Inverted Pendulum Thrust Stand
A null-type stand differs from other inverted pendulum thrust stands in that it actively holds the thrust stand in a nearly stationary position at all thrust levels. This eliminates changes in the elevation angle of the thrust vector and reduces error in the thrust measurement.
Xu, K., Walker, M. L. R., “High-Power, Null-Type, Inverted Pendulum Thrust Stand,” Review of Scientific Instruments, Volume 80, Number 5, May 2009, pp. 055103 1-6.