Computer Engineering Projects I Remote Visual Surveillance Vehicle: (RVSV) Independent Learning Project Manoj Bhambwani 5/5/04 Project Description: The Remote Visual Surveillance Vehicle (RVSV) is a radio-controlled vehicle that provides the user at a PC with visual feedback. The user controls the vehicle’s direction and the camera positioning through an Internet interface The RVSV provides the visual feedback using an camera which is mounted to the vehicle. The camera transmits the video data to router or access point which supplies it to the user through an Internet connection. Project Subsystems: The subsystem described in this report is the power subsystem. Since the RVSV is completely wireless, power consumption is a major concern. All the power must be provided using batteries and therefore the systems on the vehicle must be designed to perform under a range of voltages and consume less power. In this report different battery types will be explored, and methods of designing low-power devices will also be investigated. The next major subsystem in this project is the communications system. The communications system consists of the serial interface between the PC, and radio control signals to the vehicle and the camera positioning system. This topic is covered by my partner, Tameka Thomas. In my partner’s report, the RS-232 interface to the PC, radio control, and all other signal processing will be covered in detail. Operational Specifications/Issues: The operational lifetime of the RVSV is expected to be at least 2 hours of continuous use before having to replace any of the batteries in the system. This appears to be a simple task, however this presents a major problem. The block diagram below (Figure 1.1) shows the different subsystems in the RVSV that need to be powered. Note that each one of these can consist of multiple smaller components which also need power. Figure 1.1: Now that each subsystem needing power is defined, the next step is to determine what voltages each one needs and how much current they each draw as a whole. The transmitter subsystem already must be physically connected to the PC so providing power here is not as much of a problem. It can use the same power as the PC (assuming that the PC is using 120VAC line). The same goes for the wireless router. The vehicle system, however, must be battery operated. As shown in the block diagram, the vehicle system consists of three subsystems each requiring battery power. First is the physical vehicle subsystem, since this vehicle is a pre-made device it will already have its power requirements satisfied by the batteries it was designed for. In this case, it will be 3 AA alkaline batteries. What is not known however is whether these batteries can power the vehicle through 2 hours of continuous use. This question will have to be answered through experimentation which will be covered later in this report. Next is the camera subsystem, this consists solely of the internet camera. This camera, even though wireless, was not designed for batteries and uses a 5V,2.5A wall adapter as the power supply. This presents one of the most outstanding power issues in this project. The actual power demands must be determined (technical specifications show 900mA) and a battery power supply must be created to meet these demands. Also, the voltage tolerances of the camera must also be determined as the voltage of the batteries will drop slightly as the batteries are used. Experiments must be performed to determine these characteristics in order to proceed with the design. The need for a likely large battery pack also presents another issue. Since, the RVSV is designed to be a surveillance vehicle it is desired that the battery packs be hidden away within the chassis of the vehicle. This presents a space issue if a large battery pack is needed. Also, batteries add weight to the vehicle thus reducing its speed or even preventing it from moving at all. The last subsystems here is the camera positioning subsystem. This subsystem consists of a micro controller and a DC motor, and it is responsible for moving the camera 360°. This presents another power issue as the micro controller will need about 5V as will the DC motor. A even more important issue, however, is the current draw of the DC motor which can go up to a couple amperes. All of these issues can be separated into three categories: current, voltage, and space. These categories must be weighed against each other to find the optimal solution to providing power to all of the vehicles subsystems. The power supply, in this case battery or battery pack, must be able to withstand the current demands of the subsystem for at least 2 hours. The voltage tolerances of each subsystem must be determined and/or designed to be low voltage and tolerant of changes in voltage. The battery (pack) must be as small as possible in order to fit inside the chassis of the vehicle. The next section will now investigate possible solutions to the problems stated. Power Options: The first possible solution to the problems stated above is to explore different battery types. There are four major types of batteries which are available to the average consumer, Alkaline, Nickel Metal Hydride (NiMh), Nickel Cadmium(NiCd), Lithium. The most common of these is the Alkaline battery. Alkaline batteries are the most popular and are used in many devices, however they are not rechargeable. Nickel Cadmium(NiCd) batteries are commonly used rechargeable batteries. Nickel Metal Hydride(NiMh) is a new type of rechargeable battery which offers better performance than NiCd. Lithium batteries are available in non-rechargeable and rechargeable types (Lithium-ion). The rechargeable Lithium batteries are commonly used in cell phones and other such devices. Each of these battery types will be compared to determine which is more suitable for use with this project. Also, the availability and cost of each battery type will be analyzed as the end-user will have to replace the batteries when they wear out. First, the common alkaline battery will be analyzed. Alkaline as said before are the most common of these batteries. They mostly used in everyday consumer electronics such as, alarm clocks, cd players, and cameras. The reason for this is that alkaline batteries are very economical. They offer solid performance and are fairly inexpensive. Alkaline batteries also come in many different shapes and size. The most common shapes are the cylindrical AAA, AA, C, and D type batteries. All of these cylindrical batteries provide about 1.5V.The other common alkaline battery is the rectangular 9V battery, however this battery is merely made up of six 1.5V batteries in series. This report will focus on the cylindrical type of batteries as these are the most common. Alkaline batteries are made up of maganese dioxide and zinc conductors within a alkaline electrolyte. These chemicals provide the chemical reaction necessary to produce the voltage. A fresh cylindrical alkaline battery usually has an open circuit voltage of 1 .58V. When a load is applied to a battery its voltage gradually decreases depending on the amount of current being drawn. The graph below shows the number of service hours vs. the discharge rate of a Energizer AA battery. This battery’s rated capacity is 2850mAh at a cutoff voltage of 0.8V, but as you can see the more current drawn from the battery the shorter it will last. It is also noted that this graph is nonlinear in the higher currents and demonstrates that the test current must be taken in to consideration when reviewing battery ratings. In this case the test current for the rating was 25mA which is around the linear area of the curve. Graph 1.1 Next is the Nickel Cadmium (NiCd) battery. These batteries are common rechargeable batteries used also for consumer electronics. One can find these types of batteries in toy remote control cars. These batteries also come in many different sizes and shapes including the standard cylindrical sizes (AA, C, D, AAA). These batteries are the cheapest of the rechargeable batteries. NiCd batteries consist of nickel hydroxide and cadmium electrodes within a potassium hydroxide electrolyte. Oxidation occurs on the negative electrode which equals the oxidation reduction on the positive electrode. The battery becomes recharged by applying an external voltage to the battery and thus reversing the chemical reaction. These batteries have the advantage of being rechargeable however there are a few drawbacks. First is that these batteries suffer from the memory effect if not properly used. There are two causes for this memory effect. The first is when the battery “remembers” the previous discharge and the only will discharge to that point. The second is caused by crystal growth in the battery which occurs when the battery is charged before it is fully discharged. These problems can be avoided by fully discharging the battery before it recharged. The next problem with NiCd batteries is that the have a smaller capacity. For example, a typical alkaline AA battery has a capacity of about 1000-2000 mAh while a typical NiCd battery only has about 500-1000 mAh. The last drawback is that the performance of NiCd batteries varies with temperature and also lose charge at a rate of about 2% per day. While, alkaline batteries can be stored over a year and are relatively temperature indifferent. The next battery type is the Nickel Metal Hydride (NiMh) battery. This battery is a newer rechargeable that provides better performance over the NiCd battery. The NiMh battery is constructed the same as the NiCd battery with the exception of metal hydride instead of cadmium. The NiMh battery has the advantage over the NiCd battery for a couple reasons. The first is that it does not suffer from the memory effect explained earlier. The second reason is that NiMh batteries provide a higher capacity than NiCd batteries. The typical NiCd battery had a capacity of 500mAh – 1000mAh, but the NiMh has a capacity of much closer to the alkaline batteries ranging 1000 mAh –2000mAh. These advantages are causing the NiMh battery to quickly replace NiCd batteries. Lithium batteries are also available. These batteries are not available in all of the standard cylindrical sizes (presently only AA). Lithium batteries are mostly used in cameras because of their capability to provide power surges. These batteries offer the best performance. They have a flat discharge rate and are fairly temperature tolerant. The Energizer AA battery has a capacity of about 2900mAh. This does not seem that great when compared with the rating of the alkaline battery which had a capacity of 2850mAh. However, the alkaline battery was rated at a drain of 25mA while the lithium was rated at 200mA. There are different types of lithium batteries also the one explained refers to the non-rechargeable lithium manganese dioxide battery. The rechargeable lithium batteries are lithium-ion which are commonly used in newer cell phones. Of these batteries the best for this project are the alkaline and lithium batteries. Even though they are not rechargeable they offer very solid performance and are fairly inexpensive. The lithium battery is preferred over the alkaline simply because it has a higher maximum discharge rate and has better performance in high drain applications. Power Saving Solutions: In addition to the type of battery used, there are other methods which can be implemented in order to conserve battery power. One is the implementation of a sleep mode. If there is no activity for an extended period of time, power to the camera and other high drain systems can be cut until activity resumes. This will prevent battery power being wasted while the RVSV sits idle. Also, power does not need to be applied to the entire system before it establishes a connection with the base PC. Here is another period in which battery power can be saved. The last suggestion is a circuit solution to saving battery power. A setup such as the one shown in Figure 1.2 can be used to save prevent a continuous drain on the battery. Instead the battery charges a capacitor and then rests until the capacitor discharges. This setup would save some drain on the battery however, the addition of a circuit with large capacitors take up space. Figure 1.2: Rbatt Q2 NMOS + RL Vbatt Ctot Conclusion: Powering the RVSV will be a major issue in the design. Many test and experiments will have to be done in order to find the tolerances and drain of the camera, and also to determine the life of the battery under a simulated load. The circuit solution will definitely be tested and all IC will have to be low-power. I believe that it still can be done and as long as the proper steps are taken in the early stages of design there should be no surprises.
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