can based data acquisition - western washington...

Dean Jahns

ETEC 471

CAN Based Data Acquisition 8-Dec-10

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Introduction

This project will convert physical temperature measurements into Controller

Area Network (CAN) messages following the Society of Automotive

Engineers standard, SAE-J1939, “Surface Vehicle Recommended Practice”

format. The J1939 standard is universally used in the heavy-duty trucking

and non-road vehicle industries in North America and Europe for controller to

controller communication. Any J1939 compliant data collection device will be

able to monitor and record the signals – the specific use in this case is the

engine test cell automation system at the PACCAR Technical Center.

Description

The device used for this project is a National Instruments Compact RIO

system consisting of a controller, model NI cRIO-9014, and an 8 port, 3M

gate chassis model NI-9104. To handle the I/O, 7 thermocouple modules,

model NI-9213, and one CAN module, model NI-9853, were installed into

the chassis. Each thermocouple module can accept 16 thermocouples and

the CAN module has 2 low-speed CAN channels.

The CRIO hardware is a real time microcontroller coupled with a Field

Programmable Gate Array (FPGA). The CRIO unit plugs into a chassis with

up to 8 additional slots for I/O modules, allowing for easy configuration and

reconfiguration as necessary. The chassis dimensions are 28 cm x 9 cm x 9

cm, and all modules and the controller fit within this package. See figure 1

for a photo of the hardware.

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Figure 1 – cRIO unit, complete

Hardware Description

The cRIO device consists of 3 main components – the real time controller or

microprocessor, the chassis which contains the FPGA gates, and the I/O

modules which handle the signal conditioning.

The controller uses a 400 MHz Freescale MPC5200 real-time processor, 128

MB of internal DRAM and 2 GB of non volatile memory. The controller itself

has a power consumption of 6W, and the chassis maximum power is 3W.

With a chassis equipped with 8 modules, the recommended power supply is

a 24 VDC 48W, as the unit will draw up to 20W during normal operation

after the inrush current on startup. The 16 channel thermocouple modules

have 24 bit resolution, can sample up to 1200 samples / second with 0.02°C

of resolution in high-res mode, and a maximum power consumption of

490mW and thermal dissipation of 840mW. The CAN module has a

maximum baud rate of 1 Megabit per second (Mbps), maximum power

consumption of 625mW, and thermal dissipation of 1W. As configured, this

unit will have a maximum power consumption of 13.055W. A basic block

diagram of the hardware and signals is in figure 2 below.

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Figure 2 – Hardware Layout

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Software Requirements

National Instruments LabVIEW software was used for this project. LabVIEW

is a proprietary graphical programming language used to develop

measurement, test and control systems using graphical icons and "wires"

connecting the icons, resembling a flowchart. National Instruments hardware

is designed to run LabVIEW applications created by the end user, and

LabVIEW contains hundreds of built-in libraries to create virtual

instrumentation to aid in rapid development of software.

The software will run a loop once per second, and will read in all the

thermocouples and transmit all necessary CAN messages once per loop. The

CAN message can hold up to 8 bytes of data per message, and there are 112

thermocouple channels, which require 2 bytes for each signal, so 28 CAN

messages will be required to transmit all the data. A flowchart of the process

is below in figure 3.

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Start

Case 1 to 28

Read in 4 thermocouples

from FPGA layer

Encode CAN message

Start for loop 1 to 28

Send CAN message to

FIFO buffer

End For Loop

End

Figure 3 – Main Software Loop

User Interface

When programming in LabVIEW, all the signals are automatically placed on

the top layer VI, but this project did not make much use of the LabVIEW

user interfaces. The primary use of LabVIEW VI was merely troubleshooting

the program and signal processing, as the main use of the program was to

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provide the correct CAN message output. The automation system that will

receive the CAN message has a configurable GUI for the test operator and

engineer to monitor, and a screenshot of this is in figure 4 - notice the

temperature tabular readouts in the lower left corner. The automation

system also has data acquisition built in to record all data to a file for post-

processing of the data.

Figure 4 – PUMA Automation System GUI

Communication Protocols

The standard SAE J1939 - Surface Vehicle Recommended Practice provides

in depth requirements for compliance, from the hardware design to the

abstract layers of the CAN message format. This project only deals with the

software side of the CAN message itself - particularly the encoding and

decoding of this message. The speed of this CAN message is 250 kbps with a

29 bit identifier, which consists of a parameter group number (PGN) and a

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source address, and 8 bytes of data as described in figure 5. The PGN

identifies the content of the data field and sets priority of the message.

Figure 5 - J1939 Message format

The CAN message frame components are:

• SOF - Start of Frame

• CAN-ID - 29 bit identifier, consisting of the Parameter Group Number

(PGN) and the source address

o PGN is 21 bits long, defined in the CAN database and in the SAE

J1939-71 standard

� Priority field - 3 bits, sets the priority of the message on

the network, 0 is highest priority

� Reserved - for future use, set to zero

� Data Page - used to expand the maximum number of

possible messages

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� PDU Format (PDU-F) - 8 bits, used to determine if the

message is intended for a specific device or the entire

network

� PDU Specific (PDU-S) - 8 bits, defined by PDU-F - if

intended for a specific device, this is the destination

address; if intended for the entire network, it is interpreted

as a Group Extension field

o Source Address - the address of the device sending the

message. Each device on the network must have a unique

address

• RTR - Remote Transmission Request

• Control - Contains data length code

• Data Field - up to 8 bytes (64 bits) in single frame, more data can be

sent in multiple frame messages

o This is where the data is transmitted, in this project 8 bytes are

used for 4 thermocouple channels, 2 bytes or 16 bit resolution

• CRC - Cyclic Redundancy Check

• ACK - Acknowledge

• EOF - End of Frame

The PGN is used as a reference lookup in the CAN database, which defines

the contents of the data field. The CAN database is defined by the SAE

J1939-71 document and describes standard parameters which are grouped

together in a message frame and given a PGN. Each parameter is described

by its length, resolution, range, type, Suspect Parameter Number (SPN), and

the PGN in which the parameter is grouped. Each PGN is described by its

transmission rate, data length, data page, PDU-F, PDU-S, priority, and the

data parameter bytes (the parameters included in the data field and their

byte order). The J1939-71 document provides a certain number of PGNs for

free use, and this project used 28 of those available PGNs to define the CAN

messages that I send.

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Development Plan

This project is currently underway to provide this instrumentation for my

employer, and the project will be completed long before the Spring quarter.

Week#/Year Planned activity

44 / 2010 Hardware arrived, simple assembly and begin writing code

45 / 2010 Set up FPGA for input signals

46 / 2010 Continue FPGA code development

47 / 2010 Write main control loop

48 / 2010 Continue with main control loop

49 / 2010 Set up CAN database

50 / 2010 Continue CAN database

51 / 2010 CAN message encoding

52 / 2010 CAN message encoding

1 / 2011 FIFO buffer system and FPGA configuration for output

2 / 2011 Error checking and debug

3 / 2011 Connect to automation system

4 / 2011 CAN message decoding into automation system data acquisition

5 / 2011 Error checking and debug

6 / 2011 Data collection

7 / 2011 Data collection

16 / 2011 Hardware review with class

18 / 2011 Software system presentation

21 / 2011 Code review

22 / 2011 Project Demonstration

I will be using National Instruments LabVIEW for all software code and a

CAN analysis software called CANalyzer from Vector Informatik.

For the project demonstration, I will bring the cRIO unit and several

thermocouples into the lab and the CAN message transmission can be

viewed using CANalyzer. I will also have several posters to show the

LabVIEW graphical code and the structure of the cRIO platform.

During the spring quarter for code review and other tollgates, I will present

the graphical based National Instruments LabVIEW code. This code will be

presented with a pseudo-C equivalent to demonstrate the pros and cons of

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using LabVIEW. If desired, a simple exercise could be developed for the

students to gain exposure to LabVIEW.

Industry Standards

SAE J1939

Electrical Specifications

• Range : minimum 0-1000°C

• Accuracy : +/- 2°C

• Data transmission rate : all messages sent at minimum 1/sec

• Power Supply : 24VDC / 50W

• Operating Temperatures : -40°C to 70°C

• Maximum Power Consumption : 13.055W

Part List

The price list is current, but the only hardware purchased for this project

were the thermocouple I/O modules and connector components, as my

employer already had all the other hardware, which was one reason for

choosing this option for the data acquisition hardware. All components were

purchased from National Instruments.

Part Number Description Power Quantity Price Total

NI cRIO-9014 Controller 6W 1 $2699 $2699

NI-9104 Chassis 3W 1 $2899 $2899

NI-9213 Thermocouple I/O 490mW 7 $999 $6993

NI-9853 CAN I/O 625mW 1 $1149 $1149

196740-01 Connector block n/a 7 $39 $273

779567-01 Backshell n/a 7 $29 $203

Total Cost of all Hardware $14216

Minus price of available hardware -$6747

Total Project Costs $7469

can based data acquisition - western washington...

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