16.07 Dynamics Fall 2004 Lecture D14- Accelerometers. Newtonian Relativity accelerometers An accelerometer is a device used to measure linear acceleration without an external reference. The main idea has already been illustrated in the previous lecture with the example of the boy in the elevator. Clearly, if we know the weight of the boy when the acceleration is zero, we can determine from the reading on the scale the value of the acceleration. In summary, the acceleration will produce an inertial force on a test mass, and this force can be nulled and measured with precision. Below we have a sketch of a very simple one axis accelerometer ELASTIC RESTRAINT SCA POINTER DAMPER MASS ELEMENT ACCELEROMETER CASE AXIS In addition to the restraining spring, the accelerometer has a damper which quickly eliminates the oscillations that would otherwise occur due to rapid changes in acceleration. The force of the damper is proportional to the velocity and opposes the direction of motion, so that when the mass settles to a fixed position, the effect of the damper force disappears If we are able to obtain the value of the acceleration at every instant, then, given the initial conditions two time integrations can be done in real time to yield the position. This is the basis of inertial navigatio a topic of enormous technical significance in aerospace. The problem with this simple strategy is that the test mass is also acted upon by gravity, in addition to the inertial force. The navigation problem can be esolved if we know our location exactly, and the direction in which the accelerometer's measurement axisJ. Peraire 16.07 Dynamics Fall 2004 Version 1.5 Lecture D14 - Accelerometers. Newtonian Relativity Accelerometers An accelerometer is a device used to measure linear acceleration without an external reference. The main idea has already been illustrated in the previous lecture with the example of the boy in the elevator. Clearly, if we know the weight of the boy when the acceleration is zero, we can determine from the reading on the scale the value of the acceleration. In summary, the acceleration will produce an inertial force on a test mass, and this force can be nulled and measured with precision. Below we have a sketch of a very simple one axis accelerometer. SCALE ELASTIC RESTRAINT DAMPER MASS ELEMENT POINTER ACCELEROMETER AXIS CASE In addition to the restraining spring, the accelerometer has a damper which quickly eliminates the oscillations that would otherwise occur due to rapid changes in acceleration. The force of the damper is proportional to the velocity and opposes the direction of motion, so that when the mass settles to a fixed position, the effect of the damper force disappears. If we are able to obtain the value of the acceleration at every instant, then, given the initial conditions, two time integrations can be done in real time to yield the position. This is the basis of inertial navigation, a topic of enormous technical significance in aerospace. The problem with this simple strategy is that the test mass is also acted upon by gravity, in addition to the inertial force. The navigation problem can be resolved if we know our location exactly, and the direction in which the accelerometer’s measurement axis is 1