Gaby's 2020 Summer Blog
June 25-26: I joined the BDML slack, and in the new exploratory hand channel, Mark, Rachel, Michael and I are discussing ideas for a two-finger "lobster claw" gripper. There will be one rigid finger and one longer soft finger (actuated by a tendon) that can explore the environment with tactile sensors. I also started doing tutorials on OnShape, which BDML uses for collaborative CAD projects. So far, it seems pretty similar to Solidworks, but it also has its nuances that I'll need some more practice with.
June 29-July 3: This week has involved reading a lot of past research/papers about soft robotics, finger design, active surfaces, etc. As a team, we discussed the possibility of a snap-on-snap-off digit finger (one would be super soft, while the other is semi-soft, with several rigid members). We're also currently debating whether a linear slider design or linkage design will be better for compactness and range purposes. On my own, I've been doing some sketching of different soft finger shapes and overall hand designs. I also "met" with Rachel this week, and she was super helpful in showing me some key differences between Onshape and Solidworks, especially in terms of workflow. A main distinction is that Onshape has part studios as well as assemblies, so I've learned that part studios are for designing several parts that depend on each other's geometries and positions, and parts can only move in relation to each other in assemblies. Also, Onshape has a great organizational tool to keep track of the history of your parts (you can create versions, make new branches, go back to previous iterations, etc), so that will be very helpful in designing collaboratively.
July 6-7: My supplies are here! Now I can start prototyping with foam core, cardboard, dowels, rubber bands, bowden cables, hot glue, and anything else I have around the house. I've used the cardboard to make some linkages, which I'm currently learning more about. Rachel, Michael, and I had a good brainstorming session. We discussed a cool idea of a retractable rigid thumb so that it's not in the way during exploration. This could look similar to a tape measure mechanism, so the thumb can retract and roll up into the palm of the hand. Other options include telescoping or folding/interlacing with the index finger. Shown below are my sketches for these different options.
July 8-10: After a lot of discussion and brainstorming in the last couple of weeks, it's time to start trying out some of our ideas. It seems like for the parallel jaw motion, a linear slider is best, especially if we want the option of switching out the index finger. For the wrapping/enveloping motion, we'll explore options like tendons, linkages, and direct/quasi-direct drive. I've been testing out different linkages on OnShape to see what link lengths give us the desired positions for grasping our determined smallest and largest objects. I also made a prototype that's based off of Michael's Onshape design. It's made of mostly foam core and involves a linear slider motion and a two-jointed index finger. It will be helpful to explore a fridge or cabinet in my house to gain insight on the general feel of the gripper, factors that weren't considered, and any adjustments that need to be made.
July 13-15: I've improved my prototype to make it more realistic and reliable. I added rubber bands to the backs of the proximal and distal links to simulate the behavior of torsional springs, I made a sturdier handle, and I attached two tendons to actuate the two links. The gripper can achieve the desired grasping positions for variously-sized objects, so it seems pretty promising! After meeting with Rachel and Michael, there are definitely some questions we'll raise to others in the lab meeting on Wednesday. We mainly need to figure how we want to explore an environment with the hand, i.e. determine in which directions we need a soft touch. This will directly impact which joints need to be really backdrivable and which parts need to have low inertia/stiffness. A good next step for the team will be to do some simulation in Gazebo. My next steps include analyzing forces on my 4-bar linkage design to determine if it will "unravel" when gripping objects, updating my prototype to have a single tendon (string) that actuates both links, and drafting a similar tendon-driven finger in Onshape.
July 16-17: When considering the way the 4-bar linkage will behave in an actual gripping situation, I figured it would be helpful to just make a quick cardboard prototype and apply forces to it myself. As Mark and I expected, when the linkage is in a pinch-grasping orientation (the left side pointing straight up), any normal force applied by the object to the linkage, either on the proximal or distal link, will cause it to bend at the distal joint. This is clearly an issue. If we want to continue with a 4-bar linkage design, we will likely need a sort of brake to keep it rigid while the object is in grasp. This definitely adds to the complexity of the hand, and we're trying to keep it as simple as possible. Also, regarding the movement required to actually grasp an object, it makes sense to have the parallel jaw motion set the desired wrist size, and then use the enveloping mechanism to envelop the object with the index finger. By playing around with the prototype and the Onshape linkage, it seems like the nature of the linkage does not allow for a good "wrap-around" motion, so the hand will have to simultaneously adjust its wrist size and amount of wrapping in order to get into the proper position. This also adds to the complexity of the design, so these factors are ones to keep in mind when moving forward.
July 20-23: I've updated my prototype again, this time using a single tendon that actuates both the proximal and distal links (this is what would be employed in a higher fidelity prototype). In Onshape, I'm currently designing a tendon-driven index finger that's similar to this prototype, so it's been interesting and helpful going back and forth between the two to compare details and figure out how to model different aspects digitally and physically. In Onshape, I'm creating a through hole throughout the finger, along with filleted edges and dowel pins to ensure smooth tendon movement. For the prototype, I used string and cut-up pieces of a straw to create the tendon's path on the finger. In either case, it's important to note that the farther the tendon is from the point of flexure, the less force that is required to bend the link (think about the relationship between distance and torque). In Onshape, I'm making "dog bones" that are typically made out of urethane and act as the flexure joints for the finger. In my physical prototype, I used rubber bands to act as both the flexure joints and an antagonistic tendon that pulls the finger back to its original position.
July 24: Mark sent an email to me and a few people involved with the ME102 curriculum about how my prototype is a good example of how you can use simple materials to enhance a design. Giving ME102 students a kit of materials could be a great way to learn and prototype remotely this year. Mark thought it'd be helpful for me to include a bill of materials for my prototype, so here it is: foam core, hot glue, wooden dowel rods, string, straws, rubber bands, scissors, ruler, exacto knife
July 27-28: During the TRI meeting last week, we discussed a few different things regarding a tendon-driven index finger design. We talked about the idea of one vs multiple tendons to control the proximal and distal links. While multiple tendons may allow for more varied movement of the finger, it adds complexity that we don't necessarily want, and it would likely lead to a bulkier wrist (which we want to avoid) with several motors. So, sticking with the one-tendon idea, another question is if the tendon needed to be routed around a dowel pin, or if it could be threaded straight through the proximal link. The image on the right shows the latter option. The concern for a straight through route is that the material between the tendon and the edge of the finger may be too thin. Moving the tendon closer to the center of the finger would remedy this issue, but it would also decrease the torque, therefore requiring more force to bend the joint. As long as we avoid having excessive friction (by adding a steel dowel pin and fillets on the tendon exit holes), having a single, routed tendon seems like a good design choice for moving forward with 3D printing a prototype.
July 29: While working on the finger in Onshape, I realized that with its current resting position and its hard stops, it cannot bend backwards at all. Previously, our team decided that ideally, the hand would have a small wrist and be able to grasp larger objects with the wrap-around reach of the index finger. So, I realized I should change the CAD in order to fulfill these design goals. It was fortunately a very simple fix, where I just rotated the finger 45 degrees backwards and kept the base the same. After all, the proximal joint rotates 90 degrees, and there's really no reason for the proximal link to fold until it's parallel with the linear rail. Ironically, I'd made this adjustment to my physical prototype, which you can see in the pictures above, but I'd forgotten to do the same thing in CAD until now.
July 30-31: My top priority right now is to make some final changes to the CAD design I've been working on to get it ready to be 3D printed. Talking to Michael and Rachel was really helpful in figuring out what needs to be adjusted before printing. These changes include adding fillets to all the edges, making the tendon routing holes larger, adding a counterbore hole in the distal link, and making sure all the finger base dimensions match what Michael has. I will also need to put the finger in assembly and use revolute mates to see how the links actually move. This will show me if the edges line up well as the finger closes, or if parts collide with one another, in which case I'll need to adjust the lengths of the dog bones.
August 3-4: I've made a lot of the necessary changes listed above. The biggest thing I did was put the finger in an assembly to see how the parts would actually move and interact with each other. Initially, the dog bones were too short, which caused the links to collide instead of bending the full 90 degrees that the joint should allow. So, I changed the dimensions of the dog bones (they're both 1 inch long now), and the links fold up perfectly now, with the edges lined up correctly.
I also adjusted the base of the finger so all the features and dimensions matched up with Michael's. The index finger he's been working on is a simple rigid finger, similar to the thumb, so he can keep testing the linear sliding DOF. It's important for my tendon-driven finger to have the same base as his so we can easily remove/attach the fingers and experiment with different designs. I made sure to keep two sets of holes in the base so it can attach to two carriages on the linear slider, counteracting the moment caused by the force of the object being grasped.