2009 Urban Remote Sensing Joint Event
Pulsewidth Coding Approach for Multi-sensor Synchronization of Urban Mobile Mapping System Yuwei Chen, Ruizhi Chen, Tuomo Kröger Department of Navigation and Positioning Finnish Geodetic Institute, P.O.Box 15, 02431, Masala, Finland
[email protected] Abstract: Mobile mapping system has become an irresistible trend used for 3D urban environment modeling, transportation engineering, road survey and other applications during last few years. A typical car-borne laser scanner based mobile mapping system today can be considered as a multi-sensor system that integrates various navigation devices and data acquisition sensors on a rigid, moving platform like a van or any other vehicle for determining the positions of the surrounding objects along the driving trajectory. All these sensors in the mobile mapping system are synchronized to the GPS time via geo-referencing system. With traditional synchronizing method, the hardware restriction of the geo-referencing system limits the number (kind) of sensors to be synchronized. The paper presents a synchronization approach which applies pulsewidth coding into synchronizing pulse generation. The approach enables geo-referencing system to be synchronized with more sensors and breaks the limitation of hardware. The accuracy of the approach is analyzed after a test bench is presented. The advantages and disadvantages of the approach are discussed correspondingly. KEYWORD: mobile mapping system (MMS), Geo-reference system, synchronization, 3D urban environment modeling, pulsewidth coding.
I.
INTRODUCTION
Vehicle-based Mobile Mapping Systems (MMS) have become an indispensable trend in mapping applications throughout the last years ranging from 3D urban environment modeling, transportation engineering, road survey, to tourism [1][2][3][4][8]. The state-of-the-art integrated MMS system is characteristics of fusing multiple sensors, providing sensor trajectory by georeferencing system and inevitable complex post-processing program. The integrated Global Position System/Inertial Navigation System (GPS/INS) system provides the geo-reference information for all the sensors. Precise measurements in accuracy of millimeter level are required for system calibration and data quality control. High accuracy results require a precise synchronization of all the sensors, otherwise the system performance is degraded and the data availability is reduced.
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Antero Kukko, Juha Hyyppä, Department of Remote Sensing and Photogrammetry Finnish Geodetic Institute P.O. Box 15, 02431 Masala, Finland. The geo-referencing system could offer a precise solution whose accuracy is scale of a few millimeters by using tactical Inertial Measurement Unit (IMU) and GPS Real Time Kinematics (RTK) technology. Since the measurements of the geo-reference system are in a digital form, the data fusion between sensors and geo-reference system is possible in principle, only if the precise synchronization solution is available. According with the different synchronization frequency, MMS systems use three main synchronizing methods. First method is the easiest synchronizing method—PPS (Pulse Per Second), the PPS signal of the GPS receiver suits low frequency synchronization, such signal is available even in low-end GPS receiver, which could be used to trigger the ancillary sensor by direct connection [7][9][14]. Second method is the most common method -- the event marker, that fits medium frequency application (tens of Hz); the event marker input pins could be found on some GPS receivers, which could be triggered by the external sensor on purpose of synchronization [6][12][10]. Third method, the centralized synchronization board method is the best solution for high frequency application (hundreds to thousands Hz) in which the high performance clock guarantees the accuracy of the synchronizing system [2][12]. However, restricted by the hardware limitation, all these listed synchronization approaches could synchronize limited number (kind) sensors. Is it possible to develop a synchronization solution which could break the limitation? Finnish Geodetic Institute makes its first attempt to solve the problem by applying pulsewidth coding into the procedure of synchronization signal generation. On the other word, the synchronization signals, which are inputted into event marker pins are not only contain the information of the very moment when the georeferencing system is triggered by main synchronizing sensor. The duration of the synchronization pulse contains the information which ancillary sensors else are triggered during the period of main sensor. The only limitation of this approach is the resolution of the temperature
2009 Urban Remote Sensing Joint Event consumption crystal (TCXO) and the accuracy of the time measurement of geo-reference system. II.
SOLUTION
Pulsewidth modulation (PWM) is a common technique which has been employed on a wide variety of applications, ranging from measurement and communication to power control and conversion. PWM is mainly for controlling analog circuits with a processor’s digital output [13]. And pulsewidth coding (PWC) follows almost same idea. It is a method to encode the information into the pulsewidth. While in this application, the encoded square pulse would input into the digital processor rather than analog circuits comparing with PWM. The basic idea of PWC approach for sensor’s synchronization is encoding the trigger information into the duty cycle of the synchronization square pulse. For example, if the period of synchronizing signal is 1.024 milliseconds (976.6Hz) from main sensor, the resolution of the crystal of synchronizer is 1 microsecond (1MHz). The following coding table as Tab. 1 illustrated could be designed for system integration. TABLE 1. THE SAMPLE OF CODING TABLE FOR PWC
Sensor
Pulse width Duty Cycle
Sensor 1
512 us
1/2
Sensor 2
256 us
1/4
Sensor 3
128 us
1/8
Sensor 4
64 us
1/16
Sensor 5
32 us
1/32
Sensor 6
16 us
1/64
Sensor 7
8 us
1/128
Sensor 8
4 us
1/256
Sensor 9
2 us
1/512
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Sensor 10
1 us
1/1024
Then any pulsewidth coding signal whose pulsewidth ranges from 1 microsecond to 1023 microseconds could be decoded according to this table. For instance, a 916-microseconds pulsewidth signal could be decoded as 916 = 512 + 256 +128 + 16 + 4 That means during the period of the synchronizing signal from main sensor, sensor 1, sensor 2, sensor 3, sensor 6 and sensor 9 are triggered. And the trigger frequency could be set according to the specification of the sensors. The leading edge of the pulsewidth coding signal is the very moment that the main sensor triggers the synchronizer. The total amount (or kind) of sensors that the PWC approach could synchronize could be calculated by the following equation 2N = T / t +1
(1)
where N is the total amount of the sensors that the PWC approach could synchronize, T is the period of the synchronizing signal from main sensor, t is the period of the crystal embedded in synchronizer or the resolution of time measurement of geo-reference system, 1 in the formula means that the approach could synchronize the main sensor with georeference system because the leading edge contains the very moment that the main sensor transmits the synchronizing signal. There are some practical problems about the solution that have to be considered. The first one is the accuracy of the pulsewidth measurement. The second is the accuracy and the stability of the crystal. There are a lot of commercial georeference system offers tens nanosecond accuracy for time measurement [11]. And high-performance affordable TCXO with stability of 0.5ppm or even less is also available in market. So the risks mentioned above are under control. Because most event marker pin of geo-reference systems enable leading edge trigger mode or falling edge trigger mode only. As Fig. 1 shows, after generating the pulsewidth coding trigger signal according to the coding table, the synchronizer modifies the pulsewidth coding trigger signal by duplicating it to match the hardware restriction of geo-reference system. Two similar signals would trigger the two event marker pins of georeference system alternatively. Practically, the design of trigger port to different ancillary sensors should follow the Transistor-Transistor Logic (TTL) standard for minimum modification in future applications.
2009 Urban Remote Sensing Joint Event
Figure 1. The diagram of pulsewidth code approach
III.
TEST BENCH AND RESULTS
According to the illustration above, a test bench was set up to measure the Novatel SPAN (Synchronized Position Attitude and Navigation) system’s performance. The results were analyzed to check whether the pulsewidth information could be decoded by post-processing program or not before designing the hardware. As Fig. 2 shows, in the test solution, an Agilent 33250 signal generator outputted two simulated synchronizing signals, and these simulated signals were sent into the two event marker pins of the SPAN I/O (Input/Output) port. An oscilloscope was employed to monitor the signals sent by signal generator. The logs were called MARKTIME and MARK2TIME and they were recorded when the generator triggered the corresponding event marker pins of the SPAN system. A MARKTIME record was generated when the leading edge of pulsewidth coding signal occurred on event marker 1 input, while a MARK2TIME record was generated when the falling edge of pulsewidth coding signal occurred on event marker 2 input. All georeference data from the GPS receiver was logged to a desktop through serial ports. The postprocessing program analyzed the logs to check the data integrity in MATLAB. It calculated the pulsewidth by processing the corresponding frame of the MARKTIME and MARK2TIME logs.
MMS’s synchronization in practical field data collection. The solution is a workable substitute for multi-sensor synchronization of mobile mapping systems which apply PPS signal and event marker signal as synchronizing method. It could synchronize far more sensors than traditional method with some hardware modifications. According to the simulated test results above, a reference design based on complex programmable logic device (CPLD) is being designed nowadays because the CPLD device is more flexible to change in future. And a high-performance (0.5ppm) TCXO guarantees the accuracy of the system clock.
Oscilloscope (Tektronix TDS3054B)
Signal Generator (Agilent 33250)
Event Marker 1 Event Marker 2
The test bench collected the series data in a static mode in lab, and each simulation lasted for minutes. The periods of synchronizing pulse are 1s and 100 microseconds to simulate the situations of PPS mode and event marker mode correspondingly. The signal generator modulated the duty cycle of square wave according the coding table listed in Tab. 1. Tab. 2 presents the test results. From the table 2, it is easy to conclude that the approach successfully decoded the trigger information by extracting the pulsewidth parameter from the recorded logs. In other words, the approach has been proven to be an applicable solution for
Geo-reference data store (via serial port)
Desktop
Figure 2. test bench of pulsewidth coding approach
TABLE 2 . THE TEST RESULTS OF PWC SIMULATION EXPERIMENT. Frequency of synchronizing
1 Hz
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Geo-reference system (Novetal SPAN)
10 Hz
2009 Urban Remote Sensing Joint Event pulse Duty cycle (pulsewidth)
Leading edge detection rate
Falling edge detection rate
Pulsewidth detection rate
Duty cycle (pulsewidth)
Leading edge detection rate
Falling edge detection rate
Pulsewidth detection rate
Sensor 1
1/2 (500 ms)
100%
100%
100%
1/2 (50 ms)
100%
100%
100%
Sensor 5
1/32 (31.25 ms)
100%
100%
100%
1/32 (3.125 ms)
100%
100%
100%
Sensor 10
1/1024 (976.5625 us)a
100 %
100%
100%
1/1024 (97.65625 us)b
100%
100%
100%
Sensor1,5
17/32 (531.25ms)
100%
100%
100%
17/32 (53.125ms)
100%
100%
100%
Sensor1, 2,3,4,5
31/32 (968.75ms)
100%
100%
100%
31/32 (96.875ms)
100%
100%
100%
a.
Limited by the hardware restrictions of signal generator. The cycle period is set as 976.563 microseconds during the simulation. And the pulsewidth varied a little bit because the resolution of time measurement of the geo-reference system is about 49 ns [11]
b.
Limited by the hardware restrictions of signal generator. The cycle period is set as 97.656 microseconds during the simulation. And the pulsewidth varied a little bit because the resolution of time measurement of the geo-reference system is about 49 ns [11]
IV.
ANALYSIS OF SYNCHRONIZATION ACCURACY
Because the accuracy of the synchronization overwhelms the data quality and system availability of MMS, it is obviously necessary to analyze the accuracy of such approach. Since the approach calculates pulsewidth by two measurements. The maximum measurement errors introduced by the leading edge and falling edge measurement are equal: RTCXO + A GEO
(2)
where RTCXO is the resolution of the TXCO of synchronizer, and AGEO is the accuracy of time measurement of the georeference system. So the maximum error of pulsewidth measurement is equal 2RTCXO + 2AGEO
(3)
While the mean value of the error is equal to the (2) considering that such error follows statistical uniform distribution in this case. Because lots of geo-reference systems could offer time measurement accuracy with hundreds nanosecond, and a high-frequency and high-performance TCXO could guarantee the RTCXO, correspondingly, the position error introduced by the method could easily be calculated and the position error would far less than 1 mm under normal work condition for most MMS applications
V.
ADVANTAGE AND DISADVANTAGE
The new solution embraces two advantages: 1) The biggest advantage of this solution is that it breaks the restricts of hardware and trigger far more sensors than traditional event
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marker solution, which increases the system integrity, decreases the project budget greatly and enables the system a flexible expansibility. The system applying the approach could trigger at most 18 different sensors if a 10MHz TXCO is embedded in synchronizer. 2) It makes the data integrity checking easier. Firstly, the pulsewidth information could be calculated only if when both leading edge and falling edge of the trigger’s pulse are available. In this solution, the leading edge is generated by MARKTIME log while the falling edge is generated by MARK2TIME log. Any mal-recordings of these geo-reference data would cause the failure of pulsewidth information extraction of that frame. Secondly, the pulsewidth information would be decoded into trigger map, the failure of decoding the reconstructed pulsewidth coding signal illustrates that the frame contains the date integrity problem. Such data integrity problem always occurs during the urban field surveying because of signal blockage, electromagnetic interference and traffic jamming, especially for vehicle-borne mobile mapping system. On the other hand, there are some disadvantages for the solution. Since the approach applies two event mark ports to fulfill the pulsewidth extraction procedure that means the georeference system should equip at least two external trigger ports, which restricts some low-end applications and impairs the solution’s usability. Secondly, a post-processing program should be designed. The program decodes the pulsewidth information into trigger table and checks the integrity of synchronization data in case the geo-reference system omits to record the time stamp for unexpected interference. Thirdly, extra hardware should be designed which increases the system’s complexity and budget.
2009 Urban Remote Sensing Joint Event VI.
CONCLUSIONS
The MMS method challenges traditional survey methods for road-side data collection by offering an unbeatable productivity and presumably comparable accuracy. A flexible system configuration would cope with different customer demands practically. The approach presented in paper is a good candidate for such applications without changing any hardware configuration, especially for those solutions synchronized by event marker method. The approach would increase the availability of MMS dramatically.
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