Multidisciplinary Senior DesignKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P15571

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

ABSTRACT

The 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 tracking the suns movements it can be monitored for further study. The system will move with an azimuth and declination system and a software program called Radio Eyes will interface with our system and provide it with the coordinates of the sun. A slewing drive is used for the azimuth movement and a linear actuator is used for the declination movement. The final product’s main objective is to be able to track the sun and ideally, any other celestial body can also be tracked for further study.

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. Currently, there is a blind spot for RF emission data on the east coast of the United States. A 7 foot dish in Ionia, New York has been installed to cover this blind spot. However, the current design does not provide autonomous tracking and instead requires manually adjustments for both seasonal changes and daily tracking.

The purpose of the this project is to control the aiming of the antenna towards the sun through the use of a mechanical system that can be operated both autonomously and manually. The end result of the project is a fully functional system that can collect RF emission data year round. The designed system must work with the dish currently installed in Ionia, New York and handle the weather conditions faced in that area.

PROCESSThe dish must be able to rotate 210° around the azimuth and change the altitude from 20° to 72° at minimum.

The Dish must be able to track the sun's path without any human interaction and must be resilient to the elements including rain, snow, wind, etc. There must be a control or limit on the declination and rotation of the system to

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prevent over extension or rotation. The system must be able to resist backlash from the motors from reverse loading. The system should have a backup power supply that will keep it powered for 2+ minutes after a power outage occurs. This time will be used to save data and shut down properly. The system must have low noise and must not interfere with the data collection mechanism on the dish. The dish must be accurate to within 2° of movement and must be able to be controlled from Radio Eyes software.

Several designs were suggested but it was decided that the sun tracking 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 will be controlled through a motor shield and Arduino mega 2560 Rev 3.

MECHANICAL DESIGN1. Range of Motion

The 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 pure 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 (anytime 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 would occur anytime 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 a altitude-azimuth mount (Alt-Az) which would allow a 360° view of the sky.2. Loading and Motor Selection

Loading 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 begins to act 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 lb 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.

Additionally, the motors had to be resistant to the elements. Both the linear actuator and slewing drive are rated as water-proof and rated for outdoor use. 3. Mechanical Components

A. U BracketThe 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 Arm

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There 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.

4. Failure Modes and Safety FeaturesIn 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 implemented into the component that holds the linear actuator to the bracket it connects to. These shear pins are rated to fail at a load lower than the maximum load the linear actuator can provide. This causes the system to fail in a predicted manner that causes much less harm to the system and is significantly cheaper and easier to repair.

In order to prevent the system from colliding with itself or tearing out the wires for the system, 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 but the system does not allow for full retraction which required an additional limit switch to be installed into the bracket to prevents the bracket from colliding with the linear actuator. 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. These implemented solutions address the foreseeable ways in which the 3 moving components could collide and/or fail.

ELECTRICAL DESIGN1. 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 resolution compared to counting only rising edge of a hall signal.

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

f =1

2 πRC = 106 Hz. (1.1)      

Figure 1.1 - Simple Low-Pass Filter with Bandwidth of 106 Hz

In addition, as shown in Figure 1.2, D flip-flop are used as an interface between microcontroller interrupt pins and hall signals. The D flip-flop interface will digitize 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:

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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 edgetrigger ¿ 4∗f HallSignal=400 Hz            (2)

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

2. Reference Limit Switch InterfaceFor 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 purpose. There is a need for electrical hardware solution to capture when the switch is pressed, avoiding complexity in software and mechanical design. The response of the interface is captured in waveform and flow chart as shown in Figure 1.4.

Figure 1.4 - 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 1.4 before and after the switch is pressed and microcontroller acknowledges it.

 3. 12 VDC Power Supply Filter 12 VDC power supply filter is designed with layout designed in KiCAD to reduce switching noise from 120 Watt power supply which is connected to motor shield .

4. SensorsA. Temperature and Humidity Sensor (RHT03)Temperature and humidity sensor is used to detect and/or prevent electrical component failure. Maximum

operating temperature can be set to transmit a warning message to the user when surrounding temperature becomes a concern for PC and microcontroller or one of the components catches on fire due to short circuit or other circumstances.

Humidity is also critical to determine if it is safe to run electrical components. Maximum humidity can be set in a similar fashion to prevent short circuiting. On the other hand, anyone servicing Sun Tracker must take caution to prevent electrostatic discharge (ESD) when humidity is low.

B. Accelerometer (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.

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5. I2C Buffer for Accelerometer (LTC4315)Inter-integrated circuit (I2C) buffer is used to provide noise filtering mechanism for microcontroller

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

6. Two 360-Degree Limit SwitchesTwo hardware limit switches are installed in one of the motor power line to limit number of 360 degree

rotation to one and prevent damaging wire. The two diodes are placed in parallel to each switch to limit the direction of current flow which effectively limiting the rotation of the dish. If 5-degree limit switch is pressed, the switch is open and the current can only flow from left to right. Therefore, the dish which has been moving in counter clockwise direction hits 5-degree limit switch and now the dish can only rotate in clockwise direction.

Figure 1.6 - Two 360-Degree Limti Switches7. Kill Switch

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

SOFTWARE DESIGN

1. Arduino Software1.1. The arduino provides motor control each motor, sensor feedback on the motors, current,

accelerometer, and temperature/humidity sensor. 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.

1.2. Communication1.2.1. Protocol

1.2.1.1. 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 ommitted 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.

1.2.1.2. 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.

1.2.2. 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.

1.3. Motor Control1.3.1. There are two motor control algorithms: calibration and general. Though they are very

similar in most functionality, they are seperated into two seperate 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.

1.3.2. 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

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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.

1.3.3. 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 oscilation, whenever the motor has to reverse direction from its previous speed, the acceptable error is increased. This acceptable error is reset everytime the motor starts up. As such, the motor may oscilate trying to achieve the position, but will eventually settle down as close as it can get to the position.

1.4. Sensors1.4.1. 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.

1.4.2. 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.

1.4.3. Temperature/HumidityMax temperature, min temperature, max humidity, and min humidity are all optional parameters. If any of them are defined and violated, then the system shuts down until they are no longer violated. The protocol for processing this sensor adds approximately 2ms to the processing time of the Arduino, therefore ignoring this sensor may improve overall performance.

1.4.4. 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.

1.4.5. 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.

2. Driver Software2.1. 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 seperated into 6 parts: setup, information access, positional movement, motor rate movement, pulse guide movement, tracking movement.

2.2. 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.

2.3. Information AccessASCOM information provides details about the system such as the aperature diamater 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.

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

2.5. Motor Rate Movement

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

2.6. 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 AltAz. The rate is defined in degrees per second.

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

RESULTS AND DISCUSSION

Electrical Design1. Noise Filters for Hall Signals

Figure 1.8- Slewing Drive Hall Signals without (left), with shielded cables (middle) and D flip-flop (left)

After implementing all noise filtering methods, running slewing drive motor no longer cause 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.

2. 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.

3. 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 effect 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.

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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.

CONCLUSIONS AND RECOMMENDATIONS Within eight months of the project, students from 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 challenge which require applying knowledge and co-op experience. There are more features that can 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. Dedicated weather station on the dish site or accessing local weather station online can also be integrated into the project to avoid operations during extreme weather conditions.

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 there 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.

Project P15571