Metal Gear Servos

Differences and Compatibility

There are several differences and issues you should be aware of when purchasing and installing a servo. The sections below should help eliminate some of the confusion and shed some light on some of these topics.

Servo Wiring and Plugs

Trying to figure out what servo plug will fit your application can often be a daunting task. However servo wiring seems more complicated than it truly is. With the exception of the older Airtronics plugs, all servos are wired the same. In some cases the wire colors may be slightly different, but the wires serve the same function. These functions are positive lead, negative lead, and the signal lead. If you are purchasing Airtronics equipment, your best best is to purchase items that use the new Airtronics Z connector if possible. That way you'll end up with wiring that is compatible with everything else on the market.

The Futaba J plugs are nice to have because there is a small plastic tab that prevents them from being plugged up incorrectly. The stock Traxxas equipment uses Futaba J style plugs, and that would be my recommendation for a replacement whenever possible. A reversed servo lead can damage the servo and other electronics as well. However, in some cases, you may not be able to purchase a Futaba J plug and have to use a Hitec/JR/Airtronics Z connector. If that's the case you can use these plugs, but just ensure that you are plugging it in correctly. The best way to do this is to use the wires for one of the other servos as a reference. Match up the positive, negative, and signal, making sure you are inserting the new plug in the same direction as the others.

There may also be an occasion that you need to use a Futaba J style plug with a receiver that doesn't support it. One example would be if you had purchased a JR receiver and still plan on using a servo with a Futaba-style end such as a Futaba or Traxxas. When that happens, use a hobby knife and gently carve away the plastic tab. Then you can use the plug as you would any Hitec/JR/Airtronics Z connector. Once again, as with the Hitec/JR/Z plug, make sure you insert the plug in the correct manner. The differences between the two plugs, and the tab on the Futaba plug is shown below.

For more information regarding servo wiring and plugs, I highly recommend this informative web page. It covers this topic in great detail.

Analog vs. Digital

There are two main categories of servos available, analog and digital. Analog servos have been around for quite some time, and are often the more affordable of the two types. Both types of servos work by sending pulses of power to the servo motor, similar to the way an ESC powers an electric RC truck. However a digital servo, due to a small microprocessor inside the case, can specifically tailor the pulses being sent to the servo's motor. It can also send a larger number of pulses, which means that the range of motion can be broken down into a much finer pattern. Since more pulses are being sent, the servo is more responsive and contains a significantly less amount of deadband. Deadband is the gap between when you move the stick or the wheel on the radio, and when the servo actually moves.

The response and accuracy of the servo is one big advantage to a digital servo. However there is also another. Due to the increased pulse, you'll also find that more voltage will reach the servo's motor quicker. This helps to increase the servo's torque rating and consistency as well. Most digital servos have a much higher torque rating than a similar analog servo does.

Digital servos are definitely more advanced than their analog counterparts. However there is one disadvantage to digital servos, they use more power. Digital servos will drain a battery quicker than an analog servo would. Also running multiple digital servos is generally a bad idea due to the added current draw that they can pull when operated together. In situations where multiple digital servos are used, such as planes, special wiring and extra batteries are the preferred route. For an RC truck or car I'd suggest using a digital servo for the steering, and an analog for anything else that may need a servo.

If you use a receiver pack to power your receiver, you should avoid the use of standard AA batteries when using digital servos. When using a digital servo, a NiCad or NiMH receiver pack should be used in place of the AA batteries. For more information regarding digital servos I suggest this informative article from Futaba on digital servos. To view the article you will need to use the Adobe Acrobat Reader which can be downloaded for free.

Coreless vs. 3 or 5 Pole Servos

I often hear people in the hobby complain about a brand of servos that they see as having centering issues. They proceed to complain about that brand and often consider it a "cheap brand" of servo. However that's not truly the case. The problem lies in the components of the servo itself, and this issue can be reproduced among any brand of servo. It lies in the way that the servo itself converts an instruction into a motion.

Most servos, especially the more affordable ones, are 3 or 5 pole servos. The internal components of these servos, aside from the gears, are very similar to an electric RC motor. You have an armature, which is the part with the wire windings, that spins inside a set of magnets. Most servos have 3 or 5 of these magnets which are called "poles". Just like with any electric motor when you are between the magnets the motor is noticeably weaker. When this happens in a servo application, this can often cause the centering to drift slightly in one direction or another. Factor in the fact that the magnets lose their strength over time, and you'll find that the problem can intensify as the servo gets older.

Coreless motors do not suffer from this problem. They use a wire mesh that spins around the outside of a magnet, and this provides a more consistent and smoother pattern of travel. Another side benefit of this is that holding strength is also increased. This is because you eliminate the heavy armature that would be spinning in a standard pole-type servo and the weak areas that would be between the magnets. So a coreless servo will be much more accurate when returning itself to center under load.

Whether bashing or racing a servo that is more accurate, and that can hit center perfectly time after time, can make a big difference to the way your RC performs. So if you can shell out the extra cash, a coreless servo can be a very good investment.

Part One:	Introduction
Part Two:	Differences and Compatibility
Part Three:	Installation Tips
Part Four:	Recommended Servos
Part Five:	Servo Comparision