In the case of the tri-spin solar sail satellite the sail and deployment mechanism rotate relative to the satellite bus. An electric motor attached to a pulley can slowly deploy the sail and wire booms and with attached sensors can produce feedback on the state of the deployed system. These electronics need to be placed on the rotating deployment mechanism. The satellite bus contains the power source and onboard computer. Signal and power lines are required from the satellite bus to the rotating system.
Slip rings can produce the connections required by the deployment system. This solution will supply the system with the necessary control and feedback that may be required when deploying a large structure. Slip rings are rated according to a maximum revolution rate and/or by the total number of revolutions. Such an assembly was created for MicroMAS[64], a 3U dual-spinning CubeSat satellite containing a rotating spectrometer as the main payload (see Figure 4.2). The driving interface is constructed by an Aeroflex brushless DC motor with corresponding motor controller plus angular feedback and a 12-wire slip ring to transfer power and data between the bus and the payload.
(a) Complete assembly of MicroMAS (b) MicroMAS internal workings
Figure 4.2 – Dual-spinning MicroMAS CubeSat[7]
Another option is using a wireless and independent module placed on the deployment mechanism. A module containing its own battery power source and processing abilities can be created that is only active during the deployment process. This solution provides full control of the deployment without introducing complex and expensive mechanical connections. A release pin that, when in place, isolates the battery from the deployment electronics can be included. The restriction of this solution is that the deployment must be completed within the period of time that the wireless system has battery power. A comparison between the use of slip rings and wireless modules is summarised in Table 4.1.
Slip rings Wireless module
Limits angular rate No angular rate limit
Increases mechanical complexity No effect on mechanical design Module always active - gets power
from satellite bus
Module active for short period - limited battery power
4.2.1 Active Deployment Modelling
The length of the wire booms will increase steadily during active deployment. The length of the wire will lead to an increase in the moment of inertia of the spinning load. The angular momentum of the satellite must stay constant and thus the driving motor’s speed decreases when the moment of inertia increases. If the rotation rate of the driving motor is kept constant, the angular momentum of the spinning sail or MCS will increase and thus will induce an angular rate on the central satellite body. The angular momentum of the sail structure will be
Hs= Isyyωs
=Isyy0+ 4ms(r + `) 2
ωs,
(4.2.1)
withIsyy0 the inertia of the deployment mechanism, ms the tip mass of the wire boom, r the radius of the deployment mechanism, and ` the length of the wire boom. The driving motor speed dynamics is determined by:
Isyyω˙s= Nm− Nf− ˙Isyyωs, (4.2.2)
withNmthe torque produced by the motor, andNfthe unmodelled friction present in the motor. Equations
4.2.1 and 4.2.2 describe the effect of the deployment on the speed of the motor and on the rest of the satellite system. These equations are used to investigate the effects of an active deployment through simulation. The simulation scenario will begin with the deployment mechanism, at an initial speed of
ωs0 = 3 rev/s. The wire boom will deploy at a rate of 1 cm/s. As soon as the speed drops below0.2 rev/s, a speed controller is activated to keep the motor rate constant. The maximum length of the wire boom is
3.6 mwith tip masses of20 gattached, and the initial inertia isIsyy0= 0.001 67 kg·m2. The driving motor’s
maximum torque is limited to25 mN·mand a bearing friction of1 mN·mis assumed.
Figure 4.3 reveals the results of a simulation of an active deployment mechanism. The inertia increases exponentially and the speed initially naturally decreases. The driving motor controller is activated when the speed drops below 0.2 rev/s mark and maintains this reference speed. The required torque never saturates. The required torque increases linearly during the deployment. When the deployment is
0 50 100 150 200 250 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Time(s) Moment of Inertia(kg.m 2)
(a) Moment of inertia of rotating load
0 50 100 150 200 250 0 0.005 0.01 0.015 Time(s) Motor torque(N.m)
(b) Driving motor torque
0 50 100 150 200 250 0 0.5 1 1.5 2 2.5 3 Time(s) Angular rate(rev/s) (c) Speed of load 0 50 100 150 200 250 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Time(s) Angul ar momentum(N. m.s)
(d) Angular momentum of rotating system
completed, the required torque reduces greatly and stabilises at the small bearing friction torque level. When the speed is kept constant and the inertia increases, the load angular momentum increases exponentially. To conserve the total angular momentum the rest of the satellite body must counter this load angular momentum increase.
4.2.2 Active Deployment Demonstrator
The active deployment demonstrator was built to perform a deployment by means of a wireless module. The aim of the demonstrator was to identify the components that are required to perform the deployment. The demonstrator is not suitable for a flight model and the components that are used for the demonstration are not recommended for the final design.
The deployment can only be performed without a slip ring unit if a wireless link is available. The EZ430- RF2500T from Texas Instruments was used. The small PCB (outer dimensions20 mm × 30 mm) contains a programmable micro controller, wireless transceiver and antenna. The circuit requires24 mWwhen the wireless link is active and2.6 mW when no wireless communication is required. Many other low-power, system-on-chip (SOC) solutions exist that can be investigated for further development. The EFR4D Draco from Energy Micro is a SOC with the microcontroller and transceiver in one integrated circuit (IC). A stepper motor with a reduction gearbox is connected to the pulley, and is driven by stepper motor driver electronics. The driver electronics and stepper motor are not optimised for the demonstrator. Feedback of the rotation angle of the pulley is generated by means of a magnetic rotary encoder. The microcontroller interprets the pulley rotation to wire boom length. A lithium battery is added to supply the needed power to the remote circuit.
The wire booms are wound around a pulley with a diameter of70 mm. All four wires are wound around the same pulley, but each goes individually through a follower. The follower keeps the wires apart and forces the booms to leave the deployment mechanism at each corner. Circular beads and fishing lead sinkers are used as tip masses.
The entire mechanism is attached to a driving brushed DC motor. The driving motor contains a tachometer, which returns the current angular speed. Electronics to drive the motor and to perform speed control are added. The completed demonstrator can be seen in Figure 4.4.
(a) Side view of demonstrator (b) Electronics on demonstrator