My design is going to be require that the elbow bend both directions to about 180 degrees, the max possible. The problem I’m having may be solved by just throwing a higher torque servo at it, I ordered the 133oz/in std servo today.
However, could anyone suggest a passive load reduction method similar to the tension springs? These don’t seem to work well for this design.
Thanks,
My suggestion is to use a rack and pinion.
Place a “large” pinion on the arm joint in place of the servo.
Place the “rack” inside the “upper” arm.
Use the servo to move the rack back and forth. You can either use the servo horn with a link to the rack, or better yet, put another pinion on the servo and drive the rack directly!
This will not only move the servo (weight) away from the joint into the upper arm, a rack/pinion will increase the torque tremendously.
Also, the servo will not have to work as hard at keeping the “lower” arm in position.
With the right gear ratio, you should be able to get 180 degress easily. If both the joint pinion and the servo pinion are the same, the 180 degrees from the servo pinion will translate directly (with increased torque) to the joint pinion.
You could even gear up the pinion on the joint and get better than 180 degrees if you wanted…
I think this is a nice solution.
If you have the same amount of movement on both ends of a purely mechanical linkage there is no mechanical advantage.
Moving the servo closer to the base of the arm is a good idea though. Heck, move it far enough back and it may serve as a counterweight.
Well, after getting the highest-torque (non-digital) servo that Lynxmotion sells, it seems to be okay. Also getting a regulated wall power pack helped too to alleviate errors due to lower battery power and wasting $ on new batteries.
The only issues with it are:
I still haven’t found a “neutral” position for the arm to hang when not in motion (which will be 90% of it’s life). Still working on a solution.
Due to #1, the servo gets very warm after a few minutes of constant position. I haven’t figured out yet if this has leveled off or will continue to heat up until it fails.
Thanks to everyone for the suggestions.
If you must keep the servos running during that 90% of the time, having them lie on the ground flat will be the least amount of work for them.
If they must be off the ground, then having them stand perpendicular to the ground (it would look like an “I”), is the best bet.
If you don’t actually need the servos running at the time, then your best bet is to just cut their power.
How you do that would depend on the servos.
If they’re analog, you can simply send them a command to stop.
If they’re digital, you’ll have to directly cut their power source.
You can do this a number of ways.
The simplest would probably be to use the BotBoard with a microcontroller.
Have the SSC-32 do all the servo positioning, and put a MOSFET or relay in series with the battery —> SSC-32 connection.
Have your microcontroller turn off this relay during down-time.
