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2009-2010 K10 Inductive Charger Dan Mathewson Brett Russell Kyle Ryan Brady Coyle 3 Anna Camery [email protected] Executive Summary Team Induct-Us has designed an inductive charging system for the NASA Ames K10 Lunar Rover. The inductive charging system must be able to charge the K10’s batteries, Ocean Server Intelligent Battery System (IBPS), fully. The system needs to be integrated in the current K10 rover at Ames, autonomously run, and have desired 60% power transfer efficiency. This project is split into two separate sections; mechanical and electrical. The mechanical consists of an arm design, charging station structure and docking aids. The electrical consists of appropriate components for the charging circuit, proper feedback, and power circuit design. The arm designs and station are tested for ease of use, complexity and price. The charging circuit has been tested for power requirements and optimum power transfer efficiency. The project began by researching known inductive charging examples. From there, technical specifications and calculations can be made to determine circuit components. Using the data and calculations gained and specifications given by the client, the testing can begin. After iterative testing, a final product is then constructed. This process is recounted in detail within the following report. ii Table of Contents Executive Summary ……………………………………………………………….ii Background…………………………………………………………………………1 Problem Definition………………………………………………………………....2 Conceptual Development…………………………………………………………..3 Product Description………………………………………………………………...8 Design Evaluation…………………………………………………………………13 Recommendations and Future Work.............................................................................................................16 Appendix A: Mechanical Models Appendix B: Electrical Schematics Appendix C: Datasheets iii Background The K10 lunar rover is a remote, extraterrestrial surveillance robot under development at NASA Ames in Moffett Field, California. Tasked with increasing the efficiency of future lunar missions, the K10 will deploy to the moon before manned missions. Once there, it will map the terrain and look for resources. NASA uses this data to plan manned missions, astronaut excursions, and choose sites for future infrastructure. This frees up the astronauts for tasks not completed by the rovers and eliminates unproductive astronaut outings. The K10 packs several instruments to carry out this task including, a 360°, panoramic, 3D scanning laser; a ground penetrating radar; a color panoramic camera, and a high resolution terrain imager. Once on the moon, all lunar rovers have power requirements that have to be dealt with. Most extraterrestrial rovers rely on direct solar power to charge batteries. This method can compromise the rover by taking up precious equipment space and making them far more fragile. On the moon, large portions of the surface receive little light while blocked by the Earth making solar power impractical. The K10 rover avoids these pitfalls by charging its batteries through 3 sets of manually operated plugs. This presents a problem since while the rover is remotely exploring the lunar surface, it cannot plug itself in. Before deployment, a new method of charging needs to be developed. Charging stations are a proposed idea for charging extraterrestrial rovers. Even with a charging station, a power transfer still must occur between the station and the rover. Conductive charging plugs, like the current physical connections, are hard to connect remotely and are susceptible to debris and other blockages. The University of Idaho RLEP Fellows proposed making this power transfer with induction. 1 Problem Definition The goal for this design project is to develop an inductive charging station that will be able to fully charge the K10 lunar surveillance rover provided at NASA Ames. Currently during testing, the K10 rover must be stopped to allow human intervention to replace the batteries. There are three existing charge ports on the rover that must be connected to allow recharging. When the K10 is on a mission, this task is time consuming for the operators. That is why the ability to dock and charge itself is desirable. Table 1: Specifications General Specific Acceptable Preference Charge batteries wireless charging efficiency power requirements No interaction 60% efficiency 300 watts for batteries Requirement Desired Requirement Ocean Server boards power requirements max 16 A, 120V for ocean server boards Requirement Docking station power requirements emergency stop switch 120VAC from wall shuts down within .05 sec power on/off Requirement Requirement Requirement Ease of use easily dock remotely dock within 1 minute Desired Integration movable primary core plus or minus 6 inches (mag. of x y plane) Desired Arm fits in primary core plus or minus .25 inch Desired By the end of this project, the team has a working prototype of a charging arm that could be easily integrated into the current K10 system. 2 Concept Development Mechanical Design Arm During the course of the semester we came up with three distinctive arm designs; a static arm, a planer XY arm, and a multi-axis robotic arm. The multi-axis robotic arm would remind someone of a fully functional human arm in which the core would be mounted in place of the wrist. The arm would be mounted on top of the base station. This design was originally pursued due its ability to adapt to almost any environment and position. However, do to the complexity and need of additional controls this design was dismissed. The planer XY arm is an arm that functions parallel to the rear planner face of the K10 rover. This arm would compensate for any misalignment in the horizontal or vertical directions. However, this would require additional controls, thus this design was dismissed as well. The static arm is the simplest design, requiring no additional controls. This design is simply a rigid arm that would extend from the back of the K10. This arm mounts to the secondary side circuit enclosure. The enclosure is mounted in such a way that it can shift in order to compensate for different rover placement. Do to the simplicity of this arm it was the option selected for the final design. Base Design In association with the three arm designs, two station designs where initially created; one design each for the XYZ arm and one for the static and XY arms. Both designs fulfill the same basic tasks: housing all electronics, the E-Stop, and the primary coil. Both designs sit on three main points so the stations are not over-constrained and do not rock. The primary coil for the 3 static and XY arm is attached to a vertical screw drive that helps compensate for large misalignments. Based on suggestions received at a design review with NASA Ames, the base station designs where changed to include two tire guide tracks. These ramps will insure that the K10 docks the same every time. These ramps will raise the K10 off the ground ensuring the rover is always perpendicular to base. Electrical Design The components used in the power flow diagram include: AC/DC full bridge rectifier, DC/AC inverter, step down transformer, optical feedback loop, battery server board and lithium ion batteries. The following options were pursued throughout the project design. AC/DC Full Bridge Rectifier The full wave bridge rectifier used in the circuit is made by Center Semiconductor Corporation. Alternating current from the local utility supplying 120 V and 15 A at 60 Hz is the input for the rectifier. The voltage is then dropped across two diodes on the positive and negative period of the sine wave. The negative period is inverted and made positive allowing for maximum power transfer. A capacitor is placed in parallel to the rectifier output and controls DC ripple. DC/AC Inverter The DC/AC inverter used in the circuit is made by Maxim Integrated Products Incorporated. There are two proposed selections for the inverter. Firstly, a flyback PWM inverter type was selected because it utilized a less complex parameter circuit and only requires that three windings be used on the transformer. The second option is a forward inverter PWM type selected 4 because it able to handle the needed supplied power, however, the package does not have a built in optical sensor and it requires four windings on the transformer. Core The decision of what magnetic core to use is an important part of the inductive charger. Not only does it affect the size and shape of the mechanical arm and station designs, but also provides the required power transfer efficiency that must be obtained from our given specifications. Of course, as for any design, there are many aspects that must be considered. The most important element to the core is how well the magnetic field lines are concentrated. The goal of the transformer is to get the magnetic field lines to pass through as many turns as possible to obtain maximum power transfer efficiency. The material of the core also is an important characteristic to explore. Ferrite is the ideal material to use for this given application. This allows for a higher frequency range (2 kHz – 3 MHz) and lowers core loss, as well as having a high permeability. The three specific core geometry’s that were explored for this design were, a cylindrical plain core, an E-core, and a pot core. Table 2: Ferrite core comparative geometry considerations Core Bobbin Winding Winding Assembly Mounting Heat Shielding cost cost cost flexibility flexibility dissipation Cylindrical High Low core Low Good Simple Good Good Good E core Pot core Low Low Excellent Good Simple Simple Good Good Excellent Poor Poor Excellent Low Low High Low A cylindrical plain core is a piece of ferromagnetic material that resembles a small circular disk in shape. A coil of wire could be wrapped around the core while having the 5 component attached to the end of the mechanical arm. This is a technique that is taken directly from electric vehicle chargers. While the design is fairly simple and straight forward, the field losses would be greater than desired and would not allow for maximum efficiency. Figure 1: Cylindrical core insertion The E-core was another core geometry that was investigated. This geometry design contains the magnetic flux very well as opposed to the cylindrical plain core. It would also allow room for wire leads. Wire can be wrapped in a plastic bobbin that would be placed over the center leg of the E. Among many other advantages, the E-core is a good heat dissipater. A disadvantage to using this design, however, would be the higher probability for misalignment during the docking process. Two E-cores, one being a primary and the other the secondary, would need to be brought together every time for a direct, repeatable connection. The final core design that was explored was the pot core. Pot cores are self-shielding cores that provide complete enclosure of the magnetic field lines. This allows for shielding from 6 outside electromagnetic interference (EMI) which could have a direct effect on the field lines. They provide all the major advantages that E-cores offer, while allowing for an ease of docking. Most of the requirements needed for this project seem to be met by the pot core; however there still is one pressing need that must be explored. The pot core does a poor job at heat dissipation. Ways to combat this would be to incorporate a heat sink. Figure 2: Pot core insertion The core recommendation that was decided on was the pot core. This design allows for the best possible maximum power transfer efficiency. The major loss in the electrical power system will come from the transformer, so having the best materials and highest efficient design is essential. Optical Feedback Loop The optical feedback system is composed of an infrared emitter and phototransistor detector couple with a voltage regulator. Both the emitter and detector are made by RadioShack 7 and a selection for the voltage regulator has been made. Voltage information is sent from the DC side of the second rectifier back to the inverter to regulate the power going into the battery server boards through the transformer. Battery Server Board The battery server boards are made by Ocean Server Technology Incorporated. The output from the rectifier is the input to the server boards. The two main boards are battery management modules, a low and high current version. The high current board feeds a DC-DC converter to power the computer and the low current board charges the batteries. Lithium Ion Batteries 14.4 V, 6.6 AHr Lithium ion battery packs that was also fabricated by Ocean Server Technology Incorporated. The number of battery packs used in full scale design is 24 packs but 8 are intended to be used for testing purposes. Product Description Mechanical Design Primary Side (Base Station) The final design for the base station, pictured below in Fig. 3, has three major features. The ramps, the spring loaded arm, and the basket. 8 Figure 3: Solid model of the final base station. The purpose of the ramps is to allow the rover to dock in the same position every time, enhancing alignment. They also prevent variations and changes in the terrain under the base station from causing misalignment. Each ramp features extended outside ramp walls to prevent K-10 from climbing off the station and damaging itself and the ramps. The entrances are also flared to help guide the rover’s wheels onto the ramps. Each ramp was constructed out of 1/8th inch aluminum sheet for ease of manufacturing and to save weight. The fabrication of the ramps was completed by the University of Idaho’s facilities machine shop. To lift the primary side core up to the rover’s level, a vertical arm is attached to rear of the station. The arm is connected to the base station via a spring-loaded hinge. This allows for flexibility in the horizontal direction in the event that the rover overruns the end of the ramps, 9 keeping the primary core safe from damage. The vertical arm is constructed out of slotted aluminum stock allowing the height of the primary side core to be adjusted for different size rovers. The primary side core is housed in a tapered basket. The purpose of the basket is twofold. First it assures a proper connection by guiding the secondary arm in during docking, secondly it acts as a heat sink to help cool the cores during charging. The basket was turned out of round aluminum stock. Secondary Side (Rover) The rover side of the mechanical design consists of a housing to hold circuitry and a secondary core, spaced from the housing. The secondary side features a standoff for the secondary core to ensure proper spacing from the rover to the vertical arm. It also features an externally mounted enclose, this is to protect the circuitry as well as to enable use on multiple rovers. The enclosure was purchased from an electronics store and is made from injectionmolded plastic, while the standoff was made from steel pipe. Electrical Design Power Supply It was decided that the simplest and most effective way to transfer energy was to use an existing power supply instead of attempting to build a complete circuit from scratch. The idea was that it already meets all of our needs with it being able to take the 120VAC power from the wall and outputting 15VDC for the batteries. It became more feasible and less time consuming to purchase a product that already was capable of providing the necessary functions that was required for the project. 10 The power supply that was chosen was the Mean Well SP-320-15. This power supply can take input voltage from the wall (120 VAC) and output the required voltage (15 VDC) to power the Ocean Server boards. The Ocean Server boards will then provide the voltage to the battery packs. Figure 4: Mean Well power supply The reasoning for choosing this supply was it provides the correct voltage values on the input and output sides. Also, the 320 W is enough to power the battery packs on-board the K10 rover. Since NASA Ames is currently using a similar product, it can be easily integrated into the current setup they use for testing. However, the only problem with this decision is it does not provide for the isolation necessary to allow for the rover to charge its batteries through the means of induction. So the next step was to alter the board of the power supply to allow for separation of the primary and secondary sides. PCB Separation Investigation of the power supply board showed the primary and secondary circuitries were conveniently separated. The only components that bridged the two sides were the 11 transformer and the optocouplers. Since we had planned to create our own transformer and feedback system anyways, it was decided to pull off these components and physically cut the PCB along the primary and secondary sides to allow for two divided parts. Figure 5: Milling of PCB Power Supply Board The primary side of the circuit board is located in the housing on the base station. The secondary is inside the exterior box situated on K10. Transformer Redesign In order to duplicate the existing transformers characteristics, we measured the inductance values of each winding using an RLC meter. Since our system has a fixed 100 kHz switching frequency, we needed to measure the inductance for this same frequency. 12 Using printed ABS plastic bobbins, we wrapped our own windings to match the inductance values from the existing transformer. Following each winding, we used a layer of specialized transformer tape to enclose each winding. If two windings come into contact with each other, voltage arching can occur and cause the transformer to fail. The final design took 120VAC at its input on the primary station side, transferred power across the transformer to the secondary side rover and stepped the voltage down to 15VDC. Design Evaluation Mechanical Looking at the specifications, Table 1, the mechanical evaluation can be broken down into four parts: the on/off switch, ease of docking, the movable basket, and docking tolerance. Table 3: Mechanical design evaluation Part Specification On/off switch Power on and off Ease of docking Docks in 1 min. Movable basket 12 in total vert. displacement Docking tolerance within .25 in. Ramp strength 500 lbs total Actual Spec Met? Power on/off Yes <1 min Yes 17 in. Yes .2 Yes supports rover Yes (?) max unknown When looking at the actual results of the mechanical side of the project it is a little hard to predict the actual results with the real K-10. For example docking time; we don’t have a working rover so we don’t know exactly how fast it can climb the ramps, just that it can and it does not take overly long for us to dock our rover by hand. 13 Electrical A summarized list of the electrical results from this project from the Design Failure Mode and Effects Analysis can be found in Table 4. Table 4: DFMEA summary Item Potential Failure Recommended Action Power transfer across transformer Isolated regulation of output voltage Emergency switch integration No charge, dangerous environment Circuit component overload Will not kill power to batteries Repeated design calculations and testing Have feedback, regulation circuit Insert in charging circuit and test results Action Results Success pending Pending Testing Procedure Our design had three main specifications that could possibly fail. We have conducted a series of tests to evaluate our design and they are listed below. Test 1. The first test that we did was to test the original circuit operations. For this test, it was important to measure different voltage ratings from different parts of the circuit. We left the circuit completely as it would come out of the box and opened up the lid to have access to reference points. Once the charger was powered on, we took voltage readings at the input, across the windings of the transformer, over the optocouplers, and at the output terminals. A full list of the test results can be found in the appendix, but the main results showed 122.6VAC at the input and a regulated 15.15VDC output. Test 2. The next step for our testing was to take out the filter capacitor that bridged the primary and secondary side. The reason for removing this was that, in our circuit, there would be no way to have that filter capacitor bridge our station to rover side. It would make our circuitry 14 no longer isolated. The results from this test showed that both the input and output voltage stayed consistent with the first test. Test 3. The next few tests deal with the circuit being altered. This test was to test out our new, redesigned transformer. We already pre wound both the primary and secondary cores. It was important to be able to match the inductances of the original transformer. The redesigned transformer had similar inductance values and acceptable leakage inductance values. A full list of inductances can be found in the Appendix. We then soldered on wires to both the primary and secondary circuit that would be the approximate real length that the windings and core would have to be placed on. The only change from the last test was the switched out transformer. The input value stayed the same, but the output value was approximately 10 volts too high. It was also discovered that the only way to make this circuit work, was to remove one of the optocouplers that controlled the voltage regulation. We believed that this was due to the increase of output voltage that would cause the circuit to fail. This meant that the output was unregulated and would vary from 24VDC-35VDC. This test did, however, prove effective power transfer across two completely isolated sides. See Figure 6 for the set-up of this final test Figure 6. Test 3 Circuit Set-Up 15 This is currently where we are. We have been able to get effective induced power through our circuit. The Recommendations and Future Work section explains the remaining tests that still need to be completed to finalize our project. An estimated cost of making a replicated prototype is outlined in Table 5. Table 5: Breakdown of fabrication costs Raw Materials $104.00 Labor $160.00 Electrical Components $180.00 Total $444.00 Sheet Stock Hardware Machining Fabrication Power Supply Cores Wire Recommendations and Future Work As the product must be finalized and delivered to our client, there is still some work to be completed. First, we need to continue our testing of the circuit with our new optocouplers and the circuit board separated. This would allow our circuit to be completely isolated while regulating the voltage. Another means for regulation would be to apply a regulation circuit to the output of our circuit. Further testing would have to prove which method would be most advantageous. From there, all that would be left to do would be to test the entire circuit with the station and arm design. A full systems integration test would allow the product to be finished and ready to be shipped to Ames. The purpose of our project was to design and build an inductive charging circuit. This idea is something that is completely new to the K10 project and is something that is required for actual use of the rover on the moon. Since Ames currently has to manually plug in the batteries, an isolated system is important toward having an autonomous rover. There also, currently is no 16 other system that is designed to do what ours does, so having this new system will be used until a new version is created. Since this system is in its first version, there is still future work that can be done to improve the design. We are currently outputting an unregulated voltage to the battery management system. This variation is not one that is ideal, but one that will be able to be handled by the management system. 17 Appendix A: Mechanical Models A1 A2 A3 A4 A5 Appendix B: Electrical Schematics B1 Power Loss Note: Optimal design losses, taking into account minimal losses for each component Appendix C: Datasheets C1 C2 Matched Infrared Emitter and Phototransistor Detector Model: 276-142 | Catalog #: 276-142 What's in the box 1 x infrared emitter - 2V 40mA 1 x phototransistor detector - 20V 25mA Tech Specs Dimensions: Product Height 0.34 INCHES Product Width 0.2 INCHES General Features: Model 276-142 Product Type Phototransistor Body Material Multi Fits What: Model 276-142 Miscellaneous Features: Min Operating Temperature -40 Fahrenheit Max Operating Temperature 176 Fahrenheit Supported Languages English C3 C4 C5 DC-DC Up Converter with efficiencies of up to 96% depending on input voltage. Converts from battery power or charge power to a higher output voltage for operating user devices. The device can operate at about 100 Watts with no ariflow, and 240 Watts with a small amount of airflow. The DC2U-1V provides four input power connectors to attach any of the IBPS battery management modules (BB-xx,MP-xx and XP-xx). The input connection is by the standard 8 pin Molex Mini-Fit Jr connector. Cable 19-00026-20 is an example cable for connecting the input voltage. The output connector is a 10 pin Molex Mini-Fit Jr connector. The input Voltage range is 9V - 24V (Input Voltage must be lower than the output voltage) If you need additional output power these converters can be combined in parallel to scale the output power. The modules have dynamic current sharing to distribute the load. The standard modules support 19V, 24V, 28V and 48V outputs, other voltages may be special ordered (19-48V range). RoHS Compliant C6 C7 Rechargeable "smart" lithium ion battery pack with integrated SMB fuel gauge. Integrated safety circuit monitors and reports pack, Temp, Voltage, Current, Time to Empty, Time to Full and other key parameters via the IBPS BB-xx, MP-xx and XP-xx controllers. The battery controller module continously monitors the status of the battery pack and can report this information to the user via the serial port on the battery controller. The BA95HC-FL battery pack has a cable and latching connector for attaching the battery pack to the battery controller. This is different from a standard Li-ion battery pack as users are not required to implement a backplane with a connector as used on Notebook computers. This battery pack connects directly to OceanServer's IBPS Battery Management Modules. In applications where batteries are located more than 9" from the IBPS Modules, a Battery Extension Cable, 19-00035-06, can be used.