Choosing a Servo

Choosing a servo can seem daunting at first, given the number of options to consider. This guide is intended to provide a starting point to compare different servo options, and also has some hand picked recommendations at the end.

Important

It is very important to keep the reliability of a given vendor in mind when choosing servos. It is very common for manufacturers and resellers on Amazon and other similar sites to significantly exaggerate their servos’ specs, or pick unrealistic best-case scenarios for measuring the specs. We have limited our recommendations only to vendors who historically have been reliable with publishing accurate servo specifications.

As a rule of thumb established manufacturers (Hitec, Savox, ServoCity, goBILDA, AndyMark, etc) will usually publish accurate numbers, and servos from marketplaces (Amazon, AliExpress, hobby stores, etc) should be viewed with significant skepticism.

Servo Type: Regular or Continuous

Servos that 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 in a servo form factor. They have no position control; instead, PWM signal is used to control their rotation speed.

Tip

Continuous rotation servos aren’t even always in a traditional servo form factor. The Melonbotics Super Servo Plus , while considered a continuous rotation servos from a rules and control system perspective, is essentially a small motor + spur gearbox. The 1000rpm rated output speed makes it an excellent alternative to large, heavy standard motors for small, high-speed mechanisms such as active intakes.

Many servos from FTC® vendors are Dual Mode, meaning they can switch between continuous and regular modes (often requiring the use of a sold-separately servo programmer). These servos can be used as either continuous or regular servos.

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.

Why seconds per 60 degrees?

Historically, the servos commonly used in FTC were created for RC (Radio Controlled) car setups. These cars often had steering linkages with a maximum side to side travel of 60 degrees so manufacturers would often advertise their servos using seconds for 60 degrees.

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 will generally have an inverse relationship. 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 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 internal motor 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.

Caution

You should never stall a servo against an immovable object.

Gear stripping is a problem which can occur when the torque needed to actuate a component exceeds that of the servo’s maximum torque. There are two main cases when this can occur.

  • Shock load from external force can strip the gears. An example could be the component slamming into the field wall or another robot.

  • Shock load from reversing directions on an object that is too heavy can strip the gears. Torque increases with mass and also distance from the center of rotation. If the component being actuated is far from the servo, the long lever arm means larger torque. Furthermore, if the component is moving, reversing direction also will require more torque. Thus, the principle is that components should be light and not reverse direction suddenly to prolong servo life.

Shock load resistance is impacted directly by 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 (usually Nylon): With low power servos, these can be okay if you ensure they are not subjected to shock loads. Generally used for applications in model airplanes such as ailerons. FTC applications include light load mechanisms which will not have direct contact with any game elements or the field. An example could be a servo that opens a trapdoor or moves game elements within the robot.

    • Karbonite: Hitec’s gear plastic is more durable than Nylon and wears less over time, but it can still strip easily under the shock loads found in FTC.

  • Brass: Brass gears are stronger than plastic, but also suffer greatly when faced with shock loads in FTC such as intake wrists and deposit buckets. Brass gears are found on slightly higher end servos such as the original REV Smart Robot Servo.

  • Steel: This is where we start getting big. Steel gears are very durable and you’ll have a tough time stripping these. The goBILDA Proton servos, goBILDA Dual Mode servos, REV V2 Smart Robot Servos, and Axon servos are all examples of steel gear servo.

  • Titanium: Titanium is where you get into really high end servos. Starting from $75, they can reach over $150. A common misconception is that titanium is stronger than steel, however its advantage is in strength to weight ratio (as in, titanium gearboxes will often be lighter than steel gearboxes).

Servos with weaker materials (e.g. plastic and brass) used for their gears will often strip quite easily. Servos with steel gears are significantly harder to strip; some teams have even reported stripping the aluminum hubs attached to their servos while the servos themselves survived with no damage.

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 size most commonly used in FTC is the standard size (which includes REV Smart Robot Servos and goBILDA Dual Mode Servos) servos. Micro size (which includes the Axon Micro) servos can be useful when a particularly small mechanism is needed, though they usually are significantly less durable than standard size servos. Large size (also known as quarter-scale) servos are quite uncommon in FTC.

Note that while it is easy to assume the larger the size, the more powerful the servo, it is often not the case. 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 the spline. Most servos 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 servos. Hitec servos using 24 tooth spline were historically also very popular, but are much less common today.

AndyMark servos are an exception to this, as they use a 5mm hex shaft as their output instead of a 24 or 25 tooth servo spline.

Attention

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/ .

Servo Range

The angle over which a non-continuous servo can rotate while retaining position feedback is called the range. When choosing a servo, it is important to make sure you have enough range for the movement you need.

By default, the FTC control system outputs 600-2400 microsecond signals. However, this range can be expanded to 500-2500 microseconds. When choosing a servo, it is important to make sure that its range will be usable for your application inside of 500-2500 microseconds.

Note

The default 600-2400 range of the FTC control system when selecting the “Servo” configuration limits the range of many servos to less than the advertised range. Make sure you select the “Full Range Servo” configuration instead to get the full 500-2500 range.

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 going to be the goBILDA Dual Mode Servos and REV V2 Smart Robot Servos. In addition, the goBILDA Proton servos are an excellent value for low-load mechanisms when you do not need more than 180 degrees of rotation or continuous rotation.

Axon Robotics offers brushless, highly-programmable, steel-geared servos in the $90 price range. These servos are designed specifically for FTC, and are extremely popular among teams wishing to maximize their robot’s capabilities and performance.