Servo's and EduBots?

[DanL]

I'd like to mount a servo onto the edubot for testing purposes, but alas, I don't much about servos.

This thread did a nice job explaining servos, but I still want to make sure of a few things.

About the actual connection, as I understand it, servo's don't need to be hooked up to the victors because the pwm outputs on the RC provide the necessary signal and voltage to power them. All you have to do is in the code put in the appropriate number for the pwm output, hook the servo directly to that output, and voila! serv-tastic (I think?)

Now, as for hooking a servo (not the modified edu-servos, but one of the servo's from the full robot kit) up to the EduBot controller, page 7 on the EduBot manual describes the PWM output ports. However, there are still a few things I'm shakey about servo-wise. Mainly, does a servo have two or three wires? I haven't examined them closely yet. If three wires, what are they? Also, where can I find the specs on the servos? Checking the robot documentation, the only thing they said about the servo was it was from Innovation First, but I didn't see them on the IFI documentation.

Anyways, what I'm trying to say is this: I'm going to hook a servo from the kit up to the edu controller, just I'm not sure how to go about doing it. I'm guessing a servo has two leads, and you hook it up to the rightmost and third-from-right pins on the edu pwm output (page 7, edu controller manual). What I'd like to know is if everything I've said here is right, or am I on the path to burn out our servos and possibly more?

[sanddrag]

Servos have three wires. Positive, Negative, and Signal. As for the colors and placement of these wires, you'll have to check with the manufacturer or manual (if it comes with one) The servos that come in the kit are made by this company www.hitecrcd.com Be careful to avoid the new line of digital servos as I don't think these will work with the robot controller.

And you are correct in saying that no Victor is needed.

[ChrisH]

If you use a HiTec servo like the ones in the kit, just make sure the black wire goes to the same pin the black wires on the EduBot motors do. From there it's all a matter of programing.

There was an earlier thread on this, but I figured it was easier to answer than search for it.

[DanL]

Hmm, I searched for servos on this board, but couldn't find anything that answered what I needed, so I posted this.

As for the hitec servos, what's the exact model number so I can find the specs (can't find it in the manual)? The Edubot manual says the middle PWM pin is +Battery, so that would be +9 volts, or whatever the current voltage. If the servo's in the kit aren't designed for that, well thats gonna get us some smoke.

[DanL]

I finally got access to the actual servo, so I was able to look up the specs (as well as realize the edubattery is 7.2v, not 9v... oops). Anyways, the specs for the servo's say they're rated between 4.8v and 6v, so I need to drop the voltage a bit. The problem is I've never actually done these calculations, so I need some help.

Ohm's law solved for resistance is R = E / I, or Resistance = Voltage / Amperage. Now, I have a few questions... do I use the voltage supplied, or the voltage I want to have? The latter seems kinda silly, so I'm guessing E is the voltage supplied. As for amerage, the servo specs list two opperating currents: 7.7mA @ 6v for "Idle Current" and 180mA @ 6v for "Running Current". One gets a resistance of 935 ohms, and the other gets a resistance of 40 ohms. So the question is which do I use? I'm guessing I use the higher amerage, because thats the limit and the motor can go less if it need to, but like I said, I've never done this, so all of this is a complete guess.

What I'm basically asking is to hook the servo's up to the 7.2 volt EduBot pwm ports, is the resistor I need to use 40ohm?

[Greg Perkins]

no need to worry about votage, i think the servos have a built in votage regulator, i may be mistaken but i think it will be ok to run them directly off the controller

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[Greg Perkins]

actually since you are puttign the servos on the controller the controller has a voltage regulator, and will only distribute 6 volts to serovs and for the motors, 7.2


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