How to drive a robot route from a list of moves

To drive a robot along a route without hard-coding it, keep the route as data: an array of leg lengths in encoder ticks. Even legs drive straight; odd legs pivot. A leg counter says which entry the robot is on, the tick count is compared with that entry's length, and when it's reached the robot moves to the next leg and resets the count to zero. Change the list and the same code drives a new route.

The route is a list

int len[] = {4, 2, 4};   // straight 4, pivot 2, straight 4
const int LEGS = 3;
int leg = 0;
int ticks = 0;

Legs count from 0, so leg 0 is straight, leg 1 a pivot, leg 2 straight again. The program doesn't know the route; it reads it.

The leg machine

Each pass of loop(), three steps:

if (now == LOW && last == HIGH) ticks = ticks + 1;   // 1. count a tick
last = now;

if (leg < LEGS && ticks >= len[leg]) {               // 2. leg done?
  leg = leg + 1;
  ticks = 0;                                         //    start fresh
}

if (leg >= LEGS) {                                   // 3. drive
  analogWrite(9, 0);    analogWrite(10, 0);          //    route over
} else if (leg % 2 == 1) {
  analogWrite(9, 255);  analogWrite(10, 0);          //    odd: pivot
} else {
  analogWrite(9, 255);  analogWrite(10, 255);        //    even: straight
}

leg % 2 is the remainder after dividing by two: 1 for odd legs. The leg < LEGS check comes first so the program never reads past the end of the array.

Live experiment

Edit the route

One program drives whatever list it's given: even legs straight, odd legs a pivot of 18° a tick, stop after the last. Start with 4, 2, 4, add legs, or try 8, 5, 8, 5, 8, 5, 8, 5, a square that closes within about 2 cm. Without the reset at each new leg, every leg after the first ends the moment it starts.

Try it yourself: free lesson

“A list of legs” is lesson 6 of 6 in Encoder Edge, Robotics World 2. Free with an account. Lessons unlock in order, so you’ll do the 12 before it first.

A square from a list

Make the pivots 5 ticks, about a quarter turn each, and the straights 8:

int len[] = {8, 5, 8, 5, 8, 5, 8, 5};
const int LEGS = 8;

Four sides and four corners, and the robot ends within a couple of centimetres of where it started. Nothing in the code changed: only the list, and LEGS.

Code and data

Keeping what to do in data and how to do it in code is one of programming's best habits: the code is tested once, and the data can change every day. 3D printers and CNC machines follow long lists of moves called G-code, and warehouse robots get their routes as lists of waypoints. Neither has the route written into its program.