Building Robots Part 5 pdf

[...]... RP7/1.2K R19/7 .5 , 5W SW6 "FAST" + R21/ 15 , 2W + J3 - BR1 12 volt regulated DC input LED21 - LED20 RP7/1.2K RP7/1.2K R18/7 .5 , 5W SW7 + R20/ 15 , 2W J4 "SLOW" - LED22 RP7/1.2K Figure 3.2: Battery Charger Schematic Diagram The rule of thumb for charging batteries is to charge them at a rate equal to one-tenth of the amp-hour capacity of the battery For example, if a battery is rated for 2 .5 amp-hours (as... the robot and the performance desired, dierent gear ratios might be appropriate Experimentation is probably the best way to choose the best geartrain 2.1 DC MOTORS 63 75% Duty Cycle 50 % Duty Cycle Motor on 25% Duty Cycle Motor off 1/1 25 sec Figure 2.3: Example of Several Pulse Width Modulation Waveforms 2.1.4 Pulse Width Modulation Pulse width modulation is a technique for reducing the amount of power... resistor's value The 6.270 battery charger allows switching between two resistors for each of the two battery charge circuits The 15 resistor limits current to about 250 to 300 milliamperes for a six volt battery This is the normal charge rate The 7 .5 resistor limits current to about 50 0 to 600 milliamperes This is a quick charge rate and should not be maintained after the battery is fully charged The resistors... resistor (12 volt supply minus 6 volt battery level minus 1.2 volts diode drop) For the 7 .5 resistor, the power dissipation is then 4 82 7 5, which is approximately 3 watts A 5 watt resistor was selected for use so as to allow a margin of error and to provide better heat dissipation A similar calculation can be made for the 15 resistor, for which a 2 watt rating was chosen The status LEDs are lit by the voltage... capacity of either lead acid or nickel cadmium cells Cell Type Carbon-Zinc Alkaline Lithium Nickel-Cadmium Lead-Acid Nickel-Hydride Power Internal Voltage Density Resistance Rechargeable 1 .5 volts low 1 .5 volts high 1 .5 volts very high 1.2 volts moderate 2.0 volts moderate 1.2 volts high high high low low low low no no no yes yes yes Cost low moderate high moderate moderate very high Figure 3.1: Table... time that the power is on determines the percentage of full operating power that is accomplished Figure 2.3 illustrates this concept, showing pulse width modulation signals to operate a motor at 75% , 50 %, and 25% of the full power potential A wide range of frequencies can be used for the pulse width modulation signal 6.270 system software used to control the motors operates at 1000 Hertz A PWM waveform... 2.1 DC MOTORS Specication 61 Rating Comment Voltage 5 volts normal low voltage Current up to 4 amps high current capacity Speed 4000-6000 RPM slightly slow (?) Torque unknown unusually high Figure 2.1: Polaroid Motor Specications The motor is used to eject lm in Polaroid instant cameras For this application, the fact that it is low voltage (5 volts) is very important so that only a few cells are... motors is quite powerful The servo motor provided in the 6.270 kit delivers approximately 50 ounce-inches of torque 2.3.1 Control A servo motor has three wires: power, ground, and control The power and ground wires are simply connected to a power supply Most servo motors operate from ve volts 2.3 SERVO MOTORS 65 The control signal consists of a series of pulses that indicate the desired position of... not be needed Servo motors would also be excellent for meshing with a gear rack, accomplishing highly controllable rectilinear motion 66 CHAPTER 2 MOTORS Chapter 3 Batteries Robots may be powered by a variety of methods Some large robots use internal combustion engines to generate electricity or power hydraulic or pneumatic actuators For a small robot, however, battery power oers a number of advantages... example, 9 volt alkaline batteries (which consist internally of six 1 .5 volt alkaline cells) are generally rated at about 1 ampere hour This means that the battery can continuously supply one ampere of current for one hour before \dying." In the capacity measurement, the 9 volt alkaline battery \dies" when the battery voltage drops below 5. 4 volts However, the amp-hour measurement is usually taken to assume . DC input BR1 R19/7 .5 , 5W R21/ 15 , 2W LED19 LED21 RP7/1.2K SW6 J3 RP7/1.2K R18/7 .5 , 5W R20/ 15 , 2W LED20 LED22 RP7/1.2K SW7 J4 RP7/1.2K "FAST" - - + "SLOW" +