Positioning system with stepper motor commanded from personal computer

   The positioning system consist in a transmission with tooth-belt used in order transform the rotation movement into linear movement. It is actuated by a stepper motor in order to achieve accuracy in open loop command.

The stepper motor is an electromagnetic converter of electrical energy materialized in a train of pulses and applied on phase coils as mechanical energy  manifesting discrete movements of the motor axis. For correct functioning, every pulse corresponds with a discrete movement of motor shaft.
I used a bipolar stepper motor, STH56D306, from Shinano Kenshi Co. The motor chosen must fit in its normal functioning area, (represented in a diagram provided by producer) for the application given. I preformed the calculations needed in order to verify the motor for the application.

The bipolar stepper motor need a special circuitry, called H bridges in order to be able to reverse the polarity electronically on the motor coils. The L293D circuit provided by ST Microelectronics contained two protected H bridge cells with separate commands, internally protected by fly-back voltages. The schematic of interfacing computer trough parallel port to stepper motor is given below.   

The program was written in C standard language. The reasons of programming in this languages were related mainly to secure access to I/O resources form Ms-Dos operating system, and to show that are possible to implement software for controlling processes to oldies computers who are basically at low prices.
The program written execute a positioning process with a specified velocity and is it able to calculate errors. After a positioning, by pressing a key, positioning element returns at original position. (This positioning system don't use any sensor, it is purely an open loop system. Of course, there are a lot of advantages and a disadvantages of using positioning system in open loop command). The program is listed here.

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