A Cost Effective Synchronization System for Multisensor Integration

Transcription

1 A Cost Effective Synchronization System for Multisensor Integration Binghao Li School of Surveying and Spatial Information Systems The University of New South Wales BIOGRAPHY Binghao Li is currently a Ph.D. student in the School of Surveying and Spatial Information System (formerly the School of Geomatic Engineering), The University of New South Wales, Australia. He received a B.Sc. in Electrical & Mechanical Engineering, Northern Jiaotong University, and M.Sc. in Civil Engineering, Tsinghua University, P.R. China, in 1994 and His research interests are in integrated navigation applications and mobilephone positioning. ABSTRACT Time synchronization is the primary prerequisite for multisensor integration. Many methods have been used in different systems, but they are either too expensive or inconvenient for research purposes. In this paper the system design of a Cost Effective Synchronization System (CESS) is described, and two experiments are described that have tested the synchronization accuracy of the system. They show that a synchronization accuracy of better than 0.4 ms can be achieved using the CESS. This system provides an ideal platform for researchers to build their own multisensor integration system. INTRODUCTION In the past decade GPS has established itself as the most popular technology for positioning. With a single GPS receiver, positioning accuracies of better than 10 meters can be readily achieved. However, GPS signals are easily obstructed by buildings and trees, resulting in unreliable positioning in certain environments such as downtown city areas. One way to augment GPS is to integrate it with other systems, such as an inertial sensors, or even pseudolites. For a multisensor system, time synchronization of measurements is an important task. In principle there are 3 different methods to achieve integrated-sensor time synchronization, namely (a) hardware, (b) software, and (c) a combination of the two. In a hardware method, the one pulse-per-second signal () output from a GPS receiver can be used as input to other sensors in the multisensor system (Knight, 1992). For example, the C-MIGITS inertial sensor can be synchronized with the MicroTracker GPS receiver by using both the signal and the GPS position solution data messages (Boeing, 1997). In this system the inertial measuring unit sensor is relatively expensive (costing tens of thousands of US dollars) and the synchronization process cannot be modified by the user. Therefore the system is inflexible for researchers, as it prevents modification of the system design. The second method (based on software) can be classified into two categories, the augmented Kalman filter method (Bar-Itzhack & Vitek, 1985) and the controlled trajectory method (Lee et al., 2002). In the augmented Kalman filter method, GPS/SDINS (StrapDown Inertial Navigation System) Kalman filters are augmented with a state representing the time synchronization error between GPS and SDINS. In the controlled trajectory method, vehicles follow an S- shaped trajectory and the time synchronization error

2 effects are estimated externally to the GPS/SDINS Kalman filter. Since the identification and calibration of the time synchronization error are implicitly related with state observability (Lee et al., 2002; Goshen-Meskin & Bar-Itzhack, 1992), the time calibration performance of both software methods depends strongly on the vehicle trajectory. The third and final method is to utilize time tagging by both the hardware and software. The signal from the GPS receiver can be used as the accurate time reference. This method is more suited for researchers as it allows for the development of a low-cost, flexible, and reliable system. A flexible synchronized data acquisition system can be created using a multifunction DAQ (data acquisition) card, of which there are many suitable low-cost products available. In this paper a Cost Effective Synchronization System (CESS) is proposed, based on the third method. Figure 1 shows the CESS developed by the author. Figure 1 - Setup of the CESS HOW CESS WORKS The CESS consists of 3 primary components: GPS receiver, DAQ card and support software. GPS receiver In the proposed CESS, the GPS receiver s signal is used as a trigger. In order to achieve synchronization the signal must have a fixed relationship with the output message from the GPS receiver. Also crucial is a high frequency clock signal, which can be used as an accurate time base to generate a scan signal that controls the start of each sampling epoch. Most GPS receivers can provide a signal with an accuracy of better than 1ms, and some specific for timing applications can provide a more accurate signal (at the µs level). However, not many GPS receivers can output a high frequency clock signal. The Rockwell Jupiter receiver is one that can (Conexant Inc., 2001). The Jupiter is a 12 channel low-cost GPS receiver that can output and 10kHz clock signals, and is a member of the Rockwell Zodiac family of receivers. The signal is aligned with the UTC (Universal Coordinated Time) second. This signal is derived from the 10 khz clock output, which is valid under the same conditions as the Time Mark Pulse. Additionally, the Jupiter receiver also outputs the UTC message - UTC Time Mark Pulse Output - via a serial port. This message provides the UTC seconds of week associated with the UTC synchronized pulse. This message is output approximately 400 milliseconds before the signal (Rockwell, 1996). DAQ card and support software A multifunction DAQ card consists of analogue inputs, counter/timers, digital I/O lines, analogue outputs, etc. In the CESS, the DAQ card is triggered by the and driven by the high frequency clock signal. The counter/timers are central to the synchronization process. The A/D (analog to digital) converter transforms the analogue signal to a digital signal. If the user has control of the sensor at the hardware level, then the counter/timer can generate a trigger signal to start a sampling of the sensor, and in this way very accurate synchronization can be achieved (Knight, 1992). Unfortunately most sensors do not allow low-level modification by the user, and therefore the DAQ card is instead utilized to stamp the time tag of the data from the sensor. A Low-Cost E Series Multifunction DAQ card from NI (National Instruments) is used in the system design of the CESS. This is a PCMCIA DAQ card, and when used with a laptop computer is ideal for fieldwork. Its features are summarized below (National Instruments, 2000): 16 single-ended (8 in differential) analog inputs at 200 ks/s, 12-bit resolution. Two 12-bit analog outputs. 8 digital I/O lines (5 V/TTL/CMOS). Two 24-bit counter/timers. Digital triggering. There is a variety of support software that can be used with the DAQ card, such as Visual C++, Matlab,and LabVIEW. Since LabVIEW is a product of NI, and provides good support for the DAQ card, LabVIEW was chosen for the software component of the CESS. CESS Configuration The conceptual structure of CESS is shown in figure 2. GPS antenna GPS receiver GPS message Clock output Laptop Support Software multifunction DAQ card Interface box Figure 2 - The structure of the CESS Sensor N Sensor 1 GPS receiver (Jupiter) outputs a precise signal and 10kHz pulse signal, together with GPS data raw measurements. The analogue data output from the sensor(s) is(are) connected to the interface box. The GPS receiver signal is used as a trigger, while the 10kHz signal is used as a very accurate time base to generate a scan signal. When the first signal from the GPS receiver is detected, sampling of the analogue input from

3 the sensor(s) begins at a rate of one hundred times a second (in the case of 100Hz sample rate). The sample rate can be changed from 50Hz to 2500Hz (depending on the application and the specification of the multifunction DAQ card). LabVIEW based software (LBS) logs the sensor data to a file, and also can display the data so that it can be monitored. At the same time GPS data from the receiver is read by LBS, including a message containing UTC time. This is used to time tag the sensor data, by adding the message at the end of the sensor data every second. In the data file, there is a time tag every 100 records (100Hz sample rate) of sensor s data. The time tag appears approximately 40 records before the next signal arrives. Therefore, the relationship between GPS time (or UTC time plus a number of leap seconds) and the sensor(s) is determined with the aid of the signal. Figure 3 illustrates the relationship of the signal, the 10kHz signal, the scan signal and the message 1108 (containing UTC time) from the Jupiter GPS receiver. Clock output (10kHz) Scan signal Message of UTC time (M1108 from Jupiter) 0.6sec 1sec 0.6sec 1sec Figure 3 - The timing of, 10kHz signal, scan signal and message type 1108 Figure 4 shows the timing of the scan signal when the CESS is working at 100Hz. Ideally T L is zero, but the minimum delay the counter can provide is 2 pulses of the time base (10kHz signal), so T L is 0.2ms. T L is a constant value, however T H is variable. T H changes from 19.8ms (50Hz sample rate) to 0.2ms (2500Hz sample rate). In an approximate manner the sampling of the data from the sensor(s) aligns with the signal, but when high accuracy is demanded, T L should be considered. scan signal when the sample rate was 2500Hz, where Ch1 is the and Ch2 is the scan signal. In this oscilloscope setup the is used as the trigger. It is clear that 0.2ms after the arrives, the scan signal appears. The duration of the scan signal is 0.2ms. Figure 5 - The timing of scan signal (sample rate: 2500Hz) Most of the support software runs under the Windows 9x/ 2000 operating system (OS). However, Windows is not well suited to synchronize different sensors directly because it is not a real time OS (Dekey). However, in the CESS, synchronization is achieved through the multifunction DAQ card, and the Windows OS is responsible for interfacing to the multifunction DAQ card and saving data to the hard disk. Since the volume of data logged is large (from 4Kb to 200Kb per second, depending on the sample rate), and not all computers are fast enough to perform the operations required, some optimization must be considered. First, unnecessary tasks should be disabled, including network drivers, anti-virus applications, screen savers etc (Dekey). Secondly, the multi-threaded VIs (virtual instrument) created by LabVIEW improves the performance significantly (Dorst; National Instruments). Initialize analog acquisition and clock Read data from buffer Initialize serial port Read GPS messages from Jupiter T H Get UTC time Add it to sensor data Scan signal T L Log data to sensor file Show data on screen Log data to GPS file T L =0.2ms T H =9.8ms Figure 4 - Scan signal timing All the signals referred to in this paper were monitored using an oscilloscope. For example, Figure 5 shows the Figure 6 - The structure of the CESS software The structure of the software is shown in figure 6. There are two threads. The first deals with data from the sensor(s), while the second thread is responsible for handling the GPS receiver s messages. There is one

4 junction of the two threads, where UTC time is transferred from thread 2 to thread 1. Thread 1 can then stamp the UTC time at the end of the data from the sensor(s). TESTING CESS To evaluate the synchronization accuracy of the CESS two experiments were conducted. The first was to use a reference hardware synchronized INS system (the C- MIGITS), and compare this with the CESS using a Crossbow IMU (Inertial Measurement Unit) as the test sensor. By moving the two units in the same manner, comparisons can be made between the data logged by the hardware synchronized C-MIGITS and the CESS synchronized Crossbow IMU. In the second experiment a standard reference pulses were used to test the synchronization accuracy of the CESS. Experiment one The Crossbow IMU400CC used in this test is a six-axis measurement system designed to measure linear acceleration along three orthogonal axes, and rotation rates about three orthogonal axes (Crossbow Inc.). The accelerometers and gyroscopes are of the MEMS (Micro- Electro-Mechanical Systems) variety. This IMU has both analog outputs and an RS-232 serial link. Data may be Figure 7 - Samples of the raw data and normalized data of Crossbow and C-MIGITS

5 requested via the serial link as a single measurement, or streamed continuously. The analog outputs are fully signal conditioned and may be connected directly to a data acquisition device. These analog outputs can be acquired by the CESS.The Boeing C-MIGITS is a tightly-coupled SPS (Standard Positioning Service) system. It contains a five channel, Coarse/Acquisition (C/A code), L1- frequency GPS receiver, and a Digital Quartz IMU. The two subsystems are integrated together through a Kalman filter algorithm, and synchronization is achieved at the hardware level by the GPS receiver. The system is a small, lightweight, guidance, navigation, and control system, with data output via an RS232 port (Martin & Detterich; Boeing, 1997). The C-MIGITS is used as the perfect reference system in order to evaluate CESS s performance. For the experiment both were mounted very close together on a rigid board. During the experiment, approximately every second a shock was introduced to the board. The Crossbow IMU s data was acquired by CESS, while the C-MIGITS data was logged using a laptop. The sample rate of both systems was 100Hz. The Crossbow IMU provides acceleration outputs in the X-axis, Y-axis and Z-axis, while the C-MIGITS outputs delta velocity in the X-axis, Y-axis and Z-axis. As the data was requested at 100Hz, the data represents 10ms of delta velocity. Figure 7 shows the result from the Crossbow IMU and C-MIGITS. The Figure on the left is raw data and normalized data from the Crossbow IMU (in X-axis, Y-axis and Z-axis respectively), while the right side are delta velocities from C-MIGITS (X-axis, Y-axis and Z-axis respectively). In order to find the difference in time between the data from the Crossbow IMU and the C-MIGITS, data from the Crossbow is fixed and the data of the C-MIGITS is shifted from 0.4sec to 0.4sec. The acceleration and delta velocity is multiplied at every epoch, and the result is accumulated. Correlation analysis can indicate the nature of the difference between these two systems. Figure 8 shows the correlation result; from top to bottom, the 3 figures are the results for the X-axis, Y-axis and Z-axis respectively. It is very clear that the time difference of these two systems is very small (less than 0.01sec). Since the resolution of the C-MIGITS is 0.01sec, in this test, the best result can be no better than 0.01sec. In addition there are many factors that affect the accuracy of the test, including the fact that the mounting board is not shockproof, the stimulation is not in the same position each time and (ideally) short, and the axes of the two devices were not perfectly parallel or overlapped, etc. Experiment two Test one demonstrates the proof-of-concept for the CESS system design, and indicates a synchronization accuracy of better than a few tens of milliseconds. However, this synchronization is not able to satisfy requirement for applications with high vehicle dynamics. For example, a 10 millisecond timing error translates into a shift of 28cm for a vehicle traveling at 100km per hour. In principle the system design of the CESS should ensure synchronization errors of less than 10 milliseconds, and hence a second experiment was conducted to better determine the synchronization accuracy. In this test a signal was generated as a standard reference, and the CESS was used to acquire the signal. If the synchronization accuracy of the CESS is acceptable, then the standard signal can be acquired. Figure 8 - Correlation of the data from the Crossbow IMU and the C-MIGITS First, the quality of the generated standard signal was tested. DOS version software was used to generate a reference signal, which was triggered by a signal from the Jupiter receiver. An interrupt would occur when the signal arrives, and a pulse would be output (Peacock; Lombardi). The reaction time of the computer (a PIII 1GHz) is between 5 to 12 microseconds, and the duty cycle of the signal can be changed by the software. The peak of the pulse was approximately 3.4 volt, and figure 9 shows the reaction time of the personal computer. Ref signal T d T T 25.6 ms T d 5-12 µs Figure 9 - Reaction time of the PC Sub test one The computer generates a standard signal, with a pulse duration of 0.4 millisecond, and a delay of 0.2 ms. This delay was intentionally added since the first scan signal of every second is at 0.2 ms delay with the signal (this is dictated by the quality of the DAQ card s counter). Figure 10 shows the relationship between the signal and the reference signal.

6 T RD Ref signal T Rh T RD = 0.2ms T Rh =0.4ms Figure 10 - Timing of the reference signal in sub test one The reference signal was input to one of eight possible channels of the CESS. Two different sampling rates of 50Hz and 2500Hz were used in the experiment, and the data for each was recorded in separate files. Since the first sample of every second occurs approximately 0.2ms later than the signal, the pulse is expected to be acquired in the first sample. Figure 11 plots part of the data logged at 50 Hz and 2500 Hz sample rates. In order to identify if every reference pulse has been acquired by the first sample after the signal, a Matlab program was used to generate a group of simulated (ideal) data. Then the logged data was normalized (only 0 or 1), and compared with the ideal data. Figure 12 compares the normalized logged data and the ideal data (2500Hz as an example). The figure on the top is a plot of both logged and expected data, and visually shows very good alignment. The figure on the bottom was plotted using the ideal data as X, and the normalized logged data as Y. The simple line from (0,0) to (1,1) with 2 plotted points shows X, Y is exactly the same. This is the case for all the logged data and demonstrates that the reference pulses were acquired. Figure 12 - Logged data compare with the ideal data Sub test two This sub test was similar to the first, except that the standard pulses were shifting every second. The sample rate was 100Hz, with a reference signal width of 0.4ms, and delayed by N*10ms (N=0~99). This means that at the 1st second, the reference signal was expected to be obtained at At the 2nd second, the reference signal was expected to be obtained at 1.01.At the 3rd second, the reference signal was expected to be obtained at At the100th second, the reference signal was expected to be obtained at At the101st second, the reference signal was expected to be obtained at , and so on.the timing of the reference signal is shown in figure 13. This test can also be used to assess the stability of the CESS. T RD0 T RD1 T RD2 Ref signal T Rh T RD0 = 0ms T RD1 = 10ms T RD2 = 20ms T Rh =0.4ms Figure 13 - Timing of the reference signal in sub test two Figure 11 - Pulses obtained by CESS The top half of figure 14 shows the position change of the signal obtained by the CESS every second, and changes from 1 to 100 as expected. Matlab was used to generate

7 ideal data sets as in the first test. The bottom half of figure 14 is similar to figure 11, and shows that the ideal data and acquired data are exactly the same. It means every sampling is exactly the same as expected. These two tests demonstrate that the CESS can acquire the standard pulses. Since the standard reference signal is also triggered by the signal, the time the signal is acquired is very accurate. Test one already showed that the synchronization error is below 1 second. In test 2, CESS can obtain the pulse with a width of 0.4 ms at the expected position, which means that the accuracy of the system must be better than 0.4 ms. hardware level synchronization, and is an area of further research. ACKNOWLEDGMENT The author wishes to express his deep appreciation to his supervisors, Prof. Chris Rizos and Dr. Joel Barnes at UNSW for their support and encouragement of the author s research. Also a big thanks to Dr Hyung Keun Lee of Hankuk Aviation University (Korea) for his helpful suggestions. REFERENCES Bar-Itzhack, I.Y, Vitek, Y., 1985, The Enigma of False Bias Detection in a Strapdown System during Transfer Alignment, Journal of Guidance and Control, 8, Lee, H.K., Lee, J.G., Jee, G.I., 2002, Calibration of Measurement Delay in GPS/SDINS Hybrid Navigation, AIAA Journal of guidance, control, and dynamics, 25, Goshen-Meskin, D., Bar-Itzhack, I.Y., 1992, Observability Analysis of Piece-Wise Constant Systems, Part II: Application to inertial navigation in-flight alignment, IEEE Transactions on Aerospace and Electronic Systems, 28, Knight, D., 1992, Achieving Modularity with Tightly Coupled GPS/INS, Proceedings of the IEEE PLANS 92, Monterey, CA, Peacock, C., Interfacing the Standard Parallel Port. Available at: Figure 14 - Result of sub test two CONCLUDING REMARKS A cost effective synchronization system has been built for multisensor integration. Tests have shown that an accuracy of better than 0.4 millisecond can be achieved. At a vehicle speed of 100km per hour, a 0.4 millisecond timing error translates to a shift of 1.1cm. In general this is sufficiently accurate for many land applications. Actually, some of experiments have utilized CESS to collect data in the downtown city for the research purposes. The system design of the CESS allows users to build their own multisensor system, and is therefore an ideal platform for further research and improvements. One way to improve the accuracy of the CESS is to utilize a high-speed DAQ card and use a higher frequency time base. For example, the Sigtec GPS receiver MG5001 can provide a 100kHz signal, whereas the Jupiter receiver used in these tests provided only a 10kHz signal. Moreover, the CESS can also provide a trigger signal for sensors to start sampling. This is an option for achieving Lombardi, M., Computer Time Synchronization. Available at: Martin, M.K., Detterich, B.C., C-MIGITSII Design and Performance Boeing, C-MIGITS II Integrated GPS/INS User Guide, Dekey, S., Making Windows NT a Real-Time Solution A Technical Overview, National Instruments. Available at: Dorst, N., Using LabVIEW to Create Multithreaded VIs for Maximum Performance and Reliability, National Instruments. Available at: National Instruments, Using LabVIEW to Create Multithreaded DAQ Applications. Available at: National Instruments, DAQ 6023E/6024E/6025E User Manual, 2000.

8 Available at: Conexant Inc., Jupiter GPS Receiver data sheet, Available at: Rockwell, Zodiac GPS Receiver Family Designer s Guide, Crossbow Inc., DMU User s Manual (Revision B), Available at: