So I have started working on a planetary gear head based servo for the Servo Erector Set. This idea has been haunting me for a long time, but I couldn’t find a suitable motor for the application. Now I have a very powerful motor that wil do nicely. It’s a 5watt motor with a manufactures stall torque rating of 25kg-cm. I don’t think this value is an apple to apples comparison to a hobby servos stall torque rating though… Unfortunately for some I am putting off the HS-805 bracket project in lieu of this one for now. It should provide a much smaller, more powerful, solution to a stronger axis.
The idea is that you can use a gearhead and a pot as the servo motor. Basically, an H-bridge will run the gearhead motor and be controller by a small controler board. Which is basically all a servo is. A geared motor, a speed controller and a board for the signal and pot reading.
Plus, some gearheads can be overvolted if the H-bridge will handle it so they can have alot more torque than a servo. And they don’t have end limits so it’s abilities is dependant on the Pot used.
Not that I don’t appreciate the value of having a custom high-power servo, but I really want those mega brackets. Any idea of how far they’re being pushed back? Also, any plans on making this custom servo into a mega scale package so it can be more easily integrated into projects? It almost looks from the picture the gearhead would fit into the right size casing.
I don’t know how long it will push the 1/4 scale bracket back yet. The capabilities of this custome servo far surpass the HS-705 or HS-805, so I just can’t help myself. This motor has 15,000 rpm with 231:1 gear reduction. A stall current of 3.2 amps, and an all metal planetary geartrain. The controller I will make will come from the open servo project. So we’re talking I2C, serial, or the normal servo pulse train, and a MOSFET H-bridge. We will use a pot in the cheaper version, and another version will use a new product for the feedback that will remain a secret for now. Give me a little time to get the parts designed. As soon as I have the prototype mechanics assembled I will post images on the forum.
Too early to tell. Should be much less than those HS-5955’s that it should be able to easily outperform. Also the thing will already have the servo erector set hole pattern built in, so there are no additional bracket costs…
After hearing that I guess I can wait a bit on the Mega brackets. Good luck and hope you get the design finished quickly. 8) I can pretty much say that I want one too now.
How much more torque can the gear head motor produce compared to a similar sized servo? What is the gearing ratio of the typical standard servo? Looking at this, I think it might be very simple to have a servo control a much larger reversable DC motor such as one in an 18v reversable drill, or even a 12v winch such as are attached to the front bumpers of 4X4 trucks.
In converting servo control boards to drive larger motors, the limiting factor is the current-handling capacity of the H-bridge that is used to drive the motor. In a standard servo, the motor is fairly small, and the H-bridge is sized accordingly. Try to jam more than its maximum rated current through it to the motor, and <Foof!> Larger quarter-scale servos have larger H-bridges, sometimes (often?) composed of individual switching transistors, as seen here in an earlier “monster servo” project (the control board of which was essentially just quarter-scale servo guts, driving a gearhead motor and reading a large pot for feedback, I believe). Quarter-scale and larger servos are pretty expensive though, and by the time you get yourself the guts to rip apart, the gearhead motor, the feedback pot, and the mechanics worked out, you’re probably looking at a cost comparable to building your own smart interface and driver board, which would probably be more versatile than a simple dumb servo board anyway.
That’s pretty much what it sounds like Jim is doing on this project - making a smart controller board with feedback, capable of acting like an ordinary servo while driving much larger motors, while incorporating a number of other smart features that you don’t see in off-the-shelf products.
Sounds very exciting, and I’m eager to see the outcome - it may well be exactly the sort of thing that I’ve been looking for for a couple of my projects for quite some time.
Maybe this comparison is a little better. The smaller motor is from a cheap servo, but it’s an accurate representation of the size of motor used in all standard size servos. I haven’t counted the teeth in a servo to calculate the reduction but I believe it’s in the 200:1 ish range. So I believe this planetary gearhead motor will far surpass a servo in it’s capability. We will see…
I have 4 of these gearhead motors and I can say, they are impressive little things. I haven’t tested them in any application but the torque specs on them are impressive for their size. And they are very shiney. And I believe the model that Jim has is alot more torque than mine.
SICKKKKKKK
Ooh, shiny! 
