my arm sags whenever the driver releases the joystick, how do I hold it up

To prevent an arm from sagging when the driver releases the joystick, gravity compensation can be implemented using feedforward control . This involves adding a kF constant to the motor output to counteract the effect of gravity on the arm .

The logic for gravity compensation involves calculating the current angle of the arm and using a cosine function multiplied by the kF constant to determine the necessary power output . An example of this logic is:

while True:
    error = desire_position - current_position; # no effect on gravity compensation
    current_angle = (TICKS_AT_ZERO - current_tick) * DEGREE_PER_TICK
    output = PID(error) + cos(radians(current_angle)) * kF

The kF constant approximates the power needed to counteract gravity and is typically found empirically . To tune kF, the PID gains can be set to 0, and kF can be increased until the arm can hold itself up at any position . If the arm still falls, kF should be increased; if it moves upwards, kF should be decreased .

Other Considerations

Arms require a high gear ratio and must be well-supported to manage the torque provided by the motor . Poorly supported or constructed arms can shake, making precise movements difficult . Software control, such as control loops, can help reduce the load on motor gearboxes .

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Sources6

Control Loops > Feedforward Control > Gravity Compensated Feedforward

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In gravity-compensation we derive the effect of gravity upon an arm as Fg = gcos{theta}. Here we can use that with the following logic.

while True:
    error = desire_position - current_position; # no effect on gravity compensation
    current_angle = (TICKS_AT_ZERO - current_tick) * DEGREE_PER_TICK
    output = PID(error) + cos(radians(current_angle)) * kF

This code uses a kF constant to approximate how much power the arm needs to counteract gravity. In theory, this is something related to gravity multiplied by the rotational moment of inertia of your arm. Calculating this is impractical and kF is instead found empirically. This can be done by setting the gains on the PID to 0, and increasing kF until the arm can hold itself up at any position. If your arm is still falling, increase kF, and if your arm is moving upwards, decrease kF. FTC team 16379 Kookybotz has an excellent video on Arm programming (https://youtu.be/E6H6Nqe6qJo?si=AztzVtAqShFEA4EP&t=440), where they demonstrate how to increase kF.

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Software Glossary

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Disconnect — A disconnect (DC) is when, for any reason, the robot is not able to be controlled from the gamepad. This can happen for many reasons - static buildup on the robot, a loose cable, or an error in code.

Generally, most DCs are caused by improper wiring, so wire stress relief is encouraged for all teams (USB Retention Mount). They can also be caused by WiFi disconnects, or an ESD (electrostatic discharge) shock to the electronics.

Driver Station — The Driver Station (DS) refers to both the software and device that is used by the drive team and connects to the gamepad(s). The device can either be one of the Android phones specified in Game Manual Part 1 or a REV Robotics Driver Hub.

Robot Controller — The Robot Controller (RC) refers to both the software and device that is on the robot that controls it. This can either be a REV Robotics Control Hub or one of the Android phones specified in Game Manual Part 1 connected to an Expansion Hub via a Micro USB cable.

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Driver Station Guide > Controllers

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LEGAL CONTROLLERS: As of the 2021-2022 season, the following controllers are legal for competition: (https://web.archive.org/web/20220131114951/https://www.firstinspires.org/sites/default/files/uploads/resource_library/ftc/legal-illegal-parts-list.pdf)

Xbox 360 Controller for Windows
Logitech F310 Controller
ETPark Wired Controller for PS4
Sony DualShock 4 Wireless Controller for PS4 (Must be connected wired over USB)

In the past, the Xbox 360 controller was preferred by many top teams over the Logitech F310. This is mostly as the Xbox controller has less of a dead spot. A dead spot is basically the area where the joystick can move but not communicate that the stick has shifted slightly. This means that when the joystick is moved to a position where the controller can detect it, the robot will sometimes have a tendency to lurch forward. Dead spots make it difficult for the driver to execute fine-tuned and precise movements. This is likely a non-issue for the large majority of teams. Also, keep in mind that there are very specific models of the Xbox controller that are legal, so they can be hard to find new.

Starting with the 2020-2021 season, the ETPark and Sony PS4 Controllers are legal and provide numerous advantages over Xbox 360 Controllers. Some of these advantages include, better trigger and bumper control, a more usable dpad, and trackpad support as of the 2021-2022 season. Use of the PS4 controller is slightly different because the locations of the Left Stick and DPAD are swapped and some button names are different. (https://github.com/OpenFTC/OpenRC-Turbo/blob/2d1e527d3d53c3ac7da701a73d342b85cf407835/RobotCore/src/main/java/com/qualcomm/robotcore/hardware/Gamepad.java#L884) Pro Tip: Make sure your drivers have experience with the controller they plan to use for competition.

Generally it is the opinion that the PS4 controllers are better than the other options. Among the options for PS4 controllers, the choice is really up to you. The Sony controller has slightly better buttons, but the ETPark controller is half the cost and a more solid connecting wire. Keep in mind that the drivers can use different controllers, so pick the one most comfortable to you.

A phone holder and OTG strain relief connector can be useful as it may help to prevent connections from loosening. Especially if teams are running dual controller configuration, ensuring that the USB hub is secure won't hurt.

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Arms > Arms for End Effectors

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Arms are the most common way to add flexibility and control to end effectors. These arms typically have one or more rotational joints and/or linear extension. Due to the relatively small power requirement of these arms, they are often driven by servos instead of motors.

Attention: End effectors are likely to see collisions and with arms acting as a lever, it is imperative to take extra caution to mount and support these servos properly (see /docs/power-and-electronics/servo-guide/usage-tips).

Single and multi-axis arms can add control to the vertical and horizontal distance of an end effector. In Into The Deep, this type of arm was common in intakes to allow the intake on a horizontal extension to go over the submersible barrier and then "reach down" for samples. They can be used to line up intakes and outtakes to facilitate transfers or just generally add flexibility to where end effectors can act.

Linear extension is usually seen on outtakes to increase the range of the robot's deposit.

(Figure: 15972 TehnoZ, Into The Deep, Finalist Alliance Captain (Edison), arms used on both intake and outtake, linear extension is used on outtake.)

(Figure: 12993 RoboKings Aurum, Centerstage, Finalist Alliance 1st Pick, high degree of freedom outtake arm with horizontal extension.)

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Design Tradeoffs > Single/multi axis arm Linear extension

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Important: Teams are generally advised to stay away from arms and move in the direction of linear slides, primarily due to the issue of complexity. This is because arms typically are less effective than linear extensions and are harder to implement properly.

Arms require a high gear ratio, so arms must be supported extremely well to bear the torque that the motor provides. A poorly supported and/or constructed arm will cause the driver needless pain as it is exceedingly difficult to line up an arm that constantly shakes. In contrast, linear extensions do not need to worry about gear ratios and gearboxes. They can be optimized to be more efficient than arms, and typically are more precise, as linear motion is easier to control than angular motion. Another positive is that linear slides can have more extension than arms, with some reaching 5+ feet in length.

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Arms

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Arms encompass a large variety of mechanisms. In FTC, they are used to add extension and flexibility to end effectors. Arms are especially powerful when paired with other mechanisms such as linear extension and mini-turrets.

Arms can be a useful tool for extension. Long arms need large reduction ratios to drive. The example in the Single Arm section uses a 254.5:1 reduction. It is recommended to transfer torque to the arm through a gear, chain, or belt rather than directly mounting to a motor shaft due to axial forces and the ability to get a larger reduction. Due to the large amount of momentum these arms can carry, if suddenly stopped they may be able to break a motor's gearbox or even tip your robot over. Software control can help to reduce the load on motor gearboxes (see /docs/software/concepts/control-loops).

Depending on the application and implementation, arms can be either faster or slower than other extension options.

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