The Permanent Magnet Direct Current (PMDC) motor, commonly known as just a brushed DC motor, is used in electronic devices, robotics, and toys (see Figure 3-1). The typical DC motor has only one motor coil with two wires for operation. This is by far the easiest type of motor to drive, control, and manipulate. You can also reverse the rotational direction of a DC motor output shaft by reversing the polarity of the voltage to the motor terminals.

Most DC motors have brushes that physically touch a set of spinning electrical contacts, called commutators that are electrically connected to the motor coil winding. When each brush touches a different commutator, the electrical current is passed through the motor coil, which forces the motor output shaft to spin about its axis. Permanent Magnet DC motors typically have two magnets attached to the inside of the motor casing and the coil winding and commutator mounted to the output shaft—the brushes are typically spring loaded to keep them securely mated to the commutator contacts while spinning (see Figure 3-2).

Figure 3.2. A typical DC motor disassembled for viewing. The inside of the motor casing (left) has two permanent magnets attached and a hole for the output shaft to come through. The coil winding (center) is wrapped around the commutator, which is mounted to the output shaft. The brushes (right) are attached to the ends of the motor terminals, which are mounted to the motor's plastic end cap.

A DC motor consumes as much power as it needs at a given voltage and consumes more (current) as its workload increases. A typical DC motor draws anywhere from 50mA to 50 amps of current and have speeds ranging from 1,000RPM to 20,000RPM. Using gears, we can transform the available power of a motor from high-speed, low-torque (ideal for flat grounds) into low-speed, high-torque (ideal for hills), or vice versa, which can extend the usability of the motor's power.

Because brushed DC motors use physical contact to transfer power from the motor terminals through the coil, periodic changing of the motor brushes is necessary for frequently used motors. Many DC motors have easily accessible brushes that can be replaced in a matter of minutes.

A brushless DC (BLDC) motor uses an electro-magnetic field created by energizing one of three common motor coils in sequence in order to spin the output shaft. The fact that there are no brushes means that these motors have a longer life cycle and higher reliability, giving them an advantage over the standard brushed DC motor. Because of their abnormal coil setup, they cannot be operated with the same simplicity of a standard brushed DC motor, and instead must use a specialized three-phase driver circuit.

Hobby grade BLDC motors are rated by how many RPMs they produce per volt that is present at the motor terminals, denoted "Kv". For instance, a brushless motor with a 1000Kv rating operated at 12 volts, produces 12,000RPM (12 volts × 1000Kv = 12,000RPM). These motors are commonly used to replace brushed DC motors where longevity and reliability are preferred over being easy to drive and inexpensive. They have almost completely replaced DC motors for use in computer hard drives, cooling fans, and CD/DVD ROM drives (see Figure 3-3). We use a brushless DC motor to power the Robo-boat in Chapter 9.

Figure 3.3. Brushless DC motors are commonly used for PC cooling fans and hobby airplanes. The brushless PC fan in this picture was dismantled due to a broken fan blade. You can also see the brushless DC hobby airplane motor with a BLDC 30A motor controller.

Stepper motors are brushless DC motors that have two or more independent coils instead of three common coils like a BLDC motor. These coils must be energized at set intervals to keep the motor shaft spinning, much like the BLDC motor. This means that you cannot simply apply power to the wires and watch the motor spin. These motors must be driven using a special sequence and timing provided by the Arduino.

Stepper motors have a defined number of steps or magnetic intervals; each time a coil is switched in sequence, the motor shaft rotates one step. The number of degrees per step determines how many steps are in each rotation of the output shaft. The more steps the motor is capable of, the higher the resolution or more precision it has. This is common with Computer Numeric Controlled (CNC) machines that use two or more stepper motors to control the exact X and Y coordinates of the machine head—they can return to any exact spot on the grid by counting the X and Y steps to that position.

Stepper motors are available in the following two basic types:

Figure 3.4. A few bipolar stepper motors, noted as having four wires on each motor

Stepper motors are commonly used for high-precision applications like computer printer heads, CNC machines, and some robotic applications.

The power of a motor at a given energy can be arranged to have either high speed or high torque. Think of an 18-speed mountain bike to visualize how this works: If you put the bike in first gear, your pedaling will provide a high torque that can enable you to ride up a steep hill with ease at the expense of going slowly. If you put the bike in eighteenth gear, it will be nearly impossible to pedal up a steep hill but will yield excellent speed on flat ground or going downhill.

The power produced by an electric motor can be manipulated in the same way. To convert the power, we must use a series of gears connected to the motor's output shaft or buy a motor with gears built onto the output shaft, called a gear-motor. A gear motor reduces the speed of the motor output shaft from its normally high speed (1,000–20,000RPM) to a slower output speed that is more manageable for a mobile robot (see Figure 3-5).

Figure 3.5. Here you can see a small DC gear motor (left) and again with the DC motor removed from the plastic gear box.

A gear motor can be any type of electric motor, as long as it has a gear box that reduces the output speed of the motor shaft. Each gear motor should have what a gear ratio that specifies the ratio of input speed to output speed of the motor output shaft. For instance, a gear motor with a 100:1 gear ratio implies that the actual DC motor output shaft must spin 100 times to make the geared output shaft complete one revolution.

A servo motor is a special type of DC motor that uses an encoder to determine the position of the output shaft. Hobby servo motors consist of a small DC motor, speed-reducing gear box, potentiometer shaft encoder, and motor drive circuitry, making them easy to interface directly to the Arduino (see Figure 3-6). The motor drive circuitry is not only used to decode the input signal (R/C servo pulse), but also to drive the DC motor.

These motors can move to a specific position quickly and require only three wires to interface with the Arduino (signal, power, and ground). The signal used to operate the servo is a precisely timed pulse of electricity that ranges from 1 milliseconds to 2 milliseconds – where a 1.5mS pulse yields the center position of the servo motor. The servo motor looks for an update with a new pulse about 50 times per second, or every 20 milliseconds.

Figure 3.6. A dismantled view of the larger Servo motor from Figure 3-1 reveals the gears, potentiometer, circuit board, and DC motor that are normally packed inside the plastic casing.

These motors are intended for use in hobby cars, planes, boats, and helicopters, but have made their way into robotics for their precision, durability, and ease of use. You can find a general-purpose servo motor at your local hobby store for around $15 or at several online retailers for around $5 each.

Hobby servo motors were designed to emulate the position of an R/C transmitter control stick. If the control stick is moved to its upward most position, the servo motor output shaft will likewise move 90 degrees in one direction. If the control stick is moved to its downward most position, the servo motor output shaft will move 90 degrees in the other direction. The total range of most hobby servo motors is about 180 degrees total (1/2 of a complete rotation). Trying to make the output shaft move any farther than its stopping point will likely result in a stripped gear.

A linear actuator is an electric DC motor that converts rotational motion into a linear motion. This is usually accomplished using an Acme threaded rod or other screw drive mechanism. These motors are useful for vertically lifting loads or moving an object back and forth (like a steering wheel). These can be used to automatically open a door or gate, raise/lower a hinged load, or move an object back and forth (see Figure 3-7).

Figure 3.7. Linear actuator motor from a power wheelchair, used adjust the incline of the seat

The stroke of a linear actuator refers to the maximum distance it can extend. The speed of the actuator tells you how fast it will travel, usually rated in inches/second. The power of the actuator is determined by the power rating of the motor that is driving it and is usually rated by the maximum load capacity in pounds that the actuator can lift. It is typically fine to use a relay to control a linear actuator for simple On/Off control, unless you need extremely precise control in which case you should use a motor-controller.

Because the amperage of a motor varies depending on the load, most DC motors list the voltage level at which it can safely operate. Although DC motors are usually forgiving and can be slightly over-powered without causing problems, an excess voltage level can burn up the motor coil.

As discussed in Chapter 1, the amperage that is consumed by a motor is dependent on the voltage level and the internal resistance of the motors coil. After the operating voltage is decided, you can measure the motor's coil to determine its resistance, and lastly use the voltage and resistance to calculate the amperage of the motor. With the amperage and voltage known, you can select a properly sized motor-controller for the motor.

The DC motor is the simplest motor to power; apply a positive signal to one wire and a negative signal to the other and your motor should move, as shown in Figure 3-8. If you swap the polarity of the wires, the motor will spin in the opposite direction.

The speed of the motor is dependent on the positive supply voltage level—the higher the voltage, the faster the motor shaft spins. The power of the motor is its capability to maintain its speed, even under a load and that is determined by the amperage available from the power source—as the workload of the motor increases, more amperage is drawn from the batteries.

Figure 3.8. To power a DC motor, simply connect one wire to the Positive supply and the other wire to the Negative supply.

Some of our robots have powerful motors that can operate at up to 24vdc—if the entire 24 volts is applied to the motor all at once, it is likely to spin the tires or pop a wheelie! We don't want to break any of our equipment or hit anyone nearby because our bot launches when we turn the motors on, so we will use a motor-controller to vary the voltage to the motor from 0v to the power supply voltage (in most cases, 6v, 12v, or 24v). This enables the bot to start slowly and work its way up to full speed, which causes less strain on the batteries during start-up and provides more precise control.

We can vary the voltage level to the motors by using a pulse-width modulation PWM signal to determine the output duty-cycle, or percentage of On time. Because using PWM means that the output is either fully on or fully off, the motors receive as much amperage as the power supply allows for the given duty-cycle, and the power can be varied from 0% to 100% for full speed control.

DC motors are in virtually any device that has moving parts—you can harvest useful DC motors from old cassette tape players, VCRs, toys, and cordless tools. Salvaging a DC motor is usually easy because they are rarely ever soldered to a printed circuit board (PCB), so you simply unplug the wires and remove any fasteners that are holding the motor in place. If the wires are soldered into place, just cut them leaving as much wire connected to the motor as possible (unless you plan on soldering your own wires to the motor terminals). Once removed, you can test the motor by powering it with a 6v or 12v battery (depending on its size).

As previously mentioned, gear-motors reduce the speed of a motor shaft to a usable RPM for driving a robot. When salvaging parts, you might come across a motor assembly that has plastic or metal reducing gears attached to the motor; you can re-use these gears and create your own makeshift gear-motor. Gear-motors and gear assemblies can also be found at surplus and commercial websites.

Surplus:

Commercial:

You can find 12v automotive windshield-wiper motors at your local junkyard that can be used as drive motors for a medium-sized bot. You can also find powerful motors at your local thrift-store by looking for cordless drills that have bad battery packs or cosmetic blemishes, but working motors and gear boxes.

There is also an acceptable condition called electric-braking, which refers to connecting both motor terminals to the same voltage supply—as opposed to leaving them disconnected. Because most DC motors act as a generator if you spin the motor shaft, by connecting both terminals to either the Positive supply or the Ground supply, we are essentially trying to force the generated electricity back into the same supply (see Figure 3-11). This results in the motor resisting to spin—that is, it will keep the motor shaft from moving by forcing opposing voltages into the same supply. We can tell the Arduino to keep both Low-side switches closed to form an electric brake when the bot is in Neutral to make sure it does not roll down a hill or move without being commanded. Alternatively, if all switches are left open in Neutral (coasting), there will be no resistance to the motor generating electricity—so if it is on a hill, it will roll.

Figure 3.11. Acceptable H-bridge states

Figure 3.12. Shoot-through H-bridge states—bad!

To create an H-bridge circuit, we simply need four switches—two of the switches must control the path of the current from the positive supply to each motor terminal, and the other two must control the path of the current from the negative supply to each motor terminal. These switches are labeled as S1, S2, S3, and S4 in the illustrations. We can use any type of switch that we want in the H-bridge, depending on our application. Relay switches work fine for single speed (On/Off) operation, whereas bipolar transistors or mosfets are more appropriate for full-speed control using PWM.

If you are making a smaller H-bridge with BJT transistors, you should include protection diodes from the drain to source of each transistor to protect them from Back EMF. Mosfets have built-in "body diodes" that are capable of handling the same voltages and amperage as the mosfet itself, so these are usually safe to interface directly to the Arduino.

There are four different homemade approaches to building an H-bridge circuit that we discuss, each with their own benefits and drawbacks. We start with the most simple implementation and progress to the most complex.

Method 1: Simple Switches

We can make a full H-bridge using (2) three-way (SPDT) switches from the hardware store, a DC motor, and a 9v battery. This simple bridge has built-in short-circuit protection so it cannot be commanded into a shoot-through state. It can, however, be placed into any acceptable H-bridge state: forward, reverse, electric brake (positive), electric brake (negative), or neutral. Each switch in the circuit has three positions, On/Off/On, and switches the center contact between the two outer contacts (or in this case, the positive and negative battery wires).

Figure 3.13. Here you can see a basic H-bridge circuit using two SPDT switches, a DC motor, and a 9v battery. Notice how the top terminals of each switch share a common positive supply wire, while the bottom terminals share a common negative supply wire. The center terminals of each switch are used to route the power signals to the motor terminals.

This method shows the simplicity of a basic H-bridge circuit, but does not provide speed control (it is either on or off). Although this might be a rugged circuit, its use is limited, so it is usually only good for testing and educational purposes.

Method 2: DPDT Relay with Simple

This method is wired the same as Method 1, but we combine the two SPDT switches and use one DPDT Relay, so it can be controlled by the Arduino. Also we can use the Arduino to provide a simple PWM signal for speed control of the motor (see Figure 3-14). The simplest way to do this is to add a Logic-level N-channel mosfet (or several in parallel) to control the entire circuit's path to ground. By using a PWM signal on the Ground supply to the H-bridge (Relay), we can control the speed of the motor from 0–100%, whereas the relay switches the motor's direction. The relay acts as both the High-side and Low-side switches in the bridge, so there are actually two low-side switches in this configuration—the relay used to route the power terminals and the N-channel mosfet used to provide the PWM speed control.

This provides complete 0–100% speed control and requires as few as four parts other than the relay: (2) logic level N-channel mosfets, (1) diode (for relay coil), and (1) small prototyping PCB (or you can make your own). Depending on the mosfet, you can expect to carry about 10 amperes at 24vdc with no heatsink or fan; an n-channel logic-level mosfet can be found at Digikey.com for between $0.50–$5.00 each and with an amperage rating from 100mA–200amps. I usually select power mosfets with the highest amperage rating in my price range (anything above 75 amps), a higher voltage rating than I plan to use in my project (usually 30v–55v is good), and the lowest possible on-state resistance (check the datasheet for Rds(On)).

Figure 3.14. A relay-based PWM speed control circuit

We can build this circuit with (2) FQP50N06L N-channel mosfets from Digikey. One mosfet is needed to provide PWM speed control, and the other mosfet is needed to interface the relay coil to the Arduino for direction control.

The relay mosfet can be controlled by any Arduino digital output pin, whereas the speed control mosfet should be controlled by an Arduino PWM output. Next we connect the mosfet Drain pin to the Relay as shown in Figure 3-14, and the mosfet Source pin to the main Ground supply. The prototyping PCB makes this easier to put together and you can add screw-terminals for easy wiring. The voltage and current limits of this circuit are dependent on the mosfet and relay ratings, giving this circuit potential despite using a mechanical relay switch.

Method 3: P-Channel and N-Channel Mosfets

Moving up, we have a basic solid-state H-bridge that uses P-channel mosfets for the high-side switches and n-channel mosfets for the low-side switches. This H-bridge has no internal protection against short-circuit, so you must be careful not to open both switches on the same side of the bridge because this will result in a shoot-through condition. This design can easily be implemented on a prototyping PCB as well as adding multiple mosfets in parallel to increase the current capacity. This H-bridge can be built using only two p-channel power mosfets, two n-channel power mosfets, two n-channel signal mosfets, and a few resistors (see Figure 3-15).

Figure 3.15. Notice the 10k pull-up resistors on the P-channel mosfets and the 10k pull-down resistors on the N-channel mosfets. This keeps the mosfets in the off state when not in use.

This method enables for a full solid-state circuit without using any mechanical switches or relays. If this circuit is operated within the voltage and current limits of the mosfets, it will easily outlast either of the previous methods. Even though this bridge is more complex than the previous two, it still has limitations; this design is not optimized for high PWM frequencies or high voltages, but costs little and is easy to build.

http://www.robotpower.com/products/osmc_info.html

To build your own H-bridge, but leave the designing to a professional, you might be interested in an H-bridge IC. An H-bridge IC is a complete H-bridge circuit that is contained on a tiny integrated circuit chip. These are usually fitted into a circuit with very few extra components, typically only a few resistors and a regulated power supply for the logic controls. When using an H-bridge IC, you can usually expect shoot-through protection, thermal overload protection, and high frequency capabilities. Although these H-bridge chips are far less likely to be destroyed by user error than a completely homemade design, they also have much lower power ratings than a homemade H-bridge, typically under 3amps of continuous current.

There are several H-bridge IC chips that include all four switches and a method of controlling them safely. The L293D is a Dual H-bridge IC that can handle up to 36 volts and 600 milliamp per motor. The L298N is a larger version of the L293D that can handle up to 2amps (see Figure 3-16). There are a few ICs that can control up to 25amps, but they are expensive and hard to find. There are several H-bridge ICs that work for some of the smaller projects in this book, but the larger bots require a higher powered H-bridge capable of conducting 10amps or more.

Figure 3.16. Here is the popular L298N dual 2amp H-bridge motor-controller on a homemade PCB. The other components include a 7805 5v regulator, a few EMF protection diodes, a capacitor, and some direction LEDs. This board can be used to control the speed and direction of two independent DC motors.

We talked about how higher PWM frequencies eventually lead to switching losses and cross-conduction, so what is a good PWM frequency to use? If you leave your Arduino alone and don't change anything, the PWM outputs will run at 1kHz (pins 5 and 6) and 500Hz (pins 11, 3, 9, and 10). This is considered a relatively low PWM frequency for motor-controllers because at this frequency, there is an audible "whine" that you can hear from the motor coils being switched.

Because most motor-controllers can easily handle a 1kHz PWM signal, you might want to leave the Arduino at its default values. If however, you want your motors to be silent during operation, you must use a PWM frequency that is above the audible human-hearing range, typically around 24,000Hz (24kHz). A problem arises because some motor-controllers are not capable of switching at such high frequency—switching losses increase as the PWM frequency increases. Because it is a difficult design task, motor-controllers that can operate at silent switching speeds (24kHz or higher) are usually more expensive and well built.

The frequency of each PWM output pin on the Arduino is controlled by one of three system timers that are built into the Arduino. Think of each system timer in the Arduino as a digital metronome, that determines how many beats will be in each second. The value of each timer can be changed using one line of code and a specific setting selected from Table 3-1.

To change the frequency of a PWM pin, select an available frequency from Table 3-1 and replace the <setting> in the following code with the appropriate setting from the chart. Then add the following line of code into the setup() function of your sketch, depending on the timer you want to change:

TCCR0B = TCCR0B & 0b11111000 | <setting>; //Timer 0 (PWM pins 5 & 6)
TCCR1B = TCCR1B & 0b11111000 | <setting>; //Timer 1 (PWM pins 9 & 10)
TCCR2B = TCCR2B & 0b11111000 | <setting>; //Timer 2 (PWM pins 3 & 11)

This table Table 3-1 shows the available frequencies with their corresponding settings—you might notice that some frequencies are available only on certain timers, making each PWM pin unique. For example, to change the frequency on PWM pins 9 and 10 from the default 500Hz to an ultra-sonic switching speed of 32kHz, change the setting for system timer 1 in the setup() function, as shown in the following:

void setup(){
    TCCR1B = TCCR1B & 0b11111000 | 0x01;
}

By changing timer 1 to a setting of "0x01", PWM pins 9 and 10 will now operate at 32kHz frequency anytime the analogWrite() command is used on either pin. Alternatively, you can set these same PWM pins to operate at their lowest available frequency (30Hz) by changing the <setting> to "0x05".

If you operate the PWM output at a too low of a frequency (below 100Hz), it will significantly decrease the resolution of the control—that is, a small change in the input will cause a drastic change in output and changes will appear choppy and not smooth as they do at higher frequencies. If in doubt, simply stay with the Arduino default PWM frequencies because they are sufficient for most robotics projects, even if you can hear your motors.

Back Electro-Motive Force (Back EMF) is the term used to describe the energy that must be disposed of when the electro-magnetic field of an inductor collapses. This collapse happens each time the motor is stopped or changes directions. If the voltage cannot escape through a rectifying diode, it can damage an unprotected transistor and possibly damage the Arduino pin that is driving it. A simple rectifier diode (1n4001) works for most relay coils and small BJT transistor-based H-bridges up to 1amp.

A protection diode should be placed between the motor terminal and the power supply. If using an H-bridge, a diode must be placed between each motor terminal and both the positive and negative power supply for a total of four diodes (see Figure 3-17). If you have an H-bridge that has no protection diodes, you can add the diodes directly onto the motor terminals as shown in Figure 3-18.

Figure 3.17. Protection diodes should be placed around the switches to protect them from motor back EMF, D1–D4 in the image.

Figure 3.18. Notice this implementation of Back-EMF protection diodes, soldered directly onto the motor terminals—this alleviates the need for diodes built in to the motor-controller.

Sometimes, the best way to protect a homemade H-bridge is to install a current-sensing device to monitor the level of amperage that is passing through the H-bridge. By reading the output of a current-sensor with the Arduino, we can send a stop command to each motor if the current level exceeds a given point. The over-current protection feature uses current sensing to disable the driver if the power reaches an unsafe level to protect it from overheating. Using this feature nearly eliminates user errors that can result in a destroyed motor-controller.

The simplest way to measure the amperage level in an H-bridge is to measure the voltage drop across a power resistor. This resistor must be placed in series with motor and the positive voltage supply, and the motor must be powered and running while you are measuring the voltage across the resistor (see Figure 3-19). Knowing the exact value of the resistor in ohms and the measured voltage across the resistor, we can use Ohm's law to calculate the amperage that is passing through the resistor, and therefore the circuit.

The only problem with this method is that the resistor creates heat in the process (wasting electricity). For this reason, it is ideal to pick the lowest value resistor possible (0.01–1 ohm) and it must have a power rating that is sufficient for the amount of current you will pass through it.

Figure 3.19. By measuring the voltage across a current-sensing resistor, we can calculate the amount of current that a motor is using.

For example, if the voltage drop across a current sensing resistor is 0.5 volts, and the resistor value is 0.05 ohms, how much current is passed through the resistor, and what will the power rating need to be for the resistor?

First we measure the current through the resistor:

If you measure 0.5 volts across a 0.05-ohm current sensing resistor, the amount of current that is passing through the resistor is 10amps.

Now to calculate the power dissipation of the resistor:

As you can see, the resistor must be rated for 5 watts to be able to handle 10 amperes flowing through it without failing.

There are better options available for current sensing in an H-bridge, like the hall-effect based ACS-714 current sensor that can accurately measure up to 30amps in either direction, and outputs a proportional analog output voltage that can be read using the Arduino. This sensor is mentioned in Chapter 2 as a non-autonomous sensor. It is available on a breakout board for use with an existing motor-controller or as an IC that can be soldered directly into a motor-controller design (like the ones used on the Explorer bot in Chapter 8). With this motor feedback mechanism, we can use the Arduino to monitor the motor output current and create a custom over-current protection method to keep the motor-controller from overheating.

If you do not plan to build your own motor-controller, you will still need to decide which one to buy. It is important to select a motor-controller with a voltage limit that is at least a few volts above your desired operating voltage, because a fully charged battery is usually a few volts higher than its rating. This is important because if the maximum voltage limit is exceeded even for a few seconds, it can destroy the mosfets, which will result in a broken H-bridge. The amperage rating is a bit more forgiving, in that if it is exceeded the H-bridge will simply heat up. Remember that using a heat sink or cooling fan can increase the maximum amperage limit by removing the dangerous heat, so many commercial units have heatsinks or fans built-in to aid in heat dissipation.

A commercial H-bridge can range in price from $10–$500+, but we will assume that you are not made of money, and focus mainly on the budget motor controllers. Most units accept PWM, Serial, or R/C pulse signals and some have the capability to read several different signal types using on-board jumpers to select between modes. You can find units that handle anywhere from 1amp to 150amps of continuous current and have voltage ratings from 6VDC to 80VDC.

Small (Up to 3amps)

This size H-bridge powers small hobby motors, typically not larger than a prescription medicine bottle. There are many different H-bridges available from online retailers that can handle several amps and range in price from $10–$30 (see Table 3-2). The L293D and L298N are two common H-bridge ICs that many small commercial motor-controllers are based on. The Sparkfun Ardumoto is an Arduino compatible dual motor-controller shield that is based on a surface mount version of the L298N H-bridge IC shown in Figure 3-16, and is capable of handling up to 2amps per channel (see Figure 3-20).

Figure 3.20. The Sparkfun Ardumoto is a motor-controller shield that is built around the L298N dual H-bridge IC.

This class of speed controllers also includes hobby Electronic Speed Controller (ESC. These H-bridges usually work only with voltages up to around 12v, because most hobby vehicles do not operate above 12v. They can, however, handle quite a bit of current, but accept only R/C servo pulse signals for control. These are available for both brushed and brushless DC motors, and are usually inexpensive and compact.

Table 3.2. This Chart Shows the Pricing of Some Small Commercial H-Bridges

Company	Model	Channels	Amperage Rating	Price
AdaFruit Industries	Motor-Shield	4	1-A	$19.50
Sparkfun.com	Ardumoto	2	2-A	$24.95

http://tech.groups.yahoo.com/group/osmc/

NiCad batteries have been around for several years and offer good performance and a life cycle of several thousand charges. These batteries are also used for cordless power tools, older cordless telephones, and consumer rechargeables (see Figure 3-21). NiCad is, however, prone to a condition called "memory" that occurs when it is repeatedly charged before being allowed to fully discharge. This causes the battery life to be reduced considerably and is different than reduced battery-life due to age. If you have any of these batteries that are still good, they can be used for small- to medium-sized robots—otherwise, NiMH batteries are generally a better option for around the same price.

Figure 3.21. NiCad batteries are typically available in standard alkaline battery sizes: AA, AAA, C, D, and so on, but have a cell voltage of 1.2v instead of 1.5v like a standard alkaline battery cell.

These rechargeable batteries are commonly used with cordless power tools, cordless telephones, cell phones, toys, and many consumer rechargeable batteries (AA, AAA, and so on) are still made using NiMH. These batteries offer high amp-hour ratings for their size, often in the range of 1000mAh to 4500mAh, and can typically be found in 1.2v cells, which can be arranged in a series to produce whatever voltage you need (see Figure 3-22). They can be re-charged many times but are prone to decreased output as they get older.

Figure 3.22. NiMh batteries are also available in standard.

Lithium Polymer batteries are one of the newer battery types used for their high power to weight ratio. With a typical cell voltage of 3-7v, these batteries are lightweight yet powerful and are able to deliver large amounts of current very quickly. LiPo batteries have recently become much more affordable, making them a viable option for many robotic projects, though proper charging and discharging is required to prevent overheating. They are typically arranged in series packs with up to six cells, totaling 22.2v (see Figure 3-23).

Figure 3.23. A few different sizes of hobby LiPo battery packs that I use for robotics projects. Shown are two 7.4v packs (2-cell), two 11.1v packs (3-cell), and one 18.5v pack (5-cell).

If a Lithium Polymer cell is discharged below 3-0 volts, it can become volatile and possibly catch fire. LiPo batteries must also be charged properly or again risk catching fire. For this reason, it is recommended that you learn more about lithium batteries before using them in a project. Although LiPpo's are lightweight and pack a lot of power, they can be dangerous if the proper precautions are not taken. Many people shy away from using these batteries in favor of a more forgiving chemistry.

This is the type of battery you can find in your car, boat, Power-wheels, solar power systems, and backup power applications. They are typically heavy and bulky, but have excellent power output and the highest amp/hour ratings that we use, usually ranging from 5AH to around 150AH. These batteries are typically available only in 6v, 8v, 12v, and 24v cells of which the 12v variety is the most common. They have internal lead plates arranged in series, each producing around 2 volts. The thickness of these plates determine the use of the battery.

There are several different types of lead-acid batteries to choose from, though we are interested only in the deep-cycle and AGM types for use on our bots.

Figure 3.24. A few standard AGM sealed lead-acid batteries. Pictured are two 6v, 4.5AH batteries (top) and one 12v, 7AH battery (bottom).

Each battery type has its own advantages. Notice in Table 3-5 that although NiCad and NiMh batteries are good choices for most projects, LiPo batteries are the lightest and lead-acid are the cheapest. Each has its own place in our projects, and we will use each type throughout this book.

It is best to buy a good battery charger that has multiple charging voltages and currents. A typical adjustable automotive battery charger is selectable from 6v–12v and has several amperage levels, usually 2amps and 15amps.

As a rule of thumb, a normal charging rate is not more than 1/10^th of the Amp/Hour rating. This means that a battery that is rated for 5000mAh should not be charged with a current that is more than 500mA (5000mAh / 10 = 500mA). If an automotive battery charger delivers too much current for your batteries, you might need to use a hobby-type charger that it compatible with multiple types of batteries.

I use a Dynam Supermate DC6 multi-function battery charger to charge all of my NiCad, NiMh, LiPoly, and small lead-acid batteries, because it offers both balance charging and selectable current levels for charging a variety of different battery types (see Figure 3-25). It requires a DC power source from 11V–18V, and can charge batteries up to 22.2V. Most hobby chargers also come with several different charging adapters to accommodate for the various plug types commonly found on rechargeable batteries.

Figure 3.25. A Dynam DC6 multi-battery charger. This charger is compatible with LiPo, NiMh, NiCd, and Lead-acid—as well as having an adjustable charging rate.

Lithium batteries require special attention to keep them from overheating and catching fire, so it is recommended that you buy the appropriate charger for the battery pack you have chosen. Lithium batteries have no "memory" effect, so you can charge them as often as you like without completely discharging them.

Wood is by far the easiest material to come by. You can theoretically make a robot from tree limbs if you were so inclined, or plane the limbs into planks and make a finished wood frame. Wood is generally used only for cheap prototyping and proof-of-concept project, because it is susceptible to bending and warping without proper treatment (not good for precisely mounted parts). Sheets of wood can be used to build a platform for holding electronics, because it is non-conductive and easy to drill holes in when mounting PCBs. Wood is also good for making wedges or spacers that might be too difficult to make using steel.

There are two types of metal that are of special interest to any robot builder: steel and aluminum. Both of these make strong and rigid frames that are resistant to warping and if properly painted, should last for many years. They can be either bolted together or welded and are strong enough for even the largest robots.

If using bolts, you need to pre-drill holes the same size as the bolts you use. You should buy drill bits made for drilling through steel and use the highest speed setting on your drill. You can also drill aluminum, wood, and plastics with a steel drill bit. Metals can also be "tapped" to add screw threads to a drilled hole, which can be helpful to decrease the number of bolts needed to secure each piece together.

If you need to cut steel or aluminum, you can use an angle-grinder, reciprocating-saw (with steel blade), cutting-torch, metal miter saw, jigsaw, or hacksaw depending on your tool selection. A reciprocating saw is usually the fastest and cleanest cutting of the saws (it is my personal choice for quick cuts), whereas an angle-grinder is good for cleaning up rough edges.

Regardless of what material you choose to build your robot, you are going to need some nuts and bolts to hold everything together. Using a nut and bolt to secure two pieces of metal, wood, plastic, or fiberglass together can provide excellent strength in addition to being removable if needed.

A bolt is a metal rod with precision threads, and is available in almost any length and diameter that you can think of, in both metric and SAE sizes. You can find bolts with various shaped heads and thread types but they all serve the same purpose—holding two or more objects together.

A nut is the threaded metal ring that mounts to the bolt. Like bolts, nuts are also available in various shapes and sizes, and with different thread types. You will likely need a pair of pliers or a wrench to tighten a nut to a bolt, and it is advisable to use a lock-washer to prevent the nut from loosening during use.

Figure 3.26. Here you can see a variety of nuts and bolts, like the ones used throughout this book.

PVC is a commonly available plastic that can be cut, drilled, tapped, and even welded using a stream of hot air. We use PVC plastic to make a motor-mount in Chapter 9, and clear plexiglass (clear acrylic) sheets in multiple projects as a non-conductive mounting base and protective cover.

Chain is used to transmit power from a drive sprocket on the motor, to a receiver sprocket mounted on the wheel. Chain comes in several different sizes, but we use only #25 roller chain plexiglass also labeled as 1/4in pitch. This type of chain is commonly found on electric scooters. Sprockets are also labeled by their pitch, so it is important to get matching sprockets and chain or they won't fit together. You can choose a gear-ratio for the motor sprocket and wheel sprocket pair by selecting the number of teeth on each sprocket. Using a small motor sprocket and a large wheel sprocket is a good way to gear down a drive-train, allowing for reduced speed and increased torque.

Chains are usually sold with universal connecting links so you can size the chain to fit your needs. When using a chain and sprocket setup, it is important to maintain proper chain tension. Too much tension can result in a broken chain and not enough tension creates a slack in the drive transmission that can result in broken sprockets teeth. Each of the projects that use chain drive transmissions need to have adjustable motors or wheels to tension the chains plexiglass this can complicate the building process, so it is usually desirable to use motors with the wheels attached when available.

The wheel is the final stage of the drive-train. It is what makes contact with the ground to propel the robot. As you might know, a larger wheel diameter travels farther than a shorter wheel diameter. Logically, this makes sense because each time the motor output shaft makes one complete revolution, the bot must travel the same distance as the outer circumference of the wheel. So a larger wheel travels faster than a smaller one, using the same motor RPM.

For smaller bots, you can make your own wheels from plexiglass or wood, or buy a toy car from your local thrift-store that has a salvageable set of wheels on it. Larger bots require finding wheels typically manufactured for a lawnmower, wheel-barrow, lawn-tractor, or other similarly sized vehicle that can carry the same payload. Wheels often require modification to fit onto a motor output shaft or attach a sprocket.