Servo - January 2014

A Ton of Feathers Versus a Ton of Bricks ...

If you’ve devoted much time experimenting with robot arms, then you’ve undoubtedly spent considerable time and money repairing the servos and motors. I’ve probably fused a half dozen Hi Tec servos on one robot arm alone.

I never really appreciated the abuse I subjected the servos to until I added club swinging — essentially swinging metal and wooden bats in circular paths around the body — to my exercise routine of kettle bells and free weights. It’s one thing to look at torque specs on a spreadsheet and quite another to experience dynamic variances in torque involving your own shoulder, elbow, and wrist joints.

Take a look at the photo — two clubs from my collection and a neoprene bag of lead shot. All three have the same mass — about 15 lb of lead, iron, or beech wood. As you’d expect, at rest with my arm extended, the load on my rotator cuff muscles is the same. However, when swung overhead, the clubs are much more of a challenge than the bag of lead shot. Moreover, swinging the beech wood club is most difficult of all. This makes sense, given the center of gravity of the wooden club is farther away from the club handle than is the center of gravity of the metal club.

I don’t know the rating of my shoulder joints, but a typical servo (such as the Hi Tec HS-311) is rated at 51 oz-in and 0.15 sec/60 degrees with a supply voltage of 6V. In other words, given a 6V supply and a 1” servo arm, you would expect the servo to supply 51 oz of force at right angles to the tip of the arm, and move the 51 oz weight 60 degrees in 0.15 seconds. Use a 10” servo arm and — neglecting the weight of the arm — you’d expect the servo to generate a 5.1 oz force at right angles to the tip of the arm.

The point is the real world of robotics isn’t about carefully slowly picking up a 6 oz plastic ball and placing it in a slot. Today, it’s about wielding tools and torches at high speeds, and tomorrow about lifting patients from beds and rescuing victims from collapsed buildings. Even if you’re just designing a simple arm for your next hoops competition, you’ve got to consider the real world forces involved.

Now, it’s one thing to examine the stresses on a robotic arm or other robotic device using simulations and mathematical models. It’s another to develop an intuition for what works and what doesn’t. For example, pick up a baseball bat and carefully swing it in a vertical arc. Pay attention to the stresses on your shoulder joint and muscles. At the apex of the swing — where gravity and centrifugal forces are equal but opposite — you might feel very little. You’ll probably feel a significant tug just after the club travels past your feet. Now try the same swing with your hands alone and again while holding, say, a plastic bottle filled with water.

Pay attention to your elbow,

Mind / Iron

by Bryan Bergeron, Editor ;

Published Monthly By

T & L Publications, Inc.

430 Princeland Ct., Corona, CA 92879-1300

(951) 371-8497

FAX (951) 371-3052

Webstore Only 1-800-783-4624

www.servomagazine.com

Subscriptions Toll Free 1-877-525-2539

Outside US 1-818-487-4545

P.O. Box 15277, N. Hollywood, CA 91615

PUBLISHER

Larry Lemieux

publisher@servomagazine.com

ASSOCIATE PUBLISHER

Robin Lemieux

robin@servomagazine.com

EDITOR

Bryan Bergeron

techedit-servo@yahoo.com

CONTRIBUTING EDITORS

Jeff Eckert Jenn Eckert Tom Carroll Kevin Berry Dennis Clark R. Steven Rainwater Michael Simpson Camp Peavy John Blankenship Samuel Mishal David Geer Mike Jeffries Matt Spurk Pete Smith William Henning

ADVERTISING SALES EXECUTIVE

Richard Lockhart

richard@servomagazine.com

CIRCULATION DEPARTMENT

subscribe@servomagazine.com

MARKETING COORDINATOR WEBSTORE

Brian Kirkpatrick

sales@servomagazine.com

WEB CONTENT

Michael Kaudze

website@servomagazine.com

ADMINISTRATIVE ASSISTANT

Debbie Stauffacher

PRODUCTION

T & L Publications, Inc.

All advertising is subject to publisher’s approval.

We are not responsible for mistakes, misprints, or typographical errors. SERVO Magazine assumes no responsibility for the availability or condition of advertised items or for the honesty of the advertiser. The publisher makes no claims for the legality of any item advertised in SERVO. This is the sole responsibility of the advertiser.Advertisers and their agencies agree to indemnify and protect the publisher from any and all claims, action, or expense arising from advertising placed in SERVO. Please send all editorial correspondence, UPS, overnight mail, and artwork to: 430 Princeland Court, Corona, CA 92879.