FRC Motors in Plain English

Experienced teams and mentors often forget what it’s like to be a rookie team with no knowledge of motors. We haven’t, and we want to put you on a level playing field. This is the summary of their experience with the motors allowed in current and recent FIRST Robotics Competitions. Be sure to check out Notes Regarding Encoders for some helpful tips about finer control of motors.

CIM Motors

  • Free Speed: 5310rpm
  • Stall Torque: 344oz-in
  • Dimensions: 2.5 inch diameter, 4.34 inch long
  • Features: Strength, Compatibility
  • Drawbacks: Entire quota often required for driving
  • Encoder: With gearbox

CIM Motors are the standard we use to compare with all other motors. A CIM shaft is the closest thing the FRC has to a standardized attachment mechanism, and many gearboxes for other motors are designed to mimic this shaft. The most common uses of CIM Motors are in heavy-duty gearboxes, most frequently the ToughBox and CIMple Box. These gearboxes, which have shafts that are wider than the CIM shafts but also sometimes treated as a standard, are designed to increase the torque the motors provide at the expense of free speed. Since the motors are almost always connected to gearboxes which are designed to mount to, among other things, AndyMark C-Channels, CIM Motors are not frequently mounted directly to the frame.

The best use of a CIM is for driving the robot, or another task that requires related characteristics. CIM Motors are strong enough to propel heavy robots across the field at reasonably quick speeds, and depending on the situation, sometimes strong enough to push another robot out of the way. Drive trains designed for power (4-or-more-wheel drive) or maneuverability (Mecanum, Omni, or swerve drive) usually use four CIM Motors, or in extreme cases, more. If the robot has a strength not dependent on driving ability, a two-CIM drive train allows the team to use the remaining CIM Motors on other mechanisms that require a lot of power. For at least the past two years, the limit on CIM Motors has been four, meaning teams who want CIM motors for other mechanisms need to use 2-wheel drive. (Rarely, teams will use 3-wheel drive, and it is theoretically possible to use other motors for driving. We haven’t investigated this possibility, but believe it would be either difficult or ineffective, and probably both.)

We’ve used four drive motors for Mecanum drive very successfully. We have also used two-wheel indirect drive with chains and had less success, and mounted an ineffective kicking mechanism on a CIM which lacked strength because it had little time to accelerate.

BaneBots Motors

One example of a BaneBots motor in use (the one that failed us).

We think our experience with BaneBots motors (that of pitifully low power) was atypical, and we’ll update this section after we gain more experience.

One important note corroborated by other teams is the BaneBots company’s very long ship time. Depending on time of order and location, motors or the gearboxes required to interface them with other parts may take up two or more weeks to arrive. Last year, many teams who ordered parts the first week of the competition did not receive them until the week before Stop Build Day. It is inadvisable to place an order less than two weeks before Stop Build Day. If BaneBots can be easily replaced with parts from other vendors, we strongly suggest you do so.

Window Motors

  • Free Speed: 84rpm (Compared to CIM: Very Low)
  • Stall Torque: 1501.1oz-in (Compared to CIM: Very High)
  • Dimensions: Irregular shape, about 6.6″ long by 3.5″ tall (oriented with shaft horizontally)
  • Features: Doesn’t move without power, controlled, internal heat sensor prevents danger
  • Drawbacks: Non-standard mount, slow, internal heat sensor can shut it off unexpectedly
  • Encoder: No

A window motor is incredibly useful for moving a mechanism to a position (arbitrary or specific) and leaving it there. Unlike all other motors, no concern need be given to preventing the appendage from moving. For design purposes, it can be thought of as a very large servo, but it has two important differences: First, the locking effect is mechanical, not programmatic as with a servo, which means it cannot be manually adjusted when the robot is not powered; and second, there is no encoder function. This means that a window motor cannot detect its position. If the motor is to be used in autonomous/hybrid mode, or if there is risk of danger to the robot if the driver accidentally moves the motor too far, it should be used with a sensor to detect when it has reached the desired position or the limit of its movement. This is easiest to implement with limit switches, though it may be possible to mount an encoder to the shaft.

The motor-to-robot mounting mechanism is completely nonstandard, with small screw holes in three locations that do not line up with the holes in any standard parts. It is important to note that the plastic surrounding these holes is likely to break if it experiences too much force, so teams should plan their mounting locations carefully to avoid unnecessary stress or to reinforce the mounting system. Window motors come in right and left models, which are mirror opposites of each other. Teams are allowed to use any configuration of right and left, as long as the total number is less than or equal to the maximum allowed by the rules. The motor’s shaft is also completely nonstandard, but it comes with a piece to convert that shaft into a ⅝” bore with a standard ANSI keyway. It does not come with a shaft. When mounting the window motor, consider that the plastic conversion piece is a little under 4″ wide, which may interfere with mounting. The piece may be modified to be smaller, but keep in mind that will compromise its strength.

Though it can be used for any mechanism that works with a ⅝” keyed shaft and a very slow speed, it works best for articulating robot parts, because they will remain stationary when the motor is not powered. Though useful during play, this means that repositioning robot elements will require either control over the motor or partial disassembly of the robot. Another advantage of this motor when used to articulate elements is its slow speed, which allows finer and smoother control of parts during both autonomous/hybrid mode and teleop mode.

If a window motor stops working, it may be because its internal heat sensor was triggered. This is most likely under a heavy load, which the motor is capable of supporting but not intended for. Prolonged use under heavy load may lead to the motor shutting itself off to cool down. If this happens, teams should not continue giving it power, especially if that motor is mechanically connected to a second (a common configuration) which may still be operational. If one is trying to move and the other is not, no movement will occur and the functioning motor is likely to be damaged.

We have successfully used window motors in a rack-and-pinion elevator system, though this did lead to our discovery of the weak plastic around the mounting points (the motors broke off).

Fisher-Price Motors

  • Free Speed: 20,770rpm (Compared to CIM: Very High)
  • Stall Torque: 75.4oz-in (Compared to CIM: Low)
  • Dimensions
  • Features: Very fast, compatible with CIM-mimicking gearboxes
  • Drawbacks: Low torque, limited quantity, comes pre-installed with nonstandard gear
  • Note: Specs change slightly each year this motor is provided, so if the motor is to be used with a mechanism with a low tolerance for variance, care should be taken to not confuse the current year’s motor with those from previous years.
  • Encoder: With double gearbox

Fisher-Price motors are the most powerful motor of similar type allowed in the FRC. This makes it ideal for non-driving features that have similar demands as driving, especially rapid spinning. It is capable of performing a variety of tasks depending on what is mounted to it. Examples include a shooting wheel, a belt, and a rack and pinion, but a creative team could find infinite uses for this helpful motor. A difficulty most rookie teams will face is the problem of removing the preinstalled gear. With custom mechanisms, it is best to match the gear, rack, or other part the motor will mesh with rather than attempting to remove the gear. We and our school shop have tried unsuccessfully, but ended up buying a Fisher-Price motor with the Planetary Gearbox gear and plate preinstalled from AndyMark.

This motor fits into the AndyMark Planetary Gearbox (3.67:1 reduction), the Double Doozy AMP (two motors input, 6.18:1 reduction), Double Doozy GEM Two-Stage (two motors input, 22.6:1 reduction), and the CIM-SIM (one or two motors input, 5:1 reduction). All these gearboxes mimic the output shaft of a CIM Motor, and all except the CIM-SIM make the entire motor-gearbox assembly of comparable size and shape to a CIM. The CIM-SIM itself is comparable to a CIMple Box or ToughBox, and the one or two motors are shorter than the one or two CIM motors that would mount to those gearboxes.

The other gearbox this motor fits into is a Fisher-Price Gearbox. This is the gearbox Fisher-Price uses in some of their toy cars, and as a consequence is extremely non-standard. Motor-to-gearbox attachment is simple, but gearbox-to-shaft and gearbox-to-robot attachment is not. The output is a ring with eight equally spaced thin slots. By calling the company, the we found a conversion piece from Fisher-Price that converts this shaft into something similar to, but not compatible with, AndyMark hubs. Due to the difficulty of mounting the gearbox to the robot (there are no holes and no convenient protrusions), they did not end up using this gearbox or its conversion piece. According to this chart, the gear reduction is 112:1 This would give a theoretical speed of 185rpm, and a torque of 8444oz-in. Actual specs will be lower due to inefficiencies in the gearbox. In layman’s terms, this makes the motor much slower but much stronger. As such, it is capable of performing some extreme tasks, potentially even lifting the entire robot as seen in the 2010 FRC game Breakaway.

AndyMark Motors

  • Free Speed: 16,000rpm (Compared to CIM: High)
  • Stall Torque: 60.7oz-in (Compared to CIM: Low)
  • Dimensions: 2.25″ long by 1.4″ in diameter
  • Features: Fast, compatible with CIM-mimicking gearboxes
  • Drawbacks: Low torque, limited quantity
  • Encoder: With double gearbox

The AndyMark Motor (not to be confused with the AndyMark Gearmotor) is very similar to, but less powerful than, a Fisher-Price motor. The most important difference is the speed, which is about ¾ of a Fisher-Price motor’s speed. For more information about this motor, see the Fisher-Price section above. Note that the Fisher-Price gearbox may work with the AndyMark motor, but it is not designed to and the output gear installed on the Fisher-Price motor will need to be transferred to the AndyMark motor, which is a difficult process, especially for the inexperienced.

AndyMark Gearmotors (PG71)

  • Free Speed: 75rpm (Compared to CIM: Very Low)
  • Stall Torque: 1309.4oz-in (Compared to CIM: Very Low)
  • Dimensions: 6.26″ long by 1.77″ in diameter
  • Features: Extremely strong (high torque)
  • Drawbacks: Slow, nonstandard mounting
  • Encoder: No

The AndyMark Gearmotor is a new addition to AndyMark and to the FRC. It consists of an RS775 motor, similar to but not the same as an AndyMark motor, and a 3-stage planetary gearbox. It is the only motor which comes with a removable gearbox, but tampering with the gearbox will almost certainly be forbidden every year as part of the rule against tampering with motors.

The Gearmotor’s primary feature is its strength (and its pleasingly sturdy feel, which the robot doesn’t really care about). We haven’t explored all its possible uses, but they include any heavy-duty actuation, such as a high-force arm, a large conveyor belt, an automatic bridge-balancing weight shifter, and a turret rotation motor.

The Gearmotor is nonstandard in both robot-to-motor and shaft attachments, but can be easily modified to be otherwise. AndyMark sells both a Gearmotor-to-½-inch converter and a hub that fits directly on the Gearmotor’s shaft. For mounting, it requires only four small screw holes and one large shaft hole in a metal plate. Plates designed to mount other gearboxes and motors can easily be modified by drilling the screw holes around the existing shaft hole. This is recommended for rookie teams, because it is almost guaranteed to be sturdy enough, in contrast with custom plates designed without experience. The four mounting holes take a screw that is not used elsewhere in FRC: M4x0.7 threading (a metric size) that is 9mm deep. Teams will want to buy screws that are longer than 9mm to account for the thickness of the mounting plate and still keep the connection as sturdy as possible.

Other Motors

The Kit of Parts provides several other types of motors which the RoboKnights have no experience with and cannot offer guidance on beyond brief notes judging by specs and appearance:

Denso Motors

Denso motors appear to be the motors used in a window motor assembly (which consists of a motor and a worm gearbox). Their free speed is comparable to a CIM, but they are roughly 20% as strong (20% of the torque). They are highly nonstandard to mount, with no apparent way to attach to the motor and a pre-mounted gear that will not mesh with standard FRC gears. An especially important note is the power leads, which are on the same face of the motor as the shaft. While this makes sense for mounting to the window motor, it also restricts the motor to small or limited-angle attachments. These difficulties and their low specs make them a low priority.

Vex Motors

As these motors’ small size would suggest, their capacities are limited. A free speed of 100rpm and stall torque of 215oz-in (160rpm and 134.4oz-in for the high-speed model) make them useful for only small tasks. In fact, their only difference from a servo appears to be their capacity to rotate continuously. We can’t think of a function where this motor would be preferable to a servo that would ever come up in the FRC, but that doesn’t mean your team can’t.

If you do use this motor, be sure to look up its wiring specifications before powering it. Like many electronics in the FRC, it is sensitive to power loads it was not designed to handle. Consult official instructions, don’t try to guess how it works.

Salvage Motors

As each salvage motor will be different, we cannot offer any guidance beyond making sure you familiarize yourself with the motors’ features and limitations and plan accordingly.

Notes Regarding Encoders

Encoders are a very useful motor accessory that allows for much more powerful programming. Their basic function is to track the rotation of a shaft, thus allowing the robot to be aware of its position and/or speed. Since motors are normally programmed by specifying a percentage of their maximum power, knowing this value can be of huge help to programmers who need to program precise control of motors.

The usual FIRST encoder is from US Digital and is available on AndyMark. It fits on the ¼-inch shaft that sticks out opposite the drive shaft on many CIM-input gearboxes. Installation instructions are included in the kit and should be followed closely, because encoder installation is error-prone and an incorrectly installed encoder can almost never be removed intact. There are instructions on the US Digital website that refer to an adhesive; these do not apply to the FIRST encoders. They do explain the presence of a small round part in the kit that is not mentioned in the included instructions: it is a tool for aligning the encoder base, and can be ignored unless the team is making their own encoder mounting holes. If a team is making their own holes, which is not recommended because of encoders’ low error tolerance, this part will be extremely helpful in alignment. Note: An often important part of the encoder assembly is the mounting bracket, which is not included in one of the available kits. Not all gearboxes require this bracket, but it is easily forgotten, so be sure to check at the time you order your encoders.

BaneBots sells a modified version of one of its motors that it says is capable of having an encoder mounted. We were unable to determine where to obtain this encoder.

Another type of encoder available is a magnetic encoder. It appears to be able to mount to custom shafts, but we couldn’t determine how, mostly due to lack of research.

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