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FIGURE 18.1 The basic wiring diagram for controlling twin robot drive motors. Note that the switches are DPDT and the spring return is set to center-off.
position, the motor turns clockwise. Activate the relay, and the contacts change positions, turning the motor counterclockwise. Again, you can easily control the direction relay with digital signals. Logical 0 makes the motor turn in one direction (let s say forward), and logical 1 makes the motor turn in the other direction. Both on/off and direction relay controls are shown combined in Fig. 18.4. You can quickly see how to control the operation and direction of a motor using just two data bits from a computer. Since most robot designs incorporate two drive motors, you can control the movement and direction of your robot with just four data bits. When selecting
DIRECTION CONTROL 257
Motor supply
To motor D1 1N4003 R1 1K b e c Q1 2N2222 RL1
On/Off control signal On 1 Off 0
FIGURE 18.2 Using a relay to turn a motor on and off. The input signal is TTL/microprocessor compatible.
TABLE 18.1
PARTS LIST FOR ON-OFF RELAY CONTROL.
RL1 Q1 R1 D1
SPDT relay, 5 volt coil, contacts rated 2 amps or more 2N2222 NPN transistor 1K resistor 1N4003 diode
All resistors have 5 or 10 percent tolerance, 1/4-watt.
relays, make sure the contacts are rated for the motors you are using. All relays carry contact ratings, and they vary from a low of about 0.5 amp to over 10 amps, at 125 volts. Higher-capacity relays are larger and may require bigger transistors to trigger them (the very small reed relays can often be triggered by digital control without adding the transistor). For most applications, you don t need a relay rated higher than two or three amps.
BIPOLAR TRANSISTOR CONTROL
Bipolar transistors provide true solid-state control of motors. For the purpose of motor control, you use the bipolar transistor as a simple switch. By the way, note that when I refer to a transistor in this section I m referring to a bipolar transistor. There are many kinds of transistors you can use, including the field effect transistor, or FET. In fact, we ll talk about FETs in the next section. There are two common ways to implement the transistor control of motors. One way is shown in Fig. 18.5 (see the parts list in Table 18.3). Here, two transistors do all the work. The motor is connected so that when one transistor is switched on, the shaft turns clockwise. When the other transistor is turned on, the shaft turns counterclockwise. When both transistors are off, the motor stops turning. Notice that this setup requires a dual-polarity power supply. The schematic calls for a 6-volt motor and a 6-volt and 6-volt power source. This is known as a split power supply.
258 WORKING WITH DC MOTORS
Ground +5V
Direction control signal CW 1 CCW 0
R1 1K b
D2 1N4003 c Q1 2N2222
FIGURE 18.3 Using a relay to control the direction of a motor. The input signal is TTL /microprocessor compatible.
TABLE 18.2 PARTS LIST FOR DIRECTION RELAY CONTROL
RL1 Q1 R1 D1
DPDT relay, 5 volt coil, contacts rated 2 amps or more 2N2222 NPN transistor 1K resistor 1N4003 diode
All resistors have 5 or 10 percent tolerance, 1/4-watt.
Perhaps the most common way to control DC motors is to use the H-bridge network, as shown in Fig. 18.6 (see the parts list in Table 18.4). The figure shows a simplified H-bridge; some designs get quite complicated. However, this one will do for most hobby robot applications. The H-bridge is wired in such a way that only two transistors are on at a time. When transistor 1 and 4 are on, the motor turns in one direction. When transistor 2 and 3 are on, the motor spins the other way. When all transistors are off, the motor remains still. Note that the resistor is used to bias the base of each transistor. These are necessary to prevent the transistor from pulling excessive current from the gate controlling it (computer port, logic gate, whatever). Without the resistor, the gate would overheat and be destroyed. The actual value of the bias resistor depends on the voltage and current draw of the motor, as well as the characteristics of the particular transistors used. For ballpark computations, the resistor is usually in the 1K- to 3K-ohm range. You can calculate the exact value of the resistor using Ohm s law, taking into account the gain and current output of the transistor, or you can experiment until you find a resistor value that works. Start high and work down, noting when the controlling electronics seem to get too hot. Don t go below 1K.
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