1. Field of Invention
This invention pertains generally to angular rate sensors or gyroscopes and, more particularly, to a dithering mechanism and method for eliminating the effects of zero-rate bias in a rate sensor or gyroscope.
2. Related Art
Angular rate sensors or gyroscopes exhibit a non-zero output in the absence of rotation about the input axis. This non-zero output is called bias, and it can cause significant errors in inertial measurements. At the time of manufacture, the bias for each individual gyroscope can be measured and subtracted from the output to set the zero-rate output to zero. However, bias is not fixed in time, and it tends to drift with changes such as temperature and aging of the materials employed in the sensor. This may require periodic re-zeroing of a sensor in the field.
Bias cancellation, or “washout”, mechanisms are employed in order to reduce or eliminate the effects of biases that slowly change with time in an unpredictable manner. By dithering the input, or sensitive, axis of a gyroscope, such mechanisms are able to provide automatic adjustment for changes in zero-rate output due to temperature, time or other errors.
Dithering of gyroscopes is well known in the art. A ring laser gyroscope, for example, is dithered around its sensitive axis for the purpose of avoiding frequency lock-in. However, rotating a sensor about its input axis is undesirable because it provides an angular rate input other than the one to be detected.
Dithering the input axis of a gyroscope about an axis perpendicular to the input axis to cancel bias is also well known. Continuously rotating the input axis to eliminate the effects of bias is known as carouseling , and periodically moving the input axis between discrete locations to remove or cancel bias is known as indexing.