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LabVIEW
stepper motor positioning control
First of all, how "exact" does the final position need to be? There are many types of position encoders, rotary ones mounted on the motor shaft, linear ones that measure the actual position of the item being moved, etc. The rotary ones are usually optical encoders and produce a coded number based on their rotational angle, usually you still have to keep a count of the number of rotations, but they will give you a count that isn't dependent on the motor's ability to make the steps, in the case of a stepper motor being run too fast or missing steps because of torque issues. But, since the motors rotation is usually translated into linear motion by mechanical means, either a lead screw, or gear arrangement, there is the actual mechanical error, where there may be a finite amount of "slop" or play in the mechanical system such that the motor may turn several degrees before the linear position changes, particularly after a direction change. This is known as backlash error. A linear position, which keeps track of the actual position of the moving item, is usually more accurate in this case. These can be optical or electrical, with one type of the later being a Linear Variable Differential Transformer (LVDT). Each type needs a different means to input its data into LabVIEW, but all require some data acquisition interface.Usually the reason stepper motors are chosen for a design is that they generally don't need position transducers, additional circuitry, just as the previous poster said a "home" switch or sensor. It is also a good idea to have limit switches at each end of maximum travel, since to run the unit from a unknown starting position to the home switch to start requires "guessing" which way to go at power up, and if you guessed wrong having a signal to tell you that you reached the end of travel beyond the home sensor. That is why so many printers, on power up, run the print head to the limit one way then back to home. In fact they are a good source, for a proto-type or hobby type project, to get some of the needed parts, and at least at my house, there seem to be a lot of out of commision printers!
The first link has a lot of valuable information about motion control in general. You are using a motion control system, it is just going to be one that you build, and understanding how motion control is done is fundamental to what you need to do. You may not need the complex feedback, or PID tuning or some of the more sophisticated features of a full motion control implementation, but you do need to know the basics of a positioning system, which a one form of motion control.
The simplest stepping motor positioning is "open loop" where you provide the appropriate signals (usually signals with a phase difference, applied simultaneously to the winding inputs of the stepper motor) to cause it to step in a particular direction. open loop means that you have to keep track of how many steps, and in what direction, were applied to the motor to try and "know" where it is. As was indicated in a previous post, a stepper may miss steps, due to the movement requiring too much torque, the frequency the pulses are applied, etc. so that you can become "lost". To have a closed loop system then requires adding a encoder to indicate how much actual motion occurred. I don't recall with NI-ELVIS what additional hardware might be required. Stepper motors, as with other motors, need to have driver hardware that turns a stepper signal into the current levels required to actually drive the motor.
