Low cost Servo Controller

I am working on specifications for a Servo driver board which will allow any low or medimum cost DC motor to be converted to a Servo motor and connected to a parallel I/O board. I hope this electronic assembly will allow high performance motion control for a reasonable cost. Steppers + Stepper controllers seem to be less expensive and easier to use than Servos + Servo controllers. Many of us use steppers and put up with speed - torque limitations. I know that many low cost DC motors will not perform as well as a low inertia high quality servo motor, however the board development I am considering will out perform most steppers for less or equal system cost.

I want to use a PC Parallel I/O board even though it will cost more than the parallel printer port, for two reasons. First I want some room to configure several motors into servos and handshake limit switches and other functions to the board and the PC. Next I want to get a parallel I/O board with a counter / timer so that my program will have some reasonable timing to pace things along.

The board design concept will allow the servo loop to be closed at the motor shaft in the traditional manner with an encoder or from an external feedback means from the actual position of the moving components (after backlash), such as from an encoder mounted on the table itself.

The questions I have, are about how such hardware drivers are patched into canned application programs. After writing some test routines through the new board, I'll likely get around to making it run with some CAM software which I haven't selected and know nothing about at this point. I'm sure there are multiple formats between application programs, through interface drivers, resulting in motion. No doubt that can be as simple as step and direction and get more complex from there. What makes since in terms of communication from the interface hardware that fits the kind of software a hobby CNC person may find within budget? If you wanted to hold down the cash outflow, and were in the stepper motor budget, which software might you patch to your stepper motors through the step motor driver? How do you glue the hardware and software together without 22 e-mails and the clock striking one?

Has anyone seen high performance Servos with Servo controllers which competed in price to the stepper and stepper controler counter parts? I've picked up a few surplus servos to investigate their controller requirements. The simple ones seem to be a brush type DC motor tied to an encoder wheel. These will work out of the box with the new board. The more complex servos, seem to be multi phase brushless motors and more complex encoders. They are certainly in budget at the surplus stores, but the missing controllers and drivers may not be. It seems to make since to create an electronic interface which is low in cost and will allow the use of Servo technology on a reasonable budget. If that makes since, perhaps Servo like performance can be obtained near the same cost as stepper performance.

Will someone make a list of stepper controller costs vs. Servo controller costs plus motor costs, with a little performance data one might get at each cost? Aside from the main stream sources such as Minarik or the likes. What does it look like as you travel up the performance - cost curve for "underground" motion controller components and difficulty with CAM software interfacing? Even more important, What kind of CNC performance might one end up with along the way? Frankly, I hate dissapointment, even when it's my own fault for not spending enough money.


David Howland