isolation, current and velocity, encoders

Maybe the quote below from the installation manual for the Baldor UM
series of servo amps will help explain why an isolation transformer
is called out.

"INPUT POWER TRANSFORMER. The UM series servo control requires an
external isolation transformer that provides an ungrounded AC power
source. To determine the proper size transformer (kVA rating), use
this guideline:
(Rated Continuous Current)*(Bus Voltage)*(# of Axes)
So for example a UM4-100-2 rated at 15 continuous Amps with a 100VDC
buss voltage controlling three axes would require:
(15A)*(100VDC)*(3 Axes) = 4.5kVA.
This method of transformer sizing is safe and it may be a little
oversized."

Well, OK; I guess I'll go hunt down a 4.5kVA isolation transformer;
it would seem that "ungrounded AC power source" is the key here.

------------------------------------------------------------------

I know I've raised the question of torque vs velocity control mode
before, but I'd like to nail it down. The docs claim the amp can be
operated in current (torque) mode or velocity mode, and seem to say
that in torque mode no tach is required.

"The servo control (amplifier) consists of a power amplifier, a
velocity control amplifier, and fault logic circuitry.

The power section of a PWM bridge normally acts as a low output
impedance voltage source. However, if an analog signal representing
motor current is applied to the input of the amplifier and a current
command is present, the impedance of the PWM bridge becomes high.
These high impedance bridges exhibit a unique characteristic--motor
current and therefore motor torque become proportional to commanded
input current... A current sense coupler in the power amplifier
provides a current feedback mechanism; this unit isolates the high
voltage motor signal from the low voltage circuitry.

The high gain front end velocity control amplifier (VCA) accepts a
speed command (signal) and a tachometer voltage; the VCA clamped
error signal is applied to the power amplifier as a current command.

Current mode operation differs from velocity mode operation in that
the velocity loop is basically disabled (gain is reduced from 6000
A/V to approx 6 A/V. The current loop controls the output current
proportional to the input voltage (VCS). If a motion controller is
precisely controlling the velocity loop and supplying the +-10VDC
current command signal, tachometer feedback is not required. The
motor supplied would have an encoder (and a tachometer would not be
necessary.)"

So do you read this as saying I don't need no stinkin' tach so long
as the motion controller doesn't require one?

Baldor just happens to make a device called SMCC "Smart Motion
Control Card" which is "customer programmable to perform virtually
any type of position move." Furthermore, "The SMCC will interface
with any servo amp which requires +-10VDC ... and requires three
inputs: power to the board, encoder feedback for position data (a
train of digital position pulses), and an RS232 or 8 bit parallel
line for communciation with the customer's host computer." These
boards are claimed to do all sorts of wonderful things including
software servo tuning and programmabity with "a command language that
utilizes English abbreviations and a BASIC-like programming
structure. 'Monitor' servo interface software is available from
Baldor for programming the SMCC."

I haven't tried it, but I'll bet using Monitor would induce a 1976
flashback of being back in the Caltech poring over a JCL manual. I
guess Baldor sells an interpreter for its BASIC-like language.

Jon, does it sound like your planned card(s) can substitute for this
SMCC device?

Thanks.

--Jack