Servo motors v. steppers

[Jeff]

The Lobo CNC site lists two options for motors - servo motors and stepper motors.  All of the cool features - DRO, smoother, faster operation - depend on using the servo motor option.  But the stepper option seems a lot cheaper.  Why are servos so much more expensive?  Especially since the basic motor seems to be the same NEMA 17 type motor.  There are a number of reasons:

- Servos require encoders.  The encoders are actually more expensive than the motors themselves.

- The servos use a dedicated micro-controller per motor.

- The servo controller board provides a USB interface, rather than a printer port interface

- The servo solution provides everything pre-wired

Taking these one-by-one, the encoder is what gives you the DRO feature - displaying your position continuously whether operating manually or under computer control.  This is a huge convenience.

The servo's dedicated micro-controller off-loads all of the critical real-time control from the PC.  With stepper based systems, the PC has to generate each step pulse with sub-millisecond timing accuracy - you need a beefier PC and an operating system that won't interrupt the machine control program.  With the dedicated servo controllers, the PC only has to check in with the controller board a few times per second.

The USB interface to the servo controller board is possible because the controller board is doing a lot of the processing, and only needs high-level commands from the PC.  Low-cost stepper controllers require all of the Step and Direction pulses to come directly from the PC - usually through a (now rare) printer port.

Lastly, the stepper solutions require a lot of wiring.  Faulty cables are probably the number one source of problems, and unless you have a lot of experience making cables, you will spend a lot of time first making the cables, and then trying to debug your system when a cable goes bad.

In short, the stepper option has some hidden costs, as well as missing some of the more attractive features of servo control.  If you plan on spending any time using your milling machine, you will find the extra money for the servo control to be well worth it.

-Jeff

[Ti Voi]

Did you sale PIC-SERVO S3 servo controller chip

price?

thanks  you

[Jeff]

I'm currently not selling the PIC-SERVO S3 chips, but I'm working on a new controller board that uses them, and I'll probably make them available once I go into production.

-Jeff

[Ti Voi]

thanks you jeff, waiting your new board

b.r