Thursday, February 6, 2014

Motor changes?

While I was working on the shoulder joint actuators, I was looking at what the motor requirements would end up being for different types of actuation.

The current design makes it easier to implement the shoulder rotate motion but requires a lot of torque for the shoulder arm out and in because the leverage is at the joint with a pulley of diameter 1-1.125 inches, which isn't that much compared to the overall length of the arm.

I know I mentioned that I didn't like using the linear actuator mounted further up on the forearm, I'm starting to lean towards that more. The only problem with that is in order for the shoulder rotate to work, I would need the linear actuator to go through the joint. I'll take another look when I get home.

While I was working on that, I ran some numbers in the eye socket movement as well.

The mechanism is a stepper motor connected to a set of reduction pulleys which is connected to a acme lead screw.

The stepper motor has a holding torque of 76.4 oz-in. At 1200 pps, the torque is still relatively high, maybe 50-60 oz-in. I ran the numbers with 76.4 oz-in but with that number I'm getting roughly 70 lb axial force on the lead screw running at maybe 0.75 in/sec. With the reduction gear I'll probably get a slightly higher torque with a slower ipm.

So, 70 lb is way more than what I need so I'll switch the pulleys around.. Maybe I'll get an even larger pulley if it'll fit to get a faster ipm on the lead screw. I want to aim for something at least 1 in/sec which I think I can accomplish.. I'm just worried about space now. If it doesn't make too much of a difference, I want to use a smaller, lighter motor to reduce the weight.

Alright, time for work.


So, I tried looking for a set of timing pulleys that would increase the speed of my eye socket movement but I can't seem to find a pair that would work. It looks like the set I currently have is the best I can do. The fact that the stepper has a 5mm shaft and that the lead screw is 3/8 makes it a little difficult. What I'll try next is to see if there's a 1/4" acme rod that I could use. Currently, I'm using a 3/8-8 acme rod but I don't remember seeing a 1/4 acme rod with 8 or less TPI... maybe 1/4-10 or 12 but not 8. So that would mean that I would need at least a 2:1 timing pulley to match what I currently have. Hmm.. time to search the web!

Nope. There are some but they are extremely expensive... $90+. Oh well, I guess I'll keep the eye socket as it is.

I thought about the shoulder a little more today and I think that I'll just keep the current setup, or at least work on it a little bit to see the feasibility of it a little bit more before changing it over to the other design that I mentioned earlier.

No comments:

Post a Comment