CS 154 Homework #2
Due Friday, February 13

Readings for HW 2:

Experiments in Automatic Flock Control by R. Vaughan, N. Sumpter, A. Frost, and S. Cameron,
   Proceedings, 1998 Symposium on Intelligent Robotic Systems, Edinburgh, UK.
Modular Robot Climbers by Nadeesha Ranasinghe, Jacob Everist, Wei-Min Shen,
   Proceedings, IROS 2007 Workshop on Self-Reconfigurable Robots.

Short-answer questions

Please "submit" these simply by posting them to an appropriate link, page, tab, or area of your CS154 website.

Short-answer question #1    Stepper Motors

The example stepper motor shown in class had 4 teeth on the stator and 2 on the rotor. The angle that that motor turned for each full step was 90 degrees. A full step is considered the smallest (nonzero) angular turn that causes two teeth on either side of the rotor to align with two teeth (also diametrically opposed) on the stator. For example, the diagram below shows four consecutive full steps of 15 degrees each in a stepper motor with a six-toothed rotor and eight-toothed stator.

Part A   How large is the full-step angle in a stepper motor with R rotor teeth and S stator teeth? Note that you may need to consider separate cases, depending on the values of S and R. You may assume, however, that S is an even number greater than 4 and R is an even number greater than 2. Why would the motor not be very useful if If S == 2 ?

Part B   A typical commercial stepper motor offers a full-step resolution of 1.8 degrees. What would you guess are the number of rotor and stator teeth on such a motor? In particular, try to minimize the total number of teeth needed to achieve this resolution. After all, fewer teeth mean simpler construction and control.

A typical, inexpensive servo motor might have a 180-degree range of motion. One such hobby motor (they're typically only $10 each) is disassembled in a plastic bag in the robotics lab. This question asks you to determine how many rotations of the motor shaft are required to attain the full 180-degree range of motion that the end-effector (the output shaft) actually exhibits.

In essence, this is only a gear-ratio question, but because the system of gear-reductions is so compact, there is an additional puzzle element here: you'll have to determine how the gears interact, because the pieces are partially disassembled in the bags. You don't need to actually count gear teeth (feel free to use variables to represent their number of teeth), however clever ways of counting the number of teeth would be welcome -- mostly so that I know how many are on some of the gears -- they're too small for me to confidently count one-by-one!

There is a second back with a partially-disaassembled gear train in the lab -- this one has a wheel from an iRobot Roomba inside it. For this gear train, determine how many rotations of the servo motor are required in order to turn the Roomba's wheel one full revolution. Explain the ratios involved.

Consider a two-steerable-wheel bicycle sketched in the figure below. Unlike ordinary bicycles, here the front wheel is powered (with a velocity of Vf), while the rear wheel is free to roll to keep up. Both wheels have radius 1. However, both wheels can be steered independently, so that in a general configuration, the frame could form an angle Ar (alpha-r in the diagram) with the rolling direction of the rear wheel, and it could form another angle Af (alpha-f in the diagram) with the front wheel. You should assume that the length of the axle between the wheels is a known, fixed quantity L. Theta, of course, is (90-Ar) degrees.

First, set up a convenient coordinate system for this robot. Within your coordinate system, what are the coordinates of the vehicle's instantaneous center of curvature? At what speed does the rear wheel need to rotate in order to keep up with the front wheel (to minimize wheel slippage)?

Lab Project update for HW 2

An important quote to keep in mind is the following, borrowed from roboticist Erann Gat's quote collection:

Furious activity is no substitute for understanding.

Of course, furious activity might lead to understanding -- in which case it would be considered a good thing. But it is a also good idea to step back and reflect on a project even while you're working on it... .

• First build or otherwise familiarize yourself with your hardware!     Be sure to include a picture of your final platform in your write-up, as well as any insights from building it. There is a still-photo digital camera available in the lab, as well as an MPEG camera from which I can download videos. Of course, digital cameras are all over the place, and you probably own or can borrow one more easily than using mine. If you're doing a lot of building, consider taking pictures every so often in the process to document your progress.
  
  Although it seems crazy, it's really important to have a "BEFORE" picture to compare later in the term with your "AFTER" result. Large carpetfulls of random lego pieces, a bunch of unassembled parts, a picture of nothing at all... any of these will offer an excellent contrast with the system you've created by the project's end!
  
  Movies are always even more welcome than pictures, particularly of your final product moving around... .



• Get moving, sensing, and visualizing...     Your choice of platform will be determined by your project and the computer(s) you might have at your disposal. The key for this week is to demonstrate that you can program your robot to move around and to read from and print out the values of all of its sensors. If at all possible, you should integrate a GUI of some sort -- it may be easiest simply to integrate the Python GUI from HW #1, though others are OK, too.
  
  In particular, it would be best to write a program that gives you remote-control access to all of the movement commands and sensory data that the platform has to offer. The key for this assignment is the completeness of access you can achieve, not the elegance of the interface. Thus, being able to print out sensor values is wonderful. If you're inspired to plot them in some way, all the better - but that will be more important in the third assignment, which is on robot localization.



• Wandering around     The main task for this assignment is to create a finite-state-machine controller for your platform so that it can autonomously wander around and avoid obstacles. In fact, your robot is welcome to run into obstacles, if that is important to its interaction with the environment, e.g,. for the Roomba/Create platform. But the key is that it should not get stuck in a single place or a single state.

  This part of the project will vary according to the platforms used and the tasks that they're seeking to accomplish! More detail will be worked out on a team-by-team basis. However, a few guidelines apply to all:

  • There should be at least four states in your state machine. More is certainly OK!
  • Your system should be able to wander through an area considerably larger than itself -- say, at least an order of magnitude.
  • Your system should indicate what state it is in and, if possible, the trigger that causes it to change state.
  • Include a state-transition diagram (as an image) in your write-up. This is an opportunity to revisit JFLAP, if you'd like! :-)




• Be sure to describe your progress on your webpage as you go along...     This is where I'll go to read the current status of your project. (In addition, if you have something you would like to demonstrate physically, feel free to grab me any time. I'd love to see it!)
  
  Always admired is any documentation (words, pictures, movies) of things going wrong... . This is both fun (sometimes!) and, more importantly, another great way to convey the work you have done by the end of the project!

I would encourage you to consider a website layout in which your HWs and project progress are organized and accessible - several of the previous projects had easy-to-use layouts you might consider. Creativity (or, perhaps, creative copying) is welcome for your website layout. Using a wiki is great.

• The (green) Barbie Jeep project at https://www.cs.hmc.edu/twiki/bin/view/BarbieJeepProject/WebHome
• The Mallard Bane page at http://www.cs.hmc.edu/~dodds/projects/RobS04/DuckHunt/index.html
• or the AIBO page at http://www.cs.hmc.edu/~dodds/projects/RobS05/AIBO/index.html

The following is a rough, general guide to putting together a technical report on a robotics project, e.g., for submission to a conference. Particular projects may omit some of these points and include others. (Most will be very similar for the first half of the term, however!) You don't have to have all of this right now, but you should have a paragraph of Introduction/Vision, a half-page (at least) of Progress, and some example Media.

• Introduction/Vision
  • What problem are you addressing?
  • What are the primary objectives of your work?
  • How does this project relate to previous work? (cite)
  • Why is this project new, interesting, and/or important?


• Background
  • What makes this problem difficult?
  • What have others done or what are various possible approaches? (cite)
  • Motivate the features of the problem that your solution will focus on.
  • In what ways are you limiting the scope of the problem to make it more tractable?
  • Include any background equations or less-formal relationships here, or anything else a reader might need to be reminded of.


• Approach
  • What were the key design decisions you made?
  • Why did you choose your approach over other possible approaches?
  • What were the difficulties your approach entailed?
  • How did you get around them?
  • Diagrams are often excellent summaries.


• Progress and Performance Results
  • Provide images and/or data, along with descriptions and explanations of them.
  • When constructing systems or processing data, before-and-after pictures of the robot (or raw/processed data) are often excellent summaries of progress.


• Perspective
  • How well did your system perform?
  • What were some of the key factors in its success or failure?
  • What could be done to improve your system?
  • What other interesting problems or areas of investigation does this project and its results suggest?


• Media
  • Here would be pictures, screen shots, movies, or other media (audio clips?) that document your progress on your project.
  • Insights, errors, bloopers, etc. are not only welcome, they're important to keep around!


• References
  • Here you should reference at least the papers required for CS 154! Other references and web references are, of course, welcome, too.