ECE-CSE474-Project 5 Solved

1.0 INTRODUCTION 
Consider that your team has been contracted to implement a basic battery management system.  You’ve been given this document as a basic framework / specification for what the customer wants.  The labs in this course break down the development process as ‘milestones’ for the customer.

 

1.1 Development Phases 
This project is the fourth phase in the development of a simple battery management system for high voltage electric transportation applications.  The current phase focuses on hardware sensors:  creating a real-time signal processing task as well as calibrating an actual hardware sensor.

 

2.0 REVISIONS 
Table 1 – Project Revision Table (Version Control)

Version  
Date
Author
Description
A
11 Jan

2021
J.Vining
First release of initial architecture specification: 

Working with system architecture, a round robin scheduler, task control blocks, and general I/O to implement the first phase of a battery management system  
B
19 Jan

2021
J.Vining
 

Deletions: 

1.      Project 2 guidelines sections:   

•         Development Phases  

•         Layout of the Project/Guidelines for Success

•         Recommended Design Approach

2.      Appendix sections:

•         Implementing the TCB

•         Tips for Building Circuits

 

Additions: 

1. Appendix

• Working with Interrupts  (Section 9.1)

 
 
 
2. System controller tasks

•         Hardware timer interrupt (Section 7.3):  Hardware timer to create real-time time base

•         HVIL interrupt routine (Sections 7.11):  Introduced to set flag and open contactors in the event HVIL transitions to open

 

Modifications: 

1.      Background (Section 3.0)

2.      Project Objectives (Section 4.0)

3.      System controller tasks

•         Scheduler task (Section 7.2): Uses linked list instead of round robin array for calling tasks  

•         Touch screen task (Section 7.4):   

i.           Display has new state flow for transition between screens:  If an alarm is active but not acknowledged, the display will only show the alarm screen until the alarm is acknowledged.

ii.         The alarm screen has a new button in the event that an alarm is active and needs to be acknowledged.

iii.       The alarm and measurement screens show data from realworld measurements.

•         Measurement task (Section 7.6):  Takes analog measurements:  temperature, HV current and HV voltage  

•         SOC task (Section 7.9):  Constant value of SOC=0

•         Contactor task (Section 7.10):  Contactor state diagram updated to use HVIL alarm flag as an input for state transition criteria

•         Alarm task (Section 7.12):  Calculates & reports alarms based on measurement data

 
C
10 Feb

2021
J.Vining
 

Deletions: 

1.      Scheduler section:   

•         Associating Real Time with the Scheduler

2.      Appendix section:

•         Working with Interrupts  

 
 
 
Additions: 

1.      Appendix

•         Doubly Linked Lists: Insert and Delete Functions (Section 9.1)

 

2.      System controller tasks

•         Data logging task (Section 7.7):  Write / read measurement history values to / from EEPROM  

•         Remote terminal task (Section 7.8):  Implement a remote user terminal to read or clear measurement history data  

 

Modifications: 

2.      Background (Section 3.0)

3.      Project Objectives (Section 4.0)

4.      System controller tasks

•         Startup task (Section 7.1):  Added functionality: Reads measurement history values from EEPROM at startup  

•         Scheduler task (Section 7.2):  Time varying tasks now supported.   

i.        Implement functions for adding and removing tasks dynamically from doubly linked list task queue to support time varying tasks.   

ii.      Support more than one periodic task execution rate via counters

•         ALL TASKS:  Each task will have a unique periodic execution rate associated with it

•         Measurement task (Section 7.6):  Now tracking measurement history for EEPROM data logging and remote terminal display  

•         SOC task (Section 7.9):  Now calculating SOC based on physical current, voltage, and temperature measurements

•         Contactor task (Section 7.10):  Contactor state diagram updated to use ALL alarm flags as an input for state transitions

 
D
1 March

2021
J.Vining
 

Deletions: 

1. Appendix section:

• Doubly Linked Lists
 
 
 
 

Additions: 

1. System controller task

• Accelerometer task (Section 7.7):  Real-time task to calculate position and angle from an accelerometer

 

Modifications: 

1.      Background (Section 3.0)

2.      Project Objectives (Section 4.0)

3.      System controller tasks

•         Scheduler task (Section 7.2):  System time base now determined by you in order to meet real-time task requirements.   

•         Hardware timer interrupt (Section 7.3):  System time base now determined by you in order to meet real-time task requirements. 

•         Touch Screen task (Section 7.4):  New accelerometer screen  

 
 

3.0 BACKGROUND 
Whoa, is this really our last project?!  A bittersweet ending to the quarter…  You may now begin to wonder just how complicated a fully functional high voltage battery management system really is, oie!

 

3.1 Cautions and Warnings 
Beware the code gremlins.  It is advised that you DO NOT allow them to collaborate with the smoke demons in your embedded system.  This typically results in unwanted hardware failure!

 

4.0 PROJECT OBJECTIVES 
In this lab, we will continue working with the Arduino Mega (the “System Controller”).  This work has the following goals:

 

•         Add a calibratable hardware sensor and support for several optional features

•         Work with a hard real-time constraint on one of the tasks

•         Work with digital signal processing tools  

 

This project, project report, and program are to be done as a team – play nice, share the equipment, keep any viruses (software or otherwise) to yourself, and no fighting.

 

5.0 SYSTEM ARCHITECTURE 
General System Description: 

Battery management systems (BMS) are required to ensure the safety, proper operation and long-term reliability of high voltage batteries in electric transportation applications.  The system architecture presented is a simplified but representative approach to battery management.

 

6.0 HARDWARE ARCHITECTURE 
The hardware architecture described in this subsection presents the system hardware inputs and outputs as well as the overall layout of the system.

 

We will be simulating a majority of the following inputs with the exception of the touch screen and accelerometer.   

 

6.1 Inputs 
For this stage of the project, all the analog and digital inputs listed below will be implemented in hardware.   

 

Digital: 

•                     High voltage interlock loop (HVIL) signal – digital input actuated via switch

•                     Touch screen feedback Analog: 

•                     HV terminal voltage (0-450V sensor range scaled 0-5V)

•                     HV terminal current (-25A – 25A sensor range scaled 0-5V)

•                     Temperature (-10°C – 45°C sensor range scaled 0-5V)

•                     Accelerometer (±1.5g sensor range scaled 0-3.3V)

6.2 Outputs 
Digital: 

•         Contactor on/off (shown via LEDs)

•         Touch screen display

 

6.3 I/O Interface Circuits 
The circuit diagrams used to model the hardware I/O are provided in Section 7.0 Software Architecture, under the task that accesses the circuit.  You will use these as a guide to model/simulate/create the hardware interfaces for your project.  

 

6.4 Block Diagram 
The block diagram in Figure 1 provides a high-level partially complete block diagram for the system, including all major functional blocks.   

  

  

Figure 1 – High-Level Partially Complete Block Diagram of the Battery Management System (BMS).  NOTE:  Items in light grey are implemented in this project 

 

7.0 SOFTWARE ARCHITECTURE 
Your code will include the following tasks:

 

•         Startup

•         Scheduler

•         Measurement

•         Measurement History (Data Logging)

•         Touch Screen Task:  Display & Touch Input

•         Remote Terminal

•         SOC (State of Charge Calculation)

•         Contactor

•         Alarm

•         HVIL Interrupt

•         Hardware Timer Interrupt

•         Accelerometer

 

The following subsections describe each of the major functional tasks, as they pertain to this stage of the project.  Functionality of each of these tasks shall be modified and expanded as the project moves forward.

 

7.1 Startup Task:  setup() 
The Startup Task is the first task to execute and runs only once when the embedded controller wakes up or resets.  The startup task shall reside in the Arduino language’s setup() function which is configured to run once during startup.

 

This task initializes all:

•         Hardware

o   System time base (timers, etc.) o GPIO (general purpose input/output) o Communication protocols (UART serial bus, etc) o Interrupts, etc.   

•         Software   o Measurement history shared global variables (initializes by reading values from EEPROM).  See the Data Logging Task, Section 7.7, for information on the measurement history variables.

o   Task data structures o Task control blocks (TCB)  

 

7.2 Scheduling Task 
The Scheduling Task is executed in the main loop - for the Arduino language, this is the loop() function.  This task takes care of scheduling and executing the system tasks that make the BMS function.

 

We will transition from a Scheduler with a system time base of 10Hz to one that supports the requirements of a real-time task – you will need to determine the system time base.

 

7.2.1 Dynamic Scheduler 
As before, the dynamic scheduler will:

•         Run each task from the task queue in succession.

•         Pace execution of the task queue using a hardware interrupt timer that sets the System Time Base.

•         Use a task queue composed of a doubly linked list of TCBs.   

 

The same basic rules for task execution apply:

•         Tasks deemed non-atomic shall not be pre-emptable, i.e. no interrupts shall occur during critical code sections.   

•         If a task has nothing to do, it shall exit immediately.

 

In this project, the task queue will support time varying task rates; however, the scheduler shall execute at a rate determined by you to enable the accuracy needed for the Accelerometer Task.  

 

As before, not all of the tasks will run each time step – they will execute at an integer multiple of the global system time base.  The scheduler shall determine which tasks need to run during each time step.  Tasks not running at each time step shall be dynamically added and removed from the task queue using an insert function and delete function as described in Section 7.2.2.

 

Once the scheduler has determined which tasks are to execute during the current time step, it shall execute each TCB in the linked list sequentially without delay and then return to the main loop where it shall be called again after the hardware interrupt timer sets the flag to allow the main loop to cycle again.

 

7.2.2 Supporting Time Varying Tasks:  Adding & Removing Tasks Dynamically 
from the Task Queue 
Tasks that run at a lower rate than the system time base shall be added and removed from the task queue dynamically.  A task shall be added to the queue using an insert function and removed using a delete function.

   

To determine if a dynamic task is to run or not, create a task counter to track the duration between execution periods.  If a task’s counter indicates it’s time to run, it shall be added to the queue.  Once the task has run, it shall be removed from the queue.

 

Example:   

Take a system time base of 10Hz (0.1s period).  There are two tasks running at different rates:  Task1 at 10Hz, Task2 at 1Hz.  Task1 will run every time step (the global time base).  Task2 will run every 10 time steps (10*global time base).  Task2 will be added and removed from the dynamic task queue such that it runs once every 10 time steps.

 

7.2.3 Dynamic Task Queue (TCB Linked List) Implementation 
Each element of the task queue doubly linked list shall be a TCB, representing one of the tasks identified in Section 7.0.  

 

Memory for tasks shall NOT be created dynamically – in other words, all tasks shall be declared and initialized during startup (all tasks exist in memory indefinitely until powerdown).  The dynamic scheduler algorithm simply adds or removes tasks that already exist in memory based on the execution rate of the task.

 

7.3 Hardware Timer Interrupt:  timerISR() 
A hardware time base is implemented to set the execution rate for the scheduler.  This execution rate is known as the system time base, aka the system time step, and is initialized in the Startup Task by attaching an interrupt with a fixed period.  The interrupt task for the hardware timer shall set a flag that signals the main loop to cycle again.  

 

The System Time Base shall be computed to meet the requirements for the real-time task (Acceleration Task) specified in Section 7.5. 

 

7.4 Touch Screen Task:  Display & Touch Input 
The touch screen display task shall include display and touch input functionality.  

 

Tips: 

•         You will find it advantageous to create separate functions for touch input and display. 

•         For the display:  Only update values on the screen that are changing so your code executes faster. 

 

This task shall run at a 1s rate with OPTION to run the touch sensing portion at a faster rate for better touch response. 

 

7.4.1 Touch Input 
The display shall provide user input buttons to scroll through the following screens:  

•         Measurement Screen:  Scroll thru measurements

•         Alarm Screen:  Scroll thru / acknowledge alarms

•         Battery ON/OFF Screen:  Option to turn ON / OFF battery

•         Accelerometer Screen:  Scroll thru accelerometer measurements

 

You may use “next” and “previous” buttons or a single button for each screen.

 

7.4.1.1 Screen-Specific Input:  Battery ON/OFF Screen 
The Battery ON/OFF screen has the same scroll buttons as the other screens PLUS an additional input: an ON / OFF toggle switch.  The toggle switch provides user input to turn ON or OFF the battery.  See the next section, “Display”, for more details.

 

7.4.1.2 Screen-Specific Input:  Alarm Screen 
The Alarm Screen has the same scroll buttons as the other screens PLUS one additional input:  an alarm acknowledgement button.  In the event that an alarm is active and not acknowledged, the alarm screen will display a single button to allow the user to ACKNOWLEDGE all non-acknowledged active alarms.  The next section, “Display”, contains more details

 

7.4.2 Display 
The display consists of four screens, described below.  The display task shall access shared variables from the following tasks to populate the measurement and alarm screens:  Accelerometer, Measurement, SOC, and Alarm. 

7.4.2.1 Measurement Screen 
The Measurement Screen shall display the following sensor data:

•         State of Charge:         <value>

•         Temperature:             <value>

•         HV Current:   <value>

•         HV Voltage:   <value>

•         HVIL (HV Interlock Loop) Status:  <value>

 

7.4.2.2 Alarm Screen 
The Alarm Screen shall display the value of each of the alarms as listed in Section 7.12.  

•         High Voltage Interlock Alarm:         <state>

•         Overcurrent:   <state>

•         High Voltage Out of Range:             <state>

 

If any of the alarm states = “ACTIVE, NOT ACKNOWLEDGED”, the display shall automatically navigate to this screen if it is not there already.  When this occurs, a button will appear (as described in the Touch Input section above) with the option to acknowledge the alarm.  This acknowledgement by the user is passed as a flag to the Alarm Task, indicating the alarm is acknowledged and may change state to “ACTIVE, ACKNOWLEDGED”.

 

7.4.2.3 Battery ON/OFF Screen 
The Battery ON/OFF Screen shall display the current state of the battery contactors as well as a toggle switch to allow user input to turn ON or OFF the battery.  The two toggle switch states yield the following actions:

•         Turn ON…     (CLOSE contactors by sending flag to the Contactor Task)

•         Turn OFF…   (OPEN contactors by sending flag to the Contactor Task)

 

Note that the flag is a shared variable that is passed to the Contactor Task.  This tells the Contactor Task what state the user wants the battery to be in.

 

7.4.2.4 Accelerometer Screen 
The Accelerometer Screen shall display the value of each of the calculations listed in Section

7.5.
 
 
• 
Relative position (cm):  
<x-axis value, y-axis value, z-axis value>
• 
Total distance traveled (cm):  
<value>
• 
Static angle (deg):  
<x-axis value, y-axis value, z-axis value>
 

7.5 Accelerometer Task 
This task takes a physical measurement of the 3-axis analog accelerometer circuit and converts it to:

1.      A relative position for each axis (units: cm)

(∆x, ∆y, ∆z from the origin location at powerup)

2.      Total distance traveled  (units: cm)

(sum of x, y, z displacements since powerup)

3.      Static angle of each axis w.r.t. gravity (units: deg)…. you cannot calculate angle

w.r.t. gravity if the chip is moving unless you use a gyroscope!   

( x,  y,  z with respect to gravity)

 

This task shall be real time in order to process physics calculations that require real-world timing.

 

*REAL-TIME TASK EXECUTION RATE* 

The Accelerometer Task shall execute at a rate capable of producing a distance calculation with an error of +/- 1cm out of an actual 5cm traveled within a 5sec time interval.

Your report should clearly state the frequency of operation of this task and your error rate out of 5cm. 

 

7.5.1 Accelerometer Circuit 
There are several accelerometer chips available.  Some chips use dedicated comm ports for data transmission (SPI, I2C, etc.), whereas the Freescale MMA7361 chip we are using creates an analog output corresponding to acceleration in each of three axes.  There are pros and cons to using an analog sensor approach:  On the plus side, an analog output does not necessarily need an oscilloscope for debugging whereas a comm port may likely need such a tool for debugging.  It is also a simpler approach; however, this simplicity comes at the cost of requiring ADCs on the microcontroller and having less control of and/or less data from the peripheral accelerometer chip.  

 

We will be using an evaluation board to interface with the Freescale MMA7361 chip – this is the “accelerometer” listed in your lab kit.  Using an evaluation board allows us to do a couple of things:

•         Power the 3.3V accelerometer chip with a 5V source…. but be aware that the analog output will always be on a 0-3.3V scale!

•         Interface with the chip’s wee tiny pins via standard headers with 0.1 inch inter-lead spacing.

 

Calibrating the Circuit: 

Calibration ensures that the value read within the code matches the real-world value.  This step is required for compensating deviations in the input circuitry (i.e. component tolerances, etc).   

 

Calibration Steps:

1.      Calculate the resolution of the sensor and use this in your code for converting the analog input voltage to an acceleration value.

2.      Wire the accelerometer circuit to the microcontroller.

3.      Take a test reading of the accelerometer analog input and display the measured value using one of your debugging tools (e.g. the serial monitor).

4.      Compare the measured physical value to a known, calibrated external source… which axis is gravity?... is my circuit moving, i.e. acceleration=0?

5.      Apply an offset to the measured physical value in the code to match the real-world value that you measured, this is the calibration offset.

NOTE:  The calibration offset value should be a GLOBAL variable with a name that indicates its value as well as comments describing its use.

 

7.6 Measurement Tasks:  Sensor Measurements 
Measurements shall be taken to provide both the BMS and outside world with the system state according to the block diagram in Figure 1.     

The Measurement Task shall run at 10Hz (100ms). 

 

7.6.1 Temperature, HV Current & Voltage Measurements 
At this stage in the project, we will be implementing temperature, current & voltage measurements using analog inputs.  Since we do not have actual temperature, current and voltage sensors, we will be simulating these sensors using test circuits as described in the next section.

 

7.6.1.1 Test Circuits for Simulated [0-5V] Analog Sensors:  ANALOG INPUT 
Description of circuit in real-world application: 

HV voltage, HV current, and temperature sensors are some of the most important components of the battery management system.  These sensors tell the system about the battery’s electrical and thermal state.   

 

When designing these sensors, the full output range of the sensor is scaled to the input range of the microcontroller’s analog input pins for maximum resolution.  In the case of the ATMega, the analog input range is [0, 5V].   

 

Temperature sensor analog input scaling:

•         Temperature (-10°C – 45°C sensor range scaled to analog input 0-5V)

 

HV voltage and current sensor analog input scaling:

•         HV terminal** voltage (0V – 450V sensor range scaled to analog input 0-5V) • HV terminal** current (-25A – 25A sensor range scaled to analog input 0-5V)  **Terminal means that the measurement is taken at the battery’s terminals.

 

Implementation:

These analog inputs shall be modeled using the circuit in Figure 2:

•         The potentiometer allows you to produce a variable voltage in the range [0, 5V], which covers the range of each of the sensors.   

•         Note that the circuit requires a software-enabled pullup resistor within the ATMega.  This is configured in the Startup Task when initializing the input pin.  If the pullup resistor is not enabled, you should only see 0V at the input no matter the resistance value of the potentiometer.

 

  

Figure 2 – ANALOG INPUT: Test Circuits for Sensors in Range [0, 5V]

 

7.6.2 High Voltage Interlock Loop (HVIL) 
The HV Interlock Loop circuit will remain the same as implemented in the previous lab.  As before, this measurement shall be stored in a shared variable for inter-task data exchange with the display, similar to the other measurement values.  

7.6.2.1 Test Circuit for HVIL:  DIGITAL INPUT 
Description of circuit in real-world application: 

The high voltage interlock loop is a circuit that detects whether or not all high voltage connectors are connected since its circuit runs alongside the high voltage cabling.  The circuit provides a safety check for the battery management system to ensure that no exposed high voltage cabling is present under operating conditions.   

 

There are many means to implement one of these loops and the detection circuit that reads whether the HVIL is OPEN or CLOSED.  For this project, we will simulate a HVIL detection circuit that provides a digital reading to the microcontroller of whether the loop is OPEN or CLOSED.   

 

 

Implementation: 

The circuit in Figure 3 shows how to simulate the HVIL detection circuit’s connection to the microcontroller by using a DIP switch to OPEN and CLOSE the circuit.  The expected input at the microcontroller should be (notice these are states):

•         OPEN DIP switch:   

o   Produces 5V at the digital input pin (reading logic 1) o LED will not light up

o   HVIL is OPEN!

•         CLOSED DIP switch

o   Produces 0V at the digital input pin (reading logic 0) o LED will light up o HVIL is CLOSED, yay, no danger!

 

  

Figure 3 – DIGITAL INPUT: Test Circuit for High Voltage Interlock Loop (HVIL)

 

7.6.3 Measurement History Tracking 
The Measure Task shall track whether measurement history minimum / maximum values have changed during execution (tracked measurement history values are listed in the Remote Terminal Task, Section 7.7).  These measurement history values are stored in shared global variables which are accessed by the following tasks:  Data Logging Task and Remote Terminal Task.   

 

The Measurement Task shall update the measurement history values under the following conditions:

1.      If measurement history values have changed, the Measure Task shall store the new measurement history values.  A flag shall also be set to let the Data Logging Task know which measurement history value has changed, i.e. min or max for a specific measurement has changed.   

2.      If a reset flag is set indicating the user has requested to reset EEPROM in the Remote Terminal Task, the following “reset” values shall be written to EEPROM: 

•         HV Current Reset Value              = 0

•         HV Voltage Reset Value              = -1

•         Temperature Reset Value            = 0

NOTE:  Reset value “-1” is used for measurement values whose lowest range is 0

 

7.7 Data Logging Task:  EEPROM Measurement History 
The Data Logging Task shall store measurement history values in EEPROM (on the ATMega board).  Since EEPROM memory persists when power is removed, it is good for storing measurement history values and for tracking historical operating extremes.  Measurement history shall include highest and lowest values for:  

•         HV Current  

•         HV Voltage

•         Temperature

 

This task shall run at a 5s rate since the EEPROM has limited read/write cycles 

 

Measurement history data shall be stored in shared global variables for use by both the Remote Terminal Task and Measurement Task.

 

Values stored in EEPROM shall be updated under the following conditions:

1.      If a measurement history value change flag has been set in the Measure Task, the Data Logging task shall update the corresponding value in EEPROM.

à  The EEPROM memory has a specified life of 100,000 write/erase cycles, so values are only written to EEPROM when they change

2.      If a reset flag is set indicating the user has requested to reset EEPROM in the Remote Terminal Task, the following “reset” values shall be written to EEPROM:

•         HV Current Reset Value             = 0

•         HV Voltage Reset Value            = -1

•         Temperature Reset Value           = 0

NOTE:  Reset value “-1” is used for measurement values whose lowest range is 0.

 

Once the Data Logging task has serviced a flag, it shall reset the flag.

 

 

NOTE:  The first time the program has run with the Data Logging Task added, it is a good idea to reset the measurement history values via the Remote Terminal Task.

 

7.8 Remote Terminal Task:  Formal Communication Protocol 
The Remote Terminal Task shall present a menu displaying options for requesting data from the System Controller.  The menu will prompt for user input and display the desired output.  Output values shall be collected from the System Controller using a formal communication protocol developed by you between the remote terminal and System Controller.     

 

This task shall run at a 1s rate. 

 

It is recommended that you use the Serial0 port and the Arduino IDE Serial Monitor, however you are free to implement the remote terminal menu in another operating environment and over a different communication channel.  

 

The remote terminal shall present the user with the following options:

 

[1]    Reset EEPROM 

[2]    HV Current Range [Hi, Lo] 

[3]    HV Voltage Range [Hi, Lo] 

[4]    Temperature Range [Hi, Lo] 

 

Enter your menu choice [1-4]: <char>

 

If option 1 is selected, an EEPROM reset flag shall be set – both the Measurement Task and Data Logging Task shall monitor this flag.  Otherwise, the remote terminal host shall respond with the desired values as accessed via shared global variables.

 

7.9 SOC Task:  Calculating Battery State of Charge (SOC) 
The battery management system shall track the state of charge (SOC) of the high voltage battery.  There are many methods to calculate state of charge, with more advanced systems using coulomb counting and neural networks to track the charge state of the battery.  This system shall track SOC by interpolating data within a 2D lookup table with respect to open circuit voltage and temperature as shown in Table 1.   

 

This task shall run at a 100ms rate. 

 

To calculate SOC, the program shall use the measured temperature, terminal voltage and terminal current.  Finding the correct temperature entry in the 2D table is simple; however, the open circuit voltage must be back calculated from the measured terminal voltage and current using Ohm’s Law if the current is non-zero.  ASSUME: Internal battery resistance is

0.5Ω and the battery can be modeled according to the circuit in Figure 4.

 

Table 2 – State of Charge w.r.t. Open Circuit Battery Voltage and Temperature

Temperature 

(°C)
Open Circuit Voltage (Voc) = (Vterminal + RbattIterminal) 
200V
250V
300V
350V
400V
-10°C
0%
10%
35%
100%
100%
0°C
0%
0%
20%
80%
100%
25°C
0%
0%
10%
60%
100%
45°C
0%
0%
0%
50%
100%
 

  

Figure 4 – Battery Internal Circuit Model

 

 

7.10 Contactor Task:  Setting Contactors (signified by LED output) 
As discussed in Section 7.10.1 “Simulated Contactor Output Circuit”, contactors are a safety mechanism to protect the external world from the high voltage potential inside the battery.  Within the context of this lab, the functionality of the Contactor Task is to show that logic exists for actuating the contactors properly so that when a power circuit is made available to actuate an actual contactor solenoid, it would function as specified.

 

For the purposes of this lab, the Contactor Task shall actuate the digital output pin associated with the contactor simulation circuit defined in Section 7.10.1.

 

This task shall run at a 100ms rate. 

 

States for contactors are:

1.      <state1>: “OPEN” (default/entry state)

2.      <state2>: “CLOSED”

 

The logic for moving between these states is as follows:

•         Contactors shall be initially OPEN.   

•         Contactors shall be set to OPEN if either or both of the following conditions are met:

o   Any alarm state is “ACTIVE, NOT ACKNOWLEDGED” or “ACTIVE, ACKNOWLEDGED”.

o   User requests TURN OFF BATTERY (i.e. OPEN contactors)

•         Contactors shall transition to CLOSED when the user inputs a request to TURN ON BATTERY (i.e. CLOSE contactors) and all alarms are “NOT ACTIVE”.

 

NOTE:  The alarm flags are defined in Alarm Task, Section 7.12.

NOTE:  Request to OPEN/CLOSE contactors shall come in the form of a flag from the Display Task’s Battery ON/OFF screen.  The Contactor Task shall acknowledge the flag after it has acted upon it.

 

In a real-world system, the OPEN/CLOSE command for the contactors will likely come from another embedded controller in charge of components that the high voltage battery rails interfaces with.  This exercise puts that command in the user’s hands and can be considered a debugging tool.

 

7.10.1 Simulated Contactor Output Circuit:  DIGITAL OUTPUT 
Description of circuit in real-world application: 

High voltage contactors provide a means to disconnect the battery’s high voltage rails from the external world.  A typical contactor for this application is actuated by a solenoid, which requires more current to actuate that the microcontroller can source / sink  

à Important point to note!  Microcontrollers are limited in their capability to drive digital outputs over a few watts.  Most microcontrollers require external, board-mounted FETs to drive signals requiring higher power levels.

 

Implementation: 

For the purposes of this exercise, the contactor shall be modeled using an LED in series with a resistor as shown in Figure 5.  This circuit allows for software simulation with visual confirmation that the output pin is being actuated.

 

  

Figure 5 – DIGITAL OUTPUT: Test Circuit for Simulating Contactors

 

 

7.11 High Voltage Interlock (HVIL) Interrupt Routine 
The HVIL interrupt routine shall immediately act upon a transition of the HVIL loop from CLOSED to OPEN.  This task is associated with safety and, as such, is the quickest software method available to OPEN the contactors should HVIL transition to an unsafe state.

 

Conditions for triggering interrupt:   

1. HVIL transitions from CLOSED to OPEN.   

 

Actions taken during the interrupt:   

1.      Set the HVIL Alarm to “ACTIVE, NOT ACKNOWLEDGED”, i.e. <state2> of the alarm states defined in 7.12.

2.      OPEN contactors by writing to the output port directly from the interrupt routine.  

 

Interrupt implementation:   

1.      The interrupt trigger shall be tied to the HVIL input port using conditions described above.  

2.      The interrupt routine shall be tied to this task.

3.      Use the attachInterrupt() function to initialize the interrupt.

 

7.12 Alarm Task 
The following subsections describe the three alarms provided by the battery management system.  Each alarm has three states:

1.      <state1>:   “NOT ACTIVE”            (default/entry state)

2.      <state2>:   ACTIVE, NOT ACKNOWLEDGED

3.      <state3>:  ACTIVE, ACKNOWLEDGED

 

This task shall run at a 100ms rate. 

 

7.12.1 Alarm State Transitions 
•         Upon first triggering an alarm, the alarm shall transition from “NOT ACTIVE” to

“ACTIVE, NOT ACKNOWLEDGED”.

•         Once an alarm is set to “ACTIVE, NOT ACKNOWLEDGED”, the touch screen shall transition to the alarm screen (unless it is there already) where all alarm statuses are listed.  Here the user shall have the option of acknowledging the “ACTIVE, NOT ACKNOWLEDGED” alarms.   

•         The user must acknowledge all “ACTIVE, NOT ACKNOWLEDGED” alarms in

order to navigate away from the alarm screen (alarms do not need to be acknowledged individually).  

•         Once an “ACTIVE, NOT ACKNOWLEDGED” alarm is acknowledged on the alarm screen, the alarm shall transition to “ACTIVE, ACKNOWLEDGED”.  The Alarm Task receives user input of alarm acknowledgement via a flag sent by the Touch Screen Task.

•         Alarms remain in ACTIVE states (<state2> and <state3>) until conditions are met for their dismissal.  Conditions for dismissal are described for each alarm in the subsections below.

 

NOTE: No hysteresis shall be placed on the alarms conditions; however, implementation of hysteresis is typical in a real-world system to avoid repeatedly triggering an alarm when conditions are on the edge.

 

7.12.2 High Voltage Interlock Alarm 
This subtask shall handle transitioning the HVIL alarm to the “ACTIVE, ACKNOWLEDGED” and “NOT ACTIVE” states.   

 

•         The HVIL Alarm shall be set to “NOT ACTIVE” if HVIL is CLOSED.

•         The HVIL Alarm shall be set to “ACTIVE, ACKNOWLEDGED” in the manner described in Section 7.12.1 Alarm State Transitions.   

•         *** The interrupt routine defined in Section 7.11 shall handle transitioning the HVIL Alarm to the “ACTIVE, NOT ACKNOWLEDGED” state.   ***  

 

7.12.3 Over Current Alarm 
This subtask shall handle transitioning the Overcurrent alarm between the three alarm states:  

 

•         The Overcurrent Alarm shall be set to “ACTIVE, NOT ACKNOWLEDGED” if the current measurement lies outside or equal to the range [-5A, 20A].

•         The Overcurrent Alarm shall be set to “ACTIVE, ACKNOWLEDGED” in the manner described in Section 7.12.1 Alarm State Transitions.   

•         The Overcurrent Alarm shall be set to “NOT ACTIVE” if the current measurement lies inside of the range (-5A, 20A).

 

7.12.4 High Voltage Out of Range Alarm 
This subtask shall handle transitioning the Voltage out of range alarm between the three alarm states:  

 

•         The Voltage Out of Range Alarm shall be set to “ACTIVE, NOT ACKNOWLEDGED” if the high voltage measurement lies outside or equal to the range [280V, 405V].

•         The Voltage Out of Range Alarm shall be set to “ACTIVE, ACKNOWLEDGED” in the manner described in Section 7.12.1 Alarm State Transitions.   

•         The Voltage Out of Range Alarm shall be set to “NOT ACTIVE” if the current

                measurement lies inside of the range (280V, 405V).