Download (direct link):
You can see an example R/C servo motor in Figure 15-11.
A typical standard-size R/C servo motor.
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342 Part V: A Plethora of Projects_
Going inside a servo motor
So what’s inside a servo motor? Servo motors that you use for radio-controlled planes and cars consist of some control electronics, a motor, a couple of gears, and a variable resistor (potentiometer).
Here’s how all of these parts work together:
^ The control electronics are there to receive a signal from a radio control receiver (or in our case, the BASIC Stamp microcontroller) to activate the motor.
^ The motor turns. The motor runs pretty quickly, and your robot can’t run that fast, so the action of a series of gears reduces the motor’s speed.
^ Connect the potentiometer to the final output gear. This gear protrudes outside of the servo motor and connects to a linkage, wheel, or whatever. As this output gear turns, so does the potentiometer. The position of the potentiometer tells the control electronics the position of the output gear.
Going shopping for servos
First things first — you need to go out and buy your servo motors. You can find more than a half-dozen major manufacturers of R/C servo motors, and each manufacturer offers a multitude of models. But when it comes to adapting a servo motor for robotics, only three servos stand out from the crowd as both affordable and easy to modify:
^ Futaba S148
^ Grand Wing Servo (GWS) S03 and S06 ^ Hitec HS-422
These servos share a common trait — they use a small retainer clip on the underside of the output gear to engage with the potentiometer. You can most easily modify this type of servo. When you remove the clip, the output gear no longer turns the potentiometer shaft. (You also need to clip with snippers or file off a molded-in ridge on the top of the output gear. This ridge serves as a physical stop to prevent the output gear from turning more than about 270 degrees.)
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Chapter 15: Cool Robot Projects to Amaze Your Friends and Family 343
Making servos serviceable
Now you have your servo motors, but there’s a fly in the ointment: The manufacturers designed the R/C servo motor to move back and forth about 90 degrees in each direction, and no more. To operate a robot, you need to convert the motor for continuous rotation so the wheels don’t just stop at 90 degrees. Happily, this job isn’t tough, as long as you choose the right kind of servo. (You can read more about how to modify an R/C servo motor in the following section.)
The benefit of modifying an R/C servo motor is that you don’t need to monkey around with additional interface electronics to use the motor with a microcontroller, such as the BASIC Stamp 2. In addition, R/C servo motors are fairly low-priced, as motors go. These two facts make modifying servo motors well worth your while.
An unmodified servo allows precise positioning of the output gear. In a modified servo, you sever the link between the potentiometer and the output gear. The output gear then turns freely, without stopping.
After you modify a servo, you can simply plug it into sockets on the BOE, and it works like a charm. You need to write some programming code to operate the servo, but don’t worry, we show you just what to do in the following sections.
Once you’ve done these modifications, the motors can now turn the wheels to steer Smart Rover (as shown in Figure 15-10) and move it around your living room.
Modifying the R/C servo motors
Here’s what you need to modify any one of the servos that we recommend in the section “Going shopping for servos” earlier in this chapter:
#0 Phillips screwdriver )8-inch or smaller flat-bladed screwdriver Nippy cutters, X-ACTO blade, or razor saw Small flat jeweler’s file
After you modify a servo, you void its warranty, so be sure that you have a good ‘un first. Test the servo for proper operation by plugging it into a microcontroller and sending it a command (see the section “Putting the program in place” later in this chapter for more about how to do this) before you modify it. Though it happens only rarely, a servo may fail right out of the box.
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344 Part V: A Plethora of Projects
Taking the servo apart.
Throughout the following steps, take care to avoid wiping off too much of the lubricant that you use for the servo’s internal gears. Otherwise the servo gears may not have enough lubrication for smooth operation. If you think you’ve lost too much of the lubricant, you can always add more just prior to re-assembling the motor. You can get servo grease at the same hobby store where you bought the servo motors.
Now, you’re ready to make the actual modifications to your servo. We wrote the following steps for the Hitec HS-422 servo, but you can modify most other servos using the retaining clip design in much the same way:
1. Use the Phillips screwdriver to remove the servo disc, if it attaches to the output gear/shaft.