How to customize a servo?
 

Hey guys.

I have an application that needs a servo, but it seems like all servos are predesigned for steering or simple shifting. This servo I need for a more complex shifting mechanism.

This is a rough schematic of the operation I need.

First click of shifting button: servo rotates 1/4 a rotation
Second click of button: servo rotates another quarter of a rotation
Repeat another five times.

So how can I get a servo to do full rotations and also how do I program the servo to perform these operations?

I was thinking of just getting a small motor and hooking it up to an esc that can be programmed. But I have no idea what sort of esc would do this.

Any ideas?

This is will work with maual application only
http://www.hobbycity.com/hobbycity/s...idProduct=8296

It's all about the signal pulsewidth and feedback pot in the servo itself. Servos generally have something between 90* and 180* rotation because that is the range for most potentiometers. You could get multi-rotation by using a multi-turn pot. However, the way you want the servo to operate will have some difficulties.

A servo will travel to an absolute position set by the pulsewidth, not by a relative position. So, your radio would have to have a switch with 10 positions (two clicks X 5 times). The steering and throttle are continually variable, but ch3 is usually 2 position, or at most, 3 position. If you didn't need a steering or throttle control, you could replace the pot for that control with a rotary switch that had resistor values soldered on that would give you the steps you want.

What are you trying to do exactly?
If turns 1/4 then only need 4 clicks to make full turn.

Sounds like something the robot guys could answer.

@Bondonutz - thanks, but I would need it to be via receiver for remote operation

@BrianG - I did not know that about the third channel. Do you know any with 3 positions?

@coolhandcountry - It's for a semi automatic 7 speed transmission (doing it just for fun). It's the same concept as the E-Maxx transmission with the paddle shift, however since there are more than two speeds there will need to be 3 "paddles" and thus three forks to shift in and out of gears. The key design is the ratcheting drum that would sit just outside of the transmission. This drum is basically a cylinder with slots etched with a helix-type engraving that guides the shifting forks as you spin the drum. For my current and rough design of the ratcheting drum it requires a 1/4 to guide the shifting fork from one speed to another.

@JThiessen - where can I find robot guys?

Most radios I've seen use a simple switch for CH3 and is usually 2 position. However, the Spektrum DX3r (and I only say this model because I have it and know it can do this) is able to set this switch to be a 2 position, 3 position (forward, back, and center), or linear (makes the servo move the longer you hold the button) settings. The switch is not a toggle, but rather a slide type that is momentary (returns to center when you let go).

Whether or not your particular radio has that level of CH3 functionality is up to you to figure out. But I have a feeling you will need a channel that operates more like the throttle or steering. Meaning; the servo moves in proportion to the wheel/trigger position. CH3 on the DX3r is more "software" driven rather than mechanical like the steering/throttle. Then, do like I describe above and replace the trigger/wheel with a rotary switch that has specifically chosen resistors. This would make it so that each position of the rotary switch would move the servo a set amount. The resistors needed would depend on the existing pot's value and some trial and error, not to mention you wouldn't have much tweakability (EPA and trim) other than playing with those resistor values to get the servo to move right.

Since I assume you need your current steering and throttle controls for their intended purpose, I think you'll need a stick radio so you have more channels to play with: one stick can be throttle/steering, the other can be the modified channel for your tranny.

I know you say this is for fun, but after you get it all working, I wonder if the complexity and weight of the final product will be more work than it's worth.

Oh yes, a stick radio will be essential! Thanks for your input.