Three Pin Kinematic Coupling Design

Design Overview

After the initial meeting with ABB, Prof. Slocum suggested a new coupling method that would allow the Wonder Wyler unit to connect easily to the flange. The initial design, shown in the original image initial_sketch.jpg, consisted of two pins that would rest against the inner recess of the flange, a pin that would rest inside the control pin hole, and a spring plunger that would provide a locking preload force. The design for the Wonder Wyler proceeded along these lines until restrictions on flange design were removed by ABB. However, development of this design was continued as the new coupling method could still be used as an alternate design. Furthermore, it appeared that the three pin coupling could be successfully applied on ABB - MIT projects involving the IRB 6400 wrist and base as well as elsewhere in industry.

Several different conceptual sketches and CAD models were created to visualize the concept to ABB. At the same time, a detailed mathematical analysis was under development in the form of MathCAD and Excel documents. These documents solve the force equilibrium equations and determines the contact stress for a set of coupling parameters. The finalized version of the analysis were combined with instructions and images in an Excel spreadsheet available below.

block_n_flange_bw.JPG	Early CAD design concept
pin_n_flange_bw.JPG	Early CAD design concept
spring_plunger_concept.ppt	PowerPoint file describing the basic operation of the early conceptualization of the three point coupling. This idea was to use the three pins with a spring plunger providing the force. However, preliminary designs using the plunger required a large block on the interface plate to hold it, so this idea was used only as a development point.
fbd_spring.jpg	Free body diagram that accompanies the Excel spreadsheet.
master_spring_solver.xls	Final version of Excel spreadsheet for determining coupling and geometry parameters.

To assist the conceptualization and verify the usefulness of the coupling, a wooden model of the three pin coupling was created at the MIT Hobby Shop by Patrick Willoughby. Pictures of this model are shown below. The simple design involves several crude posts pressed into a block of wood as an interface plate and two holes drilled into a separate piece as the robot flange. The wooden flange also has a threaded hole to hold the spring plunger. When placed on the posts, the coupling works extremely well with loose tolerances during manufacturing.

spring_plunger.jpg	Spring plunger capable of providing 8lbs of force. Similar plungers can be obtained from McMaster-Carr.
wooden_prototype_1.jpg	Picture shows the components of the wooden three pin coupling.
wooden_flang.jpg	Picture shows the equivalent of the robot flange for the wooden prototype. Note the lines drawn on the lower left side of the part. These lines show where the spring plunger is threaded into the wood. The small indentation on the larger circle is an additional location for a spring plunger to provide an outwards force from the circle's center.
wooden_plate.jpg	This part is the equivalent of the interface plate. The humorous looking posts are brass rod which were fatigued to taper the ends. The fourth post in the center of the block allows for additional spring plungers to be used.
wooden_coupling_1.jpg	This picture shows the engaged wooden coupling.
wooden_coupling_2.jpg	This picture shows the engaged wooden coupling from above.

As the design proceeded, it became apparent that the spring plunger design would occupy too much space on the interface plate. To reduce the size of the design, the spring plunger was removed from the design and replaced by the fourth "spring" pin. This pin would also rest inside the control pin hole on the flange and would provide the preload force by bending the pin. Using basic beam theory, the pin deflection could be translated into the preload force, which was added to the Excel spreadsheet. Using sample dimensions, a basic prototype design was created. This prototype is shown below. The length of each pin for the prototype was increased by 6mm to facilitate testing. Proposed testing involves placing the interface plate pins into the flange, but having the interface plate rest on 6mm diameter balls rather than directly to the flange. By placing this space between the two surfaces, the coupling could be loaded laterally to test the coupling stiffness and verify the spreadsheet. When manufacturing the plate, dimensions were determined without considering the bending stress in the pin. After the coupling was attempted, it was apparent that the design was not suitable as the spring pin bent. Suitable drawings and information are included here to allow ABB to manufacture a working version of the prototype. The deformation of the pin will be considered by MIT, as it may become as useful version of the design. Similar to a quasi-kinematic coupling, the deformation would create a unique repeatable coupling to the specific flange. However, the extreme force required to couple the prototype would not be acceptable.

pin.jpg	Initial concept model of the spring pin by Prof. Slocum. Although it is not visible in the drawing, the small tab at the top of the pin has a radius along the pin's length. This radius decreases the contact stress at the interface.
	This PowerPoint file, three_pin_coupling_operation.ppt, explains the three pin coupling as used on the current prototype. This coupling consists of three pins for location and a fourth "spring" pin which provides the preload force.
three_pin_unit_on_flange.jpg	This picture shows the prototype of the three pin coupling engaged with the flange.
three_pin_unit_on_flange_side.jpg	Side view of engaged three pin coupling.
flange_plate.jpg	Shows flange plate with 6 mm balls to keep a small space between the two pieces without restricting motion in the plane of the flange face.
three_pin_plate_1.jpg	Basic dimensions for three pin coupling plate. Pins were purchased from McMaster with a diameter of 2 mm for the spring pin and 5 mm for all other pins. Due to the length of these pins, a small amount of the larger pins protrudes from the reverse side of the coupling plate. This would not be present in a final design. The prototype design required a significant force to create the coupling, which caused the spring pin to become bent.

Drawings and CAD Files

	Solid Works 2000 CAD or IGES file of interface plate
	Tolerance analysis of interface plate using Solid Works 2000
	Drawings of interface plate in Solid Works 2000 and pdf

Description of Special Parts

The special parts required for this prototype are hardened steel dowel pins, which are available from McMaster-Carr. Two sizes were ordered: 2mm and 5mm diameters. The 2mm diameter pins are part number 91595A203 and are US$16.71 for a package of 100 while the 5mm diameter pins are part number 91595A424 and are US$35 for a package of 100 pins. The McMaster catalog page is here: mcmaster_pins.pdf.

Manufacturing of Prototype

Manufacturing the prototypes was rather simple. The following simplified steps were taken:

Material cut to size
Material fixtured in milling machine and all six surfaces are milled to meet dimensions and smoothed
Material is refixtured and holes are located with center drill
Holes are drilled through slightly undersized
Holes are reamed
Part is deburred and polished
Pins are pressed into corresponding holes using arbor press

New Design Files

The following files detail the new design of the three pin interface plate to be manufactured at ABB for demonstration and testing.

	Solid Works 2000 CAD or IGES file of interface plate
	Drawings of interface plate in Solid Works 2000 and pdf

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