2020-03-06

2020-03-02 Turret Construction


>>>Pi Wars 2020<<< 


1Turret and Main Weapon

With the completion of the high speed controls and low latency video streaming, the stage is set to proceed with the final phase of construction of the OrangeBot platform.
This is the biggest upgrade since the construction of the core platform, and beyond this point only minor adjustments should be required.
Mechanical upgrades:
  • Component fitting and Pi-Wars specification check
  • Turret
  • Main weapon
  • Rotation and Elevation mechanism
  • Attachments:
    • Pi-Noon Hard Point and Pi-Noon melee weapon servo
    • Lava Palaveer sensors
    • Eco Disaster forklift
  • Battery Strap
  • Maintenance hatches
Electronics:
  • Servos: Electronics, connectors, cable management and power
  • Power: Main switch, distribution, cable management, turret joint routing
  • Raspberry-Pi: Power, I2C and UART proto hat
  • Sensors: I2C network
  • LED swag!
Firmware:
  • Servo driver and messages
  • Main weapon driver and messages
  • Forklift driver and messages
  • Pi-Noon lance driver and messages
Software
  • Key bindings
  • Integration of all remote controls in the client HMI

1.1Disassembly

The platform was disassembled in preparation for the upgrade.

Illustration 1 - OrangeBot Disassembly
Several plates of the platform needs upgrading, other plates have now become obsolete and have to be rebuilt to spec to fix fitting mistakes and support new functionalities.
Electronics need minor modifications and many new cables needs to be routed.

1.2Obsolete Plates

The now obsolete “platform top plate”, “turret base plate” and “turret rear plate” needs retiring as they were place holders.

Illustration 2 - OrangeBot Retired Plates

2Main Electronics

Main board has to be upgraded to support:
  • Three servo motors
  • Switch (Main Weapon Feedback)
  • Third DC Motor (Main Weapon Spring)
The upgrade is mostly painless, requiring an handful of connectors and resistors. Testing was done to ensure the board was operational and would likely not need a second upgrade.
Unfortunately testing revealed that three full sized servos cannot be powered by the main board and require a second independent 5V3A power regulator, that was cannibalized from “Maze Runner”.
Illustration 3 - Main Board Upgrade

2.1Main Weapon Test

A key decision for the next phase of component fitting was whatever the main weapon could be mounted side ways or needed to be mounted up strainght.

Illustration 4 - Main Weapon Side Loader Test (FAIL)
The rest revealed that the side loader has reliability issues, and a top load mechanism is required.

3Turret Construction

With key decisions about the design taken, construction could move to the next phase.

3.1Turret Mockup

A Mockup of the turret mechanism was made to take measurements on the size of various plates and decide on the component placement and fitting.

Illustration 5 - Turret Mockup

3.2Turret Rotation

With the elevation mechanism and turret dimensions sorted out, it was time to go back to the original design and evaluate the place holder rotation mechanism.
The rotation is critical as it has to withstand the inertia and shock of the moving platform and has to be reliable. After careful consideration, it was decided to retire the previous mechanism and redo three plates.

Illustration 6 - Rotation Mechanism Mockup
It became necessary to turn upside down the turret joint to increase clearance for the rotation servo inside the platform and inside the turret to increase the range of the elevator mechanism, also by turning upside down mechanism, the key that connects the rotation joint with the rotation servo no longer needs a clearance with the rotation joint walls, increasing significantly the robustness of the mechanism.
The rotation joint was also moved more toward the rear of the platform. This has the benefit of looking better (swag first!) and granting better clearance to both the components inside the platform and inside the turret.
There were two ways to place the rotation joint. As Murphy dictates, the wrong way was chosen initially!

3.3“Top Plate” and Rotation Joint

A new top plate was constructed to accommodate the rotation joint mounted in reverse. The holes to fix the top plate to the platform were reduced from eight to four, with a fixed bolt and a space below them. This allows for easier maintenance, as removing four bolts allow the turret, the top plate and the rotation mechanism to come off in one piece. An air gap is maintained to get some airflow inside the platform in case thermals become a concern.

Illustration 7 - Top Plate and Flipped Rotation Joint

3.4Rotation Mechanism

It was very important to take extra care in the design and construction of the “Rotation Mechanism” as this servo and joint have to tank the rotational inertia of the turret caused by the motion of the platform.

Illustration 8 - Rotation Mechanism
The Rotation Servo is kept in place by a structure below the turret fixed by long M4x60 bolts.
A “Rotation Key” transmit the motion from the servo, through the hollow hole of the “Rotation Joint” up to the “Rotation Plate”. The key itself can be moved upward to disconnect the servo from the turret quickly for maintenance.
The “Rotation Plate” will feature additional holes for the cables to pass from the turret to the platform.

3.5Turret Component Fitting

Another critical component of the design is the “Turret Base Plate”. It's important to get the dimension and placement of the holes right as all other turret plates and components use this plate as base.

Illustration 9 - Component Fitting - Second Iteration
A second round of component fitting was performed to estimate turret dimensions and placement. In this fitting, it was decided to move the Raspberry Pi to the “Front Elevator Plate” because of the convenience of attaching the RaspiCam flat cable. The Elevation Servo and Elevation Mechanism was also validated in this phase.

Illustration 10 - Turret Plates Masks and Cut
With the dimensions confirmed, the turret plate masks were prepared, and plates were cut to shape. No holes or other cut out were made in this phase. This allowed for a third iteration of component fitting to make sure the plates were cut right

3.6Turret Elevator Plate

Because so much of the fitting depends on the elevator mechanism, the first turret plate to be finalized was the “Turret Elevator Plate”.

Illustration 11 - Turret Elevator Plate Components Fitting
A third iteration of components fitting was performed to decide on the Elevator Joint and the position of the turret components:
  • Raspberry Pi
  • Main Weapon
  • RaspiCam
  • Targeting Laser
  • Elevator Joint

3.7Turret Base Plate

With the design of the turret validated, the Turret Base Plate could finally be prepared and placed in position. The Rotation Joint was quickly tested and proved to perform flawlessly, achieving a great coupling between turret and the rotation servo below.

Illustration 12 - Turret Base Plate
The Rotation Mechanism fits nicely inside the platform with all other components in place.

Illustration 13 - Rotation Mechanism Fitting

3.8Elevator Joint

The elevator joint is constructed from Lego Technic components and M3 bolts. Lego makes it much easier to build a rotating connection. The elevator joint connects the “Turret Elevator Plate” to the “Turret Right Plate” and “Turret left Plate”.

Illustration 14 - Elevator Joint
The elevator plate is connected to the elevator servo at the bottom end point. The motion of the plates makes the butt of the main weapon move up or down. Its limits are the rotation joint and the turret top plate.

Illustration 15 - Elevator Range
The main weapon is not meant to behave as a mortar or long range artillery, so the elevator range does not have to be massive. Something in between 10° of liberty would be more than enough to aim, even when the robot wants to aim at a ground level target while climbing a ramp.
The main weapon is connected to the Turret Elevator Plate by means of an L shaped structural element with two M4 bolts.

3.9Turret Top and Rear Plates

The Top and Rear plates are straightforward. They just needs holes for the L shaped structural elements.

Illustration 16 - Turret Top and Rear Plates

3.10Turret Final Assembly

The “Turret Left Plate” was the last to be constructed, and accommodates the “Elevator Servo”.
The Elevator Servo is connected to the bottom of the “Turret Elevator Plate” with a beam made from a clips and held in place by M3 bolts.

Illustration 17 - Final Assembly Side View

Illustration 18 - Final Assembly Top View
The video below shows how the Rotation and Elevator Joints operate.


Video 1: Rotation and Elevation mechanism

4Conclusions

With the completion of the turret, the platform is finally nearing completion.
The platform dimensions:
Width: 190 [mm]
Length: 260 [mm]
Height: 310 [mm]
Weight: 3811 [g]



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