I use standard Hobby-grade R/C equipment to interface directly to the Arduino. This type of radio system requires a Transmitter (Tx) and a Receiver (Rx) used to link your inputs to the Arduino to control the motors (see Figure 8-2). The control signals are updated approximately 50 times each second, so changes to the inputs appear smooth at the motors.

Figure 8.2. A 2.4GHz radio transmitter and receiver

Use 2.4GHz radio equipment for the R/C link because it is allowed for use on surface operated vehicles and R/C aircraft.

You can purchase a good hobby radio control system online for approximately $40 (www.hobbypartz.com part# 79P-CT6B-R6B) or at your local hobby shop. Using 2.4GHz radio equipment ensures that there will be no other R/C operators nearby on the same channel that could cause interference. These systems are also easily interfaced to the Arduino using any digital input pin. The R/C receiver requires a +5v power signal, which can be supplied from the Arduino's +5v regulated supply. Remember to also connect the GND supplies together.

Although there are many good DC gear motors to choose from, I prefer a set of power wheelchair motors because they have excellent power and appropriate speed for a large robot. They are typically operated anywhere from 6V to 30V DC (rated at 24VDC) and can together carry approximately 400 to 500lbs, making them a versatile choice for medium-to-large robots that weigh between 30 to 300lbs. Depending on the weight of the bot, these motors draw anywhere from 3amps to 30amps continuously but can consume upwards of 50 to 100 amps if stalled.

I used two FRACMO brand power wheelchair motors that are rated at 24v and 9amps. The motors have six holes on each side of a flat gear-box that are used to mount the motors to the power wheelchair. I used these mounting holes to secure two 18-inch pieces of 3/4-inch angle iron to the top of the motor gear boxes, and a third piece of shaped angle iron bolted to the gear boxes, also used to mount the front caster wheel.

The motor controller for these massive motors needs to handle approximately 30 amps continuously and have some kind of feedback to determine if the motor controller is over-stressed. To accomplish this, I designed a simple dual H-bridge that incorporates a +/− 30amp current sensor into each H-bridge to tell the Arduino exactly how much power is pulled through the motors each time they are updated. If either motor consumes more current (amps) than the limit set in the Arduino code, the Arduino sends stop commands to both motors for 1 second to keep them from over-heating the motor controller. This ensures that the PCB is protected and the bot cannot be allowed to draw an excess of heat through the board that would result in a burned PCB trace. (Trust me, I have done this several times, and it is not fun to rebuild.)

The bot can be driven in plain view or using the on-board wireless video camera to transmit the video feedback to a base station. The video camera has pan and tilt functions to enable a full 180 degrees of rotation along two axes, which provides an adjustable view of the bot's surroundings. The video comes from a wireless outdoor camera with an automatic night-vision feature enabled using a built-in photo detector. When the light level is too low, the infrared LEDs automatically turn on to provide an excellent view at night.

In addition to having R/C control, this bot uses another wireless protocol called Xbee, which is used to create a Serial connection between the Arduino and your PC. This can be used to replace the R/C control completely if you want to control the bot solely from your computer. Because Xbee is a 2.4gHz device, the range is comparable to that of the R/C radio systems. Xbee works using two radios: one connected to your computer and the other connected to the Arduino controlling the robot. Using a two Xbee radios setup is like having your Arduino plugged into the computer, only wirelessly. You can send and receive values using a serial connection from your PC or programmed device (like another Arduino).

In this chapter, the Xbee modems transmit data from the Arduino to a computer for you to view the real-time current sensor and R/C readings on your serial monitor. In Chapter 13, "Alternate Control Bot, "I enlist the help of a friend to revisit the Explorer-bot and write a serial interface between the Arduino and your computer (through the Xbee link) to control the robot using a PC game pad controller.

Now take a look at what you need to build the Explorer-bot.

Before building the frame shown in Figure 8-3, review the specs and materials you need to do so:

Figure 8.3. Explorer-bot frame

The basic frame is made using five pieces of metal, sixteen bolts, the drive motor/wheel assemblies, and the front caster wheel. Steel is an excellent choice for a large robot because it is extremely strong and can be welded or bolted together. However, steel used in large quantities becomes heavy. As the bot gets heavier, the amperage draw from the motors goes up, so too much steel can over-stress your motor controller and drain the batteries more quickly, reducing run time. Try to use steel only as a skeleton for the frame, and use a plexiglass (acrylic) sheet for the deck of the robot to reduce weight.

Follow several steps to create the frame using metal stock. First, cut a piece of metal to secure the bottoms of the motors together and provide a base to mount the batteries. Next, cut two more pieces to secure the top, a support bar for the caster wheel, and finally the main frame piece that must be cut and bent to give the frame its shape. It is then only a matter of mounting the motors and caster wheel into the new metal frame to get it rolling. The following sections walk you through the steps required to build the frame.

The first thing I did was cut a 19-inch piece of flat steel bar to hold the bottom of the motor gear boxes together, as well as provide a place to mount the batteries. Because the batteries I use are 6 inches each, my frame needed to have a 12-inch minimum space between the motor gear boxes for the two batteries to sit end to end. My motors have six mounting holes on each side of the gear box to mount the bottom bracket, and I used only the two center mounting holes on each motor (see Figure 8-4). I decided to use two 12v batteries on this bot, so it can be arranged for either 24v 7Ah (series) or 12v 14Ah (parallel) operation.

Figure 8.4. The battery brace attached to the bottom of the motor gear boxes, using two bolts to secure the brace to each motor

The battery bracket is nothing more than a 19-inch piece of a 2-inch wide flat steel bar (1/8-inch thick). This piece mounts to the center holes in the bottom of the motor gear boxes. I used two bolts in each gear box to hold the bracket in place. When secured, you have a place to mount the two batteries between the motors. My batteries just happened to be the same height as the motor gear boxes, so they fit nicely between the motors (see Figure 8-5).

Figure 8.5. The two drive motors connected together by the battery brace

The bottom bracket needs to have two holes drilled in each end to mount to the bracket to the motors. Measure the distance between the two mounting holes on each motor. (My motors have a 2.5-inch spacing between mounting holes.) Now mark and drill the two holes (2.5-inches apart) on each end of the bottom bracket. You should then secure this bracket to the bottom of the motor gear boxes. Double-check to make sure both batteries fit in the new space before proceeding to the next step.

Cut two pieces of the 3/4-inch angle iron to 19-inch using a reciprocating saw if available, or a hacksaw. These two pieces can secure the motors at the top while keeping the wheels parallel to each other. These pieces also require marking and drilling two mounting holes on both ends of each brace. The mounting holes should be the same measurement as in the previous step (2.5-inch for me).

Measure, mark, and then drill holes in the upper frame braces to secure them to the motors (see Figure 8-6).

Figure 8.6. The two upper frame braces, used to hold secure the motors together on the top, which should be the same length as the bottom battery brace.

Secure only the inside bolts of the top braces for now. You use the outside mounting holes in the next step to also secure the main frame piece to the motors. (see Figure 8-7).

Figure 8.7. Start by securing the two upper frame braces using four bolts. Do not secure the bolts on the outside of the frame ye, because there will be one more frame piece installed before these bolts are secured to the motors.

The main frame piece gives the robot its shape and enables you to mount a caster wheel to the front of the bot that connects to the rear portion of the frame. I made four cuts with my reciprocating saw and a metal blade, cutting V shapes from one side of the angle iron at premeasured points (see Figure 8-8). When cut, the angle iron is easily bent; you make the shape permanent when you bold the piece to each motor gear box with two bolts per side (four bolts total). You need to measure the distance between the two mounting holes on the motors and drill two holes for each side.

Figure 8.8. Start modifying the main frame piece by cutting V shapes where shown; then drill (measured) holes to mount to each motor as shown.

To avoid having sharp corners typical to a standard square shape, I decided to go with a semi-octagonal shape that has a flat nose and slanted, less-pointed corners. Bend your newly cut piece into the shape shown in Figure 8-9, making sure the long 14-inch legs are approximately 19 inches apart from each other.

Figure 8.9. This diagram shows the dimensions of each segment of the main frame piece, between each V cut.

With the main frame piece cut, you still need to add a crossbar to provide support for the front caster wheel.

Because you use a thin 3/4-inch wide angle iron, there is only enough room to mount two of the four bolts on the front caster wheel mounting plate. Because this might leave the front wheel a bit wobbly, you need to add a crossbar from the lower bend on each side of the main frame piece, making sure that it travels across the other two mounting holes on the caster wheel mounting plate. If you have a welder, you can tack-weld the crossbar into place (see Figure 8-10). Otherwise you can place a bolt on each end of the crossbar into the main frame piece to hold them securely together.

Figure 8.10. The main frame piece ready to be mounted to the motors and wheels

You should now be ready to mount the caster wheel to the main frame piece and crossbar, drilling holes where necessary. A diagram of the top of the frame is shown in Figure 8-11.

Figure 8.11. This diagram shows the various pieces of the frame assembled together.

When selecting a caster wheel, you should measure the height of your rear wheel assemblies to determine the approximate deck height. By matching the height of the caster to the height of the robot's deck, you can avoid having a bend in the frame to compensate for a height difference. The caster wheel mounting plate typically has four holes on a rectangular plate mounted to a swivel bearing. These holes should be used to mount the caster wheel assembly to the front of the bot frame. You should keep in mind when placing objects near the front of the bot that the caster wheel must spin a full 360 degrees without hitting anything.

My caster assembly has a height of 7 inches total, with a wheel diameter of 6 inches. Because the height of the main deck is approximately 7.5 inches as mounted to the motors, the caster wheel I use gives the bot a slight downward angle that is barely noticeable.

With the caster wheel mounted to the front of the frame, you can now mount the rear of the main frame piece into the motors. You can see the main frame piece mounted to the motor gear boxes, through the remaining holes in the two top frame braces, as shown in Figure 8-12.

Figure 8.12. The main frame piece attached to the rear wheels using four bolts through the remaining mounting holes on each side.

When the main frame piece is mounted to the rear frame assembly and the caster wheel mounted to the front, the frame is complete. You can now either proceed to building the motor controller or add an optional plexiglass deck to the top of the frame.

Whether you need current limiting on your robot depends on the power of your motors. If you use a smaller gear motor that has a stall current of 15 amps, you can likely burn up the motor before a motor controller rated for 25 amps continuous current.

These power chair motors however were designed to carry not only a chair the size of a large robot, but also a human being riding in the chair–up hills, through a parking lot, and without burning up. This means that each motor can draw upward of 50 to100amps during a stall (depending on the motor), which is plenty of heat to burn a weak spot on a motor controller.

I used the ACS714 +/−30amp current sensor from Allegro Microsystems to measure the current flowing through one of the two motor outputs on each H-bridge. The AC714 current sensor operates using a +5v power supply, which can be provided by the Arduino. The sensor has three outputs connections: +5v, GND, and the analog output signal. The output voltage of each sensor is actually centered at 2.5v when the motors are not moving (0 amps) and move to 5v as the current flows positively (that is, the motor is spinning forward) or down to 0v as the current flows negatively. (That is, the motor is spinning in reverse.)

The Arduino can read this value from the current-sensor on any Analog Input pin and translate the voltage to a value between 0 and 1023, with the center value of 0 amps being 512 (1023/2). The ACS-714 datasheet lists the voltage sensitivity as 66mV per amp, so you must calculate how many millivolts are in each "step" of the 0–1023 value range of the Arduino's analog inputs to determine how many amps are measured at a given time.

There is only one catch: The current sensor chip is only available as a Surface Mount Device (SMD) chip, which takes a little finesse to solder onto the PCB. I also made an alternative PCB design without the current sensor, for those who do not want to use one.

For this H-bridge, I used standard P-channel and N-channel power mosfets both rated for 80amps and 55vdc. I used the TC4427 low-side mosfet driver as a signal buffer for the Arduino to provide the PWM signal to the N-channel mosfets (similar to the motor controller in Chapter 5, "Bump-Bot"). The TC4427 supplies 1.5 amps to the Mosfet Gate pins during high-speed PWM switching, enabling for use of ultrasonic (silent) motor operation at 32kHz PWM. The N-channel mosfets also each get a fast-recovery signal diode attached to their Gate pin, to help drain the excess current more quickly between switching cycles.

The P-channel mosfets are switched on using small N-channel signal mosfets, and pulled-up to the supply voltage using a resistor divider to turn them off. The P-channel mosfets (AHI and BHI) are only intended to be turned on or off when changing directions and should not be driven with a PWM signal. The resistor divider enables you to select resistor values based on your desired operating voltage; the resistor values used in this chapter are safe for use with 12v to 24v battery packs.

Figure 8.13. The schematic of the home made H-bridge motor controller used on the Explorer-bot. This schematic depicts one H-bridge circuit; the complete motor controller requires two of these identical circuits.

In addition to using higher-powered mosfets than the last motor controller, this board gets double mosfets. That is, I placed two mosfets in parallel on each leg of both H-bridges to double the current carrying capacity of the motor controller by halving the resistance. Remember from Chapter 1, "Introduction," lower resistance equals less heat.

It is now time to collect the needed parts and start building the motor controller! If you have not yet tried to etch a PCB, you might need to review Chapter 6. "Making PCBs," to build this circuit. When you are ready, follow these steps to print, transfer, etch, and build the PCB.

For instructions on how to etch a PCB, refer to Chapter 6. When assembling each board, make sure to check the silkscreen on the Eagle file, the motor controller schematic, and the component data sheets to verify the proper orientation of each part.

The Eagle board file in Figure 8-14 shows only one of the two h-bridges needed. You can either print out two separate boards, or use the dual-print board file that has two of these h-bridges side by side and ready to print onto a single board. They both share a common ground plane, so only one GND wire needs to connect from the Arduino.

Figure 8.14. The PCB file used to make the H-bridge schematic shown in Figure 8-13

Use the silkscreen (component outlines) from the PCB board file shown in Figure 8-14 to place each component before soldering. You should also make sure your finished motor controller looks like the one shown in Figure 8-15.

A few notes about the motor controller layout:

See Figure 8-15 for a picture of the finished dual H-bridge board. Notice the two sets of header pins near the top of the board; these connect to the ACS714 current sensors mounted to the bottom of the board. Also notice that only one of the center GND pins connect to the Arduino at the bottom.

Figure 8.15. The finished dual motor controller with eight mosfets in each H-bridge.

This motor controller design enables for individual control of each switch in the H-bridge. Though this bridge is capable of ultra-sonic PWM switching speeds (32kHz), the PWM signals should be used only on the low side inputs that feed into the high-speed mosfet driver IC. The high side inputs that control the P-channel mosfets are not set up for high-speed switching and should be switched on and off only when changing directions or stopping. Now that you have a working motor controller, it is time to set up the Arduino to control it.

Each H-bridge provides control of all four switches individually. This basic design works well, but there is no external protection against commanding the bridge into a short-circuit because this is done in the code; you should check all connections twice to ensure proper wiring. The H-bridge connections are labeled as AHI (A High Input), ALI (A Low Input), BLI (B Low Input), and BHI (B High Input). The low inputs connect to the N-channel power mosfets, and the high inputs connect to the P-channel power mosfets. Because only the N-channel power mosfets will be driven with a PWM signal, they connect to four of the six PWM outputs on the Arduino (PWM pins 3, 9, 10, and 11).

I do not use PWM pins 5 and 6 if possible because I plan to change the default PWM frequency and do not want to affect the system timer 0. To use different PWM frequencies requires changing the system timers that the PWM pins use. Because PWM pins 5 and 6 are attached to the main system timer 0, changing that timer changes the outputs of any function depending on Timer 0 (that is Delay(), millis(), and micros()). Because you need only four PWM outputs, but you need to change the default timing of all those outputs, use PWM pins 9 and 10 on timer 1 and PWM pins 3 and 11 on timer 2, changing both to a 32kHz PWM frequency for silent motor operation. Pins 5 and 6 will be used, but only as Digital Output pins; the PWM ability will not be utilized for this project.

Make each connection from the Arduino to the motor controller, Xbee, and R/C receiver according to Table 8-2.

I chose to mount all my electronics into the 5-inch x 7-inch project box from Radio Shack. This box provides a place to mount both the motor controller and the Arduino with Xbee. The box can then be moved without disconnecting the Arduino from the motor controller. The project box is optional but makes for a nicer looking installation.

To test the Xbee connection, load the code in Listing 8-1 to the Arduino and open a Serial monitor with the Xbee Explorer USB port to check the connection. With the serial monitor opened, type any character into the space and press send. If everything connects properly, whatever characters you send to the Arduino loop back through the Xbee and appear on the serial monitor.

// Code 8-1 - Xbee Test code

int incomingByte = 0;   // for incoming serial data

void setup() {
  Serial.begin(9600);    // opens serial port, sets data rate to 9600 bps
  pinMode(13, OUTPUT);

}

void loop() {

  // send data only when you receive data:
  if (Serial.available() > 0) {
    // read the incoming byte:
    incomingByte = Serial.read();

    // say what you got:
    Serial.print("I received: ");
    Serial.println(incomingByte, BYTE);

    digitalWrite(13, HIGH);  // turn on LED pin 13
    delay(1000);  // leave LED on for 1 second

  }

  else {

    digitalWrite(13, LOW); // otherwise, turn off LED pin 13

  }

}

When loaded, power the Xbee Explorer Regulated adapter board with +5v and GND, connect the DOUT pin to Arduino DP0 (rx), and the DIN pin to Arduino DP1 (tx). Or if you built the Arduino clone, simply plug the Xbee Explorer onto the FTDI programming port of the Arduino.

Remember that the serial monitor baud rate must be set to 9600bps before you can communicate through the Xbee modems. If a different baud rate is wanted, you need to reprogram both Xbee radios with the wanted speed, and change the speed in the Arduino code.

The last component that to add to the Explorer-bot is a wireless camera used to provide an audio/video link when guiding the robot to a location that cannot be seen by the operator.

To allow control of the camera position without moving the robot frame, you need a pan-and-tilt base for the camera. The pan-and-tilt functions are achieved using two standard sized hobby servo motors to create X and Y axes with 180-degrees rotation along either axis. This setup gives a full view of anything in front of the robot and enables much more precise movement than is provided by the robot base and the ability to look up and down.

By connecting the servo motors directly to the R/C receiver, you can control the orientation of the video camera using the R/C transmitter and without the extra overhead of processing the signals through the Arduino. (These servos can alternately be controlled by the Arduino if needed.)

The pan/tilt mechanism requires two standard-size hobby servo motors and two 7-inch long pieces of 2-inch wide aluminum flat bar (see Figure 8-19). The flat bar was cut and bent into two C-shaped pieces that connect together with the video camera mounted on top.

Figure 8.19. The pan/tilt mechanism is made from a few pieces of aluminum, some nuts and bolts, and two hobby servo motors.

To make the camera pan/tilt base, complete the following steps.

Cut the first piece of aluminum to 7 inches with a hacksaw or reciprocating saw and a fine-tooth metal blade. Next measure and mark the flat bar into three sections: 2 inches on each end and 3 inches in the center (2-inch left end + 3-inch center + 2 -inch right end = 7-inch total). Use a bench vise to bend this piece of flat bar at these marked points to 90-degree angles, making somewhat of a C shape cut servo mounting hole.

Now use one of the servo motors to mark the outline of the motor casing, centered onto the side of the first aluminum piece. When marked, use a Dremel tool and a metal cutoff wheel to cut out the hole for the servo motor, as shown in Figure 8-20.

Figure 8.20. Cut a hole for the servo motor using a Dremel tool and metal cutoff wheel.

After you test fit the servo motor into the mounting hole, mark the four mounting holes with a permanent marker and drill holes for them. Use four #8 bolts and nuts to secure the servo motor to the bracket, as shown in Figure 8-21.

Figure 8.21. Mount the servo motor into the first aluminum bracket.

The second bracket needs only to be bent and have a few holes drilled into it. Measure and mark the bracket at 1.75-inch on each end and 3.5-inch in the center to equal 7 inches total. Bend the bracket at the marked points to make the same shape as the first bracket. This bracket needs to be mounted to the first servo mounting plate on one side (see Figure 8-22), and have a 1/4-inch hinge bolt on the other side that enables the camera to freely pivot. The camera should mount to the top of this bracket.

Figure 8.22. Finished servo motor pan/tilt with camera installed

The second servo motor is the main motor that rotates the entire pan/tilt assembly, as shown at the bottom of Figure 8-22. I mounted this motor into an aluminum plate that is bolted to a 24-inch riser bar, mounted to the back of the robot frame. The riser bar serves to give the camera a better point of view.

To mount the servo into the aluminum plate (obtained as an extra piece from the Radio Shack project box), again mark the outline of the servo motor using a permanent marker, and cut the outline with a Dremel tool and a metal cutoff wheel. You can then mount the pan/tilt assembly to the top of this servo motor. With the servo motors installed, you can control their direction using two open channels on your R/C transmitter.

With the camera pan/tilt assembly completed, all that you need to do is to load the code and start testing.