VII

 
GRIPPER

 

 

7.1 RESEARCH AND CONCEPTS

The previous Gateway teams invested much time into gripper design and some very interesting ideas were developed.  Two designs were mainly discussed: the 1999-2000 design, and Chris Fearon’s graduate design.  Other previous designs were quickly determined not to meet the goals of this year’s team.

Chris Fearon designed a gripper that was very efficient at picking up large objects (1.5 to 3 inches) but had some trouble with small objects.  He used three fingers (as opposed to the previous Gateway teams two-finger wrench designs), which allowed for


more stability and a greater variety of objects that could be handled.  Much of the work went into the analysis of the fingers and the loads that they could handle.  Chris did extensive research and testing using ANSYS to pick the best finger design that would limit finger deflection and stress points.

 


Aluminum was chosen for the major components of the fingers and moving pieces of the palm.  The motor was also locked into place using aluminum plates.  The side cover plates were made from Acrylic simply for aesthetics.  These two materials offered high strength to weight ratios and were fairly low in cost.  Actuation was handled through the use of a stepper motor/leadscrew system.  The NEMA size 17 stepper motor from Applied Motion and a lead screw and power nut assembly from Precision Industrial Components Corp. were the products purchased.  The thrust bearing was used to eliminate the thrust force that would be placed on the motor due to heavier objects.  The high torque size 17 stepper motor provided 31.4 in-oz of torque at 300 rpm, which is more than enough to overcome the joint friction and operate the gripper.  The overall cost of material and machining for Chris’ gripper was about $620 for a prototype and about $430 for production of 50 or more assemblies.  A solid edge model of Chris Fearon’s gripper is shown in Figure 7.2.

 


 


           

 

 

 

 

 

 

 

The 1999-2000 Gateway team took a different approach to the design.  They took Chris Fearon’s design to the next level by implementing underactuated fingers.  A mechanism is underactuated if it has fewer actuators than degrees of freedom.  This type of design allows the fingers to wrap around an object as it closes and to pick up a wide variety of objects (small and large).  The group then decided to use a six bar, underactuated, two degree of freedom linkage.  This design took much kinematic research and testing.  An example of a six bar linkage closing around an object is shown in Figure 7.3.


This design could pick up cylindrical objects ranging in size from 1 to 4 inches.  Forces were studied and parts were designed to reduce stress concentration and machining costs.  This meant rounding edges to create more “dumbbell” shaped parts.


Most of the components were constructed of aluminum, but the side plates and finger pieces were made of Lexan.  The gripper was again actuated by a stepper motor/leadscrew system.  The Ht17-070 stepper motor from Applied Motion Products, and a ¼-inch leadscrew from PIC design were the products purchased.  A ¼-inch aluminum plate was moved by the leadscrew and slid in slots cut into the sideplates.  This motion is what closed the fingers and actuated the six bar linkage. 

 


            This turned out to be a very cost effective design as well.  One of the main reasons that Chris Fearon did not invest much research into the use of an underactuated system is that he thought the cost would be too high.  After all of the kinematic analysis and design was finished, the actual machining and material cost was only $547.  This was actually a lower cost than Chris Fearon’s prototype gripper.   A production cost was not calculated for the 1999-2000 gripper but would have been even lower due to the reduced cost of machining.

 

 

7.2 DESIGN AND ACTUATION

This year’s design team used most of the design from the 1999-2000 team but made several modifications to increase durability and ease of control.  The first design change was that we eliminated the use of Lexan.  We believed that this material was used more for visual purposes than for the mechanical benefits.  In examining the old gripper, cracks had formed around the areas of high stress caused by the screws.  The Lexan is also much more flexible, which allowed the side plates to bend and twist fairly easily.  Aluminum was chosen to replace these parts.  Aluminum made the frame much more stiff and durable without adding very much weight.  The cost of aluminum is also quite low, and it is easier to machine than Lexan. 

Another design change came with the use of a cordless screwdriver motor.  The motor chosen was a 2.4 VDC Johnson Electric and was found in a Black and Decker Model #9072 screwdriver. The screwdriver itself came with a gearbox that was also used in the new design.  The planetary gearing used provided an output of 20 in-lbs of torque at 150 rpm.  This was very appropriate for our application because it allowed for the gripper to move its full range of motion in about 4.8 seconds.  There was also a cost benefit associated with the use of this product over the old stepper motor.  The Ht17-070 cost $50, but the entire cordless screwdriver only cost $15.

 

Figure 7.5 – Comparison of old/new motors

 


This cost savings was offset by the addition of new components, but the main reason for choosing this motor is for consistency and ease of control.  One issue that we wanted to solve this year was that we wanted to use a consistent type of motor.  The stepper motors were very difficult to program using the OOPic controller.  A stepper motor offers the ability to know the exact position of the motor at all times.  However, we do not need to know the exact position of the motor for this application.  The opening and closing of the gripper can be controlled visually with sufficient accuracy.  The screwdriver motor has the advantage of not being back drivable.  This eliminates the need for any braking and allows the gripper to maintain a tight grasp on an object without drawing any current.   

Several components were re-designed and several more were modified to complete our final product.  One drawback of the new design is that it is about 2.7 inches longer and slightly heavier than the previous year’s gripper.  This is a tradeoff that we were willing to accept to achieve our previously discussed motor control goals.  The length of the motor and gearbox was what caused the major design changes.  This required lengthening the side plates to house the new equipment.  Since these plates were now longer, they needed to be even more rigid (another reason for choosing aluminum over Lexan).  Also, the old plate that housed the motor had to be modified and moved in order to support the new motor/gearbox assembly.  This support plate fits around the Johnson Electric motor and rests against the bottom of the plastic housing of the gearbox.  This plate was then fixed into place using epoxy.  Fastening using epoxy was not the desired way to attach the motor to the plate, but was used due to time constraints at the end of the project.  The original idea was to weld the support plate to the motor housing (A gas tungsten arc-weld, or “Tig” weld, would have been used because of the thin housing of the motor and the joining of aluminum parts).

The motor/gearbox assembly was not modified as it came from the screwdriver.  The only modification was that the fixture on the end of the gearbox output shaft was reduced in length by ½ of an inch.  Originally, a screwdriver was disected to see what parts could be eliminated and how the output of the gearbox could be fastened to the leadscrew.  These screwdrivers were difficult to disassemble without damaging the components.  Therefore the decision was made to make use of the hexagonal shaped slot that was already part of the screwdriver.  A special coupling was machined with a hexagonal insert on one side and an identical match to the leadscrew on the other end.  This allowed for the use of a coupler to join the two pieces. 

            Because the 1999-2000 gripper design was used as our baseline, the components discussed above were the only parts that were fabricated from scratch.  The remaining parts of the gripper were re-assembled and re-used, with the exception of the finger pieces.  These pieces were also re-machined from aluminum instead of Lexan for consistency of appearance.  The mounting and dynamics of the gripper are identical to those of last year’s design.  The gripper is mounted to the wrist differential by fastening the bottom plate by using six screws (the differential plate can be seen above in Figure 7.6).  This plate also serves as protection for the motor itself.  Because of interference between the gripper and the forearm housing, a spacer had to be added to create a slight separation.

            The gripper is again actuated through the use of a leadscrew that moves a plate that slides in slots milled into the side plates.  The motion of this plate is what begins the movement of the six-bar underactuated linkage.  The upward motion of the plate forces the fingers of the gripper to close until they make contact with an object.  After contact has been made, the joints in the fingers will bend.  The bending of the joints is what allows for the fingers to “wrap around” an object to create a firm hold.  This idea is shown in Figure 7.3 but can also be seen in the closing sequence of the assembled gripper that is shown in Figure 7.7. 


 


 

 

The final prototype cost of the gripper was determined to be $524.26 and $414.88 for a production volume of 50+.  Notice that the cost of the gripper is very similar to that of last year’s design ($547.05 for 99-00 design prototype).  However, the lack of cost savings is made up by the ease of control that was created by using the screwdriver motor.  The difference in price for a production volume of 50 or more assemblies is mostly due to the reduced cost of machining larger lots of parts.  This was estimated in the calculations by reducing the machining time by 25% (from 17.5 hours to 13.125 hours).  The cost of the gripper components, material, and machining is summarized in Appendix A (Bill of Materials). 

A disadvantage of this year’s gripper design is the increase in length (from 4.60-inch side plate length to 7.25-inch side plate length).  However, the arm design allowed for a maximum gripper length of 12 inches, which we are still well below.  The final Solid Edge gripper assembly is shown in Figure 7.8.

 

 

Figure 7.8 – Final Solid Edge Gripper Assembly



 

Table of Contents

Session 6: Forearm

Session 8: Motor Testing