Autonomous Sun Tracker

Jacob HalsteadComputer Engineering

Patrick ChiuElectrical Engineering

Craig BishopMechanical Engineering

Harrison SpragueMechanical Engineering

Jason YehElectrical Engineering

Tyler NicholsonElectrical Engineering

Kaung Thu (Victor)Electrical Engineering

ABSTRACTThe sun and other celestial bodies are still a great mystery to astronomers and scientists. The purpose of this

project is to help the study of the solar system, more specifically the sun. The sun emits radio-frequency signals during solar activity typically on the scale of 120 - 140 MHz, and by autonomously tracking the sun as it moves across the sky, it can be monitored for further study. The system moves with an azimuth and declination system controlled by a slewing drive and a linear actuator, respectively. A software program called Radio Eyes interfaces with our system and provides it with the coordinates of the sun, and this data is processed and translated into usable data for the motors. Hall effect sensors in each of the driving mechanisms give a reliable pulse count, which is then translated into linear or radial distance. The mechanical design of the system allows for tracking of the sun at any point in the sky, and additionally any other celestial object whose coordinates are known.

INTRODUCTION An autonomous solar tracking antenna is a device that accurately tracks the sun and collects RF emission data.

This allows for valuable data to be collected by the antenna throughout the entire day without human intervention. This data is collected all over the world and sent to a central server in Switzerland. The nearest stations are located in Alaska and in England, so there is currently a blind spot for RF data collection on the east coast of the United States. To resolve this issue, a 7 foot dish is to be installed in Ionia, NY. At the start of this project, there was a 7 foot C-band satellite dish receiver installed in Ionia, mechanically designed to allow it to track along the right ascension axis with a declination manually controlled by a bolt. This same system was intended to be used to autonomously track the sun for the purpose of RF collection. It was decided by the team that the mechanical design that the system offered was not sufficient for full tracking of the sun throughout the full course of its movement throughout the day. Due to this, an altitude and azimuth mount was chosen to be the basis for the design, with a linear actuator controlling the altitude and a slewing drive controlling the azimuth.

The main requirements of this project were to autonomously track the sun, regardless of outside variables. Since the system will be installed outdoors, and exposed to the harsh Rochester climate, it is paramount that it can withstand any of the weather conditions that it may encounter through the course of its life. The system must also operate on a day-to-day basis with minimal interruption and virtually no human interaction.

PROCESS The basic requirements of the system include: 210° of rotation around the azimuth and 52° of altitude change

(20° to 72°); ability to operate autonomously in all weather; designed to prevent damage from over-rotation/extension of motors; and able to resist backlash from the motors from reverse loading. The system is also required to hold to 2° of accuracy, and the operation of the dish should not interfere with data collection.

The method for tracking the sun is through the predictive path program called RadioEyes. RadioEyes reports the sun’s position to the drivers that operate and move the dish. The whole system can also be powered off an uninterruptable power supply (UPS) to prevent data loss in the event of a power failure. Although the UPS has not been implemented by the completion of this project, space has been provided for the customer to install one.

During the design phases of MSDI several system designs were investigated and it was decided that the system would use a slewing drive to control the azimuth and a linear actuator to control the declination. This would provide the most stable operation and the most precise movements. The two motors would be controlled through a motor shield and Arduino mega 2560 Rev 3.

Mechanical Design

I. Range of MotionThe original dish was designed with a right ascension (RA) and declination (DEC) type bracket which

allowed for the dish to easily be pointed at different geosynchronous satellites. The drawback of the original bracket design was that it limited the area of the sky in which the dish could look. As an example, if the dish was nominally facing south, the dish would not be able to view anything in the sky to the north (beyond true east and west). The dish would essentially be limited to 180° of vision. For locating geosynchronous satellites this type of mount works great, but when tracking celestial bodies like the sun, the seasonal changes can cause objects to set farther north (after the vernal equinox and before the autumnal equinox). It was calculated that using an RA-DEC mount would compromise data collection for 3 months out of the year, where compromised data collection is classified as any time that the sun is visible in the sky to a bystander but the dish cannot be aimed at it. The solution to this problem was to switch to an altitude-azimuth mount (Alt-Az) which would allow a near-360° view of the sky.

II. Loading and Motor SelectionLoading calculations for the dish were based on worst-case operating conditions. This included strong,

steady winds of 49 MPH and an estimated ice-load of 100 lb. Below 50 MPH winds, the mesh dish experiences around one third of the drag that a solid dish of the same size would. Above 50 MPH winds, the mesh dish acts like a solid dish and the loading increases significantly. To protect the dish from damage and excessive loading in winds above 49 MPH, it was agreed with the customer that the dish would shut down and not operate during such conditions.

Motors were picked that could provide more than enough torque under normal operating conditions, but also be able to perform well under heavy loading. The linear actuator can provide 400 lbs. when running but withstand more when statically loaded. The worst case calculation for the LA showed force should not exceed 350 lb. The slewing drive provides an order of magnitude more in torque than is needed but this is a consequence of another requirement being met. The slewing drive must absorb a large over-turning moment (the force of the dish trying to rock forward when tilted towards the horizon), and this happens to be the type of loading in which the slewing drive is the weakest. The slewing drive picked for this project was the smallest that met all requirements, and can withstand roughly 2000 ft-lb of overturning moment and generate 5000 ft.lb of torque. Overturning moment in worst case conditions could total 1000 ft-lb, and a holding torque requirement of 350-400 lb. The motors also had to be resistant to the elements, and both the linear actuator and slewing drive are rated as water-proof and rated for outdoor use, so this requirement was indeed met.III. Failure Modes and Safety Features

In the case of extremely high loads or malfunctioning software where failure is inevitable, the system needs to be protected. Various failure modes were determined and solutions were implemented to reduce or prevent damage to the system. One potential failure is if the linear actuator tries to provide more force than it is rated for which could cause the motor to fail. To prevent this, a pair of shear pins were incorporated into the bracket it connects to. These shear pins were meant to fail at a load lower than the maximum load that could be provided by the linear actuator. Due to additional considerations for potential wind and ice loading, however, these shear pins were made thicker. Currently, these shear pins do not serve the purpose of breaking before the maximum load could be provided by the linear actuator.

In order to prevent the system from colliding with itself or tearing out the wires for the system, hard-stop limit switches were implemented. The linear actuator comes with two built in limit switches. Upon full extension, the limit switch prevents the bracket from colliding with the slewing drive. The other switch is hit upon full retraction. The mechanics of the system allow for less retraction of the linear actuator than the linear actuator is actually capable of. To prevent potentially harmful stress on the brackets or on the linear actuator, an additional external limit switch was implemented. On the adapter plate, a pair of limit switches were mounted that shut off power in the direction that they were hit from when the slewing drive tries to rotate past them. This prevents the slewing drive from rotating more than 360° and tearing out its wires. Each of these limit switches provide a series break in current flow for the motor, and therefore offer a near fail-proof way to ensure that the motors will not extend past their desired movements. These implemented solutions address the foreseeable ways in which the 3 moving components could collide and/or fail.

IV. Mechanical ComponentsA. U Bracket

The U Bracket consists of 3 welded plates that form a U. It is bolted to the slewing drive and it provides the pivot point for the altitude adjustments. The bolt pattern on the bottom is non symmetric so that the LA Mounting Arms do not collide with slewing drives motor.

B. Static RA ArmSince the original dish bracket was meant for tracking satellites and is being re-purposed as an Alt-Az mount, the Static RA Arm was implemented to prevent the original bracket from rotating and keeping the components perpendicular. The arm is simply constructed from L-bar with 2 holes.

C. Pipe FlangeThe pipe flange is the component that mounts the rest of the system to the existing pipe in Ionia, NY. The component is slotted with 2 ears welded to it so that it can be firmly clamped around the pipe. There is also a plate welded to the bottom of it so that it can connect to the rest of the system.

D. LA Mounting ArmThere are 2 LA Mounting Arms which hold the linear actuator at the appropriate distance and angle. They are simply rectangular pieces of steel with several holes. They are thicker than many of the other components to prevent deflections since the arms are approximately 2’ long.

E. LA Mounting BlockThe mounting block sits inside the original bracket and connects to the linear actuator. In order for the linear actuator to generate the required altitude angles, the pivot point could not be in the location of the existing holes which is why this piece is necessary. Additionally, it allows for the use of two small shear pins which are discussed in “Failure Modes and Safety Features” Section.

F. Shear Pin SleeveThe shear pin sleeve fits inside the head of the linear actuator and has two small holes so that shear pins may be used to connect the linear actuator to the LA Mounting Block

G. Adapter PlateThe adapter plate allows the Pipe Flange to connect to the Slewing Drive. This component is necessary because the hole pattern on the slewing drive is approximately the same diameter as the pipe.

Electrical DesignI. Noise Filters for Hall Signals

In order to achieve tracking precision, capturing quadrature hall signals which are position feedback from motors must be carried out without errors. Both rising and falling edges are counted to achieve 4 times the resolution compared to counting only rising edges of the hall signal.

Since hall signals for both motors have approximately 80 Hz at full speed, a simple low-pass filter (LPF) was designed as shown in Figure 1.1. The corner frequency of the low-pass filter is governed by Eq (1.1). For calculation, the hall signal frequency is assumed to be 100 Hz. The parameters of LPF are shown in Figure 1.1 and Eq.(1.1).

f = 12 πRC

=106 Hz . (1.1)

Figure 1.1 - Simple Low-Pass Filter with Bandwidth of 106 Hz In addition, D flip-flops are used as an interface between microcontroller interrupt pins and hall signals.

The D flip-flop interface digitizes hall signals by sampling at the edge of the clock, eliminating false interrupts generated by noise.

The chance of getting pulse count error is significantly reduced since two conditions have to be met:1. Noise pulse width has to be larger than setup and hold time which is typically 22 ns (assuming D

flip-flops are powered by 5 VDC source or microcontroller) in order to change the output of the flip-flop.

2. The noise has to appear at the rising/falling edge of the clock and during setup and hold time.

To sample hall signals without aliasing, the minimum sampling frequency has to be twice the frequency of hall signals. However, to count both rising and falling edges (keeping 4 times resolution) using either rising or falling edge-triggered D flip-flop, the frequency is again doubled. As a result, clock needs to be 4 times maximum frequency of hall signals (Eq. 2).

Cloc kan edge trigger=4∗f Hall Signal=400 Hz (2)

According to ElectronicsTutorials website, a NAND gate astable multi-vibrator can be created using NAND gates to provide the clock for D flip-flop [1]. An 800 Hz clock is chosen to allow the use of different motors and hall sensors which has the frequency of more than 80 Hz.

II. Reference Limit Switch Interface

For the slewing drive motor, there is a reference limit switch which is being pressed by a rod attached to the moving part of the motor when it is facing south. This event is used for day-to-day calibration purposes. There is a need for electrical hardware solutions to capture when the switch is pressed, avoiding complexity in software and mechanical design. The response of the interface is captured in a waveform and flow chart as shown in Figure 1.2.

Figure 1.2 - Flow Chart and Output State for Reference Limit Switch Interface D, S, R and Q represent data, set, reset and output respectively. Set and reset pins are tied together and

controlled by the microcontroller. The output state of reference limit switch interface is shown in Figure 1.2 before and after the switch is pressed and the microcontroller acknowledges it.

III. 12 VDC Power Supply Filter

A 12 VDC power supply filter was designed with the layout designed in KiCAD to reduce switching noise from the 120 Watt power supply which is connected to motor shield.

IV. SensorsAccelerometer (MMA8452Q)The accelerometer provides position feedback apart from hall signals. A warning message can be sent to

the user if there is a discrepancy between current angle of the dish in elevation computed from hall sensor feedback and accelerometer angle. This provides an additional measure of calibration that further increases the robustness of the system.

V. I2C Buffer for Accelerometer (LTC4315)An inter-integrated circuit (I2C) buffer is used to provide noise filtering for the microcontroller collecting

data from the accelerometer. Using the buffer, events such as the accelerometer being disconnected or faulty and the bus being stuck can be detected. The accelerometer can be disabled by the user in the software by driving the ENABLE pin LOW.

VI. Two 360-Degree Limit SwitchesTwo hardware limit switches are installed in the motor power line to limit rotation of the unit to 360

degrees in order to prevent inflicting damage to the wires. Two diodes are placed in parallel to each switch to limit the direction of current flow which effectively limits the rotation of the dish. It is these diodes in unison with the limit switches that allow the system to hit a limit switch while spinning in one direction and still be able to have current flow sufficient to allow it to then rotate in the other direction. This operation is detailed in Figure 1.3.

Figure 1.3 - Two 360-Degree Limit SwitchesVII. Kill Switch

A kill switch is placed in between the motor shield and 12 V power supply. This allows the user to stop both of the motors from receiving power if necessary.

Software DesignI. Arduino Software

A. The Arduino provides motor control each motor, sensor feedback on the motors, current, and accelerometer. Depending on various defined/initialization values used, the Arduino will communicate to the driver software via serial communication, specifically USB, to inform the driver software of its state.

B. CommunicationI.B.1. Protocol

I.B.1.1. Communication is handled primarily via the Arduino Serial library. Using the provided functionality, a communication protocol was designed so that it is easy to add in extra parameters without having to worry about order of parameters. It also allows for some non-critical parameters to be omitted so that the process loop can iterate faster or if functionality is lacking, like a cable is cut and is not going to be replaced.

I.B.1.2. The Arduino also communicates back to the driver using the same communication protocol. Most Arduino communication is asynchronous, but can have a reference id defined for driver functions that need to wait for their communication to be processed.

I.B.2. FunctionsA short list of functionality is defined. The functions allow for the user to initialize the Arduino, move motors to a given position, or get state information about the Arduino.

C. Motor ControlI.C.1. There are two motor control algorithms: calibration and general. Though they are very

similar in most functionality, they are separated into two separate systems for simplicity of added additional functionality to one without affecting the other. Both systems require that the max runtime and duty cycle of each motor be defined before they can be run.

I.C.2. CalibrationThe calibration system runs the given motor at the defined speed as long as the system is in calibration mode. There is no speed alteration in this mode as the system is still being learned during calibration. It is up to the user to provide smart control of the motors when getting near critical junctions. In calibration mode as well, the motor position and accelerometer values are reported to the driver every iteration if the motor position changed. State communication functions work as well in calibration mode.

I.C.3. GeneralThe general system behaves similarly to the calibration mode except it primarily works of positional error. In order to move the motor, the user must request a position that is not where the motor is. The control algorithm then determines the speed so that it ramps up when starting and slows down the closer it gets to the required position. If the system tries to run at a speed slower than that required to move the motor, the speed is then increased regardless of how far the motor is from its destination. Also, to avoid oscillation, whenever the motor has to reverse direction from its previous speed, the acceptable error is increased. This acceptable error is reset every time the motor starts up. As such, the motor may oscillate trying to achieve the position, but will eventually settle down as close as it can get to the position.

D. SensorsI.D.1. Current

An optional max current is defined for each motor. An error is reported if the current exceeds the max current. The motor that exceeded max current will then cease running until the next time it is told to run.

I.D.2. Hall Effect FeedbackThe hall effect sensors on the motor are interpreted using interrupts and reading the sensor values to determine which direction the motor is running. It then modifies a variable to keep track of where it is.

I.D.3. AccelerometerThe accelerometer sensor is another optional sensor. If included, when the position is within acceptable error, the angle read by the accelerometer will be reported. The accelerometer adds approximately 5ms to the Arduino processing time.

I.D.4. PowerPower is sensed by running the motors at a sufficiently low speed that they do not move, but they pull current. They are run for a very short amount of time. Using this method, it can be determined for each motor independently if they have lost power or hit a limit switch.

II. Driver SoftwareA. The driver implements the ASCOM interface for telescope control. This provides a driver that is

implemented by many client applications for telescope control. ASCOM can be effectively separated into 6 parts: setup, information access, positional movement, motor rate movement, pulse guide movement, tracking movement.

B. SetupSetup handles gathering the required values to appropriately run the motors without issue. The user must provide the required values, or must run the calibration system to determine the required values. If a value changes that is dependent on by a later value, the later value is reset. This is so no mistakes are made when attempting to interpret the user values if there are multiple options.

C. Information AccessASCOM information provides details about the system such as the aperture diameter of the dish, the current position of the dish, or information about the driver. In general, these values are defined in the setup and then reported through the interface.

D. Positional MovementPositional movement is such that given an Alt- Az pair, or a RADEC pair, the system goes to that position. RADEC requires that the system also be in tracking functionality.

E. Motor Rate MovementMotor rate movement runs the given motor at a given rate. The rate is defined in degrees per second.

F. Pulse Guide MovementPulse guide movement runs the system in a given direction at a given rate for a provided duration. Directionality most closely relates to Alt- Az. The rate is defined in degrees per second.

G. TrackingTracking runs the system at the given RADEC rates.

RESULTS AND DISCUSSION

Electrical DesignI. Noise Filters for Hall Signals

Figure 1.4- Slewing Drive Hall Signals without filtering (left), with shielded cables (middle), and D flip-flop (right)

After implementing all noise filtering methods, the running of the slewing drive motor no longer causes the linear actuator pulse count to increase even when it is not running and vice versa. The noise is no longer an issue for the microcontroller since the motor rotates the same angle for a specific number of pulse count without degree of error.

II. Printed Circuit Board DesignAll the electrical design mentioned above are implemented in three PCB boards. The three PCBs have:

1. Low-pass filter, D flip-flop interface and reference limit switch interface 2. 12 V DC filter3. I2C Buffer for Accelerometer (LTC4315)

Each board is designed to meet individual requirements. For board 1, zener diode arrays are used to provide IO protection for the microcontroller. For Board 2, multiple ground vias and large traces and copper plane are employed for large inrush current and current flow drawn by the motor shield from 12V power supply when the motor starts running and continue running. For Board 3, it is designed as a shield to the microcontroller.

III. Ambient Noises and Effects of DC Motors on RF Capture Initial concern was raised about the system’s ability to continuously measure radio frequency data from the

sun while simultaneously operating its two DC motors. Operating a motor causes a certain level of noise, and depending upon how sensitive the system is, this noise could affect the data captured from the sun. If this noise proved to be too severe, then mitigation of the problem was chosen to be pausing data capture while operating the motors. Since the motors only move once every few minutes for brief periods of time, this loss of data may not have been sufficient enough to raise concern.

A study was conducted using a simple antenna set up in close proximity to the linear actuator motor. The experiment took place in a laboratory at RIT, and therefore was prone to a lot of ambient noise from the greater Rochester area. The figure below shows the data capture from this experiment.

Figure 1.7 - Data Capture of Experiment to Determine Motor-Induced NoiseIn the figure above, there are three different plots of data. The blue line was a capture done previously in

Ionia, the location of the dish. The pink line is the data plot of the frequency gathered with the antenna in close proximity (about 1 inch) from the motor of the linear actuator while in the RIT laboratory. Finally, the yellow line is the data gathered in the laboratory without any DC motor running.

It is clearly seen that the location used for the test was much noisier than Ionia, however it is apparent that the data gathered with and without the motor noise is very similar. This indicates that operating the DC motors does not provide much for ambient noise. Furthermore, there will be a dish with a high front-to-back ratio in between the receiver and the motors, introducing even further amounts of ambient noise immunity to the system. It can be declared, then, that there is no need to pause RF data capture in order to move the system.

Mechanical DesignI. Range of Motion

The range of motion of the final system is 7 to 353 degrees of rotation and 10 to 74 degrees of altitude. These values exceed the requirements needed to track the sun. The rotation is limited by two limit switches that prevent over rotation of the system. The altitude adjustment is also limited by one internal and one external limit switch that prevent over extension and collision respectively.II. Mechanical Components

All of the mechanical components mentioned above fit together and perform their intended function. The system has been successfully assembled on site and is able to track the sun. One important note is that shims were required in between the existing pipe and the pipe flange component. The pipe flange was slightly oversized but with the use of shims, it fits securely over the post. A bolt will also be put into the post to ensure the pipe flange does not rotate under high loads.

CONCLUSIONS AND RECOMMENDATIONSWithin eight months of the project, students from three different departments collaborated to solve issues

and deliver customer and engineering requirements (CR and ER). Through the process, it was a great learning experience and a challenge which required applying both knowledge obtained through course study and co-op experience. Due to the somewhat modular design of our system, there is the possibility for more features to be added to the project in the future. One of the ‘Nice-to-have’ features in ER requirement that was not delivered to the customer is joystick control for manual movement. There are still many I/O ports available on the Arduino, and therefore there can be externals added to the system in the future such as a local weather station, a temperature/humidity sensors, or other useful items. I. Mechanical Recommendations / Future Work

There are a few suggestions that would help improve the mechanical system. For starters, the component that holds the linear actuator head could be redesigned. Instead of using the 2 shear pins, one pin could be used. This is due to the fact that the shear pins will likely not break under any foreseeable loads and they are difficult to install. By replacing them with one pin, the system would be even stronger and easier to assemble. Another suggestion would be to standardize the hardware. Currently, there is a wide range of both English and metric bolts. This is confusing when assembling the system and requires a very large amount of different tools. By sticking to one unit system and bolts with the same type of head, the system would be much easier to install. A final suggestion would be to create a new pipe flange. The current pipe flange is slightly oversized and requires shims. By creating one that is a tighter fit, it would grip the post better and create a more rigid system.II. Electrical Recommendations / Future Work

While all of the system requirements were met there are many areas in which the electrical system can be improved. The majority of the improvements are centered on design-for-test and ease of assembly. Assembly of the system involves several steps across many different components. This is due to the addition of components throughout the build and test phase that were not expected. If the design of the system were to be improved, all components would be condensed into a single custom PCB. The ideal design would be a “shield”, much like those available for Arduino, which would sit on top of the motor shield. A second improvement that is compatible with the first is the addition of test ports that would allow for a soft-test. These test ports would identify that the system is set up correctly in an easy to detect manner, whether it be through physical indications like LEDs or interactive warnings through software. This would be very convenient for assembly and debug of the system, especially for the user.

REFERENCES

[1] Storr, Wayne. "Multivibrators including Monostable, Astable and Bistable." Basic Electronics Tutorials. Wayne Storr, 29 Aug. 2013. Web. 15 Apr. 2015.

ACKNOWLEDGMENTS

The team would like to thank the following people for their help in the design of the Sun Tracker system:● Professor Slack, the guide for the team and SME for the EE team.● Mr. Pepe, the customer for the Sun Tracker Team.● Dr. Hopkins, SME for the EE team.● Dr. Leipold, SME for the ME team.● Dr. Landschoot, SME for the ME team.● Mr. Hanzlik, a guide for other teams, but was kind enough to give our team mechanical

feedback.

● Dave and Rob in the machine shop, who gave the ME team materials, machining, and design feedback.