Problems with inertia

Ariel

Hello everybody.
I've been studying a lot this days and i couldn't continue with my project. But now i can continue with it again!! [noparse]:)[/noparse]

I'm doing a robotic arm. It's 5R(standard servos) and has a gripper with 2 continuous servos to move the fingers.

I'm having some big problems in here...

The first one is the inertia. The Futaba servos I'm using don't have enough power to move the arm. As you can see in the picture. Most of the parts are made with aluminum using screws to hold the parts together. This screws are adding too much weight to the arm. So I'm designing new fiber glass parts to use less screws and get the weight lower. Specially in the gripper, because it weight affects all the motors in the arm and is 90cm from the shoulder of the arm.
Lowering these weights would make possible to have smaller counterweights. All links are in balance. With these counterweights the motors don't need to do any effort to keep the links in an horizontal position.
For more info, look at the posted pictures.
Any ideas how to use a counterweight that doesn't necessary add weight to the machinery? Because with these lead counterweight I'm adding to much weight. Could i use something like a spring? Any ideas?

The second problem is that i don't have enough 5k resistors to connect pull-up resistors to all the motors.
according to the picture... can i connect one pull-up resistor to all the motors? or I'll need to connect one resistor for each motor?

allanlane5

I don't see any pictures...

Ariel

Ups!, no pictures appeared.
here they are:

allanlane5

If they're standard servo motors, with standard servo motor controls, you have a Power wire, a Ground wire, and a Signal wire.

The Power wire shouldn't have a resistor in it -- you want the servo to get all the current/power it needs. The Signal wire doesn't take that much current (maybe 1 mA or less), and *IT* shouldn't need a pull-up resistor either, the BS2 will be 'driving' the signal both ways.

One big gotcha for new people is all the grounds DO need to be connected together -- the grounds only, the power line for the BS2 and the Servos CAN be separate.

allanlane5

One other issue is that when all joints are "fully extended" is going to be the worst case in terms of the counter-balances.

Carl Hayes

Well, in many cases you could use springs (instead of weights) to counterbalance the moment of the cantilevered weight of the extended arm.

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· -- Carl, nn5i@arrl.net

Bruce Bates

In the end analysis, if you find that mechanical advantage would be helpful, these gears and sprockets will fit on the shaft-hub of any standard servo, affording that mechanical advantage easily. Here is the link: http://www.servocity.com/html/gears___sprockets.html

Regards,

Bruce Bates

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When all else fails, try inserting a new battery.

Ariel

thank you for your very fast answear.

Project5k

they make bigger servos.. look into something like a futaba jumbo steering servo.. lots of power... might help with the strength of your movements.

erco

WOW! That's a full-sized (giant!) arm driven with standard servos. Nice construction job, Ariel. My concern is, even beyond the inertia concerns, the lifting capacity of the robot will be small since all the pivoting arms are so long. At full extension, even an ounce of payload will exert a LOT of torque on your servos. The center-most servos will really take a beating, and maybe strip the gears. It may shake and quiver a lot as it struggles to hold position.

But if you keep your loads light, it might work quite nicely. Let us know how your experiment goes!

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·"If you build it, they will come."

Ariel

erco: your right
that's a problem i thought about. And thats the reason why I'm using counterweights. Because with the counterweights, the weight of the lifting objects are the only weight that requiere torque on the machinery since the hole machinery is counterweighted.

Now, with all this counterweights i added to much weight. And started the innertia problem.

fahhhhh.....

Why does it have to be sooOOOoo complicated!!· =>
I supose is part of the idea right?

Beau Schwabe

Ariel,

"My concern is, even beyond the inertia concerns, the lifting capacity of the robot will be small since all the pivoting arms are so long....·· ....that's a problem i thought about. And thats the reason why I'm using counterweights."

Without implementing·dynamic counter weight to the system, the counter weights·affect will only be valid for static positions.· i.e. When you extend and reach your own arm out way in front of you and try to pick something up, your brain automatically extends your opposite leg to prevent you from falling over.· Try doing the same·task without using your leg and suddenly it becomes much more difficult.· Take a look at how large cranes accomplish lifting very heavy objects without falling over for some ideas.


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

SN96

You can use helper springs also to help with the weight.

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Mike



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Phil Pilgrim (PhiPi)

I agree with Carl and SN96: use springs, not counterweights, in order to keep the overall weight down. Also, those gripper motors add a lot of weight at the end of the arm (i.e. the worst spot for a lot of weight). Why not put them in the base and run cables to the grippers to open and close them?

-Phil

erco

Ariel: If you really want to use servos, consider these external large-diameter gearboxes. Should last a while.

http://www.servocity.com/html/servo_power_gearboxes.html

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·"If you build it, they will come."

Ariel

how's that spring idea?
how do i use them? could you make a picture about it?


Thanks everyone!