State Machines are Your
Friends!
Sean Donovan
What is a state machine?
• A state machine is a way of expressing an
algorithm or procedure by breaking it down
into “states” and “transitions”
• A state machine has one “starting state”
What is a State?
• States usually consist of 2 components: a
name and an associated action.
• A name could be: “Goto Dentist” and the
action could be making yourself goto the
dentist.
• The “starting state” is the state where the
state machine starts in.
What is a Transition?
• Transitions are conditions that must be
satisfied to go from one state to another.
• A transition could be: “Time is greater than
2pm”, or “Wheel rotated 14 times”
• There can be more than one transition per
State
• A transition can start in one state and loop
back to that same state
An Example State Machine
• Your daily plan could be shown as a State
Machine.
Name:Sleep
Time = 8AM
Name: Work
Action: Do work
Action: Sleep
Time = 2PM
Tired = True
Hungry = True
Name: Eat Dinner
Action: Prepare dinner, eat
Name: Dentist Appointment
Action: Goto Dentist
Explanation
• The Bubbles represent States
• The Connection Arrows represent
Transitions
Robot Related Usefulness
• In an Autonomous Program, State
Machines are usually a good way of
planning what to do
Robot Related Usefulness (cont.)
• Say you wanted to follow a line and you
have 3 reflective sensors watching the
line.
Line
Sensor
Line Following Example
• One question you must ask is how many
states can there be?
1
2
5
3
6
4
Line Following Example (cont.)
• As we can see, there are six States. It is nice to
name these states, so they have been
numbered to simplify drawing later on.
• Since we’ve found out what the states are, we
must say what we want to do when each of
these states occurs.
• At this point we can determine the starting state.
We can assign state 3 to be starting state, as
ideally the robot would start positioned over the
line.
• Try to determine what to do before going on with
this slide show
Line Following Example (cont.)
•
•
•
•
•
•
For State 1: Turn to the left
For State 2: Turn a little to the left
For State 3: Go Straight
For State 4: Turn a little to the right
For State 5: Turn to the right
For State 6: At the end of the line, stop
Line Following Example (cont.)
• The next step in creating a state machine
is to create the transitions.
• In this case, the transitions will be simply
moving over the new lines, so you do not
have to give a reason.
• Try to draw a picture of the state machine
before going on.
Line Following Example (cont.)
Start
3
2
4
1
5
6
Line Following Example (cont.)
• As you can see, there is a transition from 1
to 2 and a transition from 2 to 1. This is
very possible, and quite common
• Also not that State 6 stays looping back to
itself at the end all the time.
Another Example
• Suppose this autonomous mode has been
planed out:
– Go straight for 3 seconds
– Turn left for 1 second
– Go strait for 3 seconds
– Then spin around for the remaining 8 seconds
Another Example (cont.)
• Below is code that could do just that:
Drive(60,0);
//straight
Wait(3000);
Drive(0,60);
//turn
Wait(1000);
Drive(60,0);
//straight
Wait(3000);
Drive(0,60);
//spin
Wait(8000);
Drive(0,0);
Another Example (cont.)
• This could be put into a loop that checks to
see if the robot is in autonomous mode
ever time it goes through the loop. (Soon
to be implemented in the WPI Framework)
while(IsAutonomous())
{
//run the state machine
}
Another Example (cont.)
• First step: Identify the states
• Very simple this time:
– State 1: Go straight
– State 2: Turn
– State 3: Go straight
– State 4: Spin
Another Example (cont.)
• Next: Identify the Transitions
• This is a bit more difficult. Fortunately you
may notice that there are only 2 transitions
per state. One being a loop back (to self).
• Try to determine the transitions before
going on.
Another Example (cont.)
Start
<3000 ms
1
>=3000 ms
<4000 ms
2
>=4000 ms
<7000 ms
3
>=7000 ms
IsAutonomous == True
4
IsAutonomous != True
Driver Mode
Another Example (cont.)
• This can be coded in a variety of ways
– switch statements
– if-else if statements
• Code Examples will be posted later in the
season