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 ;
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ERVO FOR THE ROBOT INNOVATOR
6 SERVO 01.2014