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Author Topic: Servo & power supply suggestions  (Read 655 times)

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Offline DrBwtsTopic starter

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Servo & power supply suggestions
« on: May 28, 2013, 07:13:13 AM »
Hi,

After years of contemplating I am finally going to make the leap into making a hexapod based creature. Before I go any further I would like advice on servos & power supplies.

Ideally I would like the hexapod to carry its own power supply so I guess this is a limiting factor?

Power is more of an issue over speed as I intend to build a fairly elaborate body on the chassis although probably no bigger than 50 cm (yes I'm metric) at the outside.

Thanks for any help you can affoard

Nic

Offline jwatte

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Re: Servo & power supply suggestions
« Reply #1 on: May 28, 2013, 11:31:22 AM »
If you carry the power, then you're likely to want LiPo batteries, as they have the highest power to weight ratio. LiFePO4 batteries come second, and have a much longer lifetime, so if this is something you have in service every day, that might be a more economical choice.

Servos: For a bot as big as 50 cm across, I would recommend Dynamixel MX-64 servos. They have the best torque-to-price ratio in that class. The MX-28 are somewhat cheaper, but have significantly less torque. The MX-106 are somewhat more powerful, but a lot more expensive. The cheaper AX series is likely not powerful enough to support that large a span (although you might be able to do something with the AX-18 -- could be worth running the numbers on that.)

An alternative might be the Herkulex DRS-0201 servos. Again, run the numbers on those before you commit, because they may not be sufficient for your load. Also note that any servo of any brand should be designed to carry 1/4 or less of the rated stall torque in "normal" situations, which in the case of a hexapod means 3 legs on the ground.

Offline DrBwtsTopic starter

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Re: Servo & power supply suggestions
« Reply #2 on: May 28, 2013, 03:08:33 PM »
Thanks for the info been looking at Hitech HS645 servos ever used them?

Offline jwatte

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Re: Servo & power supply suggestions
« Reply #3 on: May 29, 2013, 12:57:27 PM »
Those servos have two problems for this application:

1) 133 ozin stall torque does not allow you to carry the load you need over the span you need.
2) There is no feedback or digital control of these servos, so you're not going to be able to do things like read back position, or tune your control loops.

To understand 1), consider the span of 50 cm. That's about 20 inches. This means that the "knee" servo, worst case, needs to lift a 10 inch lever. This leaves you at a stall load of 13.3 ounces. The design load should be 1/4 to 1/5 of this, so about 4 ounces. Multiply by three (because you have three legs on the ground) and your total robot weight cannot be more than 12 ounces. That's not going to work well.

If you don't have a "knee" servo, just use two degrees of freedom, then you can carry a lot more load, because the load will be held by the bearings of the "ankle" servos, but then you will not be able to reach specific positions with the legs; instead your walking gait will have to slip against the ground.

Offline DrBwtsTopic starter

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Re: Servo & power supply suggestions
« Reply #4 on: June 13, 2013, 11:43:37 AM »
Thanks that's put my grandiose plans into perspective.

OK for the sake of cost and learning experience I'm going to go for a smaller design, something the cheaper servos can take.

 :)

Offline jwatte

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Re: Servo & power supply suggestions
« Reply #5 on: June 13, 2013, 12:25:02 PM »
No, No, the way to do it is to DIVE RIGHT IN!

Order eighteen MX-106 servos right now! At 1,400 ozin, they have pretty decent torque.

:-)

 


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