Thanks (and alittle help,too..)

NWUpgrades

First, I wanted to say a very heartfelt Thank you to both Ken and Dave.

Now, My question. How much torque does Parallax's standard and continuous rotatation servos have? I am in the midst of designing my daughters robotic leg and I want to make sure I have enough torque to pull up on some springs. No specs on the springs yet, as I want to verify the torque first.

James Long

NW,

I hate to say it, but it is likely the servo will not have enough torque to do what you are asking. Typically RC servos provide a small amount of torque.

The average torque for a servo from Futaba in that size is 44-99 oz in. for 5 volts. (57-124 at 6 volts). Only Parallax can state the exact torque, for they know the model being used.

I'm guessing that you probably need more torque for that, based on the project.

If you require more torque.......an AX-12 or other dynamixel servo would probably be required. Unfortunately they run 12 volts or better. Also, their interface is pretty complex.

James L

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
James L
Partner/Designer

Lil Brother SMT Assembly Services

Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.

NWUpgrades

Thanks for the input. That is what I thought. I need to keep the voltage WAY down due to the complexity and the fact that I do not want to weigh the leg down too much with batteries. I am thinking stepper motors may be a more reasonable approach. Any thoughts on that?

James Long

NWUpgrades said...
Thanks for the input. That is what I thought. I need to keep the voltage WAY down due to the complexity and the fact that I do not want to weigh the leg down too much with batteries. I am thinking stepper motors may be a more reasonable approach. Any thoughts on that?

NW,

The main problem here is the torque required. I'm not sure of your actual approach, but torque requires power (watts). To get enough power for a robotic leg, some kind of expensive battery and/or servos are going to be the case, or pure leverage (servos do not have a large enough throw for any significant travel with any real leverage).

This is the drawback to any robotic device......either large voltage at reasonable amperage, or small voltage at incredible amperage. I wish there was an easy answer to the problem, but I have found none.

As for stepper motors, if you put them with a gear head (gear reduction......more weight) you may be able to get the torque out, but then speed is sacrificed. Pound for pound servos are usually more efficient with respect to power/torque.

If you want raw power from a small controllable device, I suggest looking at brush-less motors. They have the best power to weight ratio available that I know of. (I'm not an expert, but I do get around). If you could do some kind of adaptation for linear travel, they would provide a good medium.

RC brush-less motors are available, but most of those are a continuous rotation type devices. They require a controller, and the setups are pricey. They would be relatively easy to control with most micro-controllers.

I suggest brush-less motors over steppers because of the controllers available. Also, most brush-less systems run on a nominal voltage close to what you are attempting to work with.

I know that is little help, but the best I've come up with so far. Keep us apprised of the progress,

James L

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
James L
Partner/Designer

Lil Brother SMT Assembly Services

Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.

James Long

NW,

Just as a follow up....I thought I would look to see if anyone made a brushless servo.....and they do.

cgi.ebay.com/Futaba-BLS152-Brushless-Super-High-Torque-Air-Servo-BLS_W0QQitemZ220472658924QQcmdZViewItemQQptZRadio_Control_Parts_Accessories?hash=item335531cbec&_trksid=p3286.c0.m14

This is an ebay listing......but shows a pretty strong servo...

James L

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
James L
Partner/Designer

Lil Brother SMT Assembly Services

Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.

NWUpgrades

Thanks again for the input. I tookalook at that servo and it looks like it might do the trick. The price is a bit steep and· I would need 2 of them to start with. Right now that is not in the budget. When I mentioned the steppers, my thought was for using some gears with them. Since I have a machine shop owner that has volunteered his services to this project, that may be our most practical approach. Either way, we currently have somewhat of a design layed out, with the exception being the motors we will use. I am working on a prototype out of wood. Will post pics when that is completed.

Have you ever heard of springs that produce power? I found this robotic foot/ankle designed by MIT·and the springs provide some of the power. My idea is to have springs in the foot, so that would be an added bonus if I could figure it out.

http://web.mit.edu/newsoffice/2007/robot-ankle-0723.html

James Long

NW,

It looks as if the spring provides some of the action the ankle naturally provides. I would think it would help with the push off the ball of the foot does when ending the step with that respective leg. I think the motor would rock the foot some with the spring providing the motion once the body has past the leg.

I do hope you have success with the project. I see very few which I consider important, like the challenge you have taken.

I hope to follow your progress and give input where I have knowledge. I would love to see you succeed.

James L

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
James L
Partner/Designer

Lil Brother SMT Assembly Services

Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.