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Experimental Jitter Analysis in a FlexCAN based DbW Automotive Application. Juan R. Pimentel Kettering University and Jason Paskvan Mentor Graphics Corporation. Presentation Outline. Introduction Characterization of Jitter in CAN Summary of FlexCAN How FlexCAN reduces Jitter

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experimental jitter analysis in a flexcan based dbw automotive application

Experimental Jitter Analysis in a FlexCAN based DbW Automotive Application

Juan R. Pimentel

Kettering University

and

Jason Paskvan

Mentor Graphics Corporation

presentation outline
Presentation Outline
  • Introduction
  • Characterization of Jitter in CAN
  • Summary of FlexCAN
  • How FlexCAN reduces Jitter
  • FlexCAN based Drive by Wire Application
  • Experiments to measure Jitter
  • Results
  • Conclusions
introduction
Introduction
  • CAN is a mature protocol for many small areaapplications due to its:
    • error control features
    • low latency
    • priority-based bus access
    • instant bit monitoring
  • CAN limitations:
    • Speed up to 1 Mbps
    • Limited distance (related to speed)
    • Limited dependability
  • There is an ongoing debate of whether CAN,with proper enhancements, can support safety-critical applications
introduction5
Introduction
  • Although highly advantageous, the priority-based bus access has the negative side effect of causing substantial network delay jitter
  • A large jitter can have a detrimental impact on the performance of many distributed embedded systems
  • There has been several proposals to make CAN more deterministic and dependable
  • One of such proposals is FlexCAN that combines features of:
    • CAN
    • FlexRay
slide6

CAN: Features and Limitations

Great Features:

  • Global, priority-based bus access (bit-wise arbitration)
  • Instant bit monitoring
  • Instant acknowledgement
  • Bwxdelay < 1 bit time
  • Excellent error control features

Limitations:

  • Speed (1 Mbps)
  • Distance (40 m)
  • No unidirectional communications
  • Limited error confinement
  • Large and variable jitter
  • Limited fault-tolerant and safety-critical features
message latency jitter in can
Message Latency Jitter in CAN
  • Three sources of jitter:
    • due to bit stuffing
    • due to jitter in scheduled tasks
    • due to the dynamic mixture of TT and ET traffic
  • Jitter involving jitter in scheduled tasks is due to variations in the time to actually execute software tasks in a node
  • It is assumed that software tasks are responsible for sending CAN messages
  • The third type of jitter results from periodic messages waiting for higher priority event traffic that arrive at arbitrary and unpredictable times
flexcan main features
Architecture:

Node replication (1, 2, 3, …)

Channel replication (1, 2, 3, …)

Synchronization:

CST (TT from application)

node replication

channel replication

Replication management:

Protocol: SafeCAN

Replacement for primary node is always ready thanks on an ranking protocol based on hardware addresses.

Support for Composability in time domain

Communication cycle

Reference message

Timer based

Enforcement of fail-silent behavior

Transient failures

Similar to FTT-CAN

Permanent failures: SafeCAN, Bus guardian

FlexCAN: Main Features
flexcan architecture
Node replication (1, 2, 3, …)

Channelreplication (1, 2, 3, …)

FlexCAN: Architecture

Controller

FTU

Safeware

Safeware

Sensor

Safeware

Sensor

Actuator

Safety

Network

Standard

Standard

Safety

Layer

Manager

Application

Safety

Application

Layer

2

2

2

Layer

2

2

2

2

2

2

2

2

2

2

2

1

1

1

1

1

1

1

1

1

1

1

1

1

1

Replicated CAN channels

flexcan composability
Communication Cycle (Defines Cycle Time)

Reference message(one per cycle time)

Timer based (resolution of at least 0.1 ms)

Integral number of sub-cycles per comm. cycle

In fig. below: there are four sub-cycles

Messages are scheduled into sub-cycles

Messages from different sub-cycles do not interferewith one another (principle of independence, enforcedby removing messages from transmit buffer at the endof the sub-cycle)

FlexCAN: Composability

Cycle Time

Angle, speed

Angle, speed, status

Angle, speed

Gateway

commands

and force fdk

references

M4, m5, m6

M1, m2

M3

M9

m7, m8

HW_Position

HW_Position

HW_Position

HW_Position

HW_Position

HW_Position

T

T

T

T

1

4

2

3

flexcan highly deterministic
FlexCAN: Highly Deterministic

Sampling Period Ts

Sn

Sn

WSn

Sensing

Un

WUn

WAn

Computation

An

Actuation

CSn

CUn

Bus

E

1

E

8

E

1

E

2

E

8

E

2

E

3

E

4

E

5

E

6

Communication cycle

Steering speed

,

Angle

,

speed

Angle

,

speed

Traction speed

status and force

references

,

commands

and status

fdk

Gateway

m

1

,

m

2

m

6

,

m

7

,

m

8

m

4

,

m

5

m

3

,

m

9

HW

_

Position

HW

_

Position

HW

_

Position

HW

_

Position

HW

_

Position

R

R

R

R

A

B

C

D

Network

HW

S

1

T

1

C

(

P

)

Nodes

P

S

2

T

2

C

(

S

)

FR

Sub cycle

flexcan summary
FlexCAN is an architecture that supports safety critical systems

FlexCAN and its underlying protocol (SafeCAN) has the following features:

FlexCAN Summary
  • Flexible
  • Simple
  • Deterministic
  • Cost effective
  • Dependable
  • Modular
  • Scaleable but bounded
  • Based on COTS CAN chips and tranceivers
  • Compatible with native CAN message IDs
experimental jitter measurements drive by wire dbw system
Experimental Jitter MeasurementsDrive by Wire (DbW) System
  • Drive-by-Wire (DbW) systems are electro-mechanicalsystems.
  • Expected to replace the mechanical/hydraulic means transmitting and actuating driving commands
  • DbW systems can enhance the safety of the vehicle occupants only if
    • Dependability issues are addressed
  • Main issues:
    • Assessment of suitable control and communication architectures
    • Validation of their dependability
  • safety-critical functionql units (sub-systems):
    • Steering
    • Acceleration
    • Braking
padova lift truck
Padova Lift Truck
  • Manufacturer: Cesab S.p.A.
  • Source: 48 Volt Battery pack
  • Hydraulics:
    • Steering, hoisting, braking
  • Traction: two front electric drives (IM)
  • Steering mechanism engage rear wheels.
  • Safety requirements:
    • fault-operational
    • fault-safe
dbw control architecture
DbW: Control Architecture

Force Feedback Reference

Steering Reference

Steering Command

Control

Hand Wheel

Steering

ECU

ECU

ECU

Steering Angle

Steering Status

(Command

Conditioning,

Speed Reference

Vehicle

Accelelator

Speed Command

Traction

management

Pedal

under faults)

ECU

Vehicle Speed

ECU

Drive Status

From Dashboard ECU

dbw control ecu functions
Command Conditioning

Increase stability of system

Assist driver in maneuvers

Speed is reduced to avoid overturning the vehicle if:

a tight swerve is commanded

load is up-lifted

Adaptation of steering ratio to truck speed to:

ease maneuvers at low speed

avoid quick changes of trajectories at high speed

DbW: Control ECU Functions

Vehicle Management Under Faults

  • Upon fault detection: All I/O ECU’s stop sending messages
  • This helps I/O units to be ready to receive appropriate commands from the Central ECU
  • Central ECU prepares commands to put the system in a safe state according to the fault.
dbw network specifications
Specification parameters:

communication reliability

network load

application load

data update rate

Reliability requirement:

A DBW operates properly if:

messages reach destination without error

within a bounded time interval

DbW Network Specifications
  • A wrong command could be executed with potentially dangerous consequences if:
    • message is missing or late
    • data is corrupted
    • transmission channel breaks
  • A missing message is handled by the Central ECU
  • Corrupted data is not recognized by the Central ECU and handled by the protocol via CRCs.
  • Two channels are needed
dbw message specifications
speed command

speed reference

actual speed and status (current, temperature)of the traction drives

steering angle command

steering angle reference

actual steering angle and status (curr, temp)of the steering drives

force feedback reference

An additional message is used to convey the datacoming from the CAN network through the gateway

DbW Message Specifications
dbw message definitions
Msg Size (bits) ECU Functional Description

M132 Hand wheel (HW) Steering angle command

M2 32 Pedal (P) Acceleration command

M3 64 Central (C) Acceleration Reference (32 bits)

Steering angle Ref. (32 bits)

M4 56 Traction 1 (T1) Speed and status

M5 56 Traction 2 (T2) Speed and status

M6 56 Steering 1 (S1) Speed and status

M7 56 Steering 2 (S2) Speed and status

M8 32 Force reaction (FR) Force feedback

M9 64 Central (C) Gateway message

DbW Message Definitions
dbw network layout
DbW Network Layout

Hand

Acceleration

Control

Wheel

Pedal

C

C

HW

P

CAN bus 1

CAN bus 2

S1

S2

FR

T1

T2

Steering 1

Force

Steering 2

Traction 1

Traction 2

Reaction

flexcan global mesg schedule
FlexCAN Global Mesg. Schedule

Basic Cycle

Angle, speed

Angle, speed

Steering speed,

Traction speed

references,

commands

status and force fdk

and status

Gateway

m1, m2

m6, m7, m8

m4, m5

m3,m9

HW_Position

HW_Position

HW_Position

HW_Position

HW_Position

R1

R4

R2

R3

Bus

Guardians

Network

C(S)

HW

S1

T1

C(P)

Nodes

P

S2

T2

FR

experiments
Experiments
  • EXPERIMENT 1: Only periodic traffic
  • EXPERIMENT 2: Mixed traffic
  • Size of event traffic: 8 Bytes
  • Priority of event traffic: Lower than any periodic message
  • Event traffic : Uniform distribution [2,11] ms Inter-arrival time
  • EXPERIMENT 3: Mixed traffic
  • Same as that of experiment 2 except:
  • Priority of event traffic: Higher than any periodic message
conclusions
Conclusions
  • Sources of jitter in CAN:
    • bit stuffing
    • task schedulers
    • interference from other messages
  • Simple FlexCAN message scheduling helps reduce jitter and make CAN more predictable
  • Message schedule of a safety-critical DbW application has been implemented and experiments were conducted to measure jitter
  • Jitter can be controlled in a system based on the FlexCAN architecture