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 withdrawn from sale for redesign.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