Problem Step and Direction Pulse Length


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  1. #1
    Registered arhijawi's Avatar
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    Question Step and Direction Pulse Length

    Hello,

    I am working on a project with Kflop and a Hybrid Servo Motor and Drive by leadshine, the motor drive is HBS57, it is working on a maximum pulse frequency of 200KHz, and the step pulse length should be at least 10us, phase shift between step and direction pulses should be at least 5us.

    The problem is that Kflop offers a maximum pulse length of 3.78us set using the FPGA register to 63 (63/16.667MHz) and a maximum phase shift of 1.92us. Is there anyway around this?

    Tried measuring the pulses coming out of the Kflop, I get a different edge count every run. My guess is that its too fast to count. 132.275 KHz but small pulse length. Also if there is an explanation to pulse rate and acceleration that would be appreciated. I want to know the pulse per second too.

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    Gold Member TomKerekes's Avatar
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    Default Re: Step and Direction Pulse Length

    Hi arhijawi,

    I don't think there is a requirement of the pulse being 10us. In fact it would be impossible to go 200KHz if that were the case.

    If you use the 3.78us pulse and set the polarity so the drive steps on the trailing edge of the pulse then the drive will step 5.7us after the Direction changes and meet specification.

    But better yet may be to use the CW/CCW mode which I believe is supported by those drives and KFLOP. That avoids any Direction setup time issue.

    The Axis parameters will determine the Maximum Pulse/Step rate and the Acceleration. I'm not sure exactly what you are asking for here.

    HTH
    Regards

    TK
    http://dynomotion.com


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    Registered arhijawi's Avatar
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    Default Re: Step and Direction Pulse Length

    Hello Tom,

    Thanks for your reply, my problem is persistent as the drive I'm using doesn't seem to support CW/CCW mode. The problem I believe is that the steps that the Kflop is sending are smaller than 10us, which causes the driver to miss steps.
    I am asking about the pulse rate and acceleration real time values, not the maximum. How to read them.

    Here is a link to the drive's data sheet:
    https://mecheltron.com/sites/default...r_HBS57eng.pdf

    Check page two PUL+ and DIR+ details.

    Your help is much appreciated
    ArHijawi



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    Gold Member TomKerekes's Avatar
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    Default Re: Step and Direction Pulse Length

    Hi ArHijawi,

    You are correct. I now see the 10us specification. I haven't noticed that with any other Leadshine drives. And it seems strange as with a 10us pulse with 10us between pulses the max pulse rate would be 50KHz not the 200KHz they claim.

    I"m not aware of any way to make the pulses that long without some external pulse stretcher circuit (maybe, Diode, RC, and Schmidt trigger device like 74HCT14).

    The Axis variables contain a ch->last_vel variable for current velocity. There isn't a variable for acceleration. You would need to compute it from delta velocity / delta time.

    HTh
    Regards

    TK
    http://dynomotion.com


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    Default Re: Step and Direction Pulse Length

    Something definitely does not make sense here. As others have stated and implied, a 10us pulse length could not be seen after the frequency exceeds 100Khz because the length of one cycle at that frequency is 10us. So at that point 10us pulses would run together and would only become a high level, not pulses. To get to 200Khz, you would have to have a pulse length that is shorter than 2.5us. I think that their specs. are screwy.

    If you are running at a 200Khz pulse rate, I would try shortening the pulse length to 2us or perhaps 1.25us or even less. A micro second is a millionth of a second and it seems like an awful short time. But having worked with circuits that have pulses that are 1,000 and even 10,000 times shorter, I can assure you that in the world of digital electronics that is an eternity. Your circuitry is probably based on some form of CMOS technology and it can probably operate at least ten times faster than 200Khz, probably 100 times faster or even more.

    Your problem is probably too LONG of a pulse duration, not too short.

    If shorter pulses do not work, try slowing the pulse rate down, perhaps to 100Khz, if you can.



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    Default Re: Step and Direction Pulse Length

    All of what several others have said about strange specs, but there is another possibility here.
    Does the drive have an opto-coupling on the input? If so, that may place a lower limit on pulse length. And that may mean that the claim of 200 kHz is simply false. It may have been carried over from another unit (one without optos) without checking.

    But I dunno.

    Cheers
    Roger



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    Registered arhijawi's Avatar
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    Default Re: Step and Direction Pulse Length

    Thank you all for your replies,

    But when I try to read the pulse with a sampling frequency of 125 kHz, the maximum I can have now, I see pulses of around 10us but its varying, also tried counting the number of steps, its not the same number but its always close to a factor of the move command value.

    I don't think I have an opto-coupling input. But yeah the documentation is quiet misleading.



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    Default Re: Step and Direction Pulse Length

    A sampling frequency of 125 kHz is a period of 8 uS. That is far too close to 10 uS.
    I don't know how you are doing your measurements, but something does not sound right to me. I think you need a CRO.

    Cheers
    Roger



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    Registered arhijawi's Avatar
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    Default Re: Step and Direction Pulse Length

    Quote Originally Posted by RCaffin View Post
    A sampling frequency of 125 kHz is a period of 8 uS. That is far too close to 10 uS.
    I don't know how you are doing your measurements, but something does not sound right to me. I think you need a CRO.

    Cheers
    Roger
    Well I am sure there is something wrong. But these pulses operated my motor, but at high Jog values the motor will miss some steps causing it to vibrate and that's not really what I need. And my encoder had to be wired to the driver, which has PID position control.
    I am not sure what is a CRO, but I was thinking that maybe a small change in the Kmotion C library can fix the problem, because after all, making a pulse shorter is always the challenge to hardware not the other way around. Any advice on where to start?

    Thanks,
    ArHijawi



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    Gold Member TomKerekes's Avatar
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    Default Re: Step and Direction Pulse Length

    Hi ArHijawi,

    The link to the manual shows the Leadshine input circuit using optos. Some optos respond slowly which is probably the reason for the 10us pulse requirements. I think Leadshine went to faster optos in their later designs.

    As I said I don't know of an easy way to stretch the pulses that long without adding an external circuit. KFLOP was designed for high step rates (2.5MHz) which requires short pulses.

    With pulses 3.78us or less sampling every 8us will not provide much useful information. It is unlikely that KFLOP is outputting pulses different than what is is configured for.

    At what "high Jog values" does the motor miss step? What speed are you commanding? Is it higher then 100K steps/second?

    Regards

    TK
    http://dynomotion.com


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    Registered arhijawi's Avatar
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    Default Re: Step and Direction Pulse Length

    Hello Tom,

    After I command Jog0 more than 27K, the maximum speed is set to be 40K.



  12. #12
    Gold Member TomKerekes's Avatar
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    Default Re: Step and Direction Pulse Length

    Hi ArHijawi,

    That comes out to 37us per pulse and I wouldn't expect there to be a problem.

    What RPM does that correspond to? What are your motor and encoder specifications? What motor supply voltage are you using?

    Are the KFLOP Step/Dir outputs in Open collector mode?

    Regards

    TK
    http://dynomotion.com


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