Servo vs. Moter

G’day all. I just purchased the starter kit this last week (for $99 lucky me :slight_smile: ) and am enjoying mucking about with Vex. My plans are already getting ambitious however and I would like to try and attach a simple arm to my squarebot body but I’m unsure about whether I should be using a servo or a motor for this application.

My basic thought about the servo was that it would be better for this application as the arm would only be required to move through less than 180 degrees of motion. Does this sound reasonable or should I be using a “motor”?

Also, as the motors and servo’s have clutches attached to them is it advisable for me to still use limit switches to protect the servo/motor from damage when it hits a hard-stop?

well the Servo only goes 60 degrees and it is probably advisable to go with the servo on this robot (if it is your first). However, you really don’t need to use the limit switches, just watch how far you go with the arm, that arm should not be fast in the first place enough to damage the clutch. Plus, if you use that limit switch you will need some programming to the arm to make it come back a little and not rest on that limit switch.


Thanks for the suggestions.

I thought the servos were good for 100 or so degrees though.

I thought it was 60 degrees, but you may want to check and see if I am wrong. Can anyone else enlighten us to make sure?

You can always use a servo and gear it so that the arm moves farther than the servo should allow then again I would just use a motor that way it would have 360 degrees of rotation.

I’m pretty sure the servos have 90 degrees either direction so that would be 180 total.

no, they have a total 120 degrees

I thought about the gearing issue and had intended on that anyway but the problem I saw was that gearing things such that a small rotation from the motor results in a larger rotation later down the line would also proportionately decrease the available torque (Yikes. Now that’s a run-on sentence if I ever saw one).

I suppose you’re right and I should just use a “motor” rather than a servo.

Vex Robotics Servo Kit - Technical Info
Vex Robotics Description
**Rotation 100 degrees **
Stall Torque 6.5 in-lbs (approximately)
PWM Pulse Width 1-2 ms
Max / Min Voltage 4.4 - 15 Volts (Servo life will be reduced operating outside the Vex Controller range of 5.5 - 9.0 Volts)
Weight 0.11 lbs.
Wiring Black - ground; Orange - (+) power; White - PWM Control signal
Current Draw 20mA to 1.5 A per Servo

Oh, and thanks for not teasing me about “Moter”. Sigh. :slight_smile:

Did you plan for a vertical arm or horizontal arm?
I put a 20x1 hole horizontal on a channel 6 servo on my first squarebot, and it was unusable for much beside hitting things. If you put the servo on channel 5 or 6, as soon as you push the up or down button the arm instantly swings to max.
If put the servo on channel 1 or 4, the servo will want to follow the joystick
handle exactly (thats the point of a servo), which is spring loaded to center.
For these reasons, I find servos hard to use for the applications I have tried so far, but maybe I just need gearing and practice. Now that I have a programming kit and some practice, it seems like I should be able to use a servo like a motor, by interpreting the
RXinput from a stick as request to increment or decrement a stored value, that then goes to the servo. That way center stick can be “stay where you are” and up and down can be “move up or down”
On a related note, it seems possible to use a servo to control a motor into exact positions, ie a servo amplifier. But that is beyond the scope of this topic.