Choosing a Servo¶
For many applications, you can just use one of the three common servos in FTC, REV Smart Servo or goBILDA Dual Mode Servo (Torque), or goBILDA Dual Mode Servo (Speed) However, sometimes these servos are not enough. Here are some important servo features to consider when selecting a servo.
Servo Type: Regular or Continuous¶
Servos 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 in a 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 servos (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.
Some vendors offer continuous rotation option on some servos. These options are modifications to the original servo made by the vendor and are illegal in FTC. The only legal continuous rotation servos are CR servos direct from the manufacturer/factory. If you have a seller 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).
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.
You should never stall a servo against an immovable object.
Gear stripping is a very common problem which occurs 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 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: with low power servos, these are normally okay. 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 which opens a trapdoor or moves game elements within the robot.
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. Karbonite is more durable than plastic but still suffers from shock loads.
Brass: Brass gears are stronger than plastic 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 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 a premium. The goBILDA Dual mode servos (v2) is an example of steel gear servo.
Titanium: Titanium is where you get into really high end, virtually unbreakable servos. Starting from $75, they can reach over $150.
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 Servos) and large size (also known as quarter-scale) servos. However, large servos are quite uncommon.
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. However, Hitec servos using 24 tooth spline are also very popular.
For more info about servo splines, please check https://www.servocity.com/servo-spline-info/.
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 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) and (25-3) have steel gears, but are new and aren’t as competition tested as other servos.
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.
- Low Priced (~$18)
- Best Performance ($75+)
Savox titanium servos
Hitec titanium servos