The Force Plate, like any sensor, has a limit to how quickly it can respond to a change. If a very short impulse is applied to the force plate, it will vibrate, or ring, for a short while. That ringing can interfere with your measurement.
Revis...
Ultrasonic motion detectors only measure the position of an object that is directly in front of it, within about 3 meters (10 feet). It is intended to measure the velocity of an object or person moving toward or away from the sensor. This works well ...