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microstepping circuits

Discussion in 'Electronic Basics' started by yioryos dimitriadis, Jun 11, 2007.

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  1. Dear all,

    my name is george (yioryos in greek) dimitriadis, and this is my first
    I would be grateful if everyone is interested to reply to my post and please
    excuse me for any
    non-conformities according to the newsgroup policy.... I will do my best to
    be as typical as possible.

    I am trying to build a stepper microstepping driver for my small cnc mill. I
    am not new to microstepping and
    I am aware of chips such as lmd18245 or the popular a3977 from Allegro. I am
    also aware of the L6207 with dual bridge
    and built-in monostables and comparators. The thing is that since I am a
    hobbyist and passionate with designing my
    own projects, I like building from scratch in order to both learn and have a
    tailor-made system according to my needs.

    I have experimented with L6205, a 7555 monostable triggered by a pic
    comparator, (pic comparator used stand-alone), at slow decay mode that
    worked fine.
    When I chose to put the comparator output directly, (without the
    monostable), to one of the 2 bridge logic inputs for slow decay again,
    or at enable input for fast decay, it also worked well but the currents
    should be kept low otherwise due to the relatively high Ron
    and the resulting high frequency losses, the chip's temperature would rise

    So, I decided to build my own h bridge with low Ron mosfets, being able to
    achieve high frequencies and currents, utilising fast or slow decay,
    in order to have fine chopper control of coil current, necessary for
    microstepping. (The circuits attached, are not intended as
    the stepper motor final drivers, they are just for the development and
    debugging of the chopper function).-> the circuit schematics are attached at
    a post I sent at
    **alt.binaries.schematics.electronic**with the same subject.

    I found that there was a very handy chip, HIP4081A, that would drive
    properly an n-channel fet h-bridge using charge pumps
    and bootstrap capacitors. Furthermore, deadband delays are implemented and
    input logic has been taken care of, by means that
    the bridge can be fully controlled by the lower side inputs.(ie if the lower
    inputs are high, the higher inputs are "not care", thus
    by having the higher inputs tied at logic high, the full bridge is
    controlled via the lower inputs).

    But, there was a problem.... Although I think I was careful in setting up
    the circuit; proper bootstrap capacitors according to
    mosfet input capacitance,shotky diodes at bootstrap circuit, 1n5822 shotky
    diodes at mosfets, (I was amazed that both the hip4081a
    application note and datasheet does not mention anything about diode
    protection), decoupling caps in all chips and using the same
    succesful strategy as with L6205, the results were not as expected.

    In L6205, the Vref I was setting for the current control,(fed in the
    non-inverting input of the pic comparator),was the upper magnitude limit of
    the chopper frequency measured at Rsense with my oscilloscope. The target
    current was always met with L6205.It was also dynamically linked with Vref,
    ie when I was changing Vref by turning the preset, current limiting was
    following the theory. With the HIP4081A though,
    the waveform at Rsense was very ugly: Instead of the usual chopper
    trapezoidal ripple with a magnitude=Vref voltage, there was a spiky waveform
    with no
    trapezoidal form at all, and 2 important drawbacks: I had a magnitude much
    higher than the one set by Vref, there was no whatsoever dynamic
    link between changing Vref, (it was locked at that magnitude although I was
    changing Vref), and the spike was extending in the negative region
    as well at approximatelly 1/3 of the positive spike magnitude.

    One thing is for sure: I used the exact same strategy and working hardware
    as the one that succesfully operated with L6205 but
    I am missing something....

    Although I hoped I would have an easy to read post for my first one, I ended
    up having all these.I am sorry for this.

    I would be grateful if anyone is interested to tell me his/her
    opinion/advice considering the problem I am facing.
    Please excuse both my english and drawing quality.

    Hope to hear from anyone out there!


  2. Jamie

    Jamie Guest

    almost sounds like an open collector system with no clamping diodes to
    absorb the back EMF from the motor.
    Just a guess.
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