Machinedesign 1593 Brushless Motors Vibration 0 0

Waging war on Vibration

March 1, 2000
Brushless motors, good bearing systems, and a well-balanced load are the best weapons against vibration in today's high-speed precision machinery

Vibration and noise are an inevitable part of any motor- driven motion system. Their effects can be farreaching, impacting everything from system performance to cost, reliability, and safety.

Even small mechanical vibrations – though they may not be very irritating to the ear – can be unacceptable. Working silently, such vibrations may be accelerating material fatigue, compounding the natural rate of wear on friction surfaces, and degrading overall system efficiency. In handheld devices, low-energy vibrations can pose a health risk, causing repetitive stress injuries. If vibration is in the auditory range, the resulting noise may be a health risk as well as a safety issue.

Although vibration cannot be 100% eliminated, it can be reduced to a more manageable level. The answer, as many designers of high-speed precision systems are finding out, is a combination of brushless motor technology and advanced high-grade bearing mechanisms. Understanding the dynamics of the load also helps, as does including the motor supplier in the system-design process.

Design challenges

Even the simplest motion system is subject to complex interactions between the internal vibration properties of the motor and the external physical qualities – load, mass, balance, harmonics, damping, etc. – of whatever the motor is driving. Consequently, systems designers must work in conjunction with motor suppliers, addressing both internal and external factors, to hit noise and vibration goals for their end products.

Brushless motors are currently used in a variety of applications:

• Turning fans in everything from medical ventilators to computer systems
• Spinning optical mirrors in barcode readers
• Driving buffers and polishers in semiconductor manufacturing systems
• Rotating high-speed centrifuges and dental drills
• Powering sparkless fluid pumps in explosive environments
• Positioning light loads in highspeed industrial processes.

In any of these applications, too much vibration and noise can be disastrous. In respiratory blower systems, for example, a poorly calibrated motor can transmit vibrations into the fan, producing erratic airflow and a stressful amount of noise. If, on the other hand, the fan is unbalanced, too heavy, or fails to sufficiently match the motor's specifications, even the lowest vibration brushless motor available may not be able to compensate for external design deficiencies.

In a similar manner, if the cables, belts, or ballscrews in an XY positioning system are not adequately designed to smoothly handle the overall acceleration and deceleration requirements, the resulting vibration is likely to be well beyond that of the motor and virtually impossible to compensate.

Sound of silence

It takes more than a trained ear to assess auditory noise. First you need a suitable environment optimized for noise measurements. Then you need equipment that's sensitive enough to distinguish the subtle differences between one sound and another. Someone also has to decide what's acceptable in terms of decibels, operating conditions, and test specifications.

Since the components in a motion system are often made of different materials, they can have significantly different coefficients of thermal expansion. Some may expand or contract slightly with temperature changes (ambient as well as internal), while others may respond more dramatically. Although these differences may cause only minute changes in the tolerances between parts, it's not uncommon for them to translate to significant changes in the amount of vibration.

In most instances, individual motor components are tested and certified to meet specific noise parameters, giving the system designer a reliable starting point from which to make motor component selections. But this is just a starting point. Ultimately the motor and load assembly must be tested together to ensure compliance with noise objectives.

For this reason, more and more OEMs are relying on motor suppliers for a larger portion of the assembled system. Not only does this simplify the overall manufacturing process, it delivers a fully tested and certified assembly.

Material selection in design

One of the most critical factors in controlling vibration is minimizing the overall mass of the motor and load. For example, an examination of many medical, computer, and industrial fan assemblies reveals a trend toward plastic fans, a lighter alternative to traditional metal fans. Plastic's inherently lower mass minimizes the amount of energy required to overcome the fan's natural inertia, reducing the mechanical stresses within the fan assembly that can lead to excess vibration.

On the downside, because they are inherently less rigid, plastic fans can also show a greater tendency to deform under acceleration and at higher speeds. This deformation can potentially unbalance the fan assembly, leading to intermittent vibration problems at various points within the specified operating ranges.

Tighter control over fan manufacture has also shown to be of great assistance in reducing vibration. Traditionally, it was often an accepted fact of life that some fan assemblies had to be manually balanced by removing or adding material during a testing process which was conducted after the fan was attached to the motor. Obviously, the requirement for such secondary operations could add significant cost and time to the assembly process while also decreasing overall production yields and increasing waste. By investing in more stringent up-front process control during the fan molding process, it's possible to mix-and-match fans and motors that are mutually balanced within acceptable vibration limits.

Couplings and damping effects

Another system-level factor that can inject unwanted vibration is the use of intervening couplers to transmit energy between the motor and load.

Every point at which energy is transferred between two components is an opportunity to lose power and add to system-level vibration. Obviously, in certain XY positioning systems, the use of a coupling cannot be avoided. However, for many rotary applications, all it takes is a longer motor shaft or custom mounting arrangement to let the system designer achieve direct-drive without intervening couplers or counterbalances.

In evaluating their overall systemlevel vibration budgets, system designers also need to take into account the effects of damping actions, both within the motor and throughout the system. In essence, damping refers to the natural tendency of the surrounding structure to absorb vibrational energy. When an exciting force is equal or close to one of the natural frequencies of the structure, the amplitude of vibration is influenced by the damping capacity of the structure, which effectively converts vibration energy to heat.

Inside and out

It's a common belief among some engineers that motor vibration can be treated by merely attaching a counterbalance to the shaft extending from the back of the motor. Such crude methods, however, cannot adequately compensate for internal vibrations in today's high-precision applications. Worse, the additional load of the counterweight can actually diminish torque, and place undue stress on motor parts and bearings. Clearly, the way to reduce system vibration is to start with the motor.

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For an application like a wafer polisher running at 3,000 to 4,000 rpm, the only practical way to eliminate unacceptable vibration is through precision internal motor balancing and calibration. The process actually starts long before the manufacturing phase. If every motor component isn't adequately controlled from the get go, it will compromise the overall quality of the motor. Simply put, quality can't be "added on" in a subsequent process.

A shaft that isn't perfectly straight, for example, adds incremental errors that can accumulate to an unacceptable level by the time they manifest themselves in the final system – no matter what you do. In general, for high-precision brushless motors, the use of machined parts rather than stampings can stem such cumulative effects through tighter internal tolerances.

Another key factor in optimizing internal balance is the use of specialized magnetic materials to achieve higher energy densities in the rotor within a smaller form factor. Bearings also play a role, helping achieve consistent low-vibration performance that lasts over an extended period of time.

In a brush-type motor, the brushes are generally the limiting factor, with a typical useful life ranging from 500 to several thousand hours. In contrast, a brushless motor can last between 5,000 and 30,000 hours, if the bearings are adequately designed. Low-grade brushless motors invariably fail due to the effects of vibrational wear on poor quality bearings, compounded by a failure to appropriately balance the load.

Bearing wear is essentially a function of vibration frequency, which is determined by the speed of rotation, the continuous load on the bearings, and the intensity of vibration owing to external load imbalances. Invariably, overall system requirements dictate the frequency at which loading and unloading occurs, but it is within the motor designer's realm to help mitigate the intensity. Proper attention to bearing design and use of high-quality ball bearings can reduce load intensity by a factor of 100 or more, greatly extending system life at an optimal performance level.

It takes two

Two heads are better than one, especially when it comes to vibration control. By working together, system designers and motor suppliers have a far better chance of getting motors and loads to function smoothly (as a single unit with minimal noise and vibration) than if they worked apart.

"Working together" doesn't just mean handing off a list of specs from which to order motor components, however. Particularly for precision applications, it means establishing a collaborative relationship early in the development process. Not only does this approach achieve a more optimum end product, it pays significant dividends by avoiding unnecessary design modifications.

Another benefit of collaborative engineering – where decisions are made based on a shared understanding of the application objectives – is that it tends to more quickly resolve design and performance problems as they arise. It also helps system designers better manage tradeoffs between cost and performance by keeping level-of-effort, for both design and manufacturing, in line with the specific requirements at hand.

Consider a case where a manufacturer of medical systems needed a stringent combination of low vibration and lowcost manufacturing. By tailoring the balancing process to the application, what might have taken half an hour was reduced to just five seconds. Naturally the amount of time and money saved can be leveraged to improve speed, precision, or reliability elsewhere in the design.

Frank Leong is Systems Applications Engineer and Ron Hippe is Motors Applications Engineer for the BEI Sensors and Systems Co., Kimco Magnetics Division, San Marcos, Calif.

Super magnets put a damper on vibration

Newer, more powerful magnets, such as neodymium-iron-boron, are helping in the fight against noise and vibration. Because of its higher energy density, NdFeB makes possible brushless motors with smaller rotor assemblies, achieving optimal internal balance and minimal vibration. By contrast, conventional alnico (aluminum nickel cobalt) or ferrite magnets must be larger to produce the same amount of torque, making the rotor bigger and more difficult to balance.

The key to NdFeB is its ability to reduce the diameter of the rotor with a commensurate reduction in inertia, imbalance, and the tendency to vibrate. Another benefit: Neodymium doesn't demagnetize with time like other magnet materials, so it provides more consistent performance over a longer period.

In the case of high-temperature applications, samarium-cobalt may be an even better alternative because it combines high energydensity with exceptional thermal characteristics.

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