Which one of these servos?

[lurk26]

I was looking around on ebay for some cheap servos and I came across these two :

SG-90
http://cgi.ebay.com/4x-Genuine-SG90-Micro-Servo-9g-7-5g-Fit-Airplane-Heli-/390264985734?pt=Radio_Control_Parts_Accessories&hash=item5add9b1886

ESKY EK2-0508
http://cgi.ebay.com/GENUINE-4x-ESKY-EK2-0508-Micro-Servo-Belt-CP-V2-King-RC-/390264986228?pt=Radio_Control_Parts_Accessories&hash=item5add9b1a74

Since its my first time with servos I don't understand which one is better for me.
I'll be using these servos for steering and other small tasks.
Can these servos do atleast 120degree of controlled rotation?
I might modify them for continuous rotation for use in some lightweight robots but thats secondary.

Another thing that I just can't seem to figure out is, why do we buy servos and then modify them($50 robot) for continuous rotation when you can simply get dc gearhead motors without all the circuitry?

[Joker94]

hi lurk26

If I had my time again new about Hobby King I would have defiantly gone there and not eBay. I have found they have better qualities servos at a better price. it is also a good place to get things like batteries and chargers.

going on the servos you posted i would look at the following 2 that seem suitable

http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10344

http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=662

and the shipping is a decent price too!

These servos have more torque than the ones you posted there for more power and a heavier load. you can use them on small robots but one thing i have heard is difficult to modify to continuous rotation. not that it cannot be done, it is just the small ones are more tedious to modify.

I use many of these servos for sensors and things like that. they can easily handle the load.

Hope this helps

[lurk26]

Thank you Joker94.
What is the rotation angle of those servos you listed? The HXT900 specificaly.

How much time do they generally take to ship to somewhere in US(Boston to be specific)?

I also find their collection of wheels very nice, are they good for use in robotics?

[Joker94]

The rotation of standard servos is 180 degrees. All of these servos i have bought have been app 180 degrees.

The wheels they stock are defiantly suitable for robotics. i have used those types of wheels before and found they worked really well for my application ( a small robot platform).

As for shipping time i wouldn't know to America (I love in Australia). if every thing is in stock i would say 1 - 3 weeks. if items of out of stock it can take as they say 7 - 30 days plus shipping time.

cheers

Joker94

[lurk26]

Last thing I want to know is, how and why are modified servos superior to regular gearhead motors?

Is it just the fact that a motor driver circuit is not required for servos that makes them superior?

[VegaObscura]

People typically like to use servos because, for some odd reason, they are almost always cheaper than a gearmotor with the same torque.  But when it comes to bigger motors for anything ride-on or combat-based, a gearmotor is usually the cheaper way to go.  Supply and demand I suppose.

[Joker94]

servos are simplicity. once a servo is modified, send a signal to it and you can get it to do what you want. not outside circuitry is needed. this is what makes them perfect for small, sensory based robots like the $50 robot.

If i build robots that are more complex i prefer to use geared motors. this is because they are generally more accurate and easier to use when working with encoders and more complicated tasks like mapping. when i say more accurate, modifying servos is not an exact task, no 2 servos will be the same and factors like heat and stress on the servos will throw out the "ideal" center position making them not accurate when in operation.

and another factor that leads people to servos for basic function is prices, servos are generally a lot cheaper than a geared motor that you can find.

[lurk26]

Its weird, a servo is a gearhead motor with built in electronics and it costs cheaper(?)..

I always felt that simple motors with a gearhead would be simpler, just apply voltage and it spins.. Give PWM voltage and you have speed control.

For example this motor : http://www.solarbotics.com/products/gm2/

Would you think its a better option compared to a servo of a similar price?

[VegaObscura]

Keep in mind that you can't directly connect that motor to your microcontroller.  If you only need it to operate forwards and not backwards, you will need a transistor and a resistor to connect it to your microcontroller.  If you need forwards and backwards, you'll need an H-bridge.  If you just connect an output pin of your microcontroller to a lead of the motor, and the other lead to ground, you will quickly kill your controller with a voltage spike.

[Soeren]

Hi,

Its weird, a servo is a gearhead motor with built in electronics and it costs cheaper(?)..

Servo motors are tiny - smaller than the outer part of your thumb.
Go with a regular motor, they're much more powerful.