Re: General servo and controller questions

I wish I could point you to a Manual about Hobby CNC but about the
best resource is this list, its archieves and the DIY-CNC list. I
will try to answer some of your questions.

>
> Hello,
>
> I've set up a small stepper system, and it's been running well.
> Recently I've been looking at servo systems, specifically DC
> brushless servos, but I'm overwhelmed with all the terminology.

With

> no comprehensive source of information, I've just been reading and
> gleaning information to little avail. It's been frustrating, so I'm
> posting some of my questions here, hopefully someone can point me

in

> the right direction.
>
>
> My understanding of these brushless servo systems is that they
> controlled with a PC motion controller card -> servo amplifier ->
> servo.
>
> OR
>
> If I want to use the parallel port interface with the computer, I

can

> get the rutex R991H to generate the signals to the amplifier.
> Parallel port -> Rutex board -> Servo amplifer -> Servo.
>
> I found the Rutex R991H here:
> http://www.rutex.com/us/cart.php?
> target=product&action=view&product_id=15&category_id=5
>
>
> I've purchased some of these AMC amplfiers:
> http://www.a-m-c.com/download/b30a.pdf
>
> Is it possible to wire the Rutex with the AMC amplifiers? I believe
> the AMC ones are "analog command input." I've looked at the manual
> and I'm completely lost as to how to wire these things. For

example,

> where would the +/- 10 V signal from the Rutex go?

Opening statement: the treatise below applies to BRUSHED dc motors
which are easier to control/find surplus than unbrushed ones

There are two ways to approach the parallel port to servo interface.
The first is a way a lot of the list members choose and that is to
buy an integrated step/direction to brush servo module. Examples are
the Rutex 990 series and the Gecko 320/340 series. You put Step and
Direction from the PP into the module, Supply raw DC voltage to the
module, hook the motor up and the 4 encoder wires and it will drive
the servos directly. (more about the 991 rutex later)

The reference to a "loop" is the fact that the encoder has distinct
lines that send pulses out as the shaft rotates. The line count
varies and the count is done in what is called "quadrature". There
are two sensors so that each line produces a pulse on the detection
of a line and the loss of that line (rising and falling edge. Two
sensors slightly offset give double the resolution so the net result
is 4X/ Thus a 500 CPR (Counts per revolution) encoder actually sends
2000 pulses back to the controller.

>
>
> In general, my questions are:
>
> 1. How are the servos wired to their controllers? For example, how
> would I wire the AMC amplifiers to the Rutex R991H? Or how would I
> wire the AMC amplifiers to a motion controller card?

If you have basic AMPS (put in + - 10VDC and they spin the motors)
then you need some circuitry to tell the controlling signals the
motor is where it should be. Typically the + - 10 DC ones are just
power amps to spin the motors and the position information from the
encoders goes back to a controlling circuit. You can test the amps
you bought by giving them some raw DC (12VDC would probably do fine)
and then using a separate variable power supply (VPS) for the 0 -
10VDC first hook the VPS ground to signal common and and the pos lead
to the +-10 input slowly increase the signal voltage from 0 - 10VDC.
The motor should spin at a speed porportional to the control voltage.
Now with the signal DC turn to 0, reverse the leads so that the VPS +
is on the amp common and the VPS - is on the amp signal pin. The
motor should now spin in the opposite direction at porportional
speeds to the signal.

In the case of brushless controllers there is an extra level of
control. The controller needs feedback from the motor to tell it
when to commutate (switch the signal to the next pole). This can
take the form of a hall effect sensor (senses magnetic fields) of an
encoder. You have to match the motor feedback to the kind of
brushless amp you have.

The Rutex 991 is a Step/Direction to +-10 interface circuit that
works like this: The controller software sends a pulse to move the
motor (the software "thinks" it is talking to a step and direction
motor controller). The 991 sees the pulse and reads the motor
encoder. It moves the motor using a DC analog signal until it is
happy it moved the distance that one step is supposed to move it
(calibrated in the PC controller software) You would normally hook
it up so the the + - 10VDC output on the 991 goes to the + - 10VDC
signal inputs and a common signal ground. The servo controller needs
the RAW DC to drive the motors and the analog control signal.

The other Rutex and Gecko motor controllers don't use an external amp
and the motor movement is controlled directly from the module. The
Gecko is for brushed servo's only. Rutex has a DC brushless
controller.

All of the above approaches have a shortcoming. The controller
software has no knowledge of the actual motor position. This is
where the term "loop" is used. The loop is closed inside the motor
controller (or 991 and servo amp). In a true servo system the loop
is closed inside the software.

There exists a closed loop software solution using EMC as the
controller and a product by Jon Elson on this group. He can address
how his approach works with your setup.

If you desire is to use a DOS or Windows software controller using a
parallel port then you will need to use either the Rutex 991 and
external amps or a step to servo motor controller like the Rutex 990
(or 90 series) OR the Gecko 320/340.

There is a good yahoo group over at OSMC (open source motor control)
that is into the control of brushless motors.

>
> 2. What are the meaning of terms like current, open loop voltage,
> tachometer, and Hall velocity? And are there different types of
> brushless DC servos?

The motor commutation feedback method as mentioned above would be the
primary different types. There are also AC servo motors.

>
> Sorry if my questions are too elementary. I'd really appreciate it

if

> someone could point me to a source of information that would answer
> these types of questions (books, websites...). I'd like to obtain a
> holistic understanding rather than ask questions on a case by case
> basis.
>
> Thanks,
> Jim

The web is your best source of research into understanding motor
control. You need a basic understanding of electronics and the
relationship of current; voltage; resistance; inductance and
impedence as well as magnetics to start to grasp the more complex
control concepts. If, on the other hand, you would rather spin the
motors and use them to build a machine to cut things with using a PC
as the controller all you need to do is hang around this list.

"Do you want to know what time it is or how to build a watch?" ;-)