how do i design a custom mecanum drive train with motors close to the ground with space for pollen intake and code it

To design a custom mecanum drivetrain, you can use a parallel plate configuration where the wheels and power transmission are placed between two plates. Keeping the width of these drive pods to 3 inches or less maximizes the open space in the center of the robot for mechanisms like an intake. To keep the center of gravity low, motors should be mounted as low as possible. You can face mount the motors directly to the innermost plate, or place them on a third plate situated between the outer and inner plates to save space in the middle of the robot. Ensure you maintain a ground clearance of 0.25 to 0.5 inches on flat foam tiles so the chassis does not drag. Game Manual 0 recommends using four independently powered mecanum wheels between 3 and 4 inches in diameter, arranged so that the rollers point toward the center of the robot in an "X" shape when viewed from above.

While the provided documentation does not mention a "pollen" intake specifically, Game Manual 0 notes that designing a wide center section is highly advantageous for grabbing game elements and fitting intake mechanisms. Using a parallel plate setup helps maximize this interior space.

For programming the drivetrain, Game Manual 0 advises against hardcoding individual movements. Instead, you can use inverse kinematics to translate the robot's overall desired forward velocity (vf), strafe velocity (vs), and rotational velocity (ω) into individual wheel velocities. The exact formulas are:

v_{fl} = v_f - v_s - (2r_b \cdot \omega)

v_{bl} = v_f + v_s - (2r_b \cdot \omega)

v_{br} = v_f - v_s + (2r_b \cdot \omega)

v_{fr} = v_f + v_s + (2r_b \cdot \omega)

Further details on implementing and refining this code can be found in restricted resources. The REV Robotics Docs cover these concepts in Programming Mecanum - Refined (https://docs.revrobotics.com/duo-control/hello-robot-blocks/part-4-going-beyond/programming-mecanum-refined) and Programming Mecanum - Simplified (https://docs.revrobotics.com/duo-control/hello-robot-blocks/part-4-going-beyond/programming-mecanum-simplified). Additionally, advanced kinematics and tracking are detailed in the FTCLib documentation on Mecanum Drive Kinematics (https://docs.ftclib.org/kinematics/wpilib-kinematics/mecanum-drive-kinematics) and Mecanum Drive Odometry (https://docs.ftclib.org/kinematics/wpilib-kinematics/mecanum-drive-odometry).

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Mecanum TeleOp

Game Manual 0·CC BY-NC 4.0

Note: Some common COTS mecanum drivetrain kits are the goBILDA Strafer Chassis Kit (https://www.gobilda.com/strafer-chassis-kit-v5/) and the REV Mecanum Drivetrain Kit (https://www.revrobotics.com/rev-45-2470/).

A standard mecanum drive configuration possesses 4 mecanum wheels oriented in an "X" shape. This means that the rollers are angled towards the center when looking at it from above. This configuration allows one to add up the force vectors generated by the offset rollers and derive movement in any direction. It is important to note that because of friction, perfect movement isn't possible in every direction, so a mecanum drivetrain will be able to drive slightly faster forwards/backwards than any other directions. Combining translation and rotation will also result in slower movement.

In the image above, vectors 1, 2, 3, and 4 are the force vectors created by the mecanum wheels when the chassis is instructed to drive towards the top of the image. All motors are driving forward. The blue and red lines are their X and Y components, respectively. Here are a few examples of how the wheels must be driven to achieve different movements:

Attention: It is strongly advised to not hardcode these movements in; there is a much better way described below that allows for true holonomic movement and is much more elegant.

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Holonomic Drivetrains > Mecanum Drive

Game Manual 0·CC BY-NC 4.0

Mecanum drivetrains consist of four mecanum wheels which are powered independently by one motor. This configuration angles the velocity of each wheel, allowing the robot to strafe.

Term: Mecanum Wheel — Mecanum wheels are a special type of wheel that enable maneuverability and holonomic strafing as opposed to traditional wheels. They consist of a series of rubber rollers rotated 45 degrees to either the left or right.

In a conventional mecanum drivetrain, running the wheels on one diagonal in the opposite direction to those on the other diagonal causes sideways movement. Combinations of these wheel motions allow for vehicle motion in any direction with any vehicle rotation (including no rotation at all).

The primary advantage to mecanum drive is the maneuverability it affords, especially because the robot can strafe instead of turn and drive. The rollers on mecanum wheels form a 45 degree angle with the wheel's axis of rotation, which means that mecanum drivetrains can't strafe as fast as they can drive forward.

This can be explained by discussing the forces involved. When each wheel rotates, it applies a friction force to the ground, which moves the robot. When moving forward, both sets of left wheels rotate in the same direction at the same speed, and both sets of right wheels rotate in the same direction at the same speed, meaning that the forces do not oppose each other. However, when strafing, neither the two left wheels nor the two right wheels are rotating at the same speed. In many cases, they even rotate in opposite directions.

These two opposing forces cause the rollers to slip more and more, which angles the robot's velocity at the expense of traction (more slipping results in a loss of speed). However, the wheels do still slip when moving forward but not as drastically as they do when strafing.

This is the primary disadvantage to mecanum drivetrains: they tend not to have much pushing power and thus, are vulnerable to defense by a sturdy tank drive.

Due to the fact that mecanum wheels are more likely to slip because of the diagonal rollers, an optional addition to mecanum drives is a separate odometry mechanism in order to track the robot's location during autonomous.

Attention: It is important to note that in order to maximize the efficiency and stability of mecanum drives, when viewed from above, the rollers of each wheel should point towards the center of the robot, forming an X shape, rather than a rhombus.

The primary reason for this is that it allows the drivetrain to turn significantly faster than it would otherwise be able to. When using the suggested setup, when viewed from the robot's underside, the rollers form a rhombus. This allows the force applied by the wheels on the ground to act tangent to the turn radius, leading to faster turning.

See this video (https://www.youtube.com/watch?v=xgWf_t8owI0&t=3152s) and this other video (https://youtu.be/YJaX3vQ6kHw?t=123) for a more in depth explanation.

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Programming Mecanum - Refined | REV DUO Control System

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Programming Mecanum - Simplified | REV DUO Control System

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CAD Tutorials

Game Manual 0·CC BY-NC 4.0

After learning your CAD program of choice, determine the necessary requirements for the drivetrain based on the current game. Teams should shoot for the wheelbase that works the best in that specific field's layout.

For instance, in Relic Recovery (2017-2018) a drivetrain required precision to not only grab glyphs from the center pit, but also to line up against the cryptobox. Thus, mecanum wheels and a wide center section of the robot proved an advantage over a 6 wheel tank drive. (However, it should be noted that with sufficient practice and competent drivers, any drive base can be competitive up to a certain extent).

After selecting a drivebase, determine the number of motors. Keep in mind the eight motor limit is a pain that shouldn't be ignored. A good rule of thumb is four motors for driving and four motors for the other mechanisms (e.g. intakes, linear slides, arm, etc.) For most modern FTC® games, you need minimum 7 motors to be highly competitive, although 8 is a good rule of thumb.

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Sample Drivetrains

Game Manual 0·CC BY-NC 4.0

In this section, we compile several sample drivetrains which you can use as a starting point for your own designs. You should also check the gallery of robot designs in the Appendix.

goBILDA strafer chassis (https://www.gobilda.com/strafer-chassis-kit-v4/) Basic mecanum chassis by goBILDA. Simple and easy to build chassis, providing an excellent starting point for robots built from goBILDA parts. goBILDA BeeLine Chassis (https://www.gobilda.com/beeline-chassis-kit/) Basic 6 wheel drive chassis by goBILDA. Simple and easy to build chassis, providing an excellent starting point for robots built from goBILDA parts. Strafer chassis with Dead Wheels (https://drive.google.com/file/d/1R85u8nGGmBu5_6jIztOH3-5_W4XK08Mb/view?usp=drive_open) Modification of goBILDA chassis by FTC® 9794 Wizards.exe, which adds dead wheels. Also see their video tutorials: https://www.youtube.com/watch?v=OjNvAD350M4&list=PLICNg-rquurYgWAQGhu6iC0At75vgqFJp REV Robotics 6 wheel drivetrain (https://docs.revrobotics.com/duo-build/channel-drivetrain-build-guide) Sample 6 wheel drivetrain built using REV kit. AndyMark Tile Runner chassis (https://andymark.com/products/configurable-tilerunner-chassis-options) A universal chassis kit sold by AndyMark. It is available in several modifications: 6 wheel, mecanum, tank tread. It is expensive, but using provided 3d models as inspiration for your own drivetrain is free. Wizards.exe 6 wheel drive chassis (https://drive.google.com/file/d/1iu2UUNlqoQ6bS1vnoRPtUI0Uv3lLjNec/view?usp=drive_open) Another design by FTC 9794 Wizards.exe. Combines custom side panels with Actobotics parts (gears, chain sprockets). Custom drivetrain by FTC 14875 LightSpeed (https://drive.google.com/file/d/1iu2UUNlqoQ6bS1vnoRPtUI0Uv3lLjNec/view?usp=drive_open) This design uses goBILDA mecanum wheels, motors, and some channels combined with custom side plates. NX VANTAGE-H (https://cad.onshape.com/documents/3d22333d5ba0abcc62edb57e/w/fa027f644666441544a378c6/e/693039a92658a00632996b28) Another custom drivetrain using goBILDA mecanum wheels, by redditor /u/nateless. Note that this design can be viewed but not downloaded---this is intentional; see his original reddit post (https://www.reddit.com/r/FTC/comments/c8vlsj/cad_for_nx_vantageh_is_going_public/).

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CAD Tutorials > CAD Tutorial Part 1 - Drivetrain in an Hour > Additional Considerations

Game Manual 0·CC BY-NC 4.0

Powering wheels can be done is a couple different ways through either belts and pulleys, chains and sprockets, gears, or even powered directly from the motor. Direct drive and chains are the simpler of the options, with direct drive not needing a calculated distance at all just have to set the motor exactly where the center of the wheel is. Chains allow for a little bit of slack not needing an exact center to center distance in the wrap like belts and pulleys do. Finally gears which need to be a certain distance apart from each other to mesh properly and not skip or bind.

Mounting motors is done in a plate style by face mounting the motor into the innermost drivetrain plate. It can also be done by mounting the motors to a 3rd plate, located in between the outside and inside. This allows for the motor to take up less space in the middle of the robot, but adds complexity. Motors should always be as low as possible and depending where you want the center of mass, either the middle or towards the back of the robot. It is also worth keeping in mind the type of power transmission and the expediency of doing so in light of the motor placement.

Ground clearance is all dependent on if there are any obstacles on the field, as well as what your team wants to do in that game in regards to said obstacles.

For example, in Rover Ruckus some teams with tank drivetrains decided to enter the crater. Therefore, they left enough space to not beach themselves on top of the crater, a common mistake that inexperienced teams often make.

Other teams decided to ignore driving over the crater and decided to reach over with an arm or slide system, which meant they didn't need a lot of ground clearance for their drivebase.

Typically, anywhere from .25 inches of clearance to .5 inches (if you want to be safe) on a completely flat field will allow for the weight of the robot to push into the foam tiles. Nothing else from the robot should touch the ground.

Something you can do is set the robot in CAD onto a field. Set up obstacles such as the crater and simulate driving over the crater by moving it across like you think it would in the real world.

If either of the plates intersect with the obstacle, add some more clearance so you don't get beached like a sad whale.

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Recommended

Game Manual 0·CC BY-NC 4.0

These wheels are the gold standard of mecanum wheels, if you are buying new mecanum wheels there is very little reason to buy any mecanum wheels not from this section.

goBILDA Mecanum Wheels v2 (https://www.gobilda.com/96mm-mecanum-wheel-set-70a-durometer-bearing-supported-rollers/) ($127.49 with team discount): The goBILDA v2 Mecanum Wheels are the gold standard for teams wanting a quality design in all aspects. They feature a 96mm diameter and a thickness of 38mm, with ball bearing supported, 70A durometer rollers that provide excellent strafing performance. With the use of recessed 16x16mm and 32x32mm holes, these wheels have the largest amount of mounting options and can fit all shaft styles commonly used in FTC®.
REV Robotics Mecanum Wheels (https://www.revrobotics.com/rev-45-1655/) ($127.50 with team discount): The REV Robotics Mecanum Wheels come in at only 75mm in diameter, providing a smaller mecanum wheel option to FTC teams; however, they are not the thinnest option at 40.8mm thick. They feature ball bearing supported rollers that provide exceptional strafing capabilities and traction. The hole pattern featured on these wheels is compatible with Andymark Nubs as well as the REV Robotics Universal Hex Adapter v2 (which is included); however, in order to use other shaft options the use of an adapter or physical modification will likely be required.

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Holonomic Drivetrains > X-Drive

Game Manual 0·CC BY-NC 4.0

X-Drive is a holonomic omni-wheel based drivetrain. This type of drive involves mounting 4 omni wheels at the corner of the robot at a 45 degree angle.

One notable difference between X-Drive and mecanum is strafe speed. While, as mentioned in the mecanum section, the ratio of strafe speed to forward speed is noticeably less than 1, the ratio on an X-Drive is exactly 1 due to the rotational symmetry of the wheel placement. This means that an X-Drive bot's strafe speed and forward speed are equivalent. The drivetrains are slower, however, when strafing at 45° (approximately frac{sqrt{2}}{2} of its forward speed).

Even though X-drive has good turning and acceleration, the main downside to the drive is packaging/form factor. Packaging refers to how easy/convenient the drivetrain fits into the overall design of the robot.

Ideally, the drivetrain should take up as little space as possible to make it easier to design mechanisms around. Because the omni wheels are offset, packaging a X-Drive is more difficult than other types of holonomic drive like mecanum or H-Drive. Also because of the strange packaging, it is relatively difficult to cleanly transfer power from the motors to wheels, meaning that most X-Drives end up being direct-driven, which is bad for the lifespan of the motor gearbox.

Note: When using X-Drive, the robot moves forwards/backwards/straight side-to-side sqrt{2} times faster than a drivetrain with wheels in the normal orientation (with the same gear ratio and wheel size).

For an explanation of why exactly this is, see this analysis (https://www.chiefdelphi.com/t/paper-mecanum-and-omni-kinematic-and-force-analysis/106153).

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Holonomic Drivetrains > Mecanum Drive > Advantages

Game Manual 0·CC BY-NC 4.0

Fantastic maneuverability and agility due to strafing, can avoid defense very well
Good acceleration, can have high top speed
Very versatile drivetrain for nearly any game

Suffers in traction, as mecanum rollers have a lower coefficient of friction than traction wheels; cannot traverse terrain
Able to be pushed around on defense
Wheels must be powered independently, so there is no redundancy

CAD Examples of Mecanum Drive (Click to expand) goBILDA COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/df9bcea72fcdd7e4aee4134b)

REV COTS Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/b8d4a6cfe4f26c5170f40a6c)

goBILDA Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/cc2df2fbce0e9ca393696b54)

REV Custom Click here to explore this model in OnShape online CAD (https://cad.onshape.com/documents/45549489f570f3694569a2df/w/85ff26b9fca4988ebc4df3b4/e/40c3266dab2e444011cce79c)

(Figure: Configuration for mecanum wheels, courtesy goBILDA)

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WPILib Kinematics > Mecanum Drive Kinematics

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide > Not Recommended

Game Manual 0·CC BY-NC 4.0

There is almost no reason to use these mecanum wheels---they perform very poorly and are not much cheaper than those in the Recommended section. We cannot recommend the use of these on an FTC robot, if at all possible replace them with mecanums from the Recommended section.

TETRIX Mecanum Wheels (https://www.pitsco.com/TETRIX-MAX-Mecanum-Wheels) ($149.00): These mecanums are designed with a bushing based, hard plastic roller which in turn creates poor strafing performance. The integrated hub is a set screw based design with a round bore, causing unreliability as well as taking up extra space.
Andymark Standard Duty Mecanum Wheels (https://andymark.com/products/4-in-sd-mecanum-wheel) ($77.00): These wheels barely strafe and are super fragile.

(Figure: 8103 Null Robotics, Rover Ruckus, using Nexus mecanum bearing)

(Figure: 731 Wannabee Strange, Rover Ruckus, using AndyMark HD mecanum wheels)

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WPILib Kinematics > Mecanum Drive Odometry

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Holonomic Drivetrains > Mecanum Drive > Mecanum Wheels Miniguide

Game Manual 0·CC BY-NC 4.0

One of the most important features of a mecanum wheel is the mechanism that allows the roller to spin---either bushings or ball bearings. Mecanums which use ball bearings strafe better than those which use bushings, as the rollers can spin with less friction.

Note: In the past, some teams have invested in 6 inch diameter mecanum wheels. These are generally much more expensive and generally do not provide significant performance benefits. We highly recommend sticking with wheels that are between 3 and 4 inches in diameter.

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Kinematics > Mecanum Drive > Inverse Kinematics

Game Manual 0·CC BY-NC 4.0

The inverse kinematics of a mecanum drive relate the desired velocity of the robot to the velocity required on the wheels. These are as follows:

vfl = vf - vs - (2r_b · ω)

vbl = vf + vs - (2r_b · ω)

vbr = vf - vs + (2r_b · ω)

vfr = vf + vs + (2r_b · ω)

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Drivetrains > Drivetrain Options

Game Manual 0·CC BY-NC 4.0

Omni Wheel — Omni(directional) wheels, sold by many different vendors, are a special type of wheel that prioritizes mobility and strafing (moving laterally) over traction or front-back movement. They are similar to mecanum wheels in that omni wheels have rubber rollers that rotate perpendicular to the plane of the wheel.

Thus, the robot can move sideways (although the robot is not powered in the sideways direction). It is also utilized as a low-friction wheel in 4 wheel, 6 wheel, and 8 wheel drivetrains instead of having corner traction wheels.

Furthermore, X-drive utilizes four omni wheels, though traction is at a minimum.

A mecanum wheel is technically an omnidirectional wheel, but when generally referred to, an "omni wheel" has rollers rotated 90 degrees to the rotation of the wheel, where a mecanum wheel is generally 45 degrees.

Traction wheel — A traction or grip wheel is a wheel designed for maximum grip. It has an outer ring made of rubber, and its wide track ensures a larger contact patch with the ground. Traction wheels are commonly found in tank drivetrains. They are sold in different sizes and thicknesses by different manufacturers.

Strafing — Strafing is the act of moving sideways or laterally (somewhat similar to drifting). It is possible with omni or mecanum wheels, and not possible with traction wheels.

Parallel Plate Drivetrain — A parallel plate drivetrain is a drivetrain that has drive pods that consist of 2 plates spread apart with wheels and drive transmission in between them.

These plates can be anywhere from 1" to 5" apart, depending on the space requirements of the wheels and drive system. Generally, a pod width of 3" or less is desired to maximize the space between the drive pods for mechanisms such as an intake.

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