Soft Actuation Reacher Grabber

The concept of our project is to use soft robotics actuators without automation or electronics to create a product with advantages over traditional alternatives .In our case, the PneuNets Bending Actuators allow for a greater range of motion, and thus greater gripping ability than a traditional mechanical reacher grabber, like those used by the elderly and disabled. 

In the design section, we go over our concepts, prototypes, final design, and any problems with it, and potential improvements we could make. In fabrication, we show the bill of materials, the assembly process, and building our version. finally in testing, we talk about the testing of the design, the data we gained, and how it performs compared to traditional alternatives and earlier prototypes.

We are a three member team of seniors from Walt Whitman High School in Bethesda MD, right outside of Washington DC. We started out with two people, Andrew Gates and Nathan Zimmerberg, who've been friends for years, working together both in and out of our engineering courses, which we've taken every year since 7th grade. Nathan is in charge of building, prototyping, and CAD, and Andrew is helping with designing, researching, and documentation. We now have a Third member, Tycho Gormley, who we have both known and been fairly close with for many years, and is helping with anything needed, but with a focus on prototyping, redesigning, and building.

Background

  • Origins of topic choice:

    • We had to do a project for our engineering capstone course

    • We were very late in choosing a project, somewhat limiting our options

    • But we still wanted to do something unique

    • Nathan found out about soft robotics and we decided to use it for our project

    • However, the hardware was very expensive, so we came up with cost-effective way to use it

Problem Statement

Many people can’t reach objects that have fallen on the floor or are in hard to reach places. Current products cannot effectively grab many types of objects, in particular, small or oddly shaped ones, or delicate objects which they can easily break.

 Design Criteria/Objectives

We will design, prototype, test, and refine a device to allow a person to grab a variety of objects out of their reach.

  1. Able to grab any object on ground with imprecise aim.   

  2. Safe and comfortable to operate    

  3. Able to not require excessive hand gripping power

  4. Must be durable (will undergo stress tests)

  5. Actuation should use soft robotics

  6. Must be easily storable  

Design

After we identified the problem, we decide on constraints. We decided to not use any electrical components because of cost and complexity.

We came up with many different concepts and eventually narrowed them down to this general design.Early Concepts

Then the grabber reacher was divided into three sub components: the arm, the handle, and the gripper. 

The gripper went through an iterative design. Initially a lot of effort was spend fixing the schools 3d printer. 

 

Arm and Handle Design

The structure of the Grabber Reacher is made from PVC pipe. We choose 1/2 inch PVC for the arm after simulating many different options of extruded materials for the arm under a 2lb (8.9N) load.

We simulated 3/4" PVC, 1" PVC, Extruded Aluminum, and 1/2" PVC Pipe.  The 1/2"PVC was strong enough and much lighter than the 3/4" PVC and 1" PVC though very flexible. The extruded aluminum was much less flexible but would have been harder to machine, and harder to assemble with the PVC handle.  

The bulb pump on the handle has two check valves on it to pump air from the outside into the soft robotics gripper.An additional magnet was added to the gripper to allow it to pick up metal items. The air bulb in the handle has only one moving part. This makes it simpler to manufacture than a mechanical trigger linked to a piston such as a bike inflater. In addition, the air bulb with check valves requires multiple squeezes to grab something so the gripper can have a large volume difference and relatively high force. 

There is a hole in the tube connecting the bulb to the gripper to let air out to release the gripped object. The hole can be sealed with a thumb to keep grabbing something. This is much lighter and simpler than a push button release. 

The 1/4" inner diameter tubing was chosen because it could fit on the air bulb and was relatively inexpensive. The tube could only be ordered in bulk so we had extra to use even if the inner diameter was larger than necessary to transfer the air.

Gripper Design

First, we used an Instructable by Ben Finio for a PneuNet gripper. We made a mold made of ecoflex 30 (combo of components A and B) with an air cavity inside it. The mold itself had the shape of a square cross with semi-circular tips. We made 3-5 of these molds but eventually, all failed or malfunctioned because of one or more of the following reasons: Only one of the grabber’s arms inflated, air bubbles formed in the mold, and/or the appearance of rips and tears on the edges and corners of the mold. Eventually, we decided to take on a new design from Harrison_89 from Instructables for a Fiber Reinforced design. We opted to make a grabber with only 3 arms but slightly larger and stronger. Previously, we made the mold cast using a 3D printer but this time, we made it out of cardboard. Then, we made a second, thinner mold which would serve as the base of the grabber. Once we attached the two molds and covered it with curling ribbon, it worked but not as effectively as we would have liked. In addition, a single leak appeared which complicated things. Eventually, we used our final batches of ecoflex and with all our care and passion, created the perfect 4-arm gripper which worked repeatedly. Although all the arms still did not inflate at the same time, the gripper was successful in lifting objects less than 3/2 pounds. The addition of sticky tape and magnets further amplified our successes.

We also needed to find a way to attach the gripper to the PVC pipe arm. Most glues and tapes do not stick to ecoflex, so we needed a mechanical fastening. We decided to strap down the gripper with tape attached to the PVC arm, and this worked reasonable well and gave us a mounting point for the magnets.

However, we also wanted some more support laterally, so we wanted to attach an ecoflex plug to the end of the gripper that would snugly fit inside the PVC arm.  This also helped hold the barbed connector to supply air to the gripper.

We found that we could push the barbed connectors though a pinhole in the ecoflex to make a secure barbed connection to ecoflex with our 1/4 inch barbs. This would cause the ecoflex to expand.

To design our connecting plugs, first we assumed that ecoflex was incomprehensible and kept constant volume. Then we took the total volume of the plug and subtracted out the volume of the barb. Finally, we used this volume to find the radius of a cylinder with that volume and the correct height of the plug. We 3d printed a mold for this and it worked out well.

Strengths and Weaknesses, and Improvements

In conclusion, we would call our project a general success since we accomplished our initial goal while discovering some new methods along the way. Although we did endure quite a few setbacks and made some changes to the original design, we were successful. Some of the improvements we added to our original design were: sticky tape (originally intended to secure the arms but was also good at picking up light objects), magnets underneath the tape (for picking up metallic objects) and moving them from the edges to the middle, and using paper towels instead of regular paper for the non-stretchable layer of the gripper (much better for absorbing the ecoflex). We also found out that leftover ecoflex made for very useful can-openers. If a hole is poked into the gripper, then you can use barbed connectors to hold it more securely than just using the tube. Our gripper was generally better than the store-bought one because it could pick up metallic objects with ease.

One Improvement would be durability of the gripper. While the rest of the grabber reacher was durable, the final pneunet gripper we used only lasted a month before popping. Using another gripper design such as a fiber reinforced gripper could help this problem as well as finding a more durable material.

Fabrication

The fabrication is in 4 major steps.

1. Build the arm component

2. Build the handle component

3. Build the gripper

4. Assemble the whole grabber

The Materials are at the bottom of the page

bill_of_materials.xlsx8 KB

Arm

  1. Cut ½ inch PVC pipe to 3 ft, deburring both ends.

  2. Cut the tubing to 3 ft 3in.

  3. Connect barbed connection to the end of the tube that will go in the gripper.

Handle

Handle Building instructions

  1. Cut 3/4"PVC pipe to 5 in. long, sand flat both ends

  2. Cut 6in of tubing

  3. Cut the PVC first according to this drawing.    
  4. Then finish cutting bulb pattern out of the 5 in. PVC with the bulb and PVC elbow temprarally  attatched      
  5. Mark where your thumb will hit the 90 degree PVC elbow when the bulb is in place.

  6.  Drill holes .375” in diameter straight in using the drill press so the tube can go out and back into the elbow and go over the thumb press location.

  7. Angle the holes for the tube by clamping the elbow and using the hand drill to change the hole angles to match the tube angle.

  8. Push in the PVC and drill the hole in the PVC where the hole in the elbow is, making sure the PVC is rotated the way it should be.

  9. Connect the bulb to the 6” tube so that the air is pushed into the tube.

  10. Put the bulb in it’s cut out and push the tube through the two holes, widen or angle the holes if needed.

  11. Use a screw or pointy object to poke a hole in one side of the tube where it goes over the thumb pressing space, the thumb will hold in the air in the end.

  12. Super glue and hot glue the bulb into the PVC
  13. Cut the grip tape to a shape that will cover the PVC about 2”x5”

  14. Glue the grip tape on the back of the PVC

Gripper

  1. 3d print the connecter mold piece found at the bottom of this page

  2. Follow soft robotics toolkit instructions

If not using a 3d printer use these instructions http://www.instructables.com/id/Soft-Robotic-Grabber-No-3D-Printer-Required/
 

From soft robotics toolkit

http://softroboticstoolkit.com/book/pneunets-fabrication

http://www.instructables.com/id/Air-Powered-Soft-Robotic-Gripper/

                  


 

When following these instructions it may take several tries to make a functioning gripper.

Here are our suggestions and modification to the above instructions.

Make sure the gripper is as symmetric as possible so all fingers will bend together; otherwise, one will expand first.

Cut out a piece of paper towel in the shape of the gripper and place it on the flat layer, instead of the printer paper. The paper towel absorbs and sticks to the ecoflex better, and is stronger.

                                            

 

Do not puncture the gripper yet.

Fill the connector mold with about 1/2 an inch of ecoflex to create a barb connecting cylinder. This connector is designed to expand to the size of the inside of the 1/2"PVC pipe when the 1/4" barb is in it.

Add this cylinder to the top center of the gripper using ecoflex.

                 

Use the pin to poke a hole into the gripper through the center of the cylinder but not all the way out the other side of the gripper.

                   

Use a barbed connector to connect the tube through the hole.


Test it.

Final Assembly

  1.  Follow Handle instructions to build handle
  2. Follow Arm instructions to build arm
  3. Follow Gripper instructions to assemble gripper
  4. Test gripper.
  5. Connect the arm tube to the handle tube.
  6.  Connect the arm to the handle.
  7.  Shorten the arm tube if it is too long
  8.  Connect the gripper to the arm tube, the connecting cylinder will hold it a little.
  9.  Use the double sided tape to attach the gripper to the arm.(glue and tape don’t stick to the ecoflex)                                                                      
  10.  Add magnets to the center of the gripper on the tape.
  11.  Test full grabber reacher
  12.  Make Revisions

Testing

Movie of grabbing items

We used our final batches of ecoflex and with all our care and passion, created the perfect 4-arm PneuNet gripper using paper towels which worked repeatedly. Although all the arms still did not inflate at the same time, the gripper was successful in lifting objects less than 3/2 pounds. The addition of sticky tape and magnets further amplified our successes.

We tested the different objects the gripper could grab, and the effect of loads on the arm.

Testing the Grabber

 In order to test our grabber, we gathered together a collection of common workshop items that can be found in any toolshed. We measured each item's weight in ounces using an electric scale. Next, we stated if the item was magnetic (and thereby influenced by the magnets attached to our grabber). We then proceeded to test if the grabber can successfully grasp the object and if the item could be held. We also used sticky tape to pin down the magnets but we soon noticed that the sticky tape could pick up light objects that were neither too big or too small for the grabber to grasp. 

Item to be picked up

weight

Magnetic?

Can it be grasped?

Grasped by sticky tape?

Can item be held at all?

 

     

Jar of wood stain

18 oz

yes

yes

no

yes

Jar of wood filler

6.625 oz

yes

yes

no

yes

3 pound weight

48 oz

yes

no

no

no

screwdriver

4.125 oz

yes

no

no

yes

Cup full of leftover ecoflex

4.25 oz

no

yes

no

yes

Can of spray paint

14.875 oz

yes

yes

no

yes

Glue bottle

1 oz

no

yes

yes

yes

marker

0.75 oz

no

yes

yes

yes

Gorilla wood glue bottle

19 oz

no

no

no

no

Bottle of glass cleaner

30 oz

no

no

no

no

Sheet of paper

0.125 oz

no

Yes (from side of table)

yes

yes

Can of coke

13.875 oz

no

yes

no

yes

Bundle of leaves

0.125 oz

no

yes

yes

yes

Wallet

3 oz

no

yes

no

yes

Additional Facts and Data

-The bulb must be squeezed at least 8-9 times in order for the grabber to grasp a normal sized object (size comparable to the grabber itself)

-Takes anywhere from 10-20 lbs of force to push down on the air bulb (pressing down 3/2 inches or 0.0381 meters) which is equivalent to approx 66.7 newtons of force.

-Takes 23 joules of work to grab something

-According to the Topend sports website, the poorest grip strength (26.7 lbs) in both males and females ages 10+ should still be more than enough to successfully squeeze the air bulb. 

-However, some items were shaped in such a way that they couldn't be grasped despite being very light. 

http://www.topendsports.com/testing/tests/handgrip.htm

http://www.topendsports.com/testing/norms/handgrip.htm

 

Grabber and Comparisons

In addition to testing the grabber's abilities, we also tested its durability and rigidity compared to conventional, store-bought grabbers. In order to make our grabber lighter and easier to handle, we used a tube of pvc piping. The conventional grabber used a tube of aluminum for its structure. We decided to test how much both tubes bend while holding on to a weight. As expected and as shown on the graph, our pvc pipe underwent much more displacement than the aluminum pipe. However, just this statistic does not necessarily make the aluminum pipe the better choice. The pvc pipe is able to undergo much more bending before the point of breaking; the aluminum one would most likely break if it underwent a similar amount of displacement that was experienced by the pvc.