Choosing a Servo

For many applications, you can just use one of the two most common servos in FTC, REV Smart Servo or goBILDA Dual Mode Servo. However, sometimes these servos are not enough. To choose the right servo in these cases, you need to know more about servos.

Here are some of servo features you need to pay attention to when selecting a servo for your needs.

Servo Type: Regular or Continuous

Servos described above, which can rotate to a given position based on PWM input signal, are called regular servos. In addition, there are also continuous rotation servos, which are effectively just small motors using servo form factor. They have no position control; instead, PWM signal is used to control their rotation speed.

Both REV Smart Servo and goBILDA Dual Mode servo (which are both based on FR5311m programmable servo by Feetech) can be used as either regular or continuous rotation servos. To switch between these two modes, you need to use a servo programmer, available separately from REV or goBILDA.

Note that some vendors such as Servocity offer continuous rotation option on some of the servos they sell. These options are modifications to the original servo made by the vendor and are illegal in FTC. The only legal continuous rotation servos are servos that are continuous rotation from the factory: if you have a seller like ServoCity modify a servo for continuous rotation, that servo is not legal.

Servo Torque And Speed

Servo output power is measured in both speed and torque. Speed (normally in seconds per 60°) refers to how fast the servo turns 60 degrees in Standard Rotation mode. Torque (usually measured in oz-in or in kg-cm) refers to the amount of force the servo can apply to a lever. For reference, if you put a 1” bar on a servo, then put a force gauge on the end, the torque rating of the servo (in oz/in) will be measured. As you may know, speed and torque have an inverse relationship. Generally you can find some insanely powerful servos that are pretty slow (slower than 0.20 s/60°) or some less powerful ones with faster ratios (anything faster than 0.12 s/60° is considered very fast). Finding the right servo for your application can be tough, but a good way is trying to decide if you need more speed or torque, and if your servo will experience shock loads or not.

Durability and Servo Gear Material

The two things that threaten a servo’s longevity are the motor inside burning out and more commonly, the gears stripping inside the servo. A motor burning out is pretty uncommon, but it can happen under large loads for a prolonged amount of time. This means you should never stall a servo against an immovable object.

Gear stripping is a very common problem, and it is directly to the material the gears are made from. This ranges from plastic to titanium, so let’s go down the list, starting from the weakest.

  • Plastic: with low power servos, these are normally okay. Generally used for applications in model airplanes such as ailerons.

  • Karbonite: Hitec’s gear plastic is a very durable and long lasting plastic and is very good under long use and low load. Be aware that it can strip easily under the shock loads found in FTC.

  • Brass: Brass gears are stronger than plastics but also suffer greatly when faced with shock loads in FTC, like intake wrists and deposit buckets. It’s found on slightly higher end servos in the $25-30 range, such as the REV smart servo.

  • Steel: This is where we start getting big. Steel gears are very durable and you’ll have a tough time stripping these. In general, expect to pay $40+. The best value in this category is goBILDA Dual mode servo (v2), which sells for $21 after team discount.

  • Titanium: Titanium is where you get into really high end, virtually unbreakable servos. Starting from $75, they can reach over $150.

Servo Size

Servos come in different sizes. Fortunately, almost all manufacturers use the same standard set of servo sizes, ranging from sub-micro to large. The two sizes commonly used in FTC are standard size (which includes REV Smart Servo and goBILDA Dual Mode Servo) and large size (also known as quarter-scale) servos.

Note that while in general, the larger the size, the more powerful the servo, it is not a strict rule. You can buy very powerful standard size servos - just expect to pay more for them.

Servo Spline

The output shaft of the servo is commonly called spline. Most servos you meet have industry standard 25 tooth spline (also known as F3); in particular, this is the spline used by REV Smart Servo and goBILDA Dual Mode servo. However, Hitec servos using 24 tooth spline are also very popular. Please check the spline type before you buy the servo - otherwise, your servo attachments will not fit.

For more info about servo splines, please check https://www.servocity.com/servo-spline-info/.

Cost

Servos range from cheap $7 servos for light applications, all the way up to some Hitec or Savox servos for close to $200. By far the best bang for your buck servos out there are the Feetech dual mode servos, which is a programmable type of servo. This includes both the REV SRS (Smart Robot Servo) and both goBILDA Dual Mode servos. The biggest downside to the REV SRS and the old goBILDA servos (25-1) are their brass gears. Coupled with high output power, this meant that stripping gears with any shock load was commonplace. The new goBILDA Dual Mode servos (25-2) have steel gears now, but are new and aren’t as competition tested as other servos.

The next big name in FTC Servos is Hitec, who are a huge name in hobby servos for decades and are very well trusted. Their low end servos are inexpensive but easily broken.

A mid-priced Hitec servo is the HS 485-HB/488-HB servo, with Karbonite gears. While it shouldn’t be used in high load applications, it is fine for general use such as claws or trapdoors. 485HB uses 24 tooth spline; 488 HB uses 25 tooth spline (recommended).

Where Hitec really shines is the high end market. If your budget is over $100, you can get into some very powerful Hitec servos. Most have titanium gears and are programmable, so you can dial in the performance and range to exactly what you need.

The last big player in the servo market in FTC is Savox, which produces great mid-high range servos (think $60-$100+). They are made with titanium gears (close to bulletproof) and are fast. Savox servos are mostly brushless and coreless, so they do tend to scream a little under load, but they’re definitely worth it if your budget allows for it.

Best Value

REV and goBILDA servos can be purchased from REV and goBILDA websites respectively. For all other servos, good source is servocity.com or Amazon.