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 Action: Sleep Name: Work Action: Do work Name: Dentist Appointment Action: Goto Dentist Name: Eat Dinner Action: Prepare dinner, eat Time = 8AM Time = 2PM Hungry = True Tired = True

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. Sensor Line

Line Following Example One question you must ask is how many states can there be?

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

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 Driver Mode IsAutonomous != True >=4000 ms <3000 ms <4000 ms <7000 ms IsAutonomous == True >=3000 ms >=7000 ms

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