Figure 1: Graphs Showing the Energy and Power Consumed by Two Systems on an ROV during a Mission

Transcription

1 Power Systems 2 Cornerstone Electronics Technology and Robotics III Notes primarily from Underwater Robotics Science Design and Fabrication, an excellent book for the design, fabrication, and operation of Remotely Operated Vehicles (ROVs) Administration: o Prayer Power Requirements for Your Underwater Vehicle: o Remember the difference between energy and power. Energy is the capacity of a physical system to perform work. Power is the rate at which work is performed or energy is transmitted. Energy is what is delivered and power is the rate at which it is delivered. A system onboard an ROV may consume more energy over time even though it does not demand as much power as another systems on board. Refer to Figure 1. Figure 1: Graphs Showing the Energy and Power Consumed by Two Systems on an ROV during a Mission o Power for Propulsion: A general rule of power consumption for small ROVs and AUVs. This rule is derived from empirical data for electric motor thrusters with wellmatched propellers: At speeds less than 1 m/s, a small ROV or AUV will probably use watts of electrical power per pound of thrust. Figure 2: General Rule for Power Demand of Electrical Motor Thruster on a Small ROV 1

2 Theoretical power demand for a vehicle at a fixed speed: Power = Drag x Speed Where: Power in watts (1 watt = 1 j/s = 1 Nm/s) Drag in newtons Speed in m/s o Power Consumption for Other High-Power Systems: Other small ROV systems that consume high power include motors for payloads, bright lights, and some cameras. Their power consumption will normally be consistent with their specifications. Remember that you can calculate power demand of a device knowing its required voltage and current draw, P = V x I. Power Budget: o A power budget summarizes the power consumption of your vehicle. The sample ROV power budget in Table 1 is for a 1.5 hour mission and is taken from the textbook. Table 1: Sample Power Budget o In the unlikely event that all the ROV devices turned on at one time, the total maximum power column provides the maximum power demand. o The sum of the total energy is crucial for AUVs with on-board batteries that must carry their power source. When the sum is closely matched to the battery rating, read the battery specifications to make sure that the discharge rate and other factors do not affect the battery performance while on the mission. o Notice that power-hungry devices do not automatically indicate that they consume a significant amount of energy, for example, the camera tilt motor. On the other hand, lower-power devices such as wireless Ethernet network switch may consume considerable energy. o It is better to be conservative when you develop the power budget so that the vehicle will perform consistently while on its missions. 2

3 Electric Power Sources for Small Vehicles: o There are two primary sources of electrical power to drive your vehicle, batteries or wall outlets. In the United States, electricity from the wall outlet is 115 VAC at 60 Hz. This source is not recommended for most underwater vehicle projects, especially for beginners or school groups because of the danger of fatal electric shock. Refer to the textbook for the dangers of AC power and safety procedures when working with AC electrical power. Though batteries have their own safety issues, they are usually safer when working around water and supply an excellent source of electrical power. o Introduction to Batteries: Batteries offer several advantages in addition to their relative safety: They are easy to find and relatively inexpensive. They can store plenty of energy for operating a small underwater vehicle. Most can be placed into positions other than right side-up. Batteries are clean; they are not greasy or oily. Running power through wires makes power distribution to the onboard systems easy. Information regarding battery specifications and operation is readily available on the web or other sources. Battery technology is evolving at a rapid rate because of consumer demand for smaller, lighter and more powerful batteries. This progress gives more options to the underwater vehicle designer. What is a Battery? A battery stores chemical energy, which it converts to electrical energy. A typical battery, such as a car battery, is composed of an arrangement of galvanic cells. Each cell contains two metal electrodes, separate from each other, immersed within an electrolyte containing both positive and negative ions. A chemical reaction between the electrodes and the electrolyte takes place. This gives rise to an electric potential between the electrodes, which are typically linked together in series and parallel to one another in order to provide the desired voltage at the battery terminals (for example, a 12 volt car battery). Each galvanic cell develops a voltage from less than 1 volt to a maximum of 3 volts. Individual cells are interconnected to form a battery. For example, volt alkaline batteries connected in series make up a 9 volt battery. Figure 3: Volt Cells Are Connected in Series to Form a 9 Volt Battery 3

4 Primary Batteries: A cell in which an irreversible chemical reaction generates electricity; a cell that cannot be recharged (for one-time use). Primary cells are also called dry cells. A dry cell is a cell in which the electrolyte is absorbed into a paper or made into a paste. Figure 4: Primary Battery Construction (Carbon-Zinc Battery) Common used primary batteries are alkaline dry cells and lithium batteries. Secondary Batteries: A secondary battery is a battery that produces electric current through a chemical reaction which can be reversed; a secondary battery can be recharged. A secondary cell can be recharged by forcing a current through the battery in the opposite direction of the discharge current. Chemical formulas for lead-acid battery charging: Figure 5: 12.6 Volt Lead-Acid Car Battery Internal Connections Battery Safety: Please refer to the textbook regarding this crucial topic, battery safety. 4

5 Battery Performance Characteristics: Voltage: o Common battery voltages: Familiar AAA, AA, C, and D are normally single-cell batteries with voltages between 1.2 and 1.5 volts. The transistor battery supplies 9 volts. Lead-acid batteries such as car batteries are volts. There are many specialty battery sizes and shapes for customized uses that furnish voltages from few volts to over 100 volts. o Be aware that nominal voltage on a battery may not be the actual voltage. For example, a 12 volt car battery may actually give 13.6 volts when fully charged and then drop off below 12 volts as it discharges. o No-load voltage: The voltage level present at the output terminals when a no load is applied. Most batteries are considered dead long before their no-load voltages reach zero volts. When a car battery with a no-load voltage of 10 volts is connected to a robust load, its voltage will fall significantly and act, in effect, as a dead battery. Figure 6: Checking the No-Load Voltage of a Battery 5

6 o Internal resistance: If you use a voltmeter to measure the open circuit voltage of an AA size battery, you will find that the voltage is about 1.5 V. But if you are using a circuit to draw a large current from the battery, you will find that the voltage across the battery is less than 1.5 V. This is because the battery itself has an intrinsic resistance called internal resistance. One way to think of internal resistance is to imagine a real battery as being made up of an ideal battery of voltage V i, connecting in series with a resistor R which represents the internal resistance (see Figure 7). When no current is drawn from the battery, the voltage drop across the battery is of course V i, V = V i. But when a current I is drawn from the battery, there is a voltage drop I x R across the resistor, so the voltage V across the battery is decreased to: V = V i I R Therefore, the larger the current drawn by the load, the smaller the voltage, V of the battery. The internal resistance of a battery is usually quite small. Figure 7: Representing Internal Resistance in a Battery An experiment that measures the internal resistance of a battery is found at: o Multiple voltage requirements: All of the electrical systems on a vehicle may not function at the same voltage. For example, microcontroller control circuits normally work at 3.3 or 5 VDC, while thrusters operate in general at higher voltages such as 12 or 24 VDC. One way you can supply different voltages to the vehicle is by providing separate batteries for each voltage level. Another method is to choose a battery rated at the highest voltage needed, and then reduce the high voltage for the lower voltage circuits using voltage regulators or DC-to-DC converters. 6

7 Primary Versus Secondary Batteries: o The initial cost of primary batteries is typically less than secondary batteries; however, they cannot be recharged. Over the lifetime of a project, the replacement cost for primary batteries can exceed the higher initial cost of rechargeable secondary batteries. Energy Capacity: The total amount of useful energy stored in a battery. o The energy capacity of large batteries is specified in amphours, smaller batteries in milliamp-hours. Although amps x time is not a valid energy unit, it is a convenient unit for battery capacity. Amp-hours are the product of amps multiplied by hours. For example, 1 amp-hour = 1 amp x 1 hour 1 amp-hour = 5 amp x 0.2 hour 1 amp-hour = 10 amp x 0.1 hour 200 amp-hour = 50 amp x 4 hour A battery with a capacity of 1 amp-hour should ideally be able to continuously supply a current of 1 amp to a load for exactly 1 hour before becoming completely discharged. In summary, the higher the Ah rating, the longer the battery will last. o Calculating the energy capacity of a battery in watt-hours (a valid energy unit): Energy Capacity = Power x Time Since Power = Voltage x Current, Energy Capacity = Voltage x Current x Time For example, if a 12 V battery is rated at 5 amp-hours, the capacity in watt-hours is: Energy Capacity = 12 V x 5 Ah Energy Capacity = 60 Wh Figure 8: Lead-Acid Battery with a 20-Hour Discharge Rate o Caution #1: The actual number of amp-hours a battery supplies is dependent upon the level of current draw from the battery. The manufacturer specifies the length of discharge time for their battery. For example, a lead-acid battery discharge time is frequently 20 hours. If your discharge rate is faster than the specified rate, the actual amp-hours delivered will be less than the battery rating. 7

8 o Caution #2: Different manufacturers have different definitions for a dead battery. This will affect the actual amp-hours the battery will deliver. Energy Density: Refer to the textbook. Weight: Refer to the textbook. Maximum Power Output (Power Capacity): o Remember, power is the rate at which work is done or energy is transferred. It is the work/time or energy/time ratio. A battery may have enough stored energy for your mission, yet it may not have sufficient power capacity to supply the power demand that your ROV requires. o The maximum discharge current a battery can provide is sometimes called the surge current (in amps). o C-rate (units of per hour) is another form of stating the maximum current. It is a conversion factor to convert the amp-hour rating into the maximum current. A battery with an Ah rating of 4 and a C-rating of 6 can deliver a maximum current of 4 Ah x 6/h = 24 A. The higher the C- rating, the higher the maximum current output available. Discharge Curves: o Batteries lose voltage in the process of discharge their energy. Discharge curves plots the drop in voltage as the capacity is depleted. Figure 9: Battery Discharge Curve for a Lithium Ion Battery Each line on the graph is the voltage (V) as the capacity (Ah) is expended at a constant current. Current at 1C = 8Ah x 1/hr = 8 A. Current at 18C = 8 Ah x 18/hr = 144 A. The energy capacity of the battery is reduced as the constant current draw is increased. From: 8

9 Depth of Discharge: The amount of energy that has been removed from a battery (or battery pack). o Usually expressed as a percentage of the total capacity of the battery. For example, 50% depth of discharge means that half of the energy in the battery has been used. o Some batteries, such as car batteries, are not made to be discharged very much before they must be recharged. Others batteries, called deep-cycle batteries, can be drained deeply without being damaged. These batteries are more appropriate for underwater vehicle applications where energy is needed throughout the mission and recharging occurs after the mission is completed. Maximum Charge Rate: Refer to the textbook. Temperature Performance: Be aware that battery performance typically drops as the temperature decreases. Size and Shape: Refer to the textbook. Shelf Life: Refer to the textbook. Required Maintenance: Refer to the textbook. Ease of Acquisition and Disposal: Refer to the textbook. Price: Refer to the textbook. 9

10 Series and Parallel Battery Combinations: You can add to your voltage, energy capacity, or maximum power output by connecting two or more batteries in series, parallel, or a combination of series and parallel. Case 1, Higher Voltage Batteries in Series: Figure 10: Batteries in Series: Voltages add, but the energy capacity and peak current output remain the same. Case 2, Higher Energy Capacity and Maximum Current Output Batteries in Parallel: Figure 11: Batteries in Parallel: Voltage remains the same, energy capacity and peak current outputs add. Case 3, Higher Voltage, Energy Capacity and Maximum Current Output Batteries in Combination of Series and Parallel: Figure 12: Batteries in Combination: Voltages add in the series section, energy capacity and peak current output add in the parallel section. Caution Keep voltages below 15 volts around wet conditions. 10

11 Safety and performance considerations when combining batteries: o Minimize the possibility of electrocution by keeping voltages below 15 volts around wet conditions. o Fire and explosions are possible with improper battery combinations. See textbook for an example. o The main reason to avoid mix batteries is reduced performance. Current flow through batteries in series is limited by the weakest cell. o For peak performance and safety, always use fully charged batteries of the same size, type, age, voltage, and manufacturer when connecting batteries in series and parallel. Battery Choices: Alkaline Cells: o Readily available worldwide o Relatively inexpensive o Good performance o Available in several standardized sizes, AAA, AA, C, and D o Voltage from each cell is normally 1.5 volts o Most are not rechargeable o Less popular for ROV applications due to high battery energy draw. Sealed Lead-Acid Batteries: o Use same galvanic chemistry as car batteries o The electrolytic is non-spillable o Can be placed in any orientation including upside-down o Equipped with a valve to relieve internal pressure especially when recharging o AGM (Absorbed Glass Mat) batteries are a good choice for a ROV power source. Widely available Easily recharged Relatively inexpensive Nickel Metal Hydride Cells (NiMH) and Nickel-Cadmium Cells (Ni- Cad): o Voltage from each cell is 1.2 volts o Nickel metal hydride cells are replacing the nickel-cadmium cells which have the memory effect problem. o NiMH batteries cost more than Ni-Cad batteries and have half the service life. o NiMH batteries hold 30% more storage and discharge capacity than Ni-Cad batteries. o NiMH batteries can be used to supply currents up to 30 amps. o Ni-Cads provide greater peak power output and a lower price than NiMHs. o Also available in several standardized sizes, AAA, AA, C, and D 11

12 Lithium Batteries: o Lithium battery technology is under rapid development, so the information below is subject to change. o This battery type includes Lithium-ion (Li-ion) and Lithium polymer (Li-poly). o Commonly used in laptop computers and cell phones. o One of the best energy-to-weight ratios of any battery o No memory effect o Slow charge loss when stored o Available in both primary and secondary forms o Advantages of Lithium polymer over Lithium-ion batteries: Less expensive to produce Contain higher energy Lighter More flexible in shape and size o Current disadvantages of lithium batteries: Readily catch fire if damaged, if charged or discharged incorrectly, or if their inner parts are exposed to air or water Can burn rapidly and violently They begin to age as soon as they are manufactured The charge/discharge cycle is lower than NiMH, non-spillable lead-acid, and Ni-Cad batteries Can suddenly fail Can absorb oil in oil-filled housings Li-poly must never be discharged below a set voltage to avoid irreversible damage Connecting Li-poly in combinations creates a serious risk for fire Transmission and Distribution of Electrical Power: o Major Electrical System Components: Source of electrical power Fuse or circuit breaker close to the power source ON/OFF switch Tether umbilical to deliver electrical power to vehicle Load all of the electrical equipment onboard the vehicle Figure 13: Major Components of a Vehicle s Electrical System 12

13 o Wires, Cables, and Connectors: Electrical power is conveyed through electrical wires which can be bundled into cables and joined together with connectors. Wire: A usually pliable low-resistance metallic strand often electrically insulated, used chiefly to conduct electricity. Insulator: A material with few or no free electrons which will not let electrons flow freely. Insulators provide a protective coating around a conductor. Cable: A bound or sheathed group of mutually insulated wires. Connector: A device that joins electric conductors mechanically and electrically to other conductors and to the terminals of apparatus and equipment. Makers of subsea connectors include SeaCon, Teledyne Impulse, and Birns. Figure 14: Wires, Cables, and Connectors o Fuses: A safety device that protects an electric circuit from becoming overloaded. Fuses contain a length of thin wire (usually of a metal alloy) that melts and breaks the circuit if too much current flows through it. They are used to protect electronic equipment and prevent fires. Figure 15: Blade and Cartridge Fuses with Fuse Sockets 13

14 Short Circuit: A faulty or accidental connection between two points of different potential in an electric circuit, bypassing the load and establishing a path of low resistance through which an excessive current can flow. It can cause damage to the components if the circuit is not protected by a fuse. Figure 16: Normal Current through the Load in an Unprotected Circuit Figure 17: Excessive Current through a Short Circuit in an Unprotected Circuit Protect circuits by placing a fuse as close as possible to the positive terminal of the power source to minimize the possibility of damage. Figure 18: Current flows through the load in a circuit that is protected by a fuse. Figure 19: At the instant a short circuit occurs, the excessive current flows through the fuse. Figure 20: The excessive current melts the fuse wire inside the fuse housing which opens the current path, stopping current flow. 14

15 o Choosing the Right Fuse: First, determine the maximum voltage and current during the normal operation of your circuit. Select a fuse with a voltage rating that is higher than the maximum operating voltage. Picking a fuse with the correct current rating is more crucial. Choose a fuse that has a current rating between 1.5 to 2.0 times the normal operating current. Make certain that the wires in your circuit can withstand this extra current; otherwise they may melt before the fuse blows. Slow-blow type fuses are preferable in motor and relay circuits which exhibit transitory current spikes. Circuit Breakers: A switch that automatically interrupts the flow of electric current if the current exceeds a preset limit, measured in amperes. Unlike fuses, they can usually be reset and reused. Resettable Fuses: The most obvious difference is that resettable fuses are automatically resettable whereas traditional fuses need to be replaced after they are tripped. When the conductive plastic material in a resettable fuse is at normal room temperature, there are numerous carbon chains forming conductive paths through the material. Under fault conditions, excessive current flows through the resettable fuse device. Heating causes the conductive plastic material's temperature to rise. As this self heating continues, the material's temperature continues to rise until it exceeds its phase transformation temperature. As the material passes through this phase transformation temperature, the densely packed crystalline polymer matrix changes to an amorphous structure. This phase change is accompanied by a small expansion. As the conductive particles move apart from each other, most of them no longer conduct current and the resistance of the device increases sharply. The material will stay "hot", remaining in this high resistance state as long as the power is applied. The device will remain latched, providing continuous protection, until the fault is cleared and the power is removed. Reversing the phase transformation allows the carbon chains to re-form as the polymer re-crystallizes. The resistance quickly returns to its original value. From: apkat_pg01_1.pdf Resettable fuses are convenient on deep-diving ROVs and AUVs because of reset feature. Power Switches: Locate master switch between the fuse and the remaining circuitry. Switches will be discussed in more detail in the next chapter. 15

16 o Transmitting Electrical Power over a Tether: Voltage Drop - a decrease in voltage around a circuit through which current is flowing. For example, in Figure 21, the battery supplies an electrical potential of 10 volts. The voltmeter reading across the battery (from Point A to Point D) is 10 volts, V A = 10 V. As the 10 ma current passes through the resistor R1, the voltage drops 2 volts (V 1 = I 1 *R 1 ; 2 V = 0.01 A * 200 ). The voltage at Point B (V B = 8 V) has dropped 2 volts from Point A (V A = 10V). Since the current is equal through all of the resistors, the voltage drops an additional 3 volts across resistor R2 and 5 volts across R3. If you measure the voltage at Point D (V D = 0 V), you will observe that the voltage has dropped a total of 10 volts across the three resistors. Figure 21: Voltage Drops in a Series Circuit 16

17 Although wires are considered conductors, every wire has an internal resistance. With this resistance, there is an associated power loss and voltage drop. So the voltage at the end of a tether is less than the voltage at power source. For example in Figures 22 and 23, a 60 foot long #22 gauge wire tether has an internal resistance of /ft. The internal resistance produces a 5.8 V voltage drop across the tether, leaving 6.2 volts for the ROV. Figure 22: Voltage Drop in a 60 #22 Wire Tether Figure 23: Schematic of Voltage Drops in 60 #22 Wire Tether 17

18 Options to reduce power loss in a tether: Shorten the tether Choose a larger gauge wire to reduce the internal resistance Increase the voltage Place the battery on board the vehicle o Accommodating Multiple Voltages: Your ROV may have subsystems that require different operating voltages. In Figure 24, for example, the thrusters and cameras operate at +12 volts while the microcontroller and compass circuits run on +5 volts. Figure 24: ROV Subsystems Requiring Different Voltages Fixed Output Linear Voltage Regulators: a device that provides a constant regulated output voltage (a constant voltage despite variations in input voltage or output load) as long as the input voltage is greater than the rated output voltage plus the dropout voltage. Typically, voltage regulators are surrounded by heat sinks because they generate significant heat. The unnecessary voltage is disposed by the heat. Check your voltage regulator datasheet for the connection diagram and typical applications. Figure 25: Fixed Output Voltage Regulator Schematic and a 7805 Voltage Regulator in a TO-220 Package Links to two 7805 datasheets:

19 Figure 26: A 7805 Fixed Output Voltage Regulator Circuit on a Solderless Breadboard All linear regulators require an input voltage at least some minimum amount higher than the rated output voltage. That minimum amount is called the dropout voltage. For example, a common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the input voltage remains above about 7V. Its dropout voltage is therefore 7V - 5V = 2V. DC-to-DC Converters: DC converters convert power from one DC voltage source to another DC voltage, although sometimes the output is the same voltage. They are usually regulated devices, taking a possibly varying input voltage, and providing a stable, regulated output voltage, up to a design current limit. Steps in Circuit Design and Construction: o Circuit Design: Formulate your circuit design on paper or other medium before you begin fabrication. Use standardized circuit schematic symbols so others can understand your design. Standard symbols can be found at:

20 o Circuit Prototyping: Test your design by fabricating a temporary prototype circuit. Breadboarding: an experimental arrangement of electronic circuits giving access to components so that modifications can be carried out easily. Breadboarding can be arranging larger circuit components on a piece of plywood and connecting them using alligator clip leads. Circuits with smaller circuit components tested using a solderless breadboard. The components are plugged into contact points on the breadboard, allowing for simple modification if needed. For a more detailed session on solderless breadboards, see: eek3,%20solderless%20breadboard.pdf Figure 27: Solderless Breadboard Set-up with +9V Supply Bus Remember to update your schematic after making corrections to the prototype circuit. o Robust Circuit Construction; Your Final Circuit: Now that your prototype circuit has tested successfully, you must construct the final assembly so the electrical connections and components stay intact in spite of immersion, bumps, vibrations, and strain. Securely anchor all components, wires, and connections to the chassis box or waterproof canister. Keep crucial parts accessible for repair or replacement. 20

21 Some Wire Connections Options: Figure 28: Terminal Strip with Jumpers Figure 29: Marine Busbars Figure 30: Eurostyle Terminal Strip Figure 31: Crimp-On Terminals Figure 32: PCB Terminal Blocks Figure 33: Splice Connector Figure 34: Ribbon Wire Connector Figure 35: Perforated Circuit Board Figure 36: Printed Circuit Board Figure 37: Waterproof Housing 21