Factors that cause satellite attitude control failure: mass properties error & lack of tuning before going to space

by Austin Chang

     Failures of a satellite attitude determination and control system (ADCS) occur with unexpected steady-state errors, longer response time, or even divergence on controlling outcomes. Despite the improper calibration on attitude sensors/actuators, misalignment of sensor/actuator/system reference frames, common causing factors are the inaccurate mass properties and lack of tuning for your attitude control systems. (as figure 1 shows)

Figure 1. Factors for the failure of attitude control system

     So, what are mass properties? They are critical factors representing a satellite’s mechanical characteristics, including total mass, the center of mass (COM), the moment of inertia (MOI), and the product of inertia (POI) of a satellite. By combing the last two of them, we have the so-called “inertia matrix” or “inertia tensor.” (you then know how our company is named 😊) While propagating the attitude control algorithms, the above parameters have to be specified by users no matter you are purchasing an integrated ADCS or building your own.

     People usually obtain these parameters via simulations on their Computer-Aided Design (CAD) software such as SolidWorks or Autodesk. However, In practice, two types of error exist in determining mass properties, “the variations of mass properties by the time (figure 1-c-1)” and “the differences between simulation and the integrated hardware (figure 1-c-2).”

     For the first issue, common causing factors such as thermal variation may cause the deformation of the satellite because different materials have different thermal expansion coefficients. Furthermore, flexible bodies such as solar panels may significantly cause a variation in mass properties.

     For the second issue, material densities, machining tolerances, satellite assembly errors, and other unpredictable factors will cause the actual moment of inertia and center of mass to be far from the results calculated by the simulation.

     Therefore, measured mass properties for your assembled satellite are essential for you to get rid of the second issue. Some robust control techniques may have to be embedded in your control logic to deal with the first issue. However, in our experiences, the first issue only gets significant on large satellites. (where the characteristic of a flexible body dominates)

Figure 2. The mass of satellites in five categories [3]

(In Tensor Tech, we usually classify mini and medium satellite as small satellite.)

     As a result, Tensor Tech constructs a facility to measure the moment of inertia and center of mass of a Nano-satellite. It is equipped with a tactical gyroscope, inclinometers, level instrument, and a hemispherical air bearing. Unlike conventional equipment dedicate to small/micro-satellites (most mass properties measurement instruments are designed of things that weigh more than 30kg), ours is designed of nano-satellites. (as figure 2 shows)

     The measurement method is containing the following four steps.

  1. Use the level instrument and manually adjust the platform to be vertical to the direction of the earth gravity direction vector.
  2. Use an electronic precision balance to measure the weight of the satellite. (accuracy: +/- 1mg)
  3. Put the satellite on the platform with different faces and measure the incline angle via the electronic inclinometer. Calculate the satellite center of mass via optimal solution algorithms. (COM resolution: 0.1mm & accuracy: < +/-0.1mm)
  4. Put the satellite on the platform with different faces and manually tilt the platform. Measuring the angular velocity of the platform by time and solve for the inertia tensor via optimal solution algorithms.

     Lastly, to resolve the problem in figure 1-d, this platform can be served for our clients or us for testing the performance of their attitude control system. This hemispherical air-bearing equipment may be the closest experiment device we can have on earth to simulate attitude control in a zero-gravity environment. People usually failed to tune their attitude control system by not having such devices.