How can self-driving car lidar be synchronized with GPS time?

Among the five messages defined by gPTP, Sync and Follow_UP are a group of messages, which are sent periodically and are mainly used to measure clock deviation.

01 Synchronization scheme

There are three main schemes for laser radar and GPS time synchronization, namely PPS GPRMC, PTP, gPTP

PPS GPRMC

GNSS output two One piece of information is a synchronization pulse signal PPS with a time period of 1s and a pulse width of 5ms~100ms; the other is a GPRMC standard time synchronization message output through a standard serial port.

The synchronization pulse leading edge moment is at the same moment as the GPRMC message is sent, the error is ns level, and the error can be ignored. GPRMC is a standard format message containing UTC time (accurate to seconds), longitude and latitude positioning data.

PPS second pulse is a physical level output. The time for receiving and processing the PPS signal is at the ns level and can still be ignored. However, GPRMC data is generally sent through a serial port with a baud rate of 9600. The sending, receiving, and processing time tx is at the ms level, which is the key to time synchronization.

The following is the principle of using PPS GPRMC for time synchronization.

(1) After the device receives the PPS second pulse signal, it clears the milliseconds and below in the internal system time with the crystal oscillator as the clock source, and starts calculating the millisecond time from this.

(2) After receiving the GPRMC data, extract the hour, minute, second, year, month, and day UTC time in the message.

(3) Add the time tx from receiving the second pulse to parsing the UTC time in GPRMC to the UTC whole second time, and synchronize it to the system time. So far, a time synchronization has been completed. Repeat the same process for the next second, calibrating exactly once every second.

Smart people may have suddenly realized that lidar needs to be time synchronized. Just make two wires and connect them to these two physical interfaces. This method is possible and is also a solution used by many manufacturers. , but PPS GPRMC has the following problems.

(1) PPS is a low-power pulse level signal. The driving current is as little as 0.5mA and as much as 20mA, with several synchronization nodes (lidar and other nodes that require time synchronization) , a dozen or so would be very difficult.

(2) PPS is an unshielded single-line pulse signal. More than a dozen PPS lines shuttle in the car, which is extremely susceptible to interference from the harsh electromagnetic environment in the car. It will be impossible to distinguish whether it is an interference pulse or a synchronization pulse. .

(3) GPRMC sends synchronization messages through the RS232 serial port. RS232 is a 1-to-1 full-duplex communication form, and can also achieve 1-to-several data transmission in a master-slave form. But for more than ten people, it is really rare, and it can only be verified through experiments whether it is feasible. But at least the wiring harness engineer was unwilling to agree.

(4) When the clock source is lost, all the equipment that needs time synchronization suddenly loses its backbone. Each junior can stand on his own, and there is no second leader to stand up in time and take charge of the overall situation. This is simply unacceptable for autonomous driving systems with extremely high functional safety requirements.

PTP

Therefore, it is theoretically feasible to achieve time synchronization of the entire autonomous driving system based on simple PPS and GPRMC, but it is not practical.

The network-based high-precision time synchronization protocol PTP (Precision Time Protocol, 1588 V2) can achieve sub-microsecond synchronization accuracy. For a global architecture where the backbone network is Ethernet, everything is ready. It only needs the support of the hardware PHY chips of each domain controller.

PTP is a master-slave time synchronization system that uses hardware timestamps, so it can significantly reduce software processing time. At the same time, PTP can run on the L2 layer (MAC layer) and L4 layer (UDP layer). When running on the L2 layer network, packet parsing is performed directly on the MAC layer without going through the four-layer UDP protocol stack, thus greatly reducing the protocol stack residency. time, further improving time synchronization accuracy, which is very friendly to autonomous driving systems.

An architectural solution under the global architecture is as shown below.

The network port running the PTP protocol in the device is called the PTP port. The PTP master port is used to publish the time, and the PTP slave port is used to receive the time. Three types of clock nodes are defined at the same time, boundary clock node (BC, Boundary Clock), ordinary clock node (OC, Ordinary Clock) and transparent clock node (TC, Transparent clock).

(1) The boundary clock node has multiple PTP ports, one of which is used to synchronize the time of the upstream device, and the remaining ports are used to send time to the downstream device. When the upstream time synchronization device of the boundary clock node is a GNSS receiver, the boundary clock node at this time is a master clock node (optimal clock).

(2) Ordinary clock nodes have only one PTP port, which is used to synchronize the time of the upstream clock node.

(3) Transparent clock, as its name suggests, has multiple PTP ports, what time is received, what time is forwarded, does not perform protocol analysis, and does not participate in time synchronization internally. PTP exchanges synchronization messages between the master and slave devices and records the message sending time to calculate the network transmission delay and the clock deviation between the master and slave devices.

PTP defines four synchronization messages: Sync, Follow_Up, Delay_Req, and Delay_Resp. The precise synchronization process is as follows.

(1) The PTP master port sends a Sync message to the slave port, and the Sync sending time t1 is synchronously recorded. After receiving the Sync message from the port, the reception time t2 is recorded.

(2) Then the master port puts the t1 time in the Follow_Up message and sends it to the slave port. After receiving this message, the slave port can parse out t1 and get the first equation: t1 Network delay clock deviation = t2.

(3) The slave port sends a Delay_Req message to the master port, and simultaneously records the time t3 when the Delay_Req is sent. After the main port receives the message, it records the time t4 when it was received.

(4) Then the master port puts the t4 time in the Delay_Resp message and sends it to the slave port. After receiving this message, the slave port can parse out t4 and get the first equation: t3 Network delay-clock deviation=t4. Two unknowns and two systems of equations can be solved by applying junior high school mathematics knowledge: network delay = [(t2-t1) (t4-t1)]/2, clock deviation = [(t2-t1)-(t4-t3 )]/2.

gPTP

gPTP (generalized Precision Time Protocol), a series of optimizations based on the PTP (IEEE 1588v2) protocol, has formed a more targeted time synchronization mechanism , which can achieve μs-level synchronization accuracy.

gPTP defines two device types, Time-aware-end Station and Time-aware Bridge. Each device has a local clock. The local clock is measured by the oscillation period of the crystal oscillator. The internal hardware counter of the device is responsible for counting the oscillation period. The network port on the device used to publish time synchronization messages is called the master port, and the port used to receive time synchronization messages is called the slave port.

(1) Time-aware-end Station, which can be used as either a master clock or a slave clock.

(2) Time-aware Bridge can be used as either a master clock or a bridge device, similar to a switch. After receiving the gPTP message, the bridge device will give the message a bath before sending it out. The time consumed by the message in the bridge device is called the dwell time. gPTP requires that the bridge device must have the ability to measure dwell time.

The following figure shows a simple gPTP system, including a clock source, 1 master clock, 2 bridge devices, and 4 slave clocks. The master clock is the time base within the system. It generally has a higher-precision local clock and needs to be timed by a high-precision quasi-clock source. The master clock can be dynamically allocated within the system or pre-allocated (for vehicle-mounted fixed topology application scenarios, the principle of pre-allocation is often used).

The master clock dynamic allocation mechanism specified in gPTP is BMCA (Best Master Clock Algorithm, best master clock selection algorithm). After the system is powered on and wakes up, all devices in the system can participate in the master clock election by sending a message, which contains the clock information of their respective devices. Each participating device will compare its own clock information with the clock information of other devices and determine whether it has an advantage. If not, it will withdraw from the election until the martial arts leader with the strongest comprehensive ability is born.

02 Synchronization process

gPTP defines two types of messages, event type messages (including Sync, Pdelay_Req, and Pdelay_Resp) and general type messages (including Follow_UP and Pdelay_Resp_Follow_UP). gPTP defines that the device works in the MAC (Media Acess Control) sublayer of the second data link layer in the seven-layer network model.

When the device MAC layer receives or sends an event type message, it will trigger the sampling of the hardware counter to obtain the clock oscillation cycle count value. Combined with the clock oscillation frequency and the reference time, the timestamp at this time can be obtained . General type messages are only used to carry information and will not trigger the sampling operation of the internal hardware counter.

Clock deviation measurement

Among the five messages defined by gPTP, Sync and Follow_UP are a group of messages, which are sent periodically and are mainly used to measure clock deviation. Sync is sent by the main port. When the message leaves the MAC layer of the main port, the main port is triggered to record the timestamp t1 at this time. After receiving the Sync message from the port MAC layer, the timestamp t2 at this time is recorded. Subsequently, the master port attaches the t1 value to the Follow_UP message and sends it to the slave port.

If there is no network transmission delay or delay, which can be ignored, the slave port will add the local clock value to the clock offset (the value of t1-t2) to complete the time synchronization, that is, There are no further thoughts behind it. However, for gPTP with μs-level time synchronization accuracy, transmission delay obviously cannot be ignored.

Transmission delay measurement

gPTP uses the P2P (Peer to Peer) method to measure transmission delay. In the P2P method, the transmission delay between adjacent devices is measured, and messages are not allowed to be transmitted across devices. This requires that all devices in the gPTP network need to support the gPTP function. At the same time, a set of independent messages is defined specifically for transmission delay measurement, which are Pdelay_Req, Pdelay_Resp and Pdelay_Resp_Follow_UP that are sent periodically.

The slave port first sends a Pdelay_Req message, marking the beginning of the transmission delay measurement. When the message leaves the MAC layer of the slave port, the slave port is triggered to record the timestamp t3 at this time. . After receiving the Pdelay_Req message, the MAC layer of the master port will record the timestamp t4 at this time. Then, the master port sends the value t4 to the slave port through the Pdelay_Resp message. At the same time, when the Pdelay_Resp message leaves the MAC layer of the master port, the master port triggers The port records the timestamp t5 at this time, and records the timestamp t6 at this time after receiving the Pdelay_Resp message from the port MAC layer. Subsequently, in the same routine, the master port sends the value t5 to the slave port through the Pdelay_Resp_Follow_Up message. At this point, a transmission delay measurement process has ended. Assuming that the path transmission delay is symmetrical, the transmission delay between adjacent devices can be calculated according to the following formula.

Frequency synchronization

The above transmission delay measurement is based on the premise that the clock oscillation frequency of the slave port and the master port is consistent. Now let's consider what supernatural events will occur if the oscillation frequencies of the master and slave port clocks are inconsistent. Assuming that the clock oscillation frequency of the slave port is 25MHz, one clock oscillation period is 40ns. The clock oscillation frequency of the master port is 100MHz, and the clock oscillation period of one clock is 10ns.

Assume that during a transmission delay measurement process, the difference between the oscillation periods recorded by the slave port at t6 and t3 is 200 oscillation periods. Since the clock frequency of the master port is 4 times that of the slave port, the slave port receives about 800 oscillation period differences between t5 and t4. If calculated based on the 40ns clock oscillation period of the slave port, the transmission delay is -24μs ([200x40-800x40]/2). Not only is there no delay in transmission, but it is known in advance, and there is no doubt about it from the port.

In addition to the inherent inconsistency in the clock oscillation frequency of the master and slave ports, factors such as temperature and aging can also cause instability in the crystal oscillator frequency. In order to solve the problem of frequency synchronization, gPTP uses frequency synchronization to synchronize the clock oscillation frequency of the slave port to the master port.

Frequency synchronous multiplexing transmits the Pdelay_Resp and Pdelay_Resp_Follow_UP messages of the delay measurement process. By using two sets of responses, the values ​​of t5, t6, t9, and t10 can finally be obtained. The frequency ratio of the master-slave port can be obtained from the following formula.

When the master-slave port frequency is synchronized, the frequency ratio is equal to 1. If it is greater than 1, it means that the main port is running fast; if it is less than 1, it means that the main port is running slowly. The slave port adjusts its time base according to the value of the frequency ratio to obtain the correct timestamp.

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