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IndustryArena Forum > MetalWorking Machines > CNC "do-it-yourself" > Comments on steppers with encoder feedback.
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  1. #1
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    Dec 2012
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    Comments on steppers with encoder feedback.

    Hello everyone.

    I was about to post this thread as a reply in some other thread but decided to open a new one so that it may be more readily available to those who may need it.

    One drawback of regular step motors is the fact that they run in an "open loop" mode. That is, there is no way for the controller to be aware of the actual positions where the motors are in any given moment. It just keeps sending signals expecting the motors to move according to the signals sent to them. Among other reasons that´s why servos are the preferred choice for position accuracy. The key word is feedback.

    Now then one of the major drawbacks for servos has always been the price tag attached to them as well as to the required peripheral equipment.Step motors with feedback encoders are supposed to be somewhere in the middle attempting to allow for low cost systems but accuracy similar to servos.

    Note: among other limitations steppers achieve slower speeds and lower resolution than servos.

    At some time in the past some of us used to attach encoders to the shafts of the motors so that our controllers could make any required adjustment to guarantee the precision. The newer motors simply make that unnecesary as they include them already.

    For these new steppers the encoder feedback is used for two main purposes, speed stability and position monitoring. The result is that now the controllers are only responsible for sending the pulse signals to direct the movements as for them these systems behave as if they were open loop. In standard (old) systems the controller received the feedback signals and had to process them along with the position algorithms. In my opinion that´s one of the reasons those systems were (and still are) expensive and complex.

    One question that may arise is that if it possible to use these motors with the regular drivers or that if these new controllers can be applied to standard motors.

    In the first case it should be possible but it is necessary to remember that no feedback would be in effect so that steps might be lost.

    In the second case, unless there is a way to overrride the feedback mode of operation a motor without feedback would be considered by the controller to be at still thus making the controller drive the motor to the maximum possible speed.

    I hope this clarifies things a little.

  2. #2
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    Nov 2013
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    Re: Comments on steppers with encoder feedback.

    Hi,
    there are advantages in closing the loop but way less than the manufacturers claim.

    They say the closed loop steppers are faster, more powerful and never lose steps......all rubbish. A closed loop stepper has the same torque/speed characteristics
    that an open loop stepper (with the same motor).

    A closed loop stepper can interpolate between full steps and therefore achieve greater resolution than an open loop stepper. Also a closed loop stepper
    will produce a 'following error fault' should it lose too many steps unlike an open loop stepper which will attempt to carry on despite it now being inaccurate.
    These advantages are bought with a considerable price increase and increase in complexity.

    Good quality AC servos are coming down in price.....to whit....a Delta 400W B2 series (160,000 count per rev) servo, drive and cables delivered in the US for $380.
    A Delta servo (of adequate power) will 'eat' any stepper ever made.

    Delta and DMM are two good quality brands that won't break the bank. There are cheaper Chinese made drives but of less certain quality, support and documentation.

    Craig

  3. #3
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    Dec 2012
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    Re: Comments on steppers with encoder feedback.

    Hello.

    Please correct me if I´m wrong. The closed loop steppers do not actually do the trajectory generation. All they do is use the encoders to stabilize speed and avoid or minimize step loosing.

    From that point of view they are no different than systems where we added external encoders to the steppers and have the controller do the compensation.

    In these particular motors the effort of linking the encoders to the shaft of the motors is avoided as they are already an integral part of the same. Besides by being so that eliminates some small errors introduced due to the mechanical elements used in the linking process.

    On the other hand while prices might be dropping in the US here in Mexico the price difference lies around a 4/1 ratio. In small systems it might be a 2/1 ratio but still to high a price difference for many small industrialists who have their shops in their own houses.

    I must say that those motors do not appeal to me. I´d rather use plain steppers or full servos. And in talking about prices I agree with you. From quotations I have received the price tag on those stepper closed loop systems is quite near to the full servo systems. In case I´d ever need again a more precise system that an open loop I´d stick back to linking encoders to the motors and use my old controllers. Even the motors by themselves, in my opinion, are not worth buying.

    Best regards.

  4. #4
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    Re: Comments on steppers with encoder feedback.

    A 700 W stepper motor and driver can be had for $200 if its desgined correclty but its more complex than setting up a servo.
    I'm not convinced a closed loop stepper is really the answer. If a stepper is runing "in the zone" it will never loose steps.
    If it does loose steps, it no longer has the torque to catch up so all it can do is flag an error.

    I'm looking at a stepper design now that will use a 5 amp rated stepper but the design says it will only ever use 3.5 amps "in the zone"
    But the motor manufacturer says the motor can be overdriven to 10 amps.
    So what we are thinking of doing is to run an encoder and a smart stepper drive set for 5 amps. But the drive has a boost function
    So when the motion controller senses a following error developing before a step is missed, it can tell the driver to boost the current output to 10 amps which will increase the available torque by 20%
    So then if a step is missed we are in deep water as there is nothing left in the tank!
    Rod Webster
    www.vehiclemods.net.au

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