Reaction Wheel Subsystem - Carleton-SRCL/SPOT GitHub Wiki

A reaction wheel was built for attitude control purposes. Due to the limited budget of the lab, a reaction wheel was designed and built in-house. The following table lists the parts that were machined or purchased to construct the reaction wheel.

Reaction wheel component list

Drawings of the machined parts are available in the Drawings and Solid Model folders located here. Design Calculations are in the located here and are summarized as follows:

  1. A resultant angular rate of 1 rpm on the platform due to the reaction wheel was desired.

  2. The platform inertia was estimated to be 0.96 kg m^2 at the time the reaction wheel design was begun.

  3. A large reaction wheel spinning at a lower speed was more desirable than a small reaction wheel spinning at a high speed. This is because the force associated with an unbalanced wheel increases with the square of the wheel's angular rate. Therefore, a slow wheel has a less strict balancing requirement than a fast one. Since access to a wheel balancing facility was not readily available, spinning a large wheel slowly was the approach herein taken.

  4. The maximum allowable dimensions of the wheel were determined to be 4.4 inches in diameter and 2.4 inches tall due to space constraints on the platform. This yields a wheel inertia of 0.0074 kg m^2.

  5. In order for the platform to spin at 1 rpm, the wheel must spin at 129 rpm. This drives the selection of an appropriate motor and gearbox.

  6. A Maxon brushless motor, with a no load speed of 7950 rpm was selected. It was to be operated at a maximum speed of 7000 rpm. With a gearbox with a reduction of 53:1, this would yield a maximum wheel speed of 128 rpm.

  7. A controller was chosen to perform closed loop feedback control on the motor. It was setup in rate control mode, and it uses feedback from Hall sensors embedded in the motor. A pulse width modulated signal from the on-board computer tells the control board the desired wheel speed. Escon Studio software on a laptop can be used to modify settings on the control board.

The items were then purchased, machined, and tested. The following paragraph shows performance parameters and then discusses suggested changes for future designs.

The reaction wheel was connected to the Raspberry Pi 3 as well as the author's laptop. The control board performance can be monitored in real time using the Maxon motor ESCON Studio software. Using this, the Pi 3 sent speed commands to the controller, and the Hall sensor feedback signal was recorded to determine what speed was achieved. It was found that the motor reached the desired speed to within 10 rpm over the range [0 7000] rpm. By varying the desired speed of the motor over time, any torque within the capabilities of the motor can be achieved. This was tested, and any torque could accurately be obtained to within 0.1% of the desired torque, up to a maximum torque of 0.1 Nm. Therefore, the reaction wheel's performance was demonstrated to be as expected when commanded via the Pi 3 computer.

The limiting factor in the motor speed and torque is driven by the supplied voltage. It was learned, after the components were purchased, that a 12 V motor actually requires a larger than 12 V power supply. An equation in the motor controller manual dictates the required supply voltage to achieve a desired speed and torque. With the limited 12 V supply already incorporated in the design, the speed-load curve shown in Figure below was generated. The figure shows that above 7000 rpm, the maximum torque is negative, indicating that the maximum speed possible with the available power supply is roughly 7000 rpm. Therefore, the motor speed is capped at 7000 rpm, to ensure the accuracy of the commanded values.

Reaction wheel motor controller plot

In practice, the RED platform achieved 3 rpm due to the reaction wheel. This additional performance was attributed to the actual platform inertia being roughly 1/3 of the predicted inertia.

In the future, it is recommended to purchase a 9 V motor, so that the 12 V power supply available will be able to drive the motor to its full range of speeds and torques. It is also suggested to increase the speed of the wheel by up to a factor of 5, due to the success in machining a pre-balanced reaction wheel. This would give 5 times the angular momentum capacity and possibly allow for 15 rpm angular rates on the platform.