What is Three-axis Rate able?

1. What is 3-Axis Rate Table?

The three-axis rate table consists of three frames: the outer frame, the middle frame and the inner frame. The object to be measured is usually fixed on the inner frame. Due to the fact that these three frames form a universal support, the object can undergo angular motion in any direction in space.

The three-axis rate table mainly has two structures: vertical and horizontal.

The outer frame of the vertical three-axis rate table is the azimuth frame, the middle frame is the pitch frame, and the inner frame is the roll frame. It is mainly used for testing the inertial navigation system of conventional horizontal navigation vessels.

The outer frame of the horizontal three-axis rate table is the pitch frame, the middle frame is the azimuth frame, and the inner frame is the roll frame. It is mainly used for testing the inertial navigation system of vertical launching vessels.

2. System Composition and Control Principles

The three-axis rate table is mainly composed of a base and three frame systems. Each frame system can independently control the angular rate. Their principles are basically the same. Taking the inner frame system as an example, it can be further divided into the inner frame, inner frame axis, torque motor, speed measurement motor and control circuit, etc.

The principle of the rate control system for a certain rate table is as following graph: the angular speed input specified by the user is automatically converted into a precise voltage reference signal. The speed measurement signal output by the speed measurement motor is proportional to the frame speed. When the speed fluctuates, the speed measurement signal also fluctuates. The speed measurement signal is compared with the reference signal through feedback to form an error, which is then processed by DC amplification and power amplification to control the speed of the torque motor so that it precisely equals the angular speed specified by the user. Therefore, the speed control system mainly achieves the purpose of maintaining a constant speed through speed feedback.

3. Conductive Sliders and Automation

In a three-axis rate turntable, since the inner frame can rotate 360° freely relative to the middle frame, the middle frame can rotate freely relative to the outer frame, and the outer frame can rotate freely relative to the base, especially in the state of high angular rate operation, the power and signals between the test piece on the inner frame and the ground equipment cannot be directly transmitted through wires, but must be transmitted electrically using conductive sliders installed on the frame axis. The number of sliders and the rated current are two important performance parameters of the conductive slider.

Large three-axis rate tables are usually equipped with dedicated table control cabinets or control computers to control the operation of the table and automatically collect data on the angular positions and angular rates of each axis. Therefore, no manual rotation and reading devices are installed on the table body, but locking devices are usually still provided on each frame axis.

4. Main technical specifications

The main technical specifications of the rate table are the rate range, rate accuracy and rate stability. The rate range needs to meet the output range requirements of the tested object, while the rate accuracy and rate stability need to meet the working accuracy requirements of the tested object.

• Rate Range
  • The rate range refers to the interval between the maximum and minimum rates of the rate table. In some specific application scenarios, the maximum rate of certain tables can reach 10,000°/s, while the minimum rate of certain tables can be as low as 0.00001°/s. The rate range of common rate tables is generally 0.0002 to 2000°/s, with the ratio of the maximum rate to the minimum rate being 10⁷, which can meet the testing requirements of inertial devices and inertial navigation systems.
• Rate Accuracy
  • The methods for representing the rotational speed accuracy of the table are generally two types: the graded representation method and the integral level representation method.
    • The graded representation method divides the entire rate range of the rate table into several rate segments， and the measurement error of each rate segment is different. Generally， the relative measurement error is larger when the rate is low, and smaller when the rate is high.
    • The integral representation method means that the relative error within the entire rate range is less than a certain specified value. Here, the relative error is the percentage of the absolute error to the true value，and the absolute error is the difference between the measured value and the ideal true value.
• Rate stability
  • Rate stability refers to the degree of fluctuation of the actual rate of the table relative to its average rate. Its representation method also has two forms，similar to the representation method of rate accuracy.