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ev3dev-prolog - LEGO Mindstorms ev3 API for Prolog

This is a ev3dev-based API for Prolog using the readily available swi-Prolog.

Installation

  • write ev3dev to an SD card and launch ev3dev on your brick
  • ssh into the brick
  • install swi-Prolog using sudo apt update && sudo apt-get install swi-prolog. This takes some time.
  • clone this repository onto your brick and you're good to go.

Examples and Programming

Interactive mode can be started using swipl ev3_base.pl for the low level API with sensor and actor controls similar to the base EV3 software or other ev3dev APIs, and swipl ev3.pl for the high level abstraction which is on a similar level as OpenRoberta or others (in that it includes an internal robot model).

Complex examples (including breadth-first search planning on a grid) can be found in the examples directory.

Sensor and Actor Control

  • tacho_motor(X). binds the port of the discovered tacho motors to X. Fails if no tacho motor is connected.
  • us_dist_cm(_, X). binds the measured ultrasonic distance to X. Fails if no ultrasonic sensor is connected.
  • forall(tacho_motor(M), motor_run(M, 50)), repeat, us_dist_cm(_, X), X < 5, forall(tacho_motor(M), motor_stop(M)). runs all motors until the distance sensor shows less than 5.

The available predicates are a wrapper pretty close to what's done with other ev3dev APIs and if you're familiar with ev3dev you should feel right at home.

High Level Abstraction

The high level abstraction uses an internal robot model. The predicate set_robot(WheelDiameter, AxleLength, LeftMotorPort, RightMotorPort) is used to initialize the internal model. Then we provide the following high level commands:

  • stop
  • go(Speed)
  • go(Speed, Angle)
  • go_cm(Speed, Distance)
  • turn(Speed, Angle) - This has multiple implementations depending on whether the gyro sensor is attached or not.
  • turn(Speed)

Programming Hints

If you want to write robot programs that run forever, you have two ways to do that using the Prolog evaluation Scheme (SLD-Resolution): Using the predicate repeat (which always succeeds and creates an infinite amount of choice points) you get as resolution tree that is infinitely wide. Or using recursion (a :- a. is an infinite derivation) that doesn't end. Using the latter method be careful to use a cut before the recursion step as to not leave choice points on recursing. Leftover choice points can fill up your limited program memory very fast. In our experience a mix of those two methods with the recursion into the subgoals as program states and repeat as interactivity control leads to the most readable code.

Missing Sensors

We only implemented sensors that were readily available to us. You can easily add your own sensors by using the implemented ones as a template. We welcome pull-requests.

Publications

The Prolog API was presented or will be presented at the following workshops:

The higher level abstraction is inspired by Nalepa

Authors

Related Work

  • There is a brickman fork that supports running pl-files from the graphical user interface. This fork needs some work as the development is no longer funded.

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