Society of Robots - Robot Forum

Mechanics and Construction => Mechanics and Construction => Topic started by: reefat on August 31, 2008, 04:54:29 AM

Title: Confused about Cont. Rotation Servos!
Post by: reefat on August 31, 2008, 04:54:29 AM

Hi,
I am building a Differential Drive based Small Robot. I need to buy 2 Continuous Rotation servos and found two:
    Surplus Hitec HS-5475HB (http://www.pololu.com/catalog/product/695)
    Hitec HSR-1425 CR (http://www.servocity.com/html/hsr-1425cr__continuous_rotatio.html)

The only difference I found is, HS-5475HB (61.1 oz-in~76.4 oz-in) has better torque than HSR-1425 CR (38.8 oz-in~42 oz-in). Do you find any other advantage/disadvantage of any of these two servos. Any advice which one should I buy?

Besides, I am little confused about the Cont. Rot. Servos. Are those internal circuits removed from them to make them Cont. Rot.? If so, then do I need any Motor/Servo Driver (like H-Bridge) to drive the servo? Look at  HS-5475HB, it says, "This servo contains no control electronics and no feedback potentiometer." So without the internal circuit, how the servo runs?

Title: Re: Confused about Cont. Rotation Servos!
Post by: Ro-Bot-X on August 31, 2008, 06:45:14 AM

A servo with the electronics removed is just a DC motor with a gearbox. You need an H-bridge circuit to drive it from your microcontroller.

This is the case for the HS-5475HB servo from Pololu. This servo has karbonite gears and more torque and a little more speed (54 RPM compared to the other servo's 52 RPM at 6V). You still need to clip the limitation nub from the spline gear. I am not sure if I shoud recommend this servo or not, since it does not have enough speed for a fast robot but it has a little more strenght than a regular servo. But you can use the empty space inside to add an encoder.

The HSR-1425 CR servo has the electronics inside, it is allready modified for continuous rotation, the electronics are modified to acheave fully proportional speed (verry good!). This is perfect for a small robot, based on the $50 robot design and code. You don't need to do any modifications on this one.

Title: Re: Confused about Cont. Rotation Servos!
Post by: reefat on August 31, 2008, 07:20:17 AM

@Ro-Bot-X . Thanks a lot for this advice. I am going to order 2 of those HSR-1425 CR servos.

Title: Re: Confused about Cont. Rotation Servos!
Post by: Admin on September 01, 2008, 04:30:10 PM

http://www.societyofrobots.com/actuators_modifyservo.shtml


And the 1425 is faster, at .14s/60deg