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Thread: Closed Loop Stepper Compared with Open Loop Stepper

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    Smile Closed Loop Stepper Compared with Open Loop Stepper

    Traditional open loop stepper systems operate in open loop providing position control without feedback. When stepper motors move loads in open loop, a potential loss of synchronization called "Stall" or "Loss of Steps", between commanded steps and actual steps may occur. In the past, this has been solved by adding a feedback device such as an optical encoder and feedback the "current" position for monitoring or verification. But this implementation can only "alarm" position loss and can't correct it at the end of the move. When an application requires real-time closed-loop performance throughout the move, engineers are forced to choose much higher cost and complexity of servo systems. But in many applications, because traditional servo systems lack of high torque feature of stepper systems, expensive planetary gearboxes have to be added to achieve desired torque.

    Leadshine closed loop stepper is named as Hybrid servo systems, by implementing high resolution encoders to update the motor position every 25 micro-seconds, closed loop stepper drives ensure control real-time position accuracy without lose of steps in open loop stepper system.


    Different from the constant current output of open loop stepper systems, output current in closed loop stepper systems is optimized and changes based on moved loads. Therefore, Closed Lope Stepper systems generate higher torques, lower motor heating, lower vibration, and lower noise than open-loop stepper systems.

    When compared with brushless or brush servo systems, cost for closed loop stepper systems are much lower. In addition, much higher torque, simple tuning and configuration, no overshooting, and no hunting also make closed loop stepper systems better solutions over servo systems in many low to middle applications.

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    Default Re: Closed Loop Stepper Compared with Open Loop Stepper

    Is it a new invention, or has been in the market for a long time?
    With all the good points, why to go for the expensive servo system at all?
    There must be some limitation of closed-loop stepper motors. Please elaborate.

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    Smile Re: Closed Loop Stepper Compared with Open Loop Stepper

    Hello Sinha,

    It has been in the market more than 6 months. You can check more detail from Leadshine website.

    For the limitation, I will test and elaborate later. If you have any other idea, please share with me .





    Quote Originally Posted by sinha_nsit View Post
    Is it a new invention, or has been in the market for a long time?
    With all the good points, why to go for the expensive servo system at all?
    There must be some limitation of closed-loop stepper motors. Please elaborate.

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    Question So, how exactly does it compare?

    This interesting subject had compelled me to register today so my first post here, wee-ha!

    closed loop stepper drives ensure control real-time position accuracy without lose of steps in open loop stepper system.
    I am curious about any advantages in positioning accuracy the closed loop stepper system might offer. A way to address missed steps in microstepping mode would be of particular interest to me and I'm sure other members here.

    I am actually in a middle of a small scale research project on the subject of positional accuracy in MS mode and the results I'm getting so far are not all that promising. Along with outright missed steps I'm also seeing non-linearity in angular response to different microsteps. Meaning, out of 16 MS'es in 1/16 step mode MS #1 does not usually turn the rotor for 1/16 of the complete full step angle but MS#4 may turn it 1/8th or even more.

    I've taken part in a small but interesting discussion on the subject of accuracy at the Softsolder blog (sorry not able to post links yet, pls run a quick search on the name - it'll come up) and after many an argument has been exchanged, increasing to size of the stepper (to the biggest reasonable/affordable size) has been named as the ultimate solution to positional accuracy in MS mode.

    I am wondering how the OP feels about microsteps given that the closed loop control gives (at least in theory) a way to correct the missed or wrong sized ones.

    By the way, on the subject or the actual implementation of the closed loop control: how does it even work given that unlike traditional servos, steppers are often used directly coupled to the load, without the gear box. So, is the encoder able to pick up on minute angular changes or is it itself geared up?

    Cheers!

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