Data synchronization and time-keeping Thorbjrn Jrger Lehrstuhl fr - - PowerPoint PPT Presentation

Data synchronization and time-keeping

Thorbjörn Jörger Lehrstuhl für Systemtheorie Institut für Mikrosystemtechnik - IMTEK Albert-Ludwigs-Universität Freiburg

• 2 -

Groundstation

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 3 -

Flight Controller

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 4 -

Clock synchronization

• Reference pulse which occurs simultaneous in all systems
• 1PPS signal of GPS is the natural choice for outdoor experiments

no information exchange between systems necessary Absolute and jump-free GPS time reference is also available Small jitter (15-50 ns) with cheap equipment, < 1 ns with better equipment Indoor Environment usually has to low signal strength

ARM FC GS

tN tN tN is a synchronous timing pulse tN

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 5 -

Time keeping

• Each measurement gets a timestamp in a unified time frame
• Need for synchronous clocks

Flight Controller:

• Self designed timing framework
• 1PPS pulse for synchronization
• Intervals between pulses measured with precise quartz oscillator
• Absolute deviation below 10 µs
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 6 -

Time keeping

• Each measurement gets a timestamp in a unified time frame
• Need for synchronous clocks

Groundstation:

• Synchronized with UTC via GPS
• Less than 1 µs deviation from true time
• Essentially a Stratum 1 time server
• Automatic synchronization on startup < 2 min
• GS references ticks and PPS pulses to UTC
• Keeps track of all quartz oscillators in the system
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 7 -

Time keeping

GPS devices – Furuno GT-87 / GF-8701:

• Lock on GPS, GLONASS, Galileo, QZSS, SBAS
• Precise PPS generation < 15 ns
• For PPS generation, only one satellite necessary
• Sensitive enough for indoor operation (-161 dBm)
• After startup and lock-on, separate devices generate PPS pulses with

less than 100 ns difference

• Deviation gets smaller over time, as clock disciplination progresses
• When lock is lost, drifts with 1µs/100s (hold-over)
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 8 -

Result: Nicely timed measurements

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

380000 381000 382000 383000

• 2,00E+009
• 1,50E+009
• 1,00E+009
• 5,00E+008

0,00E+000 5,00E+008 1,00E+009

Number of ticks Package number

8 10 12 14 16 18 20 22

PPS counter

• 9 -

Synchronous control system

• Measurements taken at a fixed frequency
• All clocks in the system need to be synchronous

Synchronization method:

• Measurements need to be triggered externally
• Not all sensors support this
• Difficult to implement and maintain
• Manual calibration

Change one thing Repeat previous step

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 10 -

Thor likes his hammer, so let’s look for an easy and “swift”, but effective solution

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 11 -

Methods of “Forced Synchronization”

Resampling method: Just ignore the problem

• Simply take the latest available value
• Easiness:
• Brutality:
• Effectiveness: ? (Should work well, if the signal is slowly changing)
• Introduces discretization noise and potential systematic errors

Resampling method: Ignore the problem a little less

• Same as above, but increase the sampling rate
• Easiness:
• Brutality:
• Effectiveness: ? (Linearly reduces the time since last measurement)
• That is what we do/did but lets try a more subtle approach
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 12 -

Methods of “Forced Synchronization”

Resampling method: Linear approximation

• Take the last two values and approximate the current value
• Easiness:
• Effectiveness: ? (Should work well, if the signal is slowly changing)
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 13 -

Methods of “Forced Synchronization”

Resampling method: Moving average aka. FIR filter

• The N last samples are averaged with a windowing function
• Higher sampling rates are beneficial
• Necessary for fulfilling Nyquist criterion and to prevent aliasing
• Reduces Gaussian noise
• Introduces delay
• Discontinuity of rectangular windowing function introduces side-

lobes to the signal spectrum

• Rectangular window not optimal
• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 14 -

Methods of “Forced Synchronization”

Resampling method: FIR filter with Hamming window

• Much better suppression of side-lobes
• Higher computational cost
• Possible overkill

Discussion

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 15 -

Delay overview

tAS, tAoA, tLAS, tIMU small, constant tSPI, tUART significant, jitter through interrupts tRS485 biggest contribution

ARM FC GS IMUs Airspeed AoA IMU GPS IMU

SPI I²C I²C SPI UART

GPS

UART

TAS

SPI

RS485 (Cable) RS485 (Cable)

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 16 -

Sensor fusion

• It is desirable to have all sensors fused at one point
• Time keeping and synchronisation much easier
• Easier to maintain
• Fast and low jitter connection to controller
• More time for NMPC calculations in each time step

FC Airspeed AoA IMU GPS

UART SPI I²C I²C

LAS

SPI UART Ethernet (Cable)

BB

SPI

GS

SSH

IMU

SPI

Past:

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger

• 17 -

Sensor fusion

• It is desirable to have all sensors fused at one point
• Time keeping and synchronisation much easier
• Easier to maintain
• Fast and low jitter connection to controller
• More time for NMPC calculations in each time step

FC Airspeed AoA IMU GPS

UART SPI I²C I²C RS485 (Tether)

GS

SSH

LAS

SPI

IMUs

Present:

• 7. September 2016

Group Retreat 2016 – Botanischer Garten – Thorbjörn Jörger