Grasping with Straws: Make a Robot Hand Using Drinking Straws
AbstractImagine how cool it would be to build a robot hand that could grasp a ball or pick up a toy. In this robotics engineering project, you will learn how to use drinking straws, sewing thread, and a little glue to make a remarkably lifelike and useful robot hand. What will you design your robot hand to do? Pick up a can? Move around a ping pong ball? It is up to you! With these starting instructions, you can design any type of hand. You will simulate human finger anatomy as the basis for a fully functional robot hand that is easy to build and does not require complicated tools.
Howard Eglowstein, Science Buddies
To design and build a working hand out of craft materials that demonstrates how a real robot hand might work.
Your hands are amazing. With them you can play the violin, cook a meal with your parents, or design a robot. When people design robots, one of the parts they spend the most time on is the hand, or whatever other "grabber" the robot might have. Robots do useful and important work, and unless they can grab and move things, they cannot get the job done.
In this project you will study the physiology of the hand to see how human fingers work and design a robot hand that uses human-like fingers to perform a task. Best of all, you are going to make your robot hand from everyday drinking straws. By simply applying some creativity and your engineering know-how, you can make a useful robot hand.
The straw-hand design shown in this project mimics the way that tendons bend your fingers even though the muscles that control the action are actually in your forearm. To make your finger bend, a muscle pulls on a long string of collagen called a tendon. The tendon connects the muscle to the bone in your finger. It is kind of like remote control: When the muscle pulls, the finger bends at the joint. You will use a drinking straw as both bone and skin, sewing thread as a tendon, and you will provide the muscle. When you have finished this engineering project, you will have made a working robot hand, with multiple fingers that you control just by pulling threads. (See Figure 10 at the end of the Procedure for examples.)
Terms and Concepts
- Physiology of the hand
- How does a human finger bend?
- How are fingers arranged and connected in a human hand?
- Give examples of tools that are used for picking up and/or grabbing things. How many finger-like projections do they have?
- Give examples of different animal hands and/or feet. What do those animals do with their hands and feet?
- Based on the tool and animal examples, how might you design a hand for grabbing a ball? How about for holding a fork?
- Ross, V. (n.d.) Six innovative robot hands. Popular Mechanics. Retrieved May 17, 2012.
- Wikipedia contributors. (2012, May 10). Hand. Wikipedia, the Free Encyclopedia. Retrieved May 17, 2012.
- Smith, Miranda. Human Body: The Animated 3-D Guide. Charlotte, N.C.: Silver Dolphin Books, 2012. Available online through Amazon.com.
- Muscolino, Joseph E. Musculoskeletal Anatomy Coloring Book. Philadelphia: Elsevier Health Science, 2009. Available online through Amazon.com.
Materials and Equipment
The choice of materials for this robotics project can vary depending on what type of hand you choose to build. Look at the list below and read through the Procedure before purchasing your materials.
- Disposable drinking straws (20). Look for 7 to 8 millimeters (mm) in diameter straws, sometimes called milkshake straws, that pop back into shape when you squeeze them. Straws that crack when squeezed will not work.
- Upholstery thread, heavy duty or dental floss
- Sewing needles (2). The eye of the needle should be as small as possible while still accommodating the upholstery thread/dental floss. Longer needles work better.
- Rubber washers (10). Look for rubber washers from garden hoses, knitting stitch markers, or other rings that are just large enough to put your fingers in comfortably.
- Polymer clay, 2 oz packages (2). Available from Amazon.com. (2)
- Rubber bands (2)
- Ping pong ball (1)
- Optional: Wood or cardboard tubes. These materials can be used in addition to, or instead of, the polymer clay when connecting the fingers to form a hand; see the Procedure for more details.
- Glue; the type of glue will depend on what materials (polymer clay, wood, cardboard, etc.) you chose to connect your robot fingers. Elmer's® GlueGuide can help you make an appropriate choice.
- X-ACTO® knife or similar craft knife. These knives can be purchased at craft or hardware stores or online at vendors like Amazon.com.
- Ruler with millimeter divisions
- Fine-tip permanent marker
- Sharp scissors
- Cutting surface (cardboard or a clean kitchen cutting board)
- Lab notebook
- Optional: Needle-nose or beading pliers; useful for pulling the threaded needle through the straw.
- Optional: Scrap booking punch intended to handle heavy materials.
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These instructions will help you build a robot finger from an everyday drinking straw. They will also provide some ideas on how to hook several straw fingers together to make a hand. As the robotics engineer, you will need to figure out what you want your hand to do and play around with the design to make the best hand possible for that job.
Each finger in your robot hand will be made up of a drinking straw with one or more notches cut in it to make joints. Upholstery thread or dental floss will be threaded through the finger and knotted at the joint(s) then attached to a ring. Tugging on the ring will pull on the thread and cause the straw to bend at the joint(s). This is how the finger will flex.
Deciding How to Cut Straw Joints
Before making your final design, you need to find the best way to cut joints in the drinking straw. How big should the notch be? How close should you come to cutting all the way through the straw? Use one of your straws to find out. If you have straws from different sources, try a few notches in all the different types of straws to see which straws work the best.
- To protect your work surface, place a piece of cardboard on the table (or ask an adult if you can borrow a cutting board from the kitchen).
- With the help of a parent, use the X-ACTO® knife, scissors or punch to cut several notches of different shapes and sizes. Each notch should be at least 2 centimeters (cm) apart. For safety's sake, always place the straw down on your cutting surface and point the knife blade away from you (as shown in Figure 1).
- Put your finger on the back of the straw and bend it as shown in Figure 2. Is it easy to bend? Does it spring back nicely or does it stay bent after you let go?
- Try making the cut less deep and leave more straw. Does this make it easier or harder to bend? Is it more or less springy?
- You can also try using a scrap-booking punch on half of the straw to punch out a section (as shown in Figure 3) rather than using the knife or scissors.
- Keep track of your observations in a table, like Table 1, in your lab notebook.
Figure 1. When cutting notches in the straw with a craft knife, always cut away from you for safety.
Figure 2. Press the cut straw into a curve to see whether the notches you cut make good joints.
Figure 3. As a safer alternative to using a craft knife, squeeze the straw into the scrap-booking punch about halfway and punch out a notch. One point of a star-shaped punch does a great job.
|Brand of Straw (if using more than one)||Tool Used to Make Notch||Notch Depth (shallow/medium/deep)||Does It Spring Back When Bent? (Yes/No)||Other Notes:|
Table 1. Make a table like this one to keep track of how different notching techniques work. In the end, choose the brand of straw, tool, and notch depth that is easiest to work with and results in a notch that quickly springs back when bent.
- Look at the data you have collected. Choose the straw type and notching technique that make the joint nice and springy but not too hard to bend.
Planning the Robot Hand
- Before building the robot hand you must decide what it needs to be able to do. The Science Buddies Engineering Design Process guide has a page on how to
Specify Requirements that can help you think about how to do this. A few questions you might want to ask yourself :
- What will my robot hand do? Does it pick up items? What are some of the items it can grab?
- Suggestions: empty soda cans, paper cups, ping pong balls, paper towel cores.
- Note: The straws are not strong enough to support heavy items.
- What special features should my robot hand have? Look at your own hand and think about how you grab things. If you could have as many or as few fingers as you needed, what is the very best way to grab these items?
- Can I arrange the fingers on my robot hand to make it better at picking things up?
- What will my robot hand do? Does it pick up items? What are some of the items it can grab?
- Once you have answered the questions, sketch what your robot hand will look like.
- Decide how you will test your hand.
- Do you want to attach it to an arm of some kind to make the testing easier?
- What item(s) will you have it pick up?
- How many tries will the robot hand get with each item?
Preparing the Straws
- Here is a tip we found when testing this project. In order for the fingers to bend nice and straight, it is helpful to make sure there is a straight line running down the length of the straw. Cut the notches centered on this line. If the brand of straw you chose already has a straight line or lines as part of its design, skip this section and go on to Making the First Finger. Otherwise, mark each straw with a straight line.
- Find a straight edge at least as long as the straw that is half as high as the straw is wide. For example, if you found straws 8 mm in diameter, your straight edge should be 4 mm thick. Your ruler may have a nice metal edge. If you place it flat on the table, is the edge 4 mm off the surface? You may be able to cut a thin piece of cardboard (a shim) to raise that metal edge up until it is 4 mm from the table.
- Place the ruler and the cardboard shim on the table. Have a friend help you hold it firmly to the table or tape it down. You don't want it to move during this step.
- Place one of your straws up against the edge of the ruler. Use your fine-tip marker and run a line down the length of the straw as shown in Figure 4. Hold the straw up to your eye like a telescope and look down the length. The line should be straight. Repeat this step with your remaining straws.
Figure 4. Marking a straight line down the length of the straw helps keep the joints lined up so the finger bends more naturally.
Making the First Finger
- How did you draw up your plan? Did you make a sketch of how the robot hand will look? You should include that sketch in your final presentation. However you planned your final robot hand, it is time to make the first working drinking-straw finger.
- From your experiment earlier, you have an idea of how much material to cut out of the straw for each joint. Look at your index finger. It has three sections. Whether your final design has a finger like this or not, use one of your straws to try making a finger that bends like yours. Hold the straw up next to your finger. With the other hand (you can also have a friend help you), mark the straw where the joints should go (generally about 2 cm apart). Starting at the finger tip, measure from the end and make a mark where the first joint will go. You will be cutting out the section from the side where you drew the line earlier. Make your mark on that side. Continue from there, measuring and marking the joints. An example is shown in Figure 5 below.
Figure 5. Mark each joint in the finger to make a robot finger that bends like yours.
- Somewhere in the middle of your hand, mark the straw so you know where to cut it to length. Keep the straw at its full length for now.
- Place the straw flat on the surface with the line facing up. Make the cuts for each notch according to the technique you developed earlier. (See Figure 6 for an example.) When you have finished, try bending all of the joints at the same time. The straw should gently form a nice curve if you lined up all of the cuts along the straight line you drew.
Figure 6. Cutting the joints of the finger so they nicely line up creates a finger with a nice curve.
- Cut the straw to length with your scissors. The length will depend on your design. Make sure to leave the straw long enough to attach to the 'palm' or main structure of your robot hand.
- Cut a piece of thread at least four times the length of the finger. Close to one end, tie a nice fat knot. A figure eight knot is a good knot for the job. Thread the other end through the needle.
- Stick the needle (carefully!) through the straw just above one of the joints (as shown in Figure 7a). Using the needle-nose pliers or a chop stick (as shown in Figure 7b), pass the needle down the length of the straw until it comes out of the opposite end of the straw. Repeat steps 6 and 7 with threads on the other joints. In case your nod slips trough the hole created by the needle, a drop of glue or nail polish may help secure the nod.
Figure 7 (7a, left; 7b, right). Push the needle carefully into the straw just above the joint (7a). Work it through with the help of a chop stick and pass it down the length of the straw (7b).
- Cut the threads from the joints so they are all the same length, at least a few inches from the end of the straw. Tie one of your rings to each thread (see the example in Figure 8).
Figure 8. A finger with three joints and one tendon for each joint. Pulling a ring makes the joint it is attached to bend.
- With one hand, hold the straw upright, and place fingers from the other hand through the rings. Gently pull the rings one at a time and see what happens. Can you bend each of the joints independently? Look at your index finger again and see if you can bend each of the joints independently. Is your straw finger different than your real one? Is it the same?
- Can you bend the joint closest to the fingertip without bending the others? Can you guess why? Can you bend your real finger that way? Because of the way the tendons in a real finger work, it is almost impossible to bend just the one joint.
Assembling the Robot Hand
- Now that you have made one finger that is a lot like a human finger, it is time to make your complete robot hand. Using your design sketches and the techniques you just practiced, make several fingers. Cut as many straws as you need into as many fingers as your plan calls for. You can use thread tendons on each joint if you like. Or to make the hand simpler to control, you may decide to simply use one tendon per finger. If you do that, connect it to the joint closest to the fingertip.
- Think about how your own hand has the fingers attached around the palm of your hand. Your robot hand needs some structure to join the fingers as well. Attach all of the fingers together in some way so that they look like the robot hand you designed. You can glue them to cardboard or wood, or mold some polymer clay around the straws. If you decide to use the clay, form the clay around the straws and then remove the straws, as shown in Figure 9. Have an adult help you bake the clay in the oven according to the manufacturer's directions. When the clay has cooled, reinsert the straws.
Figure 9. Mold the polymer clay around the straws first, then remove the straws and bake the clay according to directions. The small strip shown here could be part of a larger 'palm'.
Our test robot hands are shown in Figure 10, but yours will look different. Just as an idea, you can have a thumb jointed separately and have a rubber band in the back provide the return spring. A medical glove can help if the fingers are a little too slippery. Besides gloves (or just the glove fingers) for extra grip, you can also use skinny balloons of the kind used to twist balloon animals.
Figure 10. The hands we made at Science Buddies to test this project. Since our hands are shaped somewhat like a human hand, it fits inside a medical glove which makes the fingers a little less slippery.
Testing Your Design
Part of engineering is testing your creation to see if it works as well as you want it to.
- Hold the robot hand steady on a support or have a friend help you. Place your fingers in the rings and pull the tendons. Do the fingers curl correctly? Does it look like it closes and grabs the way you planned it?
- See if your robot hand will pick up the object or grab the item or do whatever you designed it to do. If it does, congratulations! Can it do a better job? What could you do to improve it? If it can't pick up or grab the object, why not? This is your chance to figure what went wrong and make changes, or just make it better. Are the fingers too long? Too short? Do they bend in the right place? Try some experiments to figure it out then go back and remake the parts.
- Use your robot hand to pat yourself on the back - you did it!
Ask an Expert
- Does your robot hand need more reach? Perhaps you can design an arm out of wood or cardboard tubes to reach farther. If your arm needs to bend, hinges from the hardware store for joints and some extra thread for tendons could be a great start.
- What other materials can you think of to make a robot hand from?
- Engineering is often inspired by nature. What else can you see around you that you can build from simple materials?
- Check out the I Like to Move It: Motorizing a Robot Hand abbreviated project idea to get inspiration on how to motorize your robot hand.
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