal measurement as opposed to the calculated value. An example motion trajectory reconstructed with the above method is shown in Fig. 3.
Fig. 3. An example reconstructed motion trajectory
6. Conclusions
The paper presented an on-board recording device developed by Automex from Gdańsk and software designed for the reconstruction and visualization of motion trajectory of a vehicle including collisions. The equipment and the software were verified through tests on a vehicle model and an actual car. The performed tests showed that proposed device of own design is capable of a satisfactory reflection of the basic parameters of a vehicle in motion. This may turn useful when reconstructing a vehicle trajectory (e.g. after a collision) as well as to ascertain the driver’s technique (particularly in relation to the driving comfort and fuel efficiency).
The use of on-board diagnostic devices such as OBDII/EOBD or other, similar (fitted in buses or commercial vehicles) seems well justified as the information obtained there from is necessary for the determining of the initial parameters of the algorithms for used for calculations. Additionally, they may be a valuable source of information as regards the vehicle condition before and after the collision – system info whether the vehicle was fully operative (error codes), what the engine throttle position was and, in some diagnostic systems, info on braking system pressure, state of the direction indicators etc. The precision of the trajectory reconstruction is very promising not to say motivating to extend the range of algorithms to a three dimensional vehicle trajectory.
Further tests are to be performed soon on real vehicles to finally implement the complete black box system to reconstruct the vehicle trajectory and the course of accidents. The applied algorithms written in the Matlab language in the testing phase are now extended and developed with C++.
References
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