Software Design Narrative

Self-Balancing Biped Robot

Seongwoon KoAnkith Cherala

Jinliang WeiKelton Stefan

Design Considerations

Main Function Flow Chart

Code Modules

Hierarchical Arrangement of Code Modules

Biped Control GUI

Outline

14 KB RAM, 512 KB Flash – large enough

Language: Embedded C◦ Compiler takes care of where to put variables and

code.

Explicitly Put into Flash (declared as const)◦ Look-up tables for trigonometric functions◦ Vector arrays of foot positions for static sub-

actions (later)

Design Considerations - Memory

8 PWM channels to control 8 servos

Problem: not fully utilize the duty resolution◦ Servo refresh period: 25 ms◦ Pulse width 0.6 ms ~ 2.4 ms controls 0 ~ 180

degrees of turning◦ Only use 7.2 % of the duty register resolution◦ Not accurate enough

Integrated Peripherals – PWM

Solution:◦ Set PWM period to 2.5 ms◦ 1 servo refresh period consists of 10 PWM periods◦ Set PWM duty register only for the first period, set

others to 0 (timer is used)

Integrated Peripherals – PWM(cont)

Connected to WiFly card for communicating with control GUI

Interrupt-driven receive◦ Not know when user will send command

Program-driven write◦ Send status message to user periodically

Integrated Peripherals - SCI

Connected to accelerometer for measuring external force.

Micro as master, accelerometer as slave

Interrupt-driven◦ Needs to take immediate action for external force

for balancing

Integrated Peripherals - SPI

1 ATD channel connected to ultrasonic range finder to detect obstacles

Program-driven◦ Checks output periodically◦ Biped walks slowly, so enough time to avoid

obstacles

Integrated Peripherals - ADC

Accumulate timer tickets to trigger the following actions:◦ Set and reset PWM duty register◦ Read output from ultrasonic range finder◦ Send status message to PC

Integrated Peripherals – Timer

User-Control Mode◦ Read control message, act accordingly◦ Assume no need to self-balance or avoid obstacle

– user should take care of it

Auto-Navigation Mode◦ Read environment data, walk freely while

avoiding obstacles and balancing itself

Two Modes of Operation

State Machine + Interrupt Driven + Pooling Loop

Two States:◦ User-control state◦ Auto-navigation state

Interrupts: WiFly, accelerometer, timer

Interrupts service routines set flags, infinite main loop checks flags and acts accordingly

Organization of Code

Four Main Actions: Walk, Turn, Stop, Balance

Each action done by doing sub-actions◦ Walk: lift left foot -> bent for balancing -> move left

foot to front -> …

Static sub-actions and Dynamic sub-actions

Variables to store foot positions and states

Periodically adjust itself

How Biped Acts?

Static sub-action: pre-defined array of foot positions and orientations (stored in flash)

Static sub-actions performed under default conditions (no significant external force other than gravity)

When a static sub-action is done, modify foot position and foot position state

If needs to change to another action, move to another sub-action based on current foot position state

How Biped Acts? (cont) Static Sub-Action

Dynamic sub-action: calculate next foot position and orientation based on current foot position and environment information (acceleration, …)

Significant external force -> Switch from static sub-action to dynamic sub-action

When possible -> switch back to static sub-action

How to calculate next foot position? – not yet know

How Biped Acts? (cont) Dynamic Sub-Action

Interrupt PriorityTimer

Accelerometer(SPI)

WiFly Card(SCI)

Interrupt Service - SPIo Balance itself under significant

external force (dynamic sub-actions).

o In SPI ISR, SPI interrupt is also handled but simply change acceleration value.

o Return with each servos at their positions before interrupt – to resume appropriate static sub-action

Read message from WiFly card and push it to message queue – possibly pending messages

Increment msgCnt by 1

Note: SCI interrupt is disabled/ignored when serving SPI interrupt

Interrupt Service - SCI

Preliminary Flowchart

PWM-Servo Control module◦ Control the servo to turn to given an angle

Inverse-Kinematics module◦ Given a foot position and orientation, calculate

servo angles

Next-Sub-Action module◦ Based on the action to perform and current feet

positions, determine next sub-action to perform

Code Modules

Dynamic-Next-Position module◦ Based on environment information and current foot

positions, find next foot positions to balance itself

SCI-WiFly module◦ Provides functions initializing SCI port 1, sending

messages to and receiving messages from WiFly card

MsgParser module◦ Parse messages received from WiFly card and set

appropriate global variables.

Code Modules (cont)

SPI-Accelerometer moduleSPI initialization; SPI interrupt service routine

ADC–Obstacle module◦ ATD channel initialization; Read and set obstacle

distance and modify other variables if needed

Timer module◦ Timer initialization; Timer interrupt service routine

Status LED module

Code Modules (cont)

Hierarchical Arrangement

Biped Control GUI