Re: [CAD_CAM_EDM_DRO] Re: Analog Output controller?

stepperman wrote:

>Hi guys,
>
>Thanks for the suggestions. I just have a few more questions...
>
>The AMC amplifiers operate in current, open loop voltage, tachometer,
>and Hall velocity. What does this mean? I know that the amplifiers
>connect to the motor's hall effect sensors to perform the brushless
>commutation, so why are there other types of input like encoder or
>tachometer? I thought the encoder input goes to the motion
>controller, which then detects the error and compensates through the
>commands given to the amplifier. Is this idea correct?
>
>

A velocity servo amp needs to know the velocity so it can figure out
how much current to send to the motor. In that system, the computer (CNC
control) would send a velocity command signal, and expect the amplifier
to see to it that the actual velocity was proportional to the command.

A torque mode amplifier gets a torque command from the CNC control,
and produces current proportional to the command. This will require the
control to vary the command to maintain constant velocity as the load
varies.

A voltage amp provides an output voltage proportional to the input
voltage. This is approximately proportional to velocity, but varies
due to the load.

The problem with a voltage-mode amplifier is that there is no feedback
between
encoder pulses, so the control has to guess what the velocity is at that
time.
A velocity servo with a good tachometer can be totally stable, without
hunting
at true zero velocity. Any system without a tachometer must hunt between
encoder positions to know what is going on. (Yes, they can settle right
between
encoder positions and be happy, but any perturbation will set them
hunting again.)
You can use a small deadband to eliminate the hunting, but that reduces
positioning
accuracy.

When computers were slow and encoder resolution was limited, this was a
great
problem. Now that inexpensive 1000 cycle/rev encoders are available,
and computers
are much faster, it is much less of a concern.

>I have some brushless servomotors that I was able to run with the AMC
>amplifiers. I connected the motors to the amplifiers, connected the
>amplifiers to a 24V power supply and spun the motors using the
>onboard potentiometer. I also spun them with 1.5V and 4.8V control
>input. I noticed that at slow speeds the motor is not smooth, and is
>not able to obtain extremely low RPM without stopping. Will this go
>away when there is a feedback encoder signal sent to the controller?
>
>

Not completely. With a high resolution encoder and a good control
algorithm,
it will certainly be invisible to a human watching a shaft turn.

>I forgot to mention, I acquired a 3-axis Kollmorgen Servostar MC
>series motion controller:
>http://www.motionvillage.com/products/technical_documentation/septech_
>docs.html#MC
>It's an ISA PC board. I believe I need some sort of breakout board to
>connect the controller to the amplifiers, but I can't find it on
>their website. Anyone know where to acquire one? Hopefully I can find
>it somewhere other than from the factory as they seem extraordinarily
>expensive for what they are.
>
>I installed the software included, it seems to have some servo tuning
>utility, but I'm not sure if there's any software to interpret G-code
>into servo motion. Will I have to program this myself? Would I be
>able to run EMC with this board?
>
>

Not without writing a driver for it.

>Also, I compiled an overview of the suggestions given, just to
>summarize:
>
>Vital Systems
>Their boards look similar to the Kollmorgen board I have, but I don't
>have the breakout board for my Kollmorgen.
>
>Pico Systems
>Here I think I will need a Quadrature Encoder Board, a Dac16 Board, a
>8-slot Motherboard, and a Card Cage for Motherboard, resulting in a
>total of $600 for 4-axis motion control, and this interfaces with the
>computer via parallel port.
>
>

Note you have to buy a digital I/O board, too, unless you want to hack
the driver.
The total will run $750. But, you need no break-out board, and you get
17 digital
inputs and a place to mount 8 Solid State Relays for servo drive control and
spindle, coolant, etc. You also get a complete emergency stop system.