Choix d’un servo

Le choix d’un servo peut sembler décourageant au début, étant donné le nombre d’options à considérer. Ce guide a pour but de fournir un point de départ pour comparer les différentes options de servo, et contient également quelques recommandations à la fin.

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.

Type de Servo : Régulier ou Continu

Servos Les servos réguliers sont ceux qui peuvent tourner jusqu’à une position donnée sur la base d’un signal d’entrée PWM. Il existe également des servos à rotation continue, qui ne sont en fait que de petits moteurs dans un servo facteur de forme. Ils n’ont pas de contrôle de position, mais le signal PWM est utilisé pour contrôler leur vitesse de rotation.

Astuce

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.

De nombreux servos des fournisseurs de FTC® sont bi-mode, ce qui signifie qu’ils peuvent passer du mode continu au mode normal (ce qui nécessite souvent l’utilisation d’un programmateur de servos vendu séparément). Ces servos peuvent être utilisés soit en mode continu, soit en mode normal.

Couple et Vitesse des Servos

La puissance de sortie du Servo est mesurée en vitesse et couple. La vitesse (normalement en secondes par 60 degrés) correspond à la vitesse à laquelle le servo tourne de 60 degrés en mode de rotation standard.

Pourquoi des secondes par 60 degrés ?

Historiquement, les servos couramment utilisés en FTC ont été créés pour les voitures RC (Radio Controlled). Ces voitures étaient souvent équipées de tringleries de direction avec un débattement latéral maximal de 60 degrés, de sorte que les fabricants annonçaient souvent leurs servos en utilisant des secondes pour 60 degrés.

Le couple (généralement mesuré en oz-in ou en kg-cm) correspond à la force que le servo peut appliquer à un levier. Par exemple, si vous placez une barre de 1 » sur un servo, puis un dynamomètre à l’extrémité, le couple nominal du servo (en oz-in) sera mesuré.

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).

Il peut être difficile de trouver le servo adapté à votre application, mais une bonne façon de procéder consiste à déterminer si vous avez besoin de plus de vitesse ou de couple, et si votre servo subira des chocs ou non.

Durabilité et matériau des servos

Les deux choses qui menacent la longévité d’un servo sont le moteur interne qui grille et, plus fréquemment, les engrenages qui s’effilochent à l’intérieur du servo. Un moteur qui grille est assez rare, mais il peut se produire sous de fortes charges pendant une période prolongée.

Prudence

Vous ne devez jamais caler un servo contre un objet inamovible.

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.

• Les chocs provoqués par l’inversion du sens de rotation d’un objet trop lourd peuvent endommager les engrenages. Le couple augmente avec la masse et la distance par rapport au centre de rotation. Si le composant actionné est éloigné du servo, le long bras de levier se traduit par un couple plus important. En outre, si le composant est en mouvement, l’inversion du sens de rotation nécessitera également un couple plus important. Le principe est donc que les composants doivent être légers et ne pas s’inverser brusquement pour prolonger la durée de vie du servo.

La résistance aux chocs est directement influencée par le matériau dont sont faites les engrenages. Cela va du plastique au titane, alors descendons la liste, en commençant par le plus faible.

• 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.

Taille des servomoteurs

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

Veuillez vérifier le type de cannelure avant d’acheter le servo - sinon votre servo ne conviendra pas.

Pour plus d’informations sur les cannelures de servo, veuillez consulter https://www.servocity.com/servo-spline-info/.

Gamme Servo

L’angle sur lequel un servo non continu peut tourner tout en conservant le retour de position s’appelle la plage. Lorsque vous choisissez un servo, il est important de vous assurer que vous disposez d’une plage suffisante pour le mouvement dont vous avez besoin.

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.

Coût

Les Servos vont de 7 $ servos bon marché pour des applications légères, jusqu’à des servos Hitec ou Savox pour près de 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.

Servomoteurs recommandés

Note

We do not recommend very low-priced servos. Due to their low strength, they end up requiring multiple replacements over time, negating any cost benefits.

• goBILDA Servos: goBILDA offers a wide variety of servos suited excellently for FTC.

  • goBILDA Dual Mode Servos are very good price to performance/capability servos. They are dual mode, meaning you can program them to either standard or continuous rotation using a goBILDA servo programmer or REV servo programmer. In standard mode, they are rated for up to 300 degrees of range. They come in three gear ratio options: Torque, Speed, and Super Speed. They are rated for a reasonably high total mechanical output power (around 2.7W for the Torque and Speed, and around 3.2W for the Super Speed), and have steel gearboxes.

  • goBILDA 5 Turn Servos are 5 turn variants of their standard Dual Mode servos, which can rotate 5 turns while still tracking position. These servos have high range, making them ideal for use with external gearboxes, but are more expensive and have a lower precision than the normal variants.

  • goBILDA Proton Servos are goBILDA’s budget servos, coming it at approximately half the price of the standard Dual Mode servos. They come in two gear ratio options, Torque and Speed. Unlike the Dual Mode servos, these are not programmable, and therefore can not be used as continuous rotation servos. While their total mechanical power output of approximately 0.7W is a fair bit less than the Dual Mode servos, and they only have a 180 degree rotation range, their low price makes them a decent option for low-load mechanisms, and their steel gearboxes greatly reduce the chances of breaking the servos from accidental shock loads compared to other brands” budget servos.

• REV Robotics V2 Smart Robot Servos: These are similar to the goBILDA Dual Mode servos in price, performance, and capability. They are also dual mode, meaning you can program them to either standard or continuous rotation using a goBILDA servo programmer or REV servo programmer. In standard mode, they are rated for up to 270 degrees of range. They come in two gear ratio options, Balanced and UltraSpeed. The UltraSpeed is rated for a relatively high total mechanical output power of approximately 3.5W, and the Balanced is rated for a somewhat lower output of approximately 2.5W. They have steel gearboxes.

• Melonbotics Super Servo Plus: This servo is different from the other servos listed here. While considered a continuous rotation servos from a rules and control system perspective, it 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.

• Axon MAX MK2: This is widely considered to be the best high performance servo for FTC. It has uses heat-treated stainless steel gears for top-notch reliability, and has a very high rated total mechanical power output of 7.5W (only 0.5W below the 8W limit defined in the Competition Manual). The servo features an analog output wire, allowing you to read the absolute position of the servo. The Axon MK2 servos are also highly programmable with the Programmer MK2, featuring a wide range of adjustable parameters and modes (including standard and continuous rotation modes).

  Astuce

  Being able to read the absolute position of the servo can have significant benefits when programming your robot. For example, you can tell the servo to go to a position, wait for it to reach that position, then do the next action; as opposed to telling the servo to go to a position, waiting a set amount of time (which may be longer or shorter than the time actually needed), then doing the next action.

  Important

  The Axon MINI MK2 is very similar to the MAX in terms of total output power (though geared at a slightly faster speed/lower torque), and has the same features (including the heat treated stainless steel gears, programmability, and analog output wire). However, this servo is a slightly different form factor to the standard form factor used by the other servos on this list. While the height is shorter than the MAX, it is slightly wider, and the servo horn is in a different position compared to the standard servo size. Due to this, we don’t recommend the MINI for anything other than fully custom-designed mechanisms not relying on standard mounts and horn position.