Single Transmit Message Buffer Data

The host can configure some message buffers of the CC as single transmit message buffers. If there is at least one transmit message buffer configured, or the sync frame register value is not 0x0, the CC can actively transmit frames out of the connected FlexRay network.

To configure a single transmit message buffer, the following next steps must be performed.

4. Configure the message buffer as a transmit message buffer, in accordance with the configuration procedure and principles.

5. Set the message buffer type bit (BT) to 0 — single transmit message buffer.

After a buffer is configured as a single transmit message buffer and a CC enters the normal mode of operation, The host can start preparation and commitment of frames for transmission. Figure 3.6 shows an example of host and CC operation on a single transmit message buffer during frame transmission in normal operation.

As shown in Figure 3.6, The host sends a lock request for a message buffer and always checks for the lock request acknowledge bit LOCK to be a ‘1’ before it starts to update the message buffer content.

After a transmit message buffer is locked, the host can update it, via the active transmit message buffer, and can commit it to transmission by setting the BUFCMT bit to ‘1’ and unlocking the message buffer.

After transmission, the CC sets the IFLG bit of a message buffer, and

clears BUFCMT. The CC changes the VALID bit after transmission depending on the TT bit.

Above-mentioned method is realized to FlexRay protocol. The next chapter will use the method to reconstruct the sin wave which produces by function generator. This experiment proves that the transmission is correct.

Further, the steer-by-wire system will be designed via FlexRay .

Chapter 4

Experiments and application

In this chapter, I implement the transmission depend on the methods which been taken before. Then, I apply this transmission in steer-by-wire system.

4.1. Transmission Rate Calculation

Now I will test our device and its performance. First, I need a function generator to produce the sin wave, then use A/D converter of the FlexNode_0 to sample the sin wave to produce the data.

Subsequently transmit data by the FlexNode_0 and receive data by the FlexNode_1. Finally use PWM output to reconstruct the sin wave. As shown in Figure 4.1. If the sin wave reconstruct completely, it represent that the data which transmit by FlexRay doesn’t lost.

The communications cycle is the fundamental element of the media access scheme within FlexRay. The static segment and NIT (network idle time) of the communications cycle is used for the experiment. The messages transferred in the static segment must be configured before starting communications, and the maximum amount of data transferred cannot exceed the length of the static segment. These configurations are set by FlexConfig tool. The FlexConfig tool is an instrument for configuring communication controllers for the time-triggered FlexRay protocol. The FlexConfig tool is designed, intended and authorized for

laboratory applications. All other use needs the prior written permission of the TZM.

In the experiment, I configure the communication cycle length, the max static payload length, the number of static slots and other parameters (For example: End of dynamic segment, End of symbol window, Start of offset correction MT...) as shown in Figure 4.2. If I transmit more frame in a fixed communication cycle length, it represent the transmission rate is faster. For example, the communication cycle length is 2 ms, and this moment if I transmit 10 frames, then the transmission rate is 1,280,000 bit/s (10 * 32 * 500 * 8). And so forth. But I only transmit 3 frames when the communication cycle length is 2 ms in this experiment. The results show in Figure 4.3. In the next section, the reason for this issue will be discussed.

4.2. Transmission Rate of The Reasons for Not Upgrading

The FlexRay specification allows a payload up to 254 bytes. But the MFR4200 I use has a limitation of 32 bytes payload. This causes under the same frame number, the rate differs about 8 times. Additional, there is also a reason. If the host has locked a receive message buffer, and two semantically valid frames for that buffer are received during this locked time. The first frame will be lost. This is because the buffer is locked for more than one communication cycle. So we know that the limitation of the FlexNode.

4.3. Steer-by-Wire System

Steer-by-wire system is a related-safety new system comparing to the traditional mechanical, hydraulic, or electric steering systems that are currently used for automotive vehicles. It provides the potential benefits of enhancing vehicle performance, improving handling behavior, and fully integrating vehicle dynamic control.

In a steer-by-wire system, there is no mechanical coupling between the steering wheel and the steering mechanism, i.e., the vehicle’s steering wheel is disengaged from the steering mechanism during normal operation.

Even though the mechanical linkage between the steering wheel and the road wheels has been eliminated, a steer-by-wire system is expected not only to implement the same functions as a conventional mechanically linked steering system, but it is also expected to provide the advanced steering functions.

4.3.1 Steering Function Requirement

There are several main steering function requirements for a steer-by-wire system:

(1) Directional control and wheel synchronization.

Directional control is the basic requirement for vehicle steering systems, including steer-by-wire system. It is required the wheels follow the driver’s input command from the steering wheel and the possible input command from the supervisory vehicle control system according to vehicle dynamics requirements. The road wheels should maintain synchronization

with the steering wheels in real time without bias, offset, or time delay.

(2) Adjustable variable steering feel.

The steering feel provides information on the force (or torque) at the road wheel tire-road surface contact and varies depending on road conditions. This force/torque information should be fed backed to the steering wheel to produce steering wheel torque that can be felt by the vehicle driver. The vehicle driver relies on the steering feel to sense the force of road wheel tire-road surface contact and maintain control of the vehicle. Thus, steering feel has been becoming one of most important vehicle attributes to maintain vehicle directional control and stability. In a steer-by-wire system, it is required to generate not only a familiar steering feel to the vehicle driver just as in the conventional steering wheel systems with mechanical connection, but also adjustable variable artificial steering feels.

(3) Adjustable steering wheel return capability.

The steering wheel should return automatically to the wheel center or a predefined angle if the hands of vehicle driver leave the steering wheel.

The return rates of the steering wheel can be adjusted based on the vehicle speed.

(4) Variable steering ratio.

The steering ratio is a ratio between steering wheel angle and road wheel angle. It is typically fixed around 16 to one in conventional steering wheel systems. A variable ratio permits a significant improvement in handling performance and vehicle dynamics. It can be a function of vehicle speed, steering wheel angle, and other variables.

Because just want to verify whether the FlexRay can be used in the steer-by-wire system.

4.3.2 Steer-by-Wire System Architecture

There are several other different architectures for steer-by-wire systems, such as a single front road wheel actuator with mechanical backup, two independent front road wheels without mechanic connections, and the possible inclusion of rear steering with two independent rear road wheels, In addition, the redundant motor actuators may be used.

Figure 4.4 shows a schematic diagram of a vehicle steer-by-wire system. The steer-by-wire system includes a steering wheel mechanism (controlled plant) and a road wheel mechanism (controlled plant). Two FlexNode (FlexRay communication controller) control the steering wheel mechanism and the road wheel mechanism in coordinated fashion. The steer-by-wire controller effectively links the steering angle sensor and road wheel mechanisms by wire through control signals.

As shown in Figure 4.4, the conventional hydraulic steering assembly has been replaced by an electric motor actuator to drive the road wheels in the road wheel mechanism. Road wheels are connected to a rack and pinion mechanism by tie rods. FlexNode_1 receives road wheel angle signals and produces a control signal to control the AC servo motor. The primary goal for controlling the road wheel mechanism is to keep the road wheel tracking for the reference road wheel angle. The reference road wheel angle signal comes from the steering wheel assembly and changes according to the vehicle driver’s intent and the vehicle dynamics requirements.

Experiment

In order to understand the properties of development and communication fault tolerance of the FlexRay node.

In this experiment, A steering angle sensor is used to sense the steering wheel angles and connected with FlexNode_0 through the CAN protocol. Then the FlexNode_0 signals transmitted to FlexNode_1 through redundant channel A and B. FlexNode_1 put the steering wheel angle signal to control the motor. Figure 4.5 shows the block diagram of steer-by-wire system. The data which transmit by FlexNode are two words. The two words represent steering cycle and steering angle individually .The communication schedule shows in Table 4.1. The hardware of steer-by-wire system shows in Figure 4.6. So we can analyze the characteristic of FlexRay through the steer-by-wire system.

Chapter 5 Conclusion

Because new electronic application quantity and complex increase in automobile, at present the mainstream automobile communications system CAN (Controller Area Network) gradually will be unable to meet the need. Consequently, the high reliability, fault- tolerant and high bandwidth of new network FlexRay is able to achieve complex applications. For example: X-by-wire...etc.

Following summed up several advantages and disadvantages of the FlexRay:

„ Advantages

„ Deterministic behavior

‹ Time driven access

‹ Real-time capability

„ Redundant transmission is possible

„ Transmission rate of 10 Mbit/s per channel

„ Flexible configurable

„ Topology structure widely open

„ Bus Guardian, “intelligent” and interference-resistant bus drivers

„ Disadvantages

„ Configuration of the network

‹ Global configuration must be identical in all participants

‹ Adding a participant possible requires a change in the

network schedule for all participants

„ When designing a network, the access times of all participants must be known beforehand

In this paper, FlexRay protocol was be introduced, including message frame, topology, transmission mechanism...etc. Then, Use the TZM Company’s FlexRay developed tool to design a FlexRay communication application – steer-by-wire system. It provide the designer to understand quickly the content of FlexRay protocol and conversant in FlexRay design process and FlexRay developed tool. So that designers can focus on the development of applications

REFERENCES

[1] FlexRay_Protocol_Specification_V2.1_Rev.A., 2005.

[2] Freescale Semiconductor, MFR4200 Data Sheet, Rev. 0, 2005

[3] Yixin Yao, “Vehicle Steer-by-Wire System Control,” SAE 2006-01-1175.

[4] Douglas Cesiel, Michael C. Gaunt, and Brian Daugherty,

“Development of a Steer-by-Wire System for the GM Sequel.” SAE 2006-01-1173

[5] Rainer Makowitz and Christopher Temple, “FlexRay-A Communication Network for Automotive Control Systems.” IEEE 2006.

[6] S Shaheen, D Heffernan and G Leen, “A Comparison of Emerging Time-triggered Protocol for Automotive X-by-Wire Control Networks.”

IMech E 2003.

[7] 陳予柔,林明志,巫志倫，”FlexRay 通訊協定與設計”，車輛研究測 試中心, 2007-11-16

Figure

(a)Passive bus

(b)Active star

(C)Active star combined with a passive bus

Figure 2.2 Architecture of FlexRay nodes

Figure 2.3 Communications Cycle

Figure 2.4 FlexRay Frame Format

Figure 2.5 Communication controller - bus driver interface

Figure 3.1 Mapping between buffer layout and active receive/transmit/FIFO message buffers

Figure 3.2 FlexNode target board

Figure 3.3 FlexNode block diagram

Figure 3.5 Operations during a frame reception

Figure 3.6 Operations with a single transmit message buffer during an event type of transmission for a static segment

Figure 4.1 Architecture of transmission test

Figure 4.2 Parameter setting

FlexRay

(a) 10 Hz

(b) 100Hz

(c) 1000Hz

Figure 4.3 The transfer rate is 384,000 bps Input(yellow), output(green)

(a) 10Hz (b) 100Hz (C) 1000Hz

Figure 4.4 A Schematic Diagram of the Steer-by-Wire System

(a) a road wheel mechanism

(b) AC servo motor: Fuji Electric motor GYS401DC2-T2

(C) Driver: Fuji Electric FA RYC401D3 – VVT2

(D) Steer

(E) steer angle sensor

Figure 4.6 The hardware of steer-by-wire system

(a) a road wheel mechanism (b) AC servo motor (c) Driver (d) Steer (E) steer angle sensor

Table

Table 4.1 The communication schedule