Group SD0613

From ECE Department Wiki

Overview

Daktronics Inc. Solar Powered Scoreboard

 There is no elevator to success. You have to take the stairs. -Anonymous

Members

From left: Sudhanshu Singh, Chris Pierson, Corey Rood

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Advisors

[Dr. S. Yuvarajan]
[Dr. C. You]

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The Project

Overview

Daktronics is a designer, manufacturer, and retailer of sports scoreboards, electronic displays, and large screen video systems. Their BA-624 is a baseball scoreboard designed for small-scale operations such as a high schools or city fields. Currently, the scoreboard is powered through a standard 120V outlet; however the control console is capable of wireless connectivity. A standalone power source would make the entire system a truly wireless product. We have been tasked with creating a solar powered power source for the BA-624.

Figure 1

Background

Several Daktronics competitors have begun marketing displays similar to the BA-624 that operate on solar power. Sportable Scoreboards offers the option for nearly all of their outdoor sports scoreboards. Nevco has recently begun producing solar powered versions of their scoreboards as well. Daktronics would like to determine the feasibility of adding a solar powered option to their scoreboards, specifically the BA-624. Additionally, Daktronics would like to verify whether the operating statistics given by these competing manufacturers are realistic.

Solar Panels

To understand the basis of the final design, some background information must be given regarding the operation and characteristics of a solar panel. The panel can be thought of as a voltage source with a fluctuating maximum current capability. This fluctuation is caused by the varying amounts of sunlight the panel is exposed to, as shown in Figure 1. Additionally, much like a real battery, as current draw increases from the open-circuit state the voltage will decrease slightly – see right side of Figure 1. However, unlike a battery, the panel will reach a point of maximum current and plateau all the way to a short circuit (0V) – see left side of Figure 1

Figure 2

Using a simple power equation of V*I, a power curve can be plotted for varying amounts of maximum current, as shown in Figure 2. The peak points of each of these curves represent the maximum power that can be attained from the current conditions. As shown in Figure 2, the peak power is achieved at slightly varying voltage levels. By varying the load to achieve the correct point on the curve, maximum power can be drawn from the panels. It is this concept that is the basis for our design.

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Our Design

To allow nighttime usage the solar panels will be connected to batteries that will supply power when insufficient sun is available. Charging of the batteries will occur when either the scoreboard is not drawing current or when the solar panels are supplying more than the scoreboard is using. The effective load is a parallel combination of the internal battery resistance (approx. 0.1 Ohms) and the resistance of the scoreboard (4-8 Ohms, depending on operation state). Due to the variation of the effective load and current sunlight conditions, the solar panel does not typically operate at its maximum power point. Thus, a main goal of our circuit is to force the solar panel into the maximum power point of operation. Achievement of this goal is accomplished through the use of a DC-DC converter, which allows the effective load seen by the panels to be varied. Another benefit of the converter is the ability to step-up the output voltage to the necessary level to charge a 24-volt battery when the panels are not producing more than 24 volts. We chose a Single Ended Primary Inductor Circuit (SEPIC) for the converter (Figure 3). While a simple buck/boost converter would have also worked, the SEPIC converter has the added benefit of producing non-inverted voltages at its outputs. These can then be used directly for charging the batteries and supplying power to the scoreboard.

Figure 3

Voltage output for the SEPIC converter is controlled by a duty cycle (k) fed into a MOSFET (S1): Full control of the circuit is implemented through a PIC microcontroller (PIC16F876A). Measurements of the panel voltage and panel current levels are read using the analog inputs of the PIC. These readings are then used to calculate the power being generated by the panels. Additionally, two digital outputs of the PIC are used for disconnecting the load from the circuit and outputting a duty cycle to the SEPIC. A block diagram of basic circuit operation is given in Figure 4.

Figure 4

Common batteries available are rated at 12V. Two of these batteries are wired in series to present a 24V source to the scoreboard. When the PIC reads less than 11.5 volts from the center node of the batteries, a relay that connects the scoreboard to the batteries is deactivated. This prevents the batteries from being discharged to a damaging level. Likewise, when the PIC reads greater than 13.6V, the charging is adjusted to a trickle charge mode. Trickle charging consists of low level current pulse several times per second. If the battery voltage reading ever surpasses 14V, the charging is completely disconnected. Batteries have now reached full charge. Overall circuit operation is depicted in Figure 6.

Circuit block diagram

Circuit schematic

PCB Layout

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Programming/Algorithms

Figure 5

Figure 6

Determining the Maximum Power Point

Several methods were considered for determining the maximum power point of the solar panels. One method was to apply the technical specifications of the specific panels being used, along with readings of current conditions and load characteristics, to determine the parameters needed for maximum power. Ultimately, this method was not chosen due to several significant drawbacks. The first issue was the additional inputs needed to read the characteristics of the load and current conditions. While the PIC has sufficient analog inputs to accommodate the additional readings, the additional circuitry needed to implement these readings was considerably more than on the method chosen for the final design. Lack of flexibility was another major drawback of using specific panel parameters. If the panel were to be replaced with a different model (to provide more/less power, etc.) then the code for calculating the maximum power point would have to be reconfigured for the new panel. Even though this project is specifically designed for the BA-624, Daktronics produces a considerable number of scoreboards and displays that may benefit from solar powered capabilities. A significant benefit for future designs could be attained by not having to completely redesign the circuit simply because a different solar panel is being implemented. The method chosen for calculating the maximum power point in our final design consisted of a “brute force” algorithm. The main benefit of this method was it’s independence from the panels being used. The same circuit could be used for many applications simply by swapping the solar panels and batteries with those that matched the necessary power requirements of the application. To determine the maximum power point, the PIC began by reading the panel voltage and panel current. Multiplication of these values gives the power being delivered by the panel ( ). The duty cycle was then increased by approximately 0.4% (the smallest change allowable on the PIC), and the power was again calculated at this new setting. This increase should cause one of two scenarios to occur: power delivered will either increase or decrease. Comparing the initial power value to the new power value indicates whether the delivered power has increased from the previous setting. If it does indeed increase, then the point of operation has been shifted closer to peak of the power curve shown in Figure 2. The process is repeated until the new power measurement indicates a decrease in power. At this point, the duty cycle had been increased until just beyond the peak of the curve in Figure 2 (maximum power point). Since it is now one increment past the maximum power point, the duty cycle is reduced by the correction adjustment of 0.4% to achieve maximum power draw from the panels. If the alternative scenario occurs (power decreases when the duty cycle is increased) then the current power point is being shifted away from the peak in Figure 2. To cause the power to shift toward the peak, a second algorithm is implemented. This algorithm is identical to the first algorithm; however the duty cycle is decreased instead of increased. With each duty cycle decrease, the power is again calculated and compared to the previous value. This process repeats as long as more power is being generated with each decrease in duty cycle. When the maximum point is reached (power begins to decrease), the correction factor is again applied to return to maximum operating power. The process of determining the maximum power point is illustrated in Figure 6.

Code

Coding was completed in C, using MPLAB as the main development tool. Use of the NDSU PIC evaluation boards allowed in-circuit programming, greatly aiding the testing of code. PICLITE C compiler educational version was used for the vast majority of the project. This worked fine for functionality purposes, however it was limited to 2 kilobytes of program memory. During the final weeks of the project, a non-educational version was available, permitting the full eight kilobytes of memory on the PIC to be used. This allowed expanded capabilities with regard to presentation on the LCD, however the overall functionality of the circuit remained unchanged. For full project code, please see the Documents Section

Project Comments

There were a couple of major aspects of the project that were unexpected at the beginning and caused problems in our later work. The biggest problem was the switching noise that was caused by the SEPIC converter. The PIC, op-amp, and buffer needed to have a supply voltage of 5 V and if there were large voltage spikes in the supply voltage these integrated circuits would not work properly. Since the PIC controls everything in the circuit, its functionality is imperative for the project to work properly. This problem was not seen in the early stages of design because we were using the programmable PIC board which has a supply cord that plugs into the wall. Once the final design was put together there was a lot of noise on the supply voltage which caused the PIC, op-amps, buffer, and display to malfunction. To remedy this, a resistor and zener diode was added to the 5 V supply. This cut down on some of the larger voltage spikes that were causing problems. Another problem in the early design was continuity between the heat sink and the breadboard. One of the bolts holding the heat sink was conducting to the circuit and causing components to fail. Plastic bolts were used to attach the MOSFET and diode to the heat sink, allowing the use of one heat sink for both parts. The heat sink was repositioned so the bolts mounting it to the perforated board were not in contact with any other part of the circuit.

Despite all of our time and research, there are a couple design flaws in our project. Most noticeably is the electrical tape holding the LCD display on the enclosure. When the display was first mounted using metal bolts the screen was shorted to the enclosure and was not working. Since the display was implemented primarily for troubleshooting and presentation purposes, and would not be on the final product, we did not pursue an alternative mounting. Another design flaw would be the size of the product. The enclosure is very large relative to the circuitry inside it. In a final product it would be more appealing to have a smaller enclosure. However, to adequately demonstrate and test our circuit, it was more efficient to have room to work on the internals when it was operating. This is the main reason for the larger enclosure.

There were many valuable lessons learned from this project. First of all was the use of solar power, and how solar panels work. No group members had any previous experience working with solar power. Renewable energy is a very important issue and solar panels are very easy to use, so it is kind of foolish to not use this free energy. Another lesson was time management and not leaving tasks till the last minute because everything that can go wrong usually does. There were many unexpected bumps in the road that would cause delays. It is amazing how the smallest component can cause the longest delays. Good documentation is another very time consuming lesson that was learned. Keeping neatly detailed and accurate records during design and testing can save a time when dealing with recurring problems. It takes more time to keep neat and accurate data, but it is a lot better than having to do the same steps over and over again.

Future Developments

While our project meets all requirements set forth by Daktronics, there are several areas that could be improved with regards to general circuit design. Many of these center around the switching noise within the PCB caused by the SEPIC converter. With each oscillation of the duty cycle, a significant spike occurs in the readings of the analog inputs on the PIC. Significant improvements were made in the reduction of these spikes and their impact on the readings, however they are still present. To reduce them even more, a digital filter could be implemented to attenuate drastic spikes in voltage readings. A digital signal processor (DSP) would be well suited to this task, however limitations of available memory and calculation speed on the PIC prevented this option from being implemented. Time constraints prevented the exploration of using a DSP in the final design. Within the SEPIC, a possible source of increased switching noise could possibly be caused by the choice of inductors. To reduce cost, hand-wound inductors were used in place of ready-made options. This reduced the overall expenditures by over $160, however the windings were surely not as uniform as a machine-wound device. The increased quality may lead to a reduction in the overall switching noise. Another area that could be developed further is the addition of safety checks within the circuit. For instance, there is currently no protection against reverse polarity connection of the solar panels, batteries, or load. In a production design, these features would need to be added for the safety of the consumer. In addition, there should be some failsafe to prevent the SEPIC from operating if no load is connected to the outputs. During a testing phase, one of the connections to the SEPIC output became loose and disconnected. Within a few seconds the circulating current caused drastic increases in the voltages across the large capacitors, resulting in failed components throughout the circuit. Finally, due to the overwhelming dimensions of the BA-624 relative to the controlling circuit, there were few size restrictions in the initial specifications. As such, the PCB layout was not fully optimized for space. A smaller PCB footprint could likely be used.