Building a Remote Controlled Robot
|
Building a Remote Controlled Robot |
 |
Practical review of wireless standards, technologies and devices for robot builders.
Sooner or later, every robot builder realizes that he needs some kind of remote control subsystem, either for controlling his machine, retrieving operational data or communicating with other robots. No matter how self-contained and autonomous your robot is, the chances are that you'll explore the world of wireless communications anytime soon.
Just a few years ago, options for building remote control devices that can be fitted into small mobile robots were quite limited. You could either use some variation of infrared devices (cheap but limited in range), or buy a rather expensive RF modem that allowed reliable two-way communication up to several miles of distance. We will disregard the IR approach right now, and analyze the configurations of some of the popular off-the-shelf RF systems. Let us just mention that IrDA links usually require accurately pointed IR pairs for high bandwidths. However, I'm aware of at least one product that provides near RF modem service using diffuse optical signals: the Spectrix Wireless Optical Network. Diffuse optical behave and perform very much like RF wireless networks except that optical signals do not transmit through solid objects. Unlike inferior point-and-shoot technology that most people associate with infrared TV remote controls, diffuse optical is omnidirectional just like RF transmissions. The most critical advantage of optical over RF is that separate wireless network segments can operate in adjoining rooms without cross talk or interference, thereby maximizing data throughout performance without using additional channels. If that's a kind of service you are looking for, please check the Spectrix site for more details.
Many of the robotics projects built today are using high end computers that fit on a small boards with integrated network adapters and loads of other goodies. The main advantage of this approach is the ease of programming, as you don't have to worry how to implement the networking part of your application. Most of the real-world projects today use 802.11b, a spread-spectrum, high bandwidth standard optimized for rapid transfer of large amounts of data. It typically works at 2.4 GHz, achieving bandwidths up to 11 Mbps, and new enhancements can effectively double that speed under the optimal circumstances. The use of a spread spectrum technique allows for reliable communication even with external interference. The 802.11b wireless adapters are now available from variety of sources and come in the form of PCI, PCMCIA, and ISA cards and also as standalone units. The prices for wireless adapters and access points, as well as other, more exotic parts of the equipment drop rapidly, making 802.11b the most attractive choice for robot builders at this time. For an excellent example of advanced robotic design using 802.11b, see CMU's Trikebot. Some of the TrikeBot's striking features include three large wheels and large base, designed for easy but stable maneuvering, just like a tricycle. The long neck projects from the front driving and steering wheel to support the Trikebot's new, low-cost camera, CMUcam.
Another wireless standard is also making its way into robotic applications: Bluetooth. Designed for close proximity on a desktop (and not really intended to be a wireless LAN technology), the Bluetooth radios transmit only 1 mW of power for a range of about 30 ft and transfer rates up to 400 kb/s. The technology is picking up steam, but still has some maturing/price reduction to do. For an excellent discussion on Bluetooth issues, see the article "Bluetooth Real World" at ExtremeTech. In the meantime, we already got first (although experimental) robot that use this technology: Murata Manufacturing recently announced Morph, a 13.6-inch tall Bluetooth-enabled humanoid robot that could be used as part of recovery missions in disaster areas.
The spread-spectrum, high-data-burst handling capabilities, and range of 802.11b make it considerably more complex. Today, it takes typically three chips to implement, compared with the two or one of Class 2 Bluetooth silicon. See Smarthome's excellent discussion "Bluetooth vs. 802.11b: And the winner is?" for more details on both standards. However, embedded 802.11b is also on the horizon: for example, Troy Wireless offers EtherWind-Plus, an intelligent 802.11b board-level product on a 2.6-inch x 3.8-inch board that includes processor, memory, parallel & serial interfaces and 10BASE-T Ethernet.
One of the best sources of information on how to build all types of small wireless networks, using 802.11b, Bluetooth and other standards, is Tim Higgins' Small Net Builder.
When implementing robots on small platforms, microcontrollers will probably be your best choice, due to their price, flexibility and small power consumption. RF modems were the obvious choice in such configurations, connecting directly to the microcontroller's serial port. The price of standard RF modem is typically a bit higher than other networking components described here, but so is their reliability and range. A wide variety of these devices are available on the market today from manufacturers such as ABACOM Technologies, Linx Technologies, RF Industries and Wireless Mountain Laboratories, to name just a few.
An interesting and affordable alternative to full-featured RF modem comes in the form of a specialized RF modules, available for all types of popular microcontrollers: here are the examples of such devices that extend the functionality of Basic Stamp and OOPic. Rob Arnold wrote an excellent article on building RuF-Bot, a wirelessly controlled robot based on Microchip PIC controller, using a serial data link sending at 9600 bps. He looked into variety of RF products by MING Microsystems and Radio Shack, but the Reynolds Electronics TWS-434 modules proved to be far superior in performance, cost and ease of use.
There are other interesting sources of information on building a wireless robotic solutions including a Remote Control Robotics" book, (this link points to summary and excerpt from the first chapter) and an article on wireless communication technologies targeted at robot builders. As always, don't underestimate the power of Usenet - the newsgroup comp.robotics.misc will probably be the best starting point for all topics related to robotics.