Thumb Design Submitted to 3D Printer

After the successful printing of the two fingered gripper the thumb was tweaked based on observations made from the printed gripper.  The thumb is included in the previous post, this post will describe the design features of the thumb.  Refer to previous post for positioning and integration with the rest of the gripper and hand.

Figure P7.1: Rear View of Thumb

The thumb is supported by a rib on each side that extended from the tip of the thumb to its base.  These ribs are angled to better transfer the force that will act on the thumb when an object is being grasped to the base of the thumb, the area of the thumb that is secured to the frame of the hand.  The ribs are stiffened at the top of the finger with a horizontal bar that connects them. There is a triangular pattern of three holes that will serve as mounting holes for the thumb to its mounting plate, which is shown and described in the previous post.

Figure P7.2: Side View of Thumb

Figure P7.3: Curvature of the Thumb and Two Fingered Gripper

The tip of the thumb extends further than the base of the thumb.  This is designed to help keep objects in the gripper; the protrusion of the tip of the thumb acts as a stop to prevent objects from pushing out of the gripper when it closes.  The two fingered gripper has the same design, the two protrusions acting together allow there to be less of a gap around larger objects as the tips of both sides of the gripper are closer than their bases.

Wrist Design

The wrist of the robotic arm has two degrees of freedom and the movement of the gripper, utilizing a total of three servos.  The wrist allows the hand to swing horizontally as well as rotate along the vertical access.  The vertical rotation servo is the first servo in the sequence so that the entire remainder of the wrist, and subsequently the hand, can be rotated allowing for the swing of the gripper to be horizontal, vertical, or anywhere in between.  For ease the swing of the gripper is referred to as horizontal rotation, the direction of swing when the vertical rotation servo is in its default position.

Figure P6.1: Annotated Top View of Gripper

This image displays the set up of the wrist.  Each servo is mounted to a standard servo bracket (S.S.B.).  The gripper servo's S.S.B. is connected to the horizontal rotation servo by a "C" bracket, with a ball bearing at the lower connection point with the S.S.B. of that sevo.  the vertical rotation servo is directly connected to the S.S.B. of the horizontal rotation motor.  The vertical rotation bracket will be connected to the forearm of the robot.

Figure P6.2: Alternative View of Wrist

This image shows how the thumb is mounted to its mounting plate, which fits between the S.S.B. and the "C" bracket that connect the horizontal rotation servo to the gripper servo.  The thumb pertrudes over the servo so as to be closer to the pivot (servo disk) of the two fingered gripper, allowing for tighter closing of the hand.

Successful 3D Printing

Our two finger gripper was printed late last week and works without any complications, fitting all of our screw holes, pivot points, etc.  The renderings of the two fingered gripper posted on May 6, 2014 do not include all of the features that are described below.  These additional features are the cut-out for servo mount hardware/mounting flange and the depressions for mounting hardware, both are described below.  Dimensions were also altered.

Figure P5.1: Two Finger Mobile Gripper Mounted on Servo

Figure P5.2: Annotated Image of Printed Gripper

The annotated features are described below

The top of this bracket will be fastened directly to the servo disk, but the bottom will be mounted to a ball bearing secured to the standard servo bracket.

A single rib along each finger adds strength allowing the fingers to be thinner (3mm) and reduce weight.

The 1mm depressions around the mounting holes allow for the use of short screws that was provided with the servos, keep waste and cost down.  There appears to be no loss of strength due to these depressions.

The tips of the fingers are rounded, this is intended to reduce the snagging of the edges on objects in the gripper and also the third finger.

Figure P5.2: Annotated Image of Printed Gripper and Servo

This image displays the considerations that allow for the gripper to close fully around the servo's mounting flange.  Not pictures in this photo is he bolt or push rivet head that would project above the top of the upper surface of the mounting flange.  This consideration was made, rather than ending the figures further away from the servo, to reduce the gap between the servo and the gripper when it is closed.  In this design the fingers are ended so that they just clear the corner of the servo.  The "C" brackets that this gripper was based off of ended where the vertical bar is in the gripper, far away from the gripper.  This left a large gap for objects to pass through, greatly reducing the effectiveness of the gripper.

Additional Materials Order

This order consisted of the components that will make up the span of the arm and the connection between it and the servo motors.

Figure P4.1: Breakdown of cost for second material order

* During  our observation and brief tests the aluminum tubing displayed excessive strength for the current lifting capacity of the arm.  

This would allow for smaller, lighter, and cheaper tubing to be used in the production of future versions.

It is expected that only one more materials order will be needed.

This will consist of nuts, bolts, and other necessary hardware.

The tubing can be found here.

The "L" brackets can be found here.

Gripper Design

This week our group has begun creating three-dimensional models of our robotic arm's gripper.  The gripper is being designed to have a three finger design where the two outer fingers are the moving fingers connected to a servo while the third finger, or thumb, is stationary and fits between the other two fingers when they are completely closed.  At this time we have not modeled the stationary thumb, so the renderings included in this post are only of the two fingered portion.

 Figure P3.1: Honeybee Robotic's Universal Gripper Anchor

Although its appearance is much different, the three finger design is similar to our current plan.

The third finger, which is stationary in our design, is the middle finger of the three that the other two will close around, similarly to the gripper above.

Click the picture to view Parabolic Arc's article on this project from Honeybee Robotics

Figure P3.2: Two Finger Gripper Rendering, Version 1

This is our current design for the moving portion of the gripper that will clamp objects between it and the stationary 'thumb' that fits between the two 'fingers' of this piece.  This design grew of a standard "C" servo bracket (pictured below).  The ribs added to the back of the two fingers will add strength and rigidity to the fingers as the clamp down on an object.  This part will be printed on Drexel University Engineering Laboratory's 3D printer, and has been designed with a 3mm thickness for greater strength, where an aluminum version of the same gripper could be thinner.

Figure P3.3: Lynxmotion Aluminum "C" Servo Bracket

These are the brackets that will be used in the joints of the robotic arm and also served as a foundation for our gripper design.

Click the picture to see this bracket at RobotShop.com

Figure P3.4: Two Finger Gripper Version 1 Mounted on Servo

This image shows how the two finger gripper will attach to the servo.  The gripper itself is highlighted in blue while the servo has a slightly darker shade of gray than the standard servo bracket to which it is mounted.  As shown in this image, the gripper's upper mount will be directly to the servo, and this will be how the gripper gains its motion.  The lower gripper mount will be a ball bearing that is connected to the standard servo bracket.  The ball bearings used will be the same as those that some with the bracket shown in the previous image.

All parts that have been purchased and mentioned in this posting can be found by following the links included in the Materials Order post.