Traditional R/C systems are rated by the number ofchannelsthey can control.
Channels refer to the number of independent servo signals the system can send si-multaneously to the receiver. Most of the low-cost radio sets meant for R/C cars are two-channel radios. The radio transmitter can send command information for two separate servo positions at once to the receiver to control both steering and motor speed (or throttle) simultaneously. The next level for R/C cars is three-channel ra-dios; the third channel is intended to control a gearshift, air horn, lights, or other on-board accessories. Most of these radio transmitters use a pistol-grip configura-tion, in which a gun-style finger trigger controls the throttle channel and a miniature wheel on the side of the transmitter controls the steering channel. A pistol-grip transmitter is shown in Figure 8-2.
The next step up is the model aircraft radios that typically have four channels.
The transmitters have a two-stick type configuration. The primary control is con-ducted through the two sticks, calledjoysticks. More advanced transmitters include additional channels that consist of switches and knobs for extra R/C capabilities.
Figure 8-3 shows a stick-style transmitter.
FIGURE 8-2 Futaba’s top
of-the-line 3-channel, pistol-grip–style computer transmitter model 3PJFS.(courtesy of Futaba)
FIGURE 8-3
A 6-channel, computer-controlled, dual-stick–style transmitter from Futaba.
(courtesy of Futaba)
Each of the two joysticks controls two channels—one channel with the hori-zontal direction, and one with the vertical. The top-of-the-line R/C sets, usually intended for the R/C helicopter market, can have up to nine channels of servo con-trol. Most of the high-end radio sets also have computerized control interfaces that allow the driver to configure the channel allocation, and change mixing set-tings, and the R/C system can be programmed for custom control sequences.
Whether you are independently controlling each of the channels that control the left and right motors, or you are controlling the robot speed with one stick and steering with the other stick, two channels are the minimum needed to drive a robot in a controlled fashion. Some more-complex robots that involve omni-directional wheels or multi-legged walking mechanisms need more than two channels for drive control.
Most competitions require that weapons are controllable via remote control, so you will need to include at least one channel for each weapon. Complex weap-ons—such as saws on moveable arms or spring-loaded rams with separately con-trolled release mechanisms—will need more than one channel. Gasoline engines may require several control channels—one for the throttle, a second to start the engine remotely, and a third to shut down the engine remotely. A general rule to remember is that you will need a separate servo command channel for each action that you want to control separately.
Radio Control Frequencies
The frequency bands for R/C systems are established by Federal Communica-tions Commission (FCC) regulaCommunica-tions. Specific bands of the radio spectrum are al-located for use by R/C hobbyists, and radio manufacturers have standardized specific frequencies inside these bands for use by hobby radios.Channel number in a radio refers to a specific frequency within the allowed range of the frequency band. The channel number should not be confused with the number of servo channels the radio set can control. Frequency bandwidth allocation varies by country; a radio operating on a legal frequency in the United States will not be legal for use in the United Kingdom, and vice versa.
27-MHz Radio Frequency Band
The 27-MHz radio band is usually used for small R/C toy cars, planes, and tanks.
This frequency band crosses into the lower channels on the citizens band (CB) radio frequencies, so there is a chance of interference by CB radio operators.
Both ground and aircraft vehicles are allowed to use the 27-MHz radio fre-quency band, which is divided into six separate channels. The first channel operates on 26.995 MHz, and each of the other channels are spaced every 0.05 MHz. Radio sets for the 27-MHz band are available in both amplitude modulation (AM) and frequency modulation (FM) configurations, and are usually low power and lim-ited to two or three channels. Although they can be used for combat robots, this is
not recommended. The antenna on the radio transmitter must have an attached flag indicating the frequency with which they are transmitting. The flag colors for channels 1 through 6 are brown, red, orange, yellow, green, and blue, respectively.
50-MHz Radio Frequency Band
This channel band is licensed for use by air or surface models, although it is usually used for R/C airplanes and helicopters. The 50-MHz band is divided into 10 fre-quency channels starting from 50.800 MHz and spaced every .020 MHz. Al-though several high-quality radios are available for this band, use of them requires a ham radio amateur license from the FCC. Although this band is rarely used for competition, the individual lucky enough to use this channel will be virtually as-sured of a clear channel, with no other robot builders using the same frequency.
Two flags must be flying on a 50-MHz radio transmitter antenna: a flag with a number between 00 and 09 to identify the frequency number, along with a black streamer to identify the 50-MHz radio frequency band.
72-MHz Radio Frequency Band
The 72-MHz radio band is reserved by the FCC for aircraft use only. In other words, ground vehicles, including combat robots, are not allowed to use this fre-quency band. A total of 50 different channels are available in the 72-MHz radio band with frequencies ranging from 72.010 MHz to 72.990 MHz, and with each channel number spaced every 0.020 MHz. The channel numbers range from 11 to 60. The channel identification flags include one with the channel number and a white streamer, attached to the transmitter’s antenna.
For all modern 72-MHz radios, changing the frequency requires changing the frequency crystals. The transmitter uses a crystal marked with “TX” and the re-ceiver’s crystal is marked with “RX.” When changing the crystals, they must both have the same radio frequency. (More on crystals in the upcoming section “Radio Frequency Crystals.”
75-MHz Radio Frequency Band
The 75-MHz radio band is reserved by the FCC for ground use only. Thirty differ-ent channels are available in the 75-MHz radio band with frequencies ranging from 75.410 MHz to 75.990 MHz, and with each channel number spaced every 0.020 MHz. The channel numbers range from 61 to 90. The channel identifica-tion flags are the ones with the channel number and a red streamer.
Changing the channel frequency or channel number within the 75-MHz fre-quency band also requires changing the frefre-quency crystals, as with the 72-MHz radios. However, you cannot change a 72-MHz band radio into a 75-MHz band radio by swapping frequency crystals. Although the crystals look identical in size
and shape, swapping the crystals between the two radio frequency bands will not work. Switching from one band to another requires retuning the radio, which should be done only by an FCC licensed technician.
If your robot is going to use a traditional R/C system, the frequency bands that you are allowed to use by law are 75 MHz, 27 MHz, and 50 MHz. The dilemma in this scenario is the fact that R/C systems that are meant for ground applications usually have only a few channels available for driving two or three servos. The high-quality, multi-channel radios are almost exclusively made for aircraft use. In the early days of robot competition, many robot builders used aircraft frequency (72 MHz) radios exclusively, because good-quality ground frequency (75 MHz) radios were not available. In recent years, however, competition organizers have begun enforcing FCC regulations about channel number and frequency band use, forcing robot builders to switch to non-aircraft frequencies.
Most 72-MHz R/C systems can be converted to operate on 75 MHz, but only after an extensive retuning process. Legally, retuning for 75 MHz has to be done by an FCC licensed technician. In most cases, this is most easily done by the radio’s original manufacturer—although some third-party shops, such as Vantec, can do the conversion process. For a nominal fee, some radio manufacturers will retune a radio for the 75-MHz ground frequency band when the radio is sold.
United Kingdom Radio Frequency Bands
Radio control systems in the United Kingdom are similar to those in the United States, but the particular radio frequencies used are different. The UK hobby radio control system runs on the 35-MHz and 40-MHz bands. The 35-MHz frequency band is reserved for aircraft use, and the 40-MHz band is reserved for ground ap-plications such as combat robots. The 40-MHz band is separated into radio con-trol channels every .010 MHz, from 40.665 to 40.995 MHz. As with those in the United States, robot builders in the U.K. must either purchase a 40-MHz ground radio or have a 35-MHz aircraft radio set converted into a 40-MHz system for ground channel use.
Radio Frequency Crystals
Within the frequency bands is a set of individual channel numbers that can be used for R/C applications. For example, 30 different radio channel numbers can be used in the 75-MHz frequency band. The specific channel number frequency is controlled by an oscillator called afrequency crystal, which is shown in Figure 8-4.
The frequency crystals come in pairs: one for the transmitter and one for the receiver.
To change the channel number on your radio, you simply replace the frequency crystals. Both the transmitter and receiver must use the same channel number, or the system will not work. The 72-MHz and 75-MHz crystals look identical, but the crystals are not interchangeable between frequency bands. In other words, putting a 75-MHz crystal in a 72-MHz radio will not work.
When selecting a radio system, make sure it will allow you to change the trans-mitting frequencies. Because it’s likely that at least one other person at a competition will be using the same frequency that you want to use, you will want to be able to change the frequency of your R/C equipment to avoid frequency conflicts. When this happens at a match, everyone loses control of their robots. This is also why you display the frequency number flags on your transmitter’s antenna so that ev-eryone else will know what frequency you are currently using.
At some matches, organizers control the frequencies that can be used and will issue the appropriate frequency crystals prior to each match. Other organizations, such asBattleBots, will impound your transmitter when you show up. Your trans-mitter will be returned to you prior to a match, during the 15-minute testing session and safety inspections, and after the event is over. Impounding transmitters is an extreme, but effective, method for preventing radio frequency interference.
Prior to competing, you should have at least two different sets of crystals so that you can change them to avoid frequency conflicts during the competition—espe-cially if you are competing in multiple-robot rumbles.
FIGURE 8-4 Typical radio frequency crystal pair.(courtesy of Futaba)
True Story: Stephen Felk and Voltronic
Stephen Felk had a wild ride on the wayto his most memorable fight—his veryfirst.
Although Stephen started out as an engineering student at Northwestern University way back in 1970, engineering studies didn’t keep his attention. He soon switched to the arts, and found himself in San Francisco dabbling with a variety of artistic endeavors: sculpture, music, even acting. But then a chance event changed his life.
Stephen Felk and Voltronic (continued)
“I was about to join another band when I drove byFort Mason one Fridaynight and saw a sign forRobot Wars,”says Stephen. “I bought a ticket and went in. I knew I was in trouble as soon as I walked in the door. It was the perfect combination of wrong elements. It had the engineering side, the sculpting side, the competition side, and some reallygreat camaraderie. I knew I had to do it.” So addicted was he, Stephen tried returning on Sunday. “I got there late and it was already sold out. I kept badgering the guy at the door, and finally, he says, ‘I’m going to turn my back. Whatever you do is up to you, but just leave me alone.’ So I snuck in—I’d never done anything like that before. But I watched the whole event, and couldn’t sleep for like a week afterwards.”
Stephen started working on his first robot shortlythereafter, beginning with a wheelchair he managed to pick up second-hand for just $100. “In this sport, sometimes the robot guys shine on you and I got off to a great start. I had no experience at all with the electrical/mechanical thing. But I thought about wheelchairs, and realized they’re designed to do basicallythe same thing as these robots. They’re designed for the same power-to-weight ratio, carrythe same weight, go about the same speed.”
Unfortunately, Stephen underestimated the time needed to build his creation;
and while he worked obsessively right up until the weekend beforeRobot Wars ‘97, he simply couldn’t get his creation completed in time. “I got in completely over my head. It was way too complicated, I had to learn too much, and a few days before the competition I thought, ‘My god, I’m not going to make it.’ Nothing could ever be as terrible as that.” The following year wasn’t to be either; but by 1999, he and Voltronicwere ready to rumble.
“My very first match was againstRazer,a really famous English robot, and it was far and away the best match I’ve ever been in. It was a really, really great battle.
There were four or five major turning points, points where we switched superiority, and it was incredibly exciting.”
Unfortunately, at its debut,Voltronichad a sheet metal skirt, a design element that Stephen describes now as “a really stupid idea.Razercomes slamming into me and rips the sheet metal right off. I’m driving around with these three pieces of sheet metal skirt just flapping in the wind.” The fight turned around, though, and Stephen says, “It ends up withVoltronic picking upRazerand slamming him into the wall. And that’s how the match ended: I had him two feet up in the air, pinned against the wall.”
Despite the triumphant ending, the winner was declared by audience vote—and Voltronicofficially lost toRazer. “But it was so exhilarating,” says Stephen, “going through this three-year ordeal, all that frustration, maxing out all my credit cards, and the battle was so incredible and so addicting, it was such a great reward and a vindication that this whole thing was really worth it.”
Stephen adds that he understood—even at that moment—whyhe lost. “He had a great-looking robot, and I just had a simple wedge. Worse, the entire time we were fighting, he was tearing off great sheets of sheet metal. It looked like I was torn up even though he didn’t reallyhurt me. But I was so proud to have this great fight against these great guys. They were great competitors, great sportsmen . . . and that first match instantlyjustified all the work that I’d put into it. It erased anydoubts I ever had.”