Re: lost steps

Hi Mariss,
Do you have a fix for us who are having trouble using Gecko's? losing
steps.
Colin


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
<mariss92705@y...>" <mariss92705@y...> wrote:

> Jon,
>
> Good evening and Happy New Year to you! The direction input may

seem

> strange but it really is not.
>
> The step pulses are processed (delayed) by the midband

stabilization

> circuit. A little bit of theory is need to explain why.
>
> Step motors are first-order mechanically resonant, (mass-spring)
> systems. This simply means there is an inheirent 90 degrees phase

lag

> between between velocity and torque.
>
> Think of a coil spring suspended from the ceiling with a weight
> attached to the end. Leave it alone and the weight has a rest
> position. Now, pull down on the weight and then let go. The weight
> will oscillate up and down.
>
> When the weight passes its former rest position it will have

maximum

> velocity and the spring will have zero force. When the weight

reaches

> the top and bottom extent of its travel, it's velocity will be zero
> and the spring will have its maximum force. That is what I mean by

a

> 90 degree phase lag between velocity and torque (force).
>
> At low speeds a drive presents itself to the motor as a current
> source. As you know, current mode amplifiers have a near zero phase
> lag and all is well with the motor.
>
> As speed increases, the current source runs out of "head room" and
> reverts to a voltage mode amplifier. Voltage mode amplifiers have

an

> implicit 90 degree phase lag of their own.
>
> As a system, the motor and amplifier phase lags total 180 degrees

at

> this point. That is a setup for positive feedback and unconstrained
> oscillation if the system gain is above unity (it is).
>
> The G201 drive introduces a phase lead component of about 60 to 80
> degrees to the system loop to stabilize it.
>
> This done by sensing the rate of motor load change (the first
> derivative of torque) and then phase modulating the step pulses

with

> this sensed component. Phase modulation means you slightly advance

or

> retard the timing of each received step pulse so as to damp out
> oscillation (loop phase lag of 120 degrees or less).
>
> The way it works out is from the time a step pulse is sent until it
> is "clocked in" by the drive can be a variable period of time

between

> 5uS to 20uS. As long as the direction is "true" at the active edge

of

> the step pulse until the next active edge, the step will be taken

in

> the correct direction.
>
> Mariss
>
>
>
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...>
> wrote:
>
> > > Gecko drivers have a strange requirement for the direction
> signal.
> > It
> > > must stay in the right
> > > state AFTER the step pulse is issued, until the step actually
> goes
> > out
> > > to the motor. With
> > > microstepping and step multiplier drives, that can be a long
> time,
> > > depending on the conditions
> > > at that moment.
> > > Most other step drivers require the direction signal to be

stable

> > some
> > > microseconds before
> > > and hold a microsecond or so after the leading edge of the step
> > signal,
> > > and then it can
> > > change. This causes trouble with some programs. I had to

modify

> > my
> > > stepper controller
> > > board to accomodate these drives. It has a flip flop that
> latches
> > the
> > > direction signal when
> > > the step pulse occurs and holds it in that state until the next
> > step pulse.
> > >
> > > Jon