Servo - May 2015

introduced in 1965 for use in model aircraft. In 1965, integrated circuits had only just been introduced; the first microprocessors were still a number of years away; battery technology consisted of disposable carbon cells (alkaline batteries weren’t available until the 1970s); rechargeable Nickel-Cadmium batteries were known as “NIFE” cells, and weren’t available to consumers in the “AA” form factor until 1970. The technology of the time is important to keep in mind when you are planning on using servos, as it affects how you interface to them.

The first misconception that many people have about servos is that they are digital devices. They are actually analog devices. This is confused by the name that is applied to the servos. They are called “digital proportional” which is really marketing speak to make servos sound like modern devices, and not something that came out when the Beatles were still playing live shows.

In regard to the control signal passed to servos, I’m sure that you’ve seen something like Figure 2 showing that the length of the pulse which is sent to the servo is used by the servo to set its output position. The pulse length is a continuous function; it is not broken into discrete steps (as in many software representations, including one of the Arduino libraries that I use in the example programs discussed below). This continuous output is both a blessing and a curse.

Being an analog device, servos are not completely accurate nor are they precise. If you have two servos, chances are the same pulse width will not set both of the servos to the same position, and if you move them to new positions and then back, you’ll find that they may not return to exactly the same place.

This was compensated for in radio control models with “trim pots” which minutely varied the timing of the signal sent to the servos to ensure the model would return to a neutral position. I’m pointing this out because it can be an issue in applications that require very precise and accurate actuator movements. If that is the case, then you should be looking at stepper motors or more modern servos which provide much more accurate positioning.

I’m sure that by now you’re starting to get bored with my prattling on, so let’s change the pace and start building some circuits and gain some empirical knowledge regarding servos.

The first bit of wisdom that I want to impart to you is the need for a servo connector that you can plug into a breadboard. Servos require header pins that are at least

1/4” ( 6 mm) to 3/8” ( 9 mm) in length, and consist of three pins that are on 0.1” ( 2. 54 mm) centers.

As I can never reliably find headers that are long enough on both sides, I usually make my own from soldering together the short ends of two standard printed

By Myke Predko Post comments on this section and find any associated files and/or downloads at www.servomagazine.com/index.php/magazine/article/may2015_Predko.