Thread: Servo help

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I want to buy some servos for head, mouth and eyes movement and I see there are so many types can anyone help me with what model numbers to look for and maybe what they mean if they mean anything.

Hey, right up my alley!
Servos come in different prices, sizes, horn splines, speed, direction, range of movement and torque. Some also have different voltage but most are in the 4.8 to 6 volt range. And there are programmable digital servos vs analog servos.

Your first determination is size. If at all possible stay with standard size servos. All the manufacturers make a standard size. The "standard" servos all match in size and placement of the shaft and mounting holes. Hence the name. If you elect to go with smaller or larger servos you will most likely lock yourself into one model from one manufacturer. That can be bad, such as when we bought jaw servos from GWS and then their quality went in the toilet and we had to redesign our product. Not good....

From there you need to determine how much weight you need to lift and how fast. If you buy high torque servos you trade speed for power. And vice versa. Take a look at the servo chart on the http://hitecrcd.com web site for comparison. You'll need speed for a jaw servo and torque for head movement.

If you are working on a skull you'll probably find that the skull is front heavy and requires a lot of torque to lift. So whatever servo you pick for the nod function will have to be pretty beefy. Servo torque is measured in inch/ounce. So an object that is 6 inches away from the servo shaft and weighs one ounce will take 6 ounces of torque to lift. Standard servos range from 20 to 440 ounces of torque. Expect to spend more as you go up the scale. Prices rise from $7 to $130 each.

Hitec and Futaba are the largest and best quality servo companies. But their servos operate in opposite directions. So you can't just swap one for the other unless you have a control board that allows you to reverse a servo. Such as our BOC board (ad).

As power goes up so does the need for a higher amp power supply. If you are not running on battery you need a regulated power supply with an amperage rating at least twice what your total power drain will be. For 6 low cost servos in a skull figure at least a 3 amp supply at 5 volts.

For our SkeleTron that has 6 - 440 oz/in servos we need 22 amps at 7.4 volts . That's $75 just for the power supply. And we need a separate BEC controller to drop the voltage for the control board. You can't just pump 22 amps through servo control boards. You have to run power separately to the servos. 22 amps requires very hefty wire. We use 12 AWG ultra flexible wire. $3 a foot.

The horns are the little arms that mount on the servos. The shaft size and number of splines differs by manufacturer. So best to just pick Hitec and stay with them.

Range of movement is from 90 to 180 degrees on standard servos. Programmable servos sometimes let you change this. If you are translating circular motion to linear motion most of the useable torque happens in a 90 degree arc. If you can design your mechanics to stay with a 90 range you will be better off.

So stay with standard servos from Hitec. Pick the model by torque and speed needed. Make sure you get much more torque than you think you need. High priced servos often have both speed and torque. They also have better gears, ball bearing support and coreless motors for longevity.

Travel limits are the next concern. You need to make sure the servo doesn't move to the point that it will break itself or parts of your project by moving too far. With digital servos you can program the travel limits in. Known as end points. You'll need the hitec programmer for this. Another $80+ You don't get the end point programming option with the lower cost analog servos unless your control board supports that function. Eg: our Board of Chuckee (BOC for short).

If you are crazy enough to opt for the 300 oz/in or larger servos beware that they can hurt you if you get dangling parts of your body in the works. So no servo driven sex toys.

Last advice. For experimentation try a cheap servo like the Hitec HS475. If it holds up, fine. If not, move up to a beefier model.

Jerry Jewell
SkullTroniX
916 600-2295

and yes I know there is a lot of things I left out. But call me if you want to chat about your project

Thank you for the infomation been trying to get just what you told me online and now I understand thank you so much and I may just give you a call. thanks again.

An' DaddyWoof.. Let me know what/how many servos ya be needin' I get them $1-$7 off list prices, I charge servo cost/shippin' only, no profit..

Reminds me, Jerry, ya wrote couple months ago said ya be needin, like 645's or something beefy, bu' i didnt hear back from ya on specifics...

Capt Jack

The only thing I would add to what Jerry said is that Brookshire software's VSA allows you to reverse servo motion right in the programming.

You highlight the entire track that refers to the servo you are using and you can invert all the commands within your routine.

This makes it easier to swap out servos with different brands if for some reason you need to.

lots of good info and will be taking you all up on your offers.I have a schematic for a servo controller now I'm wondering does any one have a source for low priced electronic parts resistors,caps, IC pixs,etc.my local radio shack is really a cell phone store anymore very little in the way of parts .It was so nice back in the day to just run over and pick up what you needed.

Try here:

All Electronics | Electronic and Electro-Mechanical Parts and Supplies at Discount Prices

It's kind of like what Radio Shack used to be before they became a toy store.

Originally Posted by daddywoofdawg@hotmail.com

lots of good info and will be taking you all up on your offers.I have a schematic for a servo controller now I'm wondering does any one have a source for low priced electronic parts resistors,caps, IC pixs,etc.my local radio shack is really a cell phone store anymore very little in the way of parts .It was so nice back in the day to just run over and pick up what you needed.

Why would you build your own controller when you can buy a micro 8 servo serial controller for as little as $20? Pololu - Pololu Micro Serial Servo Controller (assembled)
Works great with VSA and it's very small so it's easy to incorporate into your props. If you need up to 16 servos or USB, it will run around $40 for a Parallex.

Mark