Servo motors

Servo motors are geared DC motors with positional control feed-back. Hobbyist servo motors are commonly used for position control for radio-controlled (R/C) models. The shaft of the motor can be positioned or rotated through a minimum of 90 degrees.

Because of their widespread use in the hobby market, servo motors are available in a number of stock sizes (see Fig. 4.9). While larger

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Vcc Vcc 47K47K 100 K .47 F .02 FSW1 Speed Fast/Slow

SW2 Direction

Vcc

Vcc U1U2 1

26

748 3 459121315863127 11 14 Vcc= 12 VDCU1 555 Timer U2 UCN 5804 䊏4.8BStepper motor

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industrial servo motors are also available, they are too expensive for most hobby applications. In this book we work with inexpen-sive and readily available hobby servo motors.

There are three wire leads to a servo motor. Two are for power, 4 to 6 V and ground. The third lead feeds a position-control signal to the motor. The control signal is a variable-width pulse between 1.0 and 2.0 milliseconds (ms). A neutral, midrange positional pulse is a 1.5-ms pulse. The pulse is sent 50 times a second (1 pulse every 20 ms or so) to the motor. This pulse signal will cause the shaft to lo-cate itself at the midway position at ±45 degrees.

䊏 4.9 Servo motor

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The shaft rotation on a servo motor is limited to approximately 90 degrees (±45 degrees from the center position). A 1-ms pulse will rotate the shaft all the way to the left (see Fig. 4.10), while a 2-ms pulse will turn the shaft all the way to the right. By varying the pulse width between 1 and 2 ms, the servo motor shaft can be rotated to any rotational degree position within its range.

You may feel that providing the pulse signal is a complex job; it isn’t.

The 16F84 PIC microcontroller, covered in Chap. 7, uses only a few lines of code to control a servo motor. And the PIC can control up to eight servo motors at a time. Another viable method is to utilize the servo motor control system used in R/C systems. Another alternative is to make your own circuit.

Making a servo motor circuit isn’t as difficult as it may first appear.

Figure 4.11 uses a 556 dual timer to control a servo motor. The 556 has two independent timers. To see the function more clearly, look at Fig. 4.12. Here two separate 555 timers are used. One timer is set in astable mode. The astable timer outputs a 55-hertz (Hz) square wave with a 1-ms negative component. The output from this timer is connected to the second 555 timer that is set up in monostable mode.

䊏 4.10 Servo motor pulse signal 1 ms Pulse

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The monostable timer outputs a positive pulse from pin 5, each time it receives a negative pulse from timer 1. The positive pulse from timer 2 can be varied from 1 to 2 ms using the 10K-ohm poten-tiometer. You may need to fiddle with the resistance values of R1 and R2 in Fig. 4.11, depending upon the servo motor being used.

As always, take caution that the servo motor isn’t stalled (pushing against its own internal rotational stops).

When working with servo motors, I have found that to achieve full rotational movement from the shaft I needed to use pulses shorter than 1 ms and greater than 2 ms.

As you work with and gain experience using servo motors, you can also run outside the standard pulse widths (shorter and longer pulses) to achieve greater (180 degree) shaft rotation. The standard pulse widths sometimes do not rotate the shaft to each end stop.

However, before you do so you need to understand that if a pulse signal falls outside of the range where the servo motor shaft can rotate, the servo motor will fight against its own internal rotational stop in an effort to rotate the shaft to the position called for by the pulse.

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For example, suppose you have a servo motor and you are providing a 2.8-ms pulse to rotate the shaft all the way to the right. As long as the shaft can rotate to that position, you’re fine. But suppose a different servo motor reaches its maximum right rotation with a 2.5-ms pulse. If you start sending a 2.8-ms pulse width, you are ordering the servo motor to turn further than it physically can.

Because the servo motor is fighting against the internal stop, cur-rent drain goes up, you increase wear on the gearing, and you may burn out the servo motor.

This problem usually occurs when the original servo motor tested is changed and the replacement motor doesn’t need the modified pulse widths. As a rule of thumb, if using a pulse outside of the rec-ommended 1- to 2-ms pulse, always check to make sure the servo motor isn’t stalled in position.

Servo motors are used to make the walker robot in Chap. 11. The walker robot uses a PIC microcontroller to control the servo motors.

The PIC microcontrollers and servo motor application are covered in Chap. 6.

䊏 4.12 Servo motor circuit 555

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DC motors

DC hobby motors can be applied to movement and locomotion (see Fig. 4.13). Specifications of most DC motors show high revo-lutions per minute (rpm) and low torque. Robotics need low rpm and high torque. Gearboxes can be attached to motors to increase their torque while reducing the rpm (see Fig. 4.14). The gearbox usually specifies a ratio that describes the rpm in to the rpm out. For instance, a DC motor with an rpm of 8000 is connected to a 1000:1 gearbox. What is the output rpm? 8000 rpm/1000 8 rpm. The

䊏 4.13 DC motor

䊏 4.14 DC motor with gearbox

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torque of the motor is substantially increased. You could estimate that the torque will increase by the same value the rpm decreased.

In reality, no conversion is 100 percent efficient; there always will be efficiency losses.

Some DC motors, called gearhead motors,are built with a gearbox attached (see Fig. 4.15).