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stepper motor positioning control

According to examples I have seen in the NI website, it states that there is no need for closed loop control for stepper motor. however If Iam making a positioning control system using a stepper motor what will I need to get for the feedback and is there way where i can include PID and other error reduction methods to stepper motor positioning control. Iam not doing anything big where I will need to  use a motion control software..all I have is the labview 8.

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Your query makes no sense.  Closed loop = feedback.  If you do not need feedback, then you run open loop.

 

One of the advantages of a stepper motor system is that it does not require feedback to maintain position (as long as it is properly designed and operated).  Stepper motor systems usually need a sensor (or feedback) at one position, often called the Home position, to define a starting point after the power has been off.  Any other position is located by counting steps.

 

If your electromechanical system can operate in a mode where the motor cannot always be assured of completing each step, then you need feedback or something other than a stepper motor.  Examples of such systems include load torque exceeding the step torque, running at or near a resonant point, running faster than the motor can guarantee each step, ...

 

Lynn

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ok well, iam just building a positioning control system using a stepper motor not a servo motor or something. what I need to do is use a stepper motor that moves to exact maximum position without any error. So for the feedback what can I use and also in order to reduce the error- can use a PID  or other methods to reduce the error in this particular case.

 

Thanks.

Shaham

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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!

Putnam
Certified LabVIEW Developer

Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5


LabVIEW Champion



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Hey Shaham,

Try taking a look at our Motion Fundamentals Tutorials. This should help you get started on how to approach your issue.

1) Fundamentals of Motion Control

2) Configuring a Motion Control System

Ricky V.

National Instruments
Applications Engineer
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well Iam not using a motion control software, iam just building a positioning control sys using a stepper motor. It is just a school project, Iam working in the lab, the hardware I have is NI ELVIS with DAQ and labview installed, with DAQMAX that's it. Iam trying to find a way to create a VI to control the position of motor with using any sopisticated motion control module.

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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.

Putnam
Certified LabVIEW Developer

Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5


LabVIEW Champion



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now for the stepper motor feedback position sensors, I was told that the sensor can be encoder that indicates the position, and the types of encoders can be optical or it can be LVTD. Iam planning to using something simple like LVTD for position feedback, can anyone please refer me to any particular LVTD sensor that is good for stepper motor position feedback.

 

Thanks

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