Archive for the ‘Solar Panel’ Category

Solar Array Deployment

At launch the satellite solar panels are folded against the body of the satellite (stowed configuration) to minimize space and allow the satellite to fit into the faring of the launch vehicle. Once the satellite has been placed into Geosynchronous orbit the ACS system is activated to point the proper axis of the satellite at the earth. This process is called ACS initialization and Earth capture activation. During this period the satellite is being powered by the batteries. Once the Earth sensors are activated and the ACS system is locked onto the Earth, the solar panel release mechanization can be actuated to allow the arrays to unfold and lock into a deployed position. With the arrays deployed the solar array drive system is activated and commanded to track the Sun. The following animation shows the release of the North panel, then the South panel, followed by the positioning of the panels to point directly at the sun.

This would be the worst case condition where the arrays are pointed 180 degrees away from the Sun, the arrays are typically deployed at a time of day that allows them to be pointed at the Sun so they only require minor pointing changes to peak their power output. With a 180 degree pointing offset as shown the arrays would be commanded to move in opposite directions (one clockwise and the other counter clockwise) at the same time. From the ACS control standpoint the torques on the body of the satellite would relatively cancel and minimize the error correction required by the ACS system. After the the arrays are peaked on the Sun the Solar array drive system is commanded to normal tracking mode to maintain this pointing through a full 360 degrees rotation over the course of the day. As soon as there is adequate power being generated from the arrays, the power load will transition from battery to array power and battery charging can be started.

Solar Array Drive

Solar Array Drive (SAD) refers to the system that positions the solar panels on the satellite to efficiently convert s the sun light into electrical energy. This system is comprised of the Solar Array Drive Assembly (SADA) and the Solar Array Drive Electronics (SADE).

The SADA includes the stepper motor, drive gear assembly, Barring And Power Transfer Assembly (BAPTA) and the mounting hardware required to mount the solar panel to the body of the satellite. The solar panel is mounted to the SADA and allowed to move by means of the barrings in the BAPTA. It also includes a slip ring assembly used to transfer the power generated by the solar panel to the EPS bus. The drive gear assembly is installed to smooth the step movement of the motor and provide a steady constant travel through 360 degrees of rotation. Stepper motors are known for their ability to provide high torque and controlled movement, this makes them ideal for this application. Rather than mounting two motors in the assembly, to increase the reliability these stepper motors are designed with redundant coils, this effectively reduces weight and provides improved redundancy.

Each SADA is controlled by independent Solar Array Drive Electronics (SADE) unit that provides the interface between it and the EPS subsystem or flight computer. It is comprised of all of the circuitry required to generate and shape the pulses needed to drive the stepper motors. Telemetry monitoring sensors collect data such as current, temperature, position, speed, rotational direction of travel, step counts, and status indicating movement or stopped for the SADA.

In operation the rotation of the solar panel is started when the SADE is enabled and step pulses are received, movement stops between steps or when it has been disabled. Control of the direction of travel, can be commanded to rotate in either forward or reverse direction to allow for pointing position optimization. The SADE is normally configured to output a consistent train of pulses at the required intervals to maintain movement through the 360 degree rotation called Normal Mode. With the panel rotating less than 15 degrees each hour it is easily commanded at less than 5 times a second allowing for rest intervals between movement. Speed and torque can be adjusted by increasing or decreasing the rate of pulses per second (pps) sent to the motor. If the motor is normally driven at 5 pps then driving it at 25 pps will increase the speed by a factor of 5. This system is typically design for the solar panel to complete a full 360 degree rotation in less than 15 minutes at the highest pps command rate. When commanding the movement of the SADA, commands are processed by the SADE, stored and then pulses are sent at a 5 pps or 25 pps rate until the complete commanded step count is reached. At completion the SADE must be commanded back to the Normal Mode or transitions back automatically.

Satellite Solar Array positioning

As a satellite travels along the orbital path the Solar Arrays must remain pointed at the Sun to produce power. This is accomplished in a number of methods based on the satellite design. Satellites with fixed arrays must point their surfaces at the Sun or spin around the axis that allows them to rotate into the Sun light. With movable solar arrays they either actively track the Sun or are driven at a constant rate to maintain their pointing.

Spin stabilized or 2 axis satellites typically have fixed solar panels and positioning is achieved by the orientation of the spin axis of the satellite. Attitude or Spin Precession Maneuvers are periodically preformed to maintain the spin axis alignment.

Solar arrays on a 3 axis stabilized satellite track the Sun by the implementation of positioning mechanisms that either step the arrays or actively position them. To simplify the tracking system a geared drive system is driven by stepper motors that are selected to provide the required torque and move the array smoothly. The number of steps are calculated using the angular step size of the stepper motor, the gear ratio, and the angular change needed. This is used to generate the required number of pulses per second. Pulses are generated in the solar array drive electronics then applied to the motor to allow them to complete one complete 360 degree revolution a day. To increase accuracy the sidereal day measurement of approximately 23:56:04 hours is used. These systems can provide accurate positioning with minimal operational requirements. Over time periodic adjustments are made to minimize any tracking errors and optimize positioning and output power.

Active positioning systems also employ the use of stepper motors and the output of the solar array is sampled and converted to pulses that are applied to the stepper motor to maintain the peak output power. One important characteristic of this system is that it can not be used during eclipse periods, during these periods the system normally reverts to a stepped mode and the active control is resumed when the satellite returns to full sunlight. When these systems are fully optimized it significantly reduces operational intervention to maintain peak power. Failures in active tracking systems can lead to a rapid loss of power and must have antiquate procedures or automated sequences to prevent the arrays from being driven off the Sun.

Solar Array basics 3-axis

The solar arrays are designed to provide power based on the load requirements of all units including the payload operating on a 3-axis satellite. To determine the load all operational configurations are assessed based on the units that will be powered on and the power they require. Solar array efficiency is maximized when the Sun’s angle of incidence is at a 90 degree angle to the panel, therefore the maximum power generated will be at equinox and the minimum power produced will be during the solstices. During eclipse periods, power is supplied by the batteries, this will require additional power for battery charging post eclipse. Eclipses occur each day over a period of approximately 45 days centered around the equinox. The Power budget is developed for each configuration and the solstice period efficiency and maximum eclipse recharging requirements are used to determine the power required from the solar arrays during the worst case. A margin is applied to account for potential damage and solar cell degradation to ensure antiquate power is available throughout the mission life.

Solar arrays are comprised of a number of panels and each panel is broken down into a number of strings. Starting with the strings, they are made up of a number of solar cells connected in series to obtain the necessary voltage. For example if you require 36 volts and each cell produces 0.5 volts, then the string would have 72 cells connected in series (in series voltage adds and current remains the same). To protect the string from a damaged cell or during eclipse diodes are installed. By connecting strings in parallel (the current adds and the voltage remains the same) the output current is increased. By use of Ohm’s Law, you can use the voltage and current to determine the panel’s output power generated. Panels are then added in parallel to achieve the total power needed.

Any excess power generated by the solar arrays is controlled by the voltage regulator and power distribution section of the EPS subsystem. Based on design a number of approaches have been taken to address this, ranging from shunting the excess current, to the use of switching transistors that add or remove strings connected to the power bus.

To maximize performance the solar arrays must maintain pointing at the Sun. Solar array drive motors are used to actively track the Sun or can be stepped to keep pace with the Sun. The drive motors are connected to the solar array drive electronics providing an interface for control by the EPS subsystem, ACS subsystem or the Flight Computer based on design.

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INFORMATION

Shining light on satellites and how they operate. Drawing from over 30 years of knowledge and experience in all phases of the life of a satellite from concept, to operations, and through end of life. You will find short topics intended to give you an understanding of how they work, the general concepts, and principals used along with information on ground systems. There is also a section dedicated to topics that can be used as basic concept training along with links to animations and 3D models I have created.