Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my Mill/Drill?

On Sun, 16 May 2004 13:45:01 -0400, you wrote:

>Harvey,
> Yes you are correct. I was referring to the current limit type Ron was
>mentioning versus a bipolar design. I'm SURE the Xylotex uses current sense
>resistors in the same way as the unipolar chip. Can't vouch for Geckos
>though. Never seen one up close.

I looked at one of the chips, think it was the same type as the
microstepper in the xylotex. Think that the driver is much the same
as far as the controller electronics is concerned, the difference is
in the output stage, and packaging. (also, I think you get a greater
level of control, but not sure since I won't look at the data sheet.

As far as the Gecko is concerned, my feeling is that it is not built
around the same chip, and that Mariss (think I remember it right) has
some exceptions to take with the algorithm in the chip.

Looking at the chip and looking at the design of the (remembered)
xylotex chip, you are right, same sense resistors, same place,
probably the same A/D driving them. They are both chopper designs.
Same thing as the 297/298 chip set, but there is a D/A built in to
take care of the microstepping.

Harvey

>
>Regards,
>Dave Rigotti
>HobbyCNC.com
>
>----- Original Message -----
>From: "Harvey White" <madyn@...>
>To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>Sent: Sunday, May 16, 2004 1:42 PM
>Subject: Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my
>Mill/Drill?
>
>
>> On Sun, 16 May 2004 12:29:25 -0400, you wrote:
>>
>> >Jon,
>> > This chip doesn't use resistors!
>> >http://www.allegromicro.com/sf/97060/
>> >Now what?
>>
>> Looks like it needs two sense resistors to ground to sense motor
>> current. Typical values are 0.5 ohms each. Motor current is
>> controlled by a variable voltage into the sense input, I think. The
>> internal DACs allow for up to 16th step operations, which is why there
>> are DACs in there, and not a direct run from the sense input to the
>> comparator.
>>
>> Harvey
>>
>>
>>
>> >
>> >Regards,
>> >Dave Rigotti
>> >HobbyCNC.com
>> >
>> >----- Original Message -----
>> >From: "Jon Elson" <elson@...>
>> >To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>> >Sent: Sunday, May 16, 2004 12:08 PM
>> >Subject: Re: [CAD_CAM_EDM_DRO] Xylotex or Gecko step driver for my
>> >Mill/Drill?
>> >
>> >
>> >>
>> >>
>> >> Roy J. Tellason wrote:
>> >>
>> >> >On Saturday 15 May 2004 11:05 pm, Jon Elson wrote:
>> >> >
>> >> >
>> >> >
>> >> >>>but I ruled that one out because it's a unipolar only board.
>> >> >>>
>> >> >>>
>> >> >
>> >> >
>> >> >
>> >> >>Good move, as most unipolar drives are very poor performers at higher
>> >> >>speeds
>> >> >>
>> >> >>
>> >> >
>> >> >Why is that?
>> >> >
>> >> >Would it have something to do with a bipolar setup energizing coils in
>> >the
>> >> >reverse direction rather than just turning them off?
>> >> >
>> >> >
>> >> >
>> >> Yup, that is basically the problem. With the typical unipolar drive,
>> >> the power
>> >> source is connected to the center tap, and transistors ground one end
>or
>> >the
>> >> other of the winding. When you ground one end, there is still
>circulating
>> >> current in the other half, fighting the field you are trying to build
>in
>> >the
>> >> newly powered winding. This energy has to go somewhere, and it ends
>> >> up being burned off in some diode-resistor arrangement. It is a hard
>> >> topology to harvest this energy constructively. If there is no
>resistor,
>> >> the current in the shut-off winding dies very slowly. If there is a
>> >> resistor,
>> >> then it gets quite hot at high step rates.
>> >>
>> >> With a bipolar drive, the power source is applied across the whole
>> >winding,
>> >> and is reversed to reverse the field. The field already in the winding
>> >> opposes the power source at the moment the transistors are switched,
>> >> and the reversed voltage across the winding collapses the existing
>field
>> >> and quickly builds it again in the opposite direction. The energy
>mostly
>> >> ends up back in the power supply, so there are no big resistors to burn
>> >> off the left-over field.
>> >>
>> >> There are ways around this, but they are generally more complicated
>than
>> >> the bipolar drive scheme. The only advantage to unipolar is it saves a
>> >> 4 transistors per motor.
>> >>
>> >> Jon
>> >>
>> >>
>> >>
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