RE: [CAD_CAM_EDM_DRO] Big servo motors drive recommendations

I have a mill with motors not quite so big. Typically mine would be 140 to
150V, 8.5Amp continuous unless I fan cool them in which case the continuous
is about double, and 46 amp peak.

They are fitted to quite a weighty machine so acceleration currents can be
quite hi for quite a long length of time.



I started with Rutex R2020 drives. These have a good PID control that uses
torque / current in the equation rather than a PWM figure. Consequently the
dynamic range is fairly good. However they suffer from noise issues, and
slow current trips (fet’s pop before the trip). They have presently been
withdrawn from sale for redesign. At one time I thought they were good, then
after problems I changed my mind, but in comparison to others they have got
more right I think than a lot of the competition, and I was machining with
them for several months until one developed senile demeter and lied about
it’s position. With no replacement available I tried another.

CNCTeknix Tek20’s, lot’s of faults here, insufficient voltage rating on some
of the fets, bad current trip design, PID is a PWM figure with no current or
speed compensation so dynamic range is not viable for a big motor. I.e. You
have to keep the gain so low to avoid current tripping when the motor is
slow that you get significant follow errors when you are into machining
speeds let alone rapids. Peter at CNC teknix has done everything he can to
help, but a simple fix isn’t going to do it. I was also assured Tek10’s
would work at reduced voltage so I tried them as well but these also suffer
from the same problems. I am sure they work OK on small motors but forget
about it at present for your size of motors.

I looked at several other DIY hobby style cards and have come to various
conclusions but generally they are not suitable for big motors, or do not
instill enough confidence in me to want to waste another £500+ on useless
cards..



I have now have spent several months researching servo drives for big
motors. Virtually all the hobby drives work their PID control to give a set
PWM. The trouble is at low speed with a big motor you only want a little
amount of PWM to avoid hitting the peak current, but as the motor speeds up
you need more gain and on a big motor PID into PWM just cannot achieve that.
You have to have very low acceleration and gain to get it to start moving,
and then you can’t get any machining speed, or you have to delay any current
trip and you risk going above peak currents which could damage the motor.

There are two solutions with a basic digital control you can use the encoder
signal to give details of motor speed and apply this to the PID to
compensate for speed. This is a digital technique dating back to the 1960’s
and works well, either by effecting the pwm, or by producing a constant on
time and phase locking the pwm frequency to the encoder signal. But no one
in the DIY sector seems aware of the technique. The second technique is to
consider the PID output a torque command as it should be and use that to
regulate current to the motor, Rutex attempt this in their R2020, but most
DIY Step direction controllers do not properly seem to do this.

If however you look at conventional analogue servo drives (+/-10V, or +/-5V
) as used in commercial machines these take the voltage input command and
use it to directly vary motor torque or current. Most even use the tach
signal to compensate for speed. They can be much more expensive but when you
see how much control is on a card you know why. In the UK I have been
offered analogue cards for my machine for £250 to £350 each. You may even
have a set that came with the machine and still work, these analogue cards
seem far more bullet proof than the DIY ones.

Then we need to be able to control these cards with a step/direction signal.
There are two firms that I know of that produce conversion cards for these.

Rutex, though I think their card may be about to go out of production. It
will take differential encoder signals, but could have some of Rutex’s other
noise issues though early cards are said to be OK. If Rutex’s PID is like
the R2020, it takes care of the typical problems of digital PID control,
with variable loop times to allow for differential sensitivity.

Skyco Pixie100 cards. These only have single ended encoder inputs, for me I
will use differential encoders with buffers since I need to take encoder
signal on to other machine controls. The PID looks good and has a variable
to be able to set it so the differential is only read after a number of
times around the loop, again compensating for normal digital PID control
problems where the differential can be missed in a high speed loop. With
Pixie cards you also have two sets of PID parameters so you can set one set
when the machine is idle to keep the motors cool, and a more aggressive set
for machining. They look very good and so far all reports have been
excellent and are $60 each.



I have looked at some step direction current mode controllers that were
impressive, but at £2500 each my machine won’t see any.



I hope that all makes sense and saves you the cost headache and lost hours I
have experienced playing with hobby DIY cards.



Regs Mark



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Zafar Salam
Sent: 26 May 2007 16:11
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Big servo motors drive recommendations



Hi,

A mill I am going to retrofit has rather big dc brush motors. These motors
are 150V, 12A cont. and 125A peak current. Which servo amplifiers do you
guys recommend for an application like this.

Zafar

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