## Choosing the Right Gimbal Balance Machine

The two primary factors affecting selection of a Space Electronics Gimbal Balance Machine are weight of the gimbal and its unbalance tolerance.

**Maximum gimbal weight**. The machine must accommodate the maximum weight and size of the gimbals to be balanced. However, it should be no larger than necessary since large machines are more expensive and less sensitive than small machines.

Sensitivity to unbalance is defined by Space Electronics as the smallest unbalance moment that can be repeatably measured for a gimbal with a rotation angle of ±45^{o}. If the gimbal rotation angle is smaller than this, sensitivity of measurement will be reduced. The table below illustrates this effect.

You should select a Gimbal Balance machine which is at least 5 times more sensitive than the balance tolerance for the gimbal. To calculate the rated machine sensitivity (S) required when given a balance tolerance (G), use the following formula:

S = 1.4(G)(F)SIN(a)

where a is the maximum rotation angle of the gimbal (measured from the midpoint of rotation) and F is the ratio between machine sensitivity and gimbal balance tolerance requirement (typically 0.2).

Rated Machine Sensitivity | Effective Gimbal Unbalance Sensitivity (g-cm) as a function of gimbal rotation angle | |||

(g-cm) | ±90^{o} | ±45^{o} | ±10^{o} | ±3^{o} |

0.1 | 0.07 | 0.1 | 0.41 | 1.4 |

1.0 | 0.70 | 1.0 | 4.10 | 14.0 |

10.0 | 7.0 | 10.0 | 41.0 | 140.0 |

EXAMPLE If the gimbal must be balanced to a tolerance of 10 g-cm, a maximum of 20% of the allowed unbalance may be used up by the machine sensitivity limit, and the gimbal rotation angle is ±25^{o}, then G = 10 g-cm, F = 0.2, a = 25^{o}. The required machine sensitivity:

S = 1.4 x 10 x 0.2 x sin 25^{o} = 1.2 g-cm

Therefore, a machine with a sensitivity better than 1.2 g-cm should be purchased (such as the Space Electronics Model GM904 with rated sensitivity of 0.5 g-cm).

**Optional automated gimbal rotation. ** The measurement process requires that the gimbal be rotated precisely to four positions. This can be done by hand. However, if the machine is to be used to balance a large number of gimbals, then it is convenient to have this rotation accomplished electrically. In addition to its convenience, this option generally improves the sensitivity of the measurement, since electrical rotation has less disturbing influence on the instrument than manual rotation. Space Electronics can provide the electronics necessary to do this automatically via commands from the computer. This option includes the software necessary to interface with the drive electronics provided by the customer. It also includes a beryllium copper flex strip to provide power to the gimbal without reducing the sensitivity of the measurement. We can also provide a means of electrically reading the actual rotation angle. This true angle is then used in the calculation of gimbal unbalance.