Prototype Definition

Owned by
Ethan Cronier
Last updated: Jul 13, 2021 by
Mathieu Harter (Deactivated)
4 min read

Iteration 1: Validate Assumptions (torque/workspace NOT backlash)

Prototype Goals

  • understand our workspace
  • validate torque requirements

Prototype Plan

  • In this prototype, we are just focused on refining our torque calculations for each joint and selecting a more suitable torque requirement (move away from an over engineering arm)
  • To validate joint torque:
    • see if arm is capable of lifting comp payload and determine which joints work the "hardest" 
    • track torque at each joint when lifting heaviest items (5kg payload, rope tow) to figure out if our initial calculations were accurate and what the true torque requirements are at each axis
    • if time permits, also develop a good data set on what the joint torques are (on average) for all expected arm calcs
    • collect data and select a more suitable safety factor
  • To define our workspace:
    • look for any control singularities for the arm when using performing comp related tasks
    • see if we can refine linkage lengths by figuring out where we see restricted workspaces (i.e. picking stuff of the ground, fitting the cache in the drawer, etc)
  • To determine optimal joint speeds
    • Determine what rpms are ideal for each joint based on what's easiest to control(eyeball it)
  • Selecting materials:
    • we aren't testing for backlash or precision, we are JUST trying to better define our workspace and torque requirements. Custom components should be conducive to rapid prototyping and fabrication. Tolerances aren't super critical
  • Controlling the Arm: 
    • probably control the arm in open loop

Task List for Prototype 1

  • Select motors and gearboxes to achieve rough joint torque requirements. FRC website should give us a lot of good options for this initial design
  • Determine how to track torque per joint (torque sensors, record speed/current and use torque curves, etc.)
  • Determine if encoders are needed for this design, and spec easy to integrate encoders if deemed necessary. We definitely do not need any high resolution data from these encoders, so pick ones that are easy to integrate and cheap (if we have to buy new ones)
  • Look into second stage transmission options. It will probably be easier to implement a chain drive if we need to add a second stage reduction on our high torque joints, so we should find out where to get these / what to use. 
  • Select a construction material for structural members. Would it be easier to work with wood, aluminum tubing, etc. How can we get these materials? Find cost and lead times
  • Figure out how/where to mount the arm. We want to simulate rover conditions when testing, so ~12" above the ground on a stable surface. We can probably just mount the arm on a wood block or something but let's specifically figure this out. 
  • Design prototype 1 rev 1 in CAD
  • Review design to check for any critical errors that will prevent functionality
  • Build prototype 1. As you assemble the prototype, we should probably test each joint to ensure that they are roughly providing the correct torque. We can just attach a member to the gearbox output and slap it on a scale to figure this out. 
  • Load the arm with a 5kg weight (assuming individual joint torque tests yield favorable results)
  • Lift the 5kg weight while collecting all necessary data. We should lift the weight to maximum height, and collected data sets multiple times (if possible at least 5). (do we need to test for loading capabilities in other arm configurations? To be discussed.)
  • Analyze data to determine "true" maximum torque at each joint
  • Get some understanding of the workspace by simulating task completion through movement of the arm. Figure out if we reach any awkward positions. We need to flesh out our workspace testing more lol, unsure how exactly to do this right now. 


Iteration 2: Flesh Out a Near Finished Project (functional form close to final)

Iteration 2 goals:

  • Have a fleshed out design incorporating all crucial elements (encoders, motors, gearboxes, reductions, end effector)
  • Verify assembly, tolerances and fits
  • Integrate electrical components, cable routing, etc
  • Verify arm specs (lifting capabilities, accuracy, repeatability)

Iteration 2 plan:

  • double check loading capabilities
  • try to minimize arm mass/maximize member stiffness in CAD and build arm mainly through aluminum tubing that can translate to CF in a future iteration
    • I don't mean FEA here, I just mean in terms of material selection / material stiffness. make educated guesses on good construction mats.
  • validate encoder integration design
    • does our encoder integration allow for accurate data? compare collected encoder data to actual arm positioning (somehow lol)
  • determine how the accurate our end effector is
    • how much backlash of the end effector is experienced at positional extremities, and our most common arm configurations?
    • how repeatable is the positioning of our end effector during comp tasks?
    • in what tasks/configurations does backlash/stiffness actually become a problem for our end effector positioning?
    • is backlash manageable with good encoders
  • Verify that the arm is performing as expected under all conditions. we should use our intended gearboxes and motors for this stage this iteration
  • see what parts/assemblies fail and what needs to be fixed
  • Verify ease of assembly/servicing
  • address quality of life issues like cable routing, electronics placement etc. 


Iteration 3: Final Touch Ups / Optimization


- integrate higher quality production materials such as carbon fiber for structural members
- consider FEA for mass optimization
- reduce unique parts and BOM
- aesthetics 

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todo list

  • refine prototype definitions
  • create tasks


  • select motors -frc motors use COTS
  • create prototype
  • chain drive for the higher