Thread: Discussion An old newbie... a new oldster... View Single Post
Dec 02, 2012, 03:01 PM
Registered User
United States, OH, Amesville
Joined Nov 2012
148 Posts
More thoughts on MEGA SUE

Well, I was looking through the catalogs for cheap servos that don't weigh much and have lots of torque... and look, it's the holy grail right next to it !!!!

I was looking at the specs for the 9 gram servos I've been using (1.7 kg/cm) and then I looked at 14, 18, etc gram servos. It sorta surprised me that the 18 grams had LESS than 2x the torque of the 9 gram servos. Then it struck me ... why not use TWO 9 gram servos...

Of course objections flooded into my mind - two inexpensive servos wouldn't necessarily have the same response, so that one might want to turn 64 degrees and the other might want to turn 61 degrees and they'd fight each other, until somebody gave up. SO, we need a compliant linkage. I thought of a number of different ones, and then I thought "Ackerman" and so I did some back of the Excel calcs... (shown below, but eminently ignorable).

The idea behind this spread sheet is to put two servos side by side 10 cm apart, hook them with a Y connector, and then put a T-bar between them. The control surface would be hooked to the T-bar. So the question was... if the linkage had 0.5 mm slop, how much angular difference could one have between two servos at about 60 degrees deflection. The chart shows the difference between the the ideal distance (10 cm) and the real distance with the mismatched angles. The allowable difference is 0.05 cm (.5 mm). The graph shows that the answer is 3 to 3.5 degrees difference between the two servos (given the same input). I checked for other angles of deflection and came up with similar answers. They don't have to be precise if there is a bit of compliance in the linkage.