http://www.ee.ui.ac.id/eecourse Body / Plant Actuator / motor Sensor Brain / controller
Microcontroller These are programmed either in assembly language or a high-level language such as Basic or C. The LEGO Mindstorms RCX is a goodexample of a robot run from a microcontroller. Single-board computer These are also programmed either in assembly language or a highlevellanguage, but they generally offer more processing power than a microcontroller. Personal computer Examples include an IBM PC compatible or an Apple Macintosh.
Sonar range finder Reflected sound waves are used to judge distances. Its effective use istypically from about a foot to 30 40 feet. Sonar proximity or movement Reflected sound waves are used to determine if the robot is close to anobject (this is called proximity detection ). Movement (a person, dog,whatever) changes the reflected sound waves and can likewise be detected.its range is from 0 inches to 20 30 feet. Infrared range finder or proximity Reflected infrared light is used to determine distance and proximity. Thedetected range is typically from 0 inches to two or three feet. Light sensors Various light sensors detect the presence or absence of light. Light sensorscan detect patterns when used in groups (called arrays ). A sensor with anarray of thousands of light-sensitive elements, like a CCD video camera,can be used to construct eyes for a robot. Pyroelectric infrared A pyroelectric infrared sensor detects changes in heat patterns and is oftenused in motion detectors. The detected range is from 0 to 30 feet and beyond. Speech input or recognition Your own voice and speech patterns can be used to command the robot. Sound Sound sources can be detected by the robot. You can tune the robot to listen to only certain sound wavelengths or to those sounds above a certain volume level.
Contact switches Used as touch sensors, when activated these switches indicate that the robot has made contact with some object. Accelerometer Used to detect changes in speed and/or the pull of the earth s gravity,accelerometers can be used to determine the traveling speed of a robot or whether it s tilted dangerously from center. Gas or smoke Gas and smoke sensors detect dangerous levels of noxious or toxic fumes and smoke. Temperature A temperature sensor can detect ambient or applied heat. Ambient heat isthe heat of the room or air; applied heat is some heat (or cold) source directly applied to the sensor. Motor Most often, one or more motors are attached to the outputs of a robotto allow the machine to move. On a mobile robot, the motors serve todrive wheels. On a stationary robot, the motors are attached to armand gripper mechanisms, allowing the robot to grasp and manipulate objects. Solenoid A robot may use solenoids to hop around a table Pump A robot uses pumps and valves to power pneumatic or hydraulic pressure systems.
Sound The robot may use sound to warn you of some impending danger( Danger, Will Robinson, danger! ) or to scare away intruders. If you vebuilt an R2-D2 like robot (from Star Wars fame), your robot might usechirps and bleeps to communicate with you. Hopefully, you ll know what bebop, pureeep! means. Voice Either synthesized or recorded, a voice lets your robot communicate in more human terms. Visual indication Using light-emitting diodes (LEDs), numeric displays, or liquid crystaldisplays (LCDs), visual indicators help the robot communicate with you in direct ways. As you graduate to building a mobile robot, you should consider thephysical properties of your creations, including their size, weight, andmode of transport. A robot that is too heavy for its frame, or a locomotion mechanism that doesn t provide sufficient stability, will greatly hinder theusefulness of your mechanical invention. The parts of a robot that contribute the most to its weight are the following, in (typical) descending order: Batteries Drive motors Frame
For wheeled and tracked robots, differential steering is the most common method for getting the machine to go in a different direction. The result is that the robot turns in the direction of the stopped or reversed wheel or tread. Because of friction effects, differential steering is most practical with two-wheel-drive systems. Additional sets of wheels, as well as rubber treads, can increase friction during steering. Robots with car-type steering are not as maneuverable as differentially steered robots, but they are better suited for outdoor uses, especially over rough terrain. This technique is called Ackerman steering and is found on most cars but not on as many robots.
Tricycle-steered robots must have a very accurate steering motor in the front. The motor must be able to position the front wheel with subdegree accuracy. Otherwise, there is no guarantee the robot will be able to travel a straight line. Most often, the steering wheel is controlled by a servo motor. To have the highest tech of all robots, you may want omnidirectional drive. It uses steerable drive wheels, usually at least three. The wheels are operated by two motors: one for locomotion and one for steering. In the usual arrangement, the drive/steering wheels are ganged together using gears, rollers, chains, or pulleys. Omnidirectional robots exhibit excellent maneuverability and steering accuracy, but they are technically more difficult to construct.
To have the highest tech of all robots, you may want omnidirectional drive. It uses steerable drive wheels, usually at least three. The wheels are operated by two motors: one for locomotion and one for steering. In the usual arrangement, the drive/steering wheels are ganged together using gears, rollers, chains, or pulleys. Omnidirectional robots exhibit excellent maneuverability and steering accuracy, but they are technically more difficult to construct. Direct current DC dominates the field of robotics, either mobile or stationary. DC is used as the main power source for operating the onboard electronics, for opening and closing solenoids, and, yes, for running motors. Few robots use motors designed to operate from AC, even those automatons used in factories. Such robots convert the AC power to DC, then distribute the DC to various subsystems of the machine.
With a continuous motor, the application of power causes the shaft to rotate continually. The shaft stops only when the power is removed or if the motor is stalled because it can no longer drive the load attached to it. A special subset of continuous motors is the servo motor, which in typical cases combines a continuous DC motor with a feedback loop to ensure the accurate positioning of the motor. A common form of servo motor is the kind used in model and hobby radio-controlled (R/C) cars and planes. With stepping motors, the application of power causes the shaft to rotate a few degrees, then stop. Continuous rotation of the shaft requires that the power be pulsed to the motor. As with continuous DC motors, there are subtypes of stepping motors.
OPERATING VOLTAGE All motors are rated by their operating voltage.with small DC hobby motors, the rating is actually a range, usually 1.5 to 6 volts. Some highquality DC motors are designed for a specific voltage, such as 12 or 24 volts. CURRENT DRAW Current draw is the amount of current, in milliamps or amps, that the motor requires from the power supply. Current draw is more important when the specification describes motor loading, that is, when the motor is turning something or doing some work. The current draw of a free-running (no-load) motor can be quite low. But have that same motor spin a wheel, which in turn moves a robot across the floor, and the current draw jumps 300, 500, even 1000 percent. How to test the current draw of a motor by measuring the voltage developed across an in-line resistor. The actual value of the resistor can vary, but it should be under about 20 ohms. Be sure the resistor is a high-wattage type.
The rotational speed of a motor is given in revolutions per minute (rpm). Most continuous DC motors have a normal operating speed of 4000 to 7000 rpm. For just about all robotic applications, these speeds are much too high. You must reduce the speed to no more than 150 rpm (even less for motors driving arms and grippers) by using a gear train. Note that the speed of stepping motors is not rated in rpm but in steps (or pulses) per second. The speed of a stepper motor is a function of the number of steps that are required to make one full revolution plus the number of steps applied to the motor each second. As a comparison, the majority of light- and medium-duty stepper motors operate at the equivalent of 100 to 140 rpm. Torque is the force the motor exerts upon its load. The higher the torque, the larger the load can be and the faster the motor will spin under that load. Reduce the torque, and the motor slows down, straining under the workload. Reduce the torque even more, and the load may prove too demanding for the motor. The motor will stall to a grinding halt, and in doing so eat up current (and put out a lot of heat). The torque of a motor is measured by attaching a weight or scale to the end of a lever and mounting the lever of the motor shaft.
We ve already discussed the fact that the normal running speed of motors is far too fast for most robotics applications. Locomotion systems need motors with running speeds of 75 to 150 rpm. Any faster than this, and the robot will skim across the floor and bash into walls and people. Arms, gripper mechanisms, and most other mechanical subsystems need even slower motors. The motor for positioning the shoulder joint of an arm needs to have a speed of less than 20 rpm; 5 to 8 rpm is even better. USING MOTORS WITH GEAR REDUCTION It s always easiest to use DC motors that already have a gear reduction box built onto them. R/C servo motors already incorporate gear reduction, and stepper motors may not require it. It s fairly easy to change the rotational direction of a DC motor. Simply switch the power lead connections to the battery, and the motor turns in reverse. RELAY CONTROL Perhaps the most straightforward approach to the automatic control of DC motors is to use relays. It may seem rather daft to install something as old-fashioned and cumbersome as relays in a high-tech robot, but it is still a useful technique.
Bipolar transistors provide true solid-state control of motors. For the purpose of motor control, you use the bipolar transistor as a simple switch.
There will be plenty of times when you ll want the motors in your robot to go a little slower, or perhaps track at a predefined speed. Speed control with continuous DC motors is a science in its own, and there are literally dozens of ways to do it. Pulse width modulation (PWM) is the most popular motor speed control technique. There are a number of ways of providing PWM. It is important to note that the frequency of the pulses does not change, just the relative on/off times. PWM frequencies of 2 khz to over 25 khz are commonly employed, depending on the motor. Unless you have a specification sheet from the manufacturer of the motor, you may have to do some experimentation to arrive at the ideal pulse frequency to use. You want to select the frequency that offers maximum power with minimum current draw.
In previous section we ve looked at powering robots using everyday continuous DC motors. DC motors are cheap, deliver a lot of torque for their size, and are easily adaptable to a variety of robot designs. By their nature, however, the common DC motor is rather imprecise. Without a servo feedback mechanism or tachometer, there s no telling how fast a DC motor is turning. Furthermore, it s difficult to command the motor to turn a specific number of revolutions, let alone a fraction of a revolution. Enter the stepper motor. Stepper motors are, in effect, DC motors with a twist. Instead of being powered by a continuous flow of current, as with regular DC motors, they are driven by pulses of electricity. Each pulse drives the shaft of the motor a little bit. The more pulses that are fed to the motor, the more the shaft turns. As such, stepper motors are inherently digital devices, a fact that will come in handy when you want to control your robot by computer. A unipolarstepper requires that a sequence of four pulses be applied to its various windings for it to rotate properly. By their nature, all stepper motors are at least two-phase. Many are four-phase; some are six-phase. Usually, but not always, the more phases in a motor, the more accurate it is.
STEP ANGLE Stepper motors vary in the amount of rotation of the shaft each time a winding is energized. The amount or rotation is called the step angle and can vary from as small as 0.9 (1.8 is more common) to 90. The step angle determines the number of steps per revolution. A stepper with a 1.8 step angle, for example, must be pulsed 200 times for the shaft to turn one complete revolution. A stepper with a 7.5 step angle must be pulsed 48 times for one revolution, and so on. PULSE RATE Obviously, the smaller the step angle is, the more accurate the motor. But the number of pulses stepper motors can accept per second has an upper limit. Heavy-duty steppers usually have a maximum pulse rate (or step rate) of 200 or 300 steps per second, so they have an effective top speed of one to three revolutions per second (60 to 180 rpm). Some smaller steppers can accept a thousand or more pulses per second, but they don t usually provide very much torque and aren t suitable as driving or steering motors. TORQUE Steppers can t deliver as much running torque as standard DC motors of the same size and weight. However, steppers are at their best when they are turning slowly. With the typical stepper, the slower the motor revolves, the higher the torque.
Servo Motor Servo motors are designed for closed feedback systems. The output of the motor is coupled to a control circuit; as the motor turns, its speed and/or position are relayed to the control circuit. If the rotation of the motor is impeded for whatever reason, the feedback mechanism senses that the output of the motor is not yet in the desired location. The control circuit continues to correct the error until the motor finally reaches its proper point. Servos and Pulse Width Modulation The motor shaft of an R/C servo is positioned by using a technique called pulse width modulation (PWM). In this system, the servo responds to the duration of a steady stream of digital pulses.
Shaft encoders allow you to measure not only the distance of travel of the motors, but their velocity. By counting the number of transitions provided by the shaft encoder, the robot s control circuits can keep track of the revolutions of the drive wheels. The typical shaft encoder is a disc that has numerous holes or slots along its outside edge. An infrared LED is placed on one side of the disc, so that its light shines through the holes. The number of holes or slots is not a consideration here, but for increased speed resolution, there should be as many holes around the outer edge of the disc as possible. The pulses from a shaft encoder do not in themselves carry distance measurement. The pulses must be counted and the count converted to distance
So far we ve investigated shaft encoders that have just one output. This output pulses as the shaft encoder turns. By using two LEDs and phototransistors, positioned 90 out of phase, you can construct a system that not only tells you the amount of travel, but the direction as well. Like the human hand, robotic grippers often need a sense of touch to determine if and when they have something in their grasp. Knowing when to close the gripper to take hold of an object is only part of the story, however. The amount of pressure exerted on the object is also important. Too little pressure and the object may slip out of grasp; too much pressure and the object may be damaged. Mechanical Switch The lowly mechanical switch is the most common, and simple, form of tactile (touch) feedback. Most any momentary, spring-loaded switch will do. When the robot makes contact, the switch closes, completing a circuit (or in some cases, the switch opens, breaking the circuit).
Optical sensors use a narrow beam of light to detect when an object is within the grasping area of a gripper. Optical sensors provide the most rudimentary form of touch sensitivity and are often used with other touch sensors, such as mechanical switches
Obviously, the home-built pressure sensors described so far leave a lot to be desired in terms of accuracy. If you need greater accuracy, you should consider commercially available strain gauges