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Electric spindles get their turn

July 1, 2000
New control methods and better designs give synchronous spindle motors the edge in high-speed machining operations.

Take away their computers, lasers, and advanced structural features, and machine tools seem no different today than they were 100 years ago. They still move "metal" against "metal" over straight lines and curved paths, and their spindles still spin just like they always have. Well, almost.

Today's machine tool spindles still go 'round and 'round, but a lot faster and more precisely than the spindles of even a decade ago. The improvements are the result of water-cooled synchronous motors and digital servo-amplifiers that regulate motor flux. Together, the components form a new breed of high-speed electric spindles that are changing the face of an industry.

Electric spindles

An electric spindle is an electromechanical assembly consisting of a motor, a spindle shaft and its bearings, and some sort of tool holder. It may also include provisions for cooling.

Electric spindles are becoming common in high-speed machining and precision milling equipment because they make machines simpler, more accurate, and more compact. They also open the door to new machining techniques such as lathemilling and large-scale material removal.

One of the drawbacks associated with electric spindles stems from the use of induction motors. During heavy use, high rotor losses heat up the spindle shaft, causing it to expand. It's not uncommon for axial expansion to introduce as much as 50 μm of tool offset. Shaft heat also flows to the bearings, dramatically shortening their service life.

Though induction motors are the norm for electric spindles, a growing number of machine-tool builders are beginning to use permanent-magnet synchronous motors instead. Unlike induction motors, these special synchronous motors, designed specifically for spindles, are not subject to rotor losses. Their permanent-magnet rotors draw no electric power. All losses occur in the stator, where the heat can be easily removed using water cooling or other techniques.

In recent tests at the University of Darmstadt in Germany, independent researchers showed that low-loss synchronous spindle motors not only run cooler than asynchronous motors, but also are more precise, producing half the axial displacement. And it could have been less.

Most spindles, including the one used for the test, incorporate angular contact bearings, which have an inherent axial offset of around 10 μm. The offset is due to uneven centrifugal forces caused by asymmetries in bearing construction. With better bearings, synchronous spindle motors can achieve total axial displacements of less than 10 μm.

Why brushless

Not long ago, it would have been unheard of to even suggest putting a brushless motor on a spindle. Brushless motors were designed for something altogether different, primarily positioning or servo applications. The cost of the amplifier alone – for a spindle, it would have to be overrated by as much as a factor of 15 – was more than enough to keep brushless motors out of the picture.

But things changed. For one, machine tool builders started turning to lighter spindle motors in the pursuit of higher feed rates. Spindles are often conveyed along feed axes, so making them lighter is one way to pick up the pace.

While feed rates were going up, cycle times were coming down. All of a sudden spindle motors were thrust into the "hurry up" mode. Instead of being able to spin up and down at a leisurely pace, spindle motors now had to burst into action and often stop just as abruptly to get set for the next cycle. Some tool builders went so far as to close the position loop, so spindles could synch up with other machine axes as well as assist in tool changes. The line between spindle and servo motors was blurring.

Meanwhile, motor manufacturers were chipping away at the amplifier problem. It's not as if there was anything inherently wrong with early servo-amplifiers; they just weren't designed with spindle applications in mind. To meet the power requirements of a spindle meant that the amplifiers had to be significantly oversized.

Amplifiers, whether they're used on spindles or feed axes, have to deliver a certain amount of power. The requirements, in terms of current and voltage, are determined by the application, motor, and control method. Usually, the application dictates the motor, which, in turn, sets minimum current and voltage levels.

Using conventional control techniques, a brushless spindle amplifier must deliver enough current to meet the motor's low-speed torque requirements, and enough voltage to meet the maximum speed requirements at nominal flux. To achieve this output calls for an amplifier three to 15 times more powerful that what it really needs to be.

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Get the flux out

Rather than work within the constraints of conventional amplifiers, researchers at Parvex started looking at new control approaches and more efficient motor designs. They figured that if they could find a way to reduce motor flux without sacrificing performance, they'd be able to relax the voltage requirement on amplifiers and subsequently bring down costs.

One thing they did was adopt a type of brushless motor that optimizes magnetic flux. Based on a stacked-plate rotor with embedded rare-earth magnets, such motors concentrate most of their flux in the air gap. With this design, it's possible to have flux densities in the gap exceeding the working flux density of the magnets.

The big breakthrough, however, was the development of a new control method called gradual flux control. The researchers found that if they tightly regulated the phasing, or sequencing, of stator currents, they could reduce the magnetic flux in the motor once it surpassed a certain speed. The benefit of such "defluxing" is that it lowers the apparent voltage on the motor created by the back electromotive force.

Without defluxing, the apparent voltage at high speeds would be huge, requiring a correspondingly large servo-amp. But by trimming this voltage, defluxing forces the motor to operate within the limits of a reasonably sized amplifier. And because it only kicks in at high speeds, it doesn't sacrifice starting torque and responsiveness.

Amplifiers that employ the patented new control method use Risc (reduced instruction set computer) processors to grind out all the calculations. For a six-pole synchronous spindle motor, the processors can analyze resolver signals and compute stator currents in under 100 μsec. This translates to operating speeds of 30,000 rpm and, because of high starting torque and constant power output, the possibility of direct electric drive.

Choices in permanent magnet motors

Brushless dc motors are available in two primary styles, differentiated mainly by the construction of their rotors. One type of rotor consists of an iron-silicon armature on which magnets, usually samarium-cobalt, are deposited and held in place by epoxy.

Another type of rotor consists of a stack of punched plates with embedded magnets. It accommodates ferrite (samarium-cobalt) as well as rare-earth (neodymium-iron-boron) magnets. With this design, it's possible to get average flux densities in the air gap that are higher than that of the magnet.

What's a servomotor?

A"servomotor" is any motor used in a closed-loop system to control position or regulate speed. Although technically it's not a motor type, one type of motor shows up most often in servo applications: permanent magnet motors.

Servomotors should possess several qualities. They should produce high torque over a wide speed range, including at rest; they should run smoothly at low speeds with the ability to closely follow paths (low torque ripple); and they should accelerate quickly owing to low rotor inertia. Putting it all in a small package with a high torque-to-weight ratio is also a plus.

For servo applications, machine tools use either dc motors or permanent-magnet synchronous motors. Dc motors seem to dominate at low torque (below 3 Nm), while synchronous motors are more common at medium to high torque (1 to 200 Nm).

Worth the Risc

At one time, servo-amplifiers consisted entirely of analog circuits. Today, they may be part analog, part digital; or even completely digital.

Many designers have a tough time choosing between digital and hybrid (analog-digital) amplifiers because they're not sure how to size up the computational complexity of their application. All they know is that the more complicated and precise the motion, the greater the processing requirements.

Because digital servo-amplifiers process all control loops in software, their performance is linked to that of the computer, or microprocessor, inside. In a machine-tool spindle application, the microprocessor has to handle millions of instructions per minute (Mips) to execute the necessary control algorithms. If the microprocessor bogs down, the cutting tool won't move the way it's supposed to, which could be disastrous.

Fortunately, chip makers have come up with low-cost, high-speed processors called Risc (reduced instruction set computer) chips. Risc chips are the classic embedded computer, designed to run printers, plotters, graphics accelerators and other PC peripherals. The reasons they excel in such applications – they combine extensive I/O with a powerful math core, plus they're easy to program and cost only a few dollars – make them ideal for industrial use.

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The main difference between a Risc processor and a microprocessor like the Intel 80386 or '486 is that a Risc chip does fewer things, but usually does them better. As the main engine in a PC, a general-purpose processor has to be more flexible and, therefore, has a bigger and more complicated instruction set.

A smaller instruction set not only makes Risc processors easier to program, it makes them more efficient. Operations that may take four or five instructions on a conventional processor, could very easily take only one instruction, corresponding to a single clock cycle, on a Risc chip.

What Risc means in motion

• dynamic speed range of 1:50,000
• speed, current, and position control over a serial interface
• more efficient servo regulators, with theoretically infinite static stiffness
• ability to compensate for friction and vibration using torque predictors
• motor control to electrical frequencies of 1,500 Hz

Frank Canestrier is CEO and James Reynolds is Marketing Director for Parvex Inc., a BTR Power Drives Co.

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