Actuators

Once the attitude and rate of the satellite are determined, actuators are needed to control the satellite to the desired orientation. The actuators included in this satellite are two active aerodynamic roll control paddles, three magnetic torque rods and three nano- reaction wheels.

The normal position of the paddles are visible in the deployed satellite model shown in Figure 2.5. During paddle control, the two paddles always turn in opposite directions about the ZB-axis and thus generate aerodynamic roll torques of the same magnitude

and sign. By controlling the angle and the rotation direction of the paddles, the size and the direction of the generated aerodynamic roll torque and consequently the roll angle of the satellite can be controlled.

Figure 2.7 illustrates the functionality of the roll paddles assuming that the atmospheric velocity vector is in the negative XB-direction, therefore into the page on the figure. When

the positive ZB-axis paddle rotates through a positive 30◦ angle from its normal position,

the airflow causes a reaction force with a positive YB-component at the paddle’s centre of

pressure. The rotated paddle therefore generates a negative torque about the XB-axis of

the satellite. At the same time, the negative ZB paddle rotates through an angle of -30◦

at the paddle’s centre of pressure. This also leads to a negative torque about the XB-axis

of the satellite. Both paddles produce a negative roll torque and the satellite starts a negative rotation about its XB-axis.

blue sideredside

blue sideredside

ZB

YB XB

normal position with largest surface area in XBZB-plane

ZB YB XB +30◦_rotated about ZB-axis −30◦_rotated about ZB-axis reaction force −YBcomponent reaction force +YBcomponent ZB YB XB +30◦_rotated about ZB-axis −30◦_rotated about ZB-axis ZB YB XB

Satellite rotates about XB-axis

due to negative aerodynamic roll torque T = r× F rotation

Figure 2.7: The operation of the aerodynamic roll control paddles.

The magnitude of the aerodynamic torque generated by the paddles is related to the surface area perpendicular to the airflow and the length of the moment arm from the COM of the satellite to the COP of the aerodynamic paddle. The calculation of this torque is discussed in more detail in Section 2.4.1. A number of configurations of the control paddles were considered of which only two will be discussed. Simulations were used to compare the various configurations. The total accumulated paddle angle is used as a measure of how active the control paddles are during the simulation. The time it takes the paddles to control the satellite to the desired zero degrees is also considered. In the first design the aerodynamic feathers are located on the sides of the back panel of the main bus as used by Psiaki [8]. When the paddles are rotated to their maximum angle they are not allowed to shadow the airflow to the feathers. This constraint led to the design of very small paddles (40 mm_{×2 mm×100 mm), mounted with an offset along} the YB-axis as shown in Figure 2.8(a). The maximum aerodynamic torque that can be

generated by these paddles is approximately 4_{× 10}−9Nm, assuming that the XB-axis

is aligned with the XO-axis of the ORC frame and that the satellite has an altitude of

500 km.

In the second configuration the aerodynamic feathers were moved to the corners of the back panel of the main bus. This move enabled the use of larger paddles without shad- owing the feathers. The designed paddle dimensions are 50 mm_{×2 mm×250 mm and they} are mounted in the middle of each side as can be seen in Figure 2.8(b). Using the same as-

sumptions as for the first design, the maximum aerodynamic torque that can be generated by this configuration is approximately 2_{× 10}−8Nm.

Z

Y

Z

Y

Figure 2.8: Two paddle configurations: (a) Small paddles at an Y-offset and centred feathers and (b) Large centred paddles with feathers at the corners.

The first configuration required a total accumulated paddle rotation θtotal of 602.9 rad to

control a roll angle of 20◦ to the desired zero roll angle within a time of 2 083 seconds. This is 36.7% of an orbit period and thus a shorter settling time is desirable. The larger control paddles of the second configuration resulted in a rise time of 921 seconds using only 238.4 rad of total accumulated paddle rotation. The rise time is 16.2% of the orbit period.

The second configuration resulted in the shortest rise time along with a 60% improve- ment on the total paddle rotation needed by the first configuration to complete the roll manoeuvre. The decision is made to use the second configuration in this project.

The satellite also makes use of three nano-reaction wheels. These wheels are also located in the ADCS cube and each wheel is aligned with one of the principle body axes. The reaction wheels are mainly used for more accurate control during imaging. The wheel aligned with the YB-axis can, however, be used as a momentum wheel to add gyroscopic

maximum torque of a reaction wheel is 0.1 mNm and the maximum angular momentum 4 mNms.

The three magnetic torque rods are positioned parallel to the principle body axes in die ADCS cube. The torque rods are used for damping of the passive aerodynamic control torque and momentum management of the reaction wheels. The magnetic moment of each rod is 0.2 Am2_{. This magnetic moment M reacts with the local geomagnetic field}

Blocal to create a control torque NM. The basic magnetic torque control equation [25] is

given by:

NM = M× Blocal (2.2.2)

A pulse width modulation method is used to control the on-time of the magnetic torque rods [26]. The minimum pulse width that can be used per sample period is 1 milli-second. The maximum pulse width is restricted to 80% of the sampling period of the controller. This restriction guarantees that the generated magnetic moments of the torque rods do not corrupt the magnetometer reading when it is sampled.