Making Things Move: DIY Mechanisms for Inventors, Hobbyists, and Artists - Dustyn Roberts (2010)
Chapter 5. Mechanical and Electrical Power, Work, and Energy
All things that move need some source of energy. This energy may be as simple as using the force of gravity to create movement (how apples fall from trees), or as complex as the internal combustion engine in a gas-powered car. A person can also supply power by cranking a handle or pedaling a bike. Our bodies turn the chemical energy from the food we eat into mechanical energy so we can walk, run, and jump. Motors turn electrical energy into mechanical energy so we can make things move and spin.
In this chapter, we’ll discuss how power, work, and energy are related, identify sources of power, and highlight some practical examples of putting these sources to work.
Mechanical energy is the sum of an object’s potential and kinetic energy. Potential energy is how much energy is stored in an object at rest. Kinetic energy is the energy an object has because of its motion.
For example, a ball stopped on top of a hill has potential energy equal to its weight multiplied by the height of the hill.
Potential Energy = Weight × Height
If the ball weighs 2 lbs, and the hill is 20 ft high, the potential energy is 40 ft-lbs. If you push the ball so it starts rolling down the hill, the potential energy is gradually turned into kinetic energy.
Kinetic Energy = 1/2 × Mass × Velocity2
While the ball is rolling down the hill, the potential energy is decreasing (because it’s losing height), while the kinetic energy is increasing (because it’s going faster). At the bottom of the hill, the ball no longer has any potential energy, because all of it was converted to kinetic energy.
Let’s consider how roller coasters work. A motor drags you up that first hill, increasing your potential energy, and then lets you go. On the other side of the hill, all this potential energy is converted into kinetic energy while the roller coaster races down the hill and makes your heart jump into your throat. When the motor pulls the cars up the first hill on the roller coaster, it does mechanical work.
Mechanical Work (W) = Change in Energy (E)
Think of energy as the capacity for doing work. In this case, the mechanism dragging the roller-coaster cars up the hill took the cars from zero potential energy to a lot of potential energy.
Now suppose the roller-coaster cars weigh 1,000 lbs all together, and they’re dragged to a height of 200 ft. By changing their elevation, the dragging mechanism did 1,000 lbs × 200 ft = 200,000 lbs-ft of work!
We can also define work as force multiplied by distance:
Work (W) = Force (F) × Distance (d) = Energy (E)
This should look familiar from Chapter 1, where we talked about simple machines. Work is just the amount of energy transferred by a force through a distance. For our roller coaster, the dragging mechanism carried the 1,000 lbs of coaster cars up 200 ft, so the work equals 1,000 lbs × 200 ft = 200,000 lbs-ft, which is the same answer as from the potential energy method. The potential energy method and the mechanical work method are two ways of thinking about the same situation.
Mechanical power is the rate that work is performed (or that energy is used):
Power (P) = Work (W) / Time (t) = Energy (E) / Time (t)
In the United States, mechanical power is usually measured in horsepower (hp). This curious unit is left over from the days when steam engines replaced horses, and equals the power required to lift 550 lbs by 1 ft in 1 second—the estimated work capacity of a horse. One horsepower also equals approximately 746 watts, or 33,000 ft-lb per minute. You will often see motors and engines rated in horsepower.
Up until now, we’ve talked about work and power only in straight lines, but what about power for a rotating motor? You might remember from Chapter 1, when we talked about bicycles, that the speed of something spinning is called rotational velocity. You just saw that work has units of force × distance, and luckily, as you learned in the previous chapter, so does torque! So in this case, you can think of torque as work being performed in a circle. Here’s the equation form:
Power (P) = Torque (T) × Rotational Velocity (ω)
You will likely need to use electricity at some point in your project, unless your creation is powered directly by wind or a human (or a hamster). Just as a ball on top of a hill will roll from a higher potential energy position to a lower one, electricity flows from a high potential source to a lower one. The high potential is called the power source (or just power). The low potential is called the ground, which doesn’t necessarily mean the ground you’re standing on, but it can—that’s where the term comes from.
Lightning starts out as a powerful charge just looking for a place of lower energy to discharge, so it finds the fastest path to the ground. When you walk across a carpet in your socks and pick up positive charge, you get a shock when you touch a doorknob for the same reason—your high charge is looking for a lower energy place to go. Your charge jumps to the doorknob and creates a spark, just like a tiny lightning bolt. Metals are good conductors of electricity, so electric charge flows through them easily. Since the doorknob is metal, possibly attached to a metal door that is hung on a metal frame, it provides a great place for your charged-up energy to travel. Your sister is also a great place to discharge built-up static electricity. Since humans are about 60% water, and water is very conductive, your high charge will escape using her body as the ground. (Note that sisters do not always appreciate the shocking result of this grounding experiment.)
For the projects in this book, the ground will not usually be the earth (or your sister), but a strip of metal that is isolated from the power source so it stays at a lower potential energy level. Sometimes the ground is just the negative terminal of a battery.
The relative difference between any high-energy and low-energy point is called the voltage and is measured in volts (V). If we compare electricity to the flow of water, voltage is like water pressure that comes from a pump, and wires are like pipes. The amount of voltage, or water pressure, gives an idea of how much work a power source can do. For batteries, you can read this voltage “pressure” on its label. Find a standard AA battery and you’ll see 1.5V marked somewhere on it.
As an experiment, take an AA battery and your multimeter (like the SparkFun item TOL-08657). Make sure the black probe is in the hole marked COM and the red probe in the hole marked HzVΩ. Turn the dial to V and hit the yellow button to turn it on.
FIGURE 5-1 Using a multimeter
Touch one probe to the (+) side of the battery, and one to the (–) side, as shown in Figure 5-1. New batteries may read up to 1.6V. If it’s an alkaline battery, it will read closer to 1.3V when drained. If you put the red lead on the (–) side of the battery, you’ll get a negative number (as shown in Figure 5-1). The reverse is true: if you put the red lead on the (+) side, you’ll get a positive reading. If you never use your multimeter for anything else, at least you know how to test for dead batteries now.
Current (I) is the amount of electrical energy passing through a point in the circuit. It is measured in amperes (A), or amps for short. Using the water analogy, the current at one point in a wire is like the amount of water flowing past one point on a water pipe. Most components in this book will use less than one amp of current, so are rated in milliamps: 1,000 milliamps (mA) = 1A.
Sometimes batteries have a current marking, which will say something like 3000mAh for a size AA (see ). The mAh stands for milliamps × hours. This means it will give you up to 3,000mA for 1 hour, or 1,500mA for 2 hours, or 750mA for 4 hours—get it?
NOTE This is the technical definition of mAh, but the internal chemistry of the battery will limit how fast you can get current out of it (see “Powering Your Projects” later in this chapter).
Motors and other components will often have a current rating that tells you how much current they need to operate, which is just as important as hooking them up to a battery or power supply set to the correct voltage.
If you connect two AA batteries end to end, so the (+) sides are facing the same way, you get 3V. When batteries are put together end to end like this, we say they are in series. Their voltages are added together, so there is more “water pressure” we can put to work. However, batteries in parallel add currents while the voltage stays the same. Figure 5-2 illustrates these concepts.
There are two flavors of electrical current:
1. Direct current (DC) is a constant flow of electricity from high energy to low energy. A battery supplies DC.
FIGURE 5-2 Batteries in series and parallel
2. Alternating current (AC) comes out of our wall sockets in the United States. It is like a wave of power that fluctuates between 0V and 120V 60 times per second.
AC power is useful when electricity must travel long distances, like from the power plant to our homes. Some appliances, like fans and blenders, use AC power directly to run AC motors. Otherwise, for our purposes, DC power is most useful. Most modern electronics use AC-to-DC converters (also called AC adaptors or DC power supplies) that convert the 120V AC from American wall sockets into around 5V to 12V DC that our electronic components can use.1 The converter is in the bulky black box on the charging cords for your laptop and cell phone.
A circuit is a closed loop containing a source of power (like a battery) and a load (like a light bulb or motor), as shown in Figure 5-3. Current flows from the positive (high energy) terminal of the battery, through the light bulb or motor, back to the negative (low energy, or ground) terminal of the battery. If you put stuff, like a light bulb or a motor, in the current’s way in a circuit, it has no choice but to travel through the light bulb or motor until it reaches a ground.
The resistance (R) of the load is measured in ohms (Ω) and represented by a squiggly line in circuit diagrams, as shown in Figure 5-4. You can think of resistance as a transition to a skinny pipe put in line after a fat pipe. All the current, or water, still must go through it, but it resists the flow, so there is higher voltage, or water pressure, before the transition than after it (see Figure 5-4). Electricity always follows the path of least resistance to ground.
FIGURE 5-3 A simple circuit
FIGURE 5-4 Water analogy ecosystem representing electricity flow through electronic components
A load (light bulb, motor, and so on) resists the flow of electricity by turning the electrical energy into some other form—light in the case of the light bulb and motion in the case of the motor. In an ideal circuit, all the electrical energy is converted into other forms of energy like this. In reality, some energy is always lost, most of the time as heat.
Technically, you can’t lose energy. It just gets turned into forms that are not useful. For example, heat is a form of energy, but when our circuit has extra power that causes our motor to heat up, that heat is not useful to us, so we say that the energy is lost. Some appliances, like a toaster, take advantage of this. A toaster is like a big resistor that just takes the current from the wall and turns it into heat to toast our bread.
Voltage, current, and resistance are related by Ohm’s law:
Voltage (V) = Current (I) × Resistance (R)
Batteries actually have internal resistance, which just means that they’re not 100% efficient, and this internal resistance makes the batteries heat up. Cheaper batteries tend to have higher internal resistance, which is a direct power loss.2
Electrical power (P), measured in watts (W), is the combination of current and voltage:
Power (P) = Current (I) × Voltage (V)
For example, a 60-watt light bulb needs 0.5A at 120V. The more work you need to do, the more power you need. Compact fluorescent light (CFL) bulbs are environmentally friendly because they use much less power than standard incandescent light bulbs to produce the same amount of light. A 13-watt CFL can produce as much light as a standard 60-watt bulb.3 The CFL bulbs are more efficient, which allows them to turn more of the input energy into light energy.
Now we can bring everything together. In the first section of this chapter, you learned that mechanical power is measured in horsepower. So, 746 watts equals 1 horsepower, and power equals torque times rotational velocity, and power also equals current times voltage—what does all this mean? It means you can calculate some important values. For example, if you have an electric motor rated in watts, you can figure out the torque for a given velocity (or figure out the velocity at a given torque).
If your eyes are starting to cross and you’re thinking of throwing this book into the closet, don’t worry. It’s not important to remember all these equations. However, it is important to know that work, energy, mechanical power, torque, rotational velocity, and electrical power are all related to each other with simple mathematical relationships. And you can use them to figure out how to make things move!
Powering Your Projects
Remember that energy can’t be created or destroyed; it just changes form. Transduction is the conversion of one form of energy to another. It follows that anything that converts energy from one form to another is called a transducer. For example, a motor is a transducer that changes electrical energy into kinetic energy, or motion. Light bulbs and light-emitting diodes (LEDs) are transducers that change electric energy into light and heat. Our bodies are transducers that change chemical energy into mechanical energy. Some of my students have created stationary bikes that power televisions and rocking sculptures that generate electricity by turning a motor. The number of ways you can convert one kind of energy to another is as endless as your creativity.
Back in Chapter 1, I defined a mechanism as an assembly of moving parts. Now you know that moving parts have kinetic energy. That energy needs to come from somewhere, right? Luckily, there are many energy sources we can use to make things move. Not all of them are practical for small-scale work, so we’ll focus on the ones that are. The electricity we get from the wall socket comes from other sources like burning coal (and possibly wind or hydropower) that are not directly useful to us. But all we need to know at this point is that the wall socket provides a source of AC power.
To determine your preferred power source for a given project, consider these questions:
• Is your project actually mobile, like a robotic car? If it needs to be truly mobile, you may choose batteries or another power supply small enough to lug around.
• Will your project move but stay in one place, like a painting rotating on the wall? If so, you can use a wall outlet, but you probably need to convert the AC power to DC power. You can do this with AC adaptors like the ones found on your cell phone or laptop computer chargers.
You can also generate your own power with a wind turbine or by pedaling a bike. We’ll talk about how much power to expect from these alternative energy sources later in the chapter.
Prototyping Power: The Variable Benchtop Supply
A variable benchtop power supply (see Figure 5-5) is ideal for prototyping— the initial phase of working out an idea. SparkFun’s () TOL-09292 is one example, and Marlin P. Jones & Associates () also has a good selection.
These power supplies are plugged into a wall outlet and act as power converters from AC to DC. They are expensive and large—hence the name benchtop supply. However, they are ideal because you will undoubtedly work with components that need different amounts of power: some motors want 3V, others want 24V, some want low or high current, and so on.
FIGURE 5-5 A variable benchtop power supply (image used with permission from SparkFun Electronics)
CAUTION Be careful with these things. They can source a lot of power.
These types of power supplies allow you to vary voltage and supply the required current while you are testing your project. A typical supply ranges from 0V to 30V and 0A to 3A. This flexibility can be a huge time-saver, so you may want to start here, even if you plan to go mobile for the final product. Once you get all the power questions ironed out in the testing and prototyping phase, you can choose batteries or a fixed power supply and be on your way.
Look for benchtop power supplies that are regulated. These ensure that the voltage won’t drop as current increases.1 This is important and will save you a lot of frustration. You’ll also see supplies listed as switching or linear. The switching type supply is more efficient, so usually more expensive, but worth it if you have the option.
NOTE Voltage is something that’s set, but current varies with varying load. For example, a 3V DC motor might come with a data sheet that says something like “no load current: 40mA, stall current: 450mA.” We’ll cover details on motor data like this in the next chapter. For now, know that a motor with nothing attached to it (no load) doesn’t have to do a lot of work so it isn’t thirsty for current. As soon as you attach something to the shaft, it is loaded and will need more current to overcome the additional strain. If you load the motor to the point where it stops spinning (with your fingers or a pair of pliers), it will be really thirsty and draw the most current. This maximum current is called the stall current. When you choose your batteries or other power supply for this motor, you should make sure the current rating is high enough to supply at least this stall current. Actually, the current rating of the power supply can be as high as you want; the motor will take only what it’s thirsty for. Using a benchtop supply during prototyping is a great way to find out how thirsty your mechanisms are and avoid draining batteries.
Mobile Options: Batteries
Batteries are great when you need your project to be mobile, but not so great when you are prototyping and testing. There’s nothing more frustrating than troubleshooting a mechanism that’s not working right, or not working at all, and finding out many hair-pulling hours later that all you needed were fresh batteries! I recommend prototyping with a variable benchtop supply, but when it comes time to go mobile, batteries are where it’s at.
Unfortunately, battery technology isn’t advancing as fast as we might hope. Batteries are relatively heavy, costly, and large compared to some of the other components we’ll talk about. If you plan to go mobile with your project, make sure you have accounted for the weight and size of the batteries within your mechanism.
CAUTION Avoid shorting batteries. This is when the positive (+) and negative (–) ends are accidentally connected, causing the power that the battery generates to flow back through itself! This will kill your battery and potentially cause a spark or even a fire. Keep your work space clear of wires and metal objects—like wrenches or screwdrivers—that could act as metal bridges and short your batteries. Also avoid using or storing batteries above 80°F. They won’t work as well, and higher temperatures will shorten their life.
The easiest way to incorporate batteries into your project is to use an off-the-shelf battery holder or snap, as shown in Figure 5-6. These holders can accommodate coin cells (like those in watches and calculators), 9V batteries, and up to eight AA, C, or D size batteries. All Electronics () usually stocks a good assortment in different sizes and configurations for less than $1 each. Many come with holes predrilled for easy mounting. All common cylindrical batteries are 1.5V (more like 1.2V for rechargeable batteries), but the larger and more expensive they are, the more amp-hours of current they provide.
FIGURE 5-6 Assorted battery holders
All batteries are not created equal. To help you find the best battery for your application, read on. We’ll look at different kinds, including rechargeable batteries, and highlight the pros and cons of each.2
Zinc batteries are super cheap, and come in all the common sizes (D, C, A, AA, and AAA). They can be rejuvenated a few times before their life is over with a special charger. However, their internal resistance is high, and they don’t last very long.
Standard alkaline batteries (think Duracell CopperTop) last three to eight times longer than zinc batteries. They also cost twice as much, and standard ones can’t be recharged. There are rechargeable alkalines, but you need a special low-current recharger that can’t be used on other rechargeable batteries. They also make high-tech alkaline batteries that last two or three times longer that standard alkalines, but they’re significantly more expensive.
Alkaline technology was not designed to handle the high levels of current that some motors require, so these batteries may not provide power fast enough. They also provide a decreasing voltage as they drain, which can cause problems if you’re already on the edge of the voltage range of a motor or other component in your project.
Nickel-metal hydride (NiMH) batteries are the best rechargeable batteries for the projects in this book and the most common kind of rechargeable battery you’ll find in stores. They have low internal resistance and can be recharged over 400 times in their life. They don’t last quite as long as alkaline batteries, but they are improving. NiMH batteries charge quickly (in an hour or two).
The main disadvantage is that NiMH batteries don’t hold a charge well. A fully charged battery will discharge all by itself just by sitting on a shelf for a few weeks or even days. For this reason, check battery voltage with your multimeter before use to avoid frustration.
Unlike zinc and alkaline batteries, which provide the standard 1.5V, NiMH batteries provide only about 1.2V. As a result, they are not always a valid replacement for nonrechargeable batteries (though most consumer products that take batteries will work with either).
Nickel-cadmium (NiCad) batteries are an older rechargeable battery technology. Some sources say they show a memory effect, which leads to diminished capacity if you fail to drain a battery completely before charging it to full. While the memory effect is debatable, these batteries more likely suffer from voltage drop. This means that if a battery is repeatedly used only partway before recharging, it will start delivering lower and lower voltages. This is true, but it’s also true of all rechargeable batteries.
Once charged, NiCads maintain their voltages reliably until they are almost completely empty (unlike alkalines). They won’t self-discharge as quickly as NiMH batteries do during periods of disuse. Like NiMH batteries, NiCad batteries are only 1.2V.
A downside is that cadmium is highly toxic and does not belong in a landfill. Disposing of NiCad batteries in the trash is illegal in many countries and states, so special recycling is necessary. For these reasons, properly charged NiMHs are usually a better choice.
Lead-acid batteries are found in your car and also come in smaller sizes like 6V, 12V, or 24V versions that power motorcycles, computers, and boats. They are useful to us because of their size. However, they are notoriously heavy. The trade-off for their weight is that these batteries are cheap and last a long time. Most lead-acid batteries can be recharged with a special high-current battery charger. Look for one that is sealed (denoted SLA for sealed lead acid or VRLA for valve-regulated lead acid) to avoid accidents with leaking battery acid.
Gel cells are just lead-acid batteries with a jello-like filling instead of a liquid acid. These are generally safer and cleaner. Look for the SLA batteries used in SADbot in Project 10-3 for reference.
Lithium, Lithium-Ion, and Polymer Lithium-Ion
Lithium-type batteries are the rechargeable batteries most commonly used in laptop computers and portable electronics. They’re relatively expensive, but pack a lot of power for their size, and they will retain a charge for many months. The little coin cell batteries in watches and calculators are also sometimes lithium cells, but they aren’t rechargeable.
SparkFun’s PRT-00339 is a polymer lithium-ion battery (LiPo for short). This is currently the most advanced battery technology with the highest energy density (energy density = energy / volume).
These batteries also need a special charger, of course, and you can get a simple one at SparkFun (PRT-08293).
The advantage of having a project that doesn’t need to move around is that you can have a big, heavy power supply, or you can plug right into the wall. The following are a few ways to turn AC power from the wall into DC power we can use.
Computer Power Supplies
If you’ve ever looked inside a desktop computer, you probably noticed a big, boxy looking thing, like the one shown in Figure 5-7, that was making some noise. This box is actually the power supply for the computer, and the noisy thing was probably the fan inside the power supply used to keep it cool. If you think your computer’s power comes from the wall socket where you plug it in, you are only partly right. Your computer wants DC power, not the AC power from the wall, so this power supply does the conversion and has some smarts built in that regulate the flow of power and avoid overloads.
SparkFun sells a computer power supply (TOL-09539), and you can get the accompanying ATX connector breakout board (BOB-09558) to take the mess of wires coming out of the box and give you a useful 3.3V, 5V, or 12V DC supply to power your mechanisms. Both of these can be had for under $35, so if you have a power-hungry immobile project, the combination is a very practical option. Although you need to plug these power supplies into the wall, they are small enough to be pseudo-mobile. The CupCake CNC from MakerBot () has one of these in it, and you can still carry the whole thing to a party if you want to.
FIGURE 5-7 A computer ATX power supply (image used with permission from SparkFun Electronics)
AC adaptors, AC-to-DC converters, and DC power supplies are names for the power cables that usually have large black boxes (wall warts) at the point they connect to the wall. You probably have one to charge your cell phone (see Figure 5-8). They take the AC power from the wall and step it down to a nonlethal DC power in a range we can use. The scary high voltage and current from the wall stays inside the plastic lump and gets dissipated by the heat, and we get nice, smooth, usable DC power out of the other end.4
All Electronics usually has a large and affordable selection of these kinds of power supplies. They typically range from 5V to 12V and come in a variety of current options, from about 300mA to 3A. You can also find supplies with selectable voltage settings, like TM03ADR4718 from Herbach and Rademan (www.herbach.com). You can plug these into your solderless breadboard to power your circuits. If you don’t know how to do this or what a solderless breadboard is yet, that’s perfectly okay. You’ll learn about solderless breadboards in Chapter 6.
FIGURE 5-8 AC adaptor (image used with permission from SparkFun Electronics)
Alternative Energy Sources
One problem with generating energy is that you must either use it or store it immediately. You don’t need to worry about that with batteries or power supplies you plug into the wall. The power companies that feed our homes will just charge more money if we use more electricity, and batteries supply power through a chemical reaction until they are drained.
Alternative energy sources provide ways to generate electric power, but you need to use that power right away or convert it to a form that you can use later. Even if you use it right away, the unsteady flow of power from something like a solar panel on a cloudy day or wind turbine might need to be smoothed out before it is useful to power mechanisms.
Capacitors for Energy Storage
There are many ways to store energy, but only a few that are practical for our purposes. Gasoline and food store energy in chemical bonds, dams store potential energy in the elevated water, and flywheels store energy in the movement of a heavy spinning wheel. However, we’re mostly interested in storing electric potential energy so we can directly use the electricity in our projects.
We’ve already talked about one way to store electric potential energy: batteries. You can use an alternative energy source, such as solar power, to charge your rechargeable batteries instead of using the energy directly. Another electrical component, a capacitor, can be useful for storing energy as well as smoothing out unsteady flows of energy.
A capacitor is like the water tower in our water analogy (see Figure 5-4). When there’s plenty of water around, it gets pumped up to the water tower and stored for later use. When there’s a shortage and the pump stops bringing in water, the water tower can drain immediately and supply the water it was storing. Capacitors store electrical energy like water towers store water. Similarly, when electric current is flowing into one side of a capacitor, it takes in all the energy and stores it. As soon as there is no current flowing in, the capacitor discharges immediately, until there is no stored up energy left. (See http://electronics.howstuffworks.com/capacitor1.htm for details of how capacitors work.)
A capacitor is a little like a battery because it stores electrical energy and has two connections. But unlike a battery, it doesn’t create energy (it only stores energy), so it’s much simpler. A capacitor is made of just two conductive plates close to each other but separated by something nonconductive. There are two kinds of capacitors: ceramic and electrolytic (see Figure 5-9). Ceramic ones don’t care which way you put them in a circuit, but electrolytic ones definitely do! Make sure the gray stripe or minus sign is on the ground side of the current flow.
FIGURE 5-9 Ceramic capacitors (left) and electrolytic capacitors (right)
The amount of energy a capacitor can store is called its capacitance (C) and is measured with a unit called a farad. We normally deal with tiny capacitors that are measured in millionths of a farad, or microfarads (µF). Compared to other means of energy storage such as batteries, the energy density of capacitors is low.
The energy a capacitor can store is calculated as follows:
Energy = 1/2 × Capacitance (C) × Voltage (V)2
This gives you energy in a unit we haven’t talked about yet, called a joule. A joule is a standard unit equivalent to the ft-lbs we measured mechanical power in earlier.
The advantages of using capacitors over batteries are that they charge much faster, are very efficient, and give you high power over a short amount of time when they discharge. The disadvantages are that they can get very big and expensive for a capacitor equivalent to just an AA battery because of their poor energy density. For example, you would need a soda-can-sized capacitor just to hold enough charge to light a standard flashlight for a minute or so.
So, how do we use capacitors? The easiest way to integrate these into your alternative energy projects is through ready-made modules called charge controllers or energy-harvesting modules. These modules take unsteady power, like that from a solar panel or a hand crank on a flashlight, and use it to charge a battery or capacitor that releases the power in a steady way that looks smooth and consistent to motors. Part 585-EH300A from Mouser Electronics (www.mouser.com) is one such module that can filter unsteady input energy and release it between 1.8V and 3.6V with up to 1A of current for a very short time.5
If you have a motor you want to power continuously, you probably want to set up a circuit that allows your alternative energy source to charge a battery through a charge controller like the one shown in Figure 5-10, and then run your motor off the smooth battery power through the charge controller. See the section on decoupling capacitors in Chapter 6 and the Wind Lantern Project 10-2 for more ways to use capacitors.
FIGURE 5-10 A solar charge controller like the one shown (from Silicon Solar) allows you to charge a battery through solar panels, and then run your motor off the battery. This model is used in the SADbot project (Project 10-3).
Solar cells convert light energy to electric energy. The amount of energy you get depends on the area of the cell, so you need a pretty big cell to power the motors we’ll talk about in this book. SparkFun’s huge solar cell (TOL-09241) can provide 5.2 watts of power (8V at 650mA current) in direct sunlight, and that’s enough to drive most of the smaller motors we’ll talk about in the next chapter. They call it huge, but at about 7 × 8.5 in, it’s smaller than a sheet of notebook paper. The solar cells used in SADbot in Project 10-3 are 13.5 × 18 in and they are from Silicon Solar (www.siliconsolar.com/).
Wind, Water, and Other Fluids
A fluid is anything that flows. It could be air, water, or maple syrup. Historically, wind and water power were used directly by using the circular motion from a water wheel or a windmill to grind flour or cut wood. Nowadays, fluid power is mostly used to generate electricity as a cleaner alternative to fossil fuels.
You can try harnessing the power of the wind to make your own wind turbine to create small amounts of electricity. For example, try connecting a small wind turbine to a small electric motor like in the Wind Lantern project in Chapter 10 (Project 10-2). When we give electricity to electric motors, they give us motion (as discussed in Chapter 6). However, if we give an electric motor motion by spinning the shaft, we get electricity. One company, Duggal Energy Solutions, created a streetlight powered by a small vertical-axis wind turbine and solar cell called the LUMI-SOLAIR (www.lumisolair.com). This is a commercial product, but there are also quite a few hobbyist sites devoted to making your own power using wind (for example, see www.gotwind.org and www.windstuffnow.com).
Compressed gases are another way you can use fluids like air and carbon dioxide to do work for you. Potato guns and bottle rockets are good examples of movement created by harnessing compressed air. Pneumatics is the name of the field concerned with using pressurized gas to create mechanical motion, but these devices are usually driven by an air compressor that works on electricity to begin with (more on this in Chapter 6).
Unless you have a large dam nearby or a river running through your backyard, hydropower will not be very useful for you in terms of powering projects in this book. I have seen a concept for a showerhead that uses a mini-hydroelectric generator to power an electronic display that shows users how much water they are consuming, but it has not been tested (see www.epmid.com).
Hydraulics is the technical name of the field concerned with creating movement from compressing fluids, but is usually reserved for heavy machinery like bulldozers and excavators. We’ll cover this a bit more in Chapter 6.
Bio-batteries: Creating Power from Food
You can make a battery out of any fruit or vegetable that’s acidic: potatoes (see Figure 5-11), tomatoes, onions, lemons, oranges, and so on. A bio-battery functions on the same basic principles as a traditional battery. When two strips of different metals (typically copper and zinc) are inserted into an acid solution (in this case, the acidic moisture inside the food), an electrochemical reaction takes place, which generates a potential difference (voltage) between the metal pieces. In a bio-battery, you can use a galvanized nail or other metal (zinc is the coating in anything galvanized) along with a penny for the copper. A pair of electrodes like these inserted into a potato will generate around 1V at a very small current (just a few milliamps). Individual bio-cells can be added in series to generate higher voltage, and in parallel to generate more current.4
FIGURE 5-11 Potato power (credit: Kaho Abe)
Your basic LED needs about 10mA at 2V just to light up, so you’ll need a whole plate full of potatoes to get enough power. As far as making things move, even tiny pager motors won’t start spinning until you feed them about 20mA, and they want more current as soon as you do anything with them (see www.solarbotics.com/products/rpm2/ for an example). So to get enough power for a motor, you’ll probably need a whole sack of potatoes, or better yet, a whole garden.
The simplest way we can create energy is to use our mechanical energy of movement to create different kinds of movement. For example, hand-cranked mechanical toys like those from the Cabaret Mechanical Theatre (www.cabaret.co.uk) in the United Kingdom range from dancing goats to flapping owls. These are powered by only a hand crank interacting with all kinds of gears, springs, and cams inside the wooden mechanisms. We’ll discuss mechanical toys and kinetic sculpture more in Chapter 8.
Wind-up toys have been around for hundreds of years. They have springs or elastic bands to store the wind-up energy we create and use it to make a bug crawl or a toy car move. Modern ones from Kikkerland (www.kikkerland.com) and Z Wind Ups (www.zwindups.com) are popular with kids as well as adults. The following section talks about how you can use springs to store energy like in these mechanisms.
We can also use our mechanical power to create electrical power to use or store for later in rechargeable batteries. Hand-cranked flashlights and radios have been around for years to eliminate the problem of dead batteries, especially in emergency or power-outage situations. Because of the low power requirements of LED technology, about 1 minute of cranking can give about an hour of light on one of these flashlights.
The problem with hand-powered mechanisms is that it’s hard to create enough electrical power to do much more than light a few LEDs. Luckily, our legs are built to convert more energy than our arms or hands—after all, we do walk around on them all day. For example, you can find bicycle-powered blenders at Fender Blender (www.bikeblender.com) that will whip up a smoothie while you work out. Other companies are exploring this space, and a couple innovative products are currently being tested. Bionic Power (www.bionic-power.com) created a knee brace with a generator in it that the company claim generates 7 watts of electricity per leg when walking. It’s generated as your leg swings in the air, so you don’t need to expend extra energy. They aim to use this power to charge cell phones and radios for hikers and soldiers, but that kind of power is enough to run the small motors we’ll talk about in the next chapter. Another small company, Lightning Packs (www.lightningpacks.com), uses the weight of a heavy backpack along with the up and down motion of walking to generate up to 7.4 watts.
Springs and Elastic Energy Storage
A spring is an energy storage device, since a spring has the ability to do work.6 Springs have many different shapes and sizes. Compression springs are the most common ones you can find inside mechanical pencils and pens. These will squish a certain amount when you put a certain force on one end. This force and squish distance tell us the spring’s stiffness :
Stiffness (k) = Force (F) / Squish Distance (x)
You can sort springs by stiffness on McMaster. We need this stiffness to tell us how much energy the spring can store. The energy storage depends just on the stiffness and the amount of distance the spring deforms:
Energy (E) = 1/2 × Stiffness (k) × Distance (x)2
Although we normally think of springs as coils of wire, the same spring equations apply to things like diving boards. They are really just long, flat springs, similar to leaf springs in the suspensions of trucks.
Torsion springs are the kind you find in mousetraps and hair clips that keep them shut.
No matter what kind of spring we’re dealing with, it stores elastic energy. Elastic just means that as soon as we stop pushing or jumping on it, the spring will return to its original state. We’ll talk more about different kinds of springs and how to use them in mechanisms in Chapter 7.
Project 5-1: Mousetrap-Powered Car
In this project, we’ll use the energy that a torsion spring can store to power a small car. Refer to Figure 5-12 as you step through the recipe.
• 1/4 in diameter wooden dowel
• Multitool with knife and file
• Two eye screws that the dowel fits into (McMaster 9496T27)
• Monofilament fishing line
• Two old CDs
• Laser-cut hub (see www.makingthingsmove.com for links to the digital files for download on Thingiverse.com, and Ponoko.com if you want to buy them) or epoxy putty
• Wooden paint stirring stick
FIGURE 5-12 Mousetrap-powered car ready to roll
• Ping-pong ball
• Duct tape
• Hot glue gun (with glue)
1. Line the edges of the CDs with duct tape to give them some traction.
2. Cut a 4 in length of the wooden dowel with your knife and file any splintered ends.
3. Attach the laser-cut hubs to the CDs with hot glue. Alternatively, use epoxy putty to bridge the gap between the wooden axle and the hole in the CD.
4. Gently insert the wooden dowel into the laser-cut hub on one wheel. It should be snug. If it’s too loose, wrap the dowel in a little duct tape and try inserting it again. If it’s too big, use the file to sand down the end until it fits (see Figure 5-12).
5. Twist the eye screws into the side of the mousetrap opposite the “bait” hook. They should be as close to the edges as you can get them without splitting the wood.
6. Duct tape the mousetrap down to one end of the paint stirring stick.
7. Insert the wooden dowel with one wheel attached through the eye screws, and attach the wheel on the other side.
8. Duct tape the ping-pong ball under the other end of the paint stick, and your car should balance.
9. In order to stop the wooden dowel from sliding back and forth, wrap strips of duct tape just inside the eye screws around the dowel. The dowel should still spin freely.
10. Cut at 2 ft length of fishing line. Tie one end around the center of the wooden dowel. Tie the other end to the center of the mousetrap arm. Secure with duct tape if necessary.
11. Spin the wheels backwards (clockwise in Figure 5-12) while guiding the fishing line so it wraps around the dowel. When you get almost to the end, keep winding the axle as you lift the mousetrap arm and flip it over.
12. To set the mousetrap, bring the long hook over the arm and catch it on the “bait” hook. This takes a delicate touch sometimes. Watch your fingers!
13. Once you’ve set your mousetrap, you’re ready to race! Set it down on the floor and use a pencil or other long object to trip the mousetrap. The fishing line attached to the arm will pull on the line wrapped around the axle and it will start to unravel. Your car should be able to go about 10 ft with this design. Now try some variations and see if you can get the car to go faster or farther!
1. Dan O’Sullivan and Tom Igoe, Physical Computing: Sensing and Controlling the Physical World with Computers (Boston: Thomson, 2004).
2. Gordon McComb, The Robot Builder’s Bonanza, ed. Michael Predko (New York: McGraw-Hill, 2006).
3. US Environmental Protection Agency and US Department of Energy, ENERGY STAR site, “How Much Light?” ().
4. Nicolas Collins, Handmade Electronic Music: The Art of Hardware Hacking, Second Edition (Routledge, New York: 2009).
5. Jeff LeBlanc, “ALDEH 300 Energy Harvesting Modules” (http://itp.nyu.edu/physcomp/Notes/ALDEH300EnergyHarvestingModules).
6. Michael Lindeburg, Mechanical Engineering Reference Manual for the PE Exam, Twelfth Edition (Professional Publications, Belmont, CA: 2006).