Plugin tutorial

This tutorial will try to explain how to write a plugin for CoppeliaSim. The CoppeliaSim scene file related to this tutorial is located in CoppeliaSim's installation folder's tutorials/BubbleRobExt. The plugin project files of this tutorial can be found here.

CoppeliaSim automatically loads all plugins that it can find in its folder (i.e. the installation folder, or the same folder as the one that contains coppeliaSim.exe) at program start-up. CoppeliaSim recognizes plugin files with following mask: "simExt*.dll" on Windows, "libsimExt*.dylib" on Mac OS and "libsimExt*.so" on Linux. Additionally a plugin's filename should not contain any underscore (except the one at the beginning obviously). The plugin file of this tutorial is simExtBubbleRob.dll. When testing it, make sure it was properly loaded at CoppeliaSim start-up: switch the console window to visible by unchecking the Hide console window item in the user settings dialog ([Menu bar --> Tools --> Settings]). This option is only available in the Windows version. On Mac, have a look at the system's console, and on Linux try to start CoppeliaSim from within a console. The console window should display something similar to this:

As you already understood, this plugin was written for BubbleRob from the BubbleRob tutorial. Load the related scene file (tutorials\BubbleRobExt\BubbleRobExt.ttt). The BubbleRob plugin adds 4 new Lua commands (custom Lua commands should follow the convention: "simXXX.YYY" for the name, e.g. simRob.start):


Description Creates an instance of a BubbleRob controller in the plugin.
Lua synopsis number bubbleRobHandle=simBubble.create(table_2 motorJointHandles,number sensorHandle,table_2 backRelativeVelocities)
Lua parameters
motorJointHandles: a table containing the handles of the left and right motor joints of the BubbleRob you wish to control.
sensorHandle: the handle of the proximity sensor or the BubbleRob you wish to control
backRelativeVelocities: when BubbleRob detects an obstacle, it will move backwards for some time. relativeBackVelocities[1] is the relative velocity of the left wheel when moving back. relativeBackVelocities[2] is the relative velocity of the right wheel when moving back
Lua return values
result: -1 in case of an error, otherwise the handle of the plugin's BubbleRob controller.


Description Destroys an instance of a BubbleRob controller previously created with simBubble.create.
Lua synopsis boolean result=simBubble.destroy(number bubbleRobHandle)
Lua parameters
bubbleRobHandle: the handle of a BubbleRob instance previously returned from simBubble.create.
Lua return values
result: false in case of an error


Description Sets a BubbleRob into an automatic movement mode for a given duration
Lua synopsis boolean result=simBubble.moveAndAvoid(number bubbleRobHandle,number movementDuration,boolean returnDirectly=false)
Lua parameters
bubbleRobHandle: the handle of a BubbleRob instance previously returned from simBubble.create.
movementDuration: duration in seconds of the automatic movement
returnDirectly: if true, the function returns directly (non-blocking operation), otherwise the function only returns when the automatic movement finished (this however requires the calling script to be threaded!). Can be nil or omitted, in which case the operation is blocking.
Lua return values
result: false in case of an error


Description Stops the automatic movement of a BubbleRob
Lua synopsis boolean result=simBubble.stop(number bubbleRobHandle)
Lua parameters
bubbleRobHandle: the handle of a BubbleRob instance previously returned from simBubble.create.
Lua return values
result: false in case of an error

Now open the threaded child script attached to the BubbleRob model in the scene (e.g. double-click the script icon next to object bubbleRob in the scene hierarchy). Inspect the code:

function sysCall_threadmain()
    -- Check if the required plugin is there:
    while moduleName do
        if (moduleName=='BubbleRob') then
    if (bubbleRobModuleNotFound) then
        sim.displayDialog('Error','BubbleRob plugin was not found. (simExtBubbleRob.dll)&&nSimulation will not run properly',
        local jointHandles={sim.getObjectHandle('leftMotor'),sim.getObjectHandle('rightMotor')}
        local sensorHandle=sim.getObjectHandle('sensingNose')
        local robHandle=simBubble.create(jointHandles,sensorHandle,{0.5,0.25}) -- create a BubbleRob instance
        if robHandle>=0 then
            simBubble.start(robHandle,20) -- control happens here
            simBubble.destroy(robHandle) -- destroy the BubbleRob instance

The first part of the code is in charge of checking whether the plugin required to run this script (i.e. simExtBubbleRob.dll) is available (i.e. was found and successfully loaded). If not, an error message is displayed. Otherwise, joint and sensor handles are retrieved and given to the custom Lua function that creates a controller instance of our BubbleRob in the plugin. If the call was successfull, then we can call simBubble.moveAndAvoid. The function instructs the plugin to move the BubbleRob model while avoiding obstacles, and this for a duration of 20 seconds. The function is blocking (the omitted third argument is false by default), which means that the call doesn't return until the function has finished (i.e. after 20 seconds). Run the simulation: BubbleRob moves for 20 seconds then stops, as expected. Now leave CoppeliaSim. Temporarily rename the plugin to TEMP_simExtBubbleRob.dll so that CoppeliaSim won't load it anymore, then start CoppeliaSim again. Load the previous scene and run the simulation: an error message now appears, indicating that the required plugin could not be found. Leave CoppeliaSim again, rename back the plugin to simExtBubbleRob.dll and start CoppeliaSim again.

Let's have a look at how the plugin registers and handles the above 4 custom Lua functions. Open the BubbleRob plugin project, and have a look at file simExtBubbleRob.cpp:

Notice the 3 required plugin entry points: simStart, simEnd, and simMessage: simStart is called once when the plugin is loaded (initialization), simEnd is called once when the plugin is unloaded (clean-up), and simMessage is called on a regular basis with several type of messages.

During the initialization phase, the plugin loads the CoppeliaSim library (in order to have access to all CoppeliaSim's API functions), then registers the 4 custom Lua functions. A custom Lua function is registered by specifying:

  • a function name
  • a calling tip string
  • a list of expected arguments
  • a callback address
  • When a script calls the specified function name, then CoppeliaSim will try to convert the provided arguments to what is expected by the callback, then calls the callback address. The most difficult task inside of a callback function is to correctly read the input arguments, and correctly write the output values. To ease the task, two helper classes are used, that will be in charge of that: CLuaFunctionData and CLuaFunctionDataItem, located in programming/common and programming/include.

    When writing your own custom Lua functions, try to use the same code layout/skeleton as was done in file simExtBubbleRob.cpp.

    Control of a BubbleRob instance does not happen in any of the 4 custom Lua function callbacks: the callbacks just initialize/destroy/update data structures. The control happens in simMessage, with message sim_message_eventcallback_modulehandle: that message is called for all plugins when the main script calls sim.handleModule(sim.handle_all,false), which happens once per simulation pass.

    In general, callback routines should execute as fast as possible, and control should then be given back to CoppeliaSim, otherwise the whole simulator will halt.