Proceedings of the Multi-Disciplinary Senior Design Conference Page 7 s2

Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

Project Number: 11310

Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

PARABOLIC DISH AUTOPOINT SOLUTION

Trae Rogers – ME/Team Leader / Pat Ryan – EE/Project Engineer
Kyle Norlin -- ME / Chris Reed -- CE / Ric Schmeelk -- EE

Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

Abstract

The objective of this project is to create a fully functional prototype parabolic dish autopoint solution for an antenna system used in military SATCOM applications. The prototype will be an improvement upon a currently fielded product used by US armed forces. A lightweight, yet rugged system is to be developed in order to minimize the amount of time needed by military personnel to be exposed to a potentially hostile environment while establishing a communications link. The system will be remotely controlled from a Toughbook laptop system with a GUI for the user to input the coordinates of the target satellite. The system will then direct the dish in both azimuth and elevation to point to the satellite with a very high degree of accuracy. The GUI will provide both control and feedback of the system to the user over an Ethernet connection. Achieving an accurate pointing solution while keeping weight down are the prime concerns of the project. This paper will describe in detail the design, fabrication and testing processes and results. Figures and diagrams are included to facilitate a better understanding of the challenges faced as well as the solutions which were implemented.

introduction

Reliable communications are a necessity on the modern battlefield, whether for requesting assistance or providing up-to-the-minute situational information to command and control staff. Accessing satellite communication channels is essential to ensure a global reach for military leaders to have accurate, timely information and allowing them to make informed decisions based on that information. The specific system under consideration in this project works in the X-, Ka-, and Ku-bands of the RF spectrum, covering a range between 8 and 40GHz. The satellites used are typically found in equatorial, geosynchronous orbits (approximately 22,800 miles above the Earth). Due to the great distance involved, even very small discrepancies in position of the antenna can lead to large errors. This is why an extremely high degree of accuracy is required to establish a link with the proper satellite. The primary objective of this prototype is to demonstrate that such a degree of accuracy can be obtained quickly, and repeatedly.

Due to the nature of their job, military personnel often find themselves in hostile environments. The system currently fielded requires personnel to determine the direction to the satellite and then manually position the antenna. The time spent performing such actions exposes them to greater risks and stress, increasing the chances of making errors. The main objective of this project is to improve upon the current product by automating the pointing process, allowing the soldiers to take cover out of harm’s way. Another important objective is to keep the weight increase to a minimum as the currently fielded system is carried by a single soldier. This prototype will not exceed the carrying capacity of a single service member and allow the same level of portability to be maintained.

Nomenclature

A: ampere, unit of electrical current

Azimuth: as used here, this represents a rotational dimension as in a compass direction

e: error figure, used for calculating physical distance on the targets used for testing

Elevation: measured in degrees from the horizon, at 90⁰, to a point directly overhead, 0⁰

EMI: electromagnetic interference

GUI: graphical user interface, how the end-user interacts with the system

LED: light-emitting diode

RF: radio frequency

SATCOM: satellite communications

T: torque

VDC: volts, direct current

process (or methodology)

Overall system design:

Brainstorming for ideas on the physical implementation of the overall system led to several different possible configurations. Feasibility studies and weighted assessments of the features of the candidate designs were accomplished as part of the design selection process, which ultimately led to the design of the final prototype.

One idea was to simply adapt the currently used system of hand-positioning the antenna elevation mechanism onto a platform which could be rotated by a motor, controlling the azimuth. This concept was deemed overly complex and the requirement that the system be rugged also meant that it could not be susceptible to environmental factors such as windblown particles such as sand getting into areas of movement. Such factors could lead to degraded performance or even system failure.

In the end, two concepts won out over the others and were selected for further analysis and were presented to the customer at the final design review at the end of the first portion of the course. The customers provided valuable feedback and the team learned about some additional difficulties in implementing the first of two solutions which were presented. The latter design, which became the one chosen to build, was not as fully developed, leaving the team with some work yet to do, but not an insurmountable challenge.

The final design selected by the team consisted of a rotating platform, driven by a motor which the plan would be to house within the base unit of the system. Atop this platform rests an enclosed motor and gearbox assembly which will be used to elevate an armature assembly which is designed to hold the components of the reflector portion of the antenna assembly (feed horn and petals). The design is shown in the figure below:

Figure 1: Final system design

Mechanical system:

Chief among the customers needs was the requirement for accurate pointing. This meant that build tolerances would have to be kept very tight. The design would also need to be robust enough to withstand winds from any direction of up to 25 mph. The overall system design was also constrained by a weight limit Backlash would need to be eliminated or at worst, kept to a minimum in order to maintain pointing accuracy. Such design considerations pushed selection of certain mechanical parts, such as the motors and gears.

The torque, T, required (in-lbs) is given by the following equation:

T=ma∙lcga2+md+mf∙lf (1)

Where, in equation (1), the subscript a refers to properties of the antenna arm, d refers to the dish, cg is for center of gravity, and f is for the feed. Early parts searches for the motors found motors capable of moving the amount of weight estimated for the prototype, but such motors were too large and heavy themselves to meet the specifications of the prototype. Therefore, gearing was deemed necessary to reduce the overall torque required by the motors. Gear ratios from 10 to 100 in steps of 5 were then analyzed for the amount of torque provided and for the overall weight of the arm with the antenna attached. Arm weight, also being a concern for the design, led to an open-air type design, which minimized weight while still providing the required strength.

Figure 2: Final arm design

Torque and backlash concerns led to the selection of stepper motors and worm gears. As mentioned earlier, loading from wind also needed to be taken into account. The open, ‘C’-shaped section of the arm shown above in Figure 2 is where the feed of the antenna would be secured by a clamping assembly which could be secured by an operator without the need of any special tools. Figure 3 below shows the results of stress analysis on the arm assembly:

Figure 3: Arm stress analysis

Gear boxes were then selected with different ratios each for azimuth and elevation in order to reduce the torque required by the motors. A higher gear ratio was required for the elevation due to the weight and need for stability.

Based on the required torque, the team selected the MDrive 14 motors from Schneider electric. This unit is a stepper motor designed to operate on a 12-48VDC input voltage with 20 microstep resolutions ranging from 200 to 51,200 steps per revolution [1]. Such resolution is key to obtaining an accurate system.

Electrical system:

Deployable military systems such as this are often utilized in environments where constant electrical power is either inconsistent or not available at all. Therefore the system had to be designed to be able to run on currently available power, i.e., batteries carried along with military personnel. The system was required to run on 12VDC drawing no more than 6.0A, keeping total power consumption below 150W.

In most such systems, complex control circuitry would have to have been designed in. The motors selected included their own on-board motor controllers, which limited the amount of electrical design which needed to be done. Routing of power and control signals, as well as monitoring for overcurrent conditions were of course still necessary. The power budget was some cause for concern in the early design phase before weights and required torque could be adequately estimated. Examination of the motor specifications gave the team relief as the motors are rated to draw only 1.0A of current. The motors were anticipated to be the largest draws of electrical current in the system, and operating only one motor at a time will serve to keep the overall current draw to a minimum. The simple act of connecting the control circuitry to a power supply capable of displaying current draw was enough to show that contribution was in the neighborhood of 300mA. Even in a case where both motors were to run simultaneously, the maximum current draw of the entire system would still be less than half of the maximum value listed in the customer requirements.

Some electrical work still needed to be done though, as the GUI would need to interface with the microcontroller which would handle the computation of a pointing solution based on inputs from the user, as well as controlling each motor separately. An interface board was designed and built which incorporated two RS-422 driver chips as well as an inverter chip to process the control signals from the microcontroller.

Figure 4: Early schematic

The schematic shown above is one which implements an external microcontroller and also has provisions for shaft encoders to determine the absolute position, both azimuth and elevation, of the arm assembly. The power regulation shown in the schematic is representing circuitry which in the design is based on the development board which also holds the microcontroller.

In the physical casing which houses the azimuth motor and gearbox, it was found that there existed ample room in which to incorporate both a circuit board containing the microcontroller (along with supporting circuitry) as well as the aforementioned interface board, which was laid out on a piece of proto-board and wired according to the required inputs and outputs running between the microcontroller board and the motors. For the prototype work, a development kit microcontroller board was used. In the case where the customer decides to implement this design, the interface circuitry and the microcontroller would likely be place on a single circuit board which could be easily mounted within the motor housing. It is possible the customer would be able to implement this solution in even a smaller form factor than what the team has designed.

Despite confidence that the system will not exceed rated current draws, it was requested that overcurrent protection be included in the system in order to prevent catastrophic damage in the case of some sort of failure. A power kill switch and external LEDs were installed along with the circuit breaker as a means of providing means for manual control and monitoring of the system by its users.

Interface system:

The customer requested a simple to operate graphical user interface (GUI), which gave the end-user full functionality, was descriptive and provided relevant feedback. The GUI should operate over either a serial or Ethernet connection. It would need to allow the user to input target coordinates, give controls for deploying and stowing the antenna, and also have a method to stop operation at any point.

The GUI has been developed using Java programming language to meet all of these requirements as well as to give indications of various fault conditions; one for each motor, battery condition, as well as a tilt sensor for system being level. This latter condition is not currently implemented in the prototype, but is something which the customer will likely install in a final product design. This aspect was one which was deemed by the customer as a nice to have feature, and is something for which the design leaves room for future implementation and integration.

A screenshot of a working version of the GUI is provided in the figure below:

Figure 5: Early working model of GUI

The GUI continued to be updated, both in features and in the layout as work progressed, requirements were reviewed and clarified, as well as making cosmetic changes. For instance, the customer requested that the terms, “point” and “tear-down” be changed to “deploy” and “stow”, respectively. Also added was the ability to ‘jog’ the system, or move in very small increments. This feature could be used in manual fine-tuning, even though the customer had said that an automated fine-tuning solution would likely be implemented in a final product, using a box-search method to locate the satellite. Further updates were made to give the user indications about system health, specifically informing the user about supply voltages and whether or not the system was moving. Fields for current position were also added. A ‘STOP’ button was kept to allow for an emergency interrupt of the system, should such a need arise. The system should also be able to sense when it runs into excessive stresses due to some mechanical failure or other unforeseen circumstance which inhibits motion, thus threatening to damage the unit. In such cases, the system should be able to recognize this and shut down.