Let's Make Robots!

Servo Standards?

Hey all,

Just graduated from college and started a new job. I finally have time to pursue projects I've always wanted to do.

ANYWHO. I bought a cheap aluminum hexapod frame from the internet that recommended I use Tower Pro MG995 servos. Unfortunately I just found out through online reviews that these servos and their improved models are not so hot, to put it extremely lightly.

I noticed however that a Futuba S3003 I had lying around seems to have a casing with mounting holes that fit flush with the holes in the frame as well.

Are servos in that general size range constructed to have similar mounting setups? Is there a name for the system or class they lie in? Ideally I'd just find better servos to bolt to the frame I already bought.

Bonus points if you can recommend some servos! I'd like to run the hexapod off an Arduino Mega with a driver shield attached.

Thanks!

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When I started planning my hexapod project I looked in to the different servos available. I contacted a European Dynamixel distributor and asked them how much 20 AX18 servos would cost me. They wanted 800€ including shipping, that's a lot more then I wanted to spent on a hobby project/toy. So I went for the cheapest option and bought me the cheapest Chinese servos. I bought 30 servos so I had some spare ones in case I brake a couple of servos.

Oddbot, can you tell us which servos you used on your CNC machined polycarbonate octopod?

I am just using the metal geared servos we use in the DAGU 6DOF robot arm (S06NF STD). They are just an analog servo, nothing special. Just a good balance between quality and price.

If you cannot find them in a local online store and want to buy 20 or more then it might be worth contacting Claudia to get a quote that includes shipping cost.

Unfortunately I cannot tell you what is the best servo to use, I can only tell you what I learned from experience. One thing I have learned from making walking robots is that metal gears, ball bearings and a good quality motor are essential!

Although I still tend to use 9g miniature servos for small robots because of the low cost for a big walking robot metal gears are essential! Analog servos are ok, that's all I've ever used because of the price difference. I did check out a sample digital servo with a coreless motor once. It was fast, lightweight (used aluminium gears), quiet and smooth. The only thing that turned me off buying some was that the corless motor had wimpy brushes that would only last a day or two in a walking robot before they burn out.

This is a big issue because you cannot tell what type of brushes your servo motors use without pulling one apart and possibly damaging it. The cheaper motors just have a thin brass strip that touches the commutator. This will wear through or burn out due to the high current in a few days because walking robots put a lot of load on the servo motors. The good brushes will have a small metal or carbon block on the end that will last a lot longer.

In my octopod Chopsticks I started by trying to use cheap 6kg/cm servos with plastic gears. The worked ok (for a while) for the hips and knees but the thigh servos could not handle the weight and I quickly ended up with broken servos. You can see in the earlier videos that I used a gait that only lifted 1 leg at a time to compensate for the weak servos.

Later I built a new version from CNC machined polycarbonate. This robot used 13Kg/cm servos with brass gears and double ball bearings. This made a big difference to the strength and allowed the robot to carry up to 2Kg load despite weighing about 5KG itself.

Both of these robots use 24 cheap analog servos for the legs plus several small 9g servos for eyes and sensors. Make sure your leg servos are rated to work from 7.4V (2S) LiPo batteries. Although the original Chopsticks robot ran from a 7.2V NiMh sub C battery pack you can't beat a LiPo for the shear amount of current needed to drive all those servos.

Both of my robots use a Spider controller with a HD servo sheild and a high power FET power switch that was originally designed for the Wild Thumper robot chassis. The switch can be controlled directly from the processor which allows the robot to switch power on for the servos after the servo control pins have been initialized. This prevents the servos twitching when the robot is turned on.

 

Wow, what a post!

I hadn't considered servo lifetimes, I wrongly assumed that if the specs checked out performance/power wise they'd last a while. This is pretty eye opening and puts a little more stress in servo selection. I'm not too keen on dropping $600 on servos to find out they burn out after a day, haha.

I may contact Lynxmotion or another company and ask them what their estimated lifetimes are with their hexapods. I was looking at their servo choices for their bots, one would hope that their selection/setup allows for more than a day of operation! 

Another commentor recommended the same board, I'm considering it!

Great robots, by the way! Thanks for the comment!

Here is a photo I took of a dead servo motor that died in less than a week. As you can see the brushes were very flimsy. After sending this photo to the servo factory they did change to a different motor with good brushes. Unfortunately this is not something that can be easily checked.

Walking robots place a big strain on servo motors and gear trains. The load and direction are constantly changing. The servo this motor was from had a nice strong metal gear train and dual bearings on the output shaft. Unfortunately a servo is only as good as it's weakest link.

Hi

To answer you initial question, hobby servos come in a few standard sizes:

- Standard size - aprox. 40x20x36 mm

- Mini size - aprox. 36x15x30 mm

- Micro size - aprox. 28x13x30 mm

- Nano size - aprox. 22x11x20 mm

- Pico size - aprox. 20x9x20 mm

- MAXI size - aprox. 66x30x57 mm

And there are some special sizes and purposes servos, but in a smaller number.

As for MG996R servos, this is a 11kg/cm Standard servo with metal gears. Metal gears are usually used in high power servos, they did not strip very easy but produce much noise. I have 3-4 servo from this type and I'm pretty satisfied with them. One servo though, is not very accurate at finding it's equilibrium. I guess potentiometers used in these servos are not very accurate.

If you have enough money to put into servos, go for digital ones. These are more precise and powerful than analogic ones, but they are priced accordingly.

Or you can go for something like Dynamixel servos, AX-12 for example.

Thanks, that cleared up that mystery for me. I checked some dimensioned drawings for 'standard' servos and confirmed the sizing similarities. Good to know I can plug different servos into this chassis!

I started looking at price tags for servos, and I see why so many people attempted the $10 996Rs at first. I'll probably abandon my plan of choosing servos ridiculously stronger than what I thought I'd need, haha.

I'll still probably avoid the 996Rs as the servos for this project. I'd rather shell out for decent servos and have the option of reusing them for something else in the future.

Thanks for the comment!

Hi, I use MG996R servos on my Bajdupod 996R :) These should be the improved version of the MG995. These servos don't have the best reputation in the RC world. Mainly because they are not very accurate. This is not much of a problem for a hexapod. I chose these servos because they are so cheap and have lots of torque. I bought mine on Ebay and payed less then 5€ per servo. The biggest issue is powering 18 of these power hungry servos. When walking they will draw somewhere between 10 and 20A of current.

Interesting. More than one review has called the 996R the worst servo they've ever used, but I haven't found something that has the claimed torque for the price offered. I might buy one just to play with and test for myself!

I wasn't going to use the S3003's, they don't have the specs required for what I expect/plan, especially considering the frame is aluminum. I was mostly just curious if 'standard' servos all have the same mounting dimensioning. 

Incidentally, that DIY hexapod you linked has the same exact frame I have. It's neat to see a video of it in action. Most of the reviews I've seen for the servos note that the zeroing and the accuracy are butt, which makes me extremely hesitant about using them. They're cheap at $10 a pop, but I'd rather front more money for something better made.

Also, that shield looks sweet. Definitely going to consider that!

Great reply, thanks a lot!