The CNC88 Servo System
The servo system is the heart of the CNC88
and 32MP Control. The development started back prior to the
introduction of the VMC45.
In 1974 that we began working on what was to be the heart of the CNC 88 control. Adrian de Caussin wrote the software for
assisting our NC programmers to create paper tape machining
programs. At the time the Teletype was the common means for creating
a paper tape. Editing was virtually non-existent, to edit a
program required cutting and splicing the actual tape.
Computer technology was in it's infancy, seven years before the
first IBM PC. We used a computer kit that you bought and
assembled which at the time was quite expensive. The first
unit was the IMSA 8080 with a whopping 8K of memory. We had to
even develop our own DOS to operate the Disk Drive.
The software allowed us to create and edit programs before punching
a paper tape for our Slo-Syn controls. As the software
progress we were able to actually do Cutter Radius Compensation,
this was a tremendous benefit since we used a lot of reground
endmills in the job shop. We were even able to program
backlash compensation in our paper tape which was unheard of at the
time but was tremendous in helping with the close tolerances we
needed.
As the control logic developed, the CNC boards we're being
developed, Dave and Larry de Caussin worked on the mechanical
designs of the VMC45. Everything came together about 1980.
The servos used at first were state of the art for the time, analog
amplifier with DC brush motors. Digital Brushless was years
away!
Today the Fadal machines out in the field use both DC Brush and DC
Brushless designs.
The DC Brush servo system:
The history of the DC motors goes back to the late 1970s, when we
first introduce the DC brush axis drive system with the VMC45.
Later with the VMC40, after working directly with Glentek engineers
in the design and development of a low cost/high performance servo
package, we looked for a second source supplier. After trying many
different suppliers, Baldor was selected as a second source OEM
supplier for both the large and small versions of the DC and AC axis
motors. Ordering a MTR-0002 or MTR-0010 never signified either a
Glentek or a Baldor motor; inventory determined which manufactures
product was shipped.
Interesting DC Motor Facts:
Many wonder what's the reason for a Resolver feedback?
At the time, the resolver feedback was much more common than the
encoder feedback technology. With the resolver being more common,
the cost differences was substantial. It wasn't so much just the
cost of the encoder itself but the electronics needed to process the
encoder inputs were expensive and slow. Using the resolver allows a
1 millisecond servo update cycle, which at the time was
unprecedented! The servo update time directly affects the accuracy
of servo contouring at higher feedrates. As always with CNCs; the
faster the better!
The Brushless servo system:
The history of the AC motors goes back to 1997 when we first
introduce the brushless axis drive system. After working directly
with Glentek engineers in the design, development and production,
Baldor was selected as a second source supplier for the large and
small versions of axis motors.
Interesting AC Motor Facts:
Many wonder what's the significance of the 8192 line
encoder. When going from the Brush System to the Brushless system,
we no longer had the DC tachometer on the motor. The encoder
replaces both the position and tachometer feedback. To match the
performance of a DC tachometer, using a digital feedback requires a
high line count (resolution). With an encoder, 8192 lines per 360
degrees results in 32768 counts per turn. This extreme detail
allows the digital recreation of a very accurate tachometer (the
heart of a servo system). Also the 8192 encoder gave the axis
controller (1010 card) an internal resolution of .000010", the plan
was to also achieve a programmable 10 millionths resolution in the
future. Unfortunately it was never taken advantage of in the CNC
control (1400) but it was used with the axis controller board. The
same 10 millionths resolution was achieved with the 5000 line
encoder because it was used with a .200 pitch ballscrew.
Is it AC Brushless?
Few know that technically, what's called "AC Brushless" is really
more accurately described as a "Permanent Magnet DC Brushless
System". It was a marketing decision to call it simply "AC
Brushless" to keep with industry standard terms.
Historical Reference of the Amplifier System
1700's
It's generally agreed that the beginning of the industrial revolution started around 1760. Of course this depends on which reference is used. Ultimately, the drive to automate repetitive tasks started about when humans did. The "end" of the industrial revolution supposedly occurred about 100 years ago, though looking around today it hardly seems over. Today's level of industry and automation easily surpasses the dreams of early inventors. Inventors that, through the course of the 1700 and 1800's, brought advancements in machine technology and primed the creation of today's motion control industry.
1800's
Motion control was non-existent and automation took the form of
crude motors with belt and pulley drive trains. Powering an
industrial building required a large water wheel outside or steam
engine sitting in the basement. Usually a vertical drive train ran
through the building from a steam engine in the basement to transmit
mechanical power to each floor. At the floor level, a transmission
converted power from the vertical drive train to a horizontal train
that spanned the floor. Each department needing mechanical power
tapped off the main line with a clutch mechanism. Sewing machine
operators, for example, used a foot clutch to engage individual
sewing machines to the power source.
1900's
Engineers used the momentum of the late 1800's to bring electrical
powered appliances to consumers. Edison's invention of the DC
generator in the 1870's, public electricity and Tesla's AC motor in
the 1880's, and the first electric hand drill in the 1890's gave way
to electric washing machines and refrigerators around 1915. By this
time Henry Ford had only recently realized a mobile production line
where parts were standardized and factory efficiency soared.
The Discovery of Feedback
It was 1927 when Harold Black revolutionized communications with the
concept of negative feedback in amplifiers. He was not the first to
close a feedback loop though, because thermostats and furnaces had
been regulating room temperature using feedback since the late
1800's. James Watt had worked on a mechanical feedback loop for his
steam engine even before that. In story like fashion Harold Black
had an epiphany on the way home from work one evening that applying
a portion of an amplifiers output back into the input could
substantially reduce signal distortion. Soon after Blacks discovery,
the first pneumatic motion control products arrived in the 1930's
employing feedback for closed loop control.
At this point, proportional-integral-derivative (PID) control was
just surfacing as a conscious thought for most of the world. J.C
Maxwell wrote a detailed mathematical analysis about PID in 1886,
but it took about 50 years for products intentionally using PID
tuning to arrive. The 40's and 50's marked the beginning of major
strides in PID control. People finally recognized the importance of
mathematical analysis and began developing control theory as a
science. This was, of coarse, a very crude period of PID control.
During the 50's, 60's and 70's, space flight and war helped spur the
effort to develop optimized control algorithms. Solid-state devices
and motor technology developed in the 60's to a point where PID
control migrated into microcontrollers. Various improvements and
optimizations continued until the late1970's when pulse width
modulation (PWM) switching technology was introduced along with
brush-less permanent magnet motors. Motion control hasn't been the
same since.
Digital Motion Control
During the last 20 years DSP, networking, and PWM switching
technology have created an exponential increase in the use of closed
loop motion control. PWM switching technology in amplifiers and
power supplies made high efficiency, low heat power transmission
possible. In just a few years, the size of a 2kW motor amplifier
shrank from 100 pounds or more, to something that could be hand
carried and bolted to a panel.
In about 1990, DSP based motion control products started allowing
sophisticated motion profiling and digital communication via serial
networks. Such rapid changes in technology created a breakdown in
standardizing motion control products. Network protocols such as
Profibus (1989), DeviceNet (1994), and Smart Distributed Systems
(1994), for example, attempted to take over the Control Area Network
(CAN) market. One of the first networks, CAN, had been around since
the mid 80's for automotive communication; it proved so versatile
that it moved into the automation world in the 90's. Sercos came out
in the early 90's using it's own hardware layer with fiber optic
transmission lines while other proprietary networks arrived using an
RS-485 hardware layer.
Today the industry is far from standardized with an incredible
availability in smart motion controller cards, servo amplifiers,
motors, feedback devices and mechanical linkages. See "Motion
Control Today" for a brief update on all of these options.
Brushless versus Brush-type Comparison
There are two basic types of motor design that are used for
high-performance motion control systems:
Brush-Type PM (permanent magnet), and Brushless-Type PM.
As you can
see in the figure, a brushtype motor has windings on the rotor
(shaft) and magnets in the stator (frame). In a brushless-type
motor, the magnets are on the rotor and the windings are in the
stator.
To produce optimal torque in a motor, it is necessary to direct the
flow of current to the appropriate windings with respect to the
magnetic fields of the permanent magnets. In a Brush-Type motor,
this is accomplished by using a commutator and brushes. The brushes,
which are mounted in the stator, are connected to the motor wires,
and the commutator contacts, which are mounted on the rotor, are
connected to the windings. As the rotor turns, the brushes switch
the current flow to the windings which are optimally oriented with
respect to the magnetic field, which in turn produces maximum
torque.
In a brushless motor there is no commutator to direct the current
flow through the windings. Instead, an encoder, hall sensors or a
resolver on the motor shaft senses the rotor position ( and thus the
magnet orientation). The position data is fed to the amplifier which
in turn commutates the motor electronically by directing the current
through the appropriate windings to produce maximum torque. The
effect is analogous to a string of sequencing Christmas lights: the
lights seem to chase each other around the string. In this case, the
magnets on rotor “chase” the magnetic fields of the windings as the
fields “move” around the stator.
The brushless motors are more reliable as Brush maintenance is
eliminated and no brush dust is generated.
The brushless motor can be driven to much higher RPM limits and
typically have lower inertia. The brushless motor also dissipates
heat more efficiently since the stator windings are thermally
connected to the outside of the motor case. It is also safer for
explosive atmospheres and quieter and less electrical noise
generated as there is no brush arcing in a brushless motor.
