Redshift Project Depot

FRC 2018 (Power Up) => Robot Software => Topic started by: Louis L on January 11, 2018, 12:38:45 PM

Title: How the arm will work
Post by: Louis L on January 11, 2018, 12:38:45 PM
This thread will document how the arm will operate from a software point of view. The goal is to describe in as much detail as possible, the operation of the arm as it relates to game play, software states, state machines,  etc. There will be some cross-pollination with the related intake (http://www.team4048.org/smf/index.php?topic=254.0 (http://www.team4048.org/smf/index.php?topic=254.0)) thread.
Title: Re: How the arm will work
Post by: Louis L on January 11, 2018, 01:01:46 PM
Are there fixed heights that we pre-program for the arm to simplify the work on the user? For example, here are some I'm thinking of:
Title: Re: How the arm will work
Post by: Louis L on January 14, 2018, 01:44:59 PM
The arm can extend in and out in order to prevent the arm from extending beyond the 16 inch size restriction. As the arm rises, the extension mechanism needs to shorten the arm and later extend. The movement of the extension in and out needs to be automatic and under software control. The user must not have be allowed to raise the arm without the extension mechanism working. This will strictly fly-by-wire; no direct control of the arm will be allowed.
Title: Re: How the arm will work
Post by: Louis L on January 24, 2018, 10:45:50 AM
Following the 1/23 HW meeting, here's how the arm will work:

Theory of Operation

The arm consists of 3 parts - a fixed vertical tower, an extendable rotating arm, and a gripper mounted at the end of the arm. Together, the arm grabs a Cube from the Intake, tucks it away for transport, and delivers it to various heights as needed. In its motion up and down, the arm, with the help of software, will not break the 16 inch rule.

Everything needs to move quickly and the Cube needs to be held securely. Together with the Intake, the overall system needs to be highly efficient at getting a Cube and delivering it to its destination in the shortest amount of time.

The arm is the tallest structure on the robot. If we advance to the World Championship in Detroit, we will need to make this fit inside the shipping container. Anything that doesn't fit needs to be easily disassembled for shipping.

Mechanical and Electrical Details
The tower is at a height that makes it still legal per FRC Power Up rule. The rotating arm is mounted to the top of the tower and moves via a chain driven system. The arm's motion is limited at each end by limit switches that stop the motor when tripped. The arm consists of a fixed part and an extension. The extension is build onto drawer slides whose movement is controlled by a lead screw. Limit switches define the range of motion of the extension. The extension exists because the rules allow only 16 inches of protrusion from the frame perimeter. The arc traversed by the arm would violate this rule if the arm's length was not somehow reduced; hence the extension.

Movement of the extension in and out will be software controlled at all times. The user's interaction with the arm is entirely fly-by-wire; the user has no direct interaction with the extension. This is intentional and prevents the user from breaking the 16 inch rule.

At the end of the arm is the gripper. The gripper uses a pinching mechanism to grab a Cube from the Intake. The grip position is the top edge of the Cube. Either or both of a pressure sensor and motor current sensing will be used to determine how hard to grip the Cube without destroying something. Limit switches define the min and max grip width. Another limit switch detects when the gripper makes contact with the top of a Cube.

The Gripper can rotate the the Cube 180 degrees such that it rests more securely on a surface rather than relying entirely on grip friction. A motor performs the rotation and it too is travel limited by limit switches. An accelerometer mounted to the Gripper is used to determine how level the Gripper is. This information is used to continuously rotate the assembly to keep it level.
Title: Re: How the arm will work
Post by: Louis L on February 03, 2018, 09:43:16 PM
We had a long discussion with Peter & Sarah & Ed today and it seems we had the wrong idea of how the Gripper / claw works. What this means is that from a software point of view, we have to rethink how the software will operate.

Get Cube, Deliver it to a Scale or Switch, Release the Cube.

The most basic act in the game is to get a Cube from the ground and deliver it to a Scale or Switch. Here's what the software will do.Caveat - when the arm is raised from its Home position and power is cut once the correct height is reached, gravity will pull the arm back down. We will need to apply some amount of power to the motor to stall it - not enough to burn it but enough to counter gravity. The amount of power has to be experimentally derived. It will also vary with the angle of the arm.
Title: Re: How the arm will work
Post by: ohad z on February 03, 2018, 10:56:19 PM
One addition to the software list is to raise the intake out of the way between “move arm to home” and “driver now drives”

Title: Re: How the arm will work
Post by: Louis L on February 03, 2018, 11:02:30 PM
One addition to the software list is to raise the intake out of the way between “move arm to home” and “driver now drives”

thanks,
added to previous post.
Title: Re: How the arm will work
Post by: Louis L on February 03, 2018, 11:12:40 PM
Also update definitions of "home" to include the Intake assembly.