About Servos

by David M. de Caussin

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 starting in late 1995, we were ready to begin introducing the first Brushless systems in 1997. It was later that Baldor was selected as a second source supplier for the large and small versions of axis motors. 

The  benefits of the AC Brushless system verses DC Brush system is as follows:
1) Higher resolution feedback provided a future for more precise motor control.
2) Simplification - the Brushless motor eliminates the complication of the brush commutator and the Brush tachometer. This greatly improves reliability and reduces costs.
3) Duty Cycle - with the Brush motors, the magnets are stationary, the coils are wound around the motors armature and rotate inside the motor. With the Brushless motor, it is the opposite, the coils are stationary and the magnets are attached to the armature and rotate inside the motor. With the Brushless system having to coils stationary they are closer to the outside of the motor. This greatly improves the ability of the heat that is generated by the coils during operation to escape the motor. Where as with the Brush motors having the coils deeper inside the motor makes it difficult for the heat to escape and thus reduces the duty cycle of the motor. Duty Cycle is a matter of managing the heat produced by the electromagnetic torque, the less heat, the longer the motor can operate without damaging the coils.

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

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.

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.

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.