Basic Concept¶
Simulation Function¶
Choreonoid is equipped with the simulation function, which can be used as a robot simulator. The function simulates how a robot and environment objects move though calculations and presents the result in animation by 3DCG or output as data. Using this function, it becomes possible to perform hardware design and software-related verifications as well as practice operations or use for training.
The simulation targets are the “body models”, which were introduced in Basics of Robot/Environment Models . By importing body models as body items and make them belong to a virtual world as world items, it will be possible to make simulations targeting this virtual world.
Physics Engine¶
The core part of the simulator is referred to as “Physics Engine”, which is the part that calculates how an object moves physically. Various algorithms for physical calculations are devised and many different physics engines, which are the implementations of such algorithms, have been developed. Models and physical phenomena that can be simulated depend on the engine and the simulation characteristics including the accuracy, the stability and the calculation speed also varies depending on the engine. So, it is important to use a proper engine for the target and the purpose of the simulation. From this point of view, Choreonoid is so designed that various physics engines can be used.
Simulator Items¶
In Choreonoid, a physics engine is represented as a project item called a “simulator item”. It is an item to incorporate physics engines to the simulation function of Choreonoid and is equipped with APIs, which is the base for this purpose. What will be actually used is an item that inherits a simulator item and a corresponding simulator item is defined for each physics engine. In concrete terms, the following simulator items are available:
AIST Simulator Item
It is a standard simulator item of Choreonoid, which makes simulations using its unique physics engine.
ODE Simulator Item, Bullet Simulator Item, PhysX Simulator Item
These are simulator items that utilize Open Dynamics Engine (ODE) , Bullet Physics Library and PhysX respectively, which are physics engines that can be used externally as a library. They become available by installing the corresponding library and building ODE plug-in, Bullet plug-in or PhysX plug-in.
Note
For physical calculations, it is necessary to detect collisions that occur between the objects to be simulated. Normally, the collision detector that performs such operations is also included in a physics engine. On the other hand, as was described in Collision Detection under Basics of Robot/Environment Models, the function to detect collisions is prepared as a basic function of Choreonoid and various collision detectors are available there. (Switching Collision Detectors) With some simulator items, any collision detector of the collision detection function as the basic function can be used.
Sub-simulator¶
A physical calculation is basically performed by a simulator item, but “sub-simulators” are also available as an item that can realize diversified simulation functions supplementary thereto.
For example, you may simulate a robot equipped with cameras or laser range sensors and retrieve their image data even during the simulation. As a sub-simulator that adds this function, Vision Simulator item is available. This simulates the sensor outputs by performing internally a drawing process similar to 3DCG view from the perspectives of a camera or a laser range sensor. In contrast to a “physics engine”, such a sub-simulator can be called as a “vision engine”. This function is not dependent on a physical calculation algorithm and can be used in combination with any simulator item.
Sub-simulators can realize many other different functions in the framework where the situation of a virtual world is monitored so that the corresponding outputs can be provided or the virtual world can be modified.
Controller¶
To move a robot, a program to control it is required. Such a program is called a “Controller”. In a simulation, a controller is also required to move a virtual robot. In general, the same controller is used for both a real robot and a simulated robot. By doing so, we aim to carry out the development and the verification of a controller efficiently on the simulation. Also, by doing so, it will be possible for users to practice the operation and maneuver of the robot system developed on the simulator.
Anyway, a controller is required to move a robot and also is a main element of a simulation.
Controller Item¶
In the simulation function of Choreonoid, a controller is represented as a project item called a “controller item”. Actually, a main controller module implemented separately from the controller item is operated using an item type that inherits the base “ControllerItem” class. There can be various formats of controller module, and controllers in a certain format can be used as long as a controller item type that supports the format is prepared. For example, for “RT Component”, which is a software component of RT-middleware, the corresponding controller item “Body RTC Item” can be used.
How to use controller items will be described in Introduction of Controller .
Input and Output between Robot and Controller¶
What is necessary first for a controller to control a robot is to input and output various data with the robot. That is to say, the controller retrieves the status of the robot or its environment from the input from the different sensors mounted to the robot first and then it outputs the command value decided as a result of the control calculation based on the input to the actuator, etc. of the robot.
In concrete, the following elements can be the actual input:
Joint angle of revolute joint
Joint displacement of prismatic joint
Force sensor
Acceleration sensor
Angular acceleration sensor (rate gyro)
Camera image
Range sensor distance image
The following elements are the output targets:
Torque at revolute joint
Force at prismatic joint
Command of various devices (ex. on/off of a light)
You may well regard a controller item as something that defines the interface for input/output.
The actual input and output methods will be described under Controller implementation .
Utilization of Plugin¶
It is possible to add an inheriting item type to a simulator item, a sub-simulator item and a controller item using a plugin. Using a plugin,
Addition of a physics engine available;
Expansion of a simulation function; and/or
Addition of a supportable controller format
can be supported. In other words, Choreonoid is a platform on which the simulation function per se can be expanded.