Servo Brands: Metal gears, dual bearings

Bill Chennault

All--

What are the quality, all metal gear, dual bearing digital servo brands? (I'm a servo newbie.)

Thanks!

--Bill

erco

I'll leave specific brand recommendations to more informed people, but I like this page for misc info on various servo types: http://www.rchelimag.com/pages/howto.php?howto=1&page=3

Not surprising that servos with metal gears are stronger and impact resistant than nylon gear servos, but they wear faster, too.

Bill Chennault

erco--

I read the entire piece. I learned a lot. Thank you very much.

Hopefully, others will chime in regarding their favorite brands. I am thinking of a Prop-driven hexapod and would like to begin experimenting with quality servos.

--Bill

schill

There are a number of pretty good servos out there. For a number of years now I've been sticking pretty much to Hitec except for some really small cheap ones. One thing to bear in mind is that the splines on different brands of servos are not the same. You can't stick Futaba compatible arms on Hitec servos, for example. So it can be a good idea to stick with one brand or at least compatible brands.

I've used digital servos for a few applications, but for a lot of applications there really isn't a need to spend the extra money. It can be interesting to adjust the parameters, though.

Hexapods, or any legged robot for that matter, usually require higher torque servos so you're already seeing the cost go up without adding digital.

wrightme

Slightly off topic, but some servos are more capable of "indexing" by using different arms in the same location. In other words, if your "center" position has an arm just slightly off where you want it, reposition the servo arm on the splines 90, 180, or 270 degrees. It isn't exactly 90 degrees.


My personal preference from the RC world is Futaba or Hitec, with a slight edge to hitec for price now and then.



http://www.servocity.com/


And for servo wiring, http://www.hansenhobbies.com/products/

W9GFO

JR, Futaba and Hitec are my preferences.

Rich H

Duane Degn

I've also thought about making a hexapod bot. If each leg has 3 dof then I'd need 18 servos. That's a lot.

Is there a good reason not to use these cheap servos from HobbyKing?

I have a few of these servos but I haven't used them much. They do seem to be easy to convert to continuous rotation. The metal peg (used to limit its rotation) on the largest gear is easily pulled out with pliers.

Duane

schill

For legged robots, the torque the servo provides becomes important.

The Hobbyking servos have a rated torque of 10 kg (which is the wrong unit - I assume they mean kg-cm but you can't tell for sure [1]).

You also need to know what voltage was supplied to the servo when the torque was measured. Usually you will see values for 4.8v and 6.0v (except for specialty servos which may have higher allowable voltages and some servos can't handle 6.0v).

If the Hobbyking servos are 10 kg-cm at 6.0v, then at least looking at that spec they are somewhat comparable to the Hitec HS-645MG (which runs $32.99 each at www.servocity.com and $91.08 for three at www.towerhobbies.com. It's also worth looking at www.lynxmotion.com - when you buy a set of 6 servos (of particular types) you get some nice metal horns for free (but the servos cost more)).

I only have a little experience with legged robots but for a lot of designs the HS-645MG seems to be a good recommendation - heavier robots will need something bigger. For lighter robots, something like an HS-485 may be ok (I've used both for joints in robots). It really comes down to how heavy the robot is going to be and how long the limbs are. I'd suggest looking at what other people are doing before making your decision. Investing in 18 servos (even cheap ones) isn't cheap. www.lynxmotion.com has a number of kits and you can see what servos they recommend for those kits.

Servocity has nice tables you can use to compare Hitec and Futaba servos:

http://www.servocity.com/html/hitec_servos.html
http://www.servocity.com/html/futaba_servos.html

[1] A lot of servo references get the units wrong. Even Hitec's site shows torque as kg/cm which should be kg-cm.

Duane Degn

Thanks for the information schill.

I think I'll make some of my own measurements with the cheap servos.

Your right that 18 cheap servos are expensive. But if they will work, I'd rather spend $180 than $546. Actually if I need to spend the latter, the robot wont be made. I just don't want it that much.

I'll make sure and share the results of my tests on the cheap servos.

schill

Another thing to note... Nothing says you have to use the same servo for every joint. For a hexapod, you may need higher torque servos at the shoulder/hip than you need lower down on the legs. For arms, you can typically get away with lower torques as you move further out from the shoulder. With legs, it can depend more on the configuration of the legs, how vertical the segments are, etc.

Al Booth

@Duane - Paul K. had some good words to say about the Hobby King 9g micro servos. (HXT900) He built a small hexapod which you can link to a YouTube video in this thread
http://forums.parallax.com/showthread.php?126747-Propeller-Based-IK-Hexapod-Project

At $2.69 each, buying 18 for a hexapod is not as costly.
Al

Bill Chennault

schill and Al and Duane and Rich and wrightme and erco and All--

This is a very informative thread for me. I know nothing about servos other than what I've learned from "What's a Microcontroller?" and playing with various OBEX pieces of code. (That's fun stuff!)

schill, I am fascinated by Polulu's Maestro software and their "Power HD" line of servos. It seems like Maestro would make experimentation and configuration very easy. I GUESS Maestro and just about any digital servo would be compatible.

But, there is a lot of trial and error (experimentation!) between me and a functional hexapod. My thinking is based on a Propeller on a Parallax wireless serial cable talking to a desktop.

--Bill

erco

I have a Micro Maestro and it is very nice for 6 servos. EASY to use & write scripts .

Bill Chennault

erco--

Thanks for the Maestro feedback. I am sure I will try it.

--Bill

Paul K.

I've tried a ton of servos and I'll just give my opinion on what Ive experienced.

If your going with a full sized hex and if its within your budget go with the hitec hs-645mg. I've had the weaker servo in the coxa & femur joints and they eventually will blow out ex: hs-422 & hs-485 models. Usually a tooth or two will go and they will. And to replace the gearing you looking at 10 bucks or so and with that included in the servo cost you could of had a hs-645.
The cheaper servos from hobbyking that are comparable in size and strength will work but there loud as hell and the are very glitchy. I'm assuming a cheap pot inside.

If you need more powerful servos jump to the ax-12's. With there cost to strength best buy out there.

Here's a link with a video of my Hex with all 645's
http://forums.parallax.com/showthread.php?126747-Propeller-Based-IK-Hexapod-Project
My software for the prop is posted and a schematic of my board in the word doc.

With the micro servos the hextronix hxt-500 from hobbyking are the best buy. I havn't tried the 900's a little bigger twice the strength. But the 500's compare to the hitec hs-55 which are so weak. I've stripped dozens of these servos before I gave up on them. The hxt-500 have gears inside that are 2to3X as thick as the hs-55. These servos are both kinda glitchy. But for 3 bucks a piece a steal.

A video of my micro-hex wasp project.
http://www.youtube.com/user/QuantumRoboticsInc#p/a/u/1/yZpNqUuTJNI
This bot uses the hxt500's
Another vid of the micro hex before I made the wasp mods.
http://www.youtube.com/watch?v=eddcCu5X9iM&feature=related

Paul

Spiral_72

I'm a big fan of Hitec servos. They seem like a very quality piece for the price. As an added bonus there are a huge number of accessories available for them (Lynxmotion and other vendors). I've also used the cheap ones, Bluebird, Hextronic, etc. they are junk quite honestly, but do the job just fine especially on a budget..... I'll continue to use the "expendable" junk servos for some things too.

JR's are equally nice as are Futaba and Airtronics, but I'd prefer the Hitec over em all for no specific reason..... personal preference I suppose.

Bill Chennault

All--

It looks like Hitec is going to be near the top of my list.

Has anyone tried the Power HD servos from Polulu?

--Bill

vanmunch

I've always used Hitec HS-645MG for my Hexapods for the reasons the people mentioned above. You'll never say to yourself, "I wish I didn't have that much power."

Paul K. wrote: »

If you need more powerful servos jump to the ax-12's. With there cost to strength best buy out there

Paul

I've been really curious about these, how do you control them? Do you have any sample code? Thanks!

Duane Degn

The AX-12s use a serial protocol. They are really cool "servos." I think they are usually called robot controllers or something like that.

There are several code examples. You might want to search for smartmouse. Someone (Radtronic?) wrote some code to control a robot arm with a mouse. I modified his code to control a robot arm with my RC transmitter. These examples use the Propeller as a controller.

You might want to include the key word "Dynamixel" in your search.

The communication protocol is explained well in the AX-12's manual (available on the web). One of the cool things about these controllers is you can send inquiries to the AX-12s about position, voltage, temperature, torque, speed and other parameters and the controller will send the information back. I used the torque information to make a force feedback claw.

GeorgeCollins

Someone mentioned that digital servos can be calibrated, but that is not really why you would want them for your application. If you are using a micro-controller like a prop it is very easy to tune the position of servos in code. The real advantage of digital servos is that they are usually faster, have less overshoot, and have much better holding torque. Overshoot is when your servo moves quickly and goes past the point you want it to reach. Holding torque is how firmly a servo stays in place when it is not moving at all. You can really see the difference when you compare a hitec HS-5485 vs HS-485. On paper the servos are very close in performance, but when you apply power to an HS-5485 it doesn't want to move like it is stuck still. Also digital servos tend to be more quiet and more precise, but that is really a generalization. There are very good analog servos (hitec) and cheap digital ones.

If you are making a hexapod you do not need the greatest servos because you have a very stable structure of alternating tripods and a lot of legs lifting together. Depending on how much weight you want to carry you can probably do fine with HS-485s if you want. If you were making a biped you would want very fast, very accurate servos for balance. As has been mentioned, the AX-12 is a very good robot servo. My new favorite servo is the HS-5685, which is small, light, fast, and strong-- but a little more expensive.

I put together what I have learned about servos for robots here, with more notes on individual servos I have tried:
http://www.backyardrobots.com/Servo/servoguide.shtml

Good luck! You will have a lot of fun.