RE: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations

Yes I looked at Larkin.

First their PCB’s are not professional quality; it may be they are just
showing a prototype board, perhaps not.

Secondly I asked about acceptable methods to conduct an Estop without
damaging the drives. They say under no circumstances to cut the DC before or
after the capacitors or their drives will fail, despite the fact many
commercial drives do it this way. This is a massive safety issue, it is not
uncommon for fets or IGBT’s to fail in conduction, when they do the motor
goes full runaway. A big commercial drive will signal a fault and provide
secondary braking means by often dropping it’s own Estop relay. Additional
limit and Etsop manual switches will also be wired onto this relay, if the
caps can be discharged quickly enough you can cut the supply before the
caps, if not it must be after the caps. Some drives need to conduct energy
back to the supply and this causes them a problem, a big commercial servo
will use a load / braking resistor to dump this too. For a DIY servo all we
can do to be safe is cut the DC HV supply, and possible offer a resistor for
the servo card, but Larkin are against this, they say Estop their drive by
disenabling it. I challenged their statement for an alternative suggestion
but never received any reply.

So in the UK I cannot build a safe machine with a Larkin Viper drive, and I
cannot and will not use it, safety is the most prime part of a CNC machine
above all other parameters.

It may be OK with a small machine, but 600lbs+ running wild at 16+m/min on
my machine is very dangerous.



I have not covered any other issues with them, seemed pointless, but I would
expect they are PWM angle mode rather than torque current mode drives which
again means they will not have enough dynamic range for a big motor, and I
doubt somehow they are aware how to digitally compensate for this, there is
no sign of any calibration area for it. I think there were a few other
things that did not ring quite right but without looking at the spec’s again
I am unsure.



On my machine all the servo cards tried will run the light Z axis OK, many
will run the X axis though often with limited acceleration and speed, as to
the Y axis only Rutex came close and they had problems on occasions. I
think I have been through six Rutex cards in 6 months, four repairable, two
not.



Larkin will have to do a lot more to convince me they know what they are
talking about, based on present communications with them they have blown any
confidence I may have had, and my confidence in the whole DIY servo card
area for big HV motors is very weak.



The only one in the DIY scene that may be able to provide something is Jon
Elson. Jon has played a lot with big motors, he comes over as highly
confident to begin with, then he needs to do some thinking, but there is
nothing wrong with this, it just means he wants to look at an applications
based solution, rather than jumping in and selling you junk. His cards
however are more suitable for EMC than mach.



When you go into the big commercial servo world there are loads of analogue
servo cards working day in day out on big motors without any problems hence
my previous conclusions. They used to be very expensive a year ago I was
quoted £1500 each to replace my old cards. Last week I was quoted £350 for
their direct modern replacement, and £250 for a Chinese analogue drive.
These drives look after themselves when things go wrong, the DIY ones mostly
let the smoke escape and the machine is down while you await replacements.



Regs



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 John Stevenson
Sent: 26 May 2007 19:26
To: CAD_CAM_EDM_DRO@...
Subject: [CAD_CAM_EDM_DRO] Re: 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
>

Mark have you looked at the Viper drives from Larken in Canada.
I have heard good reports on their stepper cards but the servo's are quite
new.
John S





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