Brain Teaser Challenge Solution - August 2004

Butch Shadwell

At the time I am writing this, I haven’t received any answers to the last BTC. As you recall, I was reminiscing about some robotics work I did in England years ago, which brought me to this problem; “Henry … pneumatically actuated …joints were operated by air pistons. The elbow used a 1” cylinder to move his forelimb out and in again. This cylinder had a ¼” piston rod. Henry’s arm extended just fine but would hesitate while retracting. The arm retraction was powered by the cylinder’s retraction motion, as opposedto the extension motion. With a 90 pound air supply, how much force was the cylinder applying to the retraction function?”

A lot of folks don’t know that all piston actuators do not supply the same force in both directions, even with the same fluid supply pressure. First the force generated for extending the piston comes from the entire piston surface, in this case 0.785 sq. in. This results in an extension force of 70.7 lbs. When the fluid is forced into the retraction side of the cylinder, the piston area is reduced by the cross sectional area of the piston shaft. The piston size is effectively reduced by this amount at the point of attachment. So the effective piston size in this direction is 0.735 sq. in., resulting in only 66.2 lbs. of force in this direction, a 6.4% reduction. But I bet you already knew that.

Brain Teaser Challenge - October 2004

Butch Shadwell

Unlike many of you, people are always asking me, “Butch, how do you stay in such good shape and keep that youthful spring in your step?” I always reply as modestly as I can, that I owe it all to genetics and regular infusions of Dr. Pepper. You know … the carbonated soft drink. I really don’t think that the cosmetic surgery and spa treatments I use are any different than anybody else’s. But my wife and I can attest to the medicinal properties of Dr. Pepper. I’d be willing to bet we could end this debate over stem cell research if those scientists would give Dr. Pepper a proper trial. We’ve discovered many new uses for this amazingdrink. Besides cleaning battery terminals, we also find it useful at softening one’s cuticles before a manicure. George Washington Carver may have been the whiz with peanuts, but I’m going to outdo him with Dr. Pepper.

In one of my Pepper applications I needed to recover the power waveform of an acoustical signal, so I squared the waveform resulting in making it all positive instead of bipolar, and doubling the frequency. Then I put on a low pass filter. I know I should have it on the tip of my brain, but I can’t remember the equation for the cross-over frequency of a simple one pole RC filter. If I want the knee (-3db) at 5 Hz and the capacitor is 1 uF, what should my resistor be?

Reply to Butch Shadwell at     (email), 904-223-4510 (fax), 904-223-4465 (v), 3308 Queen Palm Dr., Jacksonville, FL 32250-2328. (http://www.shadtechserv.com) The names of correct respondents may be mentioned in the solution column.


IEEE Region 3 eNewsletter Volume 19 No. 5, December 2004, © 2004 IEEE
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