Although the concept and operational hardware for mobile robots have been around for decades, a rash of natural disasters and continuing military conflict have prompted designers to take a fresh look at the capabilities of this technology. The idea of an electromechanical device's replacing one or more humans in a hazardous or potentially dangerous situation has become the mantra of military, law-enforcement, aerospace, and exploration projects. Ranging from simple wheeled vehicles with a camera to unmanned aircraft with armaments, mobile-robotic technology has saved untold numbers of lives and become the favorite subject of new development projects. These applications have limited hardware resources and require real-time operation, compact mechanical design, battery operation, and economical software-requirements that embedded-system design shares. As such, robotic-control system design is a logical next step for embedded-system-design teams.
Current products represent the leading edge of mobile-robotic design. For example, iRobot Corp's rugged, lightweight PackBot EOD (explosive-ordnance-disposal) robot conducts EOD, hazardous-material handling, search and surveillance, and hostage-rescue tasks for bomb squads, SWAT (special-weaponsand- tactical) teams, and military units (Figure 1). The PackBot's manipulator system can reach as far as 2m in any direction to disrupt improvised explosive devices, military ordnance, land mines, and other incendiary devices. One operator can carry and deploy the PackBot EOD, which weighs approximately 53lbs. The chassis offers eight payload bays, each with interchangeable payload modules, such as video, audio, chemical, and biological sensors; mine detectors; ground-penetrating radar; and extra power. A ruggedized operator control unit provides wireless remote management and sensor display. Stressing the cost savings of robotics technology, Colin Angle, chief executive officer and cofounder of iRobot, says, "The US engagement in global conflicts has stressed our military and financial resources far beyond our original planning. More than half of our active-duty units are deployed abroad, and one in three reserve units were mobilized in the last four years. The bottom line is that expenditures are growing at an exponential rate, and a basic technological change is necessary to keep the cost and manpower requirements under control."
With an eye toward delivering supplies and equipment in unmanned vehicles through a hostile environment, the DARPA (Defense Advanced Research Projects Agency) sponsored its Grand Challenge on Oct 8, 2005 (Figure 2). With a $2 million prize for the team that most quickly finished a designated route across the Southwest's Mojave Desert, the contest was designed to accelerate research and development in autonomous ground-vehicle technology that will help save lives on the battlefield. DARPA Director Anthony J. Tether notes, "This event is a challenge for US ingenuity. It brings together individuals and organizations from the research-and-development community, industry, government, the armed services, academia, and professional societies and from the ranks of students, backyard inventors, and automotive enthusiasts." Stanford University's team pocketed the cash with its "Stanley" robotic car, which the team based on a diesel Volkswagen Touareg R5, which covered the 132-mile course in just less than seven hours with an average speed of 19.1mph. Stanley comes with seven Pentium M processors, a global-positioning system, a radar system, four laser-range finders, and a stereo-camera system. It also has an inertial-measurement unit that can estimate how the vehicle is tilted relative to the ground. Four other teams also completed the course.
Robot motion Motion control is the foundation of robotics, and designers most often implement it by computer control of electric motors or actuators. Embedded system designers have used electromagnetic-control circuits since the early days of electronics to energize motors, relays, solenoids, and speakers, but current motion-control techniques are more complex, because system requirements call for precision motion that is coordinated among multiple motors or actuators. Designers usually opt for dc or stepper motors for precision motion control. You can apply each motor type in open-loop situations or with feedback from the motor itself or other portions of your application to guarantee accuracy. Each of the motor types has myriad variations with corresponding advantages, drawbacks, and best suited applications.
Stepper motors are among the most popular robotic-motion control devices because they move in discrete steps, provide accurate angular-position information, and are relatively easy to control. The rotor of a stepper motor consists of permanent magnets arranged in a series of poles that determine the step size. The stator includes multiple windings to create a magnetic field that interacts with the rotor's permanent magnets. As a sequence of pulses from a control circuit turns the stator windings on and off, the motor rotates in the forward or reverse direction. Reversing the stator-pulse sequence changes the direction of rotation, and the frequency of the pulses controls the speed. To make a stepper motor rotate, you must constantly turn on and off the windings. Conversely, if you constantly energize one winding, the motor stops rotating and maintains an angular position with its specified holding torque.
Modern dc motors find wide use in robotic applications, which require precise control of rotational speed or torque. A dc motor rotates by the interaction of two magnetic fields. The stator creates a fixed magnetic field with a permanent magnet or energized electromagnet, and the rotating armature or rotor includes a series of electromagnets that generate a magnetic field when current flows through one of its windings. For a brushed-dc motor, a commutator on the rotor and brushes on the stator energize individual windings as the motor rotates. The opposite polarities of the energized rotor winding and the stator magnet attract, causing the rotor to move until it aligns with the stator field. Just as the rotor reaches alignment, the brushes move across the commutator and energize the next winding to maintain motion.
Speed in motors is proportional to the applied voltage, and output torque is proportional to the current. Robotic control of dc motors is a challenge, because you must vary the speed of the dc motor during operation. The most popular approach to efficient dcmotor operation is to apply a PWM (pulse-width-modulated) square wave with an on-to-off ratio corresponding to the desired speed. The motor acts as a lowpass filter to translate the PWM signal into an effective dc level. PWMdrive signals are popular because a microprocessor-based controller can easily generate them. Although a precise pulse width regulates the motor's speed, the actual PWM frequency is variable, and designers should optimize it to prevent motor chatter, audible noise, and RFI. To reverse a dc motor, you must reverse the direction of the current in the motor; most designers use four switching devices arranged in an H pattern for this purpose.
Brushless hall effect A popular variation of the dc motor eliminates the electrically noisy and maintenance-prone brushes but sacrifices controller simplicity. The brushless-dc motor is basically a synchronous motor with permanent magnets on the rotor and windings on the stator. Stator windings are arranged in a three-phase Y connection and produce trapezoidal torque characteristics. The energized stator windings create electromagnet poles that attract the rotor and generate torque. By using an appropriate sequence to apply voltages to the stator phases, you can create and maintain a rotating field on the stator. You must synchronize the stator current with the rotor field to generate a torque. Most brushless-dc motors use Halleffect sensors to read the motor's rotor position and enable the controller to switch the three winding phases on and off in the proper sequence to produce rotary motion.
As the motion-control problem becomes more complex through coordinated moves or intricate motion profiles, designers have turned to dedicated processors or DSPs to calculate real-time trajectories. For example, the MCK240 DSP motion-control kit from Technosoft is an evaluation and development package for the Texas Instruments TMS320F240 DSP controller. The kit comes with an onboard 50W, three-phase inverter and a brushless motor with Hall sensors and a 500-line encoder. Built-in current feedback from the inverter low-side legs or dc path allows the development and implementation of a large set of control algorithms, including sensorless approaches. All communication between the PC and the DSP board occurs through the RS-232 interface using flash resident monitoring software with downloading, uploading, debugging, and inspection capabilities. The kit includes a graphical motion-control, evaluation, and analysis software package, along with a set of ready to-run examples (Figure 3). The MCK240 costs approximately $1250.
Several motion-control vendors offer off-the-shelf controllers for mobile-robotics systems. Roboteq Inc offers a microcomputer-based, dual-channel dc-motor controller that can directly and continuously drive as much as 60A on each channel at voltages as high as 40V. The AX3500 targets designers of mobile-robotic vehicles, including automatic guided vehicles, underwater remotely operated vehicles, and mobile robots for exploration, hazardous-material handling, military, and surveillance applications (Figure 4). The OEM version comes on a 4.2 6.75-in. board and accepts commands from a serial-port interface or a standard remote-controlled radio for robot applications. With the serial port, developers can use the AX3500 to design fully autonomous or semiautonomous robots by connecting it to single-board computers, wireless modems, or wireless-LAN adapters. The controller's two channels can operate independently, or you can combine them to set the direction and rotation of a vehicle by coordinating the motion on each side of the vehicle. The motors operate in open- or closed-loop speed or position modes. The AX3500 includes inputs for two quadrature encoders and four limit switches for speed and odometer measurement. The AX3500 features intelligent current sensing and automatically limits each channel's power output to user-preset values as high as 60A. The AX3500 is available now at $395 (one) and comes with an interface cable and PC-based configuration software.
Hobby contributions Mobile robotics receives a huge amount of research, development, software, and trial-and-error testing from engineers and hobbyists having fun with combat robots. These fighting machines use dc motors, batteries, and remote control electronics, along with an array of weapons in an attempt to subdue or disable an opponent. The agility of these robots directly depends on the dc-motor controller. Several companies provide off-the shelf controllers to supply the high voltage and current that a battle situation requires. However, one of the most useful spin-offs for design engineers is the OSMC (Open Source Motor Controller) project. This project began as an Internethosted collaborative effort to design and implement a high-power, Hbridge motor-control system for permanent-magnet dc motors targeting battle robots. You can purchase kits or download free schematics, parts lists, and software for the OSMC boards from www.robot-power.com. You can also join the OSMC project and follow new developments at http://groups.yahoo.com/group/osmc/.
Mobile robotics is a new and growing industry with diverse military, law-enforcement, aerospace, medical, and home applications appearing weekly. For example, Daniel Zanini, senior vice president and program manager at research and engineering company SAIC, says, "Robotics will play a major role in our future combat systems. Unmanned aircraft, submarines, and land vehicles, along with conventional forces, will become nodes in a huge integrated network system, allowing commanders to determine the status and deploy resources from a computer screen." Targeting consumers, iRobot has shipped more than 1.2 million Roomba mobile robots for automated home vacuuming. A constant flow of new technology, such as improved motors, better control algorithms, and more efficient controllers, gives embedded-system designers plenty of tools and opportunities to participate in robotics development. Like it or not, mobile robots will continue to replace, augment, or support human activity in the future. Maybe some of those weird science-fiction stories were right on.
Author information You can reach Technical Editor Warren Webb at 1-858-513-3713 and wwebb@edn.com.
