Motion - advanced

Overview | API | Advanced

Synchronization with ALMotion cycle

When writing reactive code that needs to send a new command every cycle, or use fresh sensor data, it can be helpful to synchronize with ALMotion’s cycles.

ALMotion cycle

The ALMotion cycle contains the following steps:

  • preProcess event is raised
  • sensor data is refreshed from memory
  • Cartesian data are updated and available
  • motion tasks provide their new intentions
  • intentions are executed on the model
  • intentions are sent to the DCM
  • postProcess event is raised
  • sleep

The motion thread uses a real time priority just lower than the DCM, and will try to execute on the anniversary of start up time + CycleNumber * 20ms. Should we fall behind by more than 20ms we will stop trying to catch up, and continue from the present.

Using ALMotion cycle events

From a NAOqi module, perhaps made with the module generator, you have access to the broker, and can use it to create a connection to Motion’s preProcess or postProcess events. This allows being awakened at the beginning or end of each cycle to do some processing: get the current state of the robot, and prepare some commands.

// Your method that will be called
void YourModule::motionPreProcessCallback() {
// Do something very small every 20ms.
// You are in a real time thread, so beware.
// If you wish to do heavy processing, you should
// raise a signal that is consumed by another thread
void YourModule::subscribeMotionPreProcess() {
try {
// Register the callback
// The specialized proxy does not give access
// to the underlying module needed for this binding,
// so we get a generic proxy from the parent broker
} catch(const AL::ALError& e) {
std::cout << "Motion preProcess callback registration failed" << std::endl;

DCM and ALMotion cohabitation

It is possible to use at the same time DCM and ALMotion but it requires some precautions.
It is not really well-made and we will improve it on the next version.

Using DCM after Motion

Deactivate Smart Stiffness: ALMotion does not get the command angles directly sent to DCM and cannot manage the Stiffness for the given posture.
With Smart Stiffness, the robot should have not enough stiffness to realize the desired motion.

Using Motion after DCM

After using DCM, ALMotion’s command angles are not updated to the internal model. To avoid jerky transition between using DCM and ALMotion, there are two options: