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 HiTec 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 HiTec 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, wrist, and shoulder joints.

Once you develop a feel for what rotation can do to a mass attached to an extension of your arm — or to a robot arm — you’ll better appreciate what’s allowable before damage likely occurs. For example, I’ve come to appreciate that those small but sudden stops and starts are deadly to servos, even when the robot arm is unloaded. I’m also more careful about parking my robot arms in safe positions so they don’t lurch into position when power is applied.

If you’re the quantitative type, then you can use the iPhone’s internal accelerometer and Bluetooth peripherals to measure the forces generated during a swing. For example, I’ve measured over 3 gs with a handheld iPhone running techBASIC during a normal swing of one revolution/second. Using a TI SensorTag mounted at the 15 lb beech wood club’s center of gravity, I’ve measured over 5 gs – that’s 75 lb. Quite a tug.

For more information on techBASIC ($20) and the TI SensorTag ($99), check out [url=http://www.byteworks.com]http://www.byteworks.com[/url]. If you’re interested in the biomechanics of Indian club swinging, check out the videos and clubs on Dr. Mike Simpson’s excellent website at www.indianclubswinging.co.uk. SV