6-DOF Manipulator: Design Requirements

The design requirements for the arm are outlined below and grouped by precedence. Sub-sections establish and further define necessary design criteria to meet that particular requirement. It’s also important to note there is a strong inter-dependency between individual groupings. Many of these requirements are not unique to robotic manipulators, but are generally accepted good practice in mechanical design. 

  • Primary
    • Manipulability: The operator’s ability to position and or re-orientate the end-effector, specifically when attempting a particular task. It is essential that the operator can move the end effector almost as intuitively as they might move their own hand. This by far is the dominating requirement. The arm literally exists to perform tasks by interacting with physical objects. Considering the competition time requirements, an inability to do so means certain failure. There are several ingredients, that when properly combined, lead to an easily-manipulated-arm. I have outlined these below.
      1. Control Scheme:  The foundation of arm manipulability. An operator must interact with the arm and in turn the arm interacts with components of a task. If there exists no means for the former, the arm becomes the world’s most advanced paperweight. 
      2. Joint Mechanical Precision and Torque requirements: In order for the operator to easily manipulate the arm with confidence, we must be certain that the system controlling the arm knows and is certain of the true spatial position of all the components of the arm within some prescribed tolerance. The joints are the connection points between different sections of the arm whose job is to allow for controlled motion in strategically chosen degrees of freedom while restricting motion in all others. Often times this motion is hindered by significant loading, leading to sometimes very high torque or force requirements at the joints. Considering the output of electric motors typically spin very quickly by design, often several thousands of revolutions per minute with relatively low torque limits. If we hope to use a reasonably massive motor (typically governed by a physical power density), we must take advantage of the trade-off between speed and torque by “gearing down”. 
      3. Global Arm Stiffness: Similar to the precision joint requirement, this requirement ensures the control system is certain of the true position of the end effector. No part of the arm will be perfectly rigid. In turn, loading at the end effector results in some deflection which is a sum of all the joints and links individually deforming. The difficulty in predicting these deflections is a combination of the fact that the geometry of the deforming bodies is complex, but also that the deflections are dependent on the orientation of the arm and would require knowing exactly what the loads at the end effector arm. Again, we want to be certain of the position of the end effector in space, and because we likely can not predict deflections well, we must make the arm as rigid as possible in order to minimize deflections to the point that under normal use loading they are nearly negligible. 

A note on these requirements: Because of the drastic changes made to the arm this year, we have never in the past actually established a realm of possibility for positioning and repeatability. From our analysis, we hope to have the worst case uncertainty in position of about .010” to .030”. That is under maximum loading conditions while the arm is extended far from its base. We hope the construction and testing of this year’s arm will be a powerful tool that is used to develop a reasonable baseline for future years. 

  • Manufacturability: Although we have a huge number of resources at Cornell, we are not a professional machine shop or a legitimate manufacturing facility. Therefore, at some point independent of our design ability, our end product will be limited by the fact that we will literally not be able to manufacture some arm component and machining it out of house would dominate the entire on rover budget. For all machined components, the root question should be asked, “Is this at all feasible to manufacture in Emerson Machine Shop?”. The next most important manufacturing topic is outlined below.
    1. DFM vs. Functionality: There is a constant battle being waged in the mind of a designer, a give and take battle between “Can I design this so it is easier to manufacture?” and “If I do so, how will the functionality of the part suffer?” This was an important consideration in the design process for this year’s arm. Often times this is the case, the DFM changes either have no net effect on functionality or result in the part functioning at least slightly worse (A separate discussion can be had on how we quantify functionality). If decisions are more parallelized, and the moons of Jupiter align, it is possible to flip this adverse condition on its head. What if we can make a part function significantly better, and make it “easier” and faster to manufacture to a higher degree of accuracy. These ideas often clash with the need for a higher level of upfront manufacturing complexity and limited resources, hence the quotes around easier.
  • Secondary
    1. General reduction of part count and arm piece part complexity: Although a lot of the parts on the arm appear slightly intimidating at first, they are actually strategically designed to be very straight forward to machine. Almost all the joints and their sub-components are a combination of plain shafts, and CNC’d pockets. Things like complex setups and unique feature geometries are avoided.
    2. Reduction in mass: This design requirement is relatively straight forward. Every gram we save can be used by either the Drives or Astrotech sub-team to potentially increase functionality of their systems. An additional benefit of reducing mass means the gravity loads from the arm and the resulting inertial loads while moving are all greatly reduced. This is a secondary requirement because it is better scoped and in turn should not have as much direct influence on the design. In other words, as long as the arm weighs less than the number specified by systems, we’re okay. 
    3. Reduce the number of unique purchased parts: A reduction in unique parts like bearings and fasteners reduces organizational complexities associated with ordering, managing, and assembling components of the arm. 
    4. Ease of Assembly: Ensure that all fasteners are actually reachable, and that the order of operations to assemble different subassemblies is straightforward and easy to follow. 
  • Tertiary
    1. Aesthetic: Obviously this is not a driving design criteria, but it has been a huge oversight to but zero effort into the aesthetic appeal of the arm in past years. We are constantly trying to win over the new members we recruit, or the companies we hope to obtain as sponsors, or even the CU administrative system to give us the budget we require. We typically only have several moments for someone to look at what we have to offer in an email or at a booth, and either hook them in, or likely lose them forever. Even if we achieve perfection in terms of functionality, people can’t help but to subconsciously judge us on the appearance of the arm, and the entire rover for that matter.