ofxBlobTracker maccel gives 0

Hi all
I’m using ofxBlobTracker and looking at source there’s an acceleration public variable maccel that seems to be calculated so far (around lines 220)
in ofxBlobTracker.cpp

I simply try to access that variable that way:

void testApp::blobMoved(ofxBlob &_blob){

ofLog(OF_LOG_NOTICE, "Blob ID " + ofToString(_blob.id) + " moved at "+ofToString(_blob.maccel) );

}

I can get Id, age but no acceleration
any clue

thanks