how do I mount odometry pods

You should mount your odometry pods using a pivoting mechanism that is spring-tensioned to push the wheels into the ground, unless you have a very specific use case that justifies the complexity of vertical springing. Game Manual 0 explains that spring tensioning is highly recommended because it ensures the wheels maintain constant traction with the floor tiles to prevent slipping. Pivoting the pod around a point using a spring or rubber band is the most popular and reliable method, whereas vertical springing is much harder to design well and is not recommended for the minor accuracy improvement it yields.

You must mount the pods in a specific physical layout depending on whether you choose a two-wheel or three-wheel configuration. According to Game Manual 0, a two-wheel setup requires mounting one parallel pod and one perpendicular pod relative to the drive wheel axis, while a three-wheel setup requires mounting two parallel pods and one perpendicular pod. The lateral distance between the parallel pods (the trackwidth) and the forward-back offset of the perpendicular pod must be carefully measured and accounted for in your software.

When integrating the pods physically, they are typically designed around your specific drivetrain. Game Manual 0 notes that popular open-source designs, such as Open Odometry or goREVdometry, are compact enough to be mounted directly inside a goBILDA channel.

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Dead Wheels > Design > Spring Tensioning

Game Manual 0·CC BY-NC 4.0

It is highly recommended that your dead wheel design includes some form of spring tensioning that pushes the wheel into the ground. This ensures that the wheel is always in contact with ground and has adequate traction. Sufficient force is required to ensure constant traction to prevent the wheels from slipping. Keep in mind that too much force may lift a light drive train off the ground and disrupt driving.

The most popular method of spring tensioning is to pivot your pod around a point and provide a rotational force via a spring or rubber band.

(Figure: FTC 14320's spring tensioning)

A much more niche option is to vertically spring odometry pods. The idea is that springing around a pivot will cause the dead wheels to move in the axis parallel to the ground if the height of the dead wheels relative to the ground changes. Vertically sprung odometry pods will not experience such an issue. However, this is not really an issue that most teams will experience. Vertically springing is much harder to design well and is not recommended for the relatively minor improvement in accuracy it yields.

(Figure: FTC 18172's vertical springing)

Open Game Manual 0

Dead Wheels > Design

Game Manual 0·CC BY-NC 4.0

There are few open source dead wheel designs. Dead wheels are often designed around a team's own drive train and FTC teams seldom publicly release their own robot CADs.

Here are a few publicly available dead wheel designs:

Open Odometry by 18219

https://openodometry.weebly.com
Utilizes the REV Through-Bore Encoder
Most popular and robust publicly available design
Compact enough to fit into a goBILDA channel

Things to consider:

Utilizes Rotacaster 35mm wheels from Australia. Shipping may take a while

goREVdometry

https://discord.com/invite/Cvz3MbM9dX
Utilizes the REV Through-Bore Encoder
Compact enough to fit into a goBILDA channel

Things to consider:

Information only available through their Discord channel
Hasn't been iterated on in a while

9794 Wizards.exe Design

https://www.youtube.com/watch?list=PLICNg-rquurYgWAQGhu6iC0At75vgqFJp&v=OjNvAD350M4&feature=emb_title
Compact enough to fit into a goBILDA channel
No longer recommended as it utilizes the E8T

Open Game Manual 0

Dead Wheels

Game Manual 0·CC BY-NC 4.0

Many teams in the community have converged on a unique solution that isn't seen very much outside of FTC: the use of "dead wheels," "tracking wheels," or "odometry pods" (these terms are all synonymous). These refer to small "dead" or non-driven (not powered by a motor) wheels attached to an encoder sensor. Two or three dead wheel pods are often sprung to the ground to ensure accurate tracking. The two-wheel design utilizes one parallel and one perpendicular pod (parallel and perpendicular with respect to the drive wheel axis), measuring x and y movement respectively. Change in heading is measured via a gyroscope. The three-wheel design utilizes two parallel and one perpendicular pod, measuring x and y movement respectively. However, this design forgoes the gyroscope and instead measures heading via the difference with the two parallel wheels. This is often more accurate in the context of the FTC control system because the BNO055 IMU (used for the gyroscope in the two-wheel design) utilizes I2C which is slower than the rest of the I/O on the REV Hub and cannot be bulk read. These two issues lead to minute drift issues which can compound over time, thus leading to a more inaccurate localization system when using the two-wheel design.

However, designing consistently accurate dead wheels proves to be a difficult design challenge. It is often quite pricey. A set of three dead wheels will cost a minimum of $100 for the encoders alone, prior to any hardware.

Let's go through the advantages and disadvantages of each system.

Open Game Manual 0

Chief Delphi > FTC > Easy and Cheap Odometry Pods for FTC

Chief Delphi·Community forum posts — not openly licensed

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open Chief Delphi

Chief Delphi > FTC > odometry wheels FTC

Chief Delphi·Community forum posts — not openly licensed

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open Chief Delphi

Chief Delphi > FTC > FTC Need Odometry Help

Chief Delphi·Community forum posts — not openly licensed

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open Chief Delphi

Dead Wheels

Game Manual 0·CC BY-NC 4.0

Warning: This is a very niche aspect of design in FTC®. Generally it is something done by more experienced teams who have had time to repeatedly test their designs and mechanisms with software during the off-season.

The term dead wheels, tracking wheels, odometry pods, and odometry are often conflated in the FTC community. However, there are a few key differences one must keep in mind. Odometry is an umbrella term and refers to the general use of motion sensors for localization purposes. Meanwhile, dead wheels, tracking wheels, and odometry pods are all synonymous terms.

Term: Dead Wheel — A small unpowered wheel (usually an omni wheel) that tracks the distance the robot has traveled through the encoder attached to the wheel's axle.

Usually, there will be two or three wheels - one or two on the x and y axis each to track the front-back and left-right position relative to the starting point. Generally, odometry wheels are sprung so that the wheel is in contact with the floor tiles at all times to ensure accuracy.

Odometry refers to the use of motion sensors for localization. Localization is a means for being able to locate the position of the bot at some point in time. Localization is crucial in path following and advanced autonomous modes as one needs to know where they are to generate the necessary movements needed to reach a desired destination. Localization software plays a major role in odometry; however, in order to produce accurate results, reliable and accurate hardware design is a necessity.

The simplest form of odometry is drive encoder localization. This is the use of encoders measuring the rotation of motors that power the drive train. One is able to read the encoder data and feed it through the kinematic equation for that specific drive train to derive the body's velocity. Drive encoder localization is generally quite simple and easy to setup as almost all of the FTC legal motors have built-in encoders. Getting drive encoder localization setup is simply a matter of plugging in wires, no additional hardware needed.

Open Game Manual 0

Dead Wheels > Two-Wheel Odometry Pods

Game Manual 0·CC BY-NC 4.0

Pros:

Cheaper than 3-wheel design
Pretty good accuracy
No tuning of the heading necessary
Cons:

Subject to more drift than the 3-wheel design

Pros:

Relatively accurate tracking. Great accuracy in a 30-second autonomous mode
Cons:

Quite pricey
Tuning of the heading is very important

Open Game Manual 0

Chief Delphi > FTC > Gobilda Odometry Pod tick values persisting through restarts

Chief Delphi·Community forum posts — not openly licensed

This source does not publish a licence, so its text is not reproduced here. Open the source to read it.

Open Chief Delphi

Dead Wheels > Encoders

Game Manual 0·CC BY-NC 4.0

A lot of the localization done in software relies on readings from encoders. encoders are sensors that track "counts" or "ticks," which are values that represent a certain amount of a rotation. Different encoders might have a different number of counts per revolution (CPR), which is also sometimes also called ticks per revolution. The greater the number of counts, the more precise the data.

Encoders are plugged into the JST-PH ports in the REV hubs. These encoders can either be built-in to the motors or external. External encoders will still need to be plugged into an encoder port but are not related to the motor in that port. Through software, we can use the motor object to determine the position of the encoder. This should be done with motors that do not use encoders. If you're using dead wheels, you will not need the drive motor encoder ports, so those are potential ports you might want to use.

If one chooses to design dead wheels, there are only two recommended encoders that one should use for FTC: REV Through-Bore Encoders and U.S. Digital S4T Encoders.

Open Game Manual 0

Dead Wheels > Encoders > U.S. Digital S4T

Game Manual 0·CC BY-NC 4.0

The S4T (https://www.usdigital.com/products/encoders/incremental/shaft/S4T) miniature shaft encoder is another viable option used in dead wheel designs. These encoders are very small and may significantly reduce the footprint of your dead wheel design. Gearing these encoders is ideal to prevent shock loads.

(Figure: S4T Encoder)

***Advantages:

Very compact

Disadvantages:

More expensive (nearly double the price)
Less durable

Very thin wires. Prone to breaking easily if not secured properly

Ideally requires external gearing

Open Game Manual 0

Dead Wheels > Gallery

Game Manual 0·CC BY-NC 4.0

(Figure: FTC Team 11115 Photo Album (https://photos.google.com/share/AF1QipPx5inCdVxK6wAqtIznFE-KqvnuzgRq9rFxrhzI50r0DeYYo2o11hWB4hroYObm8A?key=UWwxd3hFdXpYaHFqaFhTSFJnWFlEWjgtV1FTN3Zn))

(Figure: FTC Team 3658 CAD)

(Figure: FTC Team 7236 CAD)

Open Game Manual 0

Odometry > Finding the Change in Position > Angle and Displacement

Game Manual 0·CC BY-NC 4.0

The displacement (or change in position) of the left sensor is Delta xl and the displacement of the right sensor is Delta xr. The lateral distance between these two sensors is called the trackwidth, notated as L. This is very important for determining angle for turning approximations. This value will need to be tuned, which means tested repeatedly and then brought to some converging value that is close to the actual measurement.

(Figure: 17508 Rising Tau's 2019/20 Skystone Bot (https://learnroadrunner.com/dead-wheels.html#three-wheel-odometry))

Deriving the value of varphi then becomes simple:

varphi = (Δ xl - Δ xr)/(L)

To perform later calculations, we need to know the displacement of the robot in the x-direction relative to its center rather than the two parallel sensors. To do this, we take the average to derive Delta xc, or the center displacement:

Δ xc = (Δ xl + Δ xr)/(2)

The final displacement we need before we can determine the change in pose is the horizontal displacement Delta xₚerp. This is the displacement of the perpendicular sensor Delta xh with a correction for forward offset F. In order to get accurate approximations, the forward offset needs to be considered. When the sensor is closer to the back, the offset is negative, but when it is closer to the front, it is positive. This is to account for the change in its position based on point-turns.

As a result of this, we can define our horizontal displacement as:

Δ x_perp = Δ xh - (F * varphi)

Note: If you do not have perpendicular sensors, which are not required if the robot cannot move in the lateral direction, Delta xₚerp is not necessary.

For this value, use 0 if you do not have a horizontal sensor.

Open Game Manual 0

Dead Wheels > Encoders > U.S. Digital E8T (deprecated)

Game Manual 0·CC BY-NC 4.0

Once the de facto option for most FTC teams, the E8T (https://www.usdigital.com/products/encoders/incremental/kit/E8T) optical encoders are no longer recommended as the REV Through-Bores are a superior option at an equivalent price. The open-hole optical disc design of these encoders face a number of frustrating design flaws that made them very fragile and prone to breaking. The only advantage that they have relative to the REV Through-Bores is their smaller footprint.

(Figure: E8T Encoder)

Open Game Manual 0

Motion Mounting > Dead Axle

Game Manual 0·CC BY-NC 4.0

Term: Dead Axle — Another form of power transmission is called "Dead Axle". In a dead axle setup, the mechanism is mounted on a fixed axle via bearings: it is free to spin around the axle. This method requires the power transmission to be physically bolted to the mechanism, because the axle itself will not rotate. For this reason, dead axle is generally recommended when powering one thing that is between two plates, as the axle itself can serve as a standoff to provide support between the plates.

Open Game Manual 0

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.

Open Game Manual 0