2.77 – Week 1

This week, I spun a servo motor; laid out the preliminary design process for a pair of parallel-jaw pliers; and ran across some neat flexure-based designs in the process.

PUPS 1

For PUPS 1, we were assigned to generate a FRDPARRC table for “…a pair of pliers [we] would like to have…”, draw FBDs for those pliers, and then lay out the basic analysis steps and equations that we’d go through to actually design these pliers. I decided to examine the design of a pair of small, needle-nose parallel-action pliers – I’ve wanted a set of these for a long time, but have yet to find a pair that meets my criteria.

Assignment, pre-peer review

Editor’s Corrections (Reviewer 1, Reviewer 2 (verbal corrections))

Assignment, post-peer review

Lab Week 1

For the first week of lab, we were assigned to get a servo motor spinning using an Arduino and a GRBL-compatible servo shield. I had the benefit of taking Victor Leung’s CNC Drawing Machines class over IAP, and so was reasonably familiar with GRBL and the system’s functionality. However, it had been a while since I’d worked with Pololu drivers, so I worked slowly.

Checking voltage of power supply

Checking the voltage of the power supply

Week1_2

Tuning the current output of the Pololu drivers to output just under 2 A (the maximum sustained current the servos are rated for). The A4988 webpage (https://www.pololu.com/product/1182) gives lots of resources for this, including the all-important Vref conversion factor.

Week1_3

One of my drivers wasn’t tuning properly – I measured no voltage at the top of the potentiometer, even if I turned the pot. Upon closer examination, I realized that the pot itself was cracked – I’m going to need to replace the pot (or maybe the driver…) before I can run three motors simultaneously.

Despite a broken driver, I did get the motor spinning on 2 of the 3 axes! Here’s a video.

Seek-And-Geek

I saw a lot of neat things this week (I work at MIT…), but something that particularly stood out to me this week was this set of flexure-based 3D-printed pliers.

From: http://www.thingiverse.com/thing:334435

Flexure-based parallel-jaw pliers were one of the ideas that I was kicking around for the PUPS assignment this week: of course, within a minute of Googling “flexure pliers,” I’d run across these. The design is from a compliant mechanisms research group at BYU, who have an interesting page containing a few different common mechanisms executed as compliant designs. (Even cooler than that, though, are the compliant-element LEGO Technic modules that they designed for prototyping compliant mechanisms with LEGO). These particular pliers are designed for crimping wire (terminals, I’m guessing – it looks like they have HUGE mechanical advantage), but the group has also built longer arm clamps and hemostats.

I’m really curious about compliant mechanism design – it’s something I’ve never worked with before – and I was delighted to find a pair of pliers executed this way. From what I’ve read in the past about flexure design and from personal experience, my primary concerns with these pliers would be:

  • Design of flexure elements: In designing the flexural elements, I’d need to balance the spring force of the flexure; avoiding plastic deformation of the flexure at full travel (critical); and the resistance of the flexure to undesired motions (torsion about axes perpendicular to axis of bending; buckling)
    • Buckling seems to be a particular risk in these plier designs. The member that the upper handle presses against will be fairly heavily loaded in compression (it’s being used as a fulcrum for the handle), and I’d worry about the flexure buckling into an S-curve under load.
  • Fatigue life of flexure elements: Flexural fatigue life of compliant devices like this could become a concern, depending on the material used (particularly an issue if you’re doing this on the cheap with plastics or aluminum), and how disposable you see these pliers as being. I’m somewhat familiar with fatigue behavior in metals (particularly wire ropes), but less so with plastics – I’ll be exploring further.
  • Ergonomics & Usability: I don’t have a good intuitive sense of how these pliers would “feel” during use. Do they provide too much/too little resistance throughout the clamping cycle? Would be fun to waterjet out a pair and see!

Final Project

I didn’t get as much time to think about my final project this week as I would like, but have generated a fair list of possible projects. I’m going to do some preliminary research and talk with my group mates to try to winnow this list down early next week.

  • Hydraulic Single-Axis Test Stage for Robotic Arm: My research focuses on control of a large-scale micro-macro manipulator system, where a large (~ 6m reach), relatively compliant hydraulic arm has a small (~1m reach), high stiffness electric arm placed on the end of it. The small arm both provides higher resolution & bandwidth for endpoint positioning, as well as helping to control the dynamic behavior of the large arm. I’d like to build a single-axis hydraulic stage to mount my group’s KUKA KR6 R1100 to, to serve as a bench-testing tool for controllers. However, this would be a hydraulics project, which I a) know will be expensive and messy, and b) don’t have any background in.
  • Bamboo Bike Tube Auto-Miter-Er: A few weeks ago, I took a class where we built bike frames out of bamboo (super cool!). One of the most time-consuming parts of this process is hand-mitering the frame tubes so that they mate together. A tool – either automated, or partially automated – that would speed this process up would be great for bamboo bike-building workshops.
  • 2- or 3-axis probe for edge-, surface-, center-finding on mill: I’d love a probe tool that I could use for finding edges, surfaces and the centers of holes while milling. This would be similar to a CMM probe, but *much* less expensive and lower accuracy (.0005″ for my purposes would be fine).
  • Self-Contained Cable Drive Robot Joint: I’ve done a lot of work designing wire rope transmissions & drivetrains for robots at my previous employer, Barrett Technology. One project I always wanted to try out, though, was to build a self-contained joint module – think Dynamixel servo, but with cables. I think I could build a really nice joint module…but I wouldn’t be learning anything particularly new.
  • CNC Control for Benchmaster Mill: I have a small Benchmaster vertical/horizontal milling machine at home. It’s a great little mill for being a benchtop machine, but I’d love to at least be able to do basic automated machining (at least 2-axis positioning). Other people have converted these mills to CNC before, but I’d like to go through the process properly to make sure that my screw & motor selection matches the capabilities of the mill itself.
  • Automatic Single-Point Threading: I also have an (ancient) Ames lathe – it’s definitely the highest-quality tool I own, but it’s also very old & consequently fairly primitive. Since it doesn’t have a lead screw to drive the carriage, I can’t single-point thread with it. I’d like to either create a separate single-point threading mechanism, or put a motor on the existing carriage to perform threading operations.
  • High-Speed Linear Stage: I’m working on a project where I need to design a linear stage that can move over ~ 250mm at a range of speeds, from slow-ish (0.5 Hz) to REALLY fast (5 Hz). I’ve done preliminary design work for this stage already and picked a basic drive technology (high-lead screw), but need to sharpen my pencils and go back to do detailed design. Particularly major concerns for this project are heat buildup in the screw and bearings (the lead nut is a polymer nut, and so could potentially melt); system life (the stage is going into a consumer product, so regular replacement of wear components isn’t an option); and control over the BLDC motor driving the screw.

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