Auto Tool-Changer Design (beta)

The Auto-Tool-Changer is the component that—with complementary tools—enables the 3-axis Visible Robot to grow into a desktop manufacturing workstation.

It is intended to be an end-effector-platform (EEP) allowing multiple tools to be simultaneously available for use with the Visible Robot. The current design allows up to three tools to be mounted on one ATC. The ATC itself is mounted on the Z-axis shafts.

Examples of tools that could be handled by the ATC might be a camera, mirroring for a laser cutter, an automatic screwdriver, a drill, a rotary cutter, a mountable 3D-printing extruder, etc.

ATC_assembly_complete-w-2-cables1Prototype Auto Tool-Changer (ATC)

Ideas going into the ATC:
The design of the ATC is intended as an outline and a “beta-stage” strawman for a desktop manufacturing workstation architecture where designers of tools, part-presentation and tool and tool-bit presentation devices can flesh out the ecosystem with their devices, licensed as the designer chooses. Their designs and devices may be given freely to the world as “free as in beer,” or sold in the marketplace, again as the designer chooses.
The ATC is intended to help the Visible Robot fulfill its role as a manufacturing workstation, capable of a number of different operations on material. In this role it serves as a computer-controlled docking station between the robot and tools available to it in the periphery of the workspace.
Tool racks and tool-bit stationing are anticipated to be available to the ATC and robot in this periphery.
Conceptually, operating the workstation is “hands-off” for the operator. Tool and tool-bit selection, mounting, registration, operation, and dismounting are envisioned to be automatic.
Tools used are expected to be programmable. Depending on their functionality, they should be able to be started, stopped, have their spindle speed regulated, change tool-bits, take time-lapse photos, be focused, grasp a part, respond with sensor information, etc. (See the Tool Controller document in the Architecture section).
Programs for the tools are expected to be available in a database from which a job-controller application can retrieve them as needed for specific tasks within specific jobs. (See the Workstation Controller document).
Both control signaling and power are to be supplied through a single cable. The ATC has two “shelves” that hold USB connectors. The shelf on the right (see photo below) holds a fixed-to-the-robot USB Type-C connector and cable, connecting at the other end—possibly through a hub or switch—to the Tool Controller. This connector is expected to provide both communication and control signal and, eventually, up to 100W of power (60W, currently) for the tool’s use. “More-capable” tools will probably use this interface. The shelf on the left is to hold a USB Micro. It is expected to serve tools with small power and bandwidth requirements. Simple tools with more limited functionality are expected to use this connector.

ATC_Design_html_46376ffCoupler panel with shelves for USB Micro and USB-C

Specifics:
Breadboard
The first (breadboard) incarnation of the ATC was designed to be 3D-printed. It was intended to use USB Mini and HDMI adapters, with an overloaded HDMI protocol. This was before I became aware of Alternate Mode and the Power Delivery functionality of USB-C.

Prototype
The current ATC—the prototype—is designed to be molded or cast. It has a floor and crown section—each with six sides. Three of these sides accommodate solenoid-actuated couplers.

ATC_Design_html_m2f39fb77Solenoid-driven clenching on the prototype ATC

Coupler panels
Each coupler body is formed of two identical or mirrored panels placed back-to-back and fastened top and bottom into the floor and crown pieces.

ATC_Design_html_m5acbcdaMirrored coupler panels

The hollow columns to the sides fit into tubes integral to both floor and crown and are fastened to them with 3mm screws.

Two “shelves” at the top of the panels are used to clench male USB Type-C and USB Type-A Micro plugs, which provide power and signaling to the tools. These shelves mate to top-caps with stops to prevent inward movement molded into the crown piece.

ATC_Design_html_m529c388b

Crown (underside) showing USB shelves & stops

ATC_Design_html_m302f3cad

 Floor w/coupler slots, solenoid attach & Z-axis shaft captures

The tool-coupler has openings for three “cammed teeth,” which are cammed into either “hold” or “release” position to hold or release a tool. Toward the center of the panels, the openings have features which, when the two panels are placed back-to-back, serve as a bearing seat for the barrel-shaped “bearing” feature of the cammed teeth. (See cammed tooth, below). These features also serve to contain the cammed teeth radially and disallow excess “slop.”

There are three 4mm holes in the intervals between the cammed tooth openings which are used to receive three pins of the tool-side of the coupler. These are intended to provide a positive registration with respect to the robot and also prevent torsional movement of the tool while the tool is in the grasp of the ATC.

ATC_assembly_front_closeup2One of the three ATC ports

The large raised circular features surrounding the 4mm holes are intended to serve as “faces,” to be mated with counterpart faces, on the tool-side coupler. These, ideally, would be polished and registered metal inserts, rather than the plastic features as shown.
Three cammed teeth provide for the capturing of the tool-side coupler. Each part has a “tooth” with an inward-facing flat side that clenches the tool-side coupler tight to the faces of both parts. A barrel-shaped “bearing” is pivoted perpendicularly to the radius extending from the center of panel. The pivot of 30 degrees is enough to clear the tool-side coupler. A roughly cylindrical recess opposite the bearing feature provides a space for a cam that, moving orthogonally to the coupler panels, levers the cammed tooth into “Hold” and “Release” positions. A center “keel” provides a guide during movement, always being within the registering slots of the bearing bedding features.

Cammed_Tooth_THREECammed teeth prior to post-processing

Tool-coupling cam
An arrangement of three posts, with an center-facing cam at each end, set on a triangular base, forms the tool-coupling cam. A center hole of 7.5mm accommodates the plunger of the solenoid. Access features allow a 3mm x 10 screw to hold the plunger to the tool-coupling cam part.

In 3D-printed form, each post is stiffened and strengthened with a 3mm x 12 screw. This eliminates the weakness in shear that is native to 3D-printed parts. A molded part, presumably could dispense with these stiffening screws.

Tool_coupling_subassembly-with_solenoid-earlyThree-post tool-coupling cam

This part,with its cams inserted into the recesses of the cammed teeth, is pressed to the mechanism’s “Hold” position with the spring of the solenoid. It is pulled back to the “Release” position by the (pull) solenoid.

Solenoid
Each port uses a 12VDC pull-type solenoid (part number ZYE1-0826) with a throw of 6mm.

ATC_Design_html_2678316eView of ATC internals

Solenoid control
The solenoids of the ATC are actuated with a signal from the Robot Control board, which sets off a timer signal in the Solenoid Control to activate the solenoid for a (short) period of time—in the case of the prototype unit—2 seconds. During this time the robot is expected to approach and mate with the tool. The Solenoid Control board has three timer circuits. At this time they are simple transistor-driven pulse generator (RC) circuits, slightly modified from the circuit shown in “The Art of Electronics” (aka H&H) 3rd Ed. p78.

Interfaces
Mechanical
The ATC attaches to the three 8mm Z-axis shafts of the Visible Robot. The shafts are at a 33mm radial distance from a circle that intersects their centers. The center-point of this circle is also on the axis of the Z-axis leadscrew. The ATC is fastened to the shafts using 3mm setscrews.

Regarding the tool-coupling panel, the three cammed teeth of a port are arranged radially at 15mm from the center of a circle which is orthogonal to the axis of the solenoid plunger. This distance is to the axis of the “bearing” feature of the cammed tooth part.

The three cammed tooth openings are at 60 deg., 180 deg., and 300 deg. clockwise from the vertical.

The holes designed to receive the three registering posts of the tool-side coupler are at 18mm  from the center of the same circle. Including both coupler panels, they are 9mm deep. They are arranged at 0 deg., 120 deg., and 240 deg. clockwise from the vertical.

The two USB shelves are located 20.5mm above and plus and minus 14.35mm to the right and left of the center-point. (The center of the triangles forming the holes for the cammed teeth and three registration holes).

Electrical
As mentioned above, both power and signaling is provided through USB connectors. A more detailed—though not thorough—description is provided in the Tool Controller document.

Program/protocol
The solenoids receive the signal to “Release” from the above-mentioned Solenoid Control. It, in turn receives its signal from D33, D35 and D37 pins of the Arduino Mega. These pins may be different on different controller boards.

  • Critical needs
    Key components will need to be made with materials and processes that can guarantee solid functionality of the device.
  • A database schema generic to tools and their capabilities and needs should be found, transformed from another, or developed.
  • A protocol, generic across a broad swath of tools will need to be found, mutated from another, or (least favored) created. A start might be to look at SECS/GEM, used for machine communication in the semiconductor industry.
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